separation preconcentration and spectrophotometry in inorganic analysis

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Separation, Preconcentration and S pect ro p hotomet ry i n I norg an ic Analysis

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Analytical Spect roscopy L ibrary - 10

Separation, Preconcentration and Spectrophotometry in Inorganic Analysis

by Zygmunt Marczenko and M a r i a B a l c e r z a k

Department of Analytical Chemistry, Warsaw University of Technology, Naokowskiego 3, 00-664 Warsaw, Poland

Translated by Eugeniusz Kr

2 0 0 0

E L S E V I E R

A m s t e r d a m - L a u s a n n e - N e w York - Ox fo rd - S h a n n o n - T o k y o

This work is an enlarged translation of Spektrofotometryczne metody w analizie nieorganicznej by Zygmunt Marczenko and Maria Balcerzak �9 Wydawnictwo Naukowe PWN, SA, Warszawa, 1998 Published by arrangement with Polish Scientific Publishers PWN

�9 2000 Elsevier Science B. V. for the English edition only

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Contents

Preface ...................................................................................................................................... 3 Abbreviations ........................................................................................................................... 4

Part I. General

Chapter 1. Chapter 2. Chapter 3. Chapter 4.

Separation and preconcentration of elements ..................................................... 5 Principles of spectrophotometry ........................................................................ 26 Spectrophotometric methods ............................................................................. 39 Spectrophotometric reagents ............................................................................. 53

Part II. Determination of Elements

Chapter 5. Chapter 6. Chapter 7. Chapter 8. Chapter 9. Chapter 10. Chapter 11. Chapter 12. Chapter 13. Chapter 14. Chapter 15. Chapter 16. Chapter 17. Chapter 18.

Chapter 19. Chapter 20. Chapter 21.

Chapter 22.

Chapter 23. Chapter 24. Chapter 25. Chapter 26.

Chapter 27.

Chapter 28.

Chapter 29.

Alkali metals ..................................................................................................... 77 Aluminium ........................................................................................................ 83 Antimony .......................................................................................................... 92 Arsenic .............................................................................................................. 99 Beryll ium ........................................................................................................ 107 Bismuth ......................................................................................................... 113 Boron .............................................................................................................. 121 Bromine .......................................................................................................... 129 Cadmium ........................................................................................................ 133 Calcium .......................................................................................................... 140 Carbon ............................................................................................................ 147 Chlorine .......................................................................................................... 152 Chromium ....................................................................................................... 159 Cobalt ............................................................................................................. 167 Copper ............................................................................................................ 177 Fluorine .......................................................................................................... 189 Gallium ........................................................................................................... 198

Germanium ..................................................................................................... 204

Gold ................................................................................................................ 210 Indium ............................................................................................................ 216 Iodine .............................................................................................................. 222 Iron ................................................................................................................. 226

Lead ................................................................................................................ 238 Magnesium ..................................................................................................... 247

Manganese ...................................................................................................... 253

Chapter 30.

Chapter 31.

Chapter 32.

Chapter 33.

Chapter 34.

Chapter 35.

Chapter 36.

Chapter 37.

Chapter 38.

Chapter 39.

Chapter 40.

Chapter 41.

Chapter 42.

Chapter 43. Chapter 44.

Chapter 45.

Chapter 46.

Chapter 47.

Chapter 48.

Chapter 49.

Chapter 50.

Chapter 51.

Chapter 52.

Chapter 53.

Chapter 54.

Chapter 55.

Chapter 56. Chapter 57.

Mercury .......................................................................................................... 262

Molybdenum and tungsten ............................................................................. 270

Nickel ............................................................................................................. 284

Niobium and tantalum .................................................................................... 293

Nitrogen .......................................................................................................... 304

Oxygen ........................................................................................................... 315

Palladium ........................................................................................................ 318

Phosphorus ..................................................................................................... 326

Platinum .......................................................................................................... 334

Rare-earth elements ........................................................................................ 341

Rhenium ......................................................................................................... 350

Rhodium and iridium ...................................................................................... 357

Ruthenium and osmium .................................................................................. 365

Scandium ........................................................................................................ 375

Selenium ......................................................................................................... 379

Silicon ............................................................................................................. 385

Silver .............................................................................................................. 392

Strontium and barium ..................................................................................... 399

Sulphur ........................................................................................................... 403

Tellurium ........................................................................................................ 412

Thall ium ......................................................................................................... 418

Thorium .......................................................................................................... 424

Tin .................................................................................................................. 431

Titanium ......................................................................................................... 438

Uranium .......................................................................................................... 446

Vanadium ....................................................................................................... 456 Zinc ................................................................................................................. 466

Zirconium and hafnium .................................................................................. 474 Appendix .............................................................................................................................. 483

Index ..................................................................................................................................... 514

Preface

Spectrophotometry enables to determine, with a good precision and sensitivity, almost all the elements present in small and trace quantities in any materials. The method is particularly useful in the determination of non-metals.

In the book, much attention has been paid to separation and preconcentration methods, since they play an essential role in increasing the selectivity and sensitivity of spectrophotometric methods. The separation and preconcentration methods have been utilised also in other determination techniques.

Modern spectrophotometers, supplied with data-processing capabilities, enable the treatment of absorption spectra in the derivative spectrophotometry. The spectrophotometric methods can be easily automatized, e.g. in the flow injection analysis.

Chapters 1-4 provide the characteristics of the separation and preconcentration methods: solvent extraction, flotation, coprecipitation with collectors, volatilization, ion exchange etc. These chapters deal also with the fundamentals of spectrophotometry, spectrophotometric methods of analysis, and most important chromogenic reagents. Chapters 5-57 have been devoted to individual elements or groups of related elements.

In the monograph much attention has been paid to the application of the methods in analytical practice. The references, listed at the end of each chapter and critically selected, cover the works published until the end of 1999.

The book has been designed for analytical chemists dealing with environment protection, geology, biology, many branches of industry, etc. It will also be a useful guide for students interested in becoming specialists in analytical chemistry.

Zygmunt Marczenko and Maria Balcerzak

Abbreviations

BPHA 5-Br-PADAP CAS CP CTA DAM DAPM DDTC DIPE DMF ECR EDTA g

FIA HzDm H2Dz HTTA

MIBK NTA oxine PAN phen ppb ppm REE SPADNS sp. abs., a TAN TAR TBP TEA TOA TOPO TPTZ

N-benzoyl-N-phenylhydroxylamine 2-(5-Br-2-pyridylazo)-5-diethylaminophenol Chrome Azurol S cetylpyridinium ion (or salt) cetyltrimethylammonium ion (or salt) diantipyrylmethane diantipyrylpropylmethane diethyldithiocarbamate di-isopropyl ether dimethylformamide Eriochrome Cyanine R ethylenediaminetetraacetic acid (or disodium salt) molar absorptivity flow-injection analysis dimethylglyoxime dithizone, diphenylthiocarbazone thenoyltrifluoroacetone wavelength methyl isobutyl ketone nitrilotriacetic acid 8-hydroxyquinoline, 8-quinolinol 1-(2-pyridylazo)resorcinol 1,10-phenanthroline parts per billion parts per million rare-earth elements 2-(4-sulphophenylazo)chromotropic acid specific absorptivity 1 -(2- thiaz olyl azo)- 2- naphthol 4-(2-thiazolylazo)resorcinol tri-n-butyl phosphate triethanolamine tri-n-octylamine tri-n-octylphosphine oxide 2,4,6-tri(2'-pyridyl)-s-triazine

Chapter 1. Separation and preconcentration of elements

The spectrophotometric determination of elements is usually preceded by their separation from major components (matrix) and from interfering elements, the effects of which cannot be eliminated by other methods such as masking or change of pH of the medium. In the trace analysis of high-purity materials, separation from the matrix involves simultaneous concentration of the trace components. General methods of preconcentrating and separating elements have been outlined in several monographs and reviews [1-4].

The present Section provides a discussion of the following separation and preconcentration methods: solvent extraction, precipitation and co-precipitation with collectors, volatilization, and methods based on the use of ion-exchangers and other sorbents. These methods are used not only with spectrophotometry, but also in conjunction with other methods of determination.

1.1. Solvent extraction

1.1.1. Introduction

The extraction process and extractive methods for separation and preconcentration of elements are described in several monographs and reviews [5,6].

Solvent extraction separation is based on differences in the solubilities of elements and their compounds between two immiscible liquid phases. Usually, the initial phase is an aqueous solution and the second phase is an organic solvent, immiscible with water. Some properties of the more common organic solvents are listed in Table 1.1. The ion to be extracted into the non-aqueous phase should first be transformed into an uncharged species.

Stripping ("re-extraction", "back-extraction", or "scrubbing") involves bringing the element from the organic extract back into the aqueous phase.

The extraction efficiency, i.e., the degree of transfer of the species from the aqueous to the organic phase, is defined in terms of the distribution- (or extraction-) coefficient, (D). The quantity D is the ratio of total concentration (i.e., the concentration of all the existing forms) of the element in the organic phase (Ec0) to the total concentration in the aqueous phase (ECw) in the aqueous phase, at equilibrium

Z C o

The extraction efficiency (%E) is also expressed as the extraction percent

100D %E =

D + ( V w / V o )

where D is the distribution coefficient, and Vw and Vo are the volumes of the aqueous and the organic phases, respectively.

6 1. Separation and preconcentration of elements

When the distribution coefficient of a given element in a specified system is large (e.g., 1,000, i.e., log D = 3), a single extraction will suffice. In most extraction systems the partition coefficients change as the concentration of the substance extracted changes; in most cases they decrease with decreasing concentration [7].

Tab le 1.1. Physical propert ies of some organic so lvents

Solvent

Acetate, n-amyl n-butyl ethyl

Acetone Alcohol, n-amyl

n-butyl ethyl methyl

Benzene Carbon tetrachloride Chloroform Cyclohexane o-Dichlorobenzene 1,2-Dichloroethane Dioxan Ethers, di(2-chloroethyl)

Diethyl di-isopropyl (DIPE)

Hexane Methyl isobutyl ketone (MIBK) Methylene chloride

(dichloromethane) Mesityl oxide Nitrobenzene 1 -Octanol Tetrachloroethylene Toluene Trichloroethylene

Density 9/ml 0.87 0.88 0.90 0.89 0.81 0.81 0.79 0.80 0.89 1.59 1.50 0.78 1.30 1.26 1.03 1.22 0.72 0.73 0.66

0.80

1.34 0.85 1.21 0.83 1.63 0.87 1.46

Boiling point o C 149 126 77 57 138 118 78 65 80 77 61 81 180 83 101 178 35 68 69

116

40 129 211 194 121 111 87

Dielectric constant

4.8 5.0 6.0

20.7 13.8 17.1 24.3 32.6 2.3 2.2 4.8 2.0 9.9 10.4 2.2

23.0 4.3 3.9 1.9

13.1

9.1 15.6 34.8 10.3 2.3 2.4 3.4

Solubility in water %

0.2 0.5 8.6

misc. 2.2 7.9

misc. misc. 0.2 0.1 1.0

0.01 0.01 0.9

misc. 1.0 7.4 0.7

0.02

2.0

2.0 3.2 0.2 0.05 0.02 0.05 0.1

misc. - completely miscible

Shaking the phases in a separating funnel during the extraction or re-extraction must be continued until equilibrium is attained. The time required for the system to reach equilibrium varies from seconds to several minutes, depending on the kinetics of the process [8-10]. When the shaking time recommended is more than two minutes, it is advisable to use a mechanical shaker.

Extraction is equally useful in the preconcentration and separation of small amounts of elements, and in the separation of macrocomponents from traces. Extraction methods generally require less time than precipitation methods. The former give also "purer" separation of elements owing to the small area of phase contact. Co-extraction occurring in some cases [ 11 ] has not been widely used in extraction separations.

1.1. Solvent extraction 7

1.1.2. Extraction systems

Extraction systems may be divided into two classes: (1) uncharged covalent species (simple molecules and chelates), and (2) ion associates (ion pairs).

Simple molecules (e.g., I2, HgC12, AsC13, BiI3, GeC14, OsO4) are extracted with non- polar solvents such as benzene, CHC13, CC14. The extraction of this type of compound is comparatively selective and is widely applied in separation of some elements [5,12,13].

Inner chelates (uncharged chelates) are formed when metal ions react with bifunctional ligands, such as dithizone (formula 1.1), 8-hydroxyquinoline [14] (formula 1.2), dithiocarbamates (formulae 1.3 and 1.4), ethyl xanthates [15,16], cupferron (formula 1.5), BPHA (N-benzoyl-N-phenylhydroxylamine) [17,18], acetylacetone (formula 1.6) and thenoyltrifluoroacetone (HTTA) (formula 1.7) [19,20].

S=c/NH--NH-~ I

(1.1)

- - N - - - - - - N H ~

Hs_C// \"-J/

II

(1.2)

CzH5 .,~S \N__C// CzH/ ~S-Na §

CzHs_ S "X~ N--ICI- S- [ NH2( Cz Hs)z ] +

CzH/

(].3) (1.4)

O" NI'I~' H3C~ N/ HC~ ,C---OH i H ~N~O

H3 C/C~''~-'O

(1.5) (1.6) (1.7)

Inner chelates are extracted with non-polar solvents (mostly with CHC13 and CC14). Synergism [21,22] is important in the extraction of some chelates.

Some inner chelates were extracted into chloroform solutions of diantipyrylmethane (DAM) (formula 1.8) [23]. The effects of salting-out agent, solvent, and temperature on the


H3C----C~C--CH2-- C ~ C - - C H 3 I . I t H3C~N~N~C---'~ 0 0~----C~ 7N---'.-CH3

i l C6Hs C~Hs

(1.8)

extraction have been discussed [20]. Selectivity can be increased by using the exchange technique, in which a less-stable metal chelate is the source of the chelating agent [24].

The extraction of chelates is usually applied to preconcentration and separation of small amounts of metals. Owing to their low solubility in organic solvents, most chelates can not be used for the extraction of macrocomponents. Cupferronates and acetylacetonates are exceptions.

Chelates of metal ions with alkyl- and arylphosphoric and thiophosphoric acids can be extracted into chloroform and other solvents [25,26]. Such systems enable one to separate, by means of extraction procedures, many metals from strongly acid solutions. Examples of such reagents are di-(2-ethylhexyl)phosphoric acid (HDEHP) and di-n-butyldithiophosphoric acid (formulae 1.9 and 1.10). HDEHP is a viscous liquid (density 0.98), slightly soluble in water, but readily soluble in benzene, hexane, and MIBK.

~ zHs CI.Hg~CH~CHz~O~ ~,0

P, C~Hg~CH~CHz~O ~ ~OH

~zHs

(1.9) (1.10)

Ion associates (ion-pairs) are formed by the electrostatic attraction of oppositely- charged ions which have, in general, high molecular weight. Ion-associates may be divided into several groups.

Halometallic acids (e.g., HFeC14, HSbC16, HAuBr4, H2CdI4) are formed in reactions of multivalent metal ions with hydrohalic acids. These compounds are extractable from acid solutions containing high concentrations of halide ions by oxygen-containing solvents such as ethers, higher alcohols, ketones, and esters [27]. The extraction of halometallic acids is made possible by solvation of the protons by the solvent molecules, and often secondary solvation of the ion-pair formed. Since the solvent molecule co-ordinates through its oxygen atom, such systems are sometimes called oxonium extraction systems.

Extractions from chloride- [28,29], bromide- [30], iodide- [29,31,32], fluoride- [33], and thiocyanate- [34] systems have been discussed.

Heteropoly acids [oxygen compounds of Mo(VI), W(VI), Si, P(V), As(V), Ge, and other elements] and their reduction products (molybdenum blues) are extracted into oxygen- containing solvents by a mechanism similar to that above.

Another group of ion-association systems is represented by solvated salts (usually nitrates, but also halides and sulphates). Solutions (1-50%) of tri-n-butyl phosphate (TBP, formula 1.11) in hexane, CC14, and solutions (1-20 %) of tri-n-octylphosphine oxide (TOPO, formula 1.12) in cyclohexane are most often used as the extractants. Solvation with TBP or TOPO (through the strongly basic oxygen atom of the phosphoryl group) enables metal salts,


such as UO2(NO3)2"2TBP, Th(NO3)4"2TBP, Zr(NO3)4"2TOPO, TiOSO4"2TOPO, to be extracted with non-polar solvents.

UO2C12-3TBP, and

C~. H9 C ) Ca Hr/ \ \

C4HgO-"-~P ----~0 (1.11) Ce HI?----p ~----0 C4 HgO / CeH(

(1.12)

High molecular-weight amines are of particular importance in the extraction [35-39]. They form ion-associates with acids (e.g., HSCN, HReO4, HI), metal-complex acids [e.g., H2PtC16, HFeC14, H2UO2(SO4)212, and heteropoly acids. These complexes are extractable into non-polar solvents (e.g., C6H6, CHC13, CC14) and polar solvents (e.g., MIBK, amyl alcohol). Tertiary amines, such as tribenzylamine (TBA) and tri-n-octylamine (TOA) are most commonly used. Tertiary and secondary amines are used for extraction of anions from acid solutions, whereas quaternary ammonium salts enable also extraction from neutral and alkaline solutions. Secondary and tertiary amines and quaternary ammonium salts are applied as solutions in non-polar solvents. Diphenylguanidine (formula 1.13) is also frequently used in extraction.

~ H N\ .C-m--NH

(1.13)

Some amines which extract anions from aqueous solutions as ion pairs are called liquid anion exchangers [40,41]. They are marketed commercially under such trade names as Aliquat-336 and Amberlite LA-1. They are mixtures of secondary or tertiary amines, or quaternary ammonium salts, with alkyl groups having 7-12 carbon atoms.

Antipyrine and its derivatives [42,43] such as diantipyrylmethane (DAM, formula 1.8) are high molecular-weight amine extractants. In acid solutions, diantipyrylmethane is protonated, and the resulting cation combines with an anion to form an ion-associate (e.g., DAM.H+] [T1Br4-], [DAM.H+]z[CdI42-].

Ion associates formed by large cations and large anions need not be solvated and are extracted with inert solvents, such as CHC13, CC14, or toluene. Examples of large cations forming non-solvated salts are the tetraphenylarsonium ion (formula 1.14) and tetraphenylphosphonium ion. These cations form extractable ion-pairs with anions such as C104-, ReO4-, SbC16-, GaC14-, CdI42-, and SCN-.

(1.14)

Macrocyclic compounds have been proposed for selective extractive separation of metals, mostly alkali and alkaline earth metals [44-51 ]. An exhaustive review of applications of these compounds is given in articles [44,49]. Macrocyclic compounds form cationic


complexes with some metal ions, and when associated with suitable anions (e.g., picrate, perchlorate, tetraphenylborate) these can be extracted into non-polar solvents such as CH2C12, CHC13, C6H6.

(1.15) (1.16) 0~6 ~o 0 H

An example of crown ether applied in selective separation of alkali metals is dibenzo- 18-crown-6 (formula 1.15). Extractive separations of metal ions are also performed with macrocyclic ligands containing nitrogen or oxygen atoms, as well as macrocycles with combinations of oxygen, nitrogen, and sulphur atoms (N-O, S-O, N-S) [45,48]. A macrocyclic compound with only nitrogen hetero-atoms (formula 1.16) is selective for copper.

Cryptands (Greek kryptd = vault, crypt) are polycyclic compounds containing oxygen and nitrogen atoms, forming extractable cationic complexes (cryptates) with metal ions (in the presence of a suitable anion).

Thiocrown ethers are also used for extraction separation of metal ions [52-57]. The cationic complexes formed by these ethers with Cu(I), Ag, Pd(II), Pt(II) are extractable from acidic media (e.g., by 1,2-dichloroethane) in the presence of, e.g., picric acid.

1.1.3. Isolation and separation of traces

The elements which can be separated from each other and from the matrix in small and trace amounts by extraction are shown in Table 1.2. The symbols of elements are accompanied by the typical compounds used in the extractions. The extraction methods provide large possibilities in separation of traces. An important role in concentrating traces by extraction is played by organic reagents which form chelates with metal ions.

Only the more important group reagents are mentioned in Table 1.2. Dithizones and dithiocarbamates, containing sulphur as the ligand atom, are particularly suitable for extraction of metals that form sparingly soluble sulphides. Ligands complexing through oxygen atoms, such as 8-hydroxyquinoline and cupferron, react preferentially with hydrolyzable metal ions.

For the isolation and separation of some individual elements there exist highly selective, and even specific, extraction systems, such as nickel with dimethylglyoxime and cobalt with 1-nitroso-2-naphthol.


Tab le 1.2. Separat ion of traces by solvent extraction

Be Bd Mg B Ca B Sr B Ba B

Sc bd Y bd La bd

Ti bd Zr bd Hf bd

B e AI. Si P S bd ge g F

V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br bcd bdh bc bcd ace acb acb acb bed eg ceg Ecf F Nb Mo Ru Rh Pd Ag Cd In Sn Sb Te I ebd deb he ech ace ac abc bae cde edc Ce F Ta W Re Os Ir Pt Au Hg TI Pb Bi ed ebg eh he ce ace aec ace ace acb ace

Ce bd

Th Bd

U bd e

The traces are extracted as: a - dithizonates, b - 8-hydroxyquinolinates, c - dithiocarbamates, d - cupferronates, e - halogen complexes, f - elements, g - heteropoly acids, h - oxygen compounds

A group of the methods for pre-concentration of traces includes the extraction of metals by the resins modified with organic reagents, e.g., 8-hydroxyquinoline, DDTK [58], Bromopyrogallol Red [59], Zincon [60], and crown ethers [61]. Reviews of the methods have been presented [62,63].

In many cases the separation of mixtures must be preceded by conversion of the main component into a stable complex, in order to retain it in the solution and prevent its extraction along with the separated microcomponents. The masking agents most appropriate for individual elements are shown in Table 3.1.

1.1.4. Separation of macrocomponents

The elements which can be extracted in larger quantities are shown in Table 1.3. The most numerous group comprises the halide and thiocyanate complexes, namely fluorides (Ta, Nb, Sn), chlorides [Fe(III), Sb, As, Ga, Ge, Au, Mo, T1], bromides (Au, In, T1, Ga), iodides (Bi, Sb, Cd, Hg, Sn), and thiocyanates [Zn, Co, Fe(III), Ti, Mo, U].

There are numerous examples of the extraction of macrocomponents as chloride complexes in the analysis of various materials. The extraction of iron(III) from hydrochloric acid medium, prior to determination of trace elements, has been thoroughly investigated [64]. Macroquantities of gallium were extracted from 6-7 M hydrochloric acid with di(2- chloroethyl) ether [65,66], and gold(m) was extracted with isoamyl acetate [67].

As bromide complexes, indium has been extracted with diethyl ether [68] or di- isopropyl ether, and gold(III) (from 3 M HBr), also with di(2-chloroethyl) ether [69].


Table 1.3. Separat ion of the matr ix by solvent extraction

Be af

Sc ba Y be La e

Ti V Cr Mn Fe Co Ni ae aef df f aef Ae ae Zr Nb Mo Ru Rh Pd ef ae ae ad A af Hf Ta W Re Os Ir Pt ef ae ae da ad A a

AI. S fe ca

Cu Zn Ga Ge As Se ae ae af a a ca Ag Cd In Sn Sb Te a a af a a a Au Hg TI Pb Bi ab ae af ae ae

Br c I c

Ce Th U e bf bef

The macrocomponents are extracted as" a - halide compounds; b - nitrates; c - free elements" d - oxygen compounds; e - cupferronates; f - acetylacetonates.

Only those organic reagents, such as cupferron and acetylacetone, which form chelates highly soluble in non-polar organic solvents, can be used in the extraction of matrix elements.

Larger amounts of some metals can be extracted as nitrates [U, Th, Ce(IV), Y, Sc] or oxides (Os, Ru). Matrix yttrium or scandium are extracted with TBP from 12-13 M HNO3 [70].

1.2. Precipitation

Precipitation methods for the separation of elements are based on the differences in solubility of their compounds in aqueous solutions. Precipitation methods are used for separating trace elements alone, as well as for separating macrocomponents from the traces. Trace elements are separated quantitatively from the solution by using collectors (scavengers or carriers). When macrocomponents are precipitated, the aim is to prevent trace elements from co-precipitating with the large mass of the macrocomponent precipitate. This prerequisite restricts the application of the method to cases in which co-precipitation of trace elements with the macrocomponent precipitate is negligible.

1.2.1. Separation of traces with the use of carriers

When a precipitant is added to a solution containing trace amounts of an ionic species (0.1- 100 ~tg in 100-250 ml, 10-8-10 .5 M), the ionic species may be only precipitated partly (or not at all), even though the solubility product has been exceeded.

The formation of a precipitate, i.e., crystal growth, is a complex and slow process [71 ]. The process begins with nucleation. When nucleation and further growth of nuclei occur

1.2. Precipitation 13

slowly, a condition of permanent supersaturation exists in the solution. Nuclei are formed when the first portion of precipitant is added to the solution. If the ions to be precipitated occur in macro amounts in the solution, further addition of the precipitant causes rapid transformation of the nuclei into crystals by the formation of more of the sparingly soluble compound. If, however, the solution contains only trace amounts of the ions to be precipitated, the process is virtually complete after the nucleation stage.

The rneehanisrn of the action of carriers depends on the nature of both the trace substance and the carrier involved. The co-precipitation consists in separation of ions co- precipitating from the solution with particles of the carrier formed in the solution. The co- precipitation may be either isomorphous (formation of solid solutions or mixed crystals) or based on adsorption phenomena.

In most cases the co-precipitation of traces consists in the formation of solid solutions. The separation by co-precipitation is not restricted by the very low concentration of the trace species. Smaller amounts can be separated by co-precipitation than by solvent extraction, which is limited by the stability of the complex extracted. The formation of a solid solution, e.g., in the separation of Pb traces with lanthanum hydroxide, consists in the replacement of some La atoms in the crystal lattice by Pb atoms.

If the trace element and the collector have opposite chemical properties (acidic, basic), the co-precipitation may be the result of formation of chemical compounds. For example, traces of germanium or vanadium form germanates or vanadates in co-precipitation with Fe(III)-, AI-, or La- hydroxides, while traces of tungsten or molybdenum, on co-precipitation with Fe(III) hydroxide, form the corresponding Fe(III) tungstate or molybdate.

Elements which can be pre-concentrated by co-precipitation with carriers are collected in Table 1.4. The most important forms for the co-precipitation of particular elements are given.

For most elements it is possible to find a suitable form that enables the separation of traces, along with an appropriate carrier. The carriers are normally selected among related elements, although this condition is not indispensable.

The elements which are co-precipitated as hydroxides can also be separated by organic reagents of the R-OH type, such as 8-hydroxyquinoline, cupferron, or B-diketones. Metal ions giving sparingly soluble sulphides may be co-precipitated by organic reagents of the R- SH type, e.g., dithiocarbamates.

Enough collector should be added to the sample solution to ensure that the precipitation is rapid, and that sufficient precipitate is formed for easy filtration or centrifugation. At the same time the amount of the collector should be sufficiently small for adsorption of interfering ions to be negligible. The amount of collector used depends on the volume of precipitate formed. This amount may be smaller if the species is separated as the 8- hydroxyquinolinate, than in cases of precipitation as hydroxide. In practice, 2-5 mg of collector are used per 50-200 ml of sample solution.

Hydroxides are often used for precipitation of traces with collectors [72-74]. With Fe(III), A1, or La as collector, traces of most analytical group I-III metals are separated by addition of excess of ammonia. Metals forming ammine-complexes, e.g., Ag, Cu, Ni, Co, Zn, and Cd remain in solution. When excess of NaOH is used for precipitation, amphoteric metals such as A1, Pb, Zn, Sn, and Cr remain unprecipitated. In this case, Fe(III), Ti, Mg, or La may be used as the collector. Lanthanum is especially convenient, since it usually does not have to be determined in the trace concentrate. It has no chromophoric properties and it does not interfere in most spectrophotometric methods.


T a b l e 1.4. Separa t ion of t races by co-prec ip i ta t ion with carr iers

Be A Mg ae Ca ec Sr ce Ba ce

Sc ae Y ae La ae

Ti V Cr ae ae ae Zr Nb Mo ae ae bea Hf Ta W ae ae ae

Mn Fe Co Ni Cu Zn ab abe ab Ab abd ab

Ru Rh Pd Ag Cd db db Db bda ab

Re Os Ir Pt Au Hg ba db db Db dab bd

AI. Si P CI ae a a f Ga Ge As Se Br ae ab b da f In Sn Sb Te I abe ab abd da f TI Pb Bi abe abc abd

Ce Th U ae ae ae

The traces are precipitated as: a; hydroxides or acids; b - sulphides; c - sulphates; d - elements after reduction" e - 8-hydroxyquinolinates; f - silver salts.

Traces of hydrolyzable metals [e.g., Sn, Sb, TI(III), Bi] are separated from acidic medium with MnO2aq as the collector [75,76].

Hydrogen sulphide or thioacetamide are used for the separation of traces of metals in the form of their sulphides. Copper, mercury, and other metals of the "hydrogen sulphide" group are used as collectors [77,78]. After the collector has been added and the pH adjusted, the solution is heated with thioacetamide or the hot solution is saturated with hydrogen sulphide. All the metals of analytical group I-III are precipitated when an ammoniacal solution (pH 8- 9) is saturated with HzS.

In the analysis of high purity metals, trace elements were pre-concentrated by partial dissolution of the matrix. The remaining small part of the matrix retains all trace elements that are electrochemically less noble than the matrix [79,80]. In this way the trace elements were pre-concentrated from silver-, cadmium-, gallium-, indium-, zinc-, lead-, manganese-, aluminium-, and lead-antimony alloys.

Trace amounts of noble and semi-noble metals (e.g., Au, Ag, Hg, and Cu) are separated electrolytically from acid media on a small platinum or gold cathode [81]. In cementation methods (i.e., reduction to metal in situ by another metal) small amounts of semi-noble metals (Cu, Bi, Sb) are deposited on less noble metals such as tin, iron, or zinc.

Trace elements can be separated from solutions of different metals by reducing a small amount of the matrix metal with sodium hydroborate (NaBH4) [82-85]. The metallic precipitate serves as a trace collector for all the elements that are electrochemically more noble than the matrix. The method has been used in the trace analysis of lead and its alloys [82-85].

Concentrates of trace amounts of many metals are precipitated with 8-hydroxyquinoline, thionalide, or dithiocarbamates [86,87].

In addition to group systems of separating elements, shown in Table 1.4, there are also specific methods for precipitation of particular elements. For example, Pd can be precipitated with Ni as collector (and vice versa) by means of dimethylglyoxime. The rare earths and

1.2. Precipitation 15

thorium can be separated as their sparingly soluble oxalates or fluorides. Molybdenum and tungsten are separated as benzoinoximates.

Active carbon can serve as collector in the separation of some trace elements precipitated in various forms [88,89]. Silicic acid has been also used as collector in preconcentration of many elements [90].

Masking plays a substantial role in the separation of traces from macrocomponents by precipitation. The aim is to retain the macrocomponents in solution while the traces are co- precipitated with a collector. The masking agent selected must complex the sample matrix without interfering with the separation of the traces and the collector.

Knowledge of the stability constants of complexes is not enough to predict their masking possibilities. The stability constants of complexes are apparent constants which vary with the pH and with the concentration of other species capable of complexation (competitive reactions).The effect of these factors is taken into account in the conditional stability constants of complexes [91].

1.2.2. Separation of traces by flotation

Ions of precipitate particles are adsorbed or attached at the surface of bubbles rising through a liquid, and are thereby separated. A substance which is not surface-active itself can be made so through union with, or adherence to, a surface-active agent (surfactant). Froth flotation involves separation (pre-concentration) by frothing. If an insoluble product is formed in interaction between the ion to be separated and a surfactant, the process is called ion flotation. If the ion is first precipitated and the precipitate is then floated with or without the addition of a surfactant, the process is called precipitate flotation. Flotation is accomplished in a special cylindrical vessel provided with a sintered glass disk at the bottom to break the gas (nitrogen, air) stream into small bubbles [92].

Ion flotation has been used as a method for pre-concentration of heavy metal ions in water. Anionic complexes of these elements are formed by adding complexing agents, and then floated by using a cationic surfactant and nitrogen. The amount of the surfactant should be greater than the stoichiometric amount, but excessive concentration may decrease trace recoveries [93].

Precipitate flotation is applied in the analysis of natural waters. Trace ions in an aqueous solution are co-precipitated with colloidal metal hydroxide collectors and floated with the aid of a gas stream [94-96]. Tiny gas bubbles are trapped in the interstitial spaces and on the surfaces of the precipitates to give sufficient buoyancy. Surfactant ions having the charge opposite to the precipitate surfaces are used to make the surface hydrophobic. Another important role of the surfactants (e.g., sodium oleate, sodium dodecylsulphate) is to form a stable froth layer to support the precipitate on the solution surface, which is important for complete separation of the precipitate.

Instead of passage of gas bubbles through the solution, the aqueous pseudo-solution can be shaken with non-polar solvent. In such cases, hydrophobic sparingly soluble compounds accumulate at the phase boundary or adhere to the wall of the separating funnel used. After careful removal of both liquid phases, and washing, the precipitate adhering to the wall can be dissolved in a polar solvent and the isolated trace elements can be determined.

Numerous elements form multicharged anionic complexes which are able to associate with hydrophobic basic dyes. They include anions of the heteropoly acids of Si, Ge, P(V), and As(V), the bromide complexes of Bi and Te(IV), and the thiocyanate complexes of Mo and W. Flotation of sparingly soluble ion-associates formed between the multicharged anionic complexes (with halides, SnC13-, and thiocyanate) of the platinum-group metals and


gold with basic dyes (Rhodamine 6G, Crystal Violet, Methylene Blue, etc.) has been described [97,98]. The floating agents often used are DIPE, benzene, cyclohexane, and toluene. Flotation of these hydrophobic ion associates is carried out without the use of surfactants.


The precipitation of macrocomponents (matrix) to separate them from trace elements is relatively uncommon [99]. In the separation of macrocomponents their quantitative precipitation is not necessary.

The largest group of elements comprises those isolated from solution in the elemental form as a result of reduction, usually electrochemical. In acid solution, the electrolytic deposition of metal on a solid cathode is limited to noble and semi-noble metals. Trace analysis of copper and its compounds may serve as an example [100]. An anodic dissolution technique may be applied for the isolation of macroscopic amounts of copper. A sample in the form of a bar, plate, or wire is the anode in the electrolytic system. When current is passed through the electrolyte (nitric acid + persulphate), Cu is deposited on the graphite cathode, while most trace elements accumulate in the solution. In the trace analysis of platinum, the matrix has been also separated on a cathode [101 ].

The use of a mercury cathode, which has a high overvoltage to hydrogen, enables a considerable number of metals to be isolated from dilute acid solutions. After electrolysis with the mercury cathode, the following metals remain in solution: A1, Ti, Zr, V, Nb, U, Th, Be, Mg, Ca, and rare-earth elements. A mercury cathode is used to separate Fe, Ni, Cr, Mo, and Mn when steel is analysed for certain elements (e.g., A1, V, Ti).

Gold, silver, mercury, and platinum metals, as well as Se and Te, can be precipitated from acid solution in the elemental form by reduction with chemical reagents such as zinc, NHzOH, NzH4, SO2, or formic acid. In the trace analysis of high purity mercury the sample (about 100 g) is dissolved in HNO3 and the solution is warmed in the presence of formic acid. First of all, nitric acid, then mercury, is reduced. The mercury forms a separate liquid phase, and the impurities remain in the aqueous solution [ 102]. In the trace analysis of silver, the sample is dissolved in nitric acid, then formic acid and mercury are added. The silver liberated on reduction dissolves in the mercury to form an amalgam [ 102].

In the trace analysis of high-purity zinc, the sample is coated with a thin layer of mercury. After dissolution of the zinc in hydrochloric acid, a drop of mercury remains that contains amalgams of many trace metals from the zinc analysed. They can be determined after volatilization of mercury [ 103].

The belief that the isolation of macrocomponents from solution as sparingly soluble compounds is inadmissible in trace analysis, because of the considerable losses of traces caused by adsorption, is not necessarily true if the precipitation is done in acid medium. This has been confirmed in the following examples.

In the trace analysis of lead, the matrix was precipitated from a nitric acid medium as lead sulphate. By using radioisotopes, it was found that none of the 24 elements investigated had co-precipitated with PbSO4 [104,105]. Most of the lead can also be separated as PbCI2 from nitric acid medium without perceptible co-precipitation of other components [ 104].

Before the determination of trace impurities in bismuth the latter is removed from nitric acid solution as the sparingly soluble iodide or the basic nitrate [106].

In the analysis for Cu, Zn, Cd, Ni, Pb, Mn, and Fe traces in silver, the matrix is precipitated as AgC1 from dilute nitric acid medium [107]. The trace elements do not collect

1.3. Volatility 17

with the precipitate. In the trace analysis of high-purity thallium, the matrix was separated as sparingly soluble TII [ 108].

Before the determination of impurities in metal chlorides (e.g., of sodium, potassium, calcium, aluminium), the matrix can be separated by saturating the solution with gaseous HC1.

1.3. Volatility

The following group of methods for separating and preconcentrating is based on differences in the vapour pressures of individual elements and their compounds. Covalent compounds are generally fairly volatile whereas ionic compounds are not. Covalently bonded compounds are also more soluble in non-polar solvents. As both volatility and solubility in solvents depend on the strength of intermolecular attractions, there is a fair resemblance between the compounds which are volatile and those which are readily soluble in non-polar solvents. Examples are AsC13, GeC14, OsO4, and certain inner chelates [ 109].

1.3.1. Separation of traces

Volatilization is usually utilized for separating individual trace elements from the sample before the determination. The methods based on volatilization are concerned mainly with non-metallic and amphoteric elements which have high vapour pressure in the elemental form (e.g., chlorine, bromine, sulphur), or in compounds with halogen, hydrogen, or oxygen. Other volatilization methods exist for the separation of certain elements, such as the distillation of boron as methyl borate.

A method has been proposed for preconcentration of traces of the more volatile elements (e.g., Zn, Cd, T1, In, Pb) by heating samples in quartz tubes to about 1,000 ~ in a stream of hydrogen. The sublimed metals are collected on a cold-finger. Many metal halides have been volatilized from aluminium samples by heating to 990 ~ [110]. Volatile acetylacetonates and other g-diketonates have also been utilized for the separation of metals [ l l l ] .

The separation of traces is done in a closed system and involves absorbing the traces in a suitable sorbent, for example hydrogen sulphide in a zinc acetate solution, ammonia in dilute HC1, and methyl borate in dilute NaOH. In the Gutzeit method traces of arsenic, liberated in the form of ASH3, are absorbed by a strip of paper saturated with a reagent giving a colour effect with ASH3. In all such procedures a carrier gas, such as hydrogen, nitrogen, chlorine, or steam is indispensable

The volatility of some elements can be reduced by binding them in complexes. The fact has been utilized for increasing the selectivity of separation of traces by distillation. For example, Ge and As can be separated from Sn by distillation as the chlorides, after Sn(IV) had been masked as the non-volatile phosphate complex.


If the macrocomponents can be removed by volatilization (distillation, sublimation) without the introduction of large amounts of reagents, the trace elements may be greatly concentrated. This favourable situation, in which relatively large samples can be used for trace analyses, arises when the sample is volatile, especially when it is a liquid such as water, an organic solvent, a volatile acid, or an ammonia solution.


During evaporation of the matrix, substances are sometimes added to reduce the volatility of the trace components which are to be retained in the vessel. A few drops of concentrated sulphuric acid are usually added, and the solution is evaporated until fuming [112].

In certain cases, the trace element is kept in solution as a less volatile complex during the distillation of the sample matrix. In the determination of boron traces in chlorosilanes, silicon is removed as the volatile fluoride complex. To prevent the formation of volatile BF3, mannitol is added to form a non-volatile complex with boron [ 113].

Chlorine, bromine, iodine, sulphur, and mercury are distilled as the elements. Examples of the distillation of major elements as the fluoride complexes can be found in the trace analysis of silicon [ 113]. Tin can be sublimed as the iodide [ 114].

In the trace analysis of titanium [115], and zirconium, volatile chlorides (TIC14 and ZrC14) are sublimed after heating the samples with chlorine.

In the trace analysis of rubidium- and caesium arsenates, arsenic is distilled off as arsine [116]. In the trace analysis of high purity cadmium, the matrix can be separated by distillation at 630~ [ 1171. By heating aluminium with ethyl bromide the metal is converted into ethylaluminium bromide, a liquid which boils at 130~ under reduced pressure [ 118].

Mineralization of organic samples [119-121], which precedes the determination of inorganic components, is also an example of separation of major components by volatilization (principally as CO2 and H20). In the mineralization of organic substances by dry ashing, the mineral residue may be so small and light that considerable losses result from the formation of "volatile" aerosols. These losses are prevented by adding a mineral collector, e.g., by wetting the sample with a solution of Mg(NO3)2, K2SO4, or Na2CO3. The temperature should not exceed 400-500~ to prevent distillation of more volatile elements [122]. At 400~ As and Hg volatilize completely, and Ag, Au, Fe, Sb, Zn, and Pb partially volatilize.

Mercury is lost during wet mineralization with acids. It volatilizes on heating a solution in sulphuric acid to fuming.

1.4. Ion-exchange and sorption

1.4.1. Introduction

Organic ion-exchangers are used for the separation of ions and for the separation and preconcentration of traces from the sample macrocomponents (matrix). Ion-exchange processes are based on the differences in ionic charge, in stability of the complexes formed, and in the associated distribution coefficients [ 123,124].

The organic ion exchangers are obtained by co-polymerization. They contain ionizable functional groups. The functional groups in cation exchangers are: -SO3H,-COOH, and -OH; those in anion exchangers are:-NR3 +, -NR2,-NHR, and NH2.

A strongly acidic cation exchanger is prepared by sulphonation of a co-polymer of styrene and divinylbenzene (DVB). Dowex 50 X8 is the ion exchanger Dowex 50 cross- linked with 8% DVB.

The hydrogen ion of acid groups is exchanged for other cations in the exchange process. The cation exchanger may be used in the hydrogen (H+), sodium (Na+), or similar form, depending on the cation attached to its acid groups. A strongly acidic character is imparted to cation exchangers by sulphonic acid groups. In anion exchangers, a strongly basic character is imparted by quaternary ammonium groups. Anion exchangers are most often used in the hydroxide (OH-) or chloride (C1-) form.

1.4. Ion exchange and sorption 19

In separation methods, ion exchange is usually performed by the column technique. The wet swelled resin is placed in a glass, quartz, or plastic column, and the solution studied is run through the column. In simple ion exchange separations some kinds of ions are retained in the column while others are eluted. In ion exchange chromatography the ions retained by the ion exchanger are gradually eluted by appropriate, selective eluents, and are collected in different portions of the eluate.

The size of the column (resin bed) is selected according to the quantity of ions to be retained in the column. The depth of the resin bed in the column should be 10-20 times its diameter. Columns 8-10 mm in inner diameter are often used in laboratories. When the quantities of ions retained are in the microgram or milligram range, columns 3-5 mm in inner diameter are sufficient. In columns of the dimensions given above, 30-50 mesh or 100-200 mesh resins may be used [ 125

The ion-exchange capacity of the column depends on the quantity of resin in the column and on the ion-exchange capacity of the resin used. The latter is expressed in meq of ionic species per gram of dry resin. For strongly basic anion-exchangers it amounts to 4-5 meq per gram. In practice, it is not the total capacity of the ion-exchange column which is important, but the breakthrough capacity, which is lower and depends on the column shape, resin particle size, elution rate, and other experimental conditions.

The distribution coefficient (D) for a particular element, on a given ion exchanger, and in the given medium, is the ratio of the element concentration in the resin bed phase, Cr (in millimoles per gram of dry resin) to the element concentration in the solution in equilibrium with the ion exchanger, c~ (in millimoles per ml of solution).

Cr D =

C s

The ability of an ion exchanger to retain an element from a solution of a particular complexing agent is shown by a graph of log D v s . complexing-agent concentration. The concentrations of complexing agents modify the affinity of the ions to the ion exchangers.

The ion exchange methods were applied also in mixed aqueous-organic media [126]. The addition of organic solvents to the aqueous system modifies the affinity of the ions for ion exchangers. The fact may be explained by change in the solvation of the ions, reduction in the dielectric constant of the medium, and stabilization of complexes which are weak in aqueous solution.

Most ion exchangers have a microporous structure, in which the pore size does not exceed 4 nm. Separations in mixed aqueous-organic media or in organic solutions are usually performed with the use of macroporous ion-exchange resins with mean pore size ranging from 130 nm. They are characterized by lesser swelling ability, more rapid exchange processes, and higher chemical resistance.

The application of ion-exchange chromatography in inorganic analysis has been discussed widely [127-132]. The development of methods involving chelating ion- exchangers and other sorbents will be discussed in further paragraphs.

The use of molecular spectroscopy as a detection method in chromatography has also been discussed [133].

1.4.2. Separation with the use of cation exchangers

Strongly acidic cation exchangers, such as Dowex 50 and Amberlite IR-120, are most often used in ion-exchange separations.


The behaviour of elements on strongly acidic cation-exchangers in hydrohalic acid solutions has been extensively investigated. Distribution curves have been reported for individual elements separated on cation exchanger in 0.5-12 M HC1 [134], and separations in mixed media HCl-water-organic solvent were studied [135,136]. Studies of ion exchange on macroporous ion-exchangers were also carried out in the media of HCl-water-acetone [137] and HCl-acetic acid-water [138].

The behaviour of metal ions on a strongly acidic cation-exchanger in the mixed medium HBr-water-organic solvents was studied [139,140]. The behaviour of metal ions in the presence of thiourea has been described [141]. In the separation of elements on strongly acidic cation-exchangers, use was also made of fluorides [142], thiocyanates in aqueous- organic media [143], perchlorates [144], oxalates [145], and tartrates [146].

An interesting application of a cation exchanger has been given in relation to trace analysis of sodium-, potassium-, barium-, and strontium chlorides [147]. Metal cations are retained by the cation exchanger. Concentrated HC1 run onto the column precipitates NaC1, KC1, BaC12, and SrCI2, whereas trace metals present are eluted.

1.4.3. Separations with use of anion exchange resins

Strongly basic anion-exchange resins, such as Dowex 1 and Amberlite IRA-400, are mostly used in the separation and preconcentrations to be discussed.

The behaviour of elements on strongly basic anion-exchangers in hydrochloric acid medium has been studied [148,149]. The behaviour of some elements in mixed media HC1- water-organic solvents has been a subject of some work [ 150,151 ].

Detailed data have been published on the behaviour of some elements on anion exchangers in the media of fluoride [152], nitric acid [153], sulphuric acid [154,155], and in the mixed medium HCl-acetic acid-water [138]. Studies were also carried out on the behaviour of elements on strongly basic anion exchangers in the media of HBr [156,157], phosphoric acid [158], oxalic acid [159], thiocyanate [160], tartrate [161], and thiosulphate [162].

Schemes have been given for the separation of iron and alloy components, and admixtures in steel of A1, Bi, Co, Cr, Mn, Mo, Nb, Ni, Sn, Ta, Ti, V, W, and Zr. Columns with strongly basic anion-exchangers and with strongly acidic cation exchangers were used, and various media (HF, HC1, and H2804 were applied) [163].

Weakly acidic anion exchangers were used in the separation of metals in chloride [ 164] and thiocyanate [165] media.

1.4.4. Chelating resins and other sorbents

Chelating resins play a substantial role in the preconcentration and separation of trace elements 166-168]. These sorbents are especially useful in trace analysis of natural waters. Chelating resins are characterized by high selectivity, which depends mainly on the chelating groups involved. Various matrices are used in the synthesis of chelating resins: copolymers of styrene and divinylbenzene, polystyrene, polymethacrylate, and fibrous materials such as cellulose. Also used are modified anion exchangers, charcoal, and other materials impregnated (loaded) with chelating agents.

Chelex-100 is a well-known resin based on styrene-divinylbenzene copolymer with iminodiacetate groups. It has found many applications in preconcentration of traces of heavy metals such as Cu, Cd, Zn, Ni, Co, Mn, Fe, Pb, and Hg [169-172].

References 21

From among numerous chelating agents used in chelating resins can be mentioned: oxine [173,174], dithiocarbamates [175-178], dithizone [179], nitrosonaphthols [180], thiazolothiol [181], Arsenazo I [182], and formazans [183]. Anion exchangers have been modified with chelating agents, such as dithizone, Arsenazo III, Sulphonazo III [184], Xylenol Orange [185], and BPHA [186]. The sorption of metal traces on resins modified with azo reagents was also studied [187,188].

Cellulose is a natural hydrophilic porous polymer. Owing to the presence of hydroxyl groups in cellulose it is possible to introduce chelating groups into it. Cellulose sorbents are characterized by good kinetic properties and high distribution coefficients [189]. Cellulose sorbents with functional groups derived from dithiocarbamates [190] or 1-(2- hydroxyphenylazo)-2-naphthol [191] are known. Fibrillar chelating sorbents based on synthetic organic products have been proposed [192].

Chelating sorbents based on silica gel have been proposed. Among organic reagents supported on silica gel are: oxine [193], rhodanine [194], and other reagents containing thiol groups [195]. The use of macrocyclic ligands (bonded with silica gel) for the separation of metal ions has been investigated [ 196,197].

Inorganic ion exchangers have not been applied widely, but in some cases they can be useful in separation and preconcentration of trace elements. They include hydrous oxides and acid salts of multivalent metals, heteropoly acids (e.g., phosphomolybdenic acid), sparingly soluble ferrocyanides, and synthetic aluminosilicates (zeolites) [198,199].

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Chapter 2. Principles of Spectrophotometry

2.1. Introduction

Spectrophotometric methods are among the oldest methods of analytical chemistry. The absorption of visible light by certain chemical substances has long been used for visual determination of their concentration. As early as in the middle of the XIX century methods were known for the determination of bromide in natural waters by oxidation and extraction of the resulting bromine into ether, of ammonia by Nessler's method, of titanium by the peroxide method, and of molybdenum by the thiocyanate method. The term "colorimetry" was used for those analytical methods, in which chemical elements were determined by comparing the colour of unknown samples with appropriate standards, either in graduated cylinders or in visual comparators. The use of photoelectric instruments has given rise to measurement of the absorption of radiation as it passes through the analysed samples and has enabled us to extend the useful radiation range outside the visible region.

Nowadays, spectrophotometry is regarded as an instrumental technique, based on the measurement of the absorption of electromagnetic radiation in the ultraviolet (UV, 200-380 nm), visible (VIS, 380-780 nm), and near infrared region. Inorganic analysis uses UV-VIS spectrophotometry. The UV region is used mostly in the analysis of organic compounds. Irrespective of their usefulness in quantitative analysis, spectrophotometric methods have also been utilized in fundamental studies. They are applied, for example, in the determination of the composition of chemical compounds, dissociation constants of acids and bases, or stability constants of complex compounds.

Spectrophotometry as a measuring technique has developed enormously as a consequence of the progress in technology, and in the development of new materials and of methods of data processing. The development of specialized optics, and of spectrophotometers coupled with microprocessors controlling their operation, has extended considerably the possibilities of using these instruments, the recording of absorption spectra, and the treatment of the data collected. Spectrophotometric methods have proved to be particularly suitable for automation, both in analytical procedures and in the treatment of data. They belong to the detection techniques most frequently used in automatic flow injection analysis (FIA).

2.2. Absorption and molecular structure

Spectrophotometric methods of identification and determination of substances are based on the existence of relationships between the position and intensity of absorption bands of electromagnetic radiation, on the one hand, and molecular structure on the other. Electronic spectra result from changes in the energy states of electrons [o, x, and free electron pairs (n)] in a molecule as a result of absorption in the UV-VIS region. The changes depend on the probability of electronic transitions between the individual energy states of the molecule. The number of absorption bands, and their positions, intensities and shapes are the spectral parameters utilized in qualitative and quantitative chemical analyses [1-3].

The positions of individual absorption bands recorded in the spectra depend on the energy of the absorbed radiation. Radiation from the near-infrared region gives rise to changes in the rotational and oscillation energy states in a molecule. Narrow bands that are due to small

2.2. Absorption and molecular structure 27

changes in wave number are connected with characteristic groups of atoms (functional groups) and are used for identification of such groups.

The UV-VIS radiation gives rise to changes in the energy of electronic states of a molecule. The probability of electronic transitions in a molecule depends on the presence of multiple bonds in the molecule and on the kind, number and positions of the substituent groups. Determination of the kind of transitions corresponding to the observed bands of absorption spectra enables one to determine the structure of the molecule.

Spectral transitions of electrons associated with absorption of radiation correspond to transitions from binding orbitals ((y, x, n) to anti-bonding orbitals of higher energy state (o*, ~*). The energy of the respective transitions decreases in the following order:

o--~ o*> n --~ o* > 7t--~ 7t*> n ~ m*

The o---> cy* transitions may take place in the far ultraviolet, which is generally not recorded in spectrophotometers. Other transitions occur in the near ultraviolet and visible regions. The n ~ ~* transitions are characterized by high intensity which varies depending on the number and kind of multiple bonds in the molecule. An increase in the number of conjugated bonds results in a reduction of the distance between the ~ and x* levels, an increase in the probability of transition, and increase of intensity of the spectrum recorded.

The considerable bandwidth and the high intensity are characteristic for absorption bands due to intermolecular charge transfer without ionization. They appear when an electron is transferred, under the effect of radiation energy, from a donor molecule to a free orbital of another molecule (acceptor). The charge transfer bands occur in the near ultraviolet and are broad in general.

The visible and the near UV regions are characterized by absorption bands owing to intra- atomic d - d transitions. This kind of transition is specific for ions of transition metals with an incomplete d shell. Splitting of the d sub-levels in the ligand field creates a possibility of transitions between the levels due to absorption of appropriate radiation quanta. The colour of solutions of transition metal ions is connected with the intra-atomic d - d transitions. The transition metals are capable of forming coloured complexes even with colourless reagents, which do not contain chromophoric groups.

Changes in the energy of electrons within the ligands, charge-transfer bands, and bands owing to intra-atomic transitions determine finally the shape of the absorption spectrum of the given compound. The position and the intensity of electronic spectra may change when substituents are introduced into the molecule or when its environment is changed.

The colour of a molecule is an effect of the presence of ehromophor ie groups. A chromophore may be a group of atoms containing easily excitable ~ electrons (formula 2.1), including the most important groups for the visible region: the azo group - N = N - and the p- quinonoid system.

,---N---~'O ~ . - -C~N /N~--~.S (2.1)

The changes in the energy levels of these electrons owing to the absorption of radiation quanta give rise to characteristic bands in the absorption spectrum. The more easily the bond electrons in a molecule are excited, the more intense is the colour of the compound. The shape of the spectrum and the intensity of the absorption band depend on the position of the

28 2. Principles of spectrophotometry

chromophores in the molecule. In isolated systems, where multiple bonds are separated from each other by at least two single bonds, the spectra contain absorption bands characteristic for individual chromophores. Where double bonds are present in conjugated systems, intense absorption bands, shifted towards longer wavelengths, appear in most cases.

The features of the absorption spectra change if the so-called auxoehromes (e.g., -NH2, -NR2, -SH, -OH, -OR) are introduced into the molecules. The presence of free electron pairs in the auxochromic group, that interact with Ic- electrons of the chromophoric group (e.g., the free electron pair at nitrogen in the-NH2 group) leads to a state of conjugation which may result in formation of a new absorption band in the spectrum.

An action of a substituent or a solvent may give rise to a shift of absorption band towards longer wavelengths - the bathochromic effect, or towards shorter wavelengths - the hypsochromic effect. An increase or a decrease of band intensity is referred to as the hyperchromic or the hypsochromic effect, respectively.

The shape of a spectrum and its intensity depend on the positions of substituents in the molecule and on their inter-relationships, as in cis- and trans- isomers. In general, higher band intensities, and shifts to the higher wavelength part of the spectrum, are characteristics of the trans isomers, in which the coupling of ~- electrons is stronger for spatial reasons.

The intensities and the positions of absorption bands may also be influenced by solvent molecules. Electrostatic dipole interactions, and specific interactions that lead to formation of complexes based on hydrogen bonds and of charge-transfer type complexes, result in changes in the spectra of chemical compounds.

As the pH is increased, spectrophotometric reagents ionize and their electronic structure becomes deformed, which often leads to a bathochromic shift of the absorption maximum. Ionization causes polarization of the chromophoric system. The formation of a chelate complex disturbs the electronic state of the organic molecule to produce, as a rule, a bathochromic shift.

2.3. Absorption laws

Spectrophotometric measurements are generally made on solutions, either in water or in organic solvents, contained in a measuring cell which is placed in the path of a beam of monochromatic radiation of chosen wavelength.

From the total radiation of intensity Io that impinges upon a layer of solution, one fraction of the beam Ia is absorbed on passing through the solution, another fraction/t is transmitted, and still another fraction/r is reflected by the cell walls and scattered:

l o= Ia + l, + lr

As absorption measurements are always made by comparison with a standard solution, and both the sample and the standard are placed in identical cells, the part of radiation denoted as/r is constant and may be neglected.

The amount of radiation absorbed depends on the thickness of the absorbing layer and on the concentration of the solution [4,5]. In 1729 Bouguer established the relationship between the amount of absorption (the absorbance) and the thickness of the absorbing layer. A mathematical formulation of this relationship was given by Lambert in 1769. In 1852, Beer settled a relationship between the absorbance and the concentration of coloured solutions. In the formula derived (the Bouguer-Lambert-Beer law) both the solution concentration and the layer thickness are taken into account.

2.3. Absorption laws 29

When a parallel beam of monochromatic radiation of intensity Io impinges upon a layer of solution of thickness dl, a part of the radiant energy is absorbed. If the layer thickness changes, the absorption changes proportionally. The fraction absorbed increases exponentially with linear increase of the layer thickness:

dI = - k d l

I where k is a constant, and the minus sign denotes that the intensity of the radiation transmitted decreases as the thickness of the layer increases.

Integration of the above equation gives the following expression:

In ~ = I~ kl I

where Io denotes the initial beam intensity (for 1 = 0). By conversion of natural logarithms to decimal ones the above equation assumes the

following form: log Io/'It - 0.434 In Id l t = 0.434 K1 - A

where K is a new constant, and A is the absorbance. The absorbanee is a logarithm of the ratio of incident beam intensity, Io' to the intensity

of the beam transmitted,/t. If the concentration, c, of the absorbing species is doubled and the absorbing layer

thickness is reduced by a factor of two, then the total number of absorbing molecules remains the same, hence the absorbance A will also remain the same. Therefore the absorbance is a function of the number of absorbing centres in the light-beam, i.e., of the product cl, and the above equation can be given the form:

A = log Io/It = ,F_cl

where e is a new constant called the molar absorptivity (or absorption coefficient), c is the concentration of absorbing species (M, in moles per litre), and l is the layer thickness (in cm).

The equation is a mathematical expression of a fundamental law of spectrophotometry, the Bougue r -Lamber t -Bee r law, which states that absorption of radiation depends on the total number of absorbing centres, i.e., on the product of concentration and layer thickness of the solution.

In spectrophotometric measurements the thickness of the sample layer is usually identical to that of the reference solution, and only Beer's law, which relates the absorbance with the concentration of the sample solution, is of practical significance.

If a solution contains more than one absorbing species and there is no interaction between the components, the total absorbance of the solution is equal to the sum of all the component absorbances. The law of additivity of absorbance (providing the optical path length is constant) is expressed by the formula:

A = (elCl + e 2 C 2 + . . . + enCn)l The additivity of absorbance constitutes the basis for studies of multicomponent systems.

If a coloured solution obeys Beer's law, the graph of A = f ( c ) is a straight line passing through the origin.

From a practical point of view it is desirable that the solution should follow Beer 's law for the concentration range corresponding to absorbances not exceeding 1 (unity).

Deviations from Beer's law may result from either chemical reasons connected with the sample, or physical ones connected with the instruments involved [6-8]. In the former case


the deviations are due to changes in the form of the determined component as a consequence of chemical reactions (e.g., hydrolysis, solvation, association, polymerization) associated with changes in the analyte concentration. Any change in the form of the substance being determined gives rise, as a rule, to changes in its optical properties. Thus, for example, dichromate ions (orange) are transformed, on dilution, into yellow chromate ions: Cr2072- + H20 ----~2CrO42- +2H +. In the case of weak complexes, a dilution of the solution leads to a dissociation and decomposition of the complex thus giving rise to deviations from Beer's law.

Beer's law is not obeyed in systems where complexes are formed in a stepwise manner. The reaction constants and the quantitative ratios of individual complexes depend on the concentration ratio of the reactants and the pH of the reaction medium.

The optical medium must be homogeneous. Turbid solutions give deviations from Beer's law. Such deviations occur in two-phase systems insufficiently homogenized by protective colloids.

Deviations from Beer's law may also arise from insufficient quality of measuring instruments, mainly from the use of non-ideal monochromatic light, improper width of the spectral band, or scattering of radiation. The detector signal should be proportional, over a wide range, to the intensity of the radiation recorded.

Despite the many possibilities of deviation from Beer's law, in the absorbance range of practical interest for analytical purposes, colour systems not conforming to Beer's law are fairly rare.

2.4. Spectrophotometric apparatus

The quality of the measuring instrument has a strong influence on the reliability of the results obtained. The standard spectrophotometric apparatus used in modern analytical laboratories is very different from the equipment used in the initial period of application of this technique. The progress in the development of spectrophotometric apparatus up to the middle of the 1980s has been described [9-11]. The trends in the development of UV-VIS spectrophotometry, with special consideration to improved detection, and modern methods of data treatment, have been discussed [12]. The following discussion will be devoted to apparatus installed in most analytical laboratories.

The set of components that enable us to record a radiation absorption spectrum consists of: radiation source, monochromator, cuvette, and detector with the data treatment system (Fig. 2.1).

Radiation ~ Monochro- source mator

hydrogen, prism or xenon, diffraction tungsten, or grating halogen lamp

Cuvette

sample solution

I-t Detector ~-~ systemMeasuring

photocell, galvanometer or photomultiplier, microprocessor photoresistor, or photodiode

Fig. 2.1. Block diagram of spectrophotometer

The intensity of light emitted by the source, the effectiveness of its monochromatization, and the sensitivity of the detector are decisive for the quality of the spectrophotometer.

2.4. Spectrophotometric apparatus 31

2.4.1. Radiation sources and monochromators

In most cases spectrophotometers are equipped with two independent radiation sources: UV and VIS. The UV source is usually a deuterium- or xenon lamp that emits radiation in the range of 180-400 nm or 190-750 nm, respectively. The development of the UV radiation sources has been reviewed [13].

The sources emitting visible light are tungsten- and halogen lamps. A feature of the halogen lamps is their wider spectral range, higher radiation intensity, and longer lifetime. In modem spectrophotometers the exchange between the UV and VIS proceeds automatically. The increasing use of lasers as high intensity sources of monochromatic radiation is observed [ 14].

The principal element of the spectrophotometer is the monochromator which serves for dispersion of the radiation emitted by the source and isolation of a beam of monochromatic radiation of definite wavelength. The monochromator comprises a system of slits, a collimator, a light-dispersing element, and lenses or mirrors to focus the dispersed radiation. The dispersing system is the essential part of the monochromator. The degree of monochromatization is an important feature of the dispersing element.

Beams of monochromatic radiation or radiation of wavelength comprised within a specified narrow range are isolated by means of filters, prisms, or diffraction gratings. The beams of radiation of a limited range of wavelength are separated from the continuous spectra by means of properly selected colour filters.

Modem spectrophotometers are equipped with diffraction gratings, whose dispersion is independent of the kind of material used and the wavelength of radiation applied. Gratings of 1,800 and 2,400 grooves/mm are used for the UV region, and those with 600 and 1,200 grooves/mm are applicable for the visible light. The separation of the grooves, denoted as the grating constant, is the parameter characteristic for the given grating. The high precision of forming the grooves and the regularity of their separations are characteristic for holographic diffraction gratings having up to 6,000 grooves/mm. The substitution of diffraction gratings for prisms enabled researchers to increase the spectral resolution and to extend the measuring range from 1 to 4 in the absorbance scale.

To record a diffraction spectrum in a required wavelength range it is necessary to change the position of the grating to isolate the beam of a given wavelength. The manual method of changing the position, used in former instruments, has been replaced by mechanical systems. Quick and precise changes in position of the diffraction grating may be obtained by means of a laser beam.

2.4.2. Measuring cuvettes

Measuring cuvettes, in which sample solutions are placed, are made of various materials depending on the range of radiation used in the measurement. Measurements in the UV are performed with the use of quartz cuvettes. Synthetic quartz, which is less contaminated with traces of metals, has better optical properties. Measurements in the VIS range are made using quartz, glass, or plastic cuvettes.

The cuvette should provide maximum transmission of radiation and definite, precisely known thickness of the light-absorbing layer. Cuvettes of different thicknesses within the range 5 ~tm - 10 cm are produced. Small cuvettes capable of accepting samples of volumes down to 100 ~tl are also available. Small volume cuvettes that enable multiple passage of the beam of radiation are of special interest [ 15].


The cuvette material should be resistant to the action of chemicals. The cuvettes are placed in measuring chambers in special holders that provide accurate and reproducible location of the sample in the path of the radiation beam.

Cuvettes of special design are used for measurements over wide ranges of temperature and pressure or under conditions of permanent flow. An automatic method for cleaning the cuvettes has been proposed [16].

2.4.3. Detectors

After traversing the measuring cuvette the radiation impinges on the detector. The role of the detector is to convert the energy of the incoming electromagnetic radiation into electrical energy. The signal transformation should be linear, which means that the electrical signal generated should be proportional to the optical signal received. This condition is successfully fulfilled by photocells, photomultipliers, photoresistors and photodiodes. A comparison of various detectors used in UV and VIS spectrophotometry has been given [ 17].

The operation of photocells and photomultipliers is based on the external photoelectric effect. Photons impinging on the surface of a photosensitive cathode (photocathode) knock out electrons which are then accelerated in the electrical field between the cathode and the anode and give rise to electric current in the outer circuit. The spectral sensitivity of a photocell depends on the material of the photocathode. The photocathode usually consists of three layers: a conductive layer (made, e.g., of silver), a semiconductive layer (bimetallic or oxide layer) and a thin absorptive surface layer (a metal from the alkali metal group, usually Cs). A photocathode of the composition, Ag, Cs-Sb alloy, Cs (blue photocell), is photosensitive in the wavelength range above 650 nm; for longer wavelengths the red photocell with Ag, Cs-O-Cs, Cs is used. The response time of the photocell (the time constant) is of the order of 10 -s s.

Photomultipliers are equipped with several supplementary diodes (dynodes) to which the electrons emitted from the photocathode are directed. The electrons impinging on the dynodes give rise to the emission of secondary electrons from the successive dynodes and they thus amplify the signal generated by a factor of up to 108.

In the photoresistors and photodiodes use is made of the internal photoelectric phenomenon and of specific properties of semiconducting materials. Photons impinging on the photosensitive element generate an electrical current, which flows through the photoconductor and is amplified by the effect of a small applied voltage. The increase of the current intensity is proportional to the intensity of photons that strike the photosensitive element.

The microcrystalline layer of lead(H) sulphide deposited on a dielectric (glass or quartz) plate may serve as an example of a photoresistor applied in the wavelength range above 700 nm. Photodiodes are made of two or three layers of semiconducting materials containing suitable admixtures. Silicon photodiodes are used in the UV-VIS range

Modern spectrophotometers are equipped with multichannel detecting devices that contain a large number of photodiodes (a photodiode array) and enable simultaneous detection over the whole range of the spectrum. Details of the design and the advantages of using such detectors in spectrophotometric measurements have been presented [17-20].

2.4.4. Data recording and processing

The application of microprocessors and the rapid development of computer techniques has made it possible to automate the analytical operations from the step of sampling up to

2.5. Spectrophotometric techniques 33

full processing of the data obtained. In modern spectrophotometers, microprocessors are applied to control many operations that were formerly operated manually.

The functions now realized by microprocessors include the control of the optical system (lamp and analytical wavelength selection), selection of the kind of data collected (e.g., absorbance, concentration), zero-adjustment, autocalibration and control of measurement parameters [21]. The microprocessor determines the equation of the regression curve and provides statistical processing of the results. It can also be programmed to measure the absorbance, the % transmittance at a selected wavelength, or the concentration based on the relationship (linear or non-linear) established between the measured absorbance and the concentration.

The advanced spectrophotometers are coupled with computers that facilitate the recording of results and the processing of the data obtained. Appropriate software enables the presentation of results on the display, smoothing of the obtained spectrum, calculation of peak heights with respect to the base-line, and mathematical processing of the results that provides the possibility of, e.g., resolving signals owing to individual components of the sample analysed (see "Derivative Spectrophotometry" in Section 2.5). The development of the computer techniques has facilitated the identification of the structures of chemical compounds by enabling quick and easy access to catalogues of UV-VIS spectra.

The data recorded and the results obtained can be stored in the computer memory. This gives the possibility of comparing the obtained results and evaluating their quality by rapid comparison with greater numbers of data. A critical evaluation of the obtained results always remains the task of the analyst.

2.5. Spectrophotometric techniques

If the value of the molar absorptivity, e, for the wavelength used in measurement of absorbance of the given system is known, it is possible to determine directly the concentration of the analyte by means of an equation based on Beer's law. The value of e is determined from the measurement of absorbance of several solutions containing precisely known amounts of the analyte under conditions identical to those used in the measurement of the sample solution.

In analytical practice, the concentration of the given analyte is, in most cases, determined by the standard curve technique. The technique is based on the determination of the relationship between the absorbance and the analyte concentration under the measuring conditions. The relationship is given in terms of the regression equation, or graphically in the form of a standard curve. For systems that obey Beer's law this curve is a straight line.

The determination error is smaller if the absorption of radiation is a consequence of the nature of the analyte itself, as with the coloured ions of transition metals. Conversion of the analyte into a form capable of absorbing radiation in proportion to its concentration requires some additional procedures (such as the use of a chromogenic reagent, pH adjustment, or addition of masking agents), that must be identical in the treatment of standard solutions and of the sample solution.

The absorbance measurements must be carried out after the equilibrium has been settled in the system. If the absorbance varies with time, the time of measurement should be strictly specified.

In analytical practice, use is sometime made of standard curves in which the changes in absorbance are inversely proportional to changes in the analyte concentration. The analyte concentration is found from the reduction of absorbance of the system, which is proportional to the amount of the analyte. The accuracy and the precision of determination depend on the


precise knowledge of the initial reagent concentration and on the reproducibility of the reaction conditions for different concentrations of the analyte.

Differential spectrophotometry [22,23] consists in the measurement of the absorbance of a solution of the given element, not with reference to the solvent used, but with reference to a solution of this element (in the form of a coloured complex) of known concentration, slightly lower than the concentration of the solution studied. In this technique, the measuring error is in proportion to the difference of concentrations (and not to the concentration of the analyte in the solution under test), which enables one to reduce the relative error. Grey filters of appropriate absorbance have been proposed as references [24].

The theoretical bases of differential spectrophotometry have been presented [22]. The relative error of absorbance measurement is 0.2-0.5%, and is less in differential spectrophotometry [25-28] than in the regular method. Hence, the precision of differential spectrophotometry is comparable with that of gravimetric and titrimetric methods. This fact enables the technique to be applied in the determination of higher contents of the analytes.

In definite coloured systems the concentrations of reference solutions are selected with a view to obtaining maximum precision of the measurements. Particular attention is required in the preparation of standard solutions. In cases where temperature variations may influence the absorbance measurements, thermostating of the system is required.

Spectrophotometric titration [29-32] consists in repeated measurement of an absorbance which changes in the course of titration of the sample solution. The use of this method depends on the existence of a linear relationship between the absorbance measured and the concentration of the absorbing substance in the solution being titrated. The course of the titration is represented graphically by two intersecting straight lines. To find the titration end-point it is necessary to determine the absorbance at two points before and two points behind it. The graphs are drawn in the system of A (absorbance) v e r s u s v (volume of titrant solution). To increase the accuracy of determination, corrections are made for the dilution caused by the addition of the titrant solution.

The sample solution is titrated, at a definite wavelength, in a titration vessel placed inside the spectrophotometer. For this reason the spectrophotometers commonly applied require some adaptations that enable one to place a suitable titration vessel, the tip of the burette, and a mixer inside it. In this technique, the parameter of primary importance is not the absolute value of the absorbance measured but its changes during the course of titration. To reduce the effect of dilution on the absorbance one is recommended to use concentrated titrant solutions and micrometric syringes.

Spectrophotometric titrations are used in cases where it is difficult to determine the end- point visually as, for example, when there is a permanent change in the colour of the system. Good results are obtained in titrations of rather dilute solutions, of the order of 10 .5 M. Spectrophotometric titrations are often performed in automatic systems.

Dual-wavelength spectrophotometry [33-36] is applied in systems where the difference in absorbance of two absorbing components at definite wavelengths may be used for determination of their concentrations in the given solution. The choice of the wavelengths depends on the system studied. Usually one wavelength corresponds to the maximum absorption of the analyte, and the other may correspond to an absorption maximum of the reagent or of an interfering species. The highest sensitivity is obtained when the absorption is measured at )Lmax of the analyte. A necessary condition is that the

2.5. Spectrophotometric techniques 35

individual components obey Beer's law. The errors associated with the use of the dual- wavelength technique have been estimated [34-36].

Derivative spectrophotometry [37-44] is an analytical technique that uses the 1 st to 5 th

(I-V) -order derivatives of absorption spectra in the VIS and UV ranges. The recorded curve of the derivative of the spectrum (the derivative spectrum) represents the values of absorbance differentials as a function of wavelength (wave-number) according to the following equation:

d"A

d,t ~ = " D x x = f (,t)

where n denotes the order of derivative and "Dx,,~ is the value of the n-th derivative of absorption spectrum of the substance X at the given wavelength, 2.

The height of the signal of the respective derivative is proportional to the analyte concentration:

d"e "Dx,,t - d2, "1. c

and is additive in cases where the system contains more than one component absorbing in the radiation range studied. The shape of the derivative spectrum depends on the shape of the zero-order spectrum. The width of the half-height band (L- band-width in the middle of its height) is an important parameter characterizing the system in derivative spectrophotometry.

The derivative spectrophotometry methods provide higher selectivity and higher sensitivity than do the methods based on normal (zero-order) absorption spectra. The increase in selectivity (with reduction or elimination of the effect of the spectrum of one substance on the spectrum of another one) results from reducing the band-width in the derivative spectra. An appropriate order of derivative spectrum may give complete separation of the spectra owing to the corresponding components of the system).

The increase of selectivity in the derivative spectrophotometry methods results from the fact that the values of derivatives increase, in the case of basic spectra characterized by sharp peaks, and decrease in cases of broad-band zero-order spectra (Fig. 2.2). The sharp-peak spectra enable one to make determinations of analytes in the presence of considerable excess of elements having flat spectra. An example may be the direct determination of traces of manganese (as MnO4-) in nickel salts, based on the fourth-order derivative spectrum [45]. An increase of selectivity may also be obtained by proper selection of the instrument setting parameters in recording the derivative spectra.

The derivative spectra are obtained in spectrophotometers fitted with microprocessors which enable digital processing of the spectra recorded [10,44-46]. Derivative spectra may also be obtained using spectrophotometers coupled with analogue differentiating systems. The instrument parameters affecting the shape of the obtained spectra are: scanning rate, integration time, distance between measurement points, and degree of amplification. The Savitzky-Golay algorithm [47,48] is that used most frequently in the treatment of basic spectra aiming at obtaining suitable derivatives.

The analytical value of the derivative is determined mostly by the zero-crossing method (determination of the derivative at the zero point of the derivative for the interfering component), by the peak-to-peak method (determination of the amplitude of the derivative spectrum in a point corresponding to the maximum difference between the derivatives of the


analyte and the interfering component), and by the baseline-to-peak method (determination of the derivative of the analyte spectrum at its maximum). The sources of errors in determinations by derivative spectrophotometry have been discussed [48].

o)

b)

c)

d)

e)

~e.eeejpme~ . . . . .

. , . . . . .

wavelength , nm

Fig. 2.2. Zero-order absorption spectra (a) and their derivatives: 1st order (b), 2nd order (c), 3rd order (d), and 4th order (e). Broken lines - substance 1 and substance 2; continuous line - mixture of substances 1 and 2.

Derivative spectrophotometry is applied more and more widely in the determination of inorganic and organic substances without preliminary separation. It is also used for the identification of organic substances.

The dependence of the recorded signal upon the instrument parameters is a disadvantage of this technique. Reproducible results are obtained on using one M and the same M type of spectrophotometer and identical conditions of spectra recording, or by adaptation of a definite method to the available apparatus.

Flow injection analysis (FIA) is an automated method which consists in the injection of the sample solution to a continuous stream of an inactive carrier (e.g., a pH buffer or water) [49-51 ]. The diluted analyte is transported to a reaction spiral where a chromogenic reagent is added to the mixture. The dimensions of the spiral, the volume of the sample injected, and

References 37

the flow rate are optimized to provide proper reaction conditions, sensitivity, and selectivity. The liquid zone formed in the spiral and carrying the analyte is transported, by means of a pump, to the detector. The signals from the detector, which are proportional to the analyte concentration in the sample injected into the carrier stream, are recorded continuously.

Spectrophotometry is a technique most frequently applied in flow injection analysis, mainly owing to the easy coupling of the two methods, and good reproducibility of the measuring conditions. Practically all rapid reactions suitable for use in spectrophotometric determinations may be utilized under conditions of flow injection analysis. FIA coupled with spectrophotometry is a rapid (several dozens of determinations per hour) and sufficiently precise analytical technique [50-54].

The method uses small sample volume (10-100 ~tl), which is the basis of high sensitivity and is particularly useful in cases where small amounts of sample material are available (e.g., physiological fluids). The automation includes often also the sample injection.

The spectrophotometry coupled with FIA has found numerous applications in determinations of chemical elements in environmental and clinical samples, especially in laboratories involved in rapid serial analyses.

Turbidimetry is a technique based on measurement of the absorbance by suspensions of sparingly soluble compounds. The media determined should be turbid systems, and not colloidal solutions stabilized by protective colloids. Turbidimetric determinations are often realized by visual methods or by comparison with standards in measuring cylinders, e.g., the determination of sulphate as BaSO4, or chloride as AgC1. The absorption of light by a suspension depends on the dispersion of the suspended solid which depends, in turn, on the concentration of the ions determined, the rate of adding the reagents, the temperature, and the ionic strength of the solution, and the presence of organic solvents miscible with water. Turbid solutions used in this technique do not generally obey Beer's law and the precision of determinations is rather low, since it is difficult to keep experimental conditions strictly reproducible. Higher sensitivity is attained in the case of coloured suspensions, such as metal sulphides.

References

1. Sommer L., Analytical Absorption Spectrophotometry in the Visible and Ultraviolet. The Principles, Elsevier, Amsterdam 1989.

2. K~cki Z., Podstawy spektroskopii molekularnej, PWN, Warszawa 1992. 3. Cygafiski A., Metody spektroskopowe w chemii analitycznej, WNT, Warszawa 1997. 4. Lothian G. F.,Analyst, 88, 678 (1963) 5. Buijs K., Maurice M. J., Anal. Chim. Acta, 47, 469 (1969). 6. Agterdenbos J., Vlogtman J., van Breekhoven L., Talanta, 21,225 (1974). 7. Agterdenbos J., Vlogtman J., Talanta, 21, 231 (1974). 8. Youmans H. L., Brown H.,Anal. Chem., 48, 1152 (1976). 9. Altemose I. R., J. Chem. Educ., 63, A 216 (1986). 10. Altemose I. R., De Long L. E., Locke L. E., J. Chem. Educ., 63, A262 (1986). 11. Nowicka-Jankowska T., Wieteska E., Gorczyfiska K., Michalik A., Spektrofotometria

UV/VIS w analizie chemicznej, PWN, Warszawa 1988. 12. Lobifiski R., Marczenko Z., Crit. Rev. Anal. Chem., 23, 55 (1992). 13. Jones K. P., Trends Anal. Chem., 9, 195 (1990). 14. Imasaka T., Shibashi N.,Anal. Chem., 62, 363A (1990). 15. Dagsupta P. K.,Anal. Chem., 56, 1401 (1984).


16. Bautz D. E., Ingle J. D. Jr., Anal. Chem., 59, 2534 (1987) 17. Grossman W. E. J., J. Chem. Educ., 66, 697 (1989). 18. Borman S. A., Anal. Chem., 55, 836A (1983). 19. Jones D. Anal. Chem., 1057A, 1207A (1985). 20. Dose E. V., Guiochon G.,Anal. Chem. 61, 2571 (1989). 21. George W. O., Willis H. A., Computer Methods in UV, VIS and IR Spectroscopy, Royal

Society of Chemistry, Cambridge, 1990. 22. Barkovski V. F., Ganopolski V. I., Spektrofotometryczna analiza r62nicowa, WNT,

Warszawa 1971. 23. Grossmann O., Z. Anal. Chem., 321,442 (1985). 24. Marczenko Z., Ramsza A., Chem. Anal. (Warsaw), 21, 805 (1976). 25. Ingle J. D., Anal. Chem., 45, 861 (1973). 26. Blank A. B., Zh. Anal. Khim., 28, 1435 (1973). 27. Kotar' N. P., Samoilov V. P., Zh. Anal. Khim., 30, 465 (1975). 28. Grossmann O., Z. Anal. Chem., 320, 112, 223,229 (1985). 29. Ringbom A., Skrifvars B., Still E.,Anal. Chem., 39, 1217 (1967). 30. Galik A., Talanta, 13, 109 (1966); 15, 771 (1968); 17, 115 (1970). 31. Sato H.,Anal. Chim. Acta, 96, 215 (1978). 32. Johns P., Price W. J., Analyst, 95,138 (1970). 33. Shibata S., Furukawa M., Honkawa T., Anal. Chim. Acta, 78, 487 (1975). 34. Ratzlaff K. L., Natusch D. F., Anal. Chem., 49, 2170 (1977). 35. Ratzlaff K. L., Daraus H. B., Anal. Chem., 51, 256 (1979). 36. Ratzlaff K. L., Natusch D. F.,Anal. Chem., 51, 1209 (1997). 37. O'Haver T. C., Green G. L., 48, 312 (1976). 38. Ishii H., Z. Anal. Chem., 319, 23 (1984). 39. Perfil'ev V. A., Mishchenko V. T., Poluektov N. S., Zh. Anal. Khim., 49, 1349 (1985). 40. Levillain P., Fompeydie D. Analusis, 14, 1 (1986). 41. Dubrovkin I. M., Zh. Anal. Khim., 43, 968 (1988). 42. Talsky G., Derivative Spectrophotometry, CCH, Weinheim 1994. 43. Bosh Ojeda C., Sanchez Rojas F., Cano Pavon J. M., Talanta, 36, 549 (1980). 44. Ku~ M., Marczenko Z., Obarski N., Chem. Anal. (Warsaw), 41,899 (1996). 45. Ku~ M., Marczenko Z., Talanta, 36, 1139 (1996). 46. Majer J. R., Azzouz A. S., Talanta, 27, 549 (1980). 47. Savitzky A., Golay M. J., Anal. Chem., 36, 1627 (1964). 48. Steinier J., Termonia Y., Dektour J.,Anal. Chem.. 44, 1906 (1972). 49. Ru~i6ka J., Hansen E.,Anal. Chim. Acta, 179, 1 (1986). 50. Trojanowicz M., Automatyzacja w analizie chemicznej, WNT, Warszawa 1992. 51. Karlberg B., Pacey G. E., Wstrzykowa analiza przeptywowa dla praktyk6w, WNT,

Warszawa 1994. 52. Ru~i6ka J., Hansen E.,Anal. Chim. Acta, 78, 145 (1975); 99, 37 (1978). 53. Ranger C. B., Anal. Chem., 53, 21A (1981). 54. Stewart K. K., Talanta, 28, 789 (1981).

Chapter 3. Spectrophotometric methods

3.1. Introduction

The spectrophotometric methods to be discussed (methods of molecular absorption spectrometry) are based on the measurement of absorption of radiation, in the visible and near ultraviolet regions, owing to coloured compounds formed, before the determination, by the elements to be determined. Only seldom is use made of the intrinsic colour of the element itself, in its ionic form. In cases where an element neither forms coloured compounds nor occurs in a coloured form, indirect spectrophotometric methods are applied.

Spectrophotometric methods are characterized by high versatility, sensitivity, and precision. They may be used for the determination of almost all chemical elements over a wide range of concentrations, from macroquantities (by means of differential spectrophotometry) to traces ranging from 10-6-10 -8 % (after suitable preconcentration). Spectrophotometric methods are among the most precise instrumental methods of chemical analysis.

The advantages mentioned of the spectrophotometric methods are made greater by their availability. A spectrophotometer, which is the basic instrument in this field, is cheaper than most other fundamental instruments used in chemical analysis. Spectrophotometric methods are extensively discussed in the literature [ 1-4].

Spectrophotometric methods were preceded by colorimetric methods. The colorimetric determinations were first performed in cylinders, then visual colorimeters with filters came into use. The first photoelectric colorimeters were introduced into laboratory practice in the 19308, and they were next replaced by spectrophotometers. Spectrophotometers do not measure or compare the colour, but measure the absorbance of solutions.

The application of organic reagents, the development of the knowledge of complexes, then the use of spectrophotometers equipped with microprocessors that enable rapid processing of absorption spectra [5-9] have led to a very rapid development of spectrophotometric methods [ 10-24].

A paper has been published on the Recommendations of the Analytical Chemistry Section of the International Union of Pure and Applied Chemistry (IUPAC) concerning the nomenclature, symbols and units applied in molecular absorption spectrometry [25].

3.2. Sensitivity

The sensitivity of an analytical method means a minimum concentration, minimum amount, or a minimum difference in concentrations, of an element that can be determined by this method. Later on, use will be made of the first meaning of the term.

The numerical value of the sensitivity of spectrophotometric methods is usually determined in terms of the molar absorption coefficient (molar absorbance, ~, a coefficient) measured at the wavelength at which the absorbance is being measured:

A ~ =

cl

40 3. Spectrophotometric methods

where A is the absorbance, c is the concentration (in mole per litre) and l is the layer thickness (in cm). The molar absorption coefficient (e) is expressed in 1.mole-l.cm -1.

Conforming to the principles of metrology, the value of the coefficient should be given with consideration to the meaningful number of digits or the precision of measurements. Therefore it is rational to give, e . g . , ~ - 4.2.104 or 4.20.104, not ~ = 42,000. In the former cases the number of significant digits is two or three, whereas in the latter case it is five.

In sensitive spectrophotometric methods the values of the molar absorption coefficient are usually greater then 2.104 , whereas values below 1.103 correspond to methods of low sensitivity. As a result of the underlying quantum theory, the value of e cannot be greater than 1.5-105 . Higher values are possible in some indirect methods, e . g . , in spectrophotometric amplification methods. The knowledge of e enables one to compare the sensitivities of the methods of determination of one and the same element or of elements having similar atomic masses.

The sensitivity of spectrophotometric methods can be expressed conveniently and compared (especially in the case of elements differing considerably in atomic mass) in terms of the specific absorbanee (a) [26] which is obtained by dividing ~ by the molar mass of the element and by 1,000:

c a - -

m o l . m . . 1 0 0 0

The value of a corresponds to the absorbance of a solution of a given element at concentration 1 lag/ml (= 1 ppm)in a cuvette of layer thickness 1 cm.

The sensitivity of the dithizone method for determination of copper (Xmax = 550, molar mass of Cu = 63.54 g/mole) may be expressed as follows:

molar absorptivity (coefficient e) specific absorptivity, a

4.52-104 1 .mole-l.cm -1 0.71 ml.~g-l.cm -1.

The value of e is most readily determined under conditions where the spectrophotometric reagent has zero absorbance at ~max of the complex, and only one stable coloured complex is formed in the system. In cases where the complex is formed stepwise [as in the system Fe(III) - SCN-] the value of e depends on the excess of the reagent.

It is more difficult to determine the value of e in cases where the reagent itself absorbs the radiation at ~max of the complex. If only one coloured complex is formed in the system, the absorbance can be measured with respect to a reference solution in which the reagent's concentration is identical with the concentration of uncombined reagent in the measured solution.

If, however, more than one coloured complex is formed in presence of a coloured reagent, absorbing partly in the region of ~max of the complex, it is difficult to find a concentration range in which Beer's law is obeyed. The values of e then have only a relatively low significance, as they depend on the contributions of individual complexes in the mixture, and hence on conditions such as the excess of reagent or the pH value.

In some indirect methods, the element being determined causes bleaching or changes the colour of the system. In such cases, the value of e is calculated from the change in absorption caused by a definite amount of the analyte.

The determination of the coefficient e is simple in the extractive spectrophotometric methods. In many cases the extraction is associated with transformation of dichromatic solution into a monochromatic one. Several complexes may coexist in an aqueous solution,

3.2. Sensitivity 41

and the extract contains usually one complex of definite composition. Extraction usually leads to an increase in the sensitivity of the method.

In cases where the element to be determined is not completely extracted into the organic phase, because its partition coefficient is too low, and the percent of extraction varies depending on the phase-volume ratio, the value of e should be specified along with the extraction conditions (phase-volume ratio, multiplicity of extractions). Despite the fact that a fraction of the analyte remains in the aqueous phase, the whole quantity of the element present in the initial solution is taken into account.

Some ambiguities occurring in the calculation of the coefficient e may arise from the definition of the mole. For example, 127 g of iodine is 1 mole of I or 0.5 mole of I2; hence it is necessary to give always the chemical form of the substance for which the value of e has been determined.

In some methods, the sensitivity depends on the quality of reagent used to produce the colour reaction. This is of particular importance in the case of reagents of natural origin, although some differences have been noticed also in synthetic organic reagents. The differences in sensitivity are a result of the presence of some admixtures that have a bearing on the reactions of the elements being determined, or that give rise to competitive reactions.

The minimum concentration of an element (in moles per litre) that can be determined by spectrophotometric methods may be calculated from the expression: A = ecl. If one assumes that the minimum measurable absorbance of a solution is A = 0.02, with a cuvette width 1 - 2 cm, and for a moderately sensitive spectrophotometric method, e = 1.104, the respective concentration will be:

c - 0.02/2.104 = 10 -6 M.

If we assume a mean molecular mass of the element as equal to 100 (in this case the specific absorptivity a -- 104/102.103 = 0.1) the minimum measurable concentration of the element will be:

106.102/103 = 10 -7 g/ml - 0.1 ~tg/ml

In order to determine the absorbance of a solution in a cuvette of thickness 2 cm one should have 6-7 ml of coloured solution and a volumetric flask of capacity 10 ml, which corresponds to 1 gg of the element.

If one assumes that saturated solutions of soluble salts have concentrations about 10 percent, then 10 ml of such a solution corresponds to 1 g of the sample. So, if 1 gg of the analyte is contained in 1 g of the sample, its concentration in the sample is 10 -4 %. In more sensitive methods, where e is of the order of 105 and the specific absorptivity is about 1.0, the limit shifts to about 10 -5 %.

Trace concentrations below 10 -4 % are below the sensitivity range of many methods. To have a possibility of determining them by spectrophotometric methods a prel iminary concentration of trace components usually becomes necessary (see Chapter 1 on methods of preconcentration and separation of elements). Depending on the kind of sample and its weight (e.g., 10 g, 100 g, or more) such an operation can increase the sensitivity (shift the limit of determination) by 1-2 or more orders of magnitude. In such a way, the sensitivity of spectrophotometric methods can be increased to 10 -6-10 -7 %.

An important role in trace analysis is played by the blank test, particularly in the determination of the more common elements such as Fe, Zn, Ca, A1, Si. It happens sometimes that the content of an element in the blank exceeds its content in the sample. In such cases the blank test determines the limiting concentration of the element in the given


material. Thus, it is possible to increase the sensitivity by reducing the value of the blank test (e.g., by purification of reagents, or the use of quartz or polyethylene vessels instead of glass).

3.3. Precision and accuracy

In analytical chemistry the term "precision" denotes the reproducibility of results, their scatter and consistency. The term, "accuracy" denotes the degree of proximity of the obtained results to the real (true) value.

The precision of spectrophotometric methods [27-29] depends on the range of the values determined. Under optimum conditions, it ranges usually from 0.5-2%. In the differential technique, precisions ranging from 0.2 to 0.5% are attained.

A matter of considerable importance for the precision of a method is the measuring error. When very low concentrations are determined the relative error is large since the (absolute) measuring error is high as compared with the value measured. When the absorbance of a strongly coloured solution is being measured, only a small part of the incident radiation is transmitted through the solution. The divisions on the logarithmic absorbance scale are so small that considerable reading errors are made. For example, absorbance 2 corresponds to 1% transmission and absorbance 3 to 0.1% transmission. Hence 0.9% of the whole scale corresponds to a 50% change in the concentration. The theoretical value of absorbance measured with maximum precision may be found in the following way:

c = a log x

d c - (a log e)dx

dc (a log e)dx 0.434dx m

c axlogx xlogx

where c is a concentration, x is a ratio of lo/It, and a is a constant. Differentiation of the above equation, and putting the second derivative equal to zero

shows that the error is minimum when log x (=A) is 0.43. In modern absorptiometric instruments, with digital reading of absorbance, the

precision depends on the noise of photomultiplier used as a detector. The signal is subjected to electronic processing. The respective curve of precision error has a broad minimum at A equal to about 0.9. Such spectrophotometers can record absorbance with good precision up to values of about 2.

The errors involved in the absorbance measurements are usually smaller than those associated with chemical operations. In some methods the colour reaction is not reproducible. In other methods, the colour varies with time, and the absorbance should be measured after a strictly determined lapse of time. In some systems even small variations of temperature (e.g., 3-5 ~ result in changes in colour. Some reactions are sensitive to changes of pH. A small change in pH, e.g., through 0.1 unit, may cause a 5% error. Other possible errors are caused by competitive reactions occurring in the system, or by changes in the ionic strength.

The total error in an analysis of a definite material is the sum of errors made in individual stages of the analytical procedure, e.g., in sampling, sample dissolution,

3.4. Selectivity 43

preconcentration and separation of the elements, and measurement of absorbance. In the course of these operations some elements may get to the sample from the environment or may be partly lost. A matter of great importance is the determination of a blank (in trace analysis) and taking it into account in calculation of the final result. The effect of the blank test on the precision of analysis increases as smaller and smaller trace amounts of elements are determined.

In the determination of concentrations ranging from 10-3-10-4% the error is usually within +10%, whereas in determinations of trace amounts in the range 10-6-10-7%, with preconcentration, the error may be as high as +30%.

The size of the total error may be evaluated by comparing the obtained results of analysis with values considered to be true. Where standard samples of known composition are not available, the accuracy of determination may be evaluated if known amounts of the elements to be determined are added to the weighed sample at the beginning of the analytical procedure. The amounts added should be similar to the amounts present originally in the sample. The chemical form of the additions should be selected so that in the chemical operations they behave similarly to the original sample components.

A model procedure recommended for the determination of the precision (and of the sensitivity and detection limit) of spectrophotometric methods, based on experimental data and IUPAC recommendations has been developed [30], and the analysis of errors occurring in determinations of mixture components by means of modem computerized techniques, has been published [31,32].

3.4. Selectivity

Conforming to a decision of IUPAC, a selective reagent is one that reacts with only a small number of elements, and a specific reagent is one that under definite conditions reacts with only one element.

The selectivity of colour reactions and of the corresponding determination methods depends on the nature of the reagent used, the degree of oxidation of the elements determined, the pH of analytical medium, and of complexing agents that mask the interfering ions. If, despite the above agents, some ions still interfere in the determination, the analyte should be isolated from the interfering elements (or conversely). Separation and isolation methods are discussed in Chapter 1.

There are very few specific reagents and reactions. Among the scarce examples are cuproine for Cu(I) and bathophenthroline for Fe(II).

A change of valence of some ions effectively prevents their reaction with certain reagents. As an example, Nb can be determined by the thiocyanate method in presence of Fe if this had been reduced to Fe(II).

The selectivity of most methods can be increased by proper selection of the pH of the analytical medium. For reagents of the R-OH type there exists a relationship between colour reactions and hydrolytic reactions of certain elements. In strongly acid solutions, colour reactions proceed with those elements that have easily hydrolyzable cations, such as Zr, Hf, Th, U(IV) and Ti. In moderately acidic solutions the reactions also proceed with Fe(III), A1, and U(IV); in weakly acid and neutral solutions, with rare-earth elements, Fe(II), Cu, Mn, and in alkaline solutions, with Ca, Sr, and Mg. Easily hydrolyzable species react in more acidic solutions, whereas other ions react only in less acidic ones.

Reagents of the R-SH type react in acid solutions with cations of the second analytical group that form the most stable sulphides. In weakly acidic and neutral solutions they react with metal cations of the third analytical group.


The acidity of the medium has an important influence on the form of certain ions in the solution (e.g., Zr, Ti, V) and this form is decisive for the kind of reaction (in the case of Zr: Zr 4+, ZrO 2+, polymerized forms). In the reactions of chemical species with some reagents, the optimum pH range may be narrow, either because of the maximum sensitivity of the colour reaction or the effect of other ions.

The pH of coloured solutions is often stabilized by means of suitable buffers. Tartrate, citrate, phosphate and phthalate solutions are usually avoided since they can bind many metals in stable complexes. Acetate solutions are often used, although they also can form complexes with some metals (e.g., A1, Be). Acetate buffers may be replaced by hexamethylenetetramine buffers.

An increase of selectivity is usually achieved by masking the interfering ions [33-36]. The masking consists in conversion of the interfering ion into a stable complex formed with the complexing agent added. Owing to this, the ion cannot participate in the colour reaction with the spectrophotometric reagent. Some most important masking reagents are presented in Table 3.1.

High selectivity of spectrophotometric methods is achieved by choosing proper masking reagents and appropriate pH values. The stability of the complexes is not constant, but it varies depending on pH and other parameters, such as the concentration of masking agent and the presence of other complexing compounds.

Table 3.1. The most common masking agents

Element Ag AI As Au B Ba

Be

Bi Ca Cd Ce Co Cr(lll) Cu F Fe(lll) Fe(ll) Ga Ge Hg In Ir Mg

Masking agent CN-, 82032, I, CI, NH3 F, acetate, tartrate, EDTA, OH- Mo S 2-, OH Nb CN-, CI, Br-, $2032 Ni F, hydroxy acids Os EDTA, citrate, SO42 Pb

Element , ,

Mn

F-, citrate, tartrate Pd

citrate, EDTA, I EDTA, citrate, tartrate EDTA, CN, I-, tartrate F-, EDTA, citrate NH3, SCN, EDTA EDTA, tartrate, acetate NH3, ON, S2032, EDTA, citrate H3BO3, AI, Be, Ti F, PO43, EDTA, tartrate CN, $2032, phenantr. EDTA, tartrate, citrate oxalate, F- I, CN, Cl- EDTA, CI-, citrate CI, SCN, NH3 EDTA, oxalate, tartrate

Pt Rh Ru Sb Sc Sn Sr Ta

T h Ti TI U V W Zn Zr

Masking agent oxalate, EDTA, citrate F, H202, citrate, EDTA F-, tartrate, H202 CN, EDTA, NH3 CN, SCN, Cl- acetate, I-, citrate, tartrate, EDTA, SO42- CN-, I-, citrate, tartrate, EDTA, SO42- I-, ON-, NO2-, NH3 CI-, citrate, tartrate CN, Cl tartrate, I-, S 2, OH-, F Citrate, EDTA tart., OH-, S 2, F- SO42-, EDTA, citrate F-, citrate, tartrate F-, EDTA, citrate, acetate H202, F-, citrate CI, EDTA, citrate, tartrate F, CO32, H202, citrate H202, EDTA, F- F, H202, tartrate, citrate CN, EDTA, OH-, NH3 F, oxalate, citrate, H202, PO43

Although the number of known specific reagents is rather small, there exist some methods based on the use of group reagents (that react with many elements), which in the presence of suitable masking agents and at proper pH have a specific action. Examples of such systems may be the determination of Zn with dithizone at pH 4-5 in the presence of

3.5. Colour systems 45

thiosulphate, or the determination of A1 with 8-hydroxyquinoline at pH 9 in the presence of EDTA, cyanide and H202 as masking agents.

The selectivity of spectrophotometric methods has been greatly increased by the development of derivative spectrophotometry (see Chapter 1.5). Derivative spectrophotometry enables one to single out, by means of various mathematical algorithms of data processing, a separate signal due to a selected component, from the sum of absorbances of the analysed mixture. This technique was successfully applied in determinations of a number of elements in mixtures such as Pd, Pt and Au [37], Pd and Pt in iodide solutions [38], Au, Pd and Pt in bromide solutions [39], Ru(III) and Rh(III) in the form of octadecyldithiocarbamate complexes [40], Ru and Os in chloride solutions [41 ], Cu, Hg and Pb as dithizonates [42], complexes of various metals with 4-(2-pyridylazo)resorcinol [43], Fe(III) with EDTA in the presence of Cr(III), A1 and Mn [44], Cr(III) and Cu(II) with EDTA [45], and Cu and Co in a flow system [46]. Derivative spectrophotometry was also used in the study of Sr- complexing reactions with various crown ethers [47].

3.5. Colour systems

Most of the colour systems, used as a basis of spectrophotometric methods are formed as a result of complexing reactions. The most important spectrophotometric reagents are presented in Chapter 4.

A large group of the methods is based on difunctional organic reagents that with metal ions form inner chelates, soluble in non-polar solvents. Such reagents are generally used in the extraction-spectrophotometric methods involving, e.g., dithizone, dithiocarbamates, 8-hydroxyquinoline, 1-nitroso-2-naphthol, dioximes and PAN [1-(2- pyridylazo)-2-naphthol].

Another group of methods is based on organic reagents giving electrically charged, water-soluble complexes. Such reagents comprise hydrophilic, mostly sulphonic groups. Such types of complexes are formed by arsenazo III, PAR [4-(2-pyridylazo)resorcinol], 5- Br-PADAP [2-(5-Br-2-pyridylazo)-5-diethylaminophenol], Eriochrome Black T, Chrome Azurol S, and Bromopyrogallol Red. Cationic complexes of metals are formed in reactions with 1,10-phenanthroline or cuproin.

A large group of sensitive methods is based on ternary systems[ 10,48]. Reactions of easily hydrolyzable metal ions [e.g., Be, A1, In, Sc, Ti, Th, Fe(III), Zr]

with some chelating chromogenic reagents (e.g., CAS, ECR, phenylfluoron) give strongly coloured ternary complexes (with a large bathochromic shift of the order of 100-200 nm) in the presence of surfaetants [49-61]. Among the most often used cationic surfactants are bromide or chloride salts of cetyltrimethylammonium (CTA, formula 3.1) cetylpyridinium (CP, formula 3.2) and tetradecyldimethylbenzylammonium (Zephiramine, formula 3.3).

CH3 C. H3

C16H33--N--CH3 (3.1) C,6H33-- (3.2) Clt.H29 m --CPI 2 I CH3 CH3

An example of a non-ionic surfactant is Triton X-100 (octylphenyl polyethyleneglycol ether).

Basic dyes (triphenylmethane, xanthene, azine, etc., dyes) form ion-association compounds (ion-pairs) with anionic halide complexes of metals and non-metals (e.g., SbC16-, AuBr4-, TaF6-, BF4-). The resulting compounds, that may be extracted into non-polar organic solvents, may serve as a basis for sensitive spectrophotometric methods [62-65].


Extractable ionic associates are also formed in reactions of heteropoly acids with basic dyes [62,66,67]. In some systems, more complex ionic associates are formed. These compounds are not extractable, but may form a basis for very sensitive flotation- spectrophotometric methods [68,69].

Another group of methods is based on ionic associates formed by acid dyes (e.g., eosine, Bengal Rose, Methyl Orange) with hydrophobic cationic complexes of some metals [e.g., Fe(II), Ag(I), Zr, Cd] with 1,10-phenanthroline and other organic bases [10,62].

With metal ions, some organic reagents form coloured compounds that are sparingly soluble both in water and in organic solvents. They are either polynuclear complexes, such as formed by phenylfluoron with Sn(IV) or with Ge(IV), or adsorption-type compounds such as those formed by titanium yellow with Mg. In such cases, the absorbance is measured for suspensions of coloured pseudo-solutions stabilized with protective colloids [e.g., gum arabic, gelatine, poly(vinyl alcohol)].

Complexes of metal ions with inorganic reagents are also applied in spectrophotometric methods, e.g., thiocyanate [Fe(III), Co, Nb, Mo, Re, W, U, Ti], iodide (Bi, Sb, Pd), or hydrogen peroxide (Ti, V, U). Determinations based on such complexes are carried out in aqueous solutions or after extraction with oxygen-containing organic solvents.

A group of elements (P, As, Si, Ge, V, W, Mo etc.) forms yellow heteropoly acids which, on being reduced, give intensely coloured blue compounds. Both the heteropoly acids and their reduced forms are used for determination of the elements. All these forms are extracted with oxygen-containing solvents. It should also be mentioned that the heteropoly acid anions form extractable ionic associates with basic dyes.

Colour systems suitable for use in the spectrophotometric method may also be formed in redox reactions. Some examples of such reactions are: the oxidation of Mn(II) to MnO4- or Cr(III) to CrO42-, oxidation of dimethylnaphthidine with vanadium(V) or chromium(VI), oxidation of o-tolidine with cerium(IV) or with chlorine. Examples of oxidation reactions are also the iodide methods, in which iodide ions are oxidized with bromine to give iodate ions which, in turn, react with the excess of iodide anions to form free iodine (see Chapter 25). A colour effect of reduction also occurs, for example, in determinations of Se and Te in the form of coloured sols produced in the reduction of Se(IV) or Te(IV) to their elementary forms.

In some methods the colour system is a product of a synthesis reaction. Examples of such methods are: formation of an indophenol dye in a method for determination of ammonia, the synthesis of azo dyes in the determination of nitrite, the formation of Methylene Blue in the determination of sulphur, the pararosaniline method for determination of sulphite, and the benzidine-pyridine method for determination of cyanide.

There are also indirect speetrophotometrie methods, in which the element determined provokes a change in colour. This group comprises most of the methods for the determination of fluoride. Being capable of forming stable complexes with some metals, fluoride anions can decompose colour complexes of those metals. Thus, in the method involving a sulphosalicylate complex of Fe(III) the solution is discoloured by F- and a change of colour is observed in the method based on the use of the Zr complex with ECR. Still another example is the determination of phosphate with the use of lanthanum chloranilate. Phosphate anions react to form the less soluble LaPO4 and release coloured chloranilate ions.

Amplification methods [70-73], owing to their high sensitivity, are used in the determination of trace amounts of certain elements, including halides and some metals (e.g., Mo, Cr, Bi).

3.5. Colour systems 47

The development of a new spectrophotometric method is usually preceded by studies of the colour system (complex composition and stability), which is the basis of the method. Fundamental physicochemical studies of the colour complexes existing in the solution enable one to establish the optimum parameters of the method [74-76]. The composition of the complex (molar ratio of metal to ligand) in the solution is determined by Job's continuous variation method, the Bent and French method of equilibrium shift, the method of mole ratio proposed by Yoe and Jones, and the method based on the ratio of slopes and isosbestic points. Studies of complex compounds in solutions are described in many works, especially [77-80].

The role of mixed media (water and water-miscible organic solvents) in colour reactions which are useful in spectrophotometric methods has also been discussed [81 ].

Problems occurring in the development and publishing of new methods have also been considered [82.

3.6. Analytical procedure

Spectrophotometric methods based on the use of calibration curves are comparative methods. Their accuracy depends to a considerable extent on the proper preparation of the standard solutions of the elements to be determined, which are used for the preparation of calibration graphs.

Standard solutions are divided into stock and working solutions. Sufficiently concentrated stock solutions may usually be stored for a long time. More dilute working solutions, obtained by suitable dilution of stock solutions cannot, in general, be stored for a long time.

Stock solutions, usually containing 1 mg of element (or ion) per ml less often, 10 mg/ml - - are obtained by dissolving sufficiently pure salts of definite composition. Water is used as a solvent, or dilute acids if the metal ions would hydrolyse or precipitate as carbonates with atmospheric carbon dioxide.

The concentrations of the working solutions are related to the sensitivity of the spectrophotometric method: 0.1 rag, 10 ~tg, or 1 ~tg of element per ml. Solutions of concentration 10 ~tg/ml, and especially 1 ~tg/ml, are prepared freshly on the day of use. The instability with time, observed in more dilute working solutions, is mainly due to sorption of ions on the surface of the vessel [83,84].

When preparing standard working solutions it is best to conform with the rule of tenfold dilution and using adequately accurate pipettes and standard flasks (e.g., a 10 ml pipette and a 100 ml flask. From a solution of concentration 1 mg/ml one obtains a solution of 0.1 mg/ml and, in turn, 10 ~tg/ml.

If there is no available salt of constant composition, such as copper(II) sulphate or iron(III), aluminium or chromium alums, it is advisable to prepare first a stock solution of an approximate concentration, slightly higher than required and to determine the concentration by a gravimetric or volumetric method. After suitable calculations the solution is diluted with pure solvent to obtain a solution containing exactly, e.g., 1 mg/ml of the given element. In some cases, standard solutions are obtained by dissolving a precisely weighed amount of the element in its pure form.

Sometimes it is necessary for the standard solution to contain no complexing anions, and therefore a perchlorate or a nitrate is used. These can be obtained by starting with any other salt available (e.g., chloride, sulphate). The salt is dissolved in water or dilute acid, the metal is precipitated as hydroxide by means of ammonia or NaOH solution, and the


precipitate is filtered off, dissolved in perchloric or nitric acid, and diluted with water to a definite volume.

In order to select the most suitable wavelength for the spectrophotometric measurements it is necessary to know the absorption spectrum of the complex, as well as that of the reagent if it is coloured.

The absorption spectrum (absorption curve) is a plot of absorbance v s . wavelength. It is obtained with a recording spectrophotometer or plotted manually from measurements made on a non-recording instrument at 10-20 nm intervals, and at 2-5 nm intervals near the absorption maximum. The solvent is used as reference. The absorption spectra are drawn in the system: wavelength (~, in nm) as abscissa, and absorbance (A) or coefficient, e, as ordinate.

When the analytical wavelength has been selected, on the basis of absorption spectra of the complex and the reagent, it is normal to prepare the standard curve.

The absorbance measurements are usually made at ~max of the coloured compound. It can happen, when the complex and the reagent absorb in the same wavelength range, that the absorbance is measured not at the ~ma• but at a wavelength corresponding to a maximum difference between the absorbances of the complex and the reagent.

The s tandard curve (calibration graph, analytical curve, calibration curve), relating the absorbance to the analyte concentration, is prepared by applying the colour reaction procedure to a set of standard solutions, regularly spaced over a concentration range that will give a maximum absorbance of 0.8-1.0. The conditions specified in the analytic procedure must be strictly adhered to. The curve is plotted with the scale arranged so that the curve is at about 45 o to the abscissa, as shown in Fig. 3.1.

0,8

0,6

O,t,

0,2

0 0,5 1,0 1,5 2.0 concentration

Fig. 3.1. Standard curve

The absorbances of standards and samples are measured in cuvettes of the same path- lengths, usually 1 or 2 cm, although 0.1-, 0.5-, and 4 cm cells are sometimes used. The choice of cuvette depends on the sensitivity of the method, the quantity of the analyte and the behaviour of the solution in relation to Beer's law. If, at low values of absorbance, deviations from Beer's law are observed, then the use of a longer pathway cuvette is recommended so that higher absorbance values are obtained for more dilute solutions.

The reference (comparison) solution in one-colour spectrophotometric measurements is usually the solvent (if the reagent is colourless), and in two-colour methods a reagent solution is used. As well as the spectrophotometric reagent the reference should contain other reagents and have pH identical to the analyte solution. It happens that at ~max the absorbance of the colour reagent is zero and pure solvent may be applied as the reference.

49 References

3.7. Trace analysis

Sensitive spectrophotometric methods are used in trace analysis either directly or after a preconcentration of the trace elements to be determined [ 10,85-88]. The value of the blank test plays an essential role in spectrophotometric analysis, especially when common elements are determined. The blank can serve as a reference in measurements of the absorbance. It also eliminates the effect of analyte admixtures introduced with the reagents, vessels or environment [89-94]. Blank determinations are made in parallel with measurements of analyte samples, and the results are obtained as the difference.

Reducing the size of the blank permits the determination of smaller quantities of traces in the samples, especially for the more common elements such as Fe, Zn, Ca, Mg, A1, Si.

The size of the blank can be reduced in various ways. The equipment used for sampling and for commination of the sample should be made, if possible, from material which does not contain the element to be determined. In some cases the admixtures can be removed by the use of an appropriate solvent, which does not react with the sample but dissolves the impurities (e.g., the use of hydrochloric acid to remove iron from a sample of silicon comminuted in a steel mortar). Quartz, polyethylene or Teflon vessels are used instead of glassware. To prevent contact of the sample with laboratory air, certain chemical operations may be conducted in closed chambers (dry boxes) flushed with purified air or inert gas [95, 96].

The reagents are the source of the major contaminants in the blank test. The water used should be distilled in a quartz apparatus after demineralisation with ion-exchangers [97]. Acids (H2SO4, HNO3, HC104, HC1, HBr) can be purified by slow distillation in quartz vessels. Hydrofluoric acid is purified by distillation in platinum or palladium vessels. Ammonia of high purity is obtained by saturation of distilled water with gaseous NH3. Reagents thus purified are kept in polyethylene bottles.

Solutions of other reagents are usually purified by solvent extraction and coprecipitation with collectors. Some volatile reagents (hydrochloric acid, hydrobromic acid, ammonia) can be brought to a very high degree of purity by isothermal (isopiestic) distillation [98,99]. Iron(III) traces can be removed from concentrated hydrochloric and hydrobromic acids by passing the acids through a strongly basic anion-exchanger.

For control of the accuracy in determination of traces of elements, standard reference materials are of great importance [100-106].

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References 51

49. Tikhonov V. N., Zh. Anal. Khim., 32, 1435 (1977). 50. Chernova R.K., Zh. Anal. Khim., 32, 1477 (1977). 51. Chernova R.K. et al., Zh. Anal. Khim., 33, 858 (1978). 52. Savvin S.B., Chernova R.K. et al., Zh. Anal. Khim., 33, 1473 (1978). 53. Nemodruk A.A. et al., Zh. Anal. Khim., 34, 1266 (1979). 54. Tikhonov V.N., Aleksandrova N.L., Zh. Anal. Khim., 36, 242 (1981). 55. Tikhonov V.N., Pavlova O.K., Zh. Anal. Khim., 37, 1809 (1982). 56. Tikhonov V.N. et al., Zh. Anal. Khim., 38, 216 (1983). 57. Chernova R.K. et al., Zh. Anal. Khim., 39, 1019 (1984). 58. Tananayko M.M. et al., Zh. Anal. Khim., 39, 1034 (1984). 59. Callahan J.H., Cook K.D.,Anal. Chem., 56, 1632 (1984). 60. Antonovich V.P., Novoselova M.M., Nazarenko V.A., Zh. Anal. Khim., 39, 1157 (1984). 61. Jarosz M., Marczenko Z., Chem. Anal. (Warsaw), 37, 63 (1992). 62. Marczenko Z., Mikrochim. Acta, 1977 II, 651. 63. Motomizu S., Fujiwara S., T6ei K., Anal. Chim. Acta, 128, 185 (1981). 64. T6ei K., Anal. Sci., 3, 479 (1987). 65. Balog I.S., Kish P.P., Bagreev V.V., Zh. Anal. Khim., 43, 1750 (1988). 66. Alimarin I.P. et al., Zh. Anal. Khim., 39, 965 (1984). 67. Lychnikov D.S., Dorokhova E.N., Gracheva N.A., Zh. Anal. Khim., 43, 802 (1988). 68. Marczenko Z., Pure Appl. Chem., 57, 849 (1985). 69. Marczenko Z., Kalinowski K., Chem. Anal. (Warsaw); 32, 451 (1987). 70. Belcher R., Talanta, 15, 357 (1968); 24, 533 (1977). 71. Weisz H., Fritsche U., Mikrochim. Acta, 19"/3, 361" 1974, 701. 72. Flaschka H.A., Hornstein J.V., Microchem. J., 23, 488 (1978). 73. Burns D.T., Townshend A., Talanta, 39, 715 (1992). 74. McBryde W.A., Talanta, 21,979 (1974). 75. Sommer L., Kuba_ V., Langov~ M., Z. Anal. Chem., 310, 51 (1982). 76. Sommer L., Langova M., Crit. Rev. Anal. Chem., 19, 225 (1988). 77. Klausen K.S., Langmyhr F.J.,Anal. Chim. Acta, 40, 167 (1968). 78. Klausen K.S., Anal. Chim. Acta, 44, 377 (1969). 79. Likussar W., Boltz D.F., Anal. Chem., 43, 1265 (1971). 80. Ringbom A., Harju L., Anal. Chim. Acta, 59, 33, 49 (1972). 81. Petrova T.V., Savvin S.B., Zh. Anal. Khim., 42, 1925 (1987). 82. Kirkbright G.F., Pure Appl. Chem., 50, 237 (1978). 83. Struempler A.W.,Anal. Chem., 45, 2251 (1973). 84. A1-Sibaai A.A., Fogg A.G.,Analyst, 98, 732 (1973). 85. Nazarenko V.A., Flyantikova G.V., Zh. Anal. Khim., 32, 1217 (1977). 86. Sandell E.B., Onishi H., Photometric Determination of Traces of Metals. General

Aspects (Chapters 1--4), Wiley, New York 1978. 87. Zolotov Yu.A., Kuz'min N.M., Preconcentration of Trace Elements, Elsevier,

Amsterdam 1990. 88. Lobifiski R., Marczenko Z., Spectrochemical Trace Analysis for Metals and Metalloids,

Elsevier, Amsterdam 1996. 89. Robertson D.E., Anal. Chem., 40, 1067 (1968). 90. Kloster M.B., Hach C.C., Anal. Chem., 44, 1061 (1972). 91. Mizuike A., Pinta M., Pure Appl. Chem., 50, 1519 (1978). 92. T61g G., Pure Appl. Chem., 50, 1075 (1978). 93. T61g G., Z. Anal. Chem., 294, 1 (1979).


94. Kosta L., Talanta, 29, 985 (1982). 95. Zief M., Mitchell J.W., Contamination Control in Trace Element Analysis, Wiley, New

York 1976. 96. Van Grieken R., Van de Velde R., Robberecht H.,Anal. Chim. Acta, 118, 137 (1980). 97. Petrick H.J., Schulze F.W., Cammenga H.K., Mikrochim. Acta, 1981 II, 277. 98. Kuehner E.C., Alvarez R., Paulsen P.J., Murphy T.J., Anal. Chem., 44, 2050 (1972). 99. Rovinskii F.Ya., Gasilina N.K., Zh. Anal. Khim., 33, 160 (1978). 100. Knowles A., Burgess C., Standards in Absorption Spectrophotometry, Chapman-Hall,

London 1981. 101. Cali J.P.,Anal. Chem., 48, 802A (1976); Z. Anal. Chem., 297, 1 (1979). 102. Koch O.G., Pure Appl. Chem., 50, 1531, 1951 (1978). 103. Quevauviller E.A., Maier E.A., Griepink B., Anal. Chim. Acta, 283, 583 (1993). 104. Michalke B., Fresenius'J. Anal. Chem., 350, 2 (1994). 105. Caroli S.,Anal. Chim. Acta, 283, 573 (1993). 106. Kuselman I., Talanta, 40, 1 (1993).

Chapter 4. Spectrophotometric reagents

Reagents which give the colour reactions, upon which spectrophotometric methods are based, are called spectrophotometric reagents. Most spectrophotometric methods are based on organic reagents [1-4].

The theory of organic reagents, the mechanism of their reactions with metal ions, the structure of the complexes formed, and the effect of the dissociation constants and of the reagents and of hydrolysis constants of metal ions upon the formation of complexes have been presented in publications [5,6]. A historical survey of the organic reagents has been given [7]. A review of reagents used for determination of inorganic species has also been published [8,9].

This Section deals with the more important groups of spectrophotometric reagents. Some important reagents are presented, with formulas, in chapters devoted to individual elements.

4.1. Azo reagents

The azo reagents form a large group of spectrophotometric organic reagents. This group comprises such reagents as 1-(2-pyridylazo)-2-naphthol (PAN), 4-(2-pyridylazo)resorcinol, (PAR), 5-Br-PADAP, Arsenazo III and Chlorophosphonazo III. Methods using azo dyes are sensitive.

4.1.1. N-Heterocyclic azo compounds

Pyridylazo- and thiazolylazo reagents have become of great importance in the methods of determination of metals [ 10,11 ].

1-(2-pyridylazo)-2-naphthol (PAN) (formula 4.1) can exist in solutions in three forms, depending on the pH. Acid solutions (pH < 2) contain the water-soluble yellow-green protonated HzR + ion. Between pH 3 and 11, PAN occurs as the neutral HR molecule, soluble in organic solvents to give a yellow colour (colloidal suspensions can be formed in aqueous systems). In alkaline solutions of pH > 11 PAN exists as the water-soluble red R ~ anion.

OH PAN is normally used in methanol or ethanol solutions. It acts as a terdentate ligand,

complexing with metals through the hydroxyl oxygen atom, pyridine nitrogen atom, and one of the azo group nitrogen atoms. Metal complexes with PAN are sparingly soluble in water. Metal ions form neutral complexes extractable with inert solvents, such as chloroform or benzene. This permits the extractive-spectrophotometric determination of Mn, Zn, Cd, Cu, Ni, Co, In, U, Ga and Pd [12-14]. The molar absorptivities of the PAN complexes lie within

54 4. Spectrophotometric reagents

the range (2-6). 104 . During extraction with an organic solvent, the metal complex and the uncombined PAN pass to the extract. The absorption maxima ~max of the complexes are usually very different from that of the reagent.

The selectivity of the methods using PAN is enhanced by suitable selection of pH and masking agents. Iron, cobalt, and nickel, for example, react with PAN at pH = 4. At such a low pH the reagent does not form complexes with Mn, Zn and Cd. Cyanide enables manganese to be determined in the presence of Ni, Zn, Cd, Co, and Cu, which form stable cyanide complexes. Zinc and cadmium can be demasked from their cyanide complexes with formaldehyde.

Unlike PAN, 4-(2-pyridylazo)resoreinol (PAR) (formula 4.2) is water-soluble and forms water-soluble complexes with metal ions [15,16]. In the range from 90% H2SO4 to pH 2 the following protonated forms are present: HsR 3+, H4R 2+ and H3R +. The neutral PAR molecule exists between pH 2.1 and 4.2 (~,max 385 nm). The anion HR- (~max 413 nm) occurs over the pH range 4.2-7. In alkaline solutions (pH 11-13) both hydroxyl groups are dissociated. The R 2- form has ~max 490 nm.

OH

(4.2)

PAR gives coloured complexes with metal ions. The determination of metals with PAR is performed in aqueous solutions. In 0.5-0.05 M H2804 the reagent reacts with Cu, Bi, Ti, Zr, Pd, and TI(III) ions. In acetate medium (pH 3-6) PAR gives colour reactions with Zn, Cd, Co, Ni, Hg, U, Pb and Ga. Solutions of PAR complexes have a red or violet colour.

Complexes of PAR with Co, Ni, Cu, Zn, and Cd can be extracted into chloroform in the presence of diphenylguanidine [ 17].

The bromo- and ehloro-derivatives pyridylazo derivatives [18-21] are very valuable, because they form the basis of highly sensitive methods. Their complexes with metal ions have molar absorptivities e about 105. The most widely applied 2-(5-bromo-2-pyridylazo)-5- diethylaminophenol (5-Br-PADAP) (formula 4.3) has become the basis for very sensitive methods of determination of Cd, Co, Ni and other metals.

<4> N.~-----. NICzI..Is) 2

/ HO

CI,,

HO

(4.4)

HzN

(4.3)

(4.5)

Other examples of reagents of this group are: diethylaminophenol (5-C1-PADAP) (formula 4.4),

2-(5-chloro-2-pyridylazo)-5- 4- (5-bromo-2-pyridylazo)- 1,3-

4.1. Azo reagents 5 5

diaminobenzene (5-Br-PADAB) (formula 4.5) and 2-(3,5-dibromo-2-pyridylazo)-5- diethylaminophenol (3,5-diBr-PADAP) used in determinations of U and Zr.

Thiazole azo compounds have similar properties to PAN and PAR as spectrophotometric reagents. Their reactions with metals are more selective, principally as a consequence of the lower stability of their complexes. The following reagents are examples: 1-(2-thiazolylazo)-2-naphthol (TAN) [22] (formula 4.6) and 4-thiazolylazo)resorcinol (TAR) [23, 24] (formula 4.7).

s I s

o .

(4.6) (4.7)

A comprehensive review on applications of thiazolylazo reagents in spectrophotometric methods has been given [25].

4.1.2. Arsonic azo compounds

The first reagents of this group were Arsenazo I (formula 4.8) and Thoron I (formula 4.9). This group of reagents is characterized by the presence of an arsonic acid group ortho to the azo group. The hydroxyl group is usually also ortho to the azo group.

AsO3Hz HO S03H

(4.8) (4.9)

The presence of the arsonic acid group, AsO3H2, causes the formation of stable complexes of some metals even in fairly strong acid solutions. The presence of the azo group ensures the colour reaction, whereas the hydroxyl group enables a second ring to be formed with the metal. This not only stabilizes the complex but also causes a considerable deepening of the colour obtained. The sulphonic acid groups render these reagents and their complexes water- soluble.

Until Arsenazo I and related reagents were introduced into spectrophotometric analysis, there were no sensitive reagents for such elements as Th, Zr, Hf, U, and rare earths.

Arsenazo I and a number of its analogues are derivatives of chromotropic acid (1,8- dihydroxynaphthalene-3,6-disulphonic acid). Arsenazo I is applied to the determination of rare earths, Th, U, Zr, Ti, Nb, and other metals.

Thoron I is less sensitive but more selective than Arsenazo I. It is most commonly applied to the determination of thorium and also Li, U, Zr and Be.


Arsenazo III (formula 4.10) is a very useful bis-azo dye based on chromotropic acid and o-aminophenylarsonic acid. It is moderately soluble in neutral and acid solutions, and readily soluble in slightly alkaline solutions. Strong oxidizing (H202, C12, Br2) and strong reducing agents (e.g., TIC13) cause decomposition. In acid solutions (from 10 M hydrochloric acid to pH 4) Arsenazo III has a purplish-red colour, while at higher pH values it is blue- violet.

~~__ s03Hz H0 OH HzO~ As

(4.10)

In strongly acid solutions (1-10 M HC1) Arsenazo III reacts only with Th, Zr, Hf and U(IV). The molar absorptivities, e, of the complexes with these metals are about 105. At pH 1-4 Arsenazo HI reacts with U(VI), Sc, Fe(III), Bi, and rare earths. The sensitivity of the colour reactions is lower in this case (e -5-104). The use of Arsenazo III in strongly acid medium overcomes difficulties connected with the hydrolysis of some multivalent metals (e.g., Zr, Th, U). In the determination of these metals the high acidity enhances the selectivity of the reagent.

The absorbance of free Arsenazo III (~max 520-530 nm) at the absorption maxima of the metal complexes (~ 655-665 nm) is very slight. The large difference (A~) between the wavelengths of the absorption maximums of the complexes and the free reagent is important. In the case of Th and U the spectrophotometric method with Arsenazo HI is specific owing to the use of masking agents (oxalic acid, HF) and an appropriate acidity of the medium.

Only one side of the symmetrical molecule Arsenazo III participates in the formation of complexes with metal ions. The metal ion bonds to the nitrogen atom of the azo group, the oxygen atom of the arsonic acid group, and the oxygen atom of the hydroxyl group. The distortion of the symmetry of the reagent molecule gives rise to two neighbouring absorption maxima in the visible spectra of the Arsenazo III metal complexes.

It is possible to determine Arsenazo III by measuring its absorbance in concentrated sulphuric acid medium at 675 nm. Arsenazo I does not absorb under these conditions. The synthesis conditions and properties of high-purity Arsenazo III have been discussed [28-31 ]. A large number of other spectrophotometric reagents containing arsonic acid groups have been suggested. The reagents and their applications are critically reviewed [27,32,33]. The solubility and mechanism of reactions of these reagents have been discussed [34].

A related group of reagents comprises azo dyes containing phosphonic acid groups, for example, the Chlorophosphonazo HI (formula 4.11). This reagent is recommended for determinations of Zr, Ti, U, Sc, Ca, and Sr, among others [27].

P03Hz H0 OH Hz03 P~

HO3S'" v ~ f "SO3H

(4.11)

4.1. Azo reagents 57

4.1.3. Other Azo Reagents

Many azo reagents of other classes, such as o,o'-dihydroxyarylazo compounds have been employed in spectrophotometric determinations of various metals An example of the latter group of reagents is Sulphochlorophenol S (formula 4.12), which is recommended for determining such elements as Nb, Zr, Mo, Cu, V, and A1 in acid media [27].

HO3S~ - - ~ HO S03H HO OH \ / N~'N N=N--~

/ .... H03 S / ' w ~ / * ~ V J ~ ' ~ S 03H \CI CI

(4.12)

An analogue of Sulphochlorophenol S is Sulphonitrophenol M, with nitro groups substituted for the two chlorine atoms of the former compound. This reagent has been applied for determination of Pb, Nb, A1, Ga, Zr, and V. Other examples of o,o'- dihydroxyarylazo reagents are: Picramine-epsilon (formula 4.13) and the known reagents for determination of magnesium, namely Eriochrome Black T and Calmagite.

OzN OH HO S03H

OzN H03S

(4.~3)

Examples of o-hydroxyarylazo compounds are: Chromotrope 2B (formula 4.14), a reagent for thorium and rare-earth elements, and 2-(4-sulphophenylazo)chromotropic acid (SPADNS) (formula 4.15) used in determinations of A1, Zn, Th, U and other metals [35]. In both these groups of azo reagents the oxygen atom of the o-hydroxyl group and the nitrogen atom of the azo group participate in complex formation with metal ions.

HO OH HO OH

(4.14) (4.15)

The presence of the sulphonic acid group in the azo reagents makes these and their complexes soluble in aqueous solutions. These complexes can be extracted in the presence of Aliquat 336 or tetrahexylammonium iodide in chloroform [36].

The sensitivity of reactions with azo reagents increases in the presence of organic solvents, such as acetone, propanol, or acetic acid [37].

8-Hydroxyquinoline azo derivatives [38], pyrocatechol azo derivatives [39], and o- thiazo derivatives of p-cresol and 2-naphthol [ 4 0 ] have been reviewed.


4.2. Triphenylmethane reagents

Owing to the presence of p-quinonoid tings triphenylmethane reagents are intensely coloured and provide the basis of spectrophotometric methods for the determination of a number of metals.

Pyrocatechol Violet (formula 4.16) is a frequently used chelating reagent, readily soluble in water and in aqueous ethanol. An aqueous solution of the reagent is yellow (pH 1- 8), and the colour of the solution changes to violet with increasing pH as a result of proton dissociation from the hydroxyl groups. Pyrocatechol Violet forms coloured (most often blue) chelates with many metals (e.g., Be, A1, Bi, Co, Cu, Fe, Ga, In, Mn, Pb, V, Zn) in weakly acidic and weakly basic solutions.

OH OH

(4.16)

SO3'H

Eriochrome Cyanine R (ECR) (formula 4.17) and Chrome Azurol S (CAS) (formula 4.18) have carboxylate groups besides hydroxyl and sulphonate groups [41,42]. These reagents can be purified by various methods [43,44]. ECR and CAS react with many metals in weakly acidic or neutral solutions [45-47]. The yellow-orange colour of the reagents changes to blue or violet solutions of the metal chelates.

C. H3 C, H3 C'H3 C'H3

HOOC"" v "C ~ ~ "COOH HO0 C OOH + SO~H I

(4.17) (4.18)

Xylenol Orange (formula 4.19) [48,49] and its relative Methylthymol Blue [48,50] are characterized by the presence of the iminodiacetic acid groups, which occur in complexones such as EDTA.

CH3 CH3

HOOCHz C CHzCOOH

HOOCI.IzC/'''~zu ~CHzCOOH S03H (4.19)

These reagents can be purified by various methods [51-53]. They are usually contaminated with unchanged starting materials and with products containing only one

4.3. Xanthene reagents 59

iminodiacetic group (e.g., Semi-Xylenol Orange). This group comprises also the Metal- phthalein, used in determinations of Sr and Ba.

Surface-active substances, cationic (CTA, CP, Zephiramine) (formulae 3.1, 3.2, 3.3) - and non-ionic (e.g., Triton X-100) surfactants make it easier to dissociate the protons of chelating triphenylmethane reagents and, in consequence, facilitate the reactions of these chromogenic reagents (R) with easily hydrolysable metals [Be, A1, Ga, In, Fe(III), Sc, Ti, Zr] [54-61]. Ternary complexes are formed, with hyperchromic (increase of absorbance) and pronounced bathochromic (shift of ~max towards longer wavelengths) effects. In these complexes the ratio R:M. is higher than in binary systems (without surfactants). The mechanism of colour reactions involving surfactants was an object of studies that have led to fairly differing hypotheses. An important feature is the formation of surfactant micelles at considerable concentrations and appropriate pH values. In the determination of metals, values exceeding 105 are attained.

4.3. Xanthene reagents

Among most frequently used chelating xanthene reagents are: Gallein (formula 4.20) [62], Bremepyregallel Red (formula 4.21) [63], and Pyregallel Red [63,64]. Xanthene reagents form chelates with many metals, such as Bi, In, Mo, Sn, Sb, Th, Ti, and Zr. The sensitivity of the determination methods increases in the presence of cationic surfactants [65].

OH OH OH OH

v "C ~" ~ B( v -C=, - v "Br

(4.20) (4.21)

Caleein (Fluorexone, Fluorescein Complexone) (formula 4.22), used as reagent for the determination of calcium, contains iminodiacetic acid groups in positions 2 and 7, and only one hydroxyl group in position 3. In the case of Calcein (like Xylenol Orange), the nitrogen atom of the iminodiacetic acid group participates in bonding the metal ion.

HO.~ 0 ~ O

HOOCHtC CH2COOH ~ N H z C ~ C ~ C I . . ~ N /

HOOCHzC / ~CHzCOOH ~ COOH

(4.22)


(4.23)

Another group of chelating xanthene reagents comprises 2,3,7-trihydroxy-6-fluorones, called simply fluorones [66]. The most often used reagents of this group are phenylfluorone (reagent for Ge and Sn) (formula 4.23) and methylfluorone, used for determination of Sb. Among more commonly known fluorones are salicylfluorone (formula 4.24 and disulfophenylfluorone (formula 4.25).

S03H

(4.24) (4.25)

The sensitivity and the selectivity of the methods involving 2,3,7-trihydroxy-6-fluorones increase markedly in the presence of surfactants [67].

4.4. Non-chelating organic reagents

4.4.1. Basic dyes

A number of elements give anionic complexes with halide and other ligands. These complexes can form ion-associates (ion pairs) with basic dyes, and are extractable into non- polar solvents. The extracts form the basis of sensitive extraction-spectrophotometric methods [68,69].

The basic dyes are generally used as their halide salts. The form in which the dye can form the ion-association compound is its singly-charged cation. The solvents most often used are benzene, toluene, chloroform, and 1,2-dichloroethane. Sometimes a small amount of a donor-active oxygen-containing solvent (e.g., MIBK, butanol, di-isopropyl ether) is added to enhance the extraction. The acidity of the aqueous phase can vary over a wide range, from moderately concentrated mineral acid solutions (HC1, H2SO4) to pH 3-5, depending on the basic dye and the extraction solvent used.

4.4. Non-chelating organic reagents 61

Triaryhnethane (most often triphenylmethane) dyes constitute the most numerous group. The main representatives are Malachite Green, Brilliant Green (C2H5 instead of CH3 group) (formula 4.26) Crystal Violet (formula 4.27) and Methyl Violet [NHCH3 instead of N(CH3)2 on one benzene ring]. This group of dyes also includes Fuchsine (formula 27.1).

4- (CzHs]zN~ ~N(CzHsIz V - C ~'- v

+ (CH3)zN~ .~NICH3)z v -C ~

N(CH3] 2 (4.26) (4.27)

The diphenylnaphthylmethane dyes comprise Victoria Blue 4R (formula 4.28) and Victoria Blue B (H instead of CH3 on the naphthyl group). Figure 4.1 shows the absorption spectra of some triarylmethane basic dyes.

+ (CH3)zN,~ ,~N(CH3lz C

H3C//'N~csHs

(4.28)

2

500 600 . 6&O, 700 wavelength, nm

Fig. 4.1. Absorption spectra of Brilliant Green (1), Crystal Violet (2) and Victoria Blue 4R (3)

The theory of extraction of the ion-associates of basic triarylmethane dyes with complex anions has been discussed [70,71 ].


Rhodamine B (formula 4.29) is a xanthene basic dye [72]. Other reagents in this class are Rhodamine 3B (Ethyl Rhodamine B) (the ethyl ester of Rhodamine B) [73], Butyl Rhodamine B (the butyl ester) and Rhodamine 6G (formula 4.30).

4- +

C z H s N H ~ 0 ~ I H C z H s

H3C" ~ "13 CH3 ~ C00CzHs

(4.29) (4.30)

The existence of several protonated forms of rhodamine dyes in acid solutions has been established [74]. Figure 4.2 shows the absorption spectra of xanthene basic dyes.

The azine basic dyes are often applied in the spectrophotometric methods. Methylene Blue (formula 48.1) and Methylene Green are thiazine dyes.

2

r J3

., 1 530

~00 S~176 nm6~176

Fig. 4.2. Absorption spectra of Rhodamine B (1) and Rhodamine 6G (2)

4" ( C z H s ) z N - ~ ~ o ~ I C H 3 ) z

,,..u3P-" v ~ 1 ~- v

(4.31) (4.32) (4.33)

Among oxazine dyes are: Capri Blue (formula 4.31), Nile Blue A (formula 4.32) and Mendola's Blue. Safranine T [75] represents a phenazine dye (formula 4.33). The absorption spectra of the three azine dyes are presented in Fig. 4.3.

4.4. Non-chelating organic reagents 63

!

500

I

600 7O0 wavelength, nm

Fig. 4.3. Absorption spectra of Methylene Blue (1), Capri Blue (2) and Safranine T (3)

Other types of basic dyes proposed for spectrophotometric determination of elements include the antipyrine dyes (Chrompyrazoles) [76,77], azo dyes [78], and the indamine dye Bindschedler' s Green (formula 30.1).

4.4.2. Acid dyes

Acid dyes are used less frequently in spectrophotometric methods than the basic ones. Acid dyes form extractable ion-associates with hydrophobic cationic metal complexes. The dyes used are generally acid-base indicators.

The acid xanthene dyes comprise Eosin (tetrabromofluorescein, formula 4.34) [79,80], Erythrosin (tetraiodofluorescein) and Rose Bengal B (formula 4.35) [79]. All these reagents are available as sodium salts.

Br Br I I

Cl" y "CI

CI

(4.34) (4.35)

Br Br

S03H

(4.36)


Of the azo dyes, Methyl Orange has been successfully applied. The formula (4.36) shows the acid triphenylmethane dye, Bromophenol Blue

(tetrabromophenolsulphophthalein) [81,82]. The cationic complexes which form ion-association compounds with acid non-

chelating dyes are most commonly those of Zn, Cd, Pd, Fe(II), Cu with 1,10-phenanthroline. The e values often exceed 5.104. Chloroform and toluene are the commonly used extractants. As in the case of basic dyes, simple salts of acid dyes (e.g., sodium salts) are practically not extracted.

4.5. Dithizone

Dithizone (HzDz, diphenylthiocarbazone, 3-mercapto-l,5-diphenylformazane) is one of the foremost organic spectrophotometric reagents [83-85]. It provides the basis of sensitive methods for the determination of Pb, Zn, Cd, Ag, Pd, Hg, Cu, Bi, and other metals. It has often been used in the extractive separation of traces of metals before their determination.

Dithizone is insoluble in aqueous solutions at pH <7. It dissolves in alkaline media, forming an orange solution of HDz- anions. Dithizone dissolves, giving green solutions, in CC14, CHC13, hydrocarbons, and alcohols. The two tautomeric forms of dithizone: keto (I) and enol (II) (formula 1.1) co-exist in organic solvents [86]. The conditions for formation of dithizone complexes with individual metals are outlined in the sections dealing with the elements concerned.

A primary or secondary* dithizonate is obtained, depending on whether the dithizonate reacts as the anion of the monobasic acid (HDz-) or the dibasic acid (Oz2-). Secondary dithizonates are formed by only a few metals. They are represented by such formulae as AgzDz and CuDz. Generally, acidic media and excess of dithizone favour the formation of primary dithizonates, whereas in alkaline media, and with insufficient amounts of the reagent, secondary dithizonates are formed.

Investigations of the structures of primary dithizonates [89] have shown the metal to be bonded to the sulphur atoms by replacement of the hydrogen in the thiol group, and also co- ordinately bonded to a nitrogen atom, according to the formula 4.38 (for M2+).

9 N ~ N .S - -C~N- -NH-~"~ ! \ ~ / I

d (4.38)

The solubility of metal dithizonates, like that of dithizone itself, is higher in CHC13 than in CC14 (10-3-10 -4 M): Cd(HDz)2 and Pb(HDz)2 are among the dithizonates most sparingly soluble in CC14.

*In the opinion of some authors [87,88] secondary dithizonates do not exist. In alkaline or acidic media, in the presence of excess of metal ions, mixed-ligand complexes are formed, e.g. HgCI(HDz) or Cu(OH)(HDz), where HDz- is the dithizone anion.These authors negate the possibility of existence of Dz e- anion.

4.5. Dithizone 65

Solutions of metal dithizonates in organic solvents are intensely coloured, their colour differing considerably from that of dithizone. An exception is the grey-green solution of Pd(HDz)2. Figure 4.4 shows the absorption spectra of dithizone and several dithizonates dissolved in CC14. The presence of CHC13 admixtures in carbon tetrachloride affects the molar absorptivity of dithizone [90].

The most stable dithizonates (those of Pt, Pd, Au, Ag, Hg, and Cu) can be extracted from strongly acid solutions. Some metals (Bi, In, Zn) are extractable from weakly acid media, whereas other metals (Co, Ni, Pb, T1, Cd) are extractable from neutral or alkaline media. The higher the excess of dithizone, the lower is the pH at which the dithizonate forms. The pH at which extraction of individual metals starts is approximately 1.5 pH units higher with dithizone in chloroform than with dithizone in carbon tetrachloride.

0

./"

4t:)0 ' 462

.'t, r ",,, . , . ! \ , . \ - -

500 550 600

1

700 wavelength, nm

Fig. 4.4. Absorption spectra of dithizone (H2Dz) (1) and dithizonates: Cu(HDz)2 (2);, Pb(HDz)2 (3); Bi(HDz)3 (4); and AgHDz (5) in CCI4

Irrespective of their thermodynamic stability, the dithizonates of certain metals (e.g., Ag, Hg, Pb, Cd) are extracted rapidly, whereas those of other metals (Pd, Cu, Zn) require prolonged shaking with the organic solution of dithizone. This is explicable in terms of the kinetics of dithizonate formation [91]. The dithizonates of some metals (e.g., mercury) in CC14 or CHC13 change colour under the action of sunlight [92].

Selectivity in spectrophotometric methods for determining metals with dithizone is attained by controlling the acidity of the medium and using masking agents such as cyanide, EDTA, thiosulphate, or iodide.

The most frequently applied method for determining metals with dithizone consists in extracting the metal from aqueous solutions with an excess of dithizone solution in CC14 (CHC13), removing the free dithizone from the non-aqueous phase by shaking with an alkaline aqueous solution, and measuring the absorbance of the coloured metal dithizonate solution.

Atmospheric oxygen and other oxidants oxidize dithizone to diphenylthiocarbodiazone which, unlike dithizone, is insoluble in aqueous alkaline solutions, but soluble in CC14 and CHC13, giving a brown solution. This compound is chemically inactive.

Commercial dithizone preparations are always contaminated with the oxidation products. The active dithizone content of a reagent preparation diminishes with time. When


preparing a dithizone solution, the oxidized product is separated, making use of the fact that diphenylthiocarbodiazone is insoluble in ammonia. (For the preparation of dithizone see section 46.2.1).

Pure stable, solid dithizone may be obtained as follows. The reagent (0.5 g) is dissolved in 50 ml CHC13, and the solution filtered through a sintered glass filter. The solution is shaken in a separating funnel with two 10-ml portions of diluted aqueous ammonia (1 + 100). The ammoniacal extracts are separated from the brown chloroform layer and acidified with dilute hydrochloric acid to precipitate the dithizone, which is then extracted with small portions of pure CHC13. The extract is shaken with two portions of water, then placed in a beaker and the solvent is evaporated off in a water-bath at about 50 ~

Carbon tetrachloride and chloroform are the normal solvents for dithizone. Its solubility is greater in CHC13 (1 g /100 ml) than in CC14 (0.08 g/100 ml). Unless otherwise stated, carbon tetrachloride is preferable as a solvent because of its lower volatility and greater specific gravity, which results in a more rapid phase-separation on shaking with the aqueous solution. Carbon tetrachloride is less soluble in water (0.08%) than is chloroform (0.8%), and is also less toxic.

Solutions of about 0.01% (10 mg/100 ml) concentration can be stored for a long time if kept cool and in the dark. In spectrophotometric determinations use is mostly made of 0.001-0.002% solutions in CC14 or CHC13. The dilute solutions must not be stored. The stability of dithizone solutions and of metal dithizonates depends on the purity of the solvents used [88]. It is very important that the carbon tetrachloride and chloroform contain no oxidizing substances (e.g., chlorine from their decomposition). The procedure for purifying and recovering CC14 and CHC13 is given in Section 46.2.1.

In the presence of surfactants it is possible to determine metals with dithizone directly in the aqueous phase, with no extraction [93].

Di-(2-naphthyl)thioearbazone (dinaphthizone), an analogue of dithizone, is also applied to the spectrophotometric determination of certain metals. The reagent solution in CC14 or CHC13 is blue. The solubility of metal dinaphthizonates in organic solvents is lower than that of dithizonates.

Reagents related to dithizone are: 1,5-diphenylcarbazide (formula 17.1), a reagent for Cr(VI) and Os, and diphenylcarbazone which is used, inter alia, for determining mercury. To the same family of reagents belong 1,5-diphenylthiosemicarbazide (formula 42.1) and 2,4-diphenylthiosemicarbazide, which are valuable spectrophotometric reagents for Ru, Re, Se, and Te. The application of thiosemicarbazones and semicarbazones in spectrophotometry has been reviewed [94,95].

4.6. Dithiocarbamates

Sodium diethyldithiocarbamate (Na-DDTC, formula 4.40) is the dithiocarbamate most commonly used in spectrophotometric analysis [96-98].

C2H5 /~S ~N ___C// CzH/ ~S-Na '+

CzHs_ S § (4.41 (4.40) CzH/

4.7. 8-Hydroxyquinoline 67

The metal diethyldithiocarbamates are sparingly soluble in water, but dissolve in organic solvents such as CHC13, CC14, amyl acetate, or acetone.

Spectrophotometric methods using Na-DDTC are rather insensitive since the colours of metal complexes with DDTC are not intense. Complexes with Cu, Bi, and Mn are among the most intensely coloured diethyl dithiocarbamates. The yellowish Ag-DDTC is used in the determination of arsenic (reaction with ASH3, see Section 8.2.2). Fig. 19.1 shows the absorption spectra of some metal diethyldithiocarbamates.

In the reactions with metal ions, chelates with the uncommon four-membered ring are formed. Carbon tetrachloride extracts the complexes of Bi, Fe, Cu, Ni, Co, Pb; Te; As, Se; and Mn over the pH ranges 4-11; 4-9; 4-6; and 6-9, respectively. Complexes of many metals can be extracted into chloroform from fairly strongly acid solutions, e.g., from 0.1 M HC1. The diethyldithiocarbamates of Nb, Ru, Rh, Os, Ir and Pt are rather poorly extractable [99,100].

Sodium diethyldithiocarbamate decomposes in acid solution, forming diethylamine and carbon disulphide:

(CzHs)zN.CS.SNa + H + ~ (CzH5)zNH + C82 -k- Na +

Hence Na-DDTC solutions are stored in dilute alkali ( p H - 9).

Diethylammonium diethyldithiocarbamate (formula 4.41), which dissolves in CHC13 and is stable in acid solution, is a more convenient reagent than Na-DDTC. In this case the reagent is not added to the aqueous medium, but the analysed solution is shaken with a dilute (e.g., 0.05%) solution of the reagent in CHC13. The acidity of the solutions being analysed may be higher than in the case of Na-DDTC [101,102].

The stability of diethyldithiocarbamate complexes with metals decreases in the following order:

Hg, Ag, Co, Cu, Ni, Bi, Pb, Cd, Fe(III), Zn, Mn

By taking advantage of the different stabilities of individual metal dithiocarbamates, it is possible to use chloroform solutions of the relatively less-stable metal dithiocarbamates for the extraction of the metals which give more stable complexes [ 103-105]. In determining copper, for example, the colourless chloroform solution of lead diethyldithiocarbamate, Pb(DDTC)2, is used as the reagent. In this way the selectivity of metal reactions with dithiocarbamates may be enhanced.

Other dithiocarbamates which have been applied as spectrophotometric reagents are: dibenzyl dithiocarbamate, pyrazoline dithiocarbamate, glycine dithiocarbamate, and pyrrolidine dithiocarbamate [ 106].

4.7. 8-Hydroxyquinoline

8-Hydroxyquinoline (HOx, oxine, formula 4.42) is amphoteric. It dissolves in alkaline solutions as the oxinate ion and in acid solutions as the oxinium cation. Oxine is soluble in CHC13, C6H6, CC14, ethanol, acetone, and other organic solvents [107,108]. The distribution of oxine between CHC13 and water depends on pH (Fig. 4.5).


BI" !

OH SH

(4.42) (4.43) (4.44)

With certain metal ions oxine forms chelates, which are in most cases extractable into CHC13 and other solvents. Metals form complexes with oxine in an M:Ox ratio, which depends on the charge on the metal ion.

~ ' 0 0 |, i

3 5 7 9 11 - pH

Fig. 4.5. Distribution of 8-hydroxyquinoline between chloroform and water in relation to pH

In chloroform solution, 8-hydroxyquinoline has an absorption maximum at 315 nm. Solutions of all the metal oxinates are coloured, yellow predominating. Extractive spectrophotometric methods for the determination of metals with oxine are moderately sensitive (cf, the determination of A1, Ce, and V). The molar absorptivities of oxinates do not in general exceed 1-10 4.

8-Hydroxyquinoline is a group reagent often applied to the precipitation or extraction of a large number of metals. The selectivity of metal reactions with oxine may be enhanced by masking agents such as EDTA, tartrate, oxalate, or cyanide.

In acid medium, and with some metal ions [e.g., Fe(III), Mo(V), Cr(IH)] oxine forms coloured water-soluble cationic complexes which can be used for the spectrophotometric determination of these metals.

Oxygen-containing organic solvents extract the charged forms of the reagent as ion- pairs. Some metals (Ni, Zn, U) form anionic oxine complexes, which form with basic dyes ion-associates, extractable into benzene [109].

Derivatives of 8-hydroxyquinoline are also used in spectrophotometric analysis. Chloro-oxine (5,7-dichloro-8-hydroxyquinoline) and bromo-oxine (5,7-dibromo-8- hydroxyquinoline, formula 4.43) react similarly to oxine. The absorption maxima of the complexes in chloroform solutions are shifted towards longer wavelengths and the sensitivity of the reactions is higher than in the case of oxine.

8-Hydroxyquinoline derivatives containing sulphonic acid groups, e.g., ferron [a reagent for iron(IN), give water-soluble complexes with metal ions.

8-Mereaptoquinoline (thio-oxine, formula 4.44), the sulphur analogue of 8- hydroxyquinoline, forms sparingly soluble chelates with metal ions (Fe, Cu, Mn, Cd, Co,

4.8. Other organic reagents 69

Mo, W, and Ni) [110,111]. Thio-oxine compounds are coloured and extractable into chloroform. The molar absorptivities range from 5-103 to 1-104. 8-Mercaptoquinolates are extracted from more acidic solutions than are oxinates, but are susceptible to oxidation.

4.8. Other organic reagents

Hydroxylamine derivatives are used in determination of certain metals [112,113]. For example, the substitution of a benzoyl group (C6H5CO) for one of hydrogen atoms in hydroxylamine gives benzohydroxamic acid. Other important reagents in this group are N- benzoyl-N-phenylhydroxylamine (BPHA), also called N-phenylbenzohydroxamic acid [114,115] and N-furoylphenylhydroxylamine. All these reagents form extractable chelates and are used for determination of metals, e.g., V(V), Mn, Ti, Fe(III), and Nb.

Hydrazones are characterized by the presence of the triatomic grouping =C=N-N=. There are many reagents in this class, and they are used for spectrophotometric determination of such metals as Ni, Co, Zn, Cd, Cu [116-119]. Examples of these reagents are pyridine-2-aldehyde-2-quinolylhydrazone (PAQH) and cuprizone (formula 19.5).

Dioximes are a group of reagents used in determination of Ni, Pd, Re, and Cu. The group is represented by dimethylglyoxime (formula 32.1) and ~-furyldioxime (formula 32.2). Their analytical applications have been reviewed [120,121]. Formaldoxime is a simple reagent [122] highly selective for manganese (see Section 29.2.2).

13-Diketones are a group of commonly applied spectrophotometric reagents, including acetylacetone (formula 1.6), thienoyltrifluoroacetone (formula 1.7), and dibenzoylmethane (formula 54.1) used as a reagent for uranium. The reagents of this group are discussed in a monograph [123]. Fluorinated ~-diketones are applied in the determination of lanthanides [124].

Thio-Miehler's ketone (formula 46.2) is a very useful reagent allowing sensitive and selective spectrophotometric determination of a limited group of metals (Ag, Au, Pd, Pt, Hg, Cu) [125,126]. Ref. 124 is not cited in the MS. Formula 4.41 is not shown in the MS.

Maeroeyeles (crown ethers, cryptands) with chromogenic groups combine the natural selectivity of macrocycles with the possibility of direct spectrophotometric determination of some metals (e.g., K, Ca) in an organic phase after extraction [127-129]. 4- Picrylaminobenzo-15-crown-5 crown ether (formula 4.41) is applied in the extraction and spectrophotometric determination of potassium. The determinations are based on extractable ion-associates of metals (e.g., Li, Na, K, Pb) with crown ethers and xanthene or sulphophthalein dyes [ 130].

r•02 r,~o._.. ~ (4.45)


4.9. Inorganic reagents

4.9.1. Thiocyanate

Thiocyanate is one of the most important spectrophotometric reagents. The availability of the reagent and the simplicity of thiocyanate methods are responsible for its great popularity in analytical laboratories [131,132]. Thiocyanate is principally used for determination of Fe(III), Mo, W, Nb, Re, Co, U, and Ti.

The determination of metals by thiocyanate is carried out in aqueous or aqueous-acetone media, or after extraction with oxygen-containing solvents. The extractability of metal complexes depends on the acidity of the medium, the concentration of thiocyanate, and the organic solvent. The more acidic is the aqueous phase, and the higher the thiocyanate concentration, the more thiocyanic acid (HSCN) is also extracted by the organic phase.

Stepwise formation of thiocyanate complexes gives cationic (e.g., FeSCN2+), neutral [e.g., Fe(SCN)3], and anionic [e.g., Fe(SCN)4-] species. The last is formed at high thiocyanate concentrations. With organic bases such as pyridine, tributylamine, and diantipyrylmethane, anionic thiocyanate complexes form ion-pairs which can be extracted into chloroform and other inert solvents.

Increased selectivity in the determination of metals by thiocyanate is obtained by the choice of acidity, thiocyanate concentration, masking agent, and metal oxidation state. For example, the presence of a reducing agent is necessary for colour reactions with Mo, W, and Re. The reducing medium precludes the colour reaction of thiocyanate with iron.

Thiocyanate methods vary widely in sensitivity. The methods for determining Te, Fe(III), and Nb are highly sensitive, whereas those for U and Co are less sensitive.

The colour stability of some thiocyanate systems is low (e.g., that with iron). This is connected with either the reducing properties of the thiocyanate or the slow polymerization of thiocyanic acid in acid solutions, which causes yellowing. Solvents miscible with water increase the colour intensity of thiocyanate complexes in aqueous solutions. This is apparently owing to the lowered dielectric constants of the media, which inhibit dissociation of the complexes.

Anionic thiocyanate complexes are extractable as ion-association species with basic dyes (see Section 4.4.1).

4.9.2. Other inorganic reagents

In the spectrophotometric determination of Si, Ge, P(V), As(V), and V(V) the yellow heteropoly adds occurring in acid solutions in the presence of an excess of molybdate or tungstate are important. The yellow heteropoly acids are the basis of less sensitive spectrophotometric methods, but the blue reduction products (e.g., phosphomolybdenum blue) are the basis of very sensitive spectrophotometric methods for determining these elements. The conditions for formation and extraction of these compounds have been investigated [ 133-135].

Other inorganic reagents utilized in spectrophotometric methods are: hydrogen peroxide (see determination of Ti, V, U, and Ce), SnCI3 ion (determination of platinum group elements [136]), iodide (see determination of Sb, Bi, and Pd), bromide (determination of Au), and also chloride and azide.

4. Spectrophotometric reagents 71

References

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(1974). 71. Poluektov N.S., Drobyazko V.N., Meshkova S.B., Zh. Anal. Khim., 28, 1408 (1973). 72. Rochat J., Rerat J.C., Alary J., Coeur A., Analusis, 4, 267 (1976). 73. Blum I.A., Brushtejn N.A., Zh. Anal. Khim., 31, 1669 (1976). 74. Zorov N.B. et al., Zh. Anal. Khim., 26, 1466 (1971). 75. Dodin E.I., Kharlamov I.P., Zh. Anal. Khim., 37, 14 (1982). 76. Chelnokova M.N., Zh. Anal. Khim., 37, 1008 (1982). 77. Zhivopistsev V.P., Zh. Anal. Khim., 50, 714 (1995). 78. Kish P.P., Pogoyda I.I., Zh. Anal. Khim., 34, 1687 (1979). 79. Matveets M.A., Shcherbov D.P., Akhmetova S.D., Zh. Anal. Khim., 34, 1049 (1979). 80. Fompeydie D., Levillain P., Talanta, 31, 1125 (1984). 81. Skorobogatov V.M., Gershuns A.L., Adamovich L.P., Zh. Anal. Khim., 32, 1450 (1977). 82. Tananayko M.M., Gorenshtein L.I., Zh. Anal. Khim., 37, 589 (1982). 83. Iwantscheff G., Das Dithizon und seine Auwendung in der Mikro- und Spurenanalyse,

2nd Ed., Verlag Chemie, Weinheim 1972. 84. Irving N.M., Dithizone, Royal Society of Chemistry, London 1977. 85. Irving N.M., Crit. Rev. Anal. Chem., 8, 321 (1980). 86. Wagler H., Koch H., Talanta, 31, 1101 (1984). 87. Freiser B.S., Freiser H., Anal. Chem., 42, 305 (1970). 88. Brand P., Freiser H., Anal. Chem., 46, 1147 (1974).

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Part II

Determination of elements

Introductory remarks

Part II, devoted to individual chemical elements, describes the methods for separation, preconcentration, and spectrophotometric determination of the elements, as well as the application of these methods in the analysis of materials. The detailed discussion comprises the theory, preparation of standard solutions, and analytical procedures.

The analytical procedures are presented in a unified form. The maximum quantities given for a corresponding analyte refer to a 25 ml standard flask and a cuvette of thickness 1 cm. The quantities of reagents should be changed if the coloured solutions are placed in a flask of capacity 10 ml or 50 ml, and a cuvette of other thickness is applied.

In the descriptions of preparations of reagents and in analytical procedures no mention is given of the purity of reagents. The assumption is made that the analyst will use reagents of purity appropriate for the given analysis.

Procedures are given for the preparation of stock standard solutions of elements of concentration 1 mg/ml. If no other statement is given, the working solutions (of concentration 100, 10, or 1 ~tg/ml) are prepared by appropriate dilution of the stock solutions.

Series of solutions for obtaining standard curves are prepared conforming to the analytical procedure, but suitable volumes of the working standard solutions are used instead of the sample solution.

The sensitivity of the spectrophotometric methods is determined in terms of the molar absorption coefficient (molar absorptivity, coefficient e), e.g., e = 4.86104 1 mole -1 cm -~ and specific absorptivity (a = e/m.mole'1000 [ml'~tg-l"cm-~]. For space-saving reasons the units of ~ and a have been omitted from the text.

The formulae of organic reagents have been given in their acid- or ionic forms, although they are supplied commercially as salts.

The letter M stands in the book for "mole/l" or "molar".

Chapter 5. Alkali metals

Lithium (Li, at. mass 6.94), sodium (Na, at. mass 22.99), potassium (K, at. mass 39.10), rubidium (Rb, at. mass 85.47), and caesium (Cs, at. mass 132.91) form colourless M + ions, readily soluble in water, and strong bases MOH. Potassium, rubidium, and caesium are very similar in chemical properties. Sodium has somewhat different properties. The properties of lithium are intermediate between those of sodium and calcium. Lithium and sodium have the ability to form weak complexes (e.g., with pyrophosphate and EDTA).

5.1. Isolation and separation of the alkali metals

Owing to the good water-solubility of the alkali metal salts, the simplest way to isolate them from the Analytical Group I-IV metals is by leaching a carefully powdered sample with very dilute acid (e.g., HC1) or dilute alkali (e.g., ammonia), depending on the nature of the sample under examination.

Multivalent metals can be separated from the alkali metals by solvent extraction or precipitation methods. Cation exchangers and electrolysis with a mercury cathode are useful in separating the metals of other groups from the alkali metals.

The alkali metals can be separated by cation-exchange chromatography using strongly acidic cation-exchangers [ 1-7]. The isolation of small amounts of lithium, and of caesium and rubidium, is described in references [8] and [9,10], respectively. The separation of Na, K, Ca, and Mg was also investigated [11 ].

In the separation of caesium, the following inorganic cation exchangers are particularly useful: potassium cobalt(H) hexacyanoferrate(II) [12,13], potassium copper(II) and nickel(II) hexacyanoferrates(II) [ 14], ammonium molybdophosphate [ 13,15], zirconium molybdoarsenate [16], thallium tungstophosphate [17], and tin molybdosilicate [18]. Rubidium has been selectively isolated on columns filled with titanium tungstoarsenate [ 19] or titanium ferricyanide [20]. Ammonium molybdoarsenate has been used to separate potassium from sodium [21 ].

Maeroeyelie compounds (some crown ethers and cryptands) are selective reagents for extractive separation of alkali metals [22-27]. These ligands form cationic complexes with alkali metal ions, and these can be extracted as ion-pairs with suitable counter-ions (e.g., picrate) [28], most often into chloroform. For potassium, p-nitrophenoxide was used as counter-ion [29]. In cases, where a coloured anionic complex is a counter-ion [30], the extract may serve as a basis for determining the alkali metal. The effect of the structure of the dibenzo-crown ether rings upon the selectivity and effectiveness of isolation of alkali metals has been studied in detail [31]. Chromogenic macrocyclic reagents applied for the isolation and separation of alkali metals have been discussed [32].

The effect of the presence of crown ethers in the eluents [7,33], or in the stationary phases [34-37] on the effectiveness of separation of alkali metals on cation exchangers has been investigated. Higher selectivity of Na separation was observed on silica resins impregnated with calixarene [38].

The alkali-metal tetraphenylborates have been separated by virtue of their different distribution coefficients when extracted with nitrobenzene [39]. The extraction of alkali

78 5. Alkali metals

metals with pyrazolone derivatives has been studied [40-42]. Lithium can be separated from sodium by extraction with a cyclohexane solution of 1-tolyl-3-methyl-4-perfluoroacyl-5- pyrazolone and TOPO [40]. Rubidium and caesium have been separated with the use of 4- butyl-2-(a-methylbenzyl)phenol and mixture of n-octane with CC14 [43].

Dioxan leaches only lithium chloride from a mixture of dried alkali metal chlorides [44].

5.2. Determination of lithium

Lithium is the only alkali metal, which gives colour reactions with some azo dyes. The first reagent used in the direct determination of lithium was Thoron I (formula 4.9) [45,46]. The reaction is performed in aqueous or acetone-water media (e-~ 6.103 at 486 nm). A fifty-fold amount of sodium and a tenfold amount of magnesium do not interfere. The solutions are made alkaline with KOH.

Nitroanthranylazo (formula 5.1) has been proposed for the determination of Li [47]. The colour reaction is carried out in aqueous acetone made alkaline with KOH. The absorbance is measured at 530 nm ( e - 1.2.104). The colour intensity depends on the medium used (water-acetone, dioxan, methanol, DMF), excess of Nitroanthranylazo, and ionic strength. In the determination of 2 pg of lithium, there is no interference of 200 pg Rb, 100 pg Mg or Ca, 50 pg Ba, Sr, or Na, The method has been applied for determination of lithium in ferrites [48].

O 2 N , ~ , , ~ ~ C O O H OH b

CH3

(5.1)

Formazan reagents are highly selective to lithium. The use of formazan enables the determination of Li in the presence of a 5,000-fold excess of Na, which is of importance in the determination of Li in blood [49,50]. Lithium was determined also with another reagent from this group, (e = 2.0-104) [51]. Quinizarine was used for determination of Li in drugs and in blood serum [52].

Macrocyclic reagents, such as chromogenic crown ethers of the type 14-crown-4 (extraction to 1,2-dichloroethane in the presence of picrate) were used for determination of Li [53]. The 14-crown-4 type derivatives have been applied for determination of Li in blood by a continuous FIA method [54,55]. The use of chromogenic reagents aza-12 (-13 or -14)- crown-4 has also been proposed [56,57]. The effect of substituents on the selectivity of separation of Li (and Na) by means of benzo-14-crown-4 and 13-crown-4 ethers was studied [58]. A review of chromogenic macrocyclic reagents used for determination of lithium (and other alkali metals) has been published [32].

5.3. Determination of sodium

Sodium is separated from other alkali metals by precipitation from acetic acid medium as the triple acetate, NaZn(UOz)3(CH3COO)9-9H20 [59]. When the potassium content exceeds the sodium content by greater than one hundred-fold, it is partly co-precipitated with the sodium. In this case, double precipitation is advisable. The precipitated triple acetate is then

5.4. Determination of potassium 79

dissolved, and sodium is determined indirectly by the spectrophotometric determination of zinc with dithizone.

Sodium can be determined directly by selective methods involving chromogenic macrocyclic reagents, e.g., crown ethers of the type 15-crown-5 (e= 1.4-104 at 422 nm) [60], of the type 12-crown-4 (with picrate counter-ion, in 1,2-dichloroethane, e = 1.8.104 at 375 nm) [61], 18-crown-6 [62,63], crown ether with azo group [64], cryptand-(2,1,1) (picrate, toluene, 350-fold excess of K does not interfere) [65], and (2.2.2)-cryptand [63]. The crown ether, benzo-18-crown-6, has been applied in the FIA technique [62]. These reagents have been used for the determination of Na in blood. The co-extraction of Cs and Na was studied with the use of various crown ethers [66]. A review of the reagents has been published [32].

Sodium has been extracted (CHC13) in the form of mixed complexes with 15-crown-5 ether or with Bromothymol Blue [67]. The FIA procedure was applied for the determination of Na (and K) with the use of 18-crown-6 and tetrabromophenolphthalein [68].

5.4. Determination of potassium

Analytical methods based on the use of crown-ethers are highly selective. It is possible to use either chromogenic crown-ethers [32,69-72] (e.g., formula 5.2), or crown-ethers without chromogenic groups [62,73-77], e.g., dibenzo-18-crown-6 (formula 1.15) in conjunction with anionic chromophoric reagents as counter-ions, e.g., Bromocresol Green [73], or Methyl Orange [74]. Potassium was determined also as an ion-pair of a complex of 18- crown-6 ether with Bromothymol Blue [78] or calmagite [79]. Methods involving the potassium complex with [2.2.2]-cryptand associated with Methyl Orange (CHC13,

= 2.2104) [80] or with a complex of Co(III) with an azo compound [81] have been proposed.

H N..~--N 0/--"~0

(5.2)

The above-mentioned methods were applied for the determination of potassium in blood serum [72,73,82], biological fluids [79], water [75,76], and cement [76]. The FIA technique was also applied [76].

Dilituric acid (5-nitrobarbituric acid) forms a sparingly soluble compound with potassium ions. The decrease in absorbance of the reagent gives a measure of the K content in the solution. Potassium ions can be precipitated by tetraphenylborate. The excess reagent forms an ion-pair with the cationic Cu(I)-neocuproine complex, and this ion-pair is extractable into methyl acetate. The absorbance of the extract is measured at 456 nm [83]. An indirect method involving the reaction of potassium tetraphenylborate with mercury(II) chloranilate is very sensitive [84].

Potassium was determined by the FIA method after precipitation with tetraphenylborate and the crown ether, 18-crown-6 [85].

80 5.Alkali metals

5.5. Determination of rubidium and caesium

Rubidium and caesium are determined, like potassium, by indirect spectrophotometric methods, with the use of dipicrylamine [86] or picric acid [87].

Caesium is separated from small amounts of rubidium and potassium as the sparingly soluble caesium tungstosilicate. The precipitate is dissolved, the tungstosilicic acid is reduced to silicotungsten blue, and the absorbance is measured at 640 nm [88]. Caesium has been also determined indirectly as phosphomolybdenum blue (absorbance measured at 805 nm) after the precipitation of, and isolation of caesium molybdophosphate [89]. Caesium was also determined indirectly by precipitation of Cs3BizI9 with subsequent determination of Bi by the iodide method [90].

Some crown ethers [70,91,92] are used for the extraction and spectrophotometric determination of Rb and Cs.

References

1. Strelow F.W., Coetzee J.H., Van Zyl C.R., Anal. Chem., 40, 196 (1968). 2. Strelow F.W., Liebenberg C.J., Toerien S.,Anal. Chim. Acta, 43, 465 (1968). 3. Huber J.F., Van Urk-Schoen A.M., Anal. Chim. Acta, 58, 395 (1972). 4. Nair L.M., Saari-Nordhaus R., Anderson J.M. Jr., J. Chromatogr., 640, 41 (1993). 5. Narbutt J., Bilewicz A., Bartog B., Chem. Anal. (Warsaw), 33, 399 (1988). 6. Dumont P.J., Fritz J.S., J. Chromatogr. A, 706, 149 (1995). 7. Steiner F., Niederlaender C., Engelhardt H., Chromatographia, 43, 117 (1996). 8. Strelow F.W., Weinert C.H., Van der Walt T.N., Anal. Chim. Acta, 71, 123 (1974). 9. Hahn R.B., Johnson J.L., McKay J.B., Talanta, 13, 1613 (1966). 10. Shestakov G.I., Zh. Anal. Khim., 26, 273 (1971). 11. Iwachido T., Shinomiya M., Zenki M., Anal. Sci., 6, 277 (1990). 12. Boni A.L., Anal. Chem., 38, 89 (1966). 13. Terada K. et al., Talanta, 17, 955 (1970). 14. Ishfaq M.M., Karim H.M., Khan M.A., J. Radioanal. Nucl. Chem., 159, 335 (1992). 15. Oldham G., Ware A.R., Sykes D.J., Talanta, 16, 430 (1969). 16. Satyanarayana J. et al., J. Radioanal. Nucl. Chem., 188, 323 (1994). 17. Caron H.L., Sugihara T.T., Anal. Chem., 34, 1082 (1962). 18. Yassine T., J. Radioanal. Nucl. Chem., 173, 387 (1993). 19. Srivastava S.K. et al., Analyst, 109, 151 (1984). 20. Saraswat I.P. et al., Anal. Sci., 2, 343 (1986). 21. Coetzee C.J., Rohwer E.F., Anal. Chim. Acta, 44, 293 (1969). 22. Kimura K., Maeda T., Shono T., Talanta, 26, 945 (1979). 23. Strzelbicki J., Bartsch R.A., Anal. Chem., 53, 1894, 2251 (1981). 24. Mamedova Yu.G. et al., Zh. Anal. Khim., 38, 1578 (1983). 25. Yakshin V.V., Fedorova A.T., Laskorin B.N., Zh. Anal. Khim., 40, 45 (1985). 26. Sakamoto H. et al., Bull. Chem. Soc. Jpn., 69, 2907 (1993). 27. Tsurubou S., Umetani S., Komatsu Y., Anal. Chim. Acta, 394, 317 (1999). 28. Olsher U., Feinberg H., Frolow F., Shoham G., Pure Appl. Chem., 68, 1195 (1996). 29. Buncel E. et al., Talanta, 31,585 (1984). 30. Szczepaniak W., Juskowiak B., Chem. Anal. (Warsaw), 32, 787 (1987). 31. Walkowiak W. et al., Anal. Chem., 62, 2018, 2022 (1990).

References 81

32. Dolman M. et al., Analyst, 121, 1775 (1996). 33. Yoshida K., Motomizu S., J. Flow Injection Anal., 7, 130 (1990). 34. Hayashita T. et al., Anal. Chem., 62, 2283 (1990). 35. Hayashita T. et al.,Anal. Chem., 64, 815 (1992). 36. Okada T., Usui T., Anal. Chem., 66, 1654 (1994). 37. Edwards B.R., Giauque A.P., Lamb J.D., J. Chromatogr., 706, 69 (1995). 38. Glennon J.D. et al.,Anal. Lett., 26, 153 (1993). 39. Sekine T., Dyrssen D., Anal. Chim. Acta, 45, 433 (1969). 40. Umetani S. et al., Talanta, 34, 779 (1987). 41. Mukai H. et al., Anal. Chim. Acta, 220, 111 (1989). 42. Bukowsky H. et al., Anal. Chim. Acta, 257, 105 (1992). 43. Rais J., Krtil J., Chotivka V., Talanta, 18, 213 (1971). 44. Blasius E., Wolf F., Z. Anal. Chem., 174, 349 (1960). 45. Apple R.F., White J.C., Talanta, 13, 43 (1966). 46. Trautman J.K., Gadzekpo V.P., Christian G.D., Talanta, 30, 587 (1983). 47. Dziomko V.M. et al., Zh. Anal. Khim., 23, 170 (1968); 24, 985 (1969). 48. Budyak N.F., Gryaznova I.S., Zh. Prikl. Khim., 44, 669 (1971). 49. Sitnikova R.V. et al., Zh. Anal. Khim., 37, 611 (1982). 50. Attiyat A.S., Ibrahim Y.A., Christian G.D., Microchem. J., 37, 114 (1988). 51. Zelichenok S.L. et al., Zh. Anal. Khim., 30, 2311 (1975). 52. Gracia L.G., Rodriguez L.C., Ceba M.R., Talanta, 44, 75 (1997). 53. Wu Y.P., Pacey G.E.,Anal. Chim. Acta, 162, 285 (1984). 54. Kimura K. et al., Anal. Sci., 4, 221 (1988). 55. Kimura K. et al.,Analyst, 115, 1251 (1990). 56. Sasaki A., Pacey G.,Anal. Chim. Acta, 174, 141 (1985). 57. Wilcox K., Pacey G.E., Talanta, 38, 1315 (1991). 58. Bartsch R. et al., Anal. Chim. Acta, 272, 285 (1993). 59. Shell H.R., Anal. Chem., 22, 575 (1950). 60. Nakamura H. et al., Anal. Chim. Acta, 139, 219 (1982). 61. Pacey G.E., Wu Y.P., Talanta, 31, 165 (1984). 62. Motomizu S., Onoda M., Anal. Chim. Acta, 214, 289 (1988). 63. Kumar A. et al., Clin. Chem., 34, 1709 (1988). 64. Katayama Y., Nakamura H., Takagi M., Anal. Sci., 1, 393 (1985). 65. Takagi M. et al., Anal. Chim. Acta, 126, 185 (1981). 66. Peimli E., J. Radioanal. Nucl. Chem., 144, 1 (1990). 67. Lamoneda C., Barragan F.J., Guiraum A., Analusis, 16, 189 (1989). 68. Motomizu S., Onoda M., Anal. Chim. Acta, 214, 289 (1988). 69. Nakamura H., Takagi M., Ueno K., Talanta, 26, 921 (1979). 70. Nakamura H., Takagi M., Ueno K., Anal. Chem., 52, 1668 (1980). 71. Pacey G.E., Wu Y.P., Bubnis B.P., Analyst, 106, 636 (1981). 72. Bubnis B.P. et al., Anal. Chim. Acta, 139, 307 (1982). 73. Sumiyoshi H., Nakahara K., Ueno K., Talanta, 24, 763 (1977). 74. Abrodo P.A. et al., Microchem J., 30, 58 (1984). 75. Shabanov A.L. et al., Zh. Anal. Khim., 39, 1621 (1984). 76. Motomizu S. et al., Analyst, 113, 743 (1988). 77. Motomizu S., Kobayashi M., Anal. Sci., 10, 187 (1994). 78. Escobar R. et al.,Analyst, 114, 533 (1989). 79. Dadfarnia S., Shamsipur M., Anal. Lett., 25, 11 (1992).

82 5. Alkali metals

80. Abrodo P.A., Gomis D.B., Medel A.S., Microchem J., 32, 296 (1985). 81. Yamada H., Kobayakawa I., Yuchi A., Wada H.,Anal. Chim. Acta, 281, 95 (1993). 82. Kumar A. et al., Clin. Chem., 34, 1709 (1988). 83. Khreish E.A., Boltz D.F., Mikrochim. Acta, 1970, 1174. 84. Huber H. et al., Mikrochim. Acta, 1982 I, 155. 85. Motomizu S., Yoshida K., T6ei K., Anal. Chim. Acta, 261, 225 (1992). 86. Kyr~ M., Rais J., Selucky P., Talanta, 16, 1169 (1969). 87. Hejtmanek H., Hozmanova E., Mikrochim. Acta, 1966, 97. 88. Gorenc B., Kosta L., Z. Anal. Chem., 206, 321 (1964). 89. Huey F., Hargis L.G., Anal. Chem., 39, 125 (1967). 90. Boguslawska K., Cieplifiski M., Cygafiski A., Chem. Anal. (Warsaw), 30, 281 (1985). 91. Mohite B.S., Khopkar S.M., Talanta, 32, 565 (1985). 92. McDowell W.J. et al., Anal. Chem., 64, 3013 (1992).

Chapter 6. Aluminium

Aluminium (A1, at. mass 26.98) occurs in solution exclusively in the HI oxidation state. The hydroxide AI(OH)3 begins to precipitate at about pH 4. Above pH 9 it is converted into the soluble tetrahydroaluminate anion. Aluminium forms stable complexes with fluoride, oxalate, tartrate, and EDTA, and weak complexes with acetate.

6.1. Methods of separation and preconcentration

6.1.1. Precipitation

By precipitation of its hydroxide, with ammonia, or with an acetate buffer at pH 4.5-5.0, A1 can be separated from metals having hydroxides precipitated at higher pH values. Traces of aluminium are separated by using Ti, La, Zr, or Fe(III) as scavenger. It is possible to precipitate AI(OH)3 with ammonia at pH --5 in the presence of Fe masked by reduction to Fe(II). In order to separate aluminium from chromium, Cr(III) can be masked by oxidation to Cr(VI). The precipitation of AI(OH)3 in the presence of H202 enables one to separate A1 from Ti and V.

Aluminium is separated as the soluble tetrahydroxo complex from Fe, Ti, and other metal ions which are insoluble in excess of NaOH [ 1]. The aluminium co-precipitated with the insoluble metal hydroxides should be recovered by dissolving the precipitate in dilute HC1, and then re-precipitating the interfering metals with excess of NaOH. Water leaches A1. (as aluminate) from a cooled melt of Na2CO3, while Fe, Ti, Zr, and other metals remain in the solid.

Small amounts of aluminium may be precipitated as the oxinate, ff fluoride is present, it is masked as the stable, soluble beryllium complex [2]. Fe(III), Ti, and Zr are separated from aluminium by precipitation with cupferron [3].

Mercury-cathode electrolysis allows small quantities of A1 to be separated from large amounts of Fe, Ni, Co, Cu, Zn, Mn, Cr, Mo, Pb, etc., while Be, V, Ti, Zr, Mg, Ca, and the rare earths remain in solution together with the aluminium [4].

6.1.2. Extraction. Ion exchange

Extraction is used mainly for the preliminary separation of macro- and microquantities of metals which interfere in the determination of aluminium. After Fe(III), Ti, Zr, and Cu cupferronates have been extracted from dilute HC1 into chloroform, aluminium cupferronate is extracted at pH --3.5 [5]. Some interfering metals are separated from A1 by extraction with trifluoroacetylacetone in CHC13 [6], and as chloride- or thiocyanate complexes in the presence of DAM [7]. Aluminium has been separated from various elements by extraction as chelates with BPHA [8], oxine [4,9], 8-hydroxyquinaldine [10], and acetylacetone [11].

Aluminium does not form a stable chloro-complex. It can be separated by passing a solution in 9 M HC1 through a strongly basic anion exchanger column. Aluminium is eluted along with Ni, Mn, Ca, Mg, Be, and Ti, whereas Fe, Cu, Zn, Co, Cd, etc., remain on the column [ 12].

84 6. Aluminium

Aluminium, gallium and indium can be retained on the anion exchanger from a solution of 2-methoxyethanol and 6 M HC1, and can then be sequentially eluted with 1 M HC1 [ 13].

Aluminium and other cations can be sorbed on cation exchangers, whereas phosphate and other interfering anions are eluted. The anionic fluoride and sulphosalicylate complexes of A1 can be separated by cation exchangers from metals that do not form the corresponding complexes.

6.2. Methods of determination

The extraction method using 8-hydroxyquinoline is not very sensitive, but it is highly selective. The really sensitive methods for spectrophotometric determination of aluminium are based on ternary systems, including triphenylmethane reagents (mainly Chrome Azurol S and Eriochrome Cyanine R) and some surfactants.

6.2.1. 8-Hydroxyquinoline method

Between pH 4.5 and 10, 8-hydroxyquinoline (oxine) forms the chelate AI(C9H6ON)3, which is sparingly soluble in water but dissolves readily in CHC13. The yellow extract of aluminium oxinate is the basis for the determination method [2,14]. Carbon tetrachloride and trichloroethylene are also used as extraction solvents. The absorption maximum of the chloroform extract is at 390 nm (e= 7.3103, a = 0.27). The absorption of oxine in CHC13 increases rapidly below 390 nm.

Although oxine is a group reagent and reacts with many metals, the use of appropriate masking agents makes the method specific for aluminium. There are many variants of the oxine method, depending on the kinds of metals that accompany aluminium in the analyte sample.

Large quantities of Fe(III) are usually extracted as the chloride, thiocyanate, cupferronate or HTTA complexes [9]. Smaller quantities of Fe may be masked with 2,2'- dipyridyl, 1,10-phenanthroline (pH 4-6) [2], or with cyanide, after reduction to Fe(II). It is also possible to extract first Fe(III) oxinate at pH 2.5-3.0 [ 15], then A1 oxinate at pH 4.5-5.0. At pH 3 Fe(III) is extracted quantitatively, while no A1 is extracted.

Hydrogen peroxide prevents oxine from reacting with Ti, V, Nb, U, and Ce. Cyanide masks Ni, Co, Cu, Zn, Cd, Ag, and Fe(II). EDTA or tartrate keeps aluminium in solution at higher pH values, at which it normally hydrolyses. The presence of EDTA does not interfere in the extraction of aluminium with oxine when the pH of the solution is above 8.

Small amounts of heavy metals can be separated from aluminium by extraction as dithiocarbamates and dithizonates. Ti and Zr can be stripped from chloroform extracts of A1, Ti, and Zr oxinates by shaking with ammoniacal solution at pH 9.2 [9]. Vanadium and titanium can be separated from A1 by extracting the cupferronates from a 1 M H2804 medium with CHC13.

Reagents

8-Hydroxyquinoline (oxine): 1% solution in chloroform. Standard aluminium solution: 1 mg/ml. Dissolve 17.5900 g of KAl(SO4)z12H20 in

water containing 5 ml of conc. H2804: dilute the solution to 1 litre with water in a standard flask. Working solution is given by suitable dilution of stock solution with -~0.01 M H2804.

EDTA (disodium salt), 5% solution.

6.2. Methods of determination 85

Procedure

To a solution containing not more than 60 lag of A1, add 2 ml of 5 % EDTA solution and 1 ml of 3 % H202 solution. After 5 min adjust the solution with ammonia to pH -- 9, add -- 0.1 g of Na2SO3 and --20 mg of KCN, and heat the solution to 70-80~ Cool the solution and readjust the pH to 9.0 +0.2 with -- 0.1 M HC1.

Transfer the solution to a separating funnel, and extract the aluminium with two portions of the chloroform-oxine solution. Wash the combined extracts with two portions of water and make the solution up to the mark with the oxine solution in a 25 ml standard flask. Measure the absorbance of the extract at 390 nm, using as reference a reagent blank solution.

Note. During the extraction and washing in the separating funnel, the liquid must not be shaken too vigorously (lest a stable emulsion be formed).

6.2.2. Chrome Azurol S method

Chrome Azurol S (CAS) (formula 4.18) reacts with aluminium ions to form (pH 4-7) a water-soluble blue complex which has been a basis for the determination of aluminium [ 16- 18].

The value of e (under the conditions specified in Procedure) is 5.2104 (a- l .9) at 545 nm. In the absence of acetate buffer, e values can be higher [16,17]. Hexamine buffer is recommended. The absorbance of the free reagent is insignificant at 545 nm. Solutions of the CAS-A1 complex do not obey Beer's law. The intensity of the colour of the complex depends on the concentration of CAS. The colour weakens with increasing concentration of acetate. Alkali-metal salts affect the colour and its stability. To compensate for this salt effect, corresponding amounts of the salts should be added to the reference calibration solutions [ 19].

Ascorbic acid, thioglycolic acid, or hydroxylamine are used to reduce Fe(III), which interferes. Copper is masked by thiosulphate. Before the determination of aluminium, Be, V, and Zr should be separated.

Reagents

Chrome Azurol S (CAS), 0 .1% solution. Standard aluminium solution: 1 mg/ml. Preparation as in Section 6.2.1. Ascorbic acid: 1% solution, freshly prepared.

Procedure

To a solution in dilute HC1, containing not more than 10 lag of A1, add 1 ml of ascorbic acid solution, and adjust the pH of the solution with ammonia to --2. After 5 min, dilute the solution with water to --15 ml, and add 2.5 ml of CAS solution and 1 ml of 20% sodium acetate solution. Adjust the pH of the solution with ammonia to 6.0 __ 0.2, dilute with water to 25 ml in a standard flask, and measure the absorbance of the solution at 545 nm, against a reagent blank as reference.

86 6. Aluminium

6.2.3. C h r o m e A z u r o l S - s u r f a c t a n t m e t h o d

A considerable increase in the sensitivity of the determination of aluminium with Chrome Azurol S is observed in the presence of cationic or non-ionic surfactants, if present in sufficient molar excess with respect to the chromogenic reagent. The presence of surfactants gives rise to a considerable bathochromic shift (Fig. 6.1).

The molar absorptivities in the presence of CTA or CP are in the range (1.1-1.3).105 (a~-4.4) at 610-640 nm, depending on the reaction conditions [20-23]. The cationic surfactant Zephiramine is also used sometimes [21,23,24]. The recommended pH values are similar to those for the binary system, i.e., 5-6.

~oo 430 500 5z.O 600 625 wavelength, nm

|

700

Fig. 6.1. Absorption spectra of Chrome Azurol S (CAS) (vs. water), (1), AI-CAS complex (vs. reagent solution) (2), and the ternary AI-CAS-CTA complex (3) (vs. reagent solution) (pH 5.3)

A number of non-ionic surfactants was used for determination of aluminium with Chrome Azurol S. They include: Dispergator BO [21], OP-10 [25], Sintanol DS-7 and Sintanol DS-10 [25-27], and others [28]. The sensitivities are similar to those found with cationic surfactants. The values of A~, can be larger than 200 nm [25].

A number of metals [including Be, Ga, In, Sc, Fe(III), U, V, and Zr] interfere with the determination of A1. Anions that form complexes with A1 (e.g. , fluoride, phosphate, citrate) prevent the formation of the ternary aluminium complex. Before the determination, A1 should be separated from interfering elements, or they should be masked.

Reagents

Chrome Azurol S (CAS), l10 -3 M (---0.05%) solution. Cetyltrimethylammonium bromide (or chloride) (CTA), 110 .2 M (--0.3%) solution. Standard aluminium solution. Preparation as in Section 6.2.1.

Procedure

Place a sample solution in dilute HC1 (pH 1-2, 10-15 ml), containing not more than 5 gg A1, in a small beaker and add --10 mg of ascorbic acid. After 5 min add --2 ml of CAS solution, and adjust the pH to 5.3_+0.2 with dilute ammonia solution. Dilute to volume in a 25 ml standard flask, mix, and measure the absorbance of the solution at 620 nm vs. a reagent blank solution.


6.2.4. Other methods

Eriochrome Cyanine R (ECR) (formula 4.17), triphenylmethane reagent, is often used for determining aluminium, like the CAS presented above [29,30]. The optimum pH for the colour reaction lies within the rather narrow limits of 6.0-6.2. The absorptivity of the complex drops rapidly at higher or lower pH values [29]. The molar absorptivity of the complex is -~6.5.104 at 535 nm. Interfering Fe(III) can be reduced with ascorbic or thioglycolic acid. Also many other elements interfere [30].

The sensitivity of determining A1 increases markedly in the methods involving ECR and cationic surfactants (Zephiramine, CTA, CP) at pH 6.0-7.5, where ~ attains the values within (1.15-1.25) -105 at -~600 nm [31,32].

Pyroeateehol Violet (formula 4.16) is a popular reagent for determining aluminium [11,31,33-36]. The value of ~ is 6.3.104 at 580 nm (pH 6.5-7.2) [35]. A relatively small increase in sensitivity is observed in ternary systems with CTA, CP, or poly(vinylbenzyltriphenylphosphonium) chloride [37-39]. The ternary complex with Zephiramine can be extracted into CHCI3 or 1,2-dichloroethane (e ~- 9-104 at 590 nm) [40]. The extractive (xylene) method with the use of tridodecylethylammonium bromide is of very high sensitivity (~ = 1.7.105 at 613 nm [41]. Aluminium was also determined by the FIA technique using Pyrocatechol Violet [42].

Aluminon (ammonium aurintricarboxylate) (formula 6.1) was formerly an important reagent for aluminium (~-~ 2.104) [4,31,43]. It forms a sparingly soluble red chelate with A1 ions in acetate buffer. Protective colloids (e.g., gelatine) are necessary to stabilize the pseudo-solution.

COOH HO ;#~IOOH

1-100C OH

/ c!

(6.1) (6.2)

Other triphenylmethane reagents include: Xylenol Orange [5,44-47], Methylthymol Blue (~; = 1.9.104) [48,49], Sulphochrome with cationic surfactant CP (~ = 1.0.105) [50], Chromoxane Blue B in the presence of CTA (~; = 1.2.105 at 645 nm) [51], and Chromal Blue G in the presence of CTA (~; = 1.6-105 at 660 nm) [52].

A number of azo reagents has been suggested for determination of aluminium, i.e., Stilbazo (~; = 7.4.104 at 570 nm) [53], Chromazol KS (~ = 3.5.104) [54-56] (in the presence of surfactant CP, ~; = 1.2.10s), Chlorophosphonazo I [57], Lumogallion (formula 6.2) ( e - 5.0.104 at 500 nm) [58], and Sulphonitrophenol S (propanol-acetic acid medium) [59].

88 6. Aluminium

Methods of determining aluminium with the use of Chrome Azurol S, Eriochrome Cyanine R, and Pyrocatechol Violet have been compared [60,61 ].

Among the 2,3,7-trihydroxyfluorones used as reagents for determination of A1 the best was 5-bromosalicylfluorone (t; = 7.5-104 at 540 nm) (formula 6.3) [62-64]. In the determination of A1 with phenylfluorone in the presence of CP (pH 5-6) a value of ~ = 1.14.105 at 560 nm was obtained [65].

Other organic spectrophotometric reagents used for the determination of A1. were: Alizarin S (formula 57.1) [66,67], ferron [68], salicyloylhydrazones [69], and haematoxyline in the presence of CTA [70,71]. The use of Bromopyrogallol Red in the presence of diphenylguanidine [72] and some cationic surfactants [73,74] has been proposed.

An extractable (toluene) ion-associate of the aluminium-pyrocatechol complex and Brilliant Green is the basis of a very sensitive method (t; = 2.4.105) [75].

~ 0 o o H

Bi

(6.3)

6.3. Analytical applications

The 8-hydroxyquinoline method was applied for determination of aluminium in: plant materials [15,76,77], soil extracts [76], silicate rocks and minerals [2], cast iron and steel [1,8,9,14], nickel- and copper alloys [1], chromium [78], beryllium [79], metallurgy products [80], titanium concentrates [7], and phosphates [81].

The Chrome Azurol S method was used to determine aluminium in: water [82], steel [83,84], uranium alloys [85], iron ores [86], and magnetic alloys [87]. Higher contents of aluminium (--10%) in magnesium and titanium compounds were determined by the differential spectrophotometry techniques [88]

The methods involving Chrome Azurol S and surfactants were used for determining aluminium in: water [27,89-93], steel [94,95], copper alloys [26], magnesium alloys [21], chromium alloys [96], and titanium [3]. Trace amounts of aluminium were determined in tap water by means of CAS and CP (pH 5.7; 30% ethanol) using the flow-injection technique (FIA) [91 ].

The method with ECR was applied for the determination of aluminium in: biological materials [12,97], natural waters [31,98], plants and soils [99,100], steel samples [29], lead- and tin alloys [101], and silicon tetrachloride [102]. The FIA technique was used for determination of aluminium, in the presence of a surfactant, in natural waters [32].

Aluminium was determined, with the use of Pyrocatechol Violet, in: water and soil [103,104], minerals [33], silica [105], steel and copper alloys [34], molybdenum and tungsten [11]. The FIA technique was applied for determining A1 in natural waters [ 106,107].

References 89

Determinations of aluminium with the use of various triphenylmethane reagents were carried out in: natural waters [47,108], soil extracts [44,109], silicate rocks [46,50], minerals [49], titanium and its compounds [5,48], copper alloys [45], glass and slags [110].

References

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90 6. Aluminium

44. Pritchard D.T., Analyst, 92, 103 (1967). 45. Mochizuki T., Kuroda R., Z. Anal. Chem., 311, 11 (1982). 46. Mochizuki T., Toda Y., Kuroda R., Talanta, 29, 659 (1982). 47. Edwards A.C., Cresser M.S., Talanta, 30, 702 (1983). 48. Tikhonov V.N., Zh. Anal. Khim., 21, 275 (1966). 49. Zolotukhina N.M., Erenpreys T.M., Zavod. Lab., 45, 297 (1979). 50. Chernova P.K., Sukhova L.K., Amelin V.G., Zh. Anal. Khim., 33, 1934 (1978). 51. Martire C., Hainberger L., Mikrochim. Acta, 1985 II, 223. 52. Miyawaki M., Uesugi K., Mikrochim. Acta, 1985 III, 319. 53. Wetlesen C.U.,Anal. Chim. Acta, 26, 191 (1962). 54. Basargin N.N., Yakovlev L.Ya., Morozova O.N., Zavod. Lab., 40, 1322 (1974). 55. Stolyarov K.L., Smirnova G.M., Basargin N.N., Zavod. Lab., 49, No 1, 10 (1983). 56. Shao-pu L., Analyst, 107, 428 (1982). 57. Ying-quan Z., Liu Z., Jun-yi L., Talanta, 30, 291 (1983). 58. Pyatnitskii I.V., Boryak A.K., Kolomiets L.L., Zh. Anal. Khim., 51, 2199 (1986). 59. Savvin S.B., Petrova T.V., Mongush K.D., Zh. Anal. Khim., 33, 1552 (1978); 35, 54

(1980). 60. Hawke D.J., Powell H.K.,Anal. Chim. Acta, 299, 257 (1994). 61. Hawke D.J., Powell H.K., Simpson S.L.,Anal. Chim. Acta, 319, 305 (1996). 62. Nazarenko V.A., Kostenko I.G., Biriuk E.A., Zh. Anal. Khim., 34, 1942 (1979). 63. Biriuk E.A., Kostenko I.G., Zavod. Lab., 46, 293 (1980). 64. Biriuk E.A., Kostenko I.G., Pshetakovskaya N.A., Zavod. Lab., 49, No 7, 20 (1983). 65. Albota_.A., Guculak R.B., Albota N.K., Ukr. Khim. Zh., 51, 1290 (1985). 66. King H.G., Pruden G.,Analyst, 93, 601 (1968). 67. Hernadez-Mendez J. et al.,Anal. Chim. Acta, 149, 379 (1983). 68. Goto K. et al., Talanta, 21, 183 (1974). 69. Gallego M., Valcarcel M., Garcia-Vargas M., Analyst, 108, 92 (1983). 70. Zaki M.T., E1-Didamony A.M.,Analyst, 113, 577 (1988). 71. E1-Sayed A.Y., Fresenius'J. Anal. Chem., 355, 29 (1996). 72. Rudzitis J. et al., Chem. Anal. (Warsaw), 26, 1045 (1981). 73. Wyganowski C., Kolczyfiska M., Microchem. J., 27, 43 (1982). 74. Wyganowski C., Motomizu S., T6ei K., Mikrochim. Acta, 1983 I, 55. 75. Biriuk E.A., Vinarova L.I., Ryavitskaya R.V., Ukr. Khim. Zh., 47, 760 (1981). 76. Paul J., Mikrochim. Acta, 1966, 1075. 77. Frink C.R., Peaslee D.E.,Analyst, 93, 469 (1968). 78. Tumanov A.A., Petukhova V.G., Zavod. Lab., 35, 654 (1969). 79. Pollock E.N., Zopatti L.P., Anal. Chim. Acta, 28, 68 (1963). 80. Angermann W. et al., Chem. Anal. (Warsaw), 16, 261 (1971). 81. Costadinnova L., Nedeltcheva T., Anal. Lab., 5, 46 (1996). 82. Molina-Diaz A., Herrador-Mariscal J.M., Pascual-Reguera M.I., Talanta, 40, 1059

(1993). 83. Kharlamov I.L., Erenina G.V., Borcheva T.A., Zavod. Lab., 38, 8 (1972). 84. Popov V.A. et al., Zavod. Lab., 56, No 2, 103 (1990). 85. Mal'tseva L.S., Kubareva L.V., Zavod. Lab., 35, 1299 (1969). 86. Bhargava O.P., Hines W.G.,Anal. Chem., 40, 413 (1968). 87. Babenko A.S., Volodchenko T.T., Zavod. Lab., 35, 650 (1969). 88. Tikhonov V.N., Zh. Anal. Khim., 19, 1204 (1964). 89. Sampson B., Fleck A., Analyst, 109, 369 (1984). 90. Ohzeki K., Uno T., Nukatsuka I., Ishida R., Analyst, 113, 1545 (1988).

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Chapter 7. Antimony

Antimony (Sb, at. mass 121.75) occurs in compounds in the oxidation states -III (stibine SbH3), III and V. The compounds of Sb(III) are the most stable. The Sb 3+ ions hydrolyse at pH values as low as -~1. The hydroxide Sb(OH)3 dissolves at pH--10 to form the antimonite ion, SbO2-. Antimony(V) is more acidic than Sb(III). Antimony (III and V) form sulphide-, halide-, tartrate-, and oxalate complexes.


7.1.1. Extract ion

The antimony(V) chloride complex can be extracted into DIPE from 1-10 M HC1 [1]; about 99% extraction is obtained in 5-7 M HC1. It is convenient to extract Sb(V) from 2M HC1 (94% extraction) since Fe(III) is not extracted under such conditions. Iron(III) can be separated by extraction from 6-7 M HC1 before the Sb(III) is oxidised to Sb(V). The chloride complex of Sb(V) can be extracted with non-polar solvents in the presence of amines, e.g. tribenzylamine or tridodecylamine. The antimony(III) can be extracted with 20% TBP from 2M HC1 and 1 M MgC12 [3].

The iodide SbI3 is extracted by benzene from 5M H2SO4 and 0.005-0.05 M KI [4-6]. Antimony(III) is then re-extracted with 0.5 M H2SO4. Sb(III) was also extracted with benzene from acid (H2SO4) iodide solution [7], and with p-toluidine in the presence of bromide [4,6,8].

Extraction of the Sb(III) dithiocarbamate complexes from acid media allows antimony to be separated from many metals [9-11]. A preliminary extraction of Bi and Cu dithiocarbamates can be carried out after oxidation of Sb(III) to Sb(V).

Antimony was also extracted as the complex with cupferron [ 12] or BPHA [ 13]. Sb(III) and As(HI) can be extracted with bis(2-ethylhexyl)dithiophosphoric acid (0.1 M) in heptane from 2-4 M HC1 in the presence of iodide. Only Sb(III) is re-extracted with 12 M HC1 [ 14]. The reagent has been applied for isolation of Sb in partition chromatography [15]. Also diethyldithiophosphoric acid has been recommended for the separation of Sb(III) and As(III) [16]. In the presence of Bi and Sn, antimony was determined after extraction (CHC13) with cyclic thiourea derivatives [17]. Sb(lJI) was also extracted with the use of the crown ether 18-crown-6 [ 18].


Trace amounts of antimony are separated from an acid medium (HNO3, H2504) as hydrated antimonic acid with MnO2 aq. as the collector [ 19-22]. The latter is formed in situ by the slow reaction of a hot solution of Mn 2+ with MnO4-. The MnO2 aq. collector can also be formed by the reaction of MnO4- with ethanol [21]. Although Sb is quantitatively precipitated within the pH range 1-7, the reaction is usually carried out at pH -~1 to prevent co-precipitation of other metals such as Bi, As, Au, Fe(III), TI(III), Pb, and Cu. Tin is precipitated with Sb. Fluoride interferes in the precipitation of Sb with MnO2 aq. The


precipitate of MnO2 aq. with Sb is dissolved in HC1 (1 + l) containing some H202. The filter paper can be mineralized in conc. H2804 containing some conc. HNO3.

Trace amounts of Sb can be co-precipitated with Fe(III) [23], A1 [22], and Zr [24,25]. In a flotation method, Fe(OH)3, surfactant, and air bubbling have been applied [26].

Traces of Sb are precipitated with HzS from 0.5-1 M HC1 or H2804 (Cu or Mo may be used as collectors). Tin does not precipitate in the presence of oxalic acid, while W and V can be masked by tartaric acid. Traces of antimony in tellurium have been co-precipitated with a small amount of tellurium as a carrier [27].

7.1.3. Distillation and other methods

Traces of Sb have been separated as the volatile hydride SbH3 [28,29]. Antimony is reduced in fairly concentrated HC1 with amalgamated zinc.

The volatile SbC13 is distilled from 11 M HC1. Under suitable conditions Sb is separated from As and Sn [30].

Antimony was separated from Bi on a cation exchanger in a tartrate medium [31]. Mixtures of antimony and tin were separated as malonates on the anion exchanger, Amberlite IRA-400. The chloride complex of Sb(V) is retained on the strongly basic anion- exchanger.

Trace amounts of Sb were sorbed on silica gel, saturated with a mixture of the amine Aliquat 336 and Eriochrome Black T [32]. Antimony and bismuth are sorbed at pH-1.

Sb(III) and Sb(V) can be separated by passing a solution through a column of polyurethane foam saturated with DDTK; only Sb(V) is retained [33]. Sb(III) was sorbed from an iodide medium by polyurethane foam saturated with dithizone [34].

7.2. M e t h o d s of d e t e r m i n a t i o n

In the methods of Sb determination use is made of ion-associates SbC16- with Rhodamine B and other basic dyes. The methods are highly sensitive and selective. A simple iodide method is commonly used to determine higher antimony concentrations.

7.2.1. Rhodamine B method

With Rhodamine B (formula 4.29) the antimony(V) chloride complex, SbC16-, forms a sparingly water-soluble ion pair, which is extractable into benzene or DIPE. The violet-pink solutions of the complex in these solvents are the basis of a sensitive spectrophotometric method for antimony [28,35]. Rhodamine B dissolves in acid, giving a pink solution, but is insoluble in benzene and DIPE.

The molar absorptivity of the ion associate in DIPE ~ = 9.7104 (a = 0.80) at )~max 552 nm. The benzene solution of this compound has )~max of 565 nm.

The solubility of [RB+][SbC16 -] is rather low, the concentration of Sb in the benzene phase must not exceed 2 gg/ml. The solubility in DIPE is much higher.

Ceric sulphate is most often used as oxidant since it oxidizes Sb(III) to Sb(V) quickly in 6 M HC1, even in the cold. Since the excess of Ce(IV) could oxidise RB, it is reduced with hydroxylamine, which does not reduce the Sb(V) under the conditions of the reaction. That Ce(IV) is reduced by the HC1 itself is indicated by the bleaching of the yellow solution even before the addition of NHzOHHC1. Hydroxylamine reduces, first of all, the chlorine formed, which, if left in the solution, would oxidise the Rhodamine B added later. As an alternative

94 7. Antimony

to Ce(IV), sodium nitrite can be used for oxidizing Sb(III) (the excess is reduced with urea) [39].

Metal ions forming chloride complexes which give the same reactions as Sb(V) with RB interfere in the determination. These are: Au(III), Tl(III). Ga([II), and Fe(III). Gold can be separated after reduction to the element with sulphite. Gallium and iron can be separated by extraction as chloride complexes before the oxidation of antimony to Sb(V). Small amounts of Fe(III) are masked with phosphoric acid.

A convenient method of determination consists in the extraction of HSbC16 by DIPE from 1.5-2 M HC1. The ether extract is shaken with an aqueous solution of RB to form a coloured complex in the ether. Fe(III) is not extracted by DIPE from 1.5-2 M HC1.

If Sb(V) is not first extracted into DIPE, Rhodamine B is added after the excess of the oxidising agent has been reduced and the solution is then extracted with benzene or DIPE. Alcohol can be added to clear any turbidity in the benzene extract.

Reagents

Rhodamine B (RB), 0.02% solution in 1 M HC1. Standard antimony solution: 1 mg/ml. Dissolve 2.668 g of potassium antimonyl tartrate

(SbO)KC4H406 in HC1 (1+1) and dilute the solution to 1 1 with this acid. Working solutions are obtained by suitable dilution of the stock solution with HC1 (1 + 1).

Ceric(IV) sulphate, 3 % solution in 0.5 M H2SO4. Hydroxylamine hydrochloride, 1% solution. The solution is unstable.

Procedure Separation of Sb by co-precipitation with Mn02 aq. To the chloride-free sample solution, containing 3 ml of conc. HNO3 per 100 ml of the solution, and heated almost to boiling, add 1 ml of 1% KMnO4 solution and 2 ml of 1% Mn(NO3)2 solution. Keep the solution just below boiling for 30 min. Filter off the precipitate and wash with hot dilute HNO3 (1 + 100). Dry the filter paper and the precipitate and ignite. Fuse the residue with 0.3 g of Na202 and a granule of NaOH in a small nickel crucible. Heat the melt till dark red. Leach the cooled melt with hot water, transfer all the contents of the crucible to a beaker, and acidify with 5 ml of conc. HC1. Evaporate the solution to a suitable volume.

Separation of Sb by extraction. To the sample solution containing not more than 25 gg of Sb add sufficient conc. HC1 for its concentration in the solution to be 6 M (the solution volume is 10-15 ml). Add 5 drops of the Ce(SO4)2 solution and stir well. After 5 min, add 5 drops of the NH2OH.HC1 solution and stir well. After 2 min dilute the solution with two volumes of water, transfer it to a separating funnel, and shake with two portions of DIPE for 30 sec. Wash the extract with two portions of 1 M HC1.

Determination of Sb. To the separating funnel with the ether solution of the antimony(V) chloride complex add 3 ml of the RB solution and shake. Transfer the coloured ether extract to a 25 ml standard flask, add DIPE to the mark, mix, and measure the absorbance at 552 nm using the solvent as a reference.

Note. When using benzene as a solvent proceed as follows: 2 min after the addition of NH2OH.HC1, add 3 ml of the RB solution, and dilute the solution with water so that the concentration of HC1 is 2M. Transfer the solution to the separating funnel and shake for 1 rain with 2 or 3 portions of benzene. Place the combined benzene extracts in a volumetric flask. If the extract is turbid, add 1 ml of ethanol. Dilute to the mark with benzene, mix, and measure the absorbance of the solution at 565 nm against benzene.


7.2.2. Iodide method

Antimony(III) forms a greenish-yellow complex with iodide in sulphuric acid medium. The colour intensity increases with increasing iodide concentration up to 5% KI, above which it remains constant. The concentration of sulphuric acid in the final solution should be 1.5-2.5 M. To prevent liberation of iodine by atmospheric oxygen, ascorbic acid, hypophosphite, or sulphite are added as reducing agents.

The aqueous acid solution of the complex of SbI3 exhibits 2 absorption maxima: the smaller one at 425 nm and a much higher one at 330 nm. The value of e at 425 nm is 4.0103, a =0.033.

Bismuth also forms a yellow iodide complex, and thus interferes in the determination of antimony. In a solution containing as little as 1% KI bismuth gives a colour reaction while antimony does not. By measuring the absorbances of sample solution containing low (1%) and high (5%) concentrations of KI, it is possible to determine both Bi and Sb.

Antimony can be separated from bismuth by extraction into benzene from iodide medium. From 5 M H2804 and 0.05 M KI solution, SbI3 is extracted in a good yield, whereas the bismuth complex is not extracted at all. Thallium is precipitated as TI(I) and separated by filtration [36]. Copper is masked with thiourea.

The extraction of SbI3 with benzene from 5 M H2804 medium in the presence of small excess of KI enables to determine Sb by the sensitive amplification method. From the washed benzene extract SbI3 is re-extracted with water. In this aqueous extract, the iodide is oxidised to iodate with bromine. After removal of excess of bromine, the IO3- ions react with KI to yield iodine which is determined by its colour reaction with starch (see Section 25.2.1). Thus 18 iodine atoms are formed for each Sb atom (in SbI3). The molar absorptivity

in this method is --3.0.105 (a = 2.6).

Reagents

Potassium iodide, 40% solution. Standard antimony solution: 1 mg/ml. The solution may be prepared either from

potassium antimonyl tartrate (Section 7.2.1) or in the following manner. Dissolve 0.1000 g of powdered antimony metal in 25 ml of conc. H2804. Dilute the cooled solution with water while stirring, cool, and make up to 100 ml in a standard flask. Working solutions are obtained by suitable dilution of the stock solution with 0.5 M H2804.

Ascorbic acid, 2% solution, freshly prepared.

Procedure

To the sample solution in a 25-ml volumetric flask containing not more than 0.5 mg of Sb, add 5 ml of H2804 (1 + 1), 2 ml of 2% ascorbic acid solution, and 5 ml of KI solution. Dilute the solution to the mark with water and stir well. After 5 min, measure the absorbance of the yellow solution at 425 nm, using water as a reference.


Sensitive extraction spectrophotometric methods for determining Sb have been based on the ion-pair formation between SbC16- and the basic triphenylmethane dyes [37]. Brilliant Green (E=7.0-104) [3,37-46], Crystal Violet [25,37,47-49], Methyl Violet [12,37], Methyl

96 7. Antimony

Green, and Malachite Green [37] are used. Antimony has also been determined with the use of Safranine T [50], Methylene Blue [51,52], and Toluidine Blue (13=8.1.104) [53]. Antimony was determined in a benzene extract with the use of Rhodamine 6G (xanthene dye) [7].

The anionic complex of Sb(III) with mandelic acid forms with Malachite Green an associate which is extractable into chlorobenzene from weakly acid solutions (13 = 6.1.104) [54]. Mandelic acid and other organic reagents have been applied for the joint determination of Sb(m) and Sb(V) [55,56].

Methylfluorone is recommended for the spectrophotometric determination of antimony [5,57,58]. In a weakly acidic medium (pH-~2), the orange Sb(IlI)-methylfluorone complex [which exists as a stable sol in the presence of gelatine or poly(vinyl alcohol)], is formed (13 = 4.0.104 at 530 nm). Dibromophenylfluorone allows one to determine Sb in the presence of Bi [59]. Sb was determined with the use of vanillylfluorone in the presence of poly(vinyl alcohol) [60].

A number of azo reagents have been proposed for determination of Sb, e.g., PAN (CHC13, 13 = 1.3.104) [61] and other pyridylazo reagents and their bromo-derivatives [62-66].

Other organic reagents have been used in determination of Sb, i.e., Bromopyrogallol Red (in the presence of CTA, (13=3.5.104 at 560 nm) [67], Pyrocatechol Violet [6,68,69] Ag- DDTK (with SbH3) (13=1.44.104) [70], Chrome Azurol S [71], and 2,2'-diquinoxalyl [with Sb(V)] [72].


The method based on the use of Rhodamine B has been applied for determination of antimony in sea water [21], rocks [28], arsenic [74], and titanium dioxide [35].

The iodide method was used for determination of antimony in: alloys and pharmacy products [75], steel [76], lead [1], white metals [77], and lubricating oils [78]. Stibine was determined in a gas mixture, by the iodide method, after oxidation to Sb(III) [79].

Methods involving triphenylmethane dyes were applied in determination of antimony in: air [74], water [44], silicates, plants, sewage and waste waters [25], copper-zinc and copper- nickel alloys [48], steel [50], lead [12], silver and gold [42], palladium [45], and tellurium [27]. Antimony was determined in duralumin alloys and steel with the use of Rhodamine 6G [7].

Antimony was determined in coal and steel with the use of mandelic acid [80]. Phenylfluorone derivatives were applied in determination of Sb in non-ferrous metal alloys [81]. Azobenzene and its derivatives were used in determinations of Sb in ores and bronzes [82].

References

1. Bassett J., Jones J.C., Analyst, 91, 176 (1966). 2. Alian A., Sanad W., Talanta, 14, 659 (1967). 3. Yadav A.A., Khopkar S.M., Bull. Chem. Soc. Jpn., 43, 693 (1971). 4. Grimanis A.P., Hadzistelios I.,Anal. Chim. Acta, 41, 15 (1968). 5. Koch O.G., Z. Anal. Chem., 265, 29 (1973). 6. Tsukahara I., Sakakibara M., Tanaka M., Anal. Chim. Acta, 92, 379 (1977). 7. Nalini S., Balasubramanian N., Ramakrishna T.V., Fresenius'J. Anal. Chem., 348, 769

(1994).

References 97

8. Skripchuk V.G., Zh. Anal. Khim., 43, 1803 (1988). 9. Kovacs E., Guyer H., Z. Anal. Chem., 186, 267 (1962); 208, 255 (1965). 10. Biriuk E.A., Zavod. Lab., 30, 651 (1964). 11. Mok W.M., Wai C.M., Talanta, 35, 183 (1988). 12. Cyrankowska M., Downarowicz J., Chem. Anal. (Warsaw), 10, 67 (1965); 12, 137

(1967). 13. Lyle S.J., Shendrikar A. D.,Anal. Chim. Acta, 36, 286 (1966). 14. Yukhin Yu. M., Sergeeva V.V., Litvinov V.P., Zavod. Lab., 45, 798 (1979). 15. Vin Y.Y., Khopkar S.M., Mikrochim. Acta, 107, 49 (1992). 16. Hayashi K. et al., Anal. Sci., 2, 347 (1986). 17. Presnyak I.S., Shelikhina E.I., Antonovich V.P., Nazarenko V.A., Zh. Anal. Khim., 45,

1548 (1990). 18. Vibhute R.G., Khopkar S.M., Talanta, 36, 957 (1989). 19. Ogden D., Reynolds G.F.,Analyst, 89, 538 (1964). 20. Reynolds G.F., Tyler F.S., Analyst, 89, 579 (1964). 21. Portmann J.E., Riley J.P., Anal. Chim. Acta, 35, 35 (1966). 22. Tiptsova-Yakovleva V.G. et al., Zh. Anal. Khim., 25, 686 (1970). 23. Inoue Y., Munemori M., Bull. Chem. Soc. Jpn., 53, 926 (1980). 24. Adamiec I., Marczenko Z., Chem. Anal. (Warsaw), 20, 985 (1975). 25. Abu-Hilal A.H., Riley J.P.,Anal. Chim. Acta, 131, 175 (1981). 26. Nakashima S., Bull. Chem. Soc. Jpn., 54, 291 (1981). 27. Busev A.I. et al., Zh. Anal. Khim., 20, 812 (1965). 28. Schnepfe M.M., Talanta, 20, 175 (1973). 29. Cabredo Pinillos S., Sanz Asensio J., Galban Barnal J., Anal. Chim. Acta, 300, 321

(1995). 30. Patek P., Mikrochim. Acta, 1969, 282. 31. Sulcek Z., B6seova M., Dole_al J., Coll. Czech. Chem. Comm., 34, 787 (1969). 32. Przeszlakowski S., Kocjan R., Cukrowski I., Chem. Anal. (Warsaw), 31,735 (1986). 33. Valente I., Boven H.J.,Analyst, 102, 842 (1977). 34. Raychaudhuri A., Roy S.K., Talanta, 41, 171 (1994). 35. Jablonski W.Z., Watson C.A.,Analyst, 95, 131 (1970). 36. Maurice M.J., Van Lingen R.L.,Anal. Chim. Acta, 28, 91 (1963). 37. Kish P.P. et al., Zh. Anal. Khim., 28, 1746 (1973); 29, 102 (1974). 38. Burke R.W., Menis O., Anal. Chem., 38, 1719 (1966). 39. Fogg A.G. et al., Analyst, 94, 768 (1969). 40. Kerr G.O., Gregory G.R., Analyst, 94, 1036 (1969). 41. Fogg A.G., Burgess C., Burns D.T.,Analyst, 98, 347 (1973). 42. Tayurskii V.A., Sedykh T.P., Zavod. Lab., 49, No 7, 6 (1983). 43. Burns D.T., Chimpalee D., Bullick H.J., Anal. Chim. Acta, 284, 195 (1993). 44. Sharma M., Patel K.S., Int. J. Environ. Anal. Chem., 50, 63 (1993). 45. Potapenko L.I., Babkina T.A., Zavod. Lab., 58, No 10, 4 (1992). 46. Abrutis A.A., Zh. Anal. Khim., 46, 842 (1991). 47. Sorokin G.Kh., Lomonosov S.A., Zavod. Lab., 40, 23 (1974). 48. Shevchuk I.A., Baskin V.N., Zaitsev S.N., Zavod. Lab., 57, No 1, 18 (1991). 49. Shevchuk I.A., Zhan L., Ukr. Khim. Zh., 55, 267 (1989). 50. Burgess C., Fogg A.G., Burns D.T.,Analyst, 98, 605 (1973). 51. Kish P.P., Onishchenko Yu. K., Zh. Anal. Khim., 23, 1651 (1968). 52. Bastrakova E.V., Popova T.V., Zavod. Lab., 56, No 3, 1 (1990). 53. Abrutis A.A. et al., Zh. Anal. Khim., 34, 1997 (1979).

98 7. Antimony

54. Sato S. et al.,Anal. Lett., 16, 827 (1983). 55. Sato S., Talanta, 32, 341 (1985). 56. Sato S., Uchikawa S., Anal. Sci., 2, 47 (1986). 57. Mayer S., Koch O.G., Z. Anal. Chem., 179, 175 (19,,_6!). 58. Asmus E., Brandt K., Z. Anal. Chem., 208, 189 (1965). 59. Antonovich V.P. et al., Zh. Anal. Khim., 33, 458 (1978). 60. Moil I. et al., Talanta, 38, 343 (1991). 61. Rakhmatullaev K., Tashmamatov Kh., Zh. Anal. Khim., 29, 2402 (1974). 62. Gusev S.I., Poplevina L.V., Pesis A.S., Zh. Anal. Khim., 22, 731 (1967). 63. Xing-Chu Qiu et al., Mikrochim. Acta, 1989 I, 349. 64. Qiu X., Liu G., Zhu Y., Int. J. Environ. Anal. Chem., 34, 99 (1988). 65. Shen N., Chu W., Wei F., Zhu Y., Chem. Anal. (Warsaw), 33, 527 (1988). 66. Qiu X., Zhu Y., Liu G., Mikrochim. Acta, 1989 I, 349. 67. Nemcova I. et al., Microchem. J., 30, 39 (1984). 68. Bailey B.W., Chester J.E., Dagnall R.M., West T.S., Talanta, 15, 1359 (1968). 69. Hayashi K. et al., Anal. Sci., 3, 333 (1987). 70. Hulanicki A., G__b S., Chem. Anal. (Warsaw), 15, 1089 (1970). 71. Gao H.W., Zhang P.F.,Analyst, 119, 2109 (1994). 72. Baranowska I., Chem. Anal. (Warsaw), 31, 245 (1986). 73. Luke C.L., Anal. Chem., 31, 1680 (1959). 74. De Souza T.L., Kerbyson J.D.,Anal. Chem., 40, 1146 (1968). 75. Barve A.D., Desai G.S., Shinde V.M., Bull. Chem. Soc. Jpn., 66, 1079 (1993). 76. Blazejak-Ditges D., Klingeleers H., Z. Anal. Chem., 248, 18 (1969). 77. Dym A.,Analyst, 88, 232 (1963). 78. Norwitz G., Galan M.,Anal. Chim. Acta, 61,413 (1972). 79. Gann W., Z. Anal. Chem., 221, 254 (1966). 80. Sato S., Talanta, 32, 447 (1985). 81. Novikova I.S. et al., Zavod. Lab., 57, No 12, 9 (1991). 82. Kolesnikova A.M., Lazarev A.I., Zh. Anal. Khim., 49, 385 (1994).

Chapter 8. Arsenic

Arsenic (As, at. wt. 74.92) occurs in its compounds in the oxidation states -HI (arsine, ASH3), HI (arsenite), and V (arsenate). Arsenic (HI and V) are amphoteric, but with much more acidic than basic character. The sulphides are characteristically capable of yielding soluble complexes (thio-salts). Arsenic(V) forms heteropoly acids.


8.1.1. Extraction

Arsenic(Hi) chloride can be extracted into CC14, CHC13, or C6H6 from 8-12 M HC1 [1,2]. GeC14 is co-extracted with AsC13; other elements (e.g. Se, Sb, Bi) have very low distribution coefficients. Arsenic and germanium may be separated from each other by extraction after oxidation of the former to As(V). As(Ill) is stripped from the organic solvent with dilute HC1, water, or dilute ammonia solution. As(HI) can also be extracted as AsBr3 [3], or AsI3 [4-6].

A chloroform solution of diethylammonium DDTC (formula 4.37) extracts As(HI), Sn(II), and Sb(llI) from 1-5 M H2SO4 medium. Arsenic(V), Sn(IV), and Sb(V) do not react with DDTC. After Cu, Bi, and Hg have been extracted, iodide and ascorbic acid are added to the aqueous solution and As(Ill) is extracted. The DDTC method enables to separate As from Ge in the presence of oxalic acid. Pyrrolidinedithiocarbamate is also recommended for separation of As [7-9]. A hexane solution of diethyldithiophosphoric acid extracts As and Sb from 1-2 M H2SO4, but only Sb is extracted at pH 1.3-2.5 [10].

As(V) has been separated from a number of elements by extraction into butanol from acidic medium as the molybdoarsenic heteropoly acid [ 11 ]. This acid has been extracted with solutions of some high molecular-weight amines in 1,2-dichloroethane [ 12].

As(Ill) has been extracted from iodide and chloride solutions by means of a benzene solution of diphenyl(2-pyridyl)methane [13]. Dialkyltin solutions in CHC13 have been proposed for extraction of As(V) [ 14-16].

8.1.2. Distillation

The separation of arsenic as the toxic gas arsine, ASH3, is the first step in both the Gutzeit method and the Ag-DDTC method for determining As. Both methods are discussed later. Arsenic has also been separated as AsH3 before its determination as arsenomobdenum blue [17,18]. Normally, arsenic is reduced by hydrogen in statu nascendi which is formed on dissolving zinc in HC1 (3-6 M), but it can also be reduced electrolytically [18]. The AsH3 generated is carried over in the hydrogen stream into an alkaline iodine absorption solution. The distilled AsH3 was also absorbed on solid mercury(II) iodide, from which arsenic was then removed with iodine solution.

Arsenic is normally separated as AsC13. Before being distilled from 5-7 M HC1 As(V) is reduced to as(l/I) by means of hydrazine. Only GeCI4 (b.p. 86 ~ and Se and Te chlorides distil along with AsC13 (b.p. 130 ~ Stannic chloride (b.p. 115 ~ is not distilled in the

100 8. Arsenic

presence of phosphoric acid. When the sample solution contains SbC13 (b.p. 220 ~ the distillation of AsC13 should be carried out at below 110 ~ Usually distillation is continued

till a half of the initial volume remains in the distilling flask. The distillate is collected either in water or dilute HNO3, or in an alkaline iodine or H202 solution.

Arsenic(Ill) may also be distilled from HBr [19] or HF [20] medium.

8.1.3. Precipitation and other methods

Traces of As(V) are precipitated as FeAsO4 with Fe(OH)3 as the collector, by adding ammonia to an acid solution till the pH reaches 8-9 [21]. Arsenic(V) can be co-precipitated with MgNH4PO4 [22]. Traces of As have been precipitated from strongly acid solution as the sulphide by use of thioacetamide and with Mo as the collector [23]. Thionalide has also been used to co-precipitate trace amounts of As [25].

Thionalide deposited on silica gel retains As(Ill) at pH 6.5-8.5, while As(V) passes through the column [26].

Separation of As(Ill) from As(V) in ground waters by means of anion exchanger in acetate form has been proposed [27,27a]. Arsenic(V) is retained in the anion exchanger bed, whereas As(Ill) passes through. As(V), P(V), and Si were also separated on ion exchange columns [28].


The arsenomolybdenum blue method is the most widely used. The method using the reaction of AsH3 with Ag-DDTC is less sensitive. In certain cases, the classical Gutzeit method is convenient. Methods based on ion-associates of molybdoarsenate with basic dyes are very sensitive.

8.2.1. Arsenomolybdenum blue method

Arsenic, oxidised to As(V) (e.g. by evaporation to dryness with HNO3) is made to react in a suitably acid solution (0.1-0.3 M H2SO4) with ammonium molybdate to form the molybdoarsenic heteropoly acid. The molybdoarsenic acid is then reduced [the Mo(VI) atoms linked with As(V) are reduced to a lower oxidation degree] and a blue reaction product, the arsenomolybdenum blue is formed. The absorbance is measured either in the aqueous solution, or after extraction into oxygen-containing organic solvent [29-33]. The reaction conditions are so selected that the unreacted molybdate ions are not reduced.

N2H4, SnC12, and ascorbic acid are employed as reducing agents. A mixture containing ammonium molybdate, hydrazine, and H2SO4 in suitable proportions, is conveniently used in the determination of As.

The best solvents for the extraction of arsenomolybdenum blue are butyl and amyl alcohols, ethers, and ethyl acetate. The molar absorptivity e of the As-Mo blue in butanol solution is 2.5"104 at Xmax=730 nm (a=0.33). Both the e and the ~max values depend on the reducing agent and the extraction solvent used.

Alternatively, arsenic may be extracted as molybdoarsenic acid, and the reducing agent may then be added to the organic phase [25].

Phosphorus(V) forms phosphomolybdenum blue under the reaction conditions used to form arsenomolybdenum blue. The corresponding silicon and germanium compounds are


formed under slightly differing conditions. Conditions for determining As(V), P(V), and Si present together have been given [34].

Reagents

Molybdate reagent: a) dissolve 1.0 g of ammonium molybdate in 100 ml of 2 M H2SO4; b) dissolve 0.10 g of hydrazine sulphate in 100 ml of water. Immediately before use, mix 10 ml of solution a) with 10 ml of solution b) and dilute the solution with water to 100 ml. Solutions a) and b) should not be kept for longer than 3-4 days.

Standard arsenic(HI) solution: 1 mg/ml. Dissolve 1.3200 g As203 in 20 ml of 2 M NaOH, dilute the solution with a little water, acidify slightly with 2M HC1, and dilute the solution with water in a volumetric flask to 1 1.

Arsenic-free conc. hydrochloric acid. To 20 ml of conc. HC1 add 1 drop of 1% KI solution, allow to stand for 5 min, and then shake it with three 5-ml portions of benzene.

Procedure

Extractive separation of As. Evaporate a sample, containing not more than 60 ~tg of As, almost to dryness, with HNO3 or H2SO4 (the solution must not contain halide ions). To the cooled solution, add 10 ml of As-free conc. HC1 and 2 drops ofl % KI solution. Mix and allow to solution to stand for 5 min, then transfer it to a separating funnel and shake with three 5-ml portions of benzene. Strip the combined extracts with two 5-ml portions of water.

Determination of As. To the aqueous phase, add 2 ml of conc. HNO3, stir, and evaporate the solution almost to dryness. Add 25 ml of molybdate reagent, stir well, and place the beaker in a boiling water-bath for 10 min. After cooling, transfer the solution to a separating funnel and extract the As-Mo blue with 2 portions (e.g. 10 ml) of butyl alcohol. Make up the blue extract to the mark in a 25-ml standard flask with the solvent, and measure the absorbance at 730 nm, using the solvent as reference.

Note: It is also possible to measure the absorbance of the aqueous solution of As-Mo blue. In this case the cooled coloured solution is transferred to a volumetric flask, and diluted to the mark with the molybdate reagent.

8.2.2. Silver diethyldithiocarbamate method

The arsine evolved by nascent hydrogen is absorbed in a pyridine solution of silver diethyldithiocarbamate. The pyridine-soluble product of reaction between AsH3 and Ag- DDTC is intensely violet, whereas the pyridine Ag-DDTC solution is pale yellow [35-38]. The molar absorptivity of the reaction product e=l.4"104 (a = 0.19) at ~max---- 535 nm, whereas the reagent absorbs at <500 nm.

The functions of the reagents used in the reduction of arsenic compounds, such as Zn, HC1, NiCI2, SnC12, and KI are discussed below in connection with the Gutzeit method. Sodium borohydride also has been proposed as the reducing agent [39-42]. Sequential spectrophotometric determination of As(III) and As(V) is possible by using borohydride [40]. Hydrogen sulphide, which interferes in the reaction, is separated from arsine on cotton wool impregnated with lead acetate.

During the extraction of AsC13 with inert solvents, Sb remains in the aqueous phase. Under the conditions specified in the procedure below, Sb present in approximately the same amount as As does not interfere. When larger amounts of Sb are present, it is advisable to

102 8. Arsenic

increase the quantity of SnCl2 added. Under these conditions Sb(lII) is reduced to the element, and not to SbH3 [22,36].

A solution of Ag-DDTC in CHC13 containing organic bases: 1-ephedrine [35,43,44], ethanolamine [45], brucine [46], or morpholine has been used as an alternative to the obnoxious pyridine solution.

Reagents

Silver diethyldithiocarbamate (Ag-DDTC). Dissolve 1 g of the reagent in 100 ml of pyridine. To facilitate dissolution, the pyridine may be heated to 50-60 ~ after the addition of crystalline Ag-DDTC. Preparation of crystalline Ag-DDTC: Dissolve 1.8 g of AgNO3 in 20 ml of water, and 2.6 g Na-DDTK'3H20 in 20 ml of water. Add the AgNO3 solution slowly, with careful stirring, to the Na-DDTC solution. Filter off the Ag-DDTC precipitate on a sintered-glass crucible, and wash several times with water. Dry the precipitate at 100 ~ to constant weight. (Nearly 2.4 g of product is obtained, yield -~90 % with respect to AgNO3.

Standard arsenic(III) solution: 1 mg/ml. Preparation as in p. 8.2.1. Zinc (As-free), granulated. Melt down granulated zinc in a quartz crucible and pour it in a

thin jet into a tall beaker filled with water of appropriate purity. Dry the comminuted zinc and store it in a stoppered vessel.

Cotton wool impregnated with lead acetate. Preparation as in p. 8.2.3.

Procedure

Distillation and determination of As. Place in a 50-ml conical flask the almost neutral sample solution containing not more than 80 gg of As. Dilute to 10 ml with water, and then add successively 5 ml of conc. HC1, 5 ml of 10 % KI solution, 4 drops of 10 % SnC12 solution in 6 M HC1, 2 drops of 10 % NiC12 solution, and 1.5 g of zinc. Close the flask with a head carrying the cotton wool impregnated with Pb(CH3COO)2, and connected to a tube immersed in the receiver, which contains 10 ml of the Ag-DDTC pyridine solution.

After 30 min (even though all the zinc may not have dissolved) disconnect the receiver and rinse the delivery tube with pyridine. Dilute the solution with pyridine in a 25-ml volumetric flask, and measure the absorbance at 535 nm, using a blank solution as reference.

8.2.3. Gutzeit method

The trace amount of arsenic, contained in the sample solution, is reduced to volatile AsH3 with zinc and HC1. A disk of paper impregnated with mercuric bromide is placed across the flow of arsine and hydrogen evolved. The reaction of AsH3 with HgBr2 gives coloured compounds such as yellow H(HgBr)2As,, brown (HgBr)3As, and black Hg3As2. The resulting coloured spot is compared with a set of standard spots corresponding to known amounts of arsenic. The method is sensitive and permits the estimation of arsenic in the range 0.1-3 ~tg [48-501.

The nascent hydrogen is produced in reaction of zinc with HC1 (3-4 M). The zinc used should be free of traces of As, and suitably comminuted to provide high rate of dissolving and liberation of hydrogen. Nickel(H) ions catalyse the zinc dissolution. SnC12 and I- facilitate the reduction of As traces.

Since also H2S gives a colour reaction with HgBr2, it must be removed from the AsH3 stream before the contact with the mercuric bromide paper. Hydrogen sulphide is absorbed by cotton wool and paper impregnated with lead acetate. PH3, SbH3, and GeH4 also interfere in


the determination of arsenic. The only phosphorus compounds which can be reduced to phosphine under the reaction conditions are phosphite and hypophosphite.

Reagents

Mercuric bromide papers. Place thin, compact filter paper disks (about 20 mm in diameter) in a freshly prepared 5 % HgBr2 solution in ethanol for 30 min. Lay the papers on a watch-glass to dry in air. The papers may be stored in an amber-glass jar for not longer than a week after preparation.

Standard arsenic(III) solution: 1 mg/ml. Preparation as in p. 8.2.1. Filter paper and cotton wool impregnated with lead acetate. Impregnate filter paper strips

(2.5 x 4 cm) for 30 min in a 10 % lead acetate solution, dry at 105 ~ and pleat into narrow folds. Soak and dry the cotton wool in the same way.

6

5

2

3 z

1

Fig. 8.1. Gutzeit apparatus

Gutzeit apparatus (Fig. 8.1): a 50-ml amber-glass bottle fitted with a rubber stopper (2) which carries a glass tube (3), 6-7 mm in inner diameter. The tube has a side hole (4) preventing closure of the passage by liquid drops carried by the gas. A neck above the stopper holds the folded filter paper (5) impregnated with lead acetate. Above the paper is placed cotton wool (6) impregnated with lead acetate. The paper disk impregnated with HgBr2 is sandwiched between evenly cut tube ends held tightly together by a pair of rubber stoppers fitted with projecting glass hooks for clamping by means of rubber bands.

Zinc (As-free). Comminution method as in p. 8.2.2.

Procedure Place the sample solution containing 0.2-3 ~tg As in the bottle of the apparatus, and dilute to 10 ml with water. Add 10 ml of HC1 (1 + 1), then 5 ml of 10 % KI solution, 4 drops of 10 % SnC12 solution in 6 M HC1, and 2 drops of 10 % NiC12 solution, and stir well. Then add 1.5 g of comminuted zinc and immediately close the bottle with a head containing the fresh disc of HgBr2 paper.

When the evolution of hydrogen is complete (30-60 min), remove the paper disk with the coloured spot from the apparatus. Compare the colour of the spot with a set of standards prepared at the same time and corresponding to 0, 0.5, 1, 2, and 3 ~tg of As. The zero standard is a blank test to allow for any arsenic in the reagents.

104 8. Arsenic


Sensitive methods for determining arsenic are based on ion-associates formed by molybdoarsenate and basic dyes. Flotation-spectrophotometric methods with the use of Crystal Violet (mixture of cyclohexane and toluene as flotation solvent, e=3.2" 105) [51] and Malachite Green (flotation with diethyl ether, ~=3.2" 105) [52] have been proposed. The ion- associate with Ethyl Violet is extracted with a mixture of cyclohexane and 4-methylpentanone (1+3) (~=2.8"105) [53].

Methods based on extractable associates of the reduced form of molybdoarsenic acid with Crystal Violet (~=3.1" 105), Methyl Violet, Brilliant Green, or Malachite Green have also been applied [54].

The iodide complex of As(HI) associated with Brilliant Green or Butylrhodamine B was filtered off and dissolved in ethanol (~=1.3"105) [55]. A complex of As(HI) with 4- nitropyrocatechol forms an extractable ion-pair with Brilliant Green.

The amplification method for arsenic is based on its extractive separation as molybdoarsenic acid and determination of Mo with Sulphonitrophenol S (e = 4.5" 105) or zinc dithiol [56]. The molybdoarsenic acid, after extraction into a mixture of butyl acetate and ethanol, is decomposed, then Mo(VI) is reduced to Mo(III) in a Jones reducer, and finally Mo is oxidised to Mo(VI) be means of Fe(HI). The Fe(II) produced in this reaction gives a coloured complex with ferrozine. The amplification factor is 36, and e=9.4" 105 [57].

In the reaction of As(HI) with IO3-, an equivalent amount of iodine is liberated; the absorbance of the violet solution in CC14 is measured [58]. In another indirect method, arsine reduces (in the presence of Ag + ions) Fe(III) to Fe(II) which gives a coloured complex with ferrozine [59].


The arsenomolybdenum blue method was applied for determination of arsenic in: biological materials [7,17,60,61], plants [24], water [24,62-64], silicates [20], petroleum products, organic compounds [24,65], steel [15,66], antimony [2,3,67,68], antimony and gallium chlorides [69], bismuth [18], zinc [70], zinc and lead concentrates [71], tungsten [72], copper alloys [73], gold and platinum [34], silicon [74], selenium [75], and boron [76].

Trace amounts of As were determined in water and sewage by the following method: The sample was treated with sodium tetraborate (NaBH4). The AsH3 formed was absorbed with a solution of iodine, and the resulting As(V) was determined by the As-Mo blue method [77].

The Ag-DDTC method was used for determination of arsenic in: waters [42,47,78-80], plant materials [41,48], food products [82], sewage and sediments [83], iron ores [44], cast iron and steel [45,84], copper and its salts [22,45], silver and gold [85], high purity reagents [21,46], germanium dioxide [86], hydrofluoric acid [87], phosphorus compounds [88], and sulphur 89].

The Gutzeit method was applied in determinations of arsenic i.a. in: germanium and its compounds [49], silicon [49,50], and petroleum products [48].

Arsenic was also determined in natural water after flotation of the ion-associate with Malachite Green [52], and Crystal Violet [90], and after extraction with Methylene Blue [91].

References

1. Beard H.C., Lyerly L.A., Anal Chem., 33, 1781 (1961). 2. Chwastowska J., Podg6rny W., Chem. Anal. (Warsaw), 20, 53 (1975).

References 105

3. Saulnier J., Bauer D., Rosset R., Analusis, 7, 242 (1979). 4. Marin L., Vernet M.,Analusis, 7, 33 (1979). 5. Suzuki N., Satoh K., Shoji H., Imura H., Anal. Chim. Acta, 185, 239 (1986). 6. Palanivelu K., Balasubramanian N., Ramakrishna T.V., Talanta, 39, 555 (1992). 7. Puttemans F., Van den Wilken P., Massart D.L., Anal. Chim. Acta, 149, 123 (1983). 8. Amankwah S.A., Fasching J.L., Talanta, 32, 111 (1985). 9. Mok W.M., Wai C.M., Talanta, 35, 183 (1988). 10. Hayashi K. et al., Anal. Sci., 2, 347 (1986). 11. Wfinsch G., Umland F., Z. Anal. Chem., 247, 287 (1969). 12. Ivanov N.A., Todorova N.G.,Anal. Chim. Acta, 91,389 (1977). 13. Jaz M.E., Siddique E., Ahmed A., Talanta, 32, 1055 (1985). 14. Shkinev V.M. et al., Zh. Anal. Khim., 33, 922 (1978). 15. Shkinev V.M. et al., Zavod. Lab., 47, No 10, 11 (1981). 16. Shkinev V.M. et al.,Anal. Chim. Acta, 167, 145 (1985). 17. Dyfverman A., Bonnichsen R., Anal. Chim. Acta, 23, 491 (1960). 18. Jackwerth E., Z. Anal. Chem., 211,254 (1965). 19. Analytical Methods Committee, Analyst, 100, 54 (1975). 20. Sala J.V., Hernandis V., Canals A., Analyst, 111,965 (1987). 21. Marczenko Z., Mojski M., Chem. Anal. (Warsaw), 14, 495 (1969). 22. Meyer J., Z. Anal. Chem., 210, 84 (1965). 23. Reay P.F., Anal. Chim. Acta, 72, 145 (1974). 24. Briska M., Hoffmeister W., Z. Anal. Chem., 268, 347 (1974). 25. Portmann J.E., Riley J.P., Anal. Chim. Acta, 31,509 (1964). 26. Terada K., Matsumoto K., Inaba T., Anal. Chim. Acta, 158, 207 (1984). 27. Ficklin W.H., Talanta, 30, 371 (1983). 27a. Abrazheev R.V., Zorin A.D., Zh. Anal. Khim., 54, 1253 (1999). 28. Narusawa Y., Anal. Chim. Acta, 204, 53 (1988). 29. Pakalns P.,Anal. Chim. Acta, 47, 225 (1969). 30. Morosanova S.A., Zh. Anal. Khim., 29, 529, 1108 (1974). 31. Shida J. et al., Bull. Chem. Soc. Jpn., 56, 633 (1983). 32. Bogdanova V.I., Mikrochim. Acta, 1984 II, 317. 33. Kunze S., Dietze U., Ackermann G., Mikrochim. Acta, 1989 III, 147. 34. Marczenko Z., Lenarczyk _., Chem. Anal. (Warsaw), 19, 679 (1974). 35. Bode H., Hachmann K., Z. Anal. Chem., 229, 261 (1967); 241, 18 (1968). 36. Dubois L. et al., Mikrochim. Acta, 1966, 415; 1969, 185. 37. Gastinger E., Mikrochim. Acta, 1972, 526. 38. Budesinsky B.W., Microchem. J., 24, 80 (1979). 39. Basadre-Pampin M. et al., Microchem. J., 23, 360 (1978). 40. Howard A.G., Arbab-Zavar M.H., Analyst, 105, 338 (1980). 41. Merry R.H., Zarcinas B.A., Analyst, 105, 558 (1980). 42. Dodoo D.K., Vrchlabsky M., Chem. Anal. (Warsaw), 26, 867 (1981). 43. Kopp J.F.,Anal. Chem., 45, 1786 (1973). 44. Bhargava O.P., Gmitro M., Hines W.G., Talanta, 27, 263 (1980). 45. Hulanicki A., G__b S., Chem. Anal. (Warsaw), 15, 1089 (1970). 46. Ackermann G., K6the J., Z. Anal. Chem., 323, 135 (1986). 47. Gupta P.K., Prabhat K. Gupta, Microchem. J., 33, 243 (1986). 48. Maranowski N.C., Snyder R.E., Clark R.O.,Anal. Chem., 29, 353 (1957). 49. Tumanov A.A., Sidorenko A.N., Taradenkova F.S., Zavod. Lab., 30, 652 (1964). 50. Rigin V.I., Mel'nichenko N.N., Zavod. Lab., 32, 394 (1966). 51. Babko A.K., Ivashkovich E.M., Zh. Anal. Khim., 27, 120 (1972).

106 8. Arsenic

52. Nasu T., Kan M., Analyst, 113, 1683 (1988). 53. Motomizu S., Wakimoto T., T6ei K., Analyst, 108, 944 (1983). 54. Ganago L.I., Ishchenko N.N., Zh. Anal. Khim., 34, 1768 (1979). 55. Nazarenko V.A. et al., Zh. Anal. Khim., 37, 1652 (1982). 56. Malyutina T.M., Savvin S.B. et al., Zh. Anal. Khim., 29, 925 (1974). 57. Pichaj M.A., Bet-Pera F., Mikrochim. Acta, 1986 I, 429. 58. Sandhu S.S., Analyst, 101, 856 (1976). 59. Kellen G.J., Jaselskis B.,Anal. Chem., 48, 1538 (1976). 60. Eichhorn G., Wolf W., Berger S.A., Mikrochim. Acta, 1976 I, 135. 61. Maher W.A., Analyst, 108, 939 (1983). 62. Johnson D.L., Pilson M.E., Anal. Chim. Acta, 58, 289 (1972). 63. Stauffer R.E.,Anal. Chem., 55, 1205 (1983). 64. Frenzel W., Titzenthaler F., Elbel S., Talanta, 41, 1965 (1994). 65. Campbell A.D., Low Choong-Pak, Mikrochim. Acta, 1980 I, 139. 66. Fogg A.G., Marriott D.R., Burns D.T., Analyst, 97, 657 (1972). 67. Marczenko Z., Mojski M., Skibe H., Chem. Anal. (Warsaw), 17, 881 (1972). 68. Mogileva M.G., Kozlova E.L., Zavod. Lab., 51, No 7, 7 (1985). 69. Goryushina V.G., Romanova E.V., Razumova L.C., Zh. Anal. Khim., 28, 601 (1973). 70. Raghavan R., Murthy S.S., Rao C.S., Talanta, 36, 951 (1989). 71. Rao V.S., Rajan S.C., Roa N.V., Talanta, 40, 653 (1993). 72. Orlova V.A. et al., Zh. Anal. Khim., 32, 1591 (1977). 73. Scholes I.R., Waterman W.R.,Analyst, 88, 374 (1963). 74. Buldini P.L., Zini Q., Ferri D., Mikrochim. Acta, 1983 II, 131. 75. Ebner E., Z. Anal. Chem., 206, 106 (1964). 76. Marczenko Z., Chem. Anal. (Warsaw), 9, 1093 (1964). 77. Haywood M.G., Riley J.P.,Anal. Chim. Acta, 85, 219 (1976). 78. Sandhu S.S., Nelson P., Anal. Chem., 50, 322 (1978). 79. Tamari Y., Yamamoto N., Tsuji H., Kusaka Y., Anal. Sci., 5, 481 (1989). 80. Chatterjee A. et al.,Analyst, 120, 643 (1995). 81. Miliadis G.E., Liapis K.S.,Anal. Chim. Acta, 283, 258 (1993). 82. Kaliszewski J., Teperek J., Chem. Anal. (Warsaw), 32, 757 (1987). 83. Lopez A., Torralba R., Palacios H.A., C~imara C., Talanta, 39, 1343 (1992). 84. Bhargava O.P., Donovan J.F., Hines W.G., Anal. Chem., 44, 2402 (1972). 85. Tayurskii V.A. et al., Zavod. Lab., 47, 18 (1981). 86. Fowler E.W., Analyst, 88, 380 (1963). 87. Meyer J., Z. Anal. Chem., 229, 409 (1967). 88. Nowicka J., Chem. Anal. (Warsaw), 39, 741 (1994). 89. Steinke J., Z. Anal. Chem., 240, 184 (1968). 90. Kunze S., Dietze U., Acta Hydrochim. Hydrobiol., 19, 17 (1991). 91. Pal T., Jana N.R., Sau T., Anal. Proc., 32, 369 (1995).

Chapter 9. Beryllium

Beryllium (Be, at. mass 9.012) forms cations Be 2+. In its chemical properties, beryllium resembles magnesium and aluminium. Beryllium hydroxide is precipitated at pH --6, and dissolves in alkali hydroxides. Freshly precipitated Be(OH)2 dissolves in NazCO3 solution to form a rather unstable carbonate complex. Beryllium also forms weak complexes with citrate, tartrate, and fluoride anions. Beryllium and its compounds are highly toxic.


9.1.1. Precipitation. Extraction

Precipitation of Be(OH)2 with ammonia at pH --8, with A1 and Fe(III) as collectors, enables beryllium to be separated from Ca, Mg, Mn, and Cr [after oxidation of Cr(lII) to Cr(VI). In the presence of EDTA, the only Analytical Group III metals which ammonia precipitates are Be, Ti, U, Nb, and Ta.

When the sample containing Be is fused with NaOH in a nickel crucible, and the cooled melt leached with water, beryllium remains in solution, while the insoluble residue contains Fe, Mn, Ni, Mg, Ti, and some other metal hydroxides. Traces of beryllium have been co- precipitated with Sn(OH)4 as collector, in the presence of EDTA at pH --13 [1 ].

Extraction with acetylacetone in CHC13, CC14 or C6H6 enables to separate Be from Mg, Ca, and phosphate. In the presence of EDTA Fe(III) and A1 remain in the aqueous phase; traces of Fe(III) and A1 are, however, also extracted. The beryllium can be back-extracted into hydrochloric acid (--5 M) [2-4]. Trifluoroacetylacetone and hexafluoroacetylacetone have also been used to extract beryllium [5].

Beryllium may be separated by extraction with benzohydroxamic acid and some of its derivatives (including heterocyclic ones [6]), in cyclohexanone [7] and with Aliquat 336S in xylene from malonic acid solution (pH 5-7) [8,9]. Beryllium has also been extracted from thiocyanate media with TBP in toluene [10] and TOA in MIBK [11].

9.1.2. Ion exchangers and other sorbents

The oxalate solution at pH 4.4 is passed through a strongly acidic cation exchanger; Fe(III) and A1 complexes are eluted. The beryllium, which is retained on the column, is then eluted with 10 % HC1. By a similar procedure Be is separated from other cations with the use of EDTA in a weakly acid solution (pH 4) [12]. The separation of beryllium from a number of cations by using cation exchangers has been achieved by consecutive elution of the metals with increasing acid concentrations. Beryllium has been separated from various metal ions by cation exchange in the systems thiocyanate-DMSO [13] and HNO3-methanol [14].

Beryllium has been separated on an anion exchanger from large quantities of uranium, by using a mixture of 5 M HNO3 and methanol. Beryllium was also separated from Fe(III) and A1 on an anion-exchanger, using a fluoride medium [15].

108 9. Beryllium

A thiocyanate medium was applied in separating Be on a silica gel column impregnated with TBP. Beryllium was eluted with 0.1 M HC1 [16]. The same sorbent impregnated with HDEHP was used for selective pre-concentration of Be and for isolation of this element from a number of other metals [ 17].


Triphenylmethane and azo reagents are used in most methods for determining beryllium. Methods using Chrome Azurol S or Eriochrome Cyanine R and some cationic surfactants are very sensitive. The selectivity of methods for beryllium determination is improved by the use of EDTA as masking agent.

9.2.1. Chrome Azurol S method

Chrome Azurol S (CAS, formula 4.18) forms a coloured chelate complex with Be 2+, and this has been used for the determination of Be [ 18-21 ]. In acetate (or hexamine) buffer and in the presence of EDTA as masking agent, the Chrome Azurol S method is highly selective for beryllium. The absorbance of the complex depends on the pH, and on the concentrations of CAS, EDTA, and the acetate buffer. The absorbance increases with increasing CAS concentration, and decreases with increasing EDTA and acetate concentrations. A pH o f - 5 is the most suitable. Below this pH, the absorbance of CAS increases considerably, and above it the absorbance of the beryllium complex is decreased more by EDTA.

Because of this, it is difficult to give a definitive value for the molar absorptivity of the complex. The absorption maximum of the complex, (vs. the reagent solution as reference), is at --570 nm. At this wavelength, and under the reaction conditions given in the procedure below, the molar absorptivity e is 1.510 4 (a - - 1.67). In the absence of EDTA, the e value increases to 2.4104.

EDTA successfully masks Cd, Co, Cu, Fe(III), Mn, Mo, Pb, V(IV), and Zn. Copper (2 mg), causes a positive error of 2% . Ascorbic acid reduces Fe(III) to Fe(II), and V(V) to V(IV). The EDTA complexes of A1 and Cr(III) are formed slowly, and it is necessary to heat the solution after the addition of EDTA. Aluminium can be masked with oxine. Zirconium interferes in the determination even in the presence of EDTA, but can be masked by adding tartaric acid. Interfering agents are: U, Th, and Ti, as well as fluoride and, to a lesser degree, phosphate. It has been found that the presence of 2,2'-bipyridyl enhances the sensitivity of the method (e = 5.4104) [22].

Reagents

Chrome Azurol S (CAS): 0.1% solution. Dissolve 100 mg of the reagent in water and dilute to 100 ml in a volumetric flask.

Standard beryllium solution: 1 mg/ml. Dissolve 1.9640 g of BeSO4.H20 in water containing 1 ml of conc. HC1, and dilute the solution with water to volume in a 100-ml standard flask

Hexamine buffer: pH 5.0. To 250 ml of 10% hexamine solution add 45 ml of 2M HC1. The buffer is not very stable.


Procedure

To the slightly acidic solution (10-15 ml) containing not more than 12 gg of Be, add -- 10 mg of ascorbic acid and 1 ml of 5% EDTA solution. After 5 min, add 2.5 ml of CAS solution, --0.1M NaOH solution to adjust the pH to --5, and 5 ml of the buffer. Dilute with water to 25 ml in the standard flask, mix well, and after 10 min measure the absorbance of the solution at 570 nm, v s . a blank solution as reference.

9.2.2. Chrome Azurol S-surfactant method

In the presence of a cationic surfactant, such as CTA, CP or Zephiramine, the reaction of Be 2+ ions with Chrome Azurol (CAS) is several times more sensitive [23-25].

Maximal and stable absorbance of the triple complex is obtained with a large excess of surfactant in relation to CAS. In these systems, ternary complexes are formed in which the ratio CAS:Be is greater than in binary systems (without surfactant). Figure 9.1 shows the absorption spectra of Chrome Azurol S and the binary and ternary (with CTA) complexes of beryllium

3

1

,~00 500 600 700 wavelengtli, nm

Fig. 9.1. Absorption spectra of Chrome Azurol S (CAS) (1), its Be complex (2), and the ternary complex Be-CAS-CTA (3) (pH 5)

Under the conditions given below the value of e is 9.45104 (a - 14.8) at 615 nm [24]. The molar absorptivity is higher (e = 1.0 105) in the absence of EDTA, but, as in the binary system, the use of EDTA is necessary to increase the selectivity of the method. The optimum pH value is --5; it is advisable to use hexamine buffer. At room temperature the reaction is slow; measurements of the absorbance should be made after--1 h.

Besides CTA [23,26], other cationic surfactants have been proposed, namely: CP [24,27], Zephiramine ( e - 1.1105)[23,25], polyoxyethylenedecylamine [28], and Sterinol (dimethyl-laurylbenzylammonium bromide) ( e - 1.06105) [29].

Reagents

Chrome Azurol S (CAS): 1.10 -3 M ( --0.05% ) solution. Cetyltrimethylammonium chloride (or bromide) (CTA): 5.10 -3 M (-~0.15%) solution. Standard beryllium solution, preparation as in Section 9.2.1. Hexamine buffer, preparation as in Section 9.2.1.

110 9. Beryllium

Procedure

To the acidic solution (10-15 ml, pH 1-2) containing not more than 2 ~tg of Be, add 3 mg of ascorbic acid, and 1 ml of 1% EDTA solution. After 5 min, add 3 ml of CAS solution and 2.5 ml of CTA solution. Adjust the pH to -~5 with 0.1M NaOH solution and add 2.5 ml of buffer. Dilute to 25 ml in a standard flask, mix well, and after 1 h measure the absorbance of the solution at 615 nm v s . a blank solution.

9.2.3 Other methods

Erioehrome Cyanine R (ECR) (formula 4.17) reacts with beryllium ions [4,9,10,16,30] similarly to Chrome Azurol S (see Section 9.2.1). At pH 9.7, ~max of ECR is 435 nm and that of its water-soluble beryllium complex is 525 nm. The molar absorptivity of the complex is 1 .510 4. EDTA, tartrate and cyanide are used as the main masking agents for interfering metals. In the presence of cationic surfactants, the sensitivity is increased several times, and significant bathochromic shifts are observed. In the case of CTA, e = 8.7.10 4 at 590 nm (pH -~7) [31,32]. Beryllium was also sorbed on anion exchange resins impregnated with ECR [33].

Tr iphenylmethane chelating reagents, other than CAS and ECR, have also been used to determine beryllium, e.g. Aluminon [34,35], Xylenol Orange [36], Brilliant Violet B [2],, and Chromal Blue G (e = 3.1.104) [37]. A considerable increase of sensitivity is obtained by using Chromal Blue G in the presence of CTA (e = 9.4.104 at 626 nm) [38].

A large number of spectrophotometric methods for determining beryllium are based on azo reagents. Beryllon II (formula 9.1) forms a blue complex with beryllium at pH 12-13 [7,30,39,40]; the molar absorptivity e = 1.2.104 at 630 nm.

H 0 3 ~ OH OH OH (9.1)

Beryllon I, Beryllon III [41,42], and Beryllon IV [43] have not been widely applied. Other azo reagents used for determining Be are: Thoron I (e = 1.4.104 at 523 nm

[8,44,45], Chlorophosphonazo R [17,46}, Eriochrome Black T (extraction with TOA in CHC13, e = 5.5-10 4) [47], Calcichrome [48], and Sulphochlorophenol S (in acetic acid and propanol medium) [49].

In other methods for determination of beryllium use is made of such organic reagents, as 2-phenoxyquinalizarin-3,4'-disulphonic acid [12], 1-hydroxy-2-carboxyanthraquinone (the first-derivative spectrum is used) [50], 8-hydroxyquinaldine (e = 3.5.103) [51], and carminic acid [52].

In an indirect method of determining beryllium a Ti(IV)-HzOz-HF reagent is added to a sample solution containing beryllium. As a result of the reaction of fluoride with beryllium, an equivalent amount of yellow peroxotitanium(IV) complex is formed [53].

References 111


The Chrome Azurol S method has been applied for determining beryllium in, for example, bronzes [54] and water [55]. In the presence of a surfactant, beryllium was determined in minerals [15], sewage [56], coal dust [57,58], and aluminium alloys [59].

Eriochrome Cyanine R was applied in determination of beryllium in water [33] and in silicates [4].

Aluminon has been used for determining beryllium in airborne dust [34]. Methods involving Beryllon II were used for determining beryllium in minerals [15] and

in natural waters [60].

References

1. Plotnikov V.I., Safonov I.I., Zh. Anal. Khim., 33, 2350 (1978). 2. Uesugi K., Anal. Chim. Acta, 49, 89 (1970). 3. Merrill J.R., Honda M., Arnold J.R., Anal. Chem., 32, 1420 (1960). 4. Sauerer A., Troll G., Talanta, 31,249 (1984). 5. Scribner W.G., Borchers M.J., Treat W.J., Anal. Chem., 38, 1779 (1966). 6. Agraval Y.K., Dallali H., Microchem. J., 43, 258 (1991). 7. Borzenkova N.P., Burmistrova L.A., Zh. Anal. Khim., 27, 682 (1972). 8. Rao R.R., Khopkar S.M.,Anal. Chem., 55, 2320 (1983). 9. Narayanan P., Khopkar S.M.,Analyst, 110, 1295 (1985). 10. Kalyanaraman S., Khophar S.M., Anal. Chem., 47, 2041 (1975). 11. Castillo J.R. et al., Talanta, 29, 485 (1982). 12. Owens E.G., Yoe J.H.,Anal. Chem., 32, 1345 (1960); Talanta, 8, 505 (1961). 13. Janauer G.E., Madrid E.O., Mikrochim. Acta, 1974, 769. 14. Strelow F.W., Weinert C.H.,Anal. Chim. Acta, 83, 179 (1976). 15. Eristavi V.D., Eristavi D.I., Brouchek F.I., Zh. Anal. Khim., 23, 782 (1968). 16. Sharma C., Khopkar S.M., Anal. Chim. Acta, 167, 403 (1985). 17. Bykhovtsova I.V. et al., Zh. Anal. Khim., 40, 814 (1985). 18. Pakalns P., Anal. Chim. Acta, 31,576 (1964). 19. Sommer L., Kubafi V., Coll. Czech. Chem. Comm., 32, 4355 (1967). 20. Sommer L., Kubafi V.,Anal. Chim. Acta, 44, 333 (1969). 21. Petrova T.V., Sokolovskaya L.A., Savvin S.B., Zh. Anal. Khim., 38, 646 (1983). 22. Buhl F., Kwapulifiska G., Mikrochim. Acta, 1980 I, 89. 23. Marczenko Z., Ka~owska H., Chem. Anal. (Warsaw), 22, 935 (1977). 24. Rudometkina T.F., Chernova I.B., Orlov V.V., Zavod. Lab., 54, No 9, 5 (1988). 25. Jarosz M., Biernat I., Chem. Anal. (Warsaw), 33, 693 (1988). 26. Callahan J.H., Cook K.D., Anal. Chem., 54, 59 (1982). 27. Mulwani H.R., Sathe R.M.,Analyst, 102, 137 (1977) 28. Nishida H., Nishida T., Ohtomo H., Bull. Chem. Soc. Jpn., 49, 571 (1976). 29. Kwapulifiska G., Buhl F., Mikrochim. Acta, 1984 I, 333. 30. Kasiura K., Chem. Anal. (Warsaw), 16, 407 (1971). 31. Marczenko Z., Ka~owska H., Microchem. J., 23, 71 (1978). 32. Tikhonov V.N., Vashurkina E.A., Zh. Anal. Khim., 33, 1298 (1978). 33. Valencia M.C., Boudra S., Bosque-Sendra J.M., Analyst, 118, 1333 (1993); Anal. Chim.

Acta, 327, 73 (1996). 34. McCloskey J.P., Microchem. J., 12, 32, 41 (1967).

112 9. Beryllium

35. Dhond P.V., Khopkar S.M., Anal. Chem., 45, 1937 (1973). 36. Otomo M., Bull. Chem. Soc. Jpn., 38, 730 (1965). 37. Uesugi K., Bull. Chem. Soc. Jpn., 42, 2998 (1969). 38. Uesugi K., Miyawaki M., Microchem. J., 21,438 (1976). 39. Adamovich L.P., Mirnaya A.P., Zh. Anal. Khim., 18, 292 (1963). 40. Xu Y.J., Chen X.G., Hu Z.D., Talanta, 40, 883 (1993). 41. Pakalns P., Flynn W.W., Analyst, 90, 300 (1965). 42. Zhu Y., Shao J.,Analyst, 114, 97 (1989). 43. Budanova L.M., Pinaeva S.N., Zavod. Lab., 32, 401 (1966). 44. Einaga H., Ishii H., Anal. Chim. Acta, 54, 113 (1971). 45. Keil R., Z. Anal. Chem., 262, 273 (1972). 46. Lukyanov V.F. et al., Zh. Anal. Khim., 18, 562 (1963). 47. Pyatnitskii I.V., Pinaeva S.G., Pospelova N.V., Zh. Anal. Khim., 30, 2316 (1975). 48. Einaga H., Ishii H., Talanta, 28, 799 (1981). 49. Petrova T.V. et al., Zh. Anal. Khim., 36, 90 (1981). 50. Salinas F., de la Pena A.M., Murillo J.A., Analyst, 112, 1391 (1987). 51. Keil R., Mikrochim. Acta, 1973, 919. 52. Kaur P., Gupta V.K., Z. Anal. Chem., 334, 447 (1989). 53. Matsubara C., Takamura K., Anal. Chim. Acta, 77, 255 (1975). 54. Egkinja I., Grabari_ Z., Grabari_ B.S., Mikrochim. Acta, 1985 II, 443. 55. Dong H., Jiang M., Zhao G., Wang M.,Anal. Sci., 7, 69 (1991). 56. Qiu X., Zhu T., Chen J., Fresenius'J. Anal. Chem., 342, 172 (1992). 57. Nishida H.,Anal. Sci., 7, 975 (1991). 58. Buhl F., Kwapulifiska G., Chem. Anal. (Warsaw), 33, 513 (1988). 59. Rudometkina T.F., Chernova I.B., Orlov V.V., Zavod. Lab., 54, No 9, 5 (1988). 60. Kornienko T.G., Samchuk A.I., Ukr. Khim. Zh., 38, 917 (1972).

Chapter 10. Bismuth

Bismuth (Bi, at. mass 208.98) occurs in its compounds in the III and V oxidation states. Bismuthate, containing Bi(V) exists only in solids (e.g., NaBiO3). Upon dissolution, only compounds of Bi(III) are found. Bismuth(m) hydrolyses at pH 1-2, and shows no amphoteric properties. Bismuth([II) forms citrate-, oxalate-, iodide-, thiosulphate-, and EDTA complexes.


10.1.1. Extraction. Precipitation

Extraction of bismuth with dithizone in CHC13 or CC14, in the presence of masking agents, is a selective method for separating traces of Bi. It is often connected with the determination of Bi as dithizonate (see below).

The extraction of bismuth diethyldi thiocarbamate into CC14 or CHC13, from alkaline solution containing tartrate, cyanide, and EDTA, is a specific method for separation of Bi. After the extraction Bi can be determined either directly as the coloured complex Bi(DDTC)3 (see below), or by other methods [ 1,2].

Bismuth can be extracted from strongly acid solutions (-2 M H2SO4 or HC104) as the iodide complex, with a mixture of isoamyl acetate and isoamyl alcohol [3]. The anionic iodide complex of Bi was associated with a cationic complex of K + with the macrocyclic polyether dibenzo-18-crown-6, and the resulting ion-associate was extracted into a (1+3) mixture of CHC13 with 1,2-dichloroethane [4].

When precipitated as the sulphide from the 2 M HC1 with Cu(II) as the collector bismuth is separated from Pb, Sn, and Cd. When the precipitation is carried out in 0.2-0.3 M HC1, other metals of the Analytical Group I and II are precipitated along with Bi.

When traces of Bi are precipitated as the hydroxide with ammonia, Fe(III), A1, and La [5] can be used as collectors. From a HNO3 solution of pH 1-2.5, traces of Bi can be separated with a MnO2 aq. Collector, which is precipitated in reaction of Mn 2+ with MNO4- [6]. Sb and Sn are precipitated along with Bi.

To determine Bi203 (-0.01%) in bismuth, the metal is dissolved in mercury, and the undissolved oxide is separated by flotation [7]. A co-precipitate of traces of bismuth with Fe(OH)3 (pH 4) is separated by flotation with the use of a surfactant [8].

10.1.2. Ion exchange

Strongly basic anion exchangers retain Bi from dilute HC1 solutions (0.1-1 M), allowing for separation of Bi from Fe(III) and other metals [9]. Bismuth is eluted from the column with thiourea in dilute H2SO4, with dilute HNO3, or with a mixture of HBr and HNO3 [ 10].

Bismuth has been separated from other metals, retained on a cation exchange column, by elution with HBr [11]. From dilute HC1 solutions cation exchangers retain Bi and other metal cations, whereas Pt is not sorbed [12]. Successive elutions with 0.3 M and 0.6 M HNO3 enables the separation ofT1 and Ag from Bi [13].

114 10. Bismuth


Trace amounts of Bi are determined conveniently by the dithizone method. The Xylenol Orange method is less sensitive. The diethyldithiocarbamate method is not very sensitive, but it is specific for Bi.

A review of bismuth determination methods has been published [14].

10.2.1 Dithizone method

With dithizone (formula 1.1), bismuth(l/I) ions form an orange-brown complex, Bi(HDz)3, which is soluble in CC14 and CHC13, and is stable over the pH range 3-9.5. This dithizonate is the basis of the spectrophotometric method [15,16].

At )~max = 490 nm, e = 7.49.104 (a = 0.38). The absorption spectrum of Bi dithizonate is shown in Fig. 4.4.

When cyanide and tartrate are present as masking agents, only Pb, TI(I), and Sn(I1) dithizonates are co-extracted with bismuth from slightly alkaline medium (pH 8-9.5). Tin(IV) does not react with dithizone. Lead and thallium can be separated from bismuth since their dithizonates are unstable in slightly acidic medium (pH 3.0-3.5). After Bi, Pb, and T1 have been extracted into CC14, the lead and thallium are stripped into an aqueous solution at pH 3.3.

If the sample solution contains considerably more lead than bismuth, it is more convenient to prevent the extraction of lead. In this case, the noble metals (Pt, Pd, Au, Ag, and Hg) and the copper in the sample are quantitatively extracted with dithizone at pH 0.5- 1.0, after which the pH of the aqueous solution is readjusted to 3.1, and the bismuth is then extracted with dithizone. If the amounts of Zn, Cd, and Pb in the aqueous solution considerably exceed that of bismuth, then traces of Zn, Cd, and Pb dithizonates are also extracted. The cadmium and lead are quantitatively removed by stripping with an aqueous solution at pH 3.3. Zinc dithizonate, however, is not decomposed at pH 3.3, and below this pH bismuth dithizonate is partly decomposed. The traces of zinc are stripped from the extract with dilute (-~0.05%) KCN solution buffered at pH 9.5. An unbuffered KCN solution is sufficiently alkaline to partly decompose Bi(HDz).

At higher concentrations, halide ions inhibit the extraction of bismuth from acid medium. The effect is most severe with iodide.

Reagents

Dithizone (H2Dz)" 0.001% solution in CC14. Preparation as in Section 47.2.1. Standard bismuth solution: lmg/ml. Dissolve 2.3210 g of Bi(NO3)3.5H20 in 100 ml of

HNO3 (1 +3), and dilute the solution to volume with water, in a 1-1itre standard flask. Working solutions are obtained by diluting the stock solution with 0.01M HNO3.

Potassium cyanide: 10% solution. Preparation as in Section 27.2.1. Wash solution (pH 3.3+0.1). Adjust an approximately 0.1% NH4C1 solution with dilute

HC1 to pH 3.3+_0.1. Buffer solution (pH 9.5). Dissolve 60 g of NH4C1 in water, add 120 ml of conc.

ammonia and dilute with water to 1 litre.


Procedure

Any Cu, Ag, Au, Pt, Pd, or Hg present in the sample should be removed by extraction with dithizone at pH 0.5-1.0. Adjust the pH of the sample solution, containing not more than 40 ~tg of Bi, with ammonia to pH 3.1+0.1. Extract Bi with several aliquots of the dithizone solution in CC14 (1 ml of 0.001% HzDz solution is equivalent to 2.7 ~tg of B i ) . Wash the combined extracts by shaking successively with the wash solution, water, and then with a solution containing 3 drops of the buffer solution (pH 9.5) and 3 drops of 10% KCN solution per 10 ml of water. Finally wash the extract with water, and dilute to volume in a 25-ml or smaller standard flask with CC14. Measure the absorbance of the extract at 490 nm, using the solvent or a blank solution as reference.

Notes. 1. If other metals (e.g., Zn, Pb, and Cd) are to be determined in the aqueous solution after extraction of Bi, add a few drops of conc. HC1 and H202 to the combined aqueous washings, evaporate the solution to 3 4 ml, and combine the residue with the aqueous solution from which bismuth was extracted.

2. Both Bi and Pb dithizonates are extracted from cyanide solution. To remove the lead, shake the extract with two portions of the pH 3.3 wash solution, and then with the pH 9.5 buffer solution to remove the free dithizone liberated by the decomposition of Pb(HDz)2.

3. Before extracting bismuth and lead from ammoniacal cyanide solution add purified potassium sodium tartrate (see Section 27.2.1) to the initial acidic solution to prevent precipitation of hydrolyzable metals.

10.2.2. Die thy ld i th iocarbamate method

Bismuth forms a chelate (soluble, at pH 4-11, in CC14 and iso-amyl alcohol) with sodium diethyldithiocarbamate (formula 4.40).

The absorption maximum of Bi(DDTC)3 in CC14 solution (absorption spectrum see Fig. 19.1) is at 370 nm, ~ - 8.6.103. At 400 nm, e = 6.7-103, a = 0.033. The method of determining bismuth as the yellow Bi(DDTC)3 complex, although not very sensitive, is valuable as being specific for Bi.

Bismuth is the only metal forming a coloured complex with DDTC, in ammonia solutions, in the presence of EDTA and KCN, at pH 9-11. The Hg(DDTC)2 complex, also extractable under these conditions, does not absorb at 400 nm. Some tartrate should be added, if A1, Ti, or other hydrolysable elements are present.

Bismuth has been determined with the use of zinc dibenzyldithiocarbamate (CC14, 8 = 1.2.104, at 370 nm) [17,18], lead tetramethylenedithiocarbamate [19], and pyrrolidinedithiocarbamate [20].

Reagents

Sodium diethyldithiocarbamate (Na-DDTC): 0.2 % solution, adjusted to pH 8-9 by adding ammonia.

Standard bismuth solution: 1 mg/ml, preparation as in Section 10.2.1.

Procedure

To a solution, containing not more than 0.5 mg Bi, add 2 ml of 10% EDTA solution, neutralize with conc. ammonia, and add 2 ml in excess. Then add 2 ml of 10% KCN and 1 ml of Na-DDTC. Shake the solution with 2 portions of CC14. Dilute the extract with the solvent up to the mark in a 25 ml volumetric flask. Mix the solution and measure its absorbance at 400 nm using the solvent as reference.

116 10. Bismuth

10.2.3. Xylenol Orange method

Xylenol Orange (formula 4.19) forms with bismuth ions, in acid medium, a water-soluble pink complex, which is a basis for determining Bi [21-25]. Maximum colour intensity of the complex is obtained in 0.05-0.1 M H2SO4 or 0.08-0.12 M HNO3.

The absorption maximum of the bismuth-Xylenol Orange complex is at 540 nm, whereas that of the reagent is at 440 nm. The molar absorptivity of the complex in 0.05-0.1 M H2SO4 is 2.3-104 (a = 0.13). Iron(II1), which interferes in the determination of bismuth, is reduced to Fe(II) with ascorbic acid. By using tartaric acid, it is possible to mask Zr, Hf, Sn, and Sb. The sensitivity of reaction of the reagent with Sb is rather low. Tin can be masked conveniently with fluoride.

Chloride, bromide, and iodide decompose the Bi-Xylenol Orange complex. Since chloride exerts no influence on the complexes of Xylenol Orange with Zr, Hf, Fe, and Sn, this fact can be used for determining Bi in the presence of these metals [22].

Reagents

Xylenol Orange, 0.05 % solution in 0.05 M H2SO4 Standard bismuth solution: 1 mg/ml. Preparation as in Section. 10.2.1.

Procedure

Place a sample at pH -~1 containing not more than 150 ~tg of Bi in a 25 ml volumetric flask. Add 1 ml of 2% ascorbic acid solution, and (after 2 min) 3 ml of the Xylenol Orange solution. Dilute with 0.05 M H2SO4 to the mark, and mix well. After 10 min, measure the absorbance at 545 nm, using a blank solution as reference.

10.2.4. Iodide method

In acid medium of 0.2-2 M H2SO4 bismuth forms the orange-yellow complex BiI4-, which is the basis for determining bismuth [26]. Up to a concentration of 3% KI in the solution, the absorbance increases; above this concentration, the absorbance remains constant. Liberation of free iodine, owing to oxidation of the iodide by atmospheric oxygen or oxidizing substances present in the sample solution, is prevented by the addition of reducing agents, such as ascorbic acid, sulphite, or hypophosphite.

The absorption spectrum of the bismuth iodide complex exhibits an intense maximum at 337 nm, and a less intense maximum at 465 nm. The molar absorptivity of the complex at 465 nm is 9.1.103 (a = 0.044).

The anionic complex of Bi has been extracted into CHC13 as an ion-pair with the cations tetra-n-butylammonium (~=1.13.104 at 490 nm) [27], tetramethylammonium [28], benzyltributylammonium [29], tetramethylenebis(triphenylphosphonium) (extraction with 1,2-dichloroethane) [30], 1-naphthylmethyltriphenylphosphonium [31], N-octylacetamide from a surfactant medium [32], and N,N'-diphenylbenzamidine [33]. Hexadecyltributyl- phosphonium cation has also been proposed as a basis for determining bismuth [34]. Bismuth was determined also as a ternary complex with iodide and 1,10-phenanthroline or 2,2'-dipyridyl (extraction with cyclohexanone or nitromethane) [35].

Antimony interferes in the determination. In 1.5% KI medium antimony does not form a coloured complex, and the intensity of the colour of the bismuth complex is reduced by only about 10%.


From a 5 M H2504 and 0.001 M KI medium the Bi complex can be extracted by benzene (as in the case of the Sb complex). At a KI concentration increased to 0.05 M no more bismuth is extracted, whereas the extraction of Sb is practically complete.

The extraction of BiI3 with benzene from H2504 solution containing small concentrations of KI forms the basis of the very sensitive indirect starch-iodine amplification method for determining bismuth [36,37]. From the benzene extract, washed with dilute H2SO4, the BiI3 is re-extracted with water. Iodide is then oxidized to IO3-, which reacts with added KI to release iodine, which is determined by the starch-iodine method (see Section 25.2.1). For each atom of bismuth (in BiI3), 18 atoms of iodine are released. The molar absorptivity e is about 3.2.105 (a = 1.5) [36]. A 48-fold increase in sensitivity of the method has been attained after re-extraction of BiI3, adding Br2, and measurement of absorbance of the I3- formed [37].

Reagents

Potassium iodide, 20% solution. Standard bismuth solution: 1 mg/ml (preparation as in Section 10.2.1).

Procedure

To a solution in a 25-ml standard flask, containing not more than 0.4 mg of Bi, add 3 ml of H2SO4 (1+1), 1 ml of 2% ascorbic acid, and 5 ml of KI solution. Dilute the solution to the mark and mix well. After 5 min, measure the absorbance of the solution at 465 nm vs. water.


Thiourea, (H2N)2C=S, forms a cationic, yellow complex with Bi, stable in acid medium. The complex is the basis of a rather insensitive method for determining bismuth [38-40]. At 470 nm, the molar absorptivity is 9.0.103. The colour of the complex varies slightly, depending on the acid present in the solution. Usually-~ 1M HNO3 is used. The concentration of thiourea in the solution must be high (-- 6% ). Coloured complexes are also formed by thiourea with Sb, Pd, Fe(III), Os, and Ru. The thiourea-Bi complex can be extracted into TBP [38] or MIBK from HC104 medium. Bismuth has been determined with thiourea after pre-concentration on an appropriate cation exchanger [41]. Cyclic urea derivatives have also been suggested for determining bismuth [42].

Many other sulphur reagents have been proposed for determination of Bi, including Bismuthiol II (formula 49.1) and its derivatives [43], 8-mercaptoquinoline (thio-oxine) (formula 4.44) [44], thiobarbituric acid (~ = 2.4-104 at 416 nm) [45] and dithiobenzoylmethane [46].

Several azo reagents have been used for determining bismuth, e.g., PAN [47], 5-Br- PADAP (formula 4.7) (e = 4.9-104 at 583 nm) [48], PAR and related reagents [49], Arsenazo III [50], and 2,3,4-trihydroxy-4'-sulphoazobenzene (e = 5.5.10 4 at 460 nm) [51 ].

Some basic dyes form ion-associates with Bil4- that can be used in sensitive methods for determining bismuth. Extractable compounds are formed with Rhodamine B, Butylrhodamine B, and Rhodamine 6G (benzene, e = 1.1.105) [52], and with the azo dye (formula 10.1, e = 9.2-104) [53]. The ion-associate of the bromide complex of Bi with Rhodamine 6G has been floated with DIPE and then dissolved in ethanol (~ = 1.5.105) [54].

118 10. Bismuth

H3C-- i ~ - -N - -~ - -~N~ N (C 2t"ls)z HC~NIN

I CH3 (10.1)

The anionic complex formed by bismuth(III) and Alizarin S gives floatable sparingly soluble ion-associates with some basic dyes. The best results have been obtained with Brilliant Green (flotation with CC14-toluene mixture, dissolution of the separated compound in ethanol, e = 2.2.105) [55]. Other organic reagents used for spectrophotometric determination of Bi, include Bromopyrogallol Red [56], Pyrogallol Red [57], 3-nitrophenylfluorone (e = 5.0.104) [58], and Pyrocatechol Violet [59].


The dithizone method has been applied for determining bismuth in natural waters [9], platinum [12], gold [15], silver [5], tin [60], and tellurium [61].

The Xylenol Orange method has been used in determinations of Bi in copper [62,63], nickel [63], lead [64], and silver [62]. Xylenol Orange has been used in determining Bi by flow injection analysis [65].

The iodide method was used in determinations of bismuth in pharmaceuticals [66], sea water, A1 and Bi alloys [37], organic compounds [67], lead and its alloys [ 1,26], copper and its alloys [1,29,68], cast iron and steel [69], antimony [70], and sulphide concentrates [1]. After extraction of the iodide complex with various organic cations, Bi was determined in water and soil [32], pharmaceuticals [31 ], soil and ores [33], and copper alloys [31 ]. Thiourea and its derivatives were used for determining Bi in various metals [39] and ores [71]. Bi was determined in pharmaceutical preparations as the ion-pair of BiI4- with the proliptinium cation (CHC13) [72].

Bismuth has been determined in copper alloys using 2,6-Dichloroarsenazo [73].

References

1. Donaldson E.M., Talanta, 25, 131 (1978); 26, 1119 (1979). 2. Karadakov B., Sakharieva M., Anal. Chim. Acta, 125, 149 (1981). 3. Mottola H.A., Sandell E.B.,Anal. Chim. Acta, 24, 301; 25, 520 (1961). 4. Poladyan V.E. et al., Ukr. Khim. Zh., 51,743 (1985). 5. Marczenko Z., Kasiura K., Chem. Anal. (Warsaw), 9, 87 (1964). 6. Blakeley S.J., Manson A., Zatka V.J.,Anal. Chem., 45, 1941 (1973). 7. Matsumoto K., Anal. Chim. Acta, 123, 297 (1981). 8. Nakashima S., Z. Anal. Chem., 303, 10 (1980). 9. Portmann J.E., Riley J.P., Anal. Chim. Acta, 34, 201 (1966). 10. Strelow F.W., Van der Walt T.N., Anal. Chem., 53, 1637 (1981). 11. Fritz J.S., Garralda B.B., Anal. Chem., 34, 102 (1962). 12. Marczenko Z., Kasiura K., Krasiejko M., Mikrochim. Acta, 1969, 625. 13. Meintjies E., Strelow F.W., Victor A.H., Talanta, 34, 401 (1987). 14. Astafeva I.N., Shcherbov D.P., Plotnikova R.N., Zh. Anal. Khim., 30, 147 (1975). 15. Marczenko Z., Kasiura K., Krasiejko M., Chem. Anal. (Warsaw), 14, 1277 (1969). 16. Bidleman T.F., Anal. Chim. Acta, 56, 221 (1971). 17. Yamane T., Suzuki T., Mukoyama T., Anal. Chim. Acta, 62, 137 (1972).

References 119

18. Yamane T., Mukoyama T., Sasamoto T., Anal. Chim. Acta, 69, 347 (1974). 19. Szpunar-Lobifiska J., Anal. Chim. Acta, 251, 275 (1991). 20. Lau H.K., Droll H.A., Lott P.F., Anal. Chim. Acta, 56, 7 (1971). 21. Onishi H., Ishiwatari N., Talanta, 8, 753 (1961). 22. Cheng K.L., Talanta, 5, 254 (1960). 23. Onishi H., Ishiwatari N., Bull. Chem. Soc. Jpn., 33, 1581 (1960). 24. Kantcheva D., Nenova P., Karadakov B., Talanta, 19, 1450 (1972). 25. Pyatnitskii I.V. et al., Zh. Anal. Khim., 37, 1458 (1982). 26. Englis D.T., Burnett B.B.,Anal. Chim. Acta, 13, 574 (1955). 27. Hasebe K., Taga M., Talanta, 29, 1135 (1982). 28. E1-Shahavi M.S., Aldaheri S.M., Fresenius'J. Anal. Chem., 354, 200 (1996). 29. Barakat S.A., Anal. Chim. Acta, 355, 167 (1997). 30. Burns D.T. et al., Anal. Chim. Acta, 225, 449 (1989). 31. Burns D.T., Chimpalee D.,Anal. Chim. Acta, 211, 305 (1988); 256, 87 (1992). 32. Sharma M., Patel K.S., Ann. Chim. (Rome), 84, 467 (1994). 33. Ghosh A., Patel K.S., Mishra R.K., Bull. Chem. Soc. Jpn., 62, 3675 (1989). 34. Burns D.T., Tungkananuruk N., Anal. Chim. Acta, 197, 285 (1987). 35. Buhl F., Kania K., Chem. Anal. (Warsaw), 18, 369 (1973); 20, 1055 (1975). 36. Marczenko Z., Zo~dek I., Limbach A., Chem. Anal. (Warsaw), 14, 741 (1969). 37. E1-Shahawi M.S., Kamal M.M., Anal. Sci., 11,323 (1995). 38. Aoki F., Tomioka H., Bull. Chem. Soc. Jpn., 38, 1557 (1965). 39. Rublev V.V., Buzina N.I., Kosychenko L.I., Zh. Anal. Khim., 28, 1351 (1973). 40. Desai G.S., Shinde V.M., Bull. Chem. Soc. Jpn., 64, 1951 (1991). 41. Hoshi S., Notoya N., Uto M., Matsubara M., Anal. Sci., 7, 657 (1991). 42. Presnyak I.S. et al., Zh. Anal. Khim., 45, 1548 (1990). 43. Busev A.I., Simonova L.N., Gaponiuk E.I., Zh. Anal. Khim., 23, 59 (1968). 44. Suprunovich V.I., Vashchenko S.T., Zh. Anal. Khim., 37, 632 (1982). 45. Morelli B., Analyst, 107, 282 (1982). 46. Dolgorev A.V., Lysak Ya.G., Zibarova Yu.F., Zavod. Lab., 44, 1050 (1978). 47. Rakhmatullaev K., Gyasov A.S., Zavod. Lab., 55, No 5, 14 (1989). 48. Salim R., Shraydeh B., Microchem. J., 32, 83 (1985). 49. Nevskaya E.M., Shelikhina E.I., Antonovich V.P., Zh. Anal. Khim., 30, 1560 (1975). 50. Barkovskii V.F., Povet'eva Z.N., Zavod. Lab., 35, 555 (1969). 51. Gambarov D.G., Guseinov A.G., Zh. Anal. Khim., 39, 837 (1984). 52. Shestidesyatnaya N.L., Milyaeva N.M., Ukr. Khim. Zh., 41, 84 (1975). 53. Busev A.I., Shestidesyatnaya N.L., Kish P.P., Zh. Anal. Khim., 27, 998 (1972). 54. Chwastowska J., Pruszkowski K., Chem. Anal. (Warsaw), 21,525 (1976). 55. Flyantikova G.V., Isakhanova A.T., Zh. Anal. Khim., 37, 1452 (1982). 56. Nemcova I., Pe~inova H., Suk V., Microchem. J., 30, 27 (1984). 57. Wyganowski C., Chem. Anal. (Warsaw), 26, 307 (1981). 58. Antonovich V.P. et al., Zh. Anal. Khim., 30, 1566 (1975). 59. Honov~ D., N6mcov~ I., Suk V., Talanta, 35, 803 (1988). 60. Marczenko Z., Kasiura K., Chem. Anal. (Warsaw), 10, 449 (1965). 61. Kasterka B., Kwiatkowska-Sienkiewicz K., Chem. Anal. (Warsaw), 18, 1153 (1973). 62. Bagdasarov K.N. et al., Zavod. Lab., 34, 1306 (1968). 63. Adamiec I., Chem. Anal. (Warsaw), 13, 147 (1968). 64. Nan Z., Yu R., Yao X., Lu Z., Talanta, 36, 733 (1989). 65. Trojanowicz M., Augustyniak W., Hulanicki A., Mikrochim. Acta, 1984 II, 17. 66. Burns D.T., Dangolle C.D., Anal. Chim. Acta, 337, 113 (1997).

120 10. Bismuth

67. Hassan H.N., Hassouna M.E., Gawargious Y.A., Talanta, 53, 311 (1988). 68. Norwitz G., Galan M., Anal. Chim. Acta, 83, 289 (1976). 69. Koch O.G., Z. Anal. Chem., 255, 269 (1971). 70. Presnyak I.S., Antonovich V.P., Nazarenko V.A., Zavod. Lab., 53, No 1, 13 (1987). 71. Shelikhina E.I., Presnyak I.S., Nazarenko V.A., Zavod. Lab., 54, No 8, 15 (1988). 72. Burns D.T., Dunford M.D., Anal. Chim. Acta, 334, 209 (1996). 73. Zhang H.-S. et al., Anal. Chim. Acta, 380, 101 (1999).

Chapter 11. Boron

Boron (B, at. mass 10.81) is a metalloid with properties somewhat similar to those of silicon. In chemical analysis, only boron(III) compounds are of importance. Boron forms complexes with fluoride and polyalcohols (e.g., mannitol and glycerol). In an anhydrous medium, boric acid reacts with methanol to form the volatile trimethyl borate.


11.1.1. Distillation

Distillation of boron as the volatile trimethyl borate (b.p. 65~ is the most common method of isolating boron before its spectrophotometric determination [ 1-4]. When separating small amounts of boron, a quartz distillation apparatus should be used since laboratory glassware contains boron. An anhydrous medium promotes the quantitative formation and distillation of methyl borate (water hydrolyses the ester). The usual procedure is to add methanol and concentrated sulphuric acid to a dried sample, and heat the still in a glycerol- or oil-bath, gradually raising the temperature to about 120~ The distillate is collected in a quartz or platinum receiver containing dilute NaOH solution (see Section 11.2.1). If the sample solution contains fluoride, boron partly distils as volatile BF3. This is prevented by masking fluoride as the stable aluminium complex. Colloidal silica partially traps boron, thus inhibiting its quantitative distillation. Traces of boron have been separated as trimethyl borate by microdiffusion methods [5].

Volatilization of trace amounts of boron as fluoride BF3 is also utilized before its determination [6,7]. Pyrohydrolysis at 900-1,200~ in a stream of superheated steam, decomposes metal borides and enables the separation of boron as the fairly volatile boric acid [8,9].

11.1.2. Extraction

With the tetraphenylarsonium cation, the fluoroborate anion forms an ion-pair, which can be extracted with chloroform. Quantitative conversion of boron into BF4-, and subsequent quantitative extraction are ensured by using an excess of fluoride (at pH 2-3), and by leaving the solution to react for sufficient time before extraction [7]. Boric acid is often extracted from acidic solutions (1-6 M HC1) with 2-ethyl-l,3-hexanediol in CHC13 [10-14], with 2,2,4-trimethyl- 1,3-pentanediol in CHC13 [ 15], or with 2-methyl-2,4-pentanediol in CHC13 [16] or in MIBK [17].

11.1.3 Ion-exchange and other methods

From a weakly acid solution A1F63-, 8042-, PO4 3-, and NO3- can be retained on anion- exchangers, while boric acid, being only very slightly dissociated, is eluted. Borate ions can be absorbed on anion-exchangers only in neutral or alkaline media. Strongly basic anion- exchangers retain traces of boron as BF4- from dilute solutions of hydrofluoric acid [18]. The

122 11. Boron

boric acid-mannitol complex can also be retained on anion-exchangers [19]. In the analysis of a number of materials (e.g., silicon- and germanium compounds and

volatile reagents) traces of boron are pre-concentrated and boron is then separated by volatilization of the matrix. Mannitol, which forms a non-volatile complex with boric acid, is added to retain all the boron present in the residue [20]. Boron is fairly volatile in acidic media. While boron traces are determined in chlorosilanes, it is advisable to add some chlorotriphenylmethane [21], which forms a non-volatile compound with boron thus preventing its volatilization, when the matrix is evaporated. Ref. 21 is not cited.

Trace amounts of boron can be concentrated by specific adsorption on a column of Sephadex G-25 [22].


The very sensitive curcumin method is often used for determining trace amounts of boron. The carmine-acid method is much less sensitive. Extraction-spectrophotometric methods based on ion-associates of BF4- with Methylene Blue and other basic dyes are of importance in the determination of boron.

11.2.1. Curcumin method

Curcumin is a natural compound extracted from the curcuma root and purified by crystallisation. It has the formula:

HO~CH---CH CH . C H = C H ~ O H \ " " / \c / ~ c / \"J/ / II I kOCH, H3CO 0 OH

(11.1)

The reagent, classified as a [3-diketone dissolves, giving a yellow colour, in methanol, ethanol, acetone, and glacial acetic acid. In acid media, curcumin and boron form a violet- red 2:1 complex called rosocyanin.

The sensitivity of the method and the reproducibility of the results obtained depend on the quality of the curcumin reagent, and on rigorous observance of the reaction conditions (temperature, time, reagent quantities) [23-25]. Commercial curcumin samples differ considerably in quality. Under the most favourable conditions, the molar absorptivity of rosocyanin is 1.8.105 at ~max = 550 nm (a = 16.6).

In a modification of the curcumin method, a ternary complex is formed between curcumin, boron, and oxalic acid [26]. The method is more rapid, but it is only about half as sensitive. The ternary complex formed (rubrocurcumin) contains curcumin, boron, and oxalate in the ratio 1:1:1. Numerous elements (e.g., Be, Fe, Ge, Mo, Ti) form coloured complexes with curcumin, and interfere in the determination of boron. Oxidants (e.g., HNO3) , and substances forming stable complexes with boron (e.g., HF), also interfere. In general, therefore, boron is first separated by distillation as trimethyl borate.


Reagents

Curcumin, 0.1% solution in glacial acetic acid (25 mg of curcumin in 25 ml of the solvent). The solution is prepared on the day of use.

Standard boron solution: 1 mg/ml. Dissolve 0.5716 g of H3BO3 in water, and dilute the solution with water to 100 ml in a volumetric flask.

Mixture of conc. H2804 and glacial CH3COOH (1 + 1). Mix equal volumes of the two acids immediately before use.

Alkaline solution. Dissolve 1 g of NaOH in 100 ml water, add 3 g of glycerol. Store the solution in a polyethylene bottle.

Methanol. Purify by distillation from solid NaOH in a quartz still. Store in a polyethylene bottle.

Quartz distillation apparatus. Distillation flask of capacity 50-75 ml. The distance between the bulb of the distillation flask and the side-arm should be at least 10 cm.

Procedure

Separation of boron by distillation. Evaporate an alkaline sample solution containing a microgram quantity of boron to dryness. If it is necessary to ignite and fuse the residue (e.g., if mannitol is present), a platinum vessel should be used. Add to the solid residue 1-2 ml of conc. H2804, stir with a glass rod, and wash the contents of the vessel with 25 ml of methanol into a distilling flask fitted with a condenser. Immerse the condenser tip in a trapping solution (2 ml of the alkaline solution and 18 ml of H20) contained in a platinum dish. Distil the contents of the still by heating on a glycerol bath; at the end of distillation the temperature should be 120~ After all the methanol has been distilled off, cool the still, add 10 ml of methanol, and repeat the distillation.

Determination of boron. Take an aliquot (containing < 1 ~tg B) of the distillate, and evaporate it to dryness in a platinum crucible. Ignite the residue until all the organic matter has been burned off and the mineral residue has been melted. Place the crucible in a water bath at exactly 60~ add from a pipette exactly 2.5 ml of curcumin solution, and keep the vessel on the bath for about 3 min with occasional stirring. To the cooled vessel, add 1 ml of the HzSO4-CH3COOH mixture, and mix thoroughly by swirling the vessel. After 20 min wash the contents with ethanol (70%) into a 25-ml volumetric flask and dilute to the mark with ethanol. Mix the solution, and measure its absorbance at 550 nm, using the blank solution as a reference.

Notes. 1. The blank solution must be prepared very carefully. 2. If boron is determined without distillation as trimethyl borate, any traces of HF or HNO3 must be

carefully removed before the addition of curcumin (e.g., by evaporating the solution 2 or 3 times with dilute HC1 in the presence of mannitol).

11.2.2. Carminic acid method

Carminic acid belongs to the group of boron reagents derived from a-hydroxyanthraquinone. These reagents give coloured complexes with boron in conc. sulphuric acid medium. Boron occurs as the B 3+ cation in conc. H2804, and as BO + in less concentrated H2804.

Carminic acid (also called carmine or Carmine Red) is a natural product obtained from cochineal. In concentrated sulphuric acid medium carminic acid reacts with boron(III):

124 11. Boron

j B z~

CH 3 0 OH CH3 O " ~

COICHOHII'CH3 B= ,

conc.s,s0,

.o- T Y T -o. .o- T T T -o. HOOC 0 OH HOOC 0 OH (11.2)

The reagent is red (~max -- 520 nm), whereas the boron complex is violet-blue [27]. The molar absorptivity of the complex at 615 nm (vs. reagent solution as the reference) is 5.5.103 (a = 0.51). Boron reacts slowly with carminic acid. The reaction may be accelerated by diluting the H2804 (e.g., to -~92%), but the acid concentration should not be lower than 90%. The absorbance reaches a maximum within 45-60 min, after which it remains constant for a few hours. Oxidising agents and fluoride interfere in the carminic acid method [28-31 ].

Before its determination, boron is normally separated either as volatile trimethyl borate or by other methods. Carminic acid solutions in conc. H2SO4 and glacial CH3COOH were added directly to the chloroform extract of boron with 2,2,4-trimethyl- 1,3-pentanediol [ 15].

Reagents

Carminic acid solution: Dissolve 25 mg of the reagent in 100 ml of conc. sulphuric acid. Standard boron solution: 1 mg/ml. Preparation as in Section 11.2.1.

Procedure

Place 2.5 ml of acidic solution (-~4 M H2804), containing not more than 25 lag of B, in a 25- ml standard flask. Add 5 drops of conc. HC1, 12.5 ml of conc. H2804, and carmine solution up to the mark. Mix the solution thoroughly and set aside for 1 h. Measure the absorbance of the solution at 650 nm, using a blank solution as reference.

Note. If boron is separated by distillation, evaporate the distillate to dryness, and dissolve the residue in 2.5 ml of 4 M H2804.

11.2.3. Methylene Blue method

The sensitive method for determination of boron has been based on an extractable ion- associate of the anionic complex BF4-with Methylene Blue (MB) (formula 48.1) [32-35].

Formation of the boron complex in acidic (H2804) solution is not very rapid after the addition of fluoride in excess; it requires some time at room temperature. The sulphuric acid concentration can be 0.2-0.8 M , and concentrations of -~0.4 M for fluoride and --2.10 -4 M for Methylene Blue are suitable. Under these conditions, BF4- is formed in --30 min. Before extraction of the ion-pair, the sample solution should be diluted to reduce the acidity. 1,2- Dichloroethane is the most recommended solvent for the extraction of the boron ion-pair; a good quality solvent is important for good extraction. Polyethylene separating funnels must be used because of the hydrofluoric acid medium.

The molar absorptivity of the 1,2-dichloroethane solution of the MB-BF4- ion-pair is 8.2.104 (a = 7.6) at 665 nm. Ions giving extractable ion-pairs with MB interfere (e.g., C104-, SCN-). Usually the method is applied after a distillative separation of boron.


Reagents

Methylene Blue (MB), 1.2-10 -3 M (--0.05%) solution. Standard boron solution: 1 mg/ml. Preparation as in Section 11.2.1.

Procedure

To a 100-ml separating funnel, add sample solution containing not more than 2.5 /ag of B, then 2.5 ml of 10% NaF solution, and enough H2SO4 to give, finally, 8 ml of the solution, at concentration of about 0.4 M. After 30 min, add 40 ml of water, 10 ml of the MB solution, and shake the solution with two 10-ml portions of 1,2-dichloroethane (shaking time 1 min). Dilute the clear extract to volume with solvent in a 25-ml standard flask, mix well, and measure the absorbance of the solution at 665 nm, using the blank solution as a reference.

11.2.4 Other Methods

Besides the Methylene Blue, other spectrophotometric methods, based on ion-associates of anionic boron complexes with basic dyes are used. Extractable associates with BF4- are obtained with Nile Blue A (formula 4.32) [7,36,37], Capri Blue (formula 4.31) [38], Malachite Green (formula 4.26, with Me instead of Et), Chrompyrazole II (CHC13, e = 6.7.104 at 595 nm) [40], etc.

In addition to BF4-, other anionic boron complexes, also forming ion-associates with basic dyes, have been applied to determine boron, namely: 2,4-dinitro-l,8-naphthalenediol and Brilliant Green (formula 4.26) (toluene, e - 1.0.105 at 637 nm [41-43], 2,6-dihydroxy- benzoic acid, and Malachite Green (chlorobenzene, e - 9.5.104 [5], 2,3-dihydroxynaphthalene and Crystal Violet (benzene, e = 8.8-104 [44]), mandelic acid, and Malachite Green (benzene, e = 6.5.104 ) [45,46], pyrocatechol derivatives, and Ethyl Violet (toluene, e = 1.05-104 [47,48]. The ion-pair of the salicylate complex of boron with ferroin has also been proposed (CHC13) [49].

1,1'-Dianthrimide (1,1'-dianthraquinonyl amine) reacts with boron in a conc. H2SO4 medium, after heating at 70-90~ for 2-4 h. A complex with the following structure is obtained:

2+

0 O'-'i'-O 0 (11.3)

The colour of 1,1'-dianthrimide in conc. H2804 is olive-green, whereas that of the boron complex is dark blue (e - 1.9.104 at 630 nm) [50].

Azomethine H, the product of the condensation of H-acid (1-amino-8-naphthol-3,6- disulphonic acid) and salicylaldehyde (formula 11.4) is often used for determining boron. The colour reaction is carried out in acetate buffer (pH --5.2) in the presence of ascorbic acid. The absorbance is measured at 415 nm after 2 h.

126 11. Boron

HO3S N=CH~ OH HO /

HO3S /

(11.4)


The curcumin method (in either the rosocyanin or rubrocurcumin version) has been applied for determining trace amounts of boron in: biological materials [10], soils and plants [ 17], waters [51 ], silicon [52], chlorosilanes [20], uranium [1,53], zirconium and its alloys [53,54], nickel [55,56], copper alloys [56], cast iron and steel [12,57-59], beryllium and magnesium [53], and phosphates [2]. This method was also used for determining boric acid admixtures (about 0.05%) in powdered boron [11]. Some synthetic compounds having the structure similar to that of curcumin, were used in determining boron in water [60].

The earminie acid method has been applied for determining boron in geological materials [6,13,61], silicon [62], nickel and cobalt [63], magnesite [64], glass [65], and fertilizers [66]. An automatic method has been applied for determining boron in sewage and in river water [67].

The Methylene Blue method has been utilized in determinations of boron in biological materials [33], soils and rocks [68], steels [32], silicon [7,69], copper and its alloys [34,35], and various chemical materials [70,71 ].

Boron has been determined by the 1,1'-dianthrimide method in biological materials [72], dairy products [73], cast iron and steel [44], and nickel alloys [74].

The method with azomethine H has been used for determining boron in plant materials [75], biological samples [76], plants [77], soils [77-79], water [80], sewage [4,81 ], rocks and bituminous [22,55,82], steel [47], copper, nickel, and cobalt alloys [9], boron nitride [83], and fertilisers [84]. Azomethine H has been utilized in automatic determination of boron [81] and in flow injection analysis (FIA) [75]. Boron has also been determined in plants and soils with the use of 4-methoxyazomethine H [85].

References

1. Onishi H., Ishiwatari N., Nagai H., Bull. Chem. Soc. Jpn, 33, 830 (1960). 2. Kocher J., Bull. Soc. Chim. France, 1962, 1247. 3. Landry J.C., Landry M.F., Monnier D., Anal. Chim. Acta, 62, 177 (1972). 4. Monzo J., Pomares F., de la Guardia M., Analyst, 113, 1069 (1988). 5. Shida J., Suzuki H., Abe S.,Anal. Chim. Acta, 169, 349 (1985). 6. Farzaneh A., Troll G., Neubauer W., Z. Anal. Chem., 296, 383 (1979). 7.Grallath E., Tsch6pel P., K61blin G., Stix U., T61g G., Z. Anal. Chem., 302, 40 (1980). 8. McKinley G.J., Wendt H.F., Anal. Chem., 37, 947 (1965). 9. Ciba J., Stankiewicz W., Matusiak H., Z. Anal. Chem., 321, 592 (1985). 10. Mair J. W., Day H.G., Anal. Chem., 44, 2015 (1972). 11. Grotheer E. W., Anal. Chem., 51, 2402 (1979).

References 127

12. Donaldson E.M., Talanta, 28, 825 (1981). 13. Troll G., Sauerer A.,Analyst, 110, 283 (1985). 14. Betty K.R., Day G.T.,Analyst, 111,455 (1986). 15. Aznarez J., Ferrer A., Rabadan J.M., Marco L., Talanta, 32, 1156 (1985). 16. Aznarez J., Mir J. M.,Analyst., 109, 183 (1984). 17. Aznarez J., Bonilla A., Vidal J.C., Analyst, 108, 368 (1983). 18. Semov M.P., Zh. Anal. Khim., 23, 245 (1968). 19. Barbier Y., Rosset R., Bull. Soc. Chim. France, 1968, 5072. 20. Marczenko Z., Mojski M., Kasiura K., Chem. Anal. (Warsaw), 14, 1331 (1969). 21. Kawasaki K., Higo M., Anal. Chim. Acta, 33, 497 (1965). 22. Yoshimura K., Kariya R., Tarutani T.,Anal. Chim. Acta, 109, I15 (1979). 23. Uppstr6m L.R., Anal. Chim. Acta, 43, 475 (1968). 24. Dyrssen D. W., Novikov Y.P., Uppstr6m L. R., Anal. Chim. Acta, 60, 139 (1972). 25. Quint P., Umland F., Z. Anal. Chem., 295, 269 (1979). 26. Quint P., Umland F., Sommer H.D., Z. Anal. Chem., 285, 356 (1977). 27. Schwing-Weill M.J.,Analusis, 14, 290 (1986). 28. Gupta H.K., Boltz D.F., Mikrochim. Acta, 1974, 415. 29. F16chon J., Kuhnast F.A., Bull. Soc. Chim. France, 1976, 739. 30. Rosenfeld H.J., Selmer-Olsen A.R., Analyst, 104, 983 (1979). 31. Samsoni Z., Szeleczky M.A., Mikrochim. Acta ,1980 I, 445. 32. Blazejak-Ditges D., Z. Anal. Chem., 247, 20 (1969). 33. Yoshino K. et al., Anal. Chem., 56, 839 (1984). 34. Ci~ek Z., Studlarov~ V., Talanta, 31,547 (1984). 35. Kleimenova O.K., Dedkov Yu D., Zavod. Lab., 55, 3 (1989). 36. Gagliardi E., Wolf E., Mikrochim. Acta, 1968, 140. 37. Nicholson R.A., Anal. Chim. Acta, 56, 147 (1971). 38. Buldini P.L., Anal. Chim. Acta, 82, 187 (1976). 39. Xing-Chu Q., Ying-Quan Z., Analusis, 14, 46 (1986). 40. Busev A.I., Yakovlev P. Ya., Kozina G. V., Zh. Anal. Khim., 22, 1227 (1967). 41. Kuwada K., Motomizu S., T6ei K., Anal. Chem., 50, 1788 (1978). 42. T6ei K., Motomizu S., Oshima M., Watari H., Analyst, 106, 776 (1981). 43. Fogg T.R., Duce R.A., Fashing J.L.,Anal. Chem., 55, 2179 (1983). 44. Sato S., Uchikawa S., Anal. Chim. Acta, 143, 283 (1982). 45. Sato S.,Anal. Chim. Acta, 151,465 (1983). 46. Sato S., Talanta, 32, 447 (1985). 47.0shima M., Motomizu S., T6ei K., Anal. Chem., 56, 948 (1984). 48. Oshima M., Shibata K., Gyouten T., Motomizu S., T6ei, Talanta, 35, 351 (1988). 49. Bassett J., Matthews P.J., Analyst, 99, 1 (1974). 50. Gupta H.K., Boltz D.F., Anal. Lett., 4,161 (1971). 51.0stling G.,Anal. Chim. Acta, 78, 507 (1975). 52. Parashar D.C., Sarkar A.K., Singh N., Anal. Lett., 22, 1961 (1989). 53. Hayes M.R., Metcalfe J., Analyst, 87, 956 (1962). 54. Ishikuro M., Kimura J., Bunseki Kaguku, 37, 498 (1988). 55. Andrew T.R., Nichols P.N., Analyst, 91, 664 (1966). 56. Pakalns P., Analyst, 94, 1130 (1969). 57. Tolk A., Tap W.A., Lingerak W.A., Talanta, 16,111 (1969). 58. Thierig D., Z. Anal. Chem. 310, 154 (1982). 59. Vialatte A., A16v~que J., Guinot H., Taupe A.,Analusis, 11,446 (1983). 60. Lambert I.L., Paukstelis J. V., Bruckdorfer R.A., Anal. Chem., 50, 820 (1978).

128 11. Boron

61. Kiss E.,Anal. Chim. Acta, 211,243 (1988). 62. Chen J.S., Lin H.M., Yang M.H., Fresenius'J. Anal. Chem., 340, 357 (1991), 63. Norwitz G., Gordon H., Anal. Chim. Acta, 94,175 (1977). 64. Shelton N.F., Reed R.A., Analyst, 101, 396 (1976). 65. Reed R.A., Analyst, 102, 831 (1977). 66. Peterson H.P., Zoromski D.W., Anal. Chem., 44, 1291 (1972). 67. Lionnel L.J., Analyst, 95, 194 (1970). 68. Stanton R.E., McDonald A.J., Analyst, 91, 775 (1966). 69. Lanza P., Buldini P.L., Anal. Chim. Acta, 70, 341 (1974). 70. Beskova E.S., Zhuravlev G.I., Zh. Anal. Khim., 28,1411 (1973). 71. K6the J., Ackerman G., Z. Anal. Chem., 320, 545 (1985). 72. Kaczmarczyk A., Messer J.R., Peirce C.E., Anal. Chem., 43,271 (1971). 73. Raber H., Likussar W., Mikrochim. Acta, 1970, 577. 74. Burke K.E., Albright C.H., Talanta, 13, 49 (1966). 75. Krug F.J. et al., Anal. Chim. Acta, 125, 29 (1981). 76. Ciba J., Chru~ciel A., Fresenius'J. Anal. Chem., 342, 147 (1992). 77. Carrero P., Burguera J.L., Burguera M., Rivas C., Talanta, 40, 1967 (1993). 78. Chen D. et al., Anal. Chim. Acta, 226, 221 (1989). 79. Kaplan D.I., et al., Soil Sci. Soc. Am. J., 54, 708 (1990). 80. Lopez F.J., Gimenez E., Hernandez F., Fresenius' J. Anal. Chem., 346, 984 (1993). 81. Edwards R.A.,Analyst, 105, 139 (1980). 82. Schucker G.D., Magliocca T.S., Yao-Sin S.,Anal. Chim. Acta, 75, 95 (1975). 83. Mikhailovskaya V.S., Martunova L.M., Buyanovskaya A.G., Sinenko Yu.A., Zh. Anal.

Khim., 47, 1331 (1992). 84. Hofer A., Brosche E., Heidinger R., Z. Anal. Chem., 253,117 (1971). 85. Zaijun L,. Zhu Z., Jan T.,Anal. Chim. Acta, 402, 253 (1999).

Chapter 12. B r o m i n e

Bromine (Br, at. mass 79.91) is a dark red-brown liquid. Saturated bromine water contains 3.6% (w/v) of bromine (at 20~ Bromine forms bromide (Br-) and hypobromite (BrO-) in alkaline solution. The most stable forms of bromine are bromide and bromate (BrO3-). Bromide has reducing properties, whereas bromine (Br2), hypobromite, and bromate are oxidants. Many bromides are sparingly soluble compounds, and soluble bromide complexes are formed with the same metals as form soluble chloride complexes.

12.1 Separation of bromide and bromine

Bromine is volatile and can be distilled from acidified solutions. Bromide is most often separated by distillation after oxidation to bromine [1].

Distillation is carried out in a stream of gas such as air, nitrogen, or carbon dioxide. It is possible to separate iodide, bromide, and chloride from each other by selective oxidation. First, the iodine produced by oxidation of iodide with hydrogen peroxide in phosphoric acid medium (pH -~1) is distilled. Then dilute nitric acid (2.5-4 M) is used to oxidize bromide to bromine. Iodide in the presence of bromide can also be oxidized with nitrite in acetic acid medium. The iodide (and subsequently the bromine) liberated can be separated by extraction into CHC13, CC14, and other solvents [1,2].

Bromide may also be separated by conversion into volatile cyanogen bromide, CNBr, in dilute H2SO4 containing cyanide and chromic acid [3].

Chloride, bromide, and iodide can be separated on a strongly basic anion exchanger, using sodium nitrate solutions of variable concentration as eluents. Bromide can be separated from many anions on the anion-exchanger Dowex 1 [4]. The sparingly soluble silver bromide allows one to separate bromide [5]. For trace amounts, chloride can be used as collector. Other methods of separating halide ions are mentioned in the Section on chlorine.

12.2. Determination of bromide and bromine

The determination of bromide is, in general, based either on indirect methods, or on methods involving preliminary oxidation to bromine. The bromine formed participates in subsequent bromination or oxidation reactions to give coloured products. The bromination of Phenol Red is described below.

12.2.1. Phenol Red method

Phenol Red (the triphenylmethane dye phenolsulphonaphthalein, formula 12.1) reacts with bromine to form tetrabromophenolsulphophthalein (Bromophenol Blue). The change in colour of a solution at pH 5.5 from yellow to violet is very pronounced. The reaction has been the basis of a sensitive spectrophotometric method of determining bromine [1,6-9]. The molar absorptivity, e, of the violet product at ~max ----- 580 nm is 1.14-10 4 (a = 0.14).

130 12. Bromine

H~ H S03H

(12.1)

The oxidation of bromide to bromine, and the bromination of Phenol Red are carried out in a weakly alkaline medium [1]. Calcium hypochlorite can be used as oxidizing agent. The periods of time specified for oxidation (2 min) and for bromination (4 min) must be adhered to strictly (see procedure below). With longer oxidation, bromine is oxidized to bromate.

An automated version of the method has been developed [8], and the FIA technique has been applied [10].

Before the distillation, bromide is oxidized to bromine with a mixture of chromic acid (0.8 M) and H2804 (7 M) [1 ]. This mixture does not oxidise chloride. During the distillation, nitrogen is used to carry the bromine into a trapping solution of 0.1 M sodium sulphite, which is a better scrubber than NaOH. Since traces of chromium(VI) are also carried into the receiver, an additional extraction of bromine is performed. Bromide in the receiver is oxidised with a cold CrO3-HzSO4 mixture, and the bromine formed is extracted into CC14. The bromine is stripped from the organic layer with 2 M ammonia, the ammonia is driven off, and the bromide determined as given below.

Reagents

Phenol Red, 0.01% solution. Dissolve 10 mg of the reagent in 1 ml of 0.1 M NaOH and dilute the solution with water to 100 ml.

Standard bromide solution: 1 mg/ml. Dissolve in water 1.4900 g of potassium bromide dried at 110~ and dilute the solution with water in a volumetric flask to 1 litre.

Borate buffer, pH 8.7-8.8; saturated borax (NazB4Ov.10H20) solution. Calcium hypochlorite, 0.4% solution, filtered. Sodium arsenite, 0.1 M solution (13 g of salt per litre of solution). Acetate buffer, pH 4.6-4.7. Dissolve 68 g of sodium acetate trihydrate in water, add 30

ml of glacial acetic acid, and dilute the solution with water to 1 litre.

Procedure

Place an approximately neutral sample solution (-~ 12 ml), containing not more than 100 gtg of Br-, in a 25-ml standard flask, add 4 ml of the borate buffer, 1 ml of the hypochlorite solution, and shake for 2 min. Add 1.0 ml of the Phenol Red solution, stir well, and let the solution stand for 4 min. Add 1.2 ml of the arsenite solution and 4 ml of the acetate buffer, and dilute to the mark with water. Measure the absorbance of the solution at 580 nm against a blank solution.

12.2.2 Other methods of determining bromide and bromine

A group of other methods for determining bromide is based on the coloured products formed by bromination of various organic compounds by bromine produced in the oxidation of bromide. The reaction with rosaniline gives tetrabromorosaniline [11,12], and fluorescein

12.3. Determination of bromate and perbromate 131

yields tetrabromofluorescein (eosin) [13]. The product of the reaction of bromine with fuchsine (hexabromofuchsine) can be extracted into CHC13 [14].

As with chlorine, it is possible to determine bromine from its oxidizing effect on o- tolidine [15,16]. Another method has been based on bleaching Methyl Orange (formula 12.2) [17,18].

(CH3)zN~N=N-~SO3 H (12.2)

Methyl Orange solutions are bleached, in the presence of bromide, under UV irradiation, and the rate of bleaching is in proportion to the bromide concentration. The method has been applied for determining bromide in the presence of other halides [ 19].

Other indirect methods involve reactions with the mercury(II) complexes of diphenylcarbazone [20] or Methylthymol Blue [21], or the displacement of thiocyanate ion from AgSCN or Hg(SCN)2 by bromide, followed by the spectrophotometric determination of thiocyanate with ferric ions [22,23].

In another method proposed, the bromide is oxidised to BrO3- by the action of persulphate [24,25]. After complete decomposition of the oxidizer, iodide is added in excess and the equivalent amount of iodine, formed in reaction with the BrO3-, is determined spectrophotometrically at 355 nm (e = 7.3.104).

Bromide can also be oxidized to bromine, which is then extracted into CC14. The absorbance of the extract is measured at 417 nm [2], but this method is of rather low sensitivity.

12.3. Determination of bromate and perbromate

The spectrophotometric determination of bromate can be based on the reactions of BrO3- ions with o-arsanilic acid [26], antipyrine (in the presence of nitrite) [27], 1,3,4- trihydroxyanthraquinone-2-carboxylic acid [28], Pyrogallol Red [29], or 2- oxyminedimedonodithiosemicarbazone [30].

Perbromate ions have been extracted into chlorobenzene as ion-associates with Crystal Violet [31], or with Brilliant Green [32]. The analytical properties of perbromate (BrO4-), have been compared with the properties of perchlorate and periodate ions [33]. Chromatographic and electrophoretic methods have been developed for the separation of perbromate from other halogen compounds [34].


The Phenol Red method has been applied for determining bromide in water [7,8,10,35-37], food products [6], sodium chloride [38], and uranium fluorides and oxides [1 ].

The bromination reaction of organic compounds has been utilized for the determination of bromine in water [11] and in alkali-metal chlorides [12].

The reaction of bromine with fluorescein has been applied for determining bromide in blood plasma [39].

Fuchsin has been used for the determination of bromate in drinking water [40].

132 12. Bromine

References

1. Larsen R.P., Ingber N.M., Anal. Chem., 31, 1084 (1959). 2. Collins A.G., Watkins J.W., Anal. Chem., 31, 1182 (1959). 3. Winefordner J.D., Tin Maung, Anal. Chem., 35, 382 (1963). 4. Lundstr6m U., Olin A., Nydahl F., Talanta, 31, 45 (1984). 5. Koch H., Schultze K., Z. Anal. Chem., 210, 90 (1965). 6. Kretzschmann F., Engst R., Mikrochim. Acta, 1970, 270. 7. P6ron A., Courtot-Coupez J., Analusis, 6, 389 (1978). 8. Basel C.L., Defreese J.D., Whittemore D.O., Anal. Chem., 54, 2090 (1982). 9. Jones D.R.,Anal. Chim. Acta, 271,315 (1993). 10. Anf~ilt T., Twengstr6m S.,Anal. Chim. Acta, 179, 453 (1986). 11. Mold~in B., Zyka J., Microchem. J., 13, 357 (1968). 12. Joy E.F., Bonn J.D., Barnard A.J., Anal. Chem., 45, 856 (1973). 13. Oosting M., Reijnders H.F., Z. Anal. Chem., 301, 28 (1980). 14. Cogan E., Anal. Chem., 34, 716 (1962). 15. Creitz E.C., Anal. Chem., 37, 1690 (1965). 16. Scheubeck E., Ernst O., Z. Anal. Chem., 249, 370 (1970). 17. Laitinen H.A., Boyer K.W., Anal. Chem., 44, 920 (1972). 18. Metters-Tuladhar C.H., Ottaway J.M., Anal. Chim. Acta, 66, 291 (1973). 19. Dodin E.I., Kharlamov I.P., Zh. Anal. Khim., 31, 102 (1976). 20. Okutani T., J. Chem. Soc. Jpn., Pure Chem. Sect., 88, 737 (1967). 21. Nomura T., Komatsu S., J. Chem. Soc. Jpn., Pure Chem. Sect., 90, 168 (1969). 22. Kirsten W.J., Lindholm-Franzen I., Microchem. J., 25, 240 (1980). 23. Almuaibed A.M., Townshend A.,Anal. Chim. Acta, 245, 115 (1991). 24. Lundstr6m U., Talanta, 29, 291 (1982). 25. Carlsson A., Lundstr6m U., Olin A., Talanta, 34, 615 (1987). 26. MacDonald J.C., Yoe J.H., Anal. Chim. Acta, 28, 383 (1963). 27. Qureshi M., Qureshi S.Z., Zehra N., Mikrochim. Acta, 1970, 831. 28. Roman Ceba M. et al., Microchem. J., 29, 19 (1984). 29. Escriche J.M., Cabeza A.S., Penella M.M., Estelles M.L., Analyst, 110, 1467 (1985). 30. Salinas F., Sanchez J.C., Gallego J.M., Microchem. J., 37, 145 (1988). 31. Brown L.C., Boyd G.E., Anal. Chem., 42, 291 (1970). 32. Borisova I.V., Zh. Anal. Khim., 36, 2347 (1981). 33. Ossicini L., Balzoni M., J. Chromatogr., 79, 311 (1973). 34. Lederer M., Sinibaldi M., J. Chromatogr., 60, 275 (1971). 35. Jones D.R., Talanta, 36, 1243 (1989). 36. Grosse Yu.I., Zavod. Lab., 59, No 2, 6 (1993). 37. Freeman P.R., Hart B.T., McKelvie I.D., Anal. Chim. Acta, 282, 379 (1993). 38. Emaus W.J., Henning H.J., Anal. Chim. Acta, 272, 245 (1993). 39. Jennings G., Elia M., Clin. Chem. (Washington D.C.), 42, 1210 (1996). 40. Romele I., Achilli M., Analyst, 123, 291, 1998.

Chapter 13. Cadmium

Cadmium (Cd, at. mass 112.40) occurs in its compounds exclusively in the II oxidation state. Unlike Zn(OH)2 and Pb(OH)2, Cd(OH)2 shows no amphoteric properties and is insoluble in excess of NaOH. Cadmium forms ammine, cyanide, halide, and EDTA complexes.


13.1.1. Extraction

Extraction of the cadmium-iodide complex from iodide-H2SO4 solutions with oxygen- containing solvents (e.g., mesityl oxide, 2-ethyl-l-butanol) is a selective recommended separation method (e.g., from zinc) [1-3]. The iodide complex of cadmium can also be extracted with high molecular weight amines in xylene [4], TBP in benzene [5], and with tetra-n-butylammonium iodide in CHC13 [6].

Cadmium has been extracted as the thiocyanate complex with TOPO in hexane. Cadmium can also be separated from numerous metals by extraction as the chloride [7-9] and bromide [7,10] complexes, mostly in the presence of organic bases.

Cadmium may be extracted into CHC13 or CC14 as the dithiocarbamate [11,12]. In the presence of tartrate and cyanide at pH -~11, only Bi, Pb, and TI(III) are co-extracted with cadmium. Dithizone allows a highly selective separation of cadmium. Separation of Cd (and Zn) from Co and Ni has been described [13].

The extraction of cadmium chelates with oxine, chloro-oxine, HTTA, dibenzoylmethane, anthranilic acid, and other chelating reagents has been investigated [14,15].

13.1.2. Ion exchange. Precipitation

Cadmium and zinc have been separated on a Dowex 1 anion-exchanger [16,17]. In 0.1 M HC1 solution (containing 100 g of NaCI per litre), Zn and Cd chloride complexes are retained in the column, while other metals are eluted. First, zinc is eluted from the column with 2 M NaOH (containing 20 g of NaC1 per litre ), then Cd is eluted with 1 M HNO3.

The anionic chloride complex has been utilized to separate Cd from other metals. The anionic bromide- [18,19] and iodide- [20] complexes of cadmium can also be retained on anion-exchangers.

When metals dissolved in 0.5 M HC1 are passed through a strongly acidic cation exchanger, Cd and Sn(IV) are eluted, whereas Zn, Cu, Mn, Ni, Co, U, and Ti are retained on the column. Cd, Sn(IV), Bi, and Hg(II) have been separated from most other metals by elution from a cation exchanger with 0.4 M HBr [21]. The effect of various media on the separation of Cd from other metals on Dowex 50 cation exchange resin has been investigated [22].

Precipitation of cadmium as the sulphide at low acidity (pH-~l.5) allows traces of cadmium to be separated from large quantities of zinc. A small amount of sodium sulphide is added to the solution to precipitate cadmium, other Group II metals, and some of the zinc, which acts as a collector. Further separation from zinc can be achieved by a dithizone extraction. Precipitation of Cd in the presence of KCN allows its separation from Cu. In all

134 13. Cadmium

these cases trace amounts of cadmium have been precipitated in the presence of a suitable carrier element [23].


Two sensitive methods for determining cadmium are presented in detail: the classical dithizone method, and a new method with use of the chelating azo reagent 5-Br-PADAP.

13.2.1. Dithizone method

The cadmium ion reacts with dithizone (Section 4.5) in neutral to strongly alkaline media to give a pink cadmium dithizonate, which is soluble in CC14 and in CHC13. The stability of Cd(HDz)2 in strongly alkaline media (5-20% NaOH) allows cadmium to be extracted from Pb, Bi, Sn(II), and Zn, the dithizonates of which cannot exist under such conditions. Dimethylglyoxime is added to mask nickel and cobalt. Tartrate prevents the precipitation of metals as hydroxides. The noble metals (Au, Pt, Pd, Ag, Hg) and Cu must be removed before cadmium is extracted. They are most simply pre-extracted with dithizone from acid medium.

The dithizone method for determining cadmium [24,25] is very sensitive. The molar absorptivity at )~max = 520 nm is 8.8.10 4 (a = 0.78). The sensitivity of the method increases in the presence of surfactants [26,27].

In the determination of cadmium with dithizone it is possible to avoid the preliminary extraction from the acidic medium. Instead, one can apply a double extraction of cadmium from alkaline solutions containing cyanide: first, from a solution containing 6-7% NaOH and 0.2% KCN, and secondly from a solution containing 6-7% NaOH and 0.01% KCN. Tartaric acid is used as stripping agent after the first extraction. A high concentration of cyanide prevents extraction of cadmium, whereas a relatively low one does not, provided sufficient dithizone is used. Such conditions allow separation of the cadmium from all but traces of Pb and Zn in the first extraction, and the stripping separates cadmium from Ag and Hg.

Dilute hydrochloric acid (pH 1.5) readily decomposes Cd (and Zn) dithizonates, whereas Ni and Co dithizonates are unaffected. Thus, cadmium (together with Zn) may be separated from Ni and Co [13]. Cadmium is separated from zinc, based on the different resistance of their dithizonates to the action of alkali.

To prevent oxidation of dithizone by oxygen during shaking with strongly alkaline solutions (especially those containing Mn) , hydroxylamine should be added to the aqueous solution.

In the presence of 1,10-phenanthroline, at pH 3, a mixed complex, Cd(HDz)2(phen), is extracted into CHC13 ( ~3 = 6.5-104 at )~max = 505 nm) [28]. Cadmium has been extracted with dithizone in the presence of excess of Zn from solutions containing methyltrioctylammonium thiocyanate [29].

Reagents

Dithizone: 0.002% solution in CC14 (preparation as in Section 46.2.1). Standard cadmium solution: 1 mg/ml. Dissolve 1.6310 g of cadmium chloride, dried at

110~ in water containing 2 ml of conc. HC1, and dilute the solution with water to 1 litre. Potassium sodium tartrate, 20% solution (preparation as in Section 27.2.1). Hydroxylamine hydrochloride, 10% solution (preparation as in Section 27.2.1).


Sodium hydroxide, 20% solution, kept in a polyethylene bottle.

Procedure

Acidify a solution containing not more than 20 gg of Cd to pH --2, and shake it with portions of the dithizone solution in CC14 until the colour of the organic phase no longer changes. Discard the CC14 extracts. To the aqueous solution, add tartrate solution (the amount depending on the quantity of metals present), 0.5 ml of 1% HzDm solution in ethanol, and ammonia until neutral. Allow to stand for 1 min, then add 1 ml of hydroxylamine solution, and sufficient NaOH to give a minimum final NaOH concentration -- 5%. Extract the cadmium with portions of dithizone in CC14 (1 ml of 0.002% HzDz corresponds to 4.4 gg of Cd) until the extract is no longer pink. Wash the combined organic extracts with 0.1 M NaOH solution, and water. Dilute the solution with the solvent to the mark in a 25-ml standard flask, and measure its absorbance at 520 nm against CC14.

Note. When determining Cd in a solution containing a considerable excess of Zn, the two-stage extraction with KCN added should be used. After tartrate, hydroxylamine, and HzDm have been added to the sample solution, add NaOH and KCN solutions to give concentrations of-10% of the first and 0.2% of the second. Extract with CC14, and then strip the cadmium with 0.02 M HC1. To the aqueous back-extract, add a little tartrate, NHzOH, NaOH (to 10% concentration), and KCN (to 0.05% concentration). Re-extract with CC14, wash this extract with dilute NaOH and water, and measure its absorbance.

13.2.2. 5 - B r o m o - P A D A P method

2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) (formula 4.3) forms with cadmium an intensely coloured complex in the pH range 9-11, which has been used for determination of cadmium [3,30-33].

The cadmium complex is formed immediately on mixing the reagents and is stable for at least 1 h. The molar excess of chromogenic reagent should be not less than threefold. The molar ratio Cd:reagent in the complex is 1:2. An increase in absorbance is observed when mixed (water-polar organic solvent) media are employed. In aqueous ethanol (50%, v/v), the molar absorptivity is 1.39.105 (a = 1.24) at 556 nm [3]. A similar sensitivity has been obtained in the presence of non-ionic surfactant polyglycol octyl phenyl ether [34]. Alternatively, the coloured complex can be extracted with isoamyl alcohol, CHC13, or benzene. The method is not very selective. A number of metals, such as Zn, Co, Ni, Mn, Cu, Pb, Fe interfere seriously. A preliminary separation of cadmium is usually necessary. A recommended highly selective separation is extraction of Cd as the iodide complex into mesityl oxide [2,3]. The best selectivity of extraction is obtained with --0.01 M KI and --0.1M H2804. In order to ensure a complete separation of interfering metals (e.g., Zn in analysis of zinc samples), the mesityl oxide extract must be washed by shaking with a dilute acidic aqueous solution of KI.

The extractive separation of cadmium from iodide medium decreases the sensitivity of the method a little (e = 1.1-105).

Reagents

5-Br-PADAP, 0.02% (--7.10-4M) solution in ethanol. Standard cadmium solution: 1 mg/ml. Preparation as in Section 13.2.1. Ammonia buffer (pH -~10): 0.5 M solution with respect to ammonia and ammonium

chloride.

136 13. Cadmium

Potassium iodide, 0.2 M (--3.3%) solution.

Procedure

Extraction o f Cd as iodide. Shake a sample solution (-~20 ml, --0.1 M H2804, 0.01 M KI) with 10 ml of mesityl oxide in a separating funnel for 30 s. Wash the extract by shaking (30 sec) with two 10-ml portions of 0.01 M KI solution in 0.1 M H2SO4.

Determination of Cd. Re-extract Cd from the extract obtained above by shaking (30 s) with 5 ml of the ammonia buffer, and then with 5 ml of water. To these aqueous solutions (in a 25-ml standard flask) containing not more than 15 ~tg of Cd, add 10 ml of ethanol, 2 ml of 5-Br-PADAP solution, dilute with water to the mark, mix well, and measure the absorbance of the solution at 556 nm vs. a reagent blank solution.

13.2.3. Other Methods

Apart from the method presented above involving 5-Br-PADAP, many other methods for cadmium determination are based on chelating azo dyes, such as PAN [35-39], PAR [40- 45] (formulas 4.1 and 4.2). The complex of Cd with PAN, in the presence of surfactants, has an absorptivity close to 5.104 [36]. The complex of Cd with PAR, in the presence of cetyldimethylbenzylammonium chloride, can be extracted into CHC13 (e = 9.8.104) [40]. Other surfactants also have been applied [43]. Among other pyridylazo reagents proposed for determining cadmium are: 2-(2-pyridylazo)-l-naphthol-4-sulphonic acid [46] and 2-(5- chloro-2-pyridylazo)-5-dimethylaminophenol (~ = 1.2-105 at 550 nm) [47].

Cadion (p-nitrobenzenediazoaminobenzene-p-azobenzene) (formula 13.1) [48,49] is the basis of a very sensitive method for cadmium. In the presence of the non-ionic surfactant, Triton X-100, the absorptivity is 1.2.105 at 477 nm. Other azo reagents used are bromobenzothiazo (extraction into toluene, ~ = 5.8.104) [50] and Arsenazo III [51 ].

02 (13.1)

Numerous spectrophotometric methods are based on the formation of ion-associates of the anionic iodide complex (CdI42- or CdI3-) [52-55] and bromide (CdBr42-) [56] complexes with basic dyes. In the old flotation-spectrophotometric method (e = 1.3.105) [52], Crystal Violet is used, DIPE as floating solvent and acetone for dissolution of the floated compound. Other dyes that have been used in association with the cadmium iodide complex include Rhodamine 6G [57], Malachite Green (extraction with benzene) [53,54], and a basic azo dye [55].

The cationic complex of cadmium with 1,10-phenanthroline forms with acid dyes the ion associates which have become a basis for extraction-spectrophotometric methods of determining cadmium. Among the dyes used are: Rose Bengal (CHC13, e = 1.0.105) [58,59], Erythrosin (~ - 9.6-104) [60], dibromofluorescein [61], Bromophenol Blue (CHC13, ~ = 5.6.104) [62,63], and Thymol Blue [64].

The macrocycle, tetramethyltetra-azacyclotetradecane (4-Me-cyclam-14), gives selectively with Cd a cationic complex that can be extracted with the acid dye Erythrosin as counter-ion into CHC13 (~ = 1.1-105 at 550 nm) [65]. The cationic complex of cadmium with cryptand(2.2.1), associated with Erythrosin, has been also applied for determining Cd (nitrobenzene-toluene, ~ = 1.05.105) [66].

13.3. Analytical applications 137

The methods of determining cadmium with the use of porphyrins are very sensitive [e = (3-5)-105] [67,68]. Still higher sensitivity is obtained when derivative spectrophotometry techniques are applied [69]. An example of a porphyrin reagent is given by the formula (19.6).

A number of other organic reagents have been applied in determining Cd. The triple complex of Cd with diphenylcarbazone and 1,10-phenanthroline is extractable into CHC13 or benzene (e = 9.4.104 at 536 nm) [70,71]. 1,10-Phenanthroline can be replaced by other organic bases [70]. Other reagents worthy of mention are 2,2'-diquinolyl-2- quinolylhydrazone (e = 9.2.104 at 552 nm) [72], 2,2'-dipyridyl-bis(2-quinolylhydrazone) (in 80% ethanol, e = 5 .1 .10 4) [16], thio-HTTA [73], thiodibenzoylmethane [74], and 2- pyridinediaminobenzene [75].


The dithizone method has been applied in determining cadmium in food products [12], natural waters [19], organic materials [76], zinc sulphide [23], beryllium [17], zirconium alloys [8], uranium compounds [77], Cd-Se and Cd-Te thin films [78]. The flow-injection technique (FIA) has also been applied in determining Cd with dithizone [79,80].

PAN has been used in determinations of Cd in environmental samples [81], industrial waste waters, renal stones, and metal alloys [38]. Cadmium has been determined with PAN by the FIA technique [82].

Cadion has been used in determining cadmium in industrial fluids (in zinc production) [20,83], and in soils [84]. The ion associate of cadmium iodide complex with Malachite Green has been used for determining Cd in fertilizers [85], and industrial fluids (in zinc production); with Crystal Violet for determining Cd in food products and in waste waters [86].

References

1. Gagliardi E., Ttimmler P., Talanta, 17, 93 (1970). 2. Rao T.P., Ramakrishna T.V., Talanta, 29, 227 (1982). 3. Jarosz M., Chem. Anal. (Warsaw), 31, 713 (1986). 4. McDonald C.W., Moore F.L.,Anal. Chem., 45, 983 (1973). 5. Kish P.P., Balog I.S., Zh. Anal. Khim., 31, 1306 (1976). 6. Marczenko Z., Mojski M., Krejzler J., J. Radioanal. Chem., 24, 9 (1975). 7. Watanabe H., Akatsuka K., Bull. Chem. Soc. Jpn., 41,620 (1968). 8. Ghersini G., Mariottini S., Talanta, 18, 442 (1971). 9. Grudpan K., Taylor C.G.,Analyst, 109, 585 (1984). 10. Nakamura K., Ozawa T., Anal. Chim. Acta, 86, 147 (1976). 11. Bajo S., Wyttenbach A., Anal. Chem., 49, 158 (1977). 12. Krylova A.N., Zhulenko V.N., Malyarova M.A., Zh. Anal. Khim., 51, 69 (1986). 13. Marczenko Z., Mojski M., Kasiura K., Zh. Anal. Khim., 22, 1805 (1967). 14. Sundar B.S. et al., Chem. Anal. (Warsaw), 36, 159 (1991). 15. Schweitzer G.K., Randalph D.R., Anal. Chim. Acta, 26, 567 (1962). 16. West K.J., Pflaum R.T., Talanta, 33, 807 (1986). 17. Hibbits J.O., Kallmann S., Oberthin H., Oberthin J., Talanta, 8, 104 (1961). 18. Strelow F.W., Louw W.J., Weinhert C.H., Anal. Chem., 40, 2021 (1968). 19. Korkisch J., Dimitriadis D., Talanta, 20, 1295 (1973).

138 13. Cadmium

20. Hayashibe Y., Sayama Y., Analyst, 121, 7 (1996). 21. Fritz J.S., Garralda B.B., Anal. Chem., 34, 102 (1962). 22. Akki S.B., Khopkar S.M., Z. Anal. Chem., 249, 228 (1970). 23. Kasiura K., Marczenko Z., Zh. Anal. Khim., 22, 1398 (1967). 24. Math K., Freiser H., Talanta, 21, 1215 (1974). 25. Wei-hua Yu., Freiser H.,Anal. Chem., 61, 1621 (1989). 26. Singh A.K., Ratnam B.K., Microchem. J., 39, 241 (1989). 27. Paradkar R.P., Williams R.R., Anal. Chem., 66, 2752 (1994). 28. Akaiwa H., Kawamoto H., Yoshimatsu E., Bull. Chem. Soc. Jpn., 52, 3718 (1979). 29. Akaiwa H., Kawamoto H., Yoshimatsu E., Anal. Sci., 1,297 (1985). 30. Shibata S., Kamata E., Nakashima R.,Anal. Chim. Acta, 82, 169 (1976). 31. Kubafi V., Macka M., Coll. Czech. Chem. Comm., 48, 52 (1983). 32. Nonova D., Stoyanov K., Mikrochim. Acta, 1984 I, 143. 33. Salim R., Shraydeh B., Microchem. J., 34, 251 (1986). 34. Sun Yi, Microchem. J., 36, 386 (1987). 35. Meus M., Rokosz A., Z. Anal. Chem., 303, 374 (1980). 36. Escriche J.M., Estelles M.L., Reig F.B., Talanta, 30, 915 (1983). 37. Rodionova T.V. et al., Zh. Anal. Khim., 44, 1053 (1989). 38. Nambiar D.C., Shinde V.M., Bull. Chem. Soc. Jpn., 68, 2277 (1995). 39. Koby~ecka J., Skiba E., Chem. Anal. (Warsaw), 38, 599 (1993). 40. Nonova D., Pavlova S.,Anal. Chim. Acta, 123, 289 (1981). 41. Tananayko M.M., Vysotskaya T.I., Ukr. Khim. Zh., 48, 629 (1982). 42. V16kova S. et al., Coll. Czech. Chem. Comm., 47, 1086 (1982). 43. Wen-bin Qi, Li-zhong Zhu, Talanta, 32, 1013 (1985). 44. Pilipenko A.T., Safronova V.G., Zakrevskaya L.V., Zh. Anal. Khim., 44, 1594 (1989). 45. Chakravarty S., Mishra R.K., Anal. Sci., 8, 609 (1992). 46. Jan6a~ L. et al., Coll. Czech. Chem. Comm., 4"7, 2654 (1982). 47. Villarreal M., Porta L., Marchevsky E., Olsina R., Talanta, 33, 375 (1986). 48. Watanabe H., Ohmori H., Talanta, 26, 959 (1979). 49. Hsu Chung-gin, Hu Chao-sheng, Jing Ji-hong, Talanta, 27, 676 (1980). 50. Yakovleva V.G., Ivanov Yu.M., Andrejchuk A.M., Zh. Anal. Khim., 31,884 (1976). 51. Michaylova V., Yuronkova L., Anal. Chim. Acta, 68, 73 (1974). 52. Courtot-Coupez J., Guerder P., Bull. Soc. Chim. France, 1961, 1942. 53. Kish P.P., Balog I.S., Zh. Anal. Khim., 32, 482 (1977); 34, 2326 (1979). 54. Garcia I.L., Navarro P., Cordoba M.H., Talanta, 35, 885 (1988). 55. Kish P.P., Balog I.S., Slivakov B.Ya., Zolotov Yu.A., Zh. Anal. Khim., 31, 1114 (1976). 56. Ishchenko N.N. et al., Zh. Anal. Khim., 45, 1557 (1990). 57. Kartikeyan S., Prasada R.T., Iyer C.S., Damodaran A.D., Talanta, 40, 771 (1993). 58. Tananayko M.M., Bilenko N.S., Zh. Anal. Khim., 30, 689 (1975). 59. Ishak C.F., Pflaum R.T.,Analyst, 113, 941 (1988). 60. Tananayko M.M., Bilenko N.S., Zh. Anal. Khim., 34, 1899 (1979). 61. Stolarov K.P., Firyulina V.V., Zh. Anal. Khim., 33, 2102 (1978). 62. Shestidesyatnaya N.L., Voronin O.G., Motyl V.A., Zh. Anal. Khim., 32, 260 (1977). 63. Buhl F., Mikula B., Chem. Anal. (Warsaw), 28, 779 (1983). 64. Mathew L., Rao T.P., Iyer C.S., Damodaran A.D., Mikrochim. Acta, 111,231 (1993). 65. Szczepaniak W., Juskowiak B., Ciszewska W., Anal. Chim. Acta, 156, 235 (1984). 66. Szczepaniak W., Juskowiak B., Chem. Anal. (Warsaw), 32, 121 (1987). 67. Komata M., Itoh J., Talanta, 35, 723 (1988). 68. Kawamura K., Igorashi S., Yotsuyanagi T.,Anal. Sci., 4, 175 (1988).

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69. Ishii H., Satoh K., Satoh Y., Koh H., Talanta, 29, 545 (1982). 70. Yamada E. et al., Anal. Chim. Acta, 138, 409 (1982). 71. Yamada E. et al., Bull. Chem. Soc. Jpn., 55, 3151 (1982). 72. Otomo M., Singh R.B., Anal. Sci., 1, 165 (1985). 73. Solanke K.R., Khopkar S.M., Sep. Sci., 8, 511 (1973). 74. Schuknecht B., R6bisch G., Uhlemann E., Anal. Chim. Acta, 69, 329 (1974). 75. Zhu Y.R. et al., Analyst, 120, 2853 (1995). 76. Analytical Methods Committee, Analyst, 94, 1153 (1969). 77. Korkisch J., Dimitriadis D., Mikrochim. Acta, 1974, 449. 78. Marczenko Z., Mojski M., Czarnecka I., Chem. Anal. (Warsaw), 18, 189 (1973). 79. Klinghoffer O., Ru2i6ka J., Hansen E.H., Talanta, 27, 169 (1980). 80. Burguera J.L., Burguera M., Anal. Chim. Acta, 153, 207 (1983). 81. Raman B., Shinde V.M.,Analyst, 115, 93 (1990). 82. Facchin I., Pasquini C., Anal. Chim. Acta, 308, 231 (1995). 83. Hayashibe Y., Sayama Y., Oguma K., Fresenius'J. Anal. Chem., 355, 144 (1996). 84. Qiu X., Zhu Y., Yan J.,Analyst, 113, 1329 (1988). 85. Gomez Neto J.A. et al.,Anal. Chim. Acta, 308, 439 (1995). 86. Hu J., Qu W.B., Pu B.Y., Mikrochim. Acta, 109, 295 (1992).

Chapter 14. Calcium

Calcium (Ca, at. mass 40.08) occurs in compounds in the II oxidation state. The hydroxide, Ca(OH)2 (solubility 1.3 g/1.), is a strong base. Calcium ions form sparingly soluble compounds with oxalate and carbonate, and also form weak complexes with EDTA and tartrate.

14.1. Methods of separation


A preliminary separation of the Analytical Group I-II metals, which interfere, is often accomplished by precipitation of their sulphides, hydroxides (pH --9), or compounds with oxine or DDTC.

Precipitation with oxalate at pH 3-4 separates calcium from metals which give soluble oxalate complexes [e.g., Fe(HI), A1, and Ti], and also from phosphate. Calcium oxalate can also be precipitated from homogenous solution [ 1 ].

Traces of Ca and Mg are separated from matrices of metals forming cyanide complexes (e.g., Ni, Zn, and Mn) by precipitation as phosphate with a lanthanum collector in an alkaline cyanide medium [2].

Small amounts of Ca can be separated from Mg by co-precipitation with strontium sulphate from aqueous ethanol [3]. Fe(III), Cr, Mn, and phosphate were separated from Ca and Mg by precipitating with a suspension of PbCO3 [4].

14.1.2. Extraction. Ion exchange

Calcium has been extracted (Sr and Ba are not extractable) with a mixture of CC14 and TBP (10-20%) from alkaline medium (0.1-1 M NaOH) as a complex with Azo-azoxy BN (formula 14.1) [5].

0 OH

CH3

(14.1)

During the extraction, the colour of the CC14-TBP phase changes from the orange-red of the Azo-azoxy BN to the red of the calcium Azo-azoxy BN complex. Calcium is stripped from the organic phase with water or dilute HC1. After this stripping, the CC14 solution of Azo- azoxy BN may be reused.


A reagent BT (related to Azo-azoxy BN) has been suggested as an extractant for Ca, Sr, and Mg. The BT reagent is used in cyclohexane-TBP (1 + 1) solution. Calcium can be extracted at lower pH than with Azo-azoxy BN [6].

Calcium can also be separated by extraction of its complexes with HTTA (CC14) [7], di(2-ethylhexyl) dithiophosphate (hexane) [8], and BPHA (CHC13) [9].

Calcium, Sr, and Ba can be extracted as crown-ether complexes [10,11], which enable one to separate those elements [12,13]. Ca (and Sr) was also extracted with pyrazolone derivatives and with trioctylphosphine oxide (cyclohexane) [14]. Calcium can be extracted with Cryptand 2.2.2 using Erythrosin as the counter-anion [14a].

Calcium is often separated from Mg, Sr, Ba, and other metals by cation-exchange chromatography in the presence of EDTA [15,16], EGTA [16,17], chloride [18], and malonate [19]. Calcium has been sorbed on cation-exchange resins with carboxylic groups [20]. Calcium has been separated from a number of metals in HCl-methanol medium [21 ].

Ca and Sr mixtures have been separated on anion-exchange resin from methanol-HNO3 medium [22].


Glyoxal bis(2-hydroxyanil) is often used for the determination of calcium either in aqueous media or after extraction. A sensitive method involving the Chlorophosphonazo III has been described.

14.2.1 Glyoxal bis(2-hydroxyanil) method

Glyoxal bis(2-hydroxyanil) (GBHA, formula 14.2) forms a sparingly soluble red complex with calcium in alkaline media. The compound dissolves in aqueous methanol (1 + 1) media by the substitution of two molecules of methanol for the two water molecules co- ordinatively bonded to the calcium.

N~C--C~N.~ ~ OH OH

~ , , s / c - c ~ ~ 'o. "'c~ o '

(14.2)

The methanol solution of GBHA is colourless. As sodium hydroxide is added and the hydroxyl groups dissociate, the solution becomes more and more intensely yellow. The optimum alkalinity of the solution G-0.04 M NaOH) is that at which the reaction of GBHA with calcium is complete, but the colour of the reagent is still weak.

The absorption maximum of the red-violet complex is at 516 nm. The molar absorptivity of the compound e - 1.8.104 (specific absorptivity 0.45).

The absorbance of the solution should be measured within 10 min of the development of the colour, otherwise a slow fading of the colour will be observed (2-3% in 30 min).

GBHA reacts with a number of metals other than calcium (e.g., barium and strontium). Magnesium and the alkali metals do not interfere. The Analytical Group I-III metals interfere and should be separated. Small quantities may be masked by adding a little KCN and NazS [and some NH2OH when Fe(III) is present]. To prevent the reaction of GBHA with Ba and Sr, a small amount of sulphate or carbonate is added to the sample solution. Phosphate, fluoride, oxalate, tartrate, citrate, and EDTA interfere in the reaction of calcium with GBHA.

142 14. Calcium

When the red complex of calcium with GBHA is extracted with 1,2-dichloroethane in the presence of Zephiramine, the absorbance of the extract remains unchanged for 6 h [26].

Before the determination with GBHA, calcium may be extracted with a 0.01% solution of Azo-azoxy BN in CC14 (containing 20% TBP), and then stripped into dilute hydrochloric acid.

Reagents

Glyoxal bis(2-hydroxyanil) (GBHA), 0.05% solution in methanol. Solutions more than a week old must be discarded.

Standard calcium solution: 1 mg/ml. Dissolve 2.4980 g of CaCO3, dried at 110~ in 40 ml of 2 M HC1. Remove CO2 by boiling, and dilute the solution with water in a volumetric flask to 1 litre.

Azo-azoxy BN, 0.01% solution. Dissolve 12 mg of reagent in 100 ml of CC14. Add 2 ml of TBP to 10 ml of the CC14 solution before it is used for the extraction.

Procedure

Extraction of Ca with Azo-azoxy BN. Add sufficient 5M NaOH to the sample solution to make the NaOH concentration 0.5-1 M. Shake the solution for 1 min with the solution of Azo-azoxy BN. Strip calcium from the extract with a small volume of 1M hydrochloric acid. Determination of Ca. The sample solution should contain not more than 30 ~tg of Ca, and be free from ammonium salts and Analytical Group I-III metals (to mask small amounts of these metals, add -10 mg each of KCN and NazS. Neutralize the sample solution (to pH 7-8) with NaOH or HC1, and add -20 mg of NazSO4. Add 12 ml of the GBHA solution and 1 ml of 1 M NaOH, and dilute the solution with water to the mark in a 25-ml standard flask. After 10 min, measure the absorbance of the solution at 516 nm against a blank solution.

Note. If the Ca is first extracted by the Azo-azoxy BN method, neutralize the acidic back- extract, add the GBHA solution, and proceed as described above.

14.2.2. Chlorophosphonazo I l l method

The colour reaction of Chlorophosphonazo HI (formula 4.11) with calcium ions gives a sensitive spectrophotometric method for calcium [28-30]. The determination may be carried out either in weakly acid solution (pH 2-3) ( ~ = 2.8.104) or in a neutral medium (pH ~-7). In this case the molar absorptivity is 5.4.104 at 668 nm (a= 1.3) [30]. The absorbance maximum of the reagent in neutral medium is at 570 nm.

Analytical Group I-III metals interfere, and, in principle, they should be separated. Microgram amounts of some of them can be masked with EDTA [29]. Magnesium scarcely reacts with Chlorophosphonazo III in acidic media, so calcium can be determined in the presence of a 10-fold amount of magnesium [29]. At about pH 7, magnesium does react with Chlorophosphonazo III. Strontium and barium ions give colour reactions over a wide pH range. Preliminary separation of calcium from Mg, Sr and Ba is possible by extraction with Azo-azoxy BN.

The concentration of alkali metal and ammonium salts influences the colour reaction; the absorbance decreases as ionic strength increases. Thus the concentrations of salts, as well as the pH, should be matched in sample- and standard solutions. Phosphate and borate buffers are recommended.


Reagents

Chlorophosphonazo III, 0.03% solution. Standard calcium solution: 1 mg/ml. Preparation as in Section 14.2.1. Buffer solution, pH 7.5. Mix 50 ml of 0.1 M KHzPO4 solution with 41 ml of 0.1 M

NaOH, and dilute with water to 100 ml.

Procedure

To the sample solution containing not more than 15 gg of Ca, add 2.5 ml of Chlorophosphonazo III solution, and water to -~20 ml. Neutralize with ammonia to pH about 7, and add 1 ml of buffer solution. Transfer the solution to a 25-ml standard flask, dilute to volume with water, and mix well. After 15 min, measure the absorbance of the solution at 668 nm vs. a blank solution.

Note. The sample solution should not contain Analytical Group I-III metals. Trace amounts can be masked by adding one drop of 1% Na2S solution. The Azo-azoxy BN method is convenient for separation of Ca from Mg, Sr and Ba (Section 14.2.1).


Calcichrome (identical with Calcion, formula 14.3) has been proposed as a highly selective, stable reagent for calcium [32-36]. The molar absorptivity is 7.6.103 at 615 nm. HO~S.~O3H

OH N,,,,N OH (14.3)

Many azo reagents have been recommended for determining calcium, e .g . , Arsenazo III (e = 4.4.104 at 650 nm) [37-41], PAR [42,43], Eriochrome Black T [44,45], Carboxynitrazo [46], Hydroxynaphthol Blue [47], and 2-(8-hydroxy-2-quinolylazo)-l-naphthol [48].

Various organic reagents are used for direct determination of calcium, such as murexide (ammonium purpurate) (E = 1.4.104 at 500 nm) [2,49], Metalphthalein [50], Calcein [51,52], Chrome Azurol S (in the presence of 1,10-phenanthroline) [53], Alizarin S [54], 8- hydroxyquinoline (extraction into CHC13 in the presence of n-butylamine or butoxyethanol) [55], and Emodine (1,3,8-trihydroxy-6-methylanthraquinone) [56]. Calcium has been determined as a complex with Emodine, in the presence of Be and Mg, by the derivative spectrophotometry technique. The anionic complexes of calcium with bromo-oxine [57] or HTTA [58] have been extracted into benzene as ion associates with Rhodamine B. Calcium was also determined as a complex with o-cresolphthalein [59-63], or thymolphthalein [64].

A very sensitive method for determining calcium is based on the complex of Ca with the chromogenic macrocyclic reagent (formula 14.4) (1,2-dichloroethane, E - 5.5.104 at 406 nm) [65]. Other diaza-crown ethers have been also used in determinations of Ca (and Mg) [66]. Calcium has been determined after extraction (CHC13 + benzene) with the crown ether and association with Propyl Orange [67].

144 14. Calcium

" _Oo .D

"i (_0 o3 z CH z

NO

(14.4)


The methods involving GBHA have been applied for determining calcium in milk [68], soil, plant material, and natural waters [69], uranium [70], silicate minerals [71], and lithium and barium salts [27].

Chlorophenazo III has been used for determination of calcium in waters [29,30,72-74], snow [74], soil [75], steel [76], molybdenum alloys [77], cobalt [78], aluminium alloys [79], and boric acid [29]. The flow injection technique has also been applied [73,80].

Calcichrome has been used for determination of calcium in soil extracts [81] and in poly(ethylene terephthalate) [82]. Calcium has been determined in fiver water with Propyl Orange after extraction with a crown ether [67].

Arsenazo III has been used for determination of Ca in blood serum [83] and in urea [40]. Calcium has been determined with PAR in waters [42,43,84], and with Eriochrome Black in soil extracts and rocks [45].

Calcium has been determined with o-cresolphthalein in proteins [59], waters [60-62], and soil extracts [62]. The flow-injection technique has been applied [60,62]. Thymolphthalein has been used for determining Ca in blood serum [64]. Calcium has been determined with metalphthalein in water, urea, and pharmaceutical samples [50].

References

1. Grzeskowiak R., Turner T.A., Talanta, 20, 35 (1973). 2. Marczenko Z., Kasiura K., Chem. Anal. (Warsaw), 7, 775 (1962). 3. Kar K.R., Singh G.,Anal. Chim. Acta, 35, 259 (1966). 4. Bezuglyi D.V. et. al., Zh. Anal. Khim., 26, 1010 (1971). 5. Gorbenko F.P., Sachko V.V., Zh. Anal. Khim., 18, 1198 (1963); 24, 15 (1969); 25, 1884

1970). 6. Gorbenko F.P., Nadezhda A.A., Vozdvizhenskaya S.A., Zh. Anal. Khim., 31, 1898 (1976). 7. Sekine T., Dyrssen D., Anal. Chim. Acta, 37, 217 (1967). 8. Shatskaya S.S., Samoilov Yu. M., Zh. Anal. Khim., 42, 272 (1987). 9. Minczewski J., Chwastowska J., R62afiska B., Chem. Anal. (Warsaw), 19, 497 (1974). 10. Strzelbicki J., Bartsch R.A., Anal. Chem., $3, 2247 (1981). 11. Maeda T., Kimura K., Shono T., Z. Anal. Chem., 313, 407 (1982). 12. Mohite B.S., Khopkar S.M., Analyst, 112, 191 (1987). 13. Katayama Y. et al.,Anal. Chim. Acta, 204, 113 (1988). 14. Sasaki T., Umetani S., Matsui M., Tsurubou S., Chem. Lett., 7, 1195 (1994). 14a. Ajgaonkar H.S., Khopkar S.M., Chem. Anal. (Warsaw), 44, 61 (1999). 15. Chwastowska J., Szymczak S., Chem. Anal. (Warsaw), 14, 1161 (1969). 16. Strelow F.W., Weinert C.H., Talanta, 17, 1 (1970). 17. Povondra P., Pfibil R., Talanta, 10, 713 (1963).

References 145

18. Strelow F.W., Van Zyl C.R., Anal. Chim. Acta, 41,529 (1968). 19. Strelow F.W., Van Zyl C.R., Nolte C.R.,Anal. Chim. Acta, 40, 145 (1968). 20. Pesavento M., Biesuz R., Cortina J.L., Anal. Chim. Acta, 298, 225 (1994). 21. Strelow F.W., Anal. Chim. Acta, 127, 63 (1981). 22. Grahek Z. et al., J. Radioanal. Nucl. Chem., 182, 401 (1994). 23. Umland F., Meckenstock K.U., Z. Anal. Chem., 176, 96 (1960). 24. Lindstrom F., Milligan C.W., Anal. Chem., 39, 132 (1967). 25. Kuczerpa A.V.,Anal. Chem., 40, 581 (1968). 26. Nishimura M., Noriki S., Muramoto S., Anal. Chim. Acta, 70, 121 (1974). 27. Gorbenko F.P., Sachko V.V., Zh. Anal. Khim., 18, 1497 (1963); 20, 309 (1965). 28. Ferguson J.W. et al., Anal. Chem., 36, 796 (1964). 29. Lukin A.M., Smirnova K.A., Vysokova N.N., Zavod. Lab., 34, 1436 (1968). 30. Marczenko Z., Ka~owska H., Chem. Anal. (Warsaw), 21, 183 (1976). 31. Novikov E.A., Shpigun L.K., Zh. Anal. Khim., 48, 1326 (1993). 32. West T.S., Analyst, 87, 630 (1962). 33. Close R.A., West T.S., Talanta, 5, 221 (1960). 34. Herrero-Lancina M., West T.S.,Anal. Chem., 35, 2131 (1963). 35. Mendes-Bezerka A.E., Stephen W.I., Analyst, 94, 1117 (1969). 36. Chau L.K., Pruski M., Porter M.D., Anal. Chim. Acta, 217, 31 (1989). 37. Michailova V., Ilkova P., Anal. Chim. Acta, 53, 194 (1971). 38. Michailova V., Kouleva N., Talanta, 21,523 (1974). 39. Morgan B., Artiss J., Zak B., Microchem. J., 44, 15 (1991). 40. Kratochvil B., He X., Can. J. Chem., 68, 1932 (1990). 41. Blanco M. et al., Anal. Chim. Acta, 224, 23 (1989). 42. Gomez E., Estela J.M., Cerda V., Anal. Chim. Acta, 12, 513 (1991). 43. Hernandez O. et al.,Anal. Chim. Acta, 320, 177 (1996). 44. Haj-Hussein A.T., Christian G.D., Microchem. J., 34, 67 (1986). 45. Zolotov Yu.A. et al., Anal. Chim. Acta, 308, 386 (1995). 46. Petrova T.V., Savvin S.B., Dzherayan T.G., Zh. Anal. Khim., 30, 918 (1975). 47. Brittain H.G., Anal. Chem., 49, 969 (1977); Anal. Chim. Acta, 96, 165 (1978). 48. Ishizuki T. et al.,Anal. Chim. Acta, 176, 63 (1985). 49. Gordon H., Norwitz G., Talanta, 19, 7 (1972). 50. Van Staden J.F., Taljaard R.E., Anal. Chim. Acta, 323, 75 (1996). 51. Robinson C., Weatherell J.A., Analyst, 93, 722 (1968). 52. Chimpalee N. et at., Anal. Chim. Acta, 271, 247 (1993). 53. Ganago L., Alinovska L.A., Zh. Anal. Khim., 43, 1972 (1988). 54. Khalifa M.E., Chem. Anal. (Warsaw), 41, 357 (1996). 55. Luke C.L., Anal. Chim. Acta, 32, 221 (1965). 56. Pal T., Jana N.R.,Analyst, 118, 1337 (1993). 57. Bel'tyukova S.V., Poluektov N.S., Zh. Anal. Khim., 25, 1714 (1970). 58. Poluektov N.S., Bel'tyukova S.V., Zh. Anal. Khim., 25, 2106 (1970). 59. Phillipou G., Raniolo E., Clin. Chem., 36, 1385 (1990). 60. Yamane T., Goto E., Talanta, 38, 139 (1991). 61. Nyman J., Ivaska A., Anal. Chim. Acta, 308, 286 (1995). 62. Nogueira A.R., Brienza S.M., Zagatto E.A., Lima J.L., J. Agric. Food Chem., 44, 165

(1996). 63. Van Staden J.F., Malan D., Talanta, 43, 881 (1996). 64. Kruchkova E.S., Pukhova V.M., Maistrenko V.N., Zh. Anal. Khim., 44, 2265 (1989). 65. Nishida H. et al., Mikrochim. Acta, 1981 I, 281.

146 14. Calcium

66. Blair T.L., Desai J., Bachas L.G., Anal. Lett., 25, 1823 (1992). 67. Motomizu S., Oshima M., Yoneda N., Iwachido T.,Anal. Sci., 6, 215 (1990). 68. Nickerson T.A., Moore E.E., Zimmer A.A., Anal. Chem., 36, 1676 (1964). 69. Jacintho A.O. et al.,Anal. Chim. Acta, 130, 361 (1981). 70. Abrao A., Anal. Chem., 37, 437 (1965). 71. King H.G., Pruden G., Analyst, 94, 39 (1969). 72. Zenki M., Anal. Chem., 53, 968 (1981). 73. Yuan Y.,Anal. Chim. Acta, 212, 291 (1988). 74. Zenki M., Ohmuro K., T6ei K., Fresenius'J. Anal. Chem., 338, 707 (1990). 75. Qiu X.C., Zhang Y.S., Zhu Y.Q.,Analusis, 12, 273 (1984). 76. Yakovlev P. Ya., Zhukova M.P., Zavod. Lab., 36, 1169 (1970); 37, 1292 (1971). 77. Zhukova M.P., Shemyakin F.M., Yakovlev P.Ya., Zh. Anal. Khim., 28, 1705 (1973). 78. Zhukova M.P., Yakovlev P.Ya., Zavod. Lab., 39, 661 (1973). 79. Zhukova M.P., Matrosova T.V., Yakovlev P.Ya., Zh. Anal. Khim., 26, 2231 (1971). 80. Yuan Y., Anal. Chim. Acta, 212, 291 (1988). 81. Pakalns P., Florence T.M., Anal. Chim. Acta, 30, 353 (1964). 82. Borshch N.A., Mal'tseva N.G., Ivanova I.V., Zavod. Lab., 47, No 4, 12 (1981). 83. Morgan B.R., Artiss J.D., Zak B., Clin. Chem., 39, 1608 (1993). 84. Gomez E. et al., Analyst, 120, 1181 (1995).

Chapter 15. Carbon

Carbon (C, at. mass 12.01) occurs in compounds principally in the IV oxidation state. Carbonate (CO32-) gives sparingly soluble salts with all cations except Group I metals and the ammonium ion, as well as soluble complexes with U and Th ions. Of the carbon compounds generally considered inorganic, the cyanides (CN-) and thiocyanates (SCN-) are of great importance in analysis. The analytical reactions of cyanide and thiocyanate are similar to those of the halogens. Cyanide gives stable complexes with such metals as Hg(II), Ag, Cu(I), Fe(II and III), Ni, and Co(HI). Coloured thiocyanate complexes [e.g. with Fe(III), Co, Mo, Ti, Re, and Nb] are of major importance in spectrophotometric analysis.

15.1. Separation of cyanide

Since cyanides are very poisonous, suitable precautions should be taken during their separation and determination.

Hydrogen cyanide is such a weak acid that it volatilises from cyanide solutions at pH < 9. Usually HCN is separated by distillation from solutions acidified with tartaric acid to pH --3. Nitrogen or air is passed through the heated solutions to accelerate the separation of HCN. HCN is absorbed in dilute NaOH or NazCO3 solutions [1-3]. Any sulphide present in the sample solution is precipitated with Zn or Cd ions before the distillation of HCN. Complex cyanides are precipitated with zinc ions if it is necessary to separate HCN derived only from simple cyanides (e.g., KCN).

To determine total cyanides in solutions containing both simple cyanides and metal cyanide complexes, more drastic conditions are necessary to decompose the complexes, before the HCN is distilled off. Decomposition of complex cyanides occurs on heating with non-volatile mineral acids (H2SO4, H3PO4) in the presence of, e.g., EDTA or tartaric acid [4]. The cyanide complexes of Zn, Cd, Ni, and Fe(III) are decomposed fairly rapidly. On the other hand, Co(HI), Fe(II), Cu, Hg, and Pd complexes are decomposed only with difficulty. Decomposition of this latter group requires a long heating.

The cyanide complexes of Fe(II), Fe(III), Ni and Zn can be decomposed by grinding them with sulphur and KC1, then heating at 300~ The thiocyanate formed is then determined as the Fe(III) complex [5]. Cyanide, complex cyanides, cyanate, and ammonia have been separated by selective steam distillation at various pH values [6]. Traces of hydrogen cyanide have been separated conveniently by the microdiffusion method (also referred to as isothermal distillation) [7]. Cyanide has been preconcentrated from water by extraction with tributyltin hydroxide in trichloroethylene [8].

15.2. Determination of cyanide

The most important spectrophotometric methods for determining cyanide are based on the formation of polymethine dyes. These methods are highly sensitive and specific. The benzidine-pyridine method is often used. Lower colour stability is a drawback of the pyrazolone method. The barbituric acid method can also be recommended.

148 15. Carbon

15.2.1. Benzidine-pyridine method

In this method, cyanide reacts with bromine water to form cyanogen bromide, CNBr, which then reacts with pyridine to yield glutaconic aldehyde. This aldehyde is condensed with an aromatic amine (benzidine) to form a red polymethine dye, which is the basis for determination of cyanide. The excess of bromine is reduced with As(III). The colour intensity reaches a maximum after 15 min, and remains constant for a further 30 min.

The method is highly sensitive. The molar absorptivity (with respect to CN-) at ) g m a x - -

530 nm is about 6.0.10 4 ( a - 2.3). The colour obtained depends on the reaction conditions [9,10]. When the sample solution is turbid or is itself coloured, the reaction product may be extracted into butanol.

Any thiocyanate present in the sample solution also reacts with bromine to form cyanogen bromide. Since thiocyanate is non-volatile, cyanide can be conveniently separated by distillation as HCN.

The use of p-phenylenediamine has been recommended instead of benzidine, which is an active carcinogen. The molar absorptivity, e = 3.1.104 [11 ].

The glutaconic aldehyde formed in the reaction of cyanogen bromide with pyridine has been condensed with anthranilic acid [ 12]. In the determination of cyanide, use has also been made of the polymethine dye which is formed in the reaction of cyanogen chloride with pyridine and barbituric acid. Chloramine T has been used for the chlorination of cyanide.

This highly sensitive method (e = 1.0.105 at 580 nm) has been studied in detail [13-16]. 4-Methylpyridine has been suggested instead of pyridine [17].

In another method for determining cyanide, cyanogen chloride (obtained by the action

of Chloramine T) reacts with a pyridine-pyrazolone reagent to form a blue dye 0~max = 630 nm) [18,19].

Reagents

Benzidine hydrochloride, 1% solution. Add 0.5 g of the reagent to 50 ml of 0.5 M HC1, heat to boiling, cool, and filter. Keep the filtrate in an amber-glass bottle.

Benzidine-pyridine reagent. Mix 18 ml of redistilled pyridine, 12 ml of water, and 3 ml of conc. HC1. Add 10 ml of the benzidine solution, and shake until a clear solution is obtained. Prepare this solution fresh daily.

Standard cyanide solution, 1 mg/ml. Dissolve 0.2503 g of KCN in cold water which has been previously boiled, and dilute the solution with similar water to volume in a 100-ml standard flask. Store the solution in a polyethylene bottle. This solution is unstable.

Procedure

Place the alkaline solution (about 10 ml) containing not more than 10 ~tg of CN- in a 25-ml standard flask. Acidify the solution with glacial acetic acid and add 1 ml in excess. Immediately add 1 ml of bromine water. Mix the solution thoroughly, and let it stand for 10 min with occasional shaking. Add a 1.5% solution of sodium arsenite dropwise to reduce the

excess of bromine, then 2-3 drops more. Add 5 ml of the benzidine-pyridine reagent, and stir the solution. After 30 sec, add 5 ml of ethanol, make up to the mark with water, and stir well. After 15 min, measure the absorbance of the solution at 530 nm against a reagent blank.

15.2 Methods of determination 149


In an extractive spectrophotometric method for determining cyanide, a coloured ion-pair formed between cyanide complex of Fe(II) and bathophenanthroline is extracted into chloroform [20].

Several indirect methods for determination of cyanide involve the displacement by cyanide of metals from their complexes with organic reagents, with subsequent change in colour of the solutions, such as the methods using Hg(II) complexes with diphenylcarbazone [21 ] and Methylthymol Blue [22].

The cyanide ion can displace Ag from its associates with phen and Bromopyrogallol Red [23], and eosin [24]. Cyanide ions liberate coloured ferroin from the solid [Fe(phen)32+][I3-]2. A method based on displacement of IO3- from the sparingly soluble Pb(IO3)2, with subsequent reaction of the IO3- with iodide added, and determining the liberated I2 as the coloured starch complex has also been proposed [26].

Cyanide enhances the redox potential of copper(II). Spectrophotometric methods for determining cyanide based on this property involve oxidation of phenolphthalein [27]. Phenolphthalein has been used in determining cyanide by the flow injection technique [27].

15.2.3. Analytical applications

The benzidine-pyridine method has been used mostly for determining cyanide in waters and sewage [9,28]. The p-phenylenediamine method has been applied for determining cyanide in water and sewage [7,29-31] and in fodder [32].

The barbituric acid method was used for determining cyanide in sewage [33]. Cyanide was also determined by this method in industrial waste-water using the FIA technique [34].

The pyridine-pyrazolone method has been used in the determinations of cyanide in water [18], sewage [18,19], blood [35], free cyanide in ferro- and ferricyanides [36] and in cyanide complexes of Pt and Pd, after decomposition of the complexes [37].

15.3. Separation and determination of other carbon compounds

Thiocyanate is separated from cyanide by distilling off HCN from a weakly acid medium. With bromine and chloramine-T, thiocyanate is converted into cyanogen bromide and cyanogen chloride, respectively, and determined as a polymethine dye by the benzidine- pyridine method or pyridine-barbituric acid method [38-40]. The latter method has been applied for continuous determination of thiocyanate in blood plasma and in urine [40].

In the absence of cyanide, thiocyanate can be determined as the thiocyanate complex FeSCN 2+, formed in acid medium with excess of Fe(III) [41 ]. Thiocyanate may be extracted and determined as the ion-association complexes formed by SCN- with Methylene Blue (1,2- dichloroethane) [42], Rhodamine B (benzene) [43], and ferroin (nitrobenzene) (44).

In an indirect method, thiocyanate and Hg(II) form a mixed-ligand mercury complex with quinoline, which can be extracted into chloroform. This complex can be treated with a CHC13 solution of dithizone to form the Hg(HDz)2 complex [45].

Ferrocyanide [hexacyanoferrate(II), Fe(CN)64-] can be determined as the Prussian Blue formed by addition of Fe(III) in hydrochloric acid solution [46]. The Prussian Blue is extractable into chloroform in the presence of dimethyl-lauryl-benzylammonium bromide [47]. Ferrocyanide decomposes, with liberation of HCN, on heating with tartaric acid and

150 15. Carbon

EDTA. The hydrogen cyanide is distilled off and determined by the barbituric acid method. Ferrieyanide [hexacyanoferrate(III), Fe(CN)63-] can be determined using its oxidising

properties, e.g., by the oxidation of 5-sulphobutylamide-2'-methoxyphenylanthranilic acid in 4 M NaOH medium [48]. Ferricyanide has been separated from ferrocyanide by extraction (butanol, amyl alcohol), associated with triphenyl-n-propylphosphonium ion. The absorbance of the extract has been measured at 420 nm [49].

Carbon monoxide can be determined by reduction of silver in alkaline solutions of Ag p-sulphanoylbenzoate and measuring the absorbance of the coloured Ag sol obtained [50- 52]. The method has been used for determining CO in air and in blood. Carbon monoxide has also been determined (in the presence of NO, SO2, and CO2) by reduction of Ag(I) from its salt with sulphanilic acid [53]. The reduction of Pd(II) by CO followed by reaction with KIO3 to form ICln- ions which form ion-pairs with Pyrronine Y (extraction with benzene) has also been used for determination of CO [54]. The yellow cacotheline is reduced to the violet dihydroxycacotheline by Pd formed in the reduction of PdC142- complex by CO [55].

Carbon dioxide is determined in gas mixtures on the basis of its acidic properties, e.g., by passing the gas through a solution of NaHCO3 in the presence of a pH indicator such as Phenol Red or phenolphthalein [56,57]. Carbon traces in titanium, zirconium, and other metals have been determined by oxidising the sample with a mixture of PbCrO4 and V205 at 1,200 ~ and absorption of the CO2 evolved in an alkaline solution of Thymol Blue [58].

References

1. Willekens G.J., Van Den Bulcke A.,Analyst, 104, 525 (1979). 2. Zhu Y.R., Wei F.S., You F., Talanta, 30, 795 (1983). 3. Blanco M., Maspoch S., Talanta, 31, 85 (1984). 4. Leschber R., Schlichting H., Z. Anal. Chem., 245, 300 (1969). 5. Tobia S.K., Gawargious Y.A., E1-Shahat M.F., Analyst, 99, 544 (1974). 6. Hilbert F., Darwish N.A., Z. Anal. Chem., 255, 357 (1971). 7. Hangos-Mahr M., Pungor E., Kuznetsov V.,Anal. Chim. Acta, 178, 289 (1985). 8. Wroflski M., Talanta, 28, 255 (1981). 9. Higson H.G., Bark L.S., Analyst, 89, 338 (1964). 10. Kaur P., Upadhyay S., Gupta V.K.,Analyst, 112, 1681 (1987). 11. Bailey P.L., Bishop E., Analyst, 97, 691 (1972). 12. Upadhyay S., Gupta V.K.,Analyst, 109, 1619 (1984). 13. Lambert J.L., Ramasamy J., Paukstelis J.V., Anal. Chem., 47, 916 (1975). 14. Nagashima S.,Anal. Chim. Acta, 99, 197 (1978). 15. Broderius S.J., Anal. Chem., 53, 1472 (1981). 16. Csikai N.J., Barnard A.J. Jr.,Anal. Chem., 55, 1677 (1983). 17. Nagashima S., Anal. Chim. Acta, 91, 303 (1977). 18. Goulden P.D., Afghan B.K., Brooksbank P., Anal. Chem., 44, 1845 (1972). 19. Nagashima S., Anal. Chem., 55, 2086 (1983). 20. Mariaud M., Levillain P., Talanta, 34, 535 (1987). 21. Okutani T., Utsumi S., J. Chem. Soc. Jpn., Pure Chem. Sect., 87, 444 (1966). 22. Nomura T., Bull. Chem. Soc. Jpn., 41, 1619 (1068). 23. Dagnall R.M., E1-Ghamry M.T., West T.S., Talanta, 15, 107 (1968). 24. Buhl F., Kania K., Chem. Anal. (Warsaw), 24, 689 (1979). 25. Lambert J.L., Manzo D.J., Anal. Chem., 40, 1354 (1968). 26. Verma K.K., Tyagi P., Ekka M.G., Talanta, 33, 1009 (1986). 27. Haj-Hussein A.T., Talanta, 44, 545 (1997).

References 151

28. Royer J.L., Twichell J.E., Muir S.M.,Anal. Lett., 6, 619 (1973). 29. Montgomery H.A., Gardiner D.K., Gregory J.G., Analyst, 94, 284 (1969). 30. Casapieri P., Scott R., Simpson E.A., Anal. Chim. Acta, 49, 188 (1970). 31. Botto R.I., Karchmer J.H., Eastwood M.W., Anal. Chem., 53, 2375 (1981). 32. Harris J.R., Morson E.H., Hardy M.J., Curtis D.J., Analyst, 105, 974 (1980). 33. Drikas M., Routlej B.I.,Analyst, 113, 1273 (1988). 34. Rios A., Luque de Castro M.D., Valcarce! M., Talanta, 31, 673 (1984). 35. Baar S.,Analyst, 91, 268 (1966). 36. Kruse J.M., Thibault L.E.,Anal. Chem., 45, 2260 (1973). 37. Gilbert B.L., Olson B.L., Reuter W., Anal. Chem., 46, 170 (1974). 38. Giraudi G., Grillo C., Anal. Chim. Acta, 128, 169 (1981). 39. Nagashima S., Anal. Chem., 56, 1944 (1984). 40. Sharma A., Thibert R.J., Mikrochim. Acta, 1985 I, 357. 41. Whiston T.G., Cherry G.W., Analyst, 87, 819 (1962). 42. Koh T., Iwasaki I., Bull. Chem. Soc. Jpn., 40, 569 (1967). 43. Guerrero A.H., Roig A.M.,Anal. Chem., 45, 1943 (1973). 44. Yamamoto Y., Tarumoto T., Hanamoto Y., Bull. Chem. Soc. Jpn., 42, 268 (1969). 45. Einaga H., Ishii H., Talanta, 20, 1017 (1973). 46. Roberts R.F., Wilson R.H., Analyst, 93, 237 (1968). 47. Galik A., Vopravilova J., Talanta, 21,307 (1974). 48. Mushtakova S.P. et al., Zavod. Lab., 51, No 6, 6 (1985). 49. Senise P., Oliveira R.L.,Anal. Chim. Acta, 81,419 (1976). 50. Ciuhandu G., Chieu A., Z. Anal. Chem., 255, 35 (1971). 51. Bock R., Bockholt B., Z. Anal. Chem., 260, 274 (1972). 52. Bell D.R., Reiszner K.D., West P.W., Anal. Chim. Acta, 77, 245 (1975). 53. Gorska R.W., Jarym-Agaeva N.T., Zh. Anal. Khim., 43, 651 (1988). 54. Selvapathy P., Pitchai R., Ramakrishna T.V., Talanta, 37, 539 (1990). 55. Lambert J.L., Chiang Y.C.,Anal. Chem., 55, 1829 (1983). 56. Aminot A., Kerouel R., Analusis, 18, 289 (1990). 57. Edwards R.T. et al., Anal. Chim. Acta, 261, 287 (1992). 58. Nazarenko V.A., Biriuk E.A., Antonovich V.P., Zavod. Lab., 33, 22 (1967).

Chapter 16. Chlorine

Chlorine (C1, at. mass 35.45) exists at room temperature as a gas (C12) which has oxidizing properties. Chlorine occurs in several oxidation states: chloride -I, hypochlorite I, chlorite III, chlorate V and perchlorate VII. Chloride exhibits reducing properties towards powerful oxidants, such as Mn(VH) and Ce(IV). In its other oxidation states, chlorine has oxidizing properties. Hypochlorite and chlorite are rather unstable, and are subject to gradual disproportionation into chloride and chlorate. Of the chloro-anions, only chloride reveals strong complex-forming capacity.

16.1. Separation of chloride and chlorine

Small amounts of chloride can be separated from many other elements by precipitation, as silver chloride, from dilute HNO3 [1 ]. Bromide, iodide, and thiocyanate are also precipitated. Chloride has been separated as PbCI2 with lead phosphate as a collector [2].

Chloride is often oxidized to chlorine and separated by distillation [3] (see the Methyl Red method). The chloride is oxidized with periodate, permanganate, or cerium(W), in a sulphuric acid medium, after separation of Br- and I- by oxidation to Br2 and I2 with potassium periodate in dilute HNO3.

Chloride may also be separated by distilling the volatile hydrogen chloride, which is collected in a solution containing excess of silver ions [4].

Chromatographic separations of mixtures of halide ions by using strongly basic anion exchange resins have been proposed [5].

16.2. Determination of chloride and chlorine

Chloride is often determined as AgC1 by the simple turbidimetric method, but spectrophotometric methods are more accurate, and more sensitive. Many such methods are based on the oxidation of chloride to chlorine, which undergoes subsequent redox reaction resulting in either the appearance or disappearance of colour (e.g., the Methyl Red method described below). In direct methods involving chloride ions, advantage is taken of the higher stability of the colourless Hg(II) chloride complex in comparison with that of other coloured Hg(II) complexes.

16.2.1. Methyl Red method

The sample solution is placed in a still with an oxidizer which oxidizes chloride to C12. The chlorine is steam-distilled and trapped in a receiver containing an acidified solution of Methyl Red (formula 16.1). The oxidation of Methyl Red by chlorine results in a partial bleaching of the solution, thus orovidin~ a basis for an indirect method of determining chloride [6]. cooH

(CH,)zN~N~---'N ~Q=~ (16.1)

One molecule of the dye reacts with two molecules of chlorine (C12). The difference in

16.2. Determination of chloride and chlorine 153

absorbance of the Methyl Red solution before and after the reaction with chlorine enables one to calculate the value of e which expresses indirectly the sensitivity of the method. The molar absorptivity ~ = 1.17.104 (a = 0.33) at ~max : 515 nm.

Chloride is oxidized with potassium periodate in a dilute sulphuric acid solution. The solution of KIO4 in H2804 is first heated to distil off any chlorine formed from chloride impurities in the reagents. The optimum acid concentration for the Methyl Red colour reaction with chlorine is 1 M H2804. Bromide and iodide interfere in this method. The presence of 5 ~tg of Br- increases the results by 60-70% in the determination of 5 ~tg of C1-. Positive errors owing to iodide are smaller and less reproducible, than those owing to bromide.

Reagents

Methyl Red, 0.0005% solution in 1M H2804. Dissolve 50.0 mg of the dye in 1 M H2804 and dilute the solution with the acid in a volumetric flask to 1 litre. Dilute 25 ml of this solution with 1 M H2804 in a volumetric flask to 250 ml.

Standard chloride solution: 1 mg/ml. Dissolve in water 1.6486 g of sodium chloride, previously ignited at 400-500~ and dilute the solution with water to volume in a 1-1itre standard flask.

Periodate reagent. Add 200 ml of conc. H2804 to 300 ml of water, and mix. Place the cooled solution in a 750-ml distillation flask, and add 12 g of KIO4. When the salt has dissolved, add a few fragments of porous porcelain, connect the flask to the condenser, and distil off and discard the first 200 ml of water. To the cooled solution in the distillation flask add 200 ml of water, with stirring. Add a fragment of porcelain, and again distil off 200 ml of water. Transfer the cooled chlorine-free reagent solution to a glass-stoppered bottle.

Procedure

Preliminary preparation. Place 25 ml of the periodate reagent in the distillation flask, add 10 ml of water, and stir well. Add a few fragments of porous porcelain and distil 10 ml of water into a receiver containing 5 ml of the Methyl Red solution. Transfer the solution from the receiver to a 25-ml standard flask, make up to the mark with water, and measure the absorbance of the solution against water. The absorbance measured should not be lower than the absorbance of the solution obtained by diluting 5 ml of the Methyl Red solution with water in a 25-ml standard flask. If the absorbance is lower, add 10 ml of water to the distillation flask, and repeat the procedure described above.

Separation and determination of C1. Introduce 10 ml of the sample solution containing not more than 50 ~tg of C1 (chloride) into the distillation flask containing the periodate reagent prepared as described above. Mix the solution thoroughly, add a fragment of porcelain, and distil 10 ml of water together with the separated chlorine into the receiver containing exactly 5 ml of the Methyl Red solution. Transfer the solution from the receiver to a 25-ml standard flask, and dilute to the mark with water. Measure the absorbance of the partly decolorized red solution at 515 nm against water.

Notes. 1. A weighed portion of the sample can be added to the distillation flask containing the periodate reagent, and 10 ml of water are then added.

2. Complete discoloration of the Methyl Red solution implies that too large a sample was taken. 3. The weight of a sample is limited by the solubility of the sample in the periodate. During distillation, the solution in the flask should remain clear.

154 16. Chlorine

4. The same aliquot of periodate reagent in the distillation flask can be used for several successive determinations of chloride (e.g., when preparing a calibration curve).

16.2.2. Turbidimetric method

The method involves comparing the turbidity formed when silver nitrate is added to an acidified (HNO3) sample solution containing chloride, with the turbidity formed in standard solutions. The method is simple but has rather low precision. Instead of visual comparison of the turbidities in colorimetric cylinders, the absorbance can be measured with a photometer.

The development of turbidity of AgC1 is a rather slow process. A time lapse of 15-20 rain after the addition of silver nitrate is necessary to allow the AgC1 precipitate to become stabilized. Variations of temperature within the range 20-30~ and of acidity between 0.01 and 0.1M HNO3 have no effect on the determination.

An aqueous acetone medium stabilizes the suspension [7], but commercial acetone may contain chloride, and should, therefore, be purified by distillation in the presence of sodium hydroxide. Higher concentrations of electrolytes and organic compounds have adverse effects on the formation of the AgC1 sol.

Ions which form precipitates with silver nitrate in acid medium, i.e., bromide and iodide, interfere in turbidimetric determinations.

It has been proposed that one might precipitate AgC1 in the presence of gelatine under exposure to short-wave ultraviolet radiation. The absorbance of the resulting violet-brown colour is measured at 435 nm [ 8].

Reagents

Silver nitrate, 0.1 M solution (--2%). Standard chloride solution: 1 mg/ml (preparation as in Section 16.2.1.). Potassium sulphate, 2% solution (chloride-free). Barium nitrate, 2% solution (chloride-free).

Procedure

Co-precipitation o f C l with a collector. Acidify the sample solution (100-200 ml) with HNO3, add 5 ml of the K2SO4 solution, and heat to -~80~ Add dropwise a mixture of 6 ml of the Ba(NO3)2 solution, and 2 ml of the AgNO3 solution with stirring. Keep the solution at -~ 60~ for 1 h, then cool and filter, using paper washed free from chloride, and wash the precipitate three times with --0.01 M HNO3 by decantation. Discard the filtrate, and add 10 ml of 2 M NH3 to a beaker containing the precipitate of BaSO4 and AgC1. Thoroughly mix the liquid and the precipitate, refilter (using the same filter paper), and wash the filter paper and the precipitate with dilute ammonia solution. The filtrate contains the separated chloride.

Determination of chloride. Place the sample solution (or the ammoniacal filtrate obtained as described above), containing not more than 40 gg of chloride, in a Nessler cylinder, acidify with dilute HNO3 (to make the final solution 0.05 M HNO3), dilute with water to 40 ml, add 2 ml of AgNO3 solution, and stir well. Prepare at the same time a series of standards covering the range 0-40 lag of C1- in exactly equivalent Nessler cylinders. Let the cylinders stand for 15 min in the dark, and then compare the turbidity obtained in the sample solution with that of the standards. The cylinders should be observed from above, against a black background, in a brightly and uniformly lit location.

16.2. Determination of chloride and chlorine 155

16.2.3 Other methods for determining chloride and chlorine

Several methods for determining small amounts of chlorine are based on the oxidation of certain organic reagents in a colour reaction, which serves as a basis for spectrophotometric determination. The same principle is used in determining chlorides after they are oxidized to chlorine. Among the organic reagents used in this procedures are: Methyl Red (discussed above), o-tolidine (formula 16.2) (e = 3.4.10 4) [1,9,10], Methyl Orange (e = 4.0.10 4) [10- 12], and dicarboxidine [13]. The method has been applied also in the FIA technique [10].

HzN ~ ' ~ . _ ~ ~ ~ ~ ~ NHz (16.2)

In the benzidine-pyridine method [14,15], chlorine reacts with cyanide ions to form cyanogen chloride (CNC1). The product of the reaction of CNC1 with pyridine (glutaconaldehyde) is condensed with primary amines to form polymethine dyes.

Chlorine in air can be determined by passing air through an alkaline solution of 4- nitroaniline and measuring the absorbance of the compound formed (e= 1.9.10 4 at 485 nm) [ ! 6]. Chlorine in tap-water has been determined by adding phenolphthalein (in the reduced form) and ferrocyanide. The ferricyanide ions formed produce an equivalent amount of red phenolphthalein (in alkaline medium) [17].

When a solution containing chloride is passed through a column of granular silver iodate, the chloride displaces an equivalent amount of IO3-, which reacts with I- added to give iodine (3 molecules of I2 per mole of C1- ion), which can be determined by the starch- iodine method (see Section. 25.2.1) [18,19].

In the indirect thioeyanate method (not very sensitive, e~5.103) the determination of chloride [20-28] has been based on the displacement of SCN- ion from the mercury(H) thiocyanate complex by chloride ions, to give a stable mercury chloride complex. After addition of Fe(III) in excess, the red Fe(SCN) 2+ complex is formed, and the absorbance is measured at 480 nm. In the FIA method the UV detection has been applied in the absence of Fe(III) ions [29].

In another indirect method for determining chloride use is made of the fact that HgC12 is more stable than the violet Hg(II) diphenylearbazone complex [30,31 ]. The violet colour of the solution is a function of the chloride concentration. In other indirect methods chloride displaces Methylthymol Blue [32] or Xylenol Orange [33] from their complexes with mercury(H).

16.3. Determination of other chlorine compounds

Perchlorate ions form extractable ion-associates with basic dyes such as Brilliant Green (e= 9.4.104) [34,35], Malachite Green [36,37], Crystal Violet [38], Methylene Blue [39], and the oxidized form of Variamine Blue [40]. Chloroform, dichloroethane, benzene, nitrobenzene and toluene have been used as extractants. Perchlorate impurities in potassium chlorate have been determined by these sensitive methods [34]. The FIA technique has been applied in the determinations [35].

Methods based on extractable ion-pairs of C104- with bathoferroin [41], and neocuproin [42] are less sensitive. In an indirect method, perchlorate has been determined by extraction

156 16. Chlorine

with the tetrabutylphosphonium ion into o-dichlorobenzene, followed by displacement of the C104- by the Fe(III)-thiocyanate complex anion to form a coloured complex. The method was used for determining 0.003% C104- in potassium chlorate [43].

The oxidizing properties of chlorate are utilized in its spectrophotometric determination. Chlorate admixtures in perchlorates have been determined by its colour reaction with benzidine [44] and bis-thiosemicarbazone dimedone (e= 1.9.104 at 417 nm) [45]. Chlorate in water has been determined with o-tolidine [46]. At a suitably high chloride concentration in 3-7 M H2804, chlorate reacts quantitatively with C1- to form C12, which decreases the absorbance of Methyl Orange [47].

Chlorine dioxide is determined from its absorbance as the coloured product ()Lmax = 490 nm) of its reaction with tyrosine [48]. Chlorine dioxide has been found to reduce the absorbance of Acid Chrome Violet K [49]. Spectrophotometric methods of determining C102 with the use of Chlorophenol Red have been proposed [50,51]. A scheme for analysing mixtures of CI-, C10-, C102-, C103-, C104-, and C102 using spectrophotometric and other methods has been devised [52].


The Methyl Red methods have been used for determining trace amounts of chloride in chemical reagents (sulphates, phosphates, oxides, hydroxides) [6]. The method cannot be used for determining traces of chloride in nitrates, as some nitric acid can be, formed which oxidizes and bleaches Methyl Red.

When free chlorine is determined in tap water, a sample of water is placed in a still (with no oxidizer) and chlorine is distilled, along with some water, into a receiver containing Methyl Red. The sample water may also be added directly to a Methyl Red solution.

The turbidimetric method has been used in automated determinations of chloride in natural and potable waters [53].

Chlorine has been determined in water [14,15] and in selenium [54] by the benzidine- pyridine method.

The indirect thiocyanate method has been applied for determining chloride in blood serum [27], water [2,55,56], plants [57,58], industrial waste waters [24], geological materials [59], organic substances [60,61], barium sulphate [28], silicate minerals [62], and catalysts [63]. The FIA technique was used in determination of chlorine in ethanol [64].

A sensitive method for determining chlorine in tap water has been based on the reaction of chlorine with thio-Michler's ketone (formula 46.2) (~ = 7.7.104 at 640 nm). The method has been applied in the FIA technique [65]. The reaction of chlorine with 4-nitroaniline has been applied in the continuous method of determining chlorine in air [66].

Another method of determining chlorine in air ias been based on the liberation of an equivalent amount of iodine, which as 13- forms an extractable ion associate with Crystal Violet [67].

References

1. Scheubeck E., Ernst O., Z. Anal. Chem., 254, 185 (1971). 2. Rodabaugh R.D., Upperman G.T., Anal. Chim. Acta, 60, 434 (1972). 3. Elsheimer H.N., Johnston A.L., Kochen R.L., Anal. Chem., 38, 1684 (1966). 4. Reichel W., Acs L., Anal. Chem., 41, 1886 (1969). 5. Zalevskaya T.L., Starobinets G.L., Zh. Anal. Khim., 24, 721 (1969).

References 157

6. Marczenko Z., Lenarczyk L., Chem. Anal. (Warsaw), 11, 1221 (1966). 7. Palei P.N., Udal'tsova N.I., Zavod. Lab., 30, 151 (1964). 8. Hoffmann H.E., Z. Chem., 17, 147 (1977). 9. Johnson J.D., Overby R.,Anal. Chem., 41, 1744 (1969). 10. Leggett D.J., Chen N.H., Mahadevappa D.S., Analyst, 107, 433 (1982). 11. Laitinen H.A., Boyer K.W., Anal. Chem., 44, 920 (1972). 12. D'Amboise M., Meyer-Grail F., Anal. Chim. Acta, 104, 355 (1979). 13. Gr6ningsson K., Analyst, 104, 367 (1979). 14. Nicolson N.J., Analyst, 90, 187 (1965). 15. Webber H.M., Wheeler E.A., Analyst, 90, 372 (1965). 16. Gabbay J., Davidson M., Donagi A.E., Analyst, 101, 128 (1976). 17. Shahine S.A., Mahmoud R.M., Microchem. J., 21, 286 (1976). 18. Mor E.D., Beccaria A.M., Poggi G., Anal. Chim. Acta, 99, 361 (1978). 19. Utsumi S., Yokota J., Isozaki A., Bunseki Kagaku, 29, 703 (1980). 20. Elsheimer H.N., Kochen R.L., Anal. Chem., 38, 145 (1966). 21. Florence T.M., Farrar Y.J., Anal. Chim. Acta, 54, 373 (1971). 22. Kirsten W.J., Lindholm-Franzen I., Microchem. J., 25, 240 (1980). 23. Vekid B., Ra~em D.,Anal. Chim. Acta, 193, 331 (1987). 24. Van Staden J.F.,Anal. Chim. Acta, 261,453 (1992). 25. Yoshinaga T., Ohta K.,Anal. Sci., 6, 57 (1990). 26. Tanase I., Ioneci I., Dumitrescu V., Rev. Chim. (Bucarest), 40, 1004 (1989). 27. Van Staden J.F., Talanta, 38, 1033 (1991). 28. Torrades F., Castellvi M., Fresenius'J. Anal. Chem., 349, 734 (1994). 29. Cirello Egamino J., Brindle I.D., Analyst, 120, 183 (1995). 30. Kemula W., Hulanicki A., Janowski A., Talanta, 7, 65 (1960). 31. Novak J., Hauptman Z., Z. Anal. Chem., 217, 340 (1966). 32. Nomura T., Komatsu S., J. Chem. Soc. Jpn. Pure Chem. Sect., 90, 168 (1969). 33. Humphrey R.E., Hinze W.L., Anal. Chem., 45, 1747 (1973). 34. Burns D.T., Tungkananuruk N., Anal. Chim. Acta, 199, 237 (1987). 35. Burns D.T., Chimpalee N., Harriott M.,Anal. Chim. Acta, 217, 177 (1989). 36. Akiyama S. et al., Bull. Chem. Soc. Jpn., 56, 947 (1983). 37. Galban J., Urarte M.L., Aznarez J., Microchem. J., 41, 84 (1990). 38. Uchikawa S., Bull. Chem. Soc. Jpn., 40, 798 (1967). 39. Iwasaki I., Utsumi S., Kang C., Bull. Chem. Soc. Jpn., 36, 325 (1963). 40. Tusakova N.N., Mushtakova S.L., Frumina N.S., Zavod. Lab., 46, 22 (1980). 41. B~kowski W., Ba:kowski A., Microchem. J., 29, 137 (1984). 42. Weiss J.A., Stanbury J.B., Anal. Chem., 44, 619 (1972). 43. Fogg A.G., Burns D.T., Yeowart E.H., Mikrochim. Acta, 1970, 974. 44. Burns E.A., Anal. Chem., 32, 1800 (1960). 45. Leyva J.A., Mochon M., Diaz N., Misiego M., Mikrochim. Acta, 1984 III, 295. 46. Urone P., Bonde E., Anal. Chem., 32, 1666 (1960). 47. Yamasaki S. et al., Bunseki Kagaku, 28, 566 (1979). 48. Tumanova T.A., Pakhomova L.N., Maiorova L.P., Zavod. Lab., 36, 1036 (1970). 49. Knechtel J.R., Janzen E.G., Davis E.R., Anal. Chem., g0, 202 (1978). 50. Fletcher I.J., Hemmings P.,Analyst, 110, 695 (1985). 51. Sweetin D.L., Sullivan E., Gordon G., Talanta, 43, 103 (1996). 52. Prince L.A., Anal. Chem., 36, 613 (1964). 53. Ramirez-Munoz J.,Anal. Chim. Acta, 74, 309 (1975). 54. Barasel D., Jaetsch K., Z. Anal. Chem., 249, 234 (1970).

158 16. Chlorine

55. Rodabaugh R.D., Upperman G.T., Anal. Chim. Acta, 60, 434 (1972). 56. Ru~i6ka J., Stewart J.W., Zagatto E.A.,Anal. Chim. Acta, 81, 387 (1976). 57. Fessel Graner C.A., Paulucci J.B.,Anal. Chim. Acta, 123, 347 (1981). 58. Allen C.R., Allen S., Anal. Biochem., 173, 54 (1988). 59. Chan C.C., Lab. Rob. Autom., 2, 83 (1990). 60. Rowe R.D., Anal. Chem., 37, 368 (1965). 61. Marquardt R.P., Anal. Chem., 43, 277 (1971). 62. Huang W.H., Johns W.D., Anal. Chim. Acta, 37, 508 (1967). 63. Koshy V.J., Garg V.N., Talanta, 34, 905 (1987). 64. Krug F.J. et al., Anal. Chim. Acta, 130, 409 (1981). 65. Zenki M., Komatsubara H., T6ei K.,Anal. Chim. Acta, 208, 317 (1988). 66. Shina A., Gabbay J.,Analyst, 111, 183 (1986). 67. Lomonosov S.A., Shukolukova N.I., Chernoukhova V.I., Zh. Anal. Khim., 28, 2389 (1973).

Chapter 17. Chromium

Chromium (Cr, at. mass 52.00) occurs in a series of coloured species corresponding to the II-, III-, and VI- oxidation states. In spectrophotometric analysis, Cr 3+, CrO42-, and Cr207 z- ions are determined. Chromium(III) hydroxide, which precipitates at pH-5, is amphoteric, dissolving in strongly alkaline media. Chromium(III) forms inert complexes, e.g., with oxalate, tartrate and EDTA. Chromium(II) is a strong reducing agent, and chromium(VI) has oxidizing properties.


17.1.1. Extraction

Comprehensive studies of the behaviour of traces of chromium, and of methods of separation, have been presented [ 1 ].

A selective, and fairly simple method for separating chromium is the extraction of chromium(VI) with MIBK from 1-3 M hydrochloric acid [1,2]. The method enables chromium to be separated from most elements (e.g., V, Fe, Mn, and Ni). Only In, T1, Sb, Hg, W, and Re are co-extracted if present in significant quantities (>1%). The recommended procedure involves extracting Cr(VI) with 2 portions of MIBK from 2 M HC1, washing the extract with 2 M HC1, and stripping the chromium with water. Chromium(VI) has been extracted with mesityl oxide from 1 M HC1 and 2.5 M KC1 medium, and stripped with dilute ammonia [4].

Inert solvents (e.g., CHC13, C6H6, 1,2-dichloroethane) extract ion-associates of chromate with high molecular-weight amines, e.g., TOA [4], tribenzylamine [5], and 4-(5- nonyl)pyridine [6], from acid media (HC1, H2SO4, HC104, HNO3). TOPO in benzene and a tetraphenylarsonium salt in CHC13 have also been used [ 1 ].

In an acidic medium (pH---1.7) and at a temperature not higher than 10~ chromium(VI) reacts with H202 (-0.02 M) to form blue perchromic acid which can be extracted into ethyl acetate, isoamyl alcohol, or similar oxygen-containing solvents. These methods permit the separation of Cr from V, Fe, and most other metals [7]. Perchromic acid can also be extracted with solutions of tertiary amines or quaternary ammonium salts [8,9].

The anionic thiocyanate complex of chromium(III) has been extracted with MIBK or cyclohexanone [10] , and with 5% TOA in CC14 [11] . The complexes of Cr(III) with NTA [12] or DCTA [13,14] have been extracted with solutions of Aliquat 336 in CHC13 or 1,2-dichloroethane.

Methods of solvent extraction of chromium have been reviewed [ 15].


Traces of chromium(III) are precipitated as the hydroxide with NaOH (in moderate excess), or with ammonia. Fe(III), A1, Zn, or La (2-4 mg) are used as collectors [1,16,17].

When a sample is fused with Na202 or NazCO3 (with or without KNO3), Cr(III) is oxidized to Cr(VI), which is leached from the melt with water, whereas Fe, Mn (reduced with ethanol from MnO42- to MnO2 aq), Cu, Ni, Co, Ti, and most other metals remain in the solid phase. Losses of chromium owing to retention in the precipitate are insignificant. Such

160 17. Chromium

elements as V, Mo, As, and A1 pass into the alkaline solution together with Cr(VI), when the melt is leached with water. Chromium(III) can be oxidized in a hot alkaline solution (NaOH) to soluble CrO42- with H202 or bromine. Traces of chromium(VI) can be separated by co- precipitation with barium or lead sulphate [18].

Since chromium(m) is not retained on anion exchangers when 0.02-12 M HC1 is used as eluent, it can be separated from a number of metals [1]. Strongly basic anion-exchangers retain V(V), Cr(VI), and Mo(VI) from acetate solution at pH 2.5-3. These metals are washed out with 0.6 M NaOH, 8 M HC1, and 1 M HC1, respectively. Many metals can be separated from chromium by retaining Cr(VI) ions on strongly basic anion exchangers [19,20]. Water samples containing chromate are adjusted to pH --5 and passed through an anion-exchange resin bed (C1-). The chromate is eluted with a solution of an acidic reductant [21 ].

The distillation of chromium as CRO2C12, normally applied for separation of macro- amounts of Cr, has also been applied with success in the separation of traces. The distillation is carried out at 200-210~ from a perchloric acid medium, through which CO2 and HC1 are bubbled [ 1 ].


Chromium is usually determined by the diphenylcarbazide method. This method is particularly useful for determining traces of chromium. Larger amounts of chromium can be determined either by the chromate method, or by the method based on the Cr(III)-EDTA complex.

17.2.1. Diphenylcarbazide method

1,5-Diphenylcarbazide reacts in acid medium with chromium(VI) ions to give a violet solution which is the basis of this sensitive method. It has been shown that the cationic Cr(III)-diphenylcarbazone complex is formed by oxidation of diphenylcarbazide with chromium(VI). It has been assumed that the reaction proceeds with nascent, not yet hydrated Cr(III) ions, obtained in the reduction of Cr(VI) with diphenylcarbazide which is oxidized to diphenylcarbazone:

0 0 (17.1)

The molar absorptivity, ~, of the coloured product of the chromium(VI) reaction with 1,5- diphenylcarbazide is 4.3.104 (a = 0.83) at )Lmax = 545 nm. The colour intensity obtained in the reaction depends on the quality of the 1,5-diphenylcarbazide used [22,23].

The oxidation of Cr 3+ to Cr2072- is critical for the determination of Cr by the diphenylcarbazone method. The oxidation is normally done in acid medium with KMnO4 or with (NH4)28208 in the presence of silver ions. The excess of MnO4- is either decomposed by azide or precipitated as MnO2 aq in the presence of Mn 2+ ions. The excess of persulphate is either decomposed by boiling the solution or reduced by azide. Fusing solid samples with sodium carbonate or sodium peroxide converts Cr(III) into Cr(VI). Since the absorbance of the solution varies with acidity, the pH should be kept constant at the optimum value of pH --1


(--0.05 M H2SO4). The presence of HC1 should be avoided. The diphenylcarbazide method is almost specific for chromium(VI). Interferences result

only from Fe, V, Mo, Cu, and Hg(II) present at much higher concentrations than the chromium. Iron(III) can be masked by phosphoric acid or EDTA. Iron(III) can also be separated as Fe(OH)3, after chromium has been oxidized to Cr(VI), or by extraction. Vanadium can be separated from Cr(VI) by extraction as its oxinate at pH --4. Molybdenum is masked with oxalic acid, and Hg(II) is converted into the chloride complex.

When small amounts of Cr(III) are to be determined in the presence of Cr(VI), the Cr(III) is first separated by precipitation of the hydroxide, with A1 or Fe(III) as the collector and ammonia as the precipitant. The precipitate is then dissolved, and the Cr(III) is oxidized to Cr(VI) and determined with diphenylcarbazide. Traces of Cr(VI) were determined in Cr(III) after Cr(VI) had been leached with 0.1 M NaOH [24].

The preliminary extraction of Cr(VI) before its determination with 1,5-diphenylcarbazide has been discussed elsewhere [3,25].

Reagents

1,5-Diphenylcarbazide, 0.2% solution in acetone. Dissolve 0.2 g of the reagent in 100 ml of acetone containing 1 ml of H2SO4 (1 +9). Keep the solution in an amber-glass bottle.

Standard Cr(VI) solution: 1 mg/ml. Dissolve in water 2.8300 g of KzCr207 previously dried at 140~ and dilute the solution with water to volume in a 1 litre standard flask.

Potassium permanganate, -.0.02 M solution. Sodium azide, NAN3, 2.5% solution.

Procedure

Place in a beaker a solution containing not more than 20 ~tg of Cr. If the solution contains chloride add a little sulphuric acid, and evaporate to fumes. Cool the residue, add --15 ml of water and 3 or 4 drops of KMnO4 solution, cover the beaker with a watch glass, and heat without boiling for 15 min. The acidity of the solution should at this point be 0.05- 0.1 M H2SO4. If the pink colour disappears in the course of heating, add more KNInO4 solution dropwise. Reduce (decolorize the pink solution) the excess of oxidant (MnO4- or MnO2 aq.) by adding sodium azide solution dropwise, waiting a few sec after the addition of each drop.

Transfer the cooled solution to a 25-ml standard flask, add 1 ml of the diphenylcarbazide solution, dilute the solution with water to the mark, and mix thoroughly. Measure the absorbance at 545 nm, using water as the reference.

17.2.2. Chromate method

The method for the determination of chromium, based on the colour of Cr2072- or CrO42- ions, is an example of a precise but rather insensitive method. It may be based either on the greenish-yellow colour of CrO42- ions present in alkaline solution, or on the orange-yellow colour of dichromate Cr2072- ions, formed from CrO42- ions by acidification of the solution.

Figure 17.1 shows the absorption spectra of dichromate (1) and chromate (2). The absorP3tion peaks lie in the near-ultraviolet at 350 nm (~ = 7.5.102) and at 373 nm (~ =

The molar absorptivity of dichromate at 400 nm is only 1.6.10 (a - 1.4.10 ), respectively. �9 �9 �9 �9 2 0.003). The concentration of the acid in dichromate solutions affects the colour. An approximately 1 M H2SO4 medium is most suitable.

If chromium is present as Cr(III), it must first be oxidized to Cr(VI). In acid media (H2SO4, HC104), chromium can be oxidized with permanganate, persulphate in the presence

162 17. Chromium

of Ag + ions as catalyst, bismuthate, or periodate. Perchloric acid oxidizes chromium(HI) when heated to boiling (-~200~

t,..)

O l

! 350 373 t , 0 0 , . , 500

wavelengm, nm

Fig. 17.1. Absorption spectra of dichromate (in 1 M H2S04) (1) and chromate (in ammoniacal medium) (2)

In alkaline media, Cr(III) can be oxidized with bromine, H202, or Na202. Fusion with an alkaline flux (NazCO3, Na202) also converts chromium into Cr(VI). The alkaline medium enables chromium(VI) to be separated easily from the majority of metals which form coloured solutions and give sparingly soluble hydroxides [Fe(III), U(VI), Ce(IV), Cu, Co, Ni]. Any coloured MnO4- ions formed during the oxidation are reduced with sodium azide or oxalic acid.

In the presence of phosphoric acid which may be added to mask Fe(III), the colour of the solution is slightly changed owing to the formation of the mixed chromate-phosphate ions HCrPO72- and HzCrPO7-.

In the extraction-spectrophotometric methods, the ion-association complexes of dichromate ions (in dilute H2SO4 medium) with tribenzylamine, TOA or Aliquat 336 [4], tetraphenylarsonium ion [26], or 1-naphthylmethyltriphenylphosphonium ion [27] are extracted into CHC13 or 1,2-dichloroethane.

Reagents

Potassium permanganate,--0.02 M solution. Sodium azide, NAN3, 2.5% solution. Standard chromium(VI) solution: 1 mg/ml. Preparation as in Section 17.2.1.

Procedure

Oxidize the chromium to Cr(VI) in a solution containing not more than 3 mg of Cr, as described in the diphenylcarbazide method. [More KMnO4 will be needed, in this case, if the Cr(III) content is greater].

Neutralize the chromium(VI) solution, and then add 5 ml of H2SO4 (1+3) . Make the solution up to the mark with water in a 25-ml standard flask, and measure the absorbance at 400 nm against water.

Notes. 1. Chromium(VI) standard solution is used to construct the calibration curve. The procedure is thereby simplified, the oxidation of chromium being unnecessary.

2. For absorbance measurement of chromate an alkaline medium is required; 0.1 M sodium or potassium carbonate solution is suitable.


17.2.3. EDTA method

Ethylenediaminetetraacetic acid (EDTA, formula 17.2) forms coloured complexes with cations which have chromophoric properties (e.g., Fe, Cr, Cu, Co, Ni). These complexes, which are not very intensely coloured, form the basis of several less sensitive spectrophotometric methods, such as that for chromium(HI)

o>.,0,, HO "N"--CH2"--CHz~N-- 0 H~~CH2 C/ "CH2C(o H

(17.2)

With chromium(III), EDTA forms a violet complex in slightly acidic medium. The complex is formed slowly in the cold, but more rapidly if the solution is heated [28,29]. The sensitivity of the method is not high. The molar absorptivity is 1.4.102 at 540 nm (a = 0.003). The colour intensity diminishes as the pH is reduced. In a hot solution EDTA reduces Cr(VI) to Cr(III). This reaction is catalyzed by traces of Mn(II).

Coloured ions, and those giving coloured complexes with EDTA, interfere in the determination of chromium as its EDTA complex. Oxalic and citric acids interfere in the colour reaction.

The chromium(III-EDTA complex has been extracted by a solution of Aliquat-336 in non-polar solvents before the spectrophotometric determination [30].

As with EDTA (Complexone III) , DCTA ( Complexone IV) has also been used as a spectrophotometric reagent for chromium [13,14,17,31,32]. The Cr(III)-DCTA complexes have been extracted with chloroform solutions of Aliquat 336 [13,14].

Reagents

EDTA, 2.5 % solution of the disodium salt. Standard chromium(HI) solution: 1 mg/ml. Dissolve 9.1970 g of chrome alum

CrNH4(SO4)2.12H20 in water containing 2 ml of conc. H2SO4, and accurately dilute the solution with water to 1 litre. Working solutions are obtained by suitable dilution of the standard solution with 0.01 M H2804.

Procedure

To the sample solution (10-15 ml) containing not more than 3 mg of Cr, add a little NHzOH.HCI and 3 ml of the EDTA solution. Adjust the solution with ammonia to pH 4-5, and heat to boiling. Continue gentle boiling for 2 min, then cool and dilute the solution to volume with water in a 25-ml standard flask. Measure the absorbance at 540 nm, using water as the reference.


The azo reagents are often used for determination of chromium as Cr(III). Cr was also determined with PAR [33]. The methods based on the ion-association compound of Cr 3+ with PAR and Zephiramine (CHC13, ~;= 4.7.104 at 540 nm) [34] or xylometazolinium cation (e = 4.8.104 at 530 nm) [35] are much more sensitive. A lesser sensitivity is attained in the method based on the ternary complex with PAR and H202. Sensitive methods are based on 5-Br-

164 17. Chromium

PADAP (formula 4.3, e = 7.9.104 at 600 nm) [37-39]. Cr(III) has also been determined with TAR (e = 5.0.104) [40], TAN [41] and Arsenazo III (~ = 2.6.104) [42]. Because of the considerable inertia of the Cr(III) complexes all the reactions involving azo reagents should be carried out after a short heating to 90-100 ~

Methods for chromium with use of triphenylmethane reagents are less sensitive; examples are: Chrome Azurol S (~ = 5.9.104) [43,44], Xylenol Orange (~ = 1.0.104) [45], and Malachite Green [46]. A much higher sensitivity is obtained when Cr(III) is determined with the use of Eriochrome Cyanine R in the presence of surfactant CTA (~ = 6.8.104) [47]. Cr(III) was determined with Eriochrome Cyanine by means of the derivative spectrophotometry [48].

Other organic reagents recommended for determining chromium include phenylfluorone [49] and o-nitrophenylfluorone (in the presence of CTA) (e = 1.1.105 at 582 nm) [50], Methylene Blue (~ = 8.3.104) [51], Pyrogallol Red [52], morin [53], and 2,2'-diquinoxalyl [54]. Chromium was also determined by a sensitive method based on the ion-associate of Cr(VI) with Rhodamine 6G in aqueous solution containing poly(vinyl alcohol) [55], and after extraction into toluene [56].

In an indirect method, the chromium(VI) is used to oxidize Fe(II) to Fe(III), which then forms a coloured complex with Tiron [57]. In another method, a known amount of iron(II) is added in excess to the Cr(VI) solution and the surplus is determined with ferrozine [58].


The flow-injection technique (FIA) has been applied for determining chromium(VI) with 1,5- diphenylcarbazide [59-64], Cr(VI) and Cr(III) in the same solution [65-67], and Cr in the presence of iron [68]. The product of the Cr reaction with diphenylcarbazide was floated with lauryl sulphate [69]. Ion exchange on a mixed ion-exchange bed has been also used for the pre-concentration of that product [70].

1,5-Diphenylcarbazide has been used for determining chromium in biological samples [71,72], water [16,18,73-77], soil [78,79], industrial waste waters [80], wood [81], various minerals [24,82,83], ilmenite [84], sapphires and rubies [85], tin [86], palladium 87], rhenium and its compounds [88], cerium dioxide [89], and salts of alkali metals [6].

Diphenylcarbazide was also applied in the determination of chromium speciation in silicon [90], and after preliminary separation of Cr(III) and Cr(VI) by HPLC [91 ].

The chromate method has been applied for determining chromium in, for example, steels [26], catalysts [92], and natural waters [93,94]. The EDTA method was used for determining Cr in soils and refractory materials [95] and in ores [96,97].

Among the other methods mentioned above, Chrome Azurol S was used for determining Cr in steel [44], and Rhodamine 6G was used for determining Cr in steels, geological samples, and sewage [51].

References

1. Beyermann K., Z. Anal. Chem., 190, 4; 191, 346 (1962). 2. Katz S.A., McNabb W.M., Hazel J.F.,Anal. Chim. Acta, 25, 193 (1961); 27, 405 (1962). 3. Shinde V.M., Khopkar S.M., Z. Anal. Chem., 249, 239 (1970). 4. Adam J., Pfibil R., Talanta, 18, 91 (1971). 5. Donaldson E.M., Talanta, 27, 779 (1980). 6. Iqbal M., Ejaz M.,Anal. Chem., 47, 936 (1975). 7. Tuck D., Anal. Chim. Acta, 27, 296 (1962). 8. Sastri M.N., Sundar D.S., Anal. Chim. Acta, 33, 340 (1965).

References 165

9. Barakat S.A., Burns D.T., Harriott M.,Anal. Chim. Acta, 272, 135 (1993). 10. Sukhanovskaya A.I. et al., Zh. Anal. Khim., 25, 1563 (1970). 11. McClellan B.E. et al., Anal. Chem., 46, 306 (1974). 12. Irving H.M., A1-Jarrah R.H., Anal. Chim. Acta, 60, 345 (1972). 13. Adam J., Pfibil R., Talanta, 21, 1205 (1974). 14. Kinhikar G.M., Dara S.S., Talanta, 21, 1208 (1974). 15. Rao V.M., Sastri M.N., Talanta, 27, 771 (1980). 16. Chuecas L., Riley J.P., Anal. Chim. Acta, 35, 240 (1966). 17. Fuhrman D.L., Latimer G.W., Talanta, 14, 1199 (1967). 18. Yamazaki H.,Anal. Chim. Acta, 113, 131 (1980). 19. Fritz J.S., Sickafoose J.P., Talanta, 19, 1573 (1972). 20. Mulokozi A.M.,Analyst, 97, 820 (1972); Talanta, 20, 1341 (1973). 21. Pankow J.F., Janauer G.E., Anal. Chim. Acta, 69, 97 (1974). 22. Willems G.J., Lontie R.A., Anal. Chim. Acta, 51, 544 (1970). 23. Friese B., Umland F., Z. Anal. Chem., 286, 107 (1977). 24. Mocak J., Vani6kova M., Labuda J., Mikrochim. Acta, 1985 II, 231. 25. Adam J., Pfibil R., Talanta, 21, 616 (1974). 26. Baishya N.K., Barnah G., J. Indian Chem. Soc., 59, 389 (1982). 27. Burns D.T., Kheawpintong S., Anal. Chim. Acta, 177, 253 (1985). 28. Den Boef G., Poeder B.C., Anal. Chim. Acta, 30, 261 (1964). 29. Fuji Y., Kyuno E., Tsuchiya R., Bull. Chem. Soc. Jpn., 42, 1569 (1969). 30. Irving H.M., A1-Jarrah R.H.,Anal. Chim. Acta, 63, 79 (1973). 31. Fuhrman D.L., Latimer G.W., Jr., Talanta, 14, 1199 (1967). 32. Panel J., Coll. Czech. Chem. Comm., 50, 2840 (1985). 33. Ivanov V.M., Golubitskii G.B., Gen'i L.I., Zh. Anal. Khim., 40, 36 (1985). 34. Yotsuyanagi T. et al.,Anal. Chim. Acta, 67, 297 (1973). 35. Anjaneyulu Y., Reddy M.R., Kavipurapu C.S.,Analyst, 111, 1167 (1986). 36. Ferng W.B., Parker G.A., Z. Anal. Chem., 304, 382 (1980). 37. Wei Fu-sheng et al., Mikrochim. Acta, 1982 II, 67. 38. Guo-zhen Fang, Cui-ying Miao, Analyst, 110, 65 (1985). 39. Rathaiah G.V., Eshwar M.C., Analyst, 111, 61 (1986). 40. Subrahmanyam B., Eshwar M.C., Mikrochim. Acta, 1976 II, 579. 41. Rathaiah G.V., Eshwar M.C., Bull. Chem. Soc. Jpn., 58, 2447 (1985). 42. Tataev O.A., Guseinov V.K., Zh. Anal. Khim., 30, 935 (1975). 43. Pantaler R.P., Pulyaeva I.V., Zh. Anal. Khim., 50, 1634 (1985). 44. Fang G.Z., Liu J.K., Talanta, 39, 1579 (1992). 45. Tataev O.A., Abdulaev P.P., Zh. Anal. Khim., 25, 930 (1970). 46. Parkash R., Bansal R., Kaur A., Rehani S.K., Talanta, 38, 1163 (1991). 47. Jarosz M., Biernat J., Chem. Anal. (Warsaw), 33, 685 (1988). 48. E1-Sayed A.Y., Abd-Elmottaleb M., Anal. Lett., 27, 1727 (1994). 49. Izquierdo-Hornillos et al., Anal. Chim. Acta, 142, 325 (1982). 50. Wen-bin Qi, Li-zhong Zhu, Talanta, 33, 694 (1986). 51. Kamburova M., Talanta, 40, 713 (1993). 52. Sicilia D., Rubio S., Perez Bendito D., Anal. Chim. Acta, 284, 149 (1993); 297, 453

(1994). 53. E1-Sayed A.Y., Khalil M.M., Talanta, 43, 583 (1996). 54. Baranowska I., Microchem. J., 26, 55 (1981). 55. Liu Shaopu, Wang Fuchang, Talanta, 38, 801 (1991). 56. Maheswari V., Balasubramanian N., Chem. Anal. (Warsaw), 41, 569 (1996). 57. Abdullah K.A., Hassan Y.I., Bashir W.A., Microchem. J., 27, 319 (1982).

166 17. Chromium

58. Bet-Pera F., Jaselskis B., Analyst, 106, 1234 (1981). 59. De Andrade J.C. et al.,Analyst, 108, 621 (1983). 60. De Andrade J.C., Rocha J.C., Baccan N., Analyst, 109, 645 (1984). 61. Whitaker M.J.,Anal. Chim. Acta, 174, 375 (1985). 62. Alonso-Chamarro J., Bartroli J., Barber R., Anal. Chim. Acta, 261, 219 (1992). 63. Peixoto C.R., Goshikem Y., Baccan N., Analyst, 117, 1029 (1992). 64. Andrade F.J., Tudino M.B., Troccoli E., Analyst, 121, 613 (1996). 65. De Andrade J.C., Rocha J.C., Baccan N., Analyst, 110, 197 (1985). 66. Ruz J. et al.,Anal. Chim. Acta, 185, 139 (1986); Talanta, 33, 199 (1986). 67. Matsouka S. et al., Analyst, 124, 787 (1999). 68. Martelli P.B. et al., Anal. Chim. Acta, 308, 397 (1995). 69. Aoyama M., Hobo T., Suzuki S., Anal. Chim. Acta, 129, 237 (1981). 70. Shijo Y., Sakai K., Bull. Chem. Soc. Jpn., 59, 1455 (1986). 71. Bryson W.G., Goodall C.M., Anal. Chim. Acta, 124, 391 (1981). 72. Agterdenbos J. et al., Talanta, 19, 341 (1972). 73. Yoshimura K., Ohashi S., Talanta, 25, 103 (1978). 74. Manzoori J.L., Sorouraddin M.H., Shemiran F., Anal. Lett., 29, 2007 (1996). 75. Ososkov V., Kebbekus B., Chesbro D., Anal. Lett., 29, 1829 (1996). 76. Savvin S.B. et al., Zh. Anal. Khim., 51,308 (1996). 77. Osaki S., Osaki T., Takashima Y., Talanta, 30, 683 (1983). 78. Solano G. et al., J. Radioanal. Nucl. Chem., 179, 173 (1994). 79. Milacic R. et al.,Analyst, 117, 125 (1992). 80. Barrais P.,Analusis, 17, 87 (1989). 81. Williams A.J., Analyst, 93, 611 (1968). 82. Nechitailov A.A., Prokof'ev V.V., Krasin'kova M.V., Zh. Anal. Khim., 50, 2007 (1985). 83. Yoshikuni N., Talanta, 43, 1949 (1996). 84. Pilkington E.S., Smith P.R., Anal. Chim. Acta, 39, 321 (1967). 85. Chirnside R.C. et al., Analyst, 88, 851 (1963). 86. Pilipenko A.T., Voronina A.I., Nabivanets B.I., Zavod. Lab., 36, 273 (1970). 87. Babkina T.A., Potapenko L.I., Zavod. Lab., 58, No 10, 6 (1992). 88. Ryabchikov D.I., Lazarev A.I., Lazareva V.I., Zh. Anal. Khim., 19, 1110 (1964). 89. Grekova I.M., Golik N.N., Serbinovich V.V., Zh. Anal. Khim., 38, 443 (1983). 90. Girard L., Hubert J., Talanta, 43, 1965 (1996). 91. Trojanowicz M., Pobozy E., Worsfold P.J., Anal. Lett., 25, 1373 (1992). 92. Haukka S., Saastamoinen A.,Analyst, 117, 1381 (1992). 93. Thomas O.,Analusis, 17, 221 (1989). 94. Thomas O., Gallot S., Naffrechoux E., Fresenius'J. Anal. Chem., 338, 241 (1990). 95. Ohls K., Riemer G., Z. Anal. Chem., 317, 774 (1984). 96. Bennet H., Marshall K., Analyst, 88, 877 (1963). 97. Patnaik U., Muralidhar J., Talanta, 42, 553 (1995).

Chapter 18. Cobalt

Cobalt (Co, at. mass 58.93) occurs predominantly in the II oxidation state. In some complexes it is readily oxidizable to Co(III). The hydroxide Co(OH)2 is precipitated at pH -~7.5 and is insoluble in excess of NaOH. Cobalt forms ammine, cyanide, tartrate and EDTA complexes. Blue chloride complexes are formed in fairly concentrated chloride solutions.


18.1.1. Extraction

1-Nitroso-2-naphthol, a popular spectrophotometric reagent for cobalt, is sometimes used in the preliminary extraction of Co from other metals before its determination [1-3].

Very small amounts of cobalt can be separated together with traces of other heavy metals by extraction with dithizone [4-6]. Within the pH range from 6 to 10 the extraction of cobalt is quantitative. The stability of Co(HDz)2 to dilute HC1 permits the separation of cobalt from other metals which form easily decomposed dithizonates (e.g., Cd, Zn, and Pb) [7]. Dithizone can be used for separating Co from Ni after the Ni has been converted into Ni(phen)32+ [5].

Thiocyanate complexes play an important role not only in the spectrophotometric determination of cobalt, but also in its separation from other metals [8-11 ]. The complex has been extracted with Alamine, TOPO [9], methyltrioctylamine [10], and DAM [ 11 ].

18.1.2. Ion exchange and other methods

When a solution of metals in 9 M HC1 is passed through a strongly basic anion-exchanger column, the chloride complexes of Co, Cu, Zn, and Fe(III) are retained by the resin, whereas Ni, Mn, and Cr are eluted since they do not form stable chloride complexes. Cobalt is eluted from the column with 4 M HC1. More dilute hydrochloric acid (e.g., 0.01 M) elutes Cu, Zn, and Fe(III) [ 12,13].

Cobalt has been separated from other metals by elution with hydrochloric acid mixed with acetone [14] or with tetrahydrofuran and glycol monomethyl ether [ 15].

Traces of cobalt can be precipitated as the hydroxide with La, A1, or Fe(III) as collectors [16,17]. Co(II) has been separated from Co(HI) by precipitating Co(II) with acetylacetone from ammoniacal medium [ 18]. A solid chelating material, 1-nitroso-2-naphthol supported on silica gel, provides a rapid and highly selective mean of separating traces of Co(H) from natural waters [19]. Open-cell polyurethane foam loaded with PAN azo reagent [20], and organic resins modified with nitroso-R salt have been used similarly [21,22].


The thiocyanate method is suitable for the determination of relatively large amounts of cobalt. Methods with nitroso-naphthols or nitroso-R salt are specific but not very sensitive. An example of a really sensitive method is that using the azo reagent, 5-Br-PADAP.

18.2.1. Thiocyanate method

In concentrated solutions of K, Na or ammonium thiocyanate, CO 2+ ions produce a blue

168 18. Cobalt

colour which fades when the solution is diluted with water, owing to dissociation of the complex. The addition of acetone or any other water-miscible organic solvent (e.g., ethanol) which suppresses the dissociation of the complex, restores the blue colour to the solution. A complex with the formula Co(SCN)42- is predominant in the blue solution. In solutions with lower SCN- concentrations, and in the absence of acetone, pale pink Co(SCN) + exists.

The colour intensity depends on the concentrations of SCN- and acetone. In solutions containing 50% acetone an increase in KSCN concentration above 10% gives no further increase of colour intensity. The acidity of the solution influences the colour intensity, so it should be kept constant (within 0.1-1 M HC1). The acidity of the solution affects the absorbance and should therefore be kept constant (between 0.1 and 1 M HC1) in the sample and the reference solutions.

At )~max : 620 nm the molar absorptivity ~ (of a solution containing 10% of KSCN and 50% of acetone) is 1.9.103 (a = 0.032).

Since iron(III) forms coloured complexes with SCN- ions, it interferes in the determination of cobalt. Larger quantities must be separated [e.g., by extraction, or by precipitation with Zn(OH)2]. Smaller amounts can be masked with fluoride, phosphate, or reduced to Fe(II) with ascorbic acid or SnCI2.

Interfering elements include other metals which form coloured thiocyanate complexes (e.g., V, Bi, U, Cu, Mo, and W), and metals which give precipitates [Ag, Cu(I)] or consume thiocyanate ions to form colourless complexes ( e.g. Hg). Finally, ions which are themselves coloured (e.g., Ni and Cr) interfere when present at high concentrations. Copper(H) can be masked by means of thiosulphate or thiourea, and vanadium can be masked with tartaric acid.

The cobalt(II) thiocyanate complex can be extracted with oxygen-containing solvents, such as a mixture of diethyl ether with isoamyl alcohol (1+1), MIBK, or acetylacetone [23,24]. The molar absorptivity, e, of the complex in the ether-isoamyl alcohol mixture is about 30% smaller than in the aqueous acetone media.

The anionic cobalt(II) thiocyanate complex reacts with many organic bases to form ion- associates which can be extracted into CHC13, or other non-polar solvents. Organic reagents used for this purpose include DAM [25], TOA [26], triphenylsulphonium ion [27], or 2,4- dichlorobenzyltriphenylphosphonium ion [28].

Reagents

Potassium or ammonium thiocyanate, 50% solution. Standard cobalt solution: 1 mg/ml. Dissolve 4.7800 g of COSO4.7H20 in water containing

2 ml of conc. H2SO4, and dilute the solution with water to 1 litre. Alternatively use 2.6300 g of anhydrous cobalt sulphate prepared by ignition of the hydrous salt at --400~ The anhydrous salt dissolves more slowly.

Thiosulphate-phosphate solution. Dissolve in water 40 g of NazSzO3.5H20 and 10 g of Na3PO4.12H20 and dilute the solution with water to 250 ml.

Procedure

Acetone variant. To the sample solution in a 25-ml standard flask containing less than 0.5 mg of Co, add enough HC1 to give a final concentration of 0.5 M. Add 5 ml of the thiocyanate solution. If a red colour appears (indicative of iron), add the ascorbic acid solution dropwise until the red colour is discharged, and then add 1 ml more. Add 12 ml of acetone, and dilute the solution with water to the mark. Measure the absorbance of the blue solution at 620 nm, using water as the reference.


Extraction variant. Slightly acidify the sample solution containing not more than 0.5 mg of Co. Add 5 ml of the thiosulphate-phosphate solution and 5 ml of the thiocyanate solution, and adjust to pH 3.5-4.0. Transfer the solution to a separating funnel, and extract the complex with two 10-ml portions of a mixture of diethyl ether and isoamyl alcohol (1 + 1). Make up the combined extracts to the mark with the solvent in a 25-ml flask, and measure the absorbance at 620 nm, using the solvent as reference.

18.2.2. Nitrosonaphthol methods

1-Nitroso-2-naphthol ((x-nitroso-[3-naphthol, Cobaltone, formula 18.1) and its isomer, 2-nitroso-l-naphthol (13-nitroso-o~-naphthol, formula 18.2) react with cobalt ions in a similar, highly selective manner, to form chelates soluble in non-polar solvents.

NO OH

(18.1) (18.2)

In the reaction with nitrosonaphthols, cobalt is oxidized by atmospheric oxygen to the (III) oxidation state. The complex of cobalt with 1-nitroso-2-naphthol is orange, whereas that with 2-nitroso-l-naphthol is brown-pink. The absorption spectra of the two chelates in chloroform solutions are shown in Fig. 18.1.

50 600 365 400 wavelength, nm

Fig. 18.1. Absorption spectra of cobalt complexes with 1-nitroso-2-naphthol (1) and with 2-nitroso-l-naphthol (2)

The molar absorptivity of the complex with 1-nitroso-2-naphthol at ~max = 415 nm is 2.9.10 4

(a = 0.49), and of the complex with 2-nitroso-naphthol at ~,max = 365 nm is 3.7.104 (a = 0.63). Chloroform is the most popular solvent, although carbon tetrachloride, benzene, toluene,

and isoamyl acetate are also used. The reaction between cobalt ions and nitroso-naphthols takes place in weakly acidic medium and proceeds rather slowly. For this reason, the sample solution is allowed to stand for about 30 min after the reagent is added, before the complex is extracted. The reaction with cobalt is done at pH -- 3 with 1-nitroso-2-naphthol, and at pH -- 4 with 2-nitroso-l-naphthol.

The cobalt complex formed is stable and does not decompose even when treated with 2 M HC1, which decomposes other complexes formed in a weakly acidic medium, e.g., those of Ni, Cu, Fe, and Cr. Uncomplexed nitroso-naphthol is scrubbed from the organic extract with NaOH solution.

Larger amounts of Fe(III) are either extracted first, or masked with fluoride. Addition of citrate prevents the precipitation of the hydroxides of metals which hydrolyse at the pH of the reaction of Co with the nitrosonaphthols.

170 18. Cobalt

Reagents

1-Nitroso-2-naphthol, 0.5% solution in glacial acetic acid. Before use, the solution is purified by shaking with active carbon.

2-Nitroso-l-naphthol, 0.5% solution in glacial acetic acid. Purify the solution as for 1- nitroso-2-naphthol.

Standard cobalt solution: 1 mg/ml. Preparation as in Section 18.2.1.

Procedure

To a solution containing not more than 30 lag of Co, add 1 ml of the 1-nitroso-2-naphthol (or 2-nitroso-l-naphthol) solution with stirring, adjust the pH of the solution with ammonia to 4 (or 5 in the case of 2-nitroso-l-naphthol), and allow to stand for 30 min. Transfer the solution to a separating funnel and extract with two portions of CHC13. Shake the combined extracts with 2 M HC1, followed by two portions of 2 M NaOH, and finally wash with water. Transfer the extract to a 25-ml standard flask, make up to the mark with chloroform, and measure the absorbance of the solution at 415 nm (in the case of 2-nitroso-l-naphthol, at 365 nm), using the solvent as the reference.

18.2.3. Nitroso-R salt method

Nitroso-R salt (formula 18.3) is a derivative of 1-nitroso-2-naphthol. Both reagents are specific for cobalt. The sulphonate groups in the molecule of nitroso-R salt render this reagent and its cobalt complex soluble in water but insoluble in non-polar solvents. Hence, nitroso-R salt is used to determine cobalt spectrophotometrically in aqueous medium [29]. In acidic solution (pH -~4), cobalt(II) is oxidized to Co(III).

N0

(18.3)

In acid solution, the reagent is yellow whereas the complex is red. Figure 18.2 shows the absorption spectra. The molar absorptivity of the red cobalt-nitroso-R salt complex at ~ m a x --

415 nm is 3.5.104 (a = 0.60). At 500 nm, where interference from the colour of the reagent is negligible, e = 1.5.104.

The absorbance of the Co(III) complex solution may be measured with higher sensitivity at 415-425 nm, or with lower sensitivity at 500-520 nm (a reagent blank, or water as reference). The reaction of cobalt with nitroso-R salt is usually done in a hot weakly acidic medium buffered with sodium acetate. The solution is then made sufficiently acidic with hydrochloric or nitric acid to decompose the nitroso-R salt complexes of other metals (e.g., Cu, Ni, Fe, and Mn), which are less stable than the cobalt(HI) complex. Phosphate or fluoride masks iron (III), which has a yellow colour in hydrochloric acid medium.


~J

o oq

/

t, O0 415

I 500 , , . 600 wavelengm, nm

Fig. 18.2. Absorption spectra of nitroso-R salt (vs. water) (1) and of its complex with cobalt (vs. the reagent) (2)

Preliminary separation of cobalt from large quantities of other metals is achieved by the methods outlined above. Alternatively, large quantities of metals, e.g., Fe(III), are separated from Co by extraction of their chloride complexes.

When the colour reaction takes place in the presence of relatively large amounts of metals which also react in weakly acidic medium with nitroso-R salt, a considerable amount of the reagent should be added to the solution.

The anionic cobalt-nitroso-R salt complex has been extracted with a (5+1) mixture of chloroform with butanol in the presence of TP. The extract contains an ion associate of composition corresponding to Co:R:CP = 1"3:6 (e = 5.0-104 at 510 nm) [30].

Related reagents, such as 2-nitroso-l-naphthol-4-sulphonic acid [31,32], 1-nitroso- naphthol-6-sulphonic acid, and 2-nitroso-l-naphthol-6-sulphonic acid [33] have also been recommended for determination of cobalt. The complexes with these reagents can be extracted into CHC13 in the presence of amines [31,33].

Reagents

Nitroso-R salt, 0.1% solution. Store the solution in an amber-glass bottle. Standard cobalt solution" 1 mg/ml. Preparation as in Section 18.2.1.

Procedure

Neutralize a solution containing less than 30 gg of Co with dilute ammonia solution, and then acidify with 2 ml of 1 M HC1. Add 3 ml of the nitroso-R salt solution, 3 ml of 25% sodium acetate solution, heat to boiling, and boil gently for 1 min. To the slightly cooled solution, add 1 ml of conc. phosphoric acid and 2 ml of HC1 (1 + 1). Boil the solution for 1 min more. Cool the solution, transfer it to a 25-ml standard flask, and make it up to the mark with water. Measure the absorbance of the solution at 415 nm against a reagent blank solution.

18.2.4. 5-Bromo-PADAP method

2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) (formula, 4.3) reacts with a fairly large group of metals (e.g., Co, Ni, Zn, Cd, Mn, Cu, Pd, V, U) to give, in general, complexes with high molar absorptivities (e -- 105), and large bathochromic shifts. A reagent with dimethyl groups (instead of diethyl) shows identical analytical properties [34]. A sensitive method for determining cobalt with 5-Br-PADAP has been developed [35].

The reagent, which is sparingly soluble in water, dissolves well in ethanol, DMF,

172 18. Cobalt

acetone, dioxan, and in mixtures of these solvents with water. In 10% DMF-aqueous medium, at pH -7, a Co(III)-5-Br-PADAP complex forms, with ~max ----

586 nm 0~max of the reagent is --460 nm). The molar absorptivity of the complex is 9.1.104 (a = 1.6).

Quantitative complex formation in the pH range 3-8 requires about 10 min; the absorbance is then stable for at least 2 h. In strongly acidic solutions and in the presence of EDTA, the cobalt complex is not formed. However, the cobalt complex formed is resistant to HzSO4 or HNO3 (up to 2 M) and EDTA (up to 0.1 M). Chelates of many other metals with 5- Br-PADAP would be decomposed under these conditions. This fact, which results from the inertness of the cobalt complex, increases the selectivity of 5-Br-PADAP for cobalt. Palladium, Cu, V, Ni, and Hg still interfere.

Acetate and pyridine buffers do not interfere at concentrations below 0.4 M. In the presence of perchlorate, an ion-associate is formed which can be extracted into CHC13.

Reagents

5-Br-PADAP, 0.03 % (- 1.103 M) solution in DMF. Standard cobalt solution: 1 mg/ml. Preparation as in Section. 18.2.1. Acetate buffer (pH 7.0), 1 M solution of ammonium acetate.

Procedure

To the neutral or weakly acid sample solution in a 25-ml standard flask, containing not more than 12 tag of Co, add 2.5 ml of buffer solution, 2 ml of 5-Br-PADAP solution, water to -15 ml, and mix. After 15 min add 5 ml of 6 M H2SO4 and 1 ml of 0.01 M EDTA solution. Dilute to the mark with water, mix, and after 10 rain measure the absorbance of the solution at 586 nm against a reagent blank solution.


Azo reagents form the most numerous group of spectrophotometric reagents for determining cobalt. PAN (~ = 2.3.104 at 580 nm)[20,36-38] and PAR (~ = 5.6.104 at 510 nm) [39-42] (formulae 4.1 and 4.2) were the first azo reagents used for determination of cobalt. The green Co(III)-PAN complex is extracted by chloroform.

Bromo-and chloro- derivatives of 2-pyridylazo reagents are more sensitive. The 5-Br- PADAP method has already been described. Other halogen-derivatives used in determining cobalt are: 2-(5-chloro-2-pyridylazo)-5-diethylaminophenol (5-C1-PADAP, formula 18.4) (E = 1.06.105) [43], 2-(5-bromo-2-pyridylazo)- 1,5-diaminobenzene (5-Br-PADAB) (formula 18.5) (~ - 1.16.105) [44,45], 2-(3,5-dichloro-2-pyridylazo-5-dimethylaminophenol) (~ = 8.4.104) [46], and 2-(3,5-dibromo-2-pyridylazo)-5-dimethylaminobenzoic acid (e = 1.55.105 at 673 nm) [47].

N=N N(CzHs) z (18.4) N-----N NHz

HO H2N

(18.5)

Other azo reagents suggested for determining cobalt include, TAR (e = 5.6-104) [48],


3-(2'-thiazolylazo)-2,6-diaminotoluene (e = 9.7.104) [49], 2-(2-benzothiazolylazo)-5-dimethyl aminobenzoic acid (~ = 1.2-105) [50], 2-(2-thiazolylazo)-4-methyl-5- (sulphomethylamino)benzoic acid (e)~ = 1.13.105 at 655 nm) [51,52], Sulpharsazen (in the presence of CP, e = 9.9.104 [53], and Eriochrome Black T [10].

Among other organic reagents, a group of nitroso compounds, related to nitrosonaphthols, is highly selective for cobalt, such as 2-nitroso-5-dimethylaminophenol (1,2- dichloroethane, e = 6.2-104) [54,55], and 3-nitroso-2,6-pyridinediol (e = 3.4-104) [56].

Dimethylglyoxime (HzDm), the very well known reagent for Ni, reacts with cobalt in ammoniacal medium to give a water-soluble brown complex, usable to determine cobalt [57].

A number of other oximes have also been recommended as spectrophotometric reagents for cobalt, namely 2,2'-dipyridylketoxime (e = 2.1.104 at 388 nm) [58], 2-pyridyl-2-thienyl-13- ketoxime [59], and 2,2-diquinolyl ketoxime ( e - 5.3.104) [60].

Spectrophotometric methods based on ion associates with basle dyes are very sensitive. The Co-thiocyanate complex was associated with Malachite Green (CC14 + cyclohexane), (e = 8.6-104) [61], Turquoise Blue (triphenylmethane dye) (toluene + DMF) [62], and 6- nitrodimethyline-carbocyanine [63,64]. The anionic complex of Co with chloro-oxine [65] associated with Rhodamine 6G was extracted with benzene. The complex of Co with 2- nitroso-l-hydroxynaphthalene-4-sulphonic acid was extracted into CHC13 as the associate with a basic azo dye (e = 1.66-105 at 566 nm) [66].

Methods based on reagents containing sulphur as a ligand atom are less sensitive than those discussed above. Those recommended include DDTC (extraction with benzene or CHC13) [67-69], in the presence of a surfactant [70,71], thiodibenzoylmethane (cyclohexane,

= 1.2.104) [72], thiobenzoylacetone [73], thio-HTTA [74], and di-2-pyridylketo- thiosemicarbazone [75].

Many hydrazone derivatives have been proposed as spectrophotometric reagents for Co, namely 2,2'-dipyridyl-2-pyridylhydrazone (e = 4.2.104), 2-furaldehyde-2-pyridylhydrazone [79], benzil-2-pyridylketone-2-pyridylhydrazone [80], and 2,2'-dipyridyl-2-benzothiazolylhydrazone [81]. Co was determined in the presence of Hg [82], and in the presence of Ni [83] by the derivative spectrophotometry using benzil-2-pyridylketo-2-quinolylhydrazone.

Many other organic reagents have been used to determine Co, including phenylfluorone (in the presence of CP, e = 1.16.105 at 620 nm) [84,85], and ferrozine [86,87]. A very sensitive method has been based on the complex of Co with a,13,~,,~5-tetrakis(4- sulphonyl)porphyrin (Co replaces Cd in the complex) (e = 2.4.105 at 432 nm) [88,89]. ]. Less sensitive methods for Co utilize EDTA, DCTA, EGTA and NTA ()~ = 200-300 nm) [90].

Among inorganic complexes, the blue chloride complex [91], green tricarbonato- cobaltate(III) (formed in the presence of EDTA) [92], and the mixed pyridine-azide complex [93] are used for determination of Co.


The thiocyanate method has been used for determining cobalt in vitamin B12 [94], steel [24,94], and nickel [25]. Cobalt present in considerable amounts in alloys with aluminium, nickel, chromium, manganese, copper, and iron was determined by the differential spectrophotometric analysis [95].

1-Nitroso-2-naphthol and 2-nitroso-l-naphthol have been used for determining cobalt in plant materials [96,97], sea water [98], cast iron and steel [99], and bronzes [100].

Cobalt has been determined by means of the nitroso-R salt in biological materials [1], natural waters [13], plants [101,102], iron ores [103], uranium minerals [15], cast iron and

174 18. Cobalt

steel [103], titanium, zirconium, and their alloys [104], and rare earth metals and their compounds [8].

The 5-Br-PADAP method was applied for determining Co in blood serum [105]. Various hydrazones were used in determinations of Co in pharmaceuticals [78,106], steel [107-109], alloys [110], and catalysts [111 ]. PAN was used for determining Co in vitamin B12. The PAR method has been applied in Co determinations in the presence of Ni [113-115], in water and in pharmaceuticals [116], and in copper selenide [117]. Dithizone was used for determining Co in Cu-Ni-Zn alloys [6]. 5-(Arsonophenylazo)-8-(4-toluenesulphonamido)quinoline has been used for determining Co in vegetables [118].

Co and Cu were determined with EDTA in the same solution using the derivative spectrophotometry technique [118]. Cobalt contents (5-55%) in steel were determined, based on the colour of the chloride complex [ 119].

References

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(1986). 101. Jacinto A.O. et al., Quim. Anal. (Barcelona), 10, 51 (1991). 102. Pyrzyfiska K., Janiszewska Z., Szpunar J., Trojanowicz M., Analyst, 119, 1553 (1994). 103. Koch K.H. et al., Z. Anal. Chem., 249, 307 (1970). 104. Vogel J., Monnier D., Haerdi W.,Anal. Chim. Acta, 24, 55 (1961). 105. Vitouchova M., Jancar L., Sommer L., Fresenius'J. Anal. Chem., 343, 274 (1992). 106. Alonso E.V. et al., Talanta, 43, 1941 (1996). 107. Cristofol E. et al., Talanta, 38, 445 (1991). 108. Ishii H., Odashima T., Kawamonzen Y., Anal. Chim. Acta, 244, 223 (1991). 109. Cristofol Alcarez E., Sanchez Rojas F., Cano Pavon J.M., Fresenius'J. Anal. Chem.,

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Chapter 19. Copper

Copper (Cu, at. mass 63.54) occurs in its compounds in the II, and less often in the I, oxidation state. The properties of copper(I) are similar to those of Ag, Au(I), and TI(I). Copper(I) forms sparingly soluble compounds with the halogens. In solution, copper(I) exists only in complexes, e.g., Cu(CN)2-, CuCI2-, and Cu(NH3)2 +. Cupric hydroxide, Cu(OH)2, begins to precipitate at pH --5 and shows no amphoteric properties. Copper(II) forms ammine, chloride, tartrate, and EDTA complexes. As a result of oxidizing properties of Cu(II), its cyanide complex is converted into copper(I) cyanide complex, and sparingly-soluble CuSCN is precipitated from Cu(II) thiocyanate.



The separation of copper by extraction is usually connected with spectrophotometric determination with the use of dithizone, dithiocarbamates, cuproine and related compounds, as well as other reagents. Sometimes copper is first preconcentrated by extraction as the dithizonate or dithiocarbamate before its determination by other methods [ 1-3]. Crown ethers [1] and porphyrin compounds [2] have also been used for extracting Cu(II).

Copper can be separated conveniently by precipitation as the sulphide CuS from either an acidic or a neutral medium. Metals yielding sparingly soluble sulphides (e.g., Hg, Cd, Pd, and Zn) can be used as collectors. Traces of copper can be isolated quantitatively as the sulphide or hydroxide with lanthanum or iron as collectors [3,4].

Since Cu forms soluble ammine complexes, it may be separated from relatively small amounts of metals, which are precipitated as hydroxides with ammonia (e.g., Fe, A1, and Ti). Reprecipitation is advisable when a more quantitative separation is required.

Small amounts of copper are separated quantitatively from less noble metals by electrodeposition on a platinum cathode in an acidic medium [5].


By exploiting the differences in stability of various complexes of Cu and other metals, it is possible to achieve separations on ion-exchange columns. Owing to the different stability of the chloride complexes, copper has been separated from Ga, Fe(III), and Zn by ion-exchange on anion- or cation-exchange resins. The anionic chloride copper(I) complex (in the presence of ascorbic acid) has been retained on anion-exchanger Dowex 1 from 0.1 M HC1 medium; copper was then eluted with 1 M nitric acid [6].

Cation-exchange chromatography in acetone-HC1 [7], or acetone-HBr [8] media has been employed for separation of copper from many other metals. Cation-exchangers have also been used for preconcentration and separation of trace amounts of copper [9,10].

Copper-N-(dithiocarboxy)sarcosine chelate has been preconcentrated with Amberlite XAD-2 resin. A cellulose sorbent with iminodiacetic acid groups has also been used for effective preconcentration of copper [ 11 ].

178 19. Copper


A great variety of spectrophotometric methods for determination of copper has been developed. Among the most widely used are the dithizone, dithiocarbamate, cuproine, and cuprizone methods described below in more detail.


In acidic medium containing an excess of dithizone (formula 1.1), copper(II) forms the violet dithizonate, Cu(HDz)2, which is soluble in non-polar solvents (CC14, CHC13) [12,13]. In alkaline medium, the less-intensely coloured, yellow-brown cupric dithizonate is formed. It can also be formed in a neutral or acidic medium deficient in dithizone.

The molar absorptivity of Cu(HDz)2 in CC14 is 4.52.104 a t )Lmax "- 550 nm (sp. abs. = 0.71). The absorption spectrum of copper dithizonate is shown in Fig. 4.4.

Although Cu(II) dithizonate is very stable, it is formed rather slowly, necessitating relatively prolonged shaking during the extraction. Higher concentrations of dithizone in the organic phase and lower acidity of the aqueous medium promote extraction. At pH 1, the rate of extraction is highest, and the solution is too acidic for bismuth and zinc to be extracted with a 0.002% solution of dithizone. Noble metals which react with dithizone (Pt, Pd, Au, Ag, and Hg) are extracted together with copper, but can be stripped from the organic extract as their stable iodide complexes with a 1% solution of potassium iodide. Extraction of silver is prevented by chloride. Citrate and tartrate slightly inhibit the extraction of copper with dithizone.

The interferences in the determination of copper, owing to noble metals, are most conveniently eliminated by extracting these metals at the start with dithizone from a 1 M mineral acid solution. Noble metals (except Pd) form yellow-orange dithizonates, and their rates of extraction are much higher than that for copper. The solution is shaken with small portions of dithizone in CC14 until the organic layer no longer rapidly becomes yellow. The free dithizone should be stripped from the extract with a very dilute ammonia solution.

Cu(II) has been determined by the dithizone method, without the extraction, in the presence of Triton X- 100 [ 14,15] and hydroxylammonium chloride [ 16].

Reagents

Dithizone, 0.002% solution in CC14. Preparation as in Section 46.2.1. Standard copper solution: 1 mg/ml. Dissolve 3.9280 g of cupric sulphate pentahydrate,

CuSO4.5H20, in water containing 1 ml of concentrated H2SO4, and dilute the solution with water to 1 litre.

Procedure

Adjust a solution containing not more than 20 ~tg of copper, from which the noble metals (Pt, Pd, Au, Ag, Hg) have been removed, to pH --1 with ammonia. Transfer the solution to a separating funnel, and extract with portions of dithizone in CC14 (1 ml of 0.002% H2Dz corresponds to 2.5 ~tg of Cu) until the last portion does not change its green colour. With each extraction, separate the organic phase before the green dithizone reagent is fully complexed as the violet Cu(HDz)2. Shake the aqueous sample with the final portion of the dithizone solution for not less than 3 min. Remove free dithizone by shaking the combined extracts with very dilute ammonia. Dilute the violet dithizonate solution with CC14 in a 25-ml standard flask. Measure the absorbance at 550 nm, using the solvent as reference.


Note. For prior separation of the noble metals, adjust the concentration of the inorganic acid in the sample solution to about 1 M, and shake the solution in a separating funnel with 0.1-0.2 ml portions of a 0.001% dithizone solution in CC14 until the green colour changes to yellow. Discontinue the extraction when the last portion of the CC14 solution turns violet, and add this portion to the subsequent (vide supra) Cu(HDz)2 extract.

19.2.2. D i t h i o c a r b a m a t e methods

The addition of an aqueous solution of sodium diethyldithioearbamate (Na-DDTC, cupral) (formula, 4.40) to a solution (at pH 4-11) containing small amounts of copper(II) ions produces a yellow-brown colour owing to a colloidal suspension of the sparingly-soluble copper 1:2 chelate with DDTC. The reagent co-ordinates with copper through the two sulphur atoms to form a chelate with four-membered tings, which is a rather rare configuration. Protective colloids (e.g., gum arabic) stabilize the pseudo-solution, and permit the spectrophotometric determination of copper. Cu (II) has been determined in aqueous solutions in the presence of surfactants [ 17].

Greater accuracy and sensitivity are attained by extracting the copper diethyldithiocarbamate into organic solvents such as CC14, CHC13, trichloroethylene, amyl acetate, and isoamyl alcohol. These solutions are fairly stable [18]. The absorption spectra of copper(II), manganese, and bismuth dithiocarbamates are shown in Fig. 19.1.

2

r

v 370 400 436 . 500 . 600 wavelength, nm

Fig. 19.1. Absorption spectra of copper (1), manganese (2), and bismuth (3) diethyldithiocarbamates in CCI4

The molar absorptivity, ~, of a carbon tetrachloride solution of the complex is 1.4.10 4 at

~max "- 436 nm (a = 0.22). The main interfering metals in the copper determination are Fe, Bi, Mn, Ni, Co, Cr, Mo

and U, which form coloured complexes. The selectivity of the method is considerably enhanced by the use of EDTA as a masking agent. In a tartrate or citrate medium at pH 8-9, Fe, Mn, Ni and Co are masked by EDTA, as are Cd, Pb, Zn, and In, which form colourless complexes with DDTC. Of the metals forming coloured compounds with DDTC, only Bi, TI(III), and Cu are not masked. Thallium, when reduced to TI(I), does not interfere. Bismuth can be stripped from the organic extract, containing copper and bismuth diethyldithiocarbamates, with 5 M HC1. Copper diethyldithiocarbamate is decomposed by cyanide, whereas the bismuth complex remains unaffected.

Apart from cyanide, interferences in the determination of copper are also caused by thiosulphate and species which reduce Cu(II) to Cu(I) or oxidize DDTC.

The disadvantage of sodium diethyldithiocarbamate lies in its insolubility in organic solvents. It is also decomposed quite readily in acidic solutions into diethylamine and carbon

180 19. Copper

disulphide. Sometimes diethylammonium diethyldithiocarbamate (formula 4.41), which is soluble in

chloroform and stable in acid, is used instead of Na-DDTC [19]. A CC14 or CHC13 solution of the reagent is shaken with the sample solution to extract copper(II).

The interference of metals such as Fe, Mn, and Zn in the determination of copper has been overcome by using lead diethyldithiocarbamate instead of Na-DDTC. When a chloroform solution of Pb(DDTC)2 is shaken with an aqueous solution containing copper, a displacement reaction occurs:

C u 2+ -k- Pb(DDTC)2 --> Cu(DDTC)2 + Pb 2+

Metals, which have DDTC complexes that are more stable than the lead complex, namely Hg, Ag, T1, and Bi, interfere.

Zinc dibenzyldithiocarbamate (formula 19.1), which is soluble in CC14 and CHC13, is less selective than Pb(DDTC)2 but more resistant to highly acidic media [20]. With this reagent, copper can be extracted from 1-2 M HC1 or H2S04. Lead dibenzyldithiocarbamate has been used similarly [21 ].

(19.1)

Other dithiocarbamates which have been suggested for determination of copper include pyrrolidinedithiocarbamate [22,23], piperazine-bis(dithiocarbamate) [24], and morpholine-N- dithiocarbamate [25].

Reagents

Sodium diethyldithiocarbamate, Na-DDTC, 0.1% solution adjusted with ammonia to pH -8.5.

Standard copper solution: 1 mg/ml. Preparation as in p. 19.2.1. EDTA (disodium salt), 0.1 M (--3.7%) solution. Sodium-potassium tartrate, 20% solution. Purified by extraction of copper traces with

Na-DDTC.

Procedure

To a solution containing not more than 70 ~tg of Cu, add 1-3 ml of the tartrate solution and 1-3 ml of the EDTA solution, adjust the solution with ammonia to pH --8.5, and add 3 ml of the Na-DDTC solution. Shake the solution in a separating funnel for about 1 min with each of two portions of CC14. Make the combined extracts to the mark with the solvent in a 25-ml flask, and measure the absorbance at 436 nm against CC14.

Notes. 1. Sufficient EDTA and tartrate must be added to mask metals which would react with DDTC, or which would be precipitated after adjustment of the pH.

2. If bismuth is absent, shaking the CC14 extract with an aqueous 1% solution of KCN will decolourise the organic phase.

3. Copper may be determined without extraction by adding 2 ml of 1% gum arabic solution to the


sample solution before the Na-DDTC is added.

1 9 . 2 . 3 . C u p r o i n e m e t h o d s

Cuproine (2,2'-biquinolyl, formula 19.2), neocuproine (2,9-dimethyl-l,10-phenanthroline, formula 19.3) , and bathocuproine (2,9-dimethyl-4,7-diphenyl- 1,10-phenanthroline, formula 19.4) react with copper(I) to form coloured cationic complexes. The methods using these reagents are specific for copper, but are of relatively low sensitivity.

H3C CH3

(19.2) (19.3) (19.4)

Hoste et al. [26] have thoroughly investigated the conditions for the extractive spectrophotometric determination of copper with e u p r o i n e . Amyl alcohols and n-hexanol are most suitable for the extraction of the Cu complex from aqueous solutions. A reducing agent (usually NHzOH.HC1), is added to the sample solution, the pH is adjusted to the optimum range of 4-7, and the sample shaken with a colourless isoamyl alcohol solution of cuproine.

At ~max 546 nm, the molar absorptivity of the copper(I)-cuproine complex in isoamy! alcohol is 6.4.103 (specific absorptivity 0.10).

Figure 19.2 shows the absorption spectra of the copper(I) complexes with cuproine, neocuproine, and bathocuproine in isoamyl alcohol.

(D r

rat)

3

t.oo ~.~ t.Tg .5oo . .~6 600 wavelength, nm

Fig. 19.2. Absorption spectra of copper(I) complexes with cuproine (1), neocuproine (2), and bathocuproine (3) in isoamyl alcohol

From among other cations, only Ti 3+ gives a greenish-coloured complex with cuproin, but its colour is much weaker than that of the purple copper(I) complex. Of the anions, cyanide, thiosulphate, oxalate, and EDTA interfere in the determination of copper [27,28].

N e o e u p r o i n e reacts with copper(I) within the pH range 3-10, to form an orange complex extractable with n-amyl and isoamyl alcohols, n-hexanol, and MIBK. Neocuproine is added to the sample as an ethanol solution, and the complex formed is extracted with CHC13 [29]. The molar absorptivity of the complex in isoamyl alcohol at )Lmax = 545 nm is 7.9-103 (a = 0.12).

Neocuproine is as specific for copper(I) as cuproine is. Substitution of methyl groups at the 2- and 9-positions in 1,10-phenanthroline renders the reagent unreactive towards iron(II).

182 19. Copper

The bathocuproine method for the determination of copper(I) [30,31] is about twice as sensitive as those based on cuproine and neocuproine. The molar absorptivity of the copper- bathocuproine complex in isoamyl alcohol a t )gmax = 479 nm is 1.42.104 (a - 0.22).

The Cu(I) bathocuproine complex, which is formed between pH 4 and 8, can be extracted with amyl alcohols or n-hexanol. Bathocuproine has been applied in the determination of Cu(I) in the presence of Cu(II). Ethylenediamine is used as a masking agent for Cu(II) [31 ].

The bathocuproinedisulphonic acid has been recommended for the determination of copper, because the reagent and its Cu(I) complex are soluble in water. The sensitivity of this method is almost the same as in the case of bathocuproine. The bathocuproinedisulphonic acid has been used in the continuous determination of copper by the flow injection technique [331.

Reagents

Cuproine, 0.03 % solution. Dissolve 30 mg of reagent in 100 ml of amyl alcohol. Standard copper solution: 1 mg/ml. Preparation as in Section 19.2.1. Acetate buffer, pH 4.5. Dissolve in water 80 g of sodium acetate and 60 ml of glacial

acetic acid; dilute the solution with water to 500 ml.

Procedure

To an acidic solution containing not more that 150 gg of Cu, add about 0.2 g of NH2OH.HC1, ammonia to pH -3, and 10 ml of acetate buffer. Transfer the solution to a separating funnel, and shake for 3 min with each of 2 or 3 portions of the cuproine solution. The last portion should remain colourless. Dilute the combined extracts with isoamyl alcohol to the mark in a 25-ml standard flask, and measure the absorbance at 546 nm, using the solvent as reference.

Note. If the isoamyl alcohol extract is turbid, clear it by adding a small amount of methanol, or by passing the solution through a dry filter.

19.2.4 Cuprizone method

Colourless cuprizone (bis-cyclohexanone-oxalyldihydrazone) (formula 19.5) reacts with copper(II) in slightly alkaline medium (optimum pH 8-9.5) to form a blue water-soluble complex which constitutes the basis of a very selective method for the determination of copper in the aqueous phase [34,35]. The complex is not formed below pH 6.5, and the colour fades above pH 12. The molar absorptivity at 600 nm is 1.6.104 (a = 0.25).

~ N - - N H - - C - - C (19.5)

Copper is usually determined in an ammoniacal citrate medium which keeps most metals in solution. The presence of citrate somewhat retards the development of the colour, but once the copper-cuprizone complex is formed, the colour does not fade.

Interferences in the determination of copper by this method include major quantities (about 10 mg) of Ni, Co, Fe, Cr, and U. Iron in a 500-fold amount relative to copper does not interfere when citrate is present. Citrate, tartrate, and oxalate do not interfere, but cyanide does.

Among related reagents recommended are: quinoline-2-aldehyde-2-quinolylhydrazone


(E = 5.8-104 at 450 nm) [36,37], bis-(ethyl acetoacetate)oxalyldihydrazone (neocuprizone) [38], and benzothiazole-2-aldehyde-2-quinolylhydrazone (e = 7.5-104 at 523 nm) [39].

Reagents

Cuprizone, 0.1% solution. Dissolve 100 mg of reagent in 20 ml of hot 50% ethanol, and dilute the solution with cold 50% ethanol to 100 ml.

Standard copper solution: 1 mg/ml. Preparation as in Section 19.2.1.

Procedure

To a solution in a 25-ml standard flask, containing not more than 70 gg of Cu, add 1-2 ml of 10% ammonium citrate solution (depending on the amount of metals in the solution which would precipitate as the pH is increased), and adjust the solution with ammonia to pH 8-9. Add 3 ml of the cuprizone solution, dilute the solution to the mark with water, and after 10 rain, measure the absorbance of the blue solution at 600 nm against water.


1,5-Diphenylcarbazide, a well-known reagent for chromium(VI), is also a highly sensitive reagent for the determination of copper ( pH 9, ~; = 15.8.104 at 495 nm). Copper(H) can be extracted with a benzene solution of 1,5-diphenylcarbazide. The colour of the complex is stable. The molar absorptivity of benzene solution of the complex is 5.5.104 at 545 nm [40]. A related reagent, 1,5-diphenylcarbazone, is also used (e = 7.5.104 at 550 nm) [41 ].

Numerous sensitive methods for the determination of copper are based on azo reagents, namely PAN (~ = 2.3.104) [42-46], PAR [47-50] 1-(2-pyridylazo)-4-cyclopenti-resorcinol) (~; = 4.9.104) [51], 5-Br-PADAP (~; = 1.0.105) [52-56], 4-methyl-2-(2'-hydroxy-l'- naphthylazo-thiazole (~; = 7.8.104) [57], 4-(3,5-dibromo-2-pyridylazo)-N-ethyl-N-(3- sulphopropyl)aniline in the presence of dodecyl sulphate (e = 1.24-105 at 638 nm) [58].

Many methods developed are based on organic reagents having sulphur as the ligand atom, such as the dithizone and dithiocarbamates, thio-Michler's ketone (in 30% DMF, e = 1.05.105 at 505 nm) [59], 2-thiophenealdehyde-2-benzothiazolylhydrazone (benzene, e = 4.4.104) [60], 1-phenyl-3-thiobenzoylthiocarbamide (CHC13, ~ = 1.3.105 at 360 nm) [61]. Copper has been determined in the presence of Cd and Zn with the use of 1,5-bis(di-2- pyridylmethylene)thiocarbonehydrazide [62].

Copper can be determined by use of ion associates, formed by the cationic complexes of Cu(I) with cuproine [63-65], neocuproine [65], bathocuproine [66] and thio-crown ethers [67,68], associated with the acid dyes such as Rose Bengal (~; = 7.8.104) [63,65,66], the ethyl ester of eosin (~; = 9.4.104) [64], and Erythrosin [63]. These ion-associates are extracted into chloroform [65,66], 1,2-dichloroethane [64,67,68], and other solvents. The ion-associates of cyanide [69] and chloride [70,71] complexes of Cu(I) with Methylene Blue (1,2- dichloroethane, ~; = 9.8.104) [69], and Ethyl Violet (toluene, E = 9.6. l04) [70] are also worth mentioning. The halide complexes of Cu(I) with azo dyes have also been extracted.

Some very sensitive methods for determination of copper with porphyr ins have been described. The molar absorptivities of the copper cationic complexes of porphyrins at the sharp Sorer bands (near 400 nm) are of the order (2-5).105 [9,10,72,73]. Examples are (~,[~,7,~-tetraphenylporphyrin trisulphonic acid (formula 19.6) [72]. The porphyrins have been utilized in the presence of various surfactants [74,75]; the value of E = 4.7.105 has been

184 19. Copper

obtained with lauryl sulphate.

H03S S03H

03

(19.6)

Many other organic reagents have been recommended for determination of copper, e.g.,

2,2'biquinoxalyl [76], ferrone [77], ferrozine [78], cyclohexylfluorone (~ = 1.9.105) [79], and Eriochrome Cyanine R (with CP) (~ = 7.4.104) [80] or CTA [81], Cromal Blue G (with CTA) [82]. The use of tetra-azamacrocyclic compounds for spectrophotometric determination of copper(II) (~ = 4.7.104) [83] is worth mentioning.

Coloured copper complexes with EDTA and related reagents can be applied for determining Cu at higher concentrations. Cu has been determined as EDTA and NTA complexes by the derivative spectrophotometry [84,85]. Relatively high concentrations of Cu can be determined as the ammino complex (e = 120). Other sensitive methods are based on copper complexes with chloride (in water-acetone) and azide [86].

The catalytic effect of Cu on the oxidation of organic compounds by H202 [87-92] and K28208 [93] has become the basis for a number of spectrophotometric determination methods.


The dithizone method has been applied for determining copper in biological materials [ 12,16], water [94], tin [3], titanium and its alloys [95]. Copper has been determined in sea sediments (in the presence of Hg and Pb) by derivative spectrophotometry [96].

The methods involving dithiocarbamates were used for determining copper in plant materials [97], foods [98,99], water [100-103], sewage [104,105], aluminium alloys and environmental samples [106], cast iron and steel [22,107,108], niobium and tantalum [109], gallium arsenide [ 110], and crude oil [ 111 ].

Copper was determined with cuproine in silicate rocks, biological materials, and sea water [112,113], in environmental samples [113], steel and cast iron [114]. Neocuproine was used for determination of copper in biological materials [12,115], foods [116], sea water [117], beryllium [118], arsenic and gallium [119], tungsten [120], aluminium alloys [117], plutonium [121], tellurium [122], and fertilisers [123]. Bathocuproine was applied in determinations of copper in blood serum [124,125], water [126,127], niobium, tantalum, molybdenum, and tungsten [128], lead and nickel [129], cast iron and steel [66].

Bathocuproine disulphonate was used for determining Cu in blood serum and in ground water [ 129a].

The cuprizone method was used for determining copper in plant materials [130], biological samples [12], lead and its alloys [131], aluminium and magnesium alloys [132], platinum alloys [133], cadmium sulphide [134], borate glass [135], and petroleum samples [1361.

PAN was applied for determining Cu in sewage [137]; derivative spectrophotometry was

References 185

involved in determinations of Cu in the presence of Zn and Ni [138]. Copper was determined, with the use of PAR, in technological solutions [139] and in zinc alloys [140]. Copper was determined in environmental samples after extraction of the ion-associate of the chloride complex with Ethyl Violet [ 141]. The derivative spectrophotometry technique was applied for determining copper in iron and copper alloys with the use of the cyanide complexes [ 142,143].

Copper was determined in amalgams with the use of bidithiolene [144].

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Chapter 20. Fluorine

Fluorine (F, at. mass 19.00), exists as the diatomic gas, F2, which is the strongest oxidizing agent known. Water reduces fluorine to hydrogen fluoride, HF. Fluoride ions form sparingly soluble compounds with a number of metals (e.g., Ba, Mg, Ce, and Th). Among the most commonly encountered fluoride complexes are those with Si, B, A1, Fe(llI), Ti, Zr, Ta, and Be.


20.1.1 Distillation

The conventional method for separating fluorine is by distillation from a solution made strongly acidic with a non-volatile mineral acid. It has been shown [1,2] that fluorine distils as hydrated hydrogen fluoride.

Organic substances with covalently bonded fluorine cannot be ashed in the usual way, but must be destroyed by special methods, e.g., by oxygen-flask combustion [3,4].

Perchloric or sulphuric acid is usually used in the distillation. HC104 is more suitable since it neither complexes nor precipitates metal ions. However, perchloric acid must not be used in the presence of organic substances for fear of explosion. When HF is distilled from sulphuric acid medium, minute amounts of H2804, carried over to the distillate, can interfere in the determination of fluoride by complexing Zr or Th if these are used as reagents. The sample volume can be reduced by evaporation from a platinum crucible, after the sample has been made slightly alkaline.

Samples containing silicate minerals are fused with NazCO3 or NaOH before HF is distilled off. Larger quantities of silica precipitate in the still and interfere in the distillation. If the silicate sample is fused with NazCO3 and ZnO, silica remains in the solid phase during subsequent leaching with water. Fluorine is distilled from the silica-free filtrate.

In the presence of metals forming strong fluoride complexes (e.g., Zr), distillation from phosphoric acid is advisable [5]. To prevent distillation of hydrogen chloride, silver nitrate is added to the still.

20.1.2 Microdiffusion and other methods

Traces of hydrogen fluoride are conveniently separated by mierodiffusion [6-9]. The sample is placed in a polypropylene container, which is then closed. The HF distils isothermally from conc. HC104 over about 20 hours and is absorbed in dilute NaOH solution, or in filter paper impregnated with NaOH. The micro-diffusion of HF has been performed in the presence of hexamethyldisiloxane, which yields the volatile trimethylfluorosilane with HF in a H2SO4 medium [ 10,11 ].

When superheated steam and oxygen are passed through a quartz-, platinum-, or nickel tube containing the sample at --1,000~ metal fluorides present are pyrolyzed, releasing hydrogen fluoride which is absorbed in dilute NaOH after the gases have been cooled. This method is useful for materials containing stable fluorine compounds from which it is difficult to release HF by distillation [ 12-14].

When phosphate, sulphate, and fluoride are sorbed on an anion-exchange column, the

190 20. Fluorine

fluoride is the easiest to elute. During the elution of fluoride from an anion-exchanger with 10 M hydrochloric acid, the anionic-chloride complexes of multivalent metals are retained in the column [15]. A Dowex-2 anion exchanger has been used for the separation of fluorine from drinking water (containing --1 mg of F-per litre) [16]. Fluoride has been separated from aluminium on an anion-exchanger. After sorption of aluminium as AI(OH)4- and F-, aluminium and fluoride were sequentially eluted with 0.2 M NaOH and 1 M NaOH, respectively.

On passage of a 1 M HC1 solution through a cation exchange column, aluminium is retained as A13+, and F- is not retained. Interfering metals have been retained on a cation exchanger before the determination of fluoride in water [17].

Fluoride ions can be separated from other halides by extraction into CHC13 with the use of triphenyltin chloride [ 18].


The direct method for determining traces of fluoride is based on the coloured ternary complex formed by fluoride with Alizarin Complexone and lanthanum or cerium(HI). Fluoride ions form stable complexes with some multivalent metals, namely Zr, Th, Ti, Fe(III), and A1. The colour changes resulting from the reactions of fluoride with coloured complexes of these metals provide indirect methods for the determination of fluoride. Examples of these methods are the Eriochrome Cyanine R-zirconium method, and the (less sensitive) Fe(l/I)- sulphosalicylate method.

20.2.1. Alizarin Complexone method

Yellow Alizarin Complexone (AC) reacts with La or Ce(l]I) ions to form a red chelate, which, in turn, reacts with fluoride ions to give a blue ternary complex (Alizarin Fluorine Blue). The fluoride anion displaces the water molecule bound with the metal ion (formula 20.1) [19,20]. Other suggestions regarding the mechanism of this reaction have also been proposed [21,22].

o /OH, 0 .... H"O 0 0 e F,,,~

~~OC/0 I O~CO F.__~ ~ ~~OC.""O 1%C0 H,O + 0 ;" ~ C H ' "K"I~i/ 0 2 - - (20.1)

An equimolar mixture of AC and La or Ce(III) is used as the reagent. There are only small differences between the sensitivities of the methods using lanthanum or cerium(III). When lanthanum is used, the optimum pH is 4.3-4.7.

In the presence of certain water-miscible organic solvents (e.g., acetone, acetonitrile, dimethyl sulphoxide), equilibrium is attained more rapidly (15-20 min instead of 2 h), and the sensitivity of the reaction and the stability of the blue ternary complex are enhanced [23-26].

The molar absorptivity e in the presence of 20% acetone is 1.4.104 (a = 0.74) at 610 nm. Figure 20.1 shows the absorption spectra of yellow Alizarin Complexone (AC), the red AC- La complex, and the blue AC-La-F complex. The maximum difference in absorbance between the AC-La-F and AC-La complex occurs at 610 nm.

Ions of metals giving stable fluoride compounds, namely A1, Fe(III), Sn, Ca, and Mg, interfere in the determination of fluoride. Phosphate, sulphate, and oxalate, which compete with fluoride in the reaction with the AC-La [or AC-Ce(l]-I)] complex, also interfere. Finally,


certain oxidants, such as nitrite, should also be avoided. Because of these many interferences, the fluoride must usually first be separated by distillation [5], pyrohydrolysis [13], or ion- exchange [ 17].

8

J

t.O0 500 .600 . 700 wavelength, nm

Fig. 20.1 Absorption spectra of Alizarin Complexone (AC) (1), La-AC complex (2) and La-AC-F complex (3) at pH 4.5.

The anionic complexes AC-La-F and AC-Ce-F can be extracted in the presence of organic bases for greater sensitivity. The following are recommended: hydroxylamine and tri-n-butylamine [27], dioctylamine (isobutanol, e = 1.8.104) [28], and diphenylguanidine (CHC13-isobutanol, e = 2.5.104 at 580 nm) [29]. Fluoride was also determined in aqueous phase in the presence of dodecylsulphonate [30]. The derivative spectrophotometry has been applied for determination of fluoride [31 ].

Sulphonated Alizarin Complexone [32-35], and the related reagent Quinalizarin Complexone [36] have also been suggested for determining fluoride. The use of Quinalizarin Complexone increases the sensitivity by about 50%. Traces of fluoride have been pre-concentrated on anion exchange resin impregnated with Alizarin Complexone [37].

Reagents

Alizarin Complexone (AC), 0.001 M solution. Dissolve 0.0963 g of the reagent in 50 ml of water containing 0.3 ml of conc. ammonia solution. Add 0.25 ml of glacial acetic acid, and dilute with water to exactly 250 ml. The solution stored in an amber-glass bottle is stable for a week.

Lanthanum nitrate: 0.001 M solution. Dissolve in water 0.1082 g of La(NO3)3.6H20, and dilute with water to 250 ml in a volumetric flask.

The La-AC reagent. Mix together equal volumes of Alizarin Complexone and lanthanum nitrate solutions. The solution may be used for 2 days

Standard fluoride solution: 1 mg/ml. Dissolve in water 2.2100 g of sodium fluoride, previously ignited at -400~ and dilute the solution with water to 1 litre in a volumetric flask.

Acetate buffer, pH 4.0. Dissolve in water 30 g CH3COONa.3H20, add 57.5 ml of glacial acetic acid, and dilute with water to 500 ml. Procedure

To a 25-ml standard flask, add 3 ml of acetate buffer, 5 ml of La-AC solution, and 5 ml of acetone. To this solution, add the sample solution (distillate) containing not more than 25 gg of F- and neutralized with dilute NaOH or HC1 in the presence of phenolphthalein. Dilute the

192 20. Fluorine

solution with water to 25 ml, mix well, and allow to stand for 20 min. Measure the absorbance at 610 nm against a reagent blank solution.

20.2.2. Eriochrome Cyanine R-zirconium method

Zirconium ions (in dilute HC1) react with Eriochrome Cyanine R (ECR) (formula 4.17) to form red complexes. The complex formed at a deficiency of ECR (~,max = 515 nm) is favoured by a more acidic medium (pH 0-1). Conversely, the complex formed in the presence of excess of ECR 0~max = 540 nm) is formed at lower acidities (pH 1-2). The absorption maximum of Eriochrome Cyanine R in dilute HC1 is at 475 nm.

Addition of fluoride to a solution of Zr-ECR complex results in a colour change owing to partial decomposition of the Zr-ECR complex and formation of the more stable, colourless zirconium fluoride complex. This reaction constitutes the basis of a sensitive method for determining fluoride [26,38--40].

To obtain reproducible results, rigorous observance of the specified conditions is necessary. Zirconium can occur in solution in various forms, because it tends to polymerize and hydrolyse.

To avoid interference by foreign ions in the reaction of fluoride with the Zr-ECR complex, preliminary distillation of fluoride is recommended. Sulphate, which complexes zirconium, and metal ions forming stable fluoride complexes, interferes.

The most suitable molar ratio of Zr-ECR in the zirconium-Eriochrome Cyanine R reagent is 1:4. The highest sensitivity of the reaction is obtained at pH 1_+0.1 [39].

The indirect molar absorptivity calculated from the change in absorbance produced by fluoride (F-) under the conditions given below in the procedure is 2.7.104 (sp. abs., a = 1.4) at 540 nm. At this wavelength, the difference in the absorbance before and after reaction is greatest (at pH 1).

It is recommended to use the ECR solution as the reference when measuring the absorbance of the Zr-ECR solution, partly decolorized with F. The absorbance is maximum at the absence of fluoride, and it decreases as the concentration of fluoride increases. It is more convenient to measure the absorbance against the reagent (ECR-Zr complex). The zero of absorbance corresponds to a zero concentration of fluoride. As the fluoride concentration increases, the difference in absorbance between the reference solution and the sample solution, partly decolorized by fluoride, increases. In this procedure the cuvette with the sample solution is placed in the spectrophotometer holder normally occupied by the reference.

The sensitivity of the ECR-Zr method for indirect determination of fluoride is more than doubled in the presence of excess of cationic surfactant, e.g., CP (e = 5.9.104 at 610 nm) [41 ].

Reagents

Eriochrome Cyanine R (ECR), 0.004 M solution. Dissolve 0.5364 g of the reagent in water containing 2.5 ml of 1 M HC1, and dilute the solution with water to 250 ml in a volumetric flask.

Zirconium solution, 0.005 M in 4 M HC1 (0.4561 g Zr in 1 litre). Dissolve a weighed portion of zirconium chloride or zirconium nitrate in 25 ml of HC1 (1 +1) and evaporate the solution until salt crystals appear, then dilute with 4 M HC1 to 1 litre. Determine the zirconium in the solution gravimetrically as ZrO2, and dilute with 4 M HC1 to give a zirconium concentration of exactly 0.005 M.

Zirconium solution: 0.001 M in 2 M HC1. Dilute 50 ml of the 0.005 M zirconium solution with exactly 1.5 M HC1 to 250 ml.

Zirconium-Eriochrome Cyanine R reagent (Zr:ECR molar ratio = 1:4), freshly prepared.


Add 25 ml of 0.004 M zirconium solution to 25 ml of ECR solution with stirring. Standard fluoride solution: 1 mg/ml. Preparation as in Section 20.2.1.

Procedure

Preparation for determination of F. Place 10-15 ml of conc. HC104 in a 50-ml still, add 15 ml of water, stir the solution well, and add a few fragments of porous porcelain. Connect the still to a condenser, and distil -~15 ml. Stop heating, when the temperature of the solution in the still reaches -150~ During the distillation, the end of the condenser should be immersed in water (5 ml made slightly alkaline to phenolphthalein with ammonia) in a measuring cylinder serving as a distillation receiver. If the solution in the receiver becomes decolorized, add ammonia until the pink colour just reappears. Place exactly 2.5 ml of the Zr-ECR reagent in a 25-ml volumetric flask. Add the distillate to the reagent, while agitating the flask, until the colour of phenolphthalein disappears. Dilute the solution to the mark with water and mix thoroughly. If the distillation system contained no fluoride, the absorbance of this solution should not differ from the absorbance of 2.5 ml of the Zr-ECR reagent diluted to the mark with water in a 25-ml standard flask. Zero absorbance should be obtained when measuring the absorbance of one of the solutions against the another one.

Distillation and determination of F. Having conducted the test above, add to the still with HC104 a known volume of solution (-~15 ml) containing not more than 12 pg of F-. Distil -~15 ml of the solution to a receiver containing 5 ml of water made slightly alkaline by addition of ammonia (as above). Add some ammonia if the solution in the receiver becomes decolorized during the distillation. Place 2.5 ml of the Zr-ECR reagent in a 25-ml volumetric flask, add the distillate with swirling, dilute to the mark with water, and mix well. Prepare a reference solution from 2.5 ml of Zr-ECR reagent diluted to volume with water in a 25-ml standard flask and measure the absorbance of this solution against the sample solution, at 540 nm. Place the cuvette with the sample solution where the reference solution is usually positioned.

Notes. 1. This method of measuring the absorbance gives a calibration curve as for direct spectrophotometric methods, i.e., zero absorbance corresponds to absence of fluoride.

2. In the preparation of the calibration curve, the standard solution of fluoride is added to the Zr-ECR reagent solution in a 25-ml volumetric flask, the solution is diluted to the mark with water, and the absorbance is measured as stated above.

20.2.3. Iron-sulphosalicylate method

In weakly acidic media (pH 2.5-3) iron(HI) forms a red-violet (1:1) complex with sulphosalicylic acid. Addition of fluoride partly decolorizes the solution owing to the formation of a stable iron-fluoride complex. A suitable pH is adjusted by means of a chloroacetate buffer. The method is recommended for determination of larger amounts of fluoride [42].

The indirect absorptivity e (calculated as the decrease of absorbance of the Fe(III)- sulphosalicylate complex at 500 nm owing to the presence of fluoride) is -~2-102 ( a - 0 . 0 1 ) .

The colour system does not obey Beer' s law. Ions of metals forming complexes with sulphosalicylic acid or fluoride, as well as anions

which react with Fe(III), interfere in the determination of fluoride. Separation of fluoride by distillation prevents the interference of foreign ions.

Methylsalicylic acid has also been proposed for determination of fluoride [43].

194 20. Fluorine

Reagents

Sulphosalicylic acid, in solution. Dissolve in water 0.95 g of the hydrated reagent (or 0.82g of the anhydrous form) and dilute with water to 100 ml.

Iron(HI) solution. Dissolve 0.30 g of Fe(NO3)3 in water containing 0.5 ml of concentrated HC104, and dilute with water to 100 ml.

Chloroacetate buffer, pH 2.85-2.9. Dissolve 18.90 g of monochloroacetic acid in 100 ml of 1 M NaOH and dilute with water up to 1 litre.

Iron-sulphosalicylate reagent. Mix 20 ml of the sulphosalicylic acid solution with 40 ml of the Fe(III) solution, and 2.8 ml of the NaOH solution. Dilute the solution with the chloroacetate buffer up to 100 ml and set aside for 5 h. The solution is stable within 10 days.

Standard fluoride solution: 1 mg/ml. Preparation as in Section 20.2.1.

Procedure

Place 12 ml of chloroacetate buffer in a 25-ml standard flask. Add 2.5 ml of the iron- sulphosalicylate reagent, then add the analyte solution (distillate) containing not more than 1 mg of F, and make up to the mark with water. Measure the absorbance of the partly decolorized solution at 500 nm using water as the reference.

20.2.4. Other determination methods

As well as the zirconium-Eriochrome Cyanine R complex, the coloured complexes of zirconium with other organic reagents are used for indirect spectrophotometric determination of fluoride. These include Alizarin S [44-46], SPADNS [6,16,47,48], Xylenol Orange [49- 52] Chrome Azurol S [53], and rutin [54].

Numerous methods for determining fluoride are based on compounds of thorium with organic reagents, such as Alizarin S [55], Xylenol Orange [48], Arsenazo I [4], and chloranilic acid [56] . An exceptionally sensitive method is based on the ternary system Th- Chrome Azurol S-CTA (e = 1.0-105 at 635 nm) [57].

In indirect methods for determination of fluoride, aluminium complexes with Eriochrome Cyanine R (ECR) [58] and Xylenol Orange [59] have been applied. The sensitivity of the ECR method increases considerably in the presence of CP [60]. The complexes of scandium with Methylthymol Blue (~; = 1.07.104 at 590 nm) and Pyrocatechol Violet [61] have also been recommended.

Direct spectrophotometric methods proposed for fluoride (similar to the Alizarin Complexone method) involve use of Arsenazo III-Zr reagent [62,63], Sulphochlorophenol S- Zr reagent (e - 3.0.104) [64], Xylenol Orange-A1 (extraction of ternary fluoride-containing complex into CHC13 in the presence of TOA, ~ = 4.1.104) [65], and Xylenol Orange-Zr reagent (e = 3.4.104) [66].

Small concentrations of fluorine in nitrogen occurring during the fluorination have been determined by the continuous method by passing the gas through a NaC1 solution and measuring at 360 nm the absorbance of chlorine produced in reaction 2C1- + F2 ---> 2F-+ C12 [67].


The Alizarin Complexone method using La or Ce(lII) has been used for determination of

References 195

fluoride in biological materials [27,68-71], waters [23,72,73], air [74,75], soils [68,76], organic matters [3,77-79], silicate rocks and minerals [13,76,80,81], hydrochloric acid [82], trifluoroacetic acid [83], and phosphates [5,28]. This method has been also applied for automatic determination of fluoride in urea [70] and in natural waters [71]; the flow injection technique (FIA) has also been applied [75].

The Zr-ECR method has been applied for determination of fluoride in water [38], air [84], rocks and minerals [85-87], iron ore and apatite [15], and various reagents [39]. Various above-mentioned coloured systems Zr-organic reagent were used for determining fluoride in bones [49], waters [16], silicate rocks [47], phosphates [44], and uranium oxides [50].

The Th complexes with organic reagents have been applied in determinations of fluoride in waters [57] and in organic compounds [4,56]. The Alizarin S method has been automated [55]. The aluminium complex with Xylenol Orange has been used for determining fluorine in phosphates [59]. Fluorine was determined in dental preparations with the use of methylsalicylic acid [43].

References

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196 20. Fluorine

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(1989). 74. Marshall B.S., Wood R., Analyst, 94, 493 (1969). 75. Wada H., Mori H., Nakagawa G., Anal. Chim. Acta, 172, 297 (1985). 76. Babas Pardillo M. et al., Microchem. J., 39, 182 (1989). 77. Kirsten W.J., Shah Z.H., Anal. Chem., 47, 184 (1975). 78. Debal E., Madelmont G., Peynot S., Poliahoff O., Mikrochim. Acta, 1978 I, 441. 79. Tanaka Y., Okazaki A., Hozumi K., Microchem. J., 43, 62 (1991). 80. Hall A., Walsh J.N., Anal. Chim. Acta, 45, 341 (1969). 81. Khalizova V.A., Polupanova L.I., Bebeshko G.I., Alekseeva A.Ya., Zh. Anal. Khim., 30,

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82. Popov V.A. et al., Zavod. Lab., 57, No. 1, 71 (1991). 83. Spirina R.I., Lyakhova K.V., Zavod. Lab., 55, No 4, 6 (1989). 84. Marshall B.S., Wood R.,Analyst, 93, 821 (1968). 85. Huang W.H., Johns W.D., Anal. Chim. Acta, 37, 508 (1967). 86. Evans W.H., Sergeant G.A., Analyst, 92, 690 (1967). 87. Gimeno Adelantado J.V. et al., Talanta, 32, 224 (1985).

Chapter 21. Gallium

Gallium (Ga, at. mass 69.72) forms colourless Ga 3+ ions. It occurs in solution exclusively in the III oxidation state. Gallium resembles aluminium and zinc in its properties. The hydroxide, Ga(OH)3, precipitates at pH -~3, but dissolves in weakly alkaline media (pH 8-9). Gallium forms halide, oxalate, tartrate, acetate, and EDTA complexes.


21.1.1. Extraction

Gallium is extracted as the chloride complex from 5.5-6.5 M HC1 with diethyl ether. In a single extraction, --95% of the gallium passes into the organic phase. When the extractant is DIPE, more than 99% of the gallium is extracted in one step (the optimum HC1 concentration is 7-8 M). The following species are also wholly or partly extracted: Fe(III), Au(III), TI(III), Ge, Mo, Re, As, Sb, Sn, and Te. Many ions [e.g., A1, Ti, Fe(II), and Zn] remain quantitatively in the aqueous phase. Before gallium is extracted, Fe(III) is reduced with TIC13 or ascorbic acid. The chloride complex of gallium may be extracted with MIBK [1] or with methyl ethyl ketone [2].

Gallium has been extracted from HC1 media into chloroform in the presence of N- benzylaniline [3,4], n-octylaniline [5], and crown ethers (e.g., dicyclohexyl-18-crown-6) [6], and into o-dichlorobenzene in the presence of the tetraphenylarsonium ion [7]. Gallium has been separated from In and other metals by extraction into non-polar solvents in the form of its chelates with cupferron [8], HTTA [9], BPHA, [3-diketones [10], and alkylphosphoric acids [11]. Gallium has been extracted also, from citric acid solutions of pH 2-3.5, into xylene solutions of Amberlit LA-2 [ 12] and Aliquat 336S [ 13].


Gallium can be separated from A1 and Zn on anion- or cation-exchange columns with the use of oxalate, tartrate, or EDTA complexes. In an ammonium carbonate medium, Ga exists as the anionic carbonate complex, whereas zinc exists as the cationic ammine complex.

Indium and gallium are separated by sequential elution from a Dowex-50 cation- exchanger with 0.4 M and 1.3 M HC1, respectively. Cation exchange in acetone-aqueous HC1 gives a very clean separation of Ga, In, A1, and T1 [14]. Ga has been separated from In, Zn, Fe(III), Cu, and other metals by cation exchange chromatography in acetone-aqueous solution of hydrobromic acid [15]. The differences between the stabilities of chloride complexes [16] have permitted anion-exchange separation of Ga from In, A1, and Fe.

When a sample is fused with Na2CO3 or NaOH, the gallium is leached almost quantitatively from the melt with water, whereas Fe(III) and Zn remain in the solid phase. Similarly, gallium remains in solution during the precipitation of In and Fe(III) with NaOH solution.

Traces of gallium have been co-precipitated with AI(OH)3 [17], Fe(OH)3 [18], and MnO2 aq. [ 19] as collectors.



The sensitive and selective spectrophotometric methods involve extraction of ion-associates of the GaC14- ion with basic dyes (e.g., Rhodamine B). Methods based on ternary complexes of gallium with chelating triphenylmethane reagents (e.g., Eriochrome Cyanine R) and some surfactants are very sensitive but less selective.


The gallium chloride complex, GaC14-, reacts with Rhodamine B (formula 4.29), to form the ion pair which can be extracted from HC1 medium into benzene or other solvents. The pink extract is the basis of a sensitive and selective method for the determination of gallium [20- 22]. Addition of NaCI improves the extraction efficiency very effectively [22]. A mixture of benzene and diethyl ether, and also o-dichlorobenzene [23], have been recommended as extractants. Since Rhodamine B is only slightly soluble in o-dichlorobenzene, but dissolves readily in the meta-isomer, the o-dichlorobenzene must not be contaminated with the meta- isomer.

The Rhodamine B chlorogallate complex is 96% extracted into the mixed C6H5C1 + CC14 solvent under the conditions given in the procedure below. Only a small amount of free Rhodamine B is co-extracted. The molar absorptivity appears to depend on the conditions used and is -~ 1.0.105 (a = 1.4) at 560 nm.

The Rhodamine B ion-associates with Fe(III), Au(III), Sb(V), and TI(III) are also extracted from 6 M HC1. In the presence of reducing agents, such as ascorbic acid, TIC13, SnCI2, or NHzOH, these interfering metals are reduced to their lower oxidation states and do not react with Rhodamine B. In the presence of larger quantities of these metals, it is advisable to separate gallium first by extraction from HC1 medium with DIPE or diethyl ether (in the presence of a reducing agent).

Reagents

Rhodamine B, 0.5 % solution in 6 M HC1. Standard gallium solution: 1 mg/ml. Dissolve 0.1000 g of gallium metal in 5 ml of HC1

(1+1), and dilute the solution with HC1 (1+1) to 100 ml in a volumetric flask. Working solutions are obtained by appropriate dilution of the stock solution with HC1 (1 + 1).

Titanium(III) chloride, 1% solution in 6.5 M HC1, stored in a closed bottle.

Procedure

Extraction separation of Ga. To the sample solution (in 7-8 M HC1) containing not more than 15 ~tg of Ga, add the Ti(III) solution [the amount depends on the Fe(III) present]. Extract the gallium with two portions of DIPE (shake with each portion for about 1 min).

Determination of Ga. Place in a beaker the ethereal extract obtained as above, add -50 mg of NaC1, and evaporate to dryness on a water-bath. Dissolve the residue in 5 ml of 6.5 M HC1 containing TIC13, transfer the solution to a separating funnel, add 1 ml of the Rhodamine B solution, and extract gallium with two 10-ml portions of chlorobenzene and CC14 (3+1) solution (shake with each portion for about 1 rain). Place the clear extract in a 25-ml standard flask, dilute to the mark with solvent, mix, and measure the absorbance of the solution at 560 nm vs. a reagent blank.

200 21. Gallium

21.2.2. Eriochrome Cyanine R and CTA method

Gallium ions form an orange-red complex with Eriochrome Cyanine R (ECR) (formula 4.17) at pH 3-7. The optimum pH is 5.1+0.2, at which ~max of the complex is 535 nm, and E~max of the reagent is 515 nm. For maximal absorbance at least a 10:1 ratio of ECR to Ga is necessary [24,25]. The molar absorptivity is about 5.104.

In the presence of CTA in sufficient excess, a bathochromic shift of ~max of the complex (to 588 nm) and an increase in the colour intensity are observed [24,26]. The absorption spectra of ECR and of the binary and ternary complexes with gallium are shown in Fig. 21.1.

7

5nn l

40O

3

600 wave length , nm

Fig. 21.1. Absorption spectra of Eriochrome Cyanine (ECR) (vs. water) (1), Ga-ECR complex (2), and Ga-ECR-CTA complex (3) (vs. reagent blank) (pH 5.4)

The optimum pH for formation of the ternary complex is 5.3_+0.2. Acetate or hexamine buffer can be used. Maximum absorbance is attained in 5 min and it is constant for 1 h. With a suitable concentration of CTA (as in the procedure), the soluble complex with E~max at 588 nm is formed. When the CTA concentration is too small, the solution becomes turbid; at CTA concentrations that are too high, the absorbance decreases. The molar absorptivity under the optimum conditions (as given in the procedure) is 1.20.105 (a = 1.7) at 588 nm.

Citrate and EDTA at concentrations of 0.4 and 0.2 mg/ml, respectively, completely suppress the reaction. Numerous metals interfere [e.g., Be, A1, Fe(III), In, Sc, U, Th, Zr, V], but these interferences can be readily avoided by preliminary extraction of gallium from 7 M HC1 with DIPE. The method with preliminary extraction has been used for determination of traces of Ga in aluminium [24].

Other cationic surfactants, cetylpyridinium bromide (CP) [27], and Zephiramine [26] have been recommended for determining Ga in a ternary system with ECR.

Reagents

Eriochrome Cyanine R (ECR), 5.10 -4 M (+0.025%) solution. Cetyltrimethylammonium bromide (or chloride) (CTA), 4.10-3M solution. Standard gallium solution: 1 mg/ml. Preparation as in Section 21.2.1. Acetate buffer (pH 5.4). Add 225 ml of 0.1 M ammonia solution to 275 ml of 0.1 M


acetic acid.

Procedure

Add 5 ml of ECR solution and 5 ml of CTA solution to a slightly acidic sample solution (pH 1-2) containing not more than 15 gg of Ga. Dilute the solution to -15 ml, adjust to pH ~-5.4 with ammonia, and add 5 ml of acetate buffer. Transfer the solution to a 25-ml standard flask, dilute to the mark with water, mix, and after 5 min measure the absorbance of the solution at 588 nm, vs. a reagent blank.


A comparison has been made of spectrophotometric methods based on extraction of the ion- associates formed by GaC14 and basic triphenylmethane or diphenylnaphthylmethane dyes. The best reagent of this class for determining gallium appears to be Crystal Violet (e = 4.9.104 at 589 nm in CHC13 : acetone = 6:1) [28]. Brilliant Green [28] and Victoria Blue 4R [28,29] can also be used.

Gallium can also be determined with Methylene Blue (formula 48.1) [30]. A very sensitive method (e =2.105) is based on an extractable (toluene) ion associate of the anionic pyrocatechol complex of gallium and Brilliant Green [31 ].

Gallium ions form coloured complexes with a number of t r iphenylmethane chelating reagents besides Eriochrome Cyanine R. Pyrocatechol Violet (PV) gives with Ga and diphenylguanidine a ternary complex, extractable into n-butanol-CHC13 (1+1) mixture (e=l.08.105) [32]. In another extractive spectrophotometric method with PV, tridodecylethylammonium bromide, and xylene, e = 8 . 0 . 1 0 4 [33]. In a method based on the binary Ga-PV complex, e = 6.4.104 [34]. Chrome Azurol S used with cationic surfactants gives sensitive methods for gallium: with CTA, e = 1.2.105 [26,35], with CP [26,36], and Zephiramine [26]. Methods which use Xylenol Orange are less sensitive (e = 2.3.10 4 at 545 nm) [37,38]. Other triphenylmethane reagents for Ga are: Methylthymol Blue [39], Chromoxane Blue B with CTA [40], and Chromal Blue G with CTA (~ = 1.4.105) [41].

Many chelating azo dyes have been proposed as reagents for determination of gallium, viz. PAR [12,13,42-44], PAN [45], TAR [46], Lumogallion (formula 21.1) [47], Picramine M (in aqueous organic media, e -7.104) [48], 1-(2-quinolylazo)-m-aminophenol (e = 6.5.104) [49], Eriochrome Black T (extraction into n-butanol-CHC13 mixture in the presence of capronic acid, e = 3.4-104) [50], and some antipyrylazo compounds [with e = (7-9)-104] [51 ].

HO3S OH HO

/ CI

(21.1)

Among other organic reagents suggested for the spectrophotometric determination of gallium are: Pyrogallol Red and Bromopyrogallol Red in the presence of CTA [~ = 1.0.105- 1.3.105] [52], phenylfluorone in the presence of CP (~ = 1.15.105) [53], and of a surfactant with pyridine [54], haematein [1], haematoxylin in the presence of CTA (~ = 1.4.105) [55], and thiocarbonhydrazide and carbonhydrazide derivatives [56].

202 21. Gallium


The Rhodamine B method has been applied for determining gallium in minerals and ores [20,21,23,57], in bauxites [58], aluminium [59], steel [60], tungsten [17], and platinum [61].

Among other organic reagents, Ga has been determined with Chrome Azurol S in aluminium, aluminium chloride, and peat bath [62], with Xylenol Orange in biological samples [63], with PAR in aluminium and its alloys [64], with haematoxylin in aluminium alloys [55], with Carminic Acid in gallium arsenide semiconductor materials [65], and with Semimethylthymol Blue in minerals and ores [66].

References

1. Graffmann G., Jackwerth E., Z. Anal. Chem., 246, 12 (1969). 2. Rafaeloff R., Anal. Chem., 43, 272 (1971). 3. Khosla M.M., Rao S.P.,Anal. Chim. Acta, 61, 156 (1972). 4. Khosla M.M., Singh S.R., Rao S.P., Talanta, 21, 411 (1974). 5. Kuchekar S.R., Chavan M.B., Talanta, 35, 357 (1988). 6. Koshima H., Onishi H.,Analyst, 111, 1261 (1986). 7. Finston H.L., Rahaman M.S., Mikrochim. Acta, 1969, 78. 8. Vadasdi K.G., Anal. Chim. Acta, 44, 471 (1969). 9. Dhond P.V., Khopkar S.M., Talanta, 23, 51 (1976). 10. Uhlemann E., Mickler W., Fischer C., Anal. Chim. Acta, 130, 177 (1981). 11. Levin I.S., Balakireva N.A., Novoseltseva L.A., Zh. Anal. Khim., 29, 1095 (1974). 12. Vithute C.P., Khopkar S.M.,Analyst, 111,435 (1986). 13. Karve M.A., Khopkar S.M., Chem. Anal. (Warsaw), 38, 469 (1993). 14. Strelow F.W., Victor A.H., Talanta, 19, 1019 (1972). 15. Strelow F.W., Talanta, 27, 231 (1980). 16. Korkisch J., Hazan I., Anal. Chem., 36, 2309 (1964). 17. Buxbaum P., Vadasdi K.G., Chem. Anal. (Warsaw), 14, 429 (1969). 18. Musi6 S., Wolf R.H., Mikrochim. Acta, 1979 I, 87. 19. Biskupsky V.S.,Anal. Chim. Acta, 46, 149 (1969). 20. Culkin F., Riley J.P., Analyst, 83, 208 (1958). 21. Lypka G.N., Chow A., Anal. Chim. Acta, 60, 65 (1972). 22. Hasegawa Y., Inagake T., Karasawa Y., Fujita A., Talanta, 30, 721 (1983). 23. Rutkowski W., Basifiska M., Chem. Anal. (Warsaw), 13, 641 (1968). 24. Marczenko Z., Ka~owska H., Mikrochim. Acta, 1979 II, 507. 25. Tikhonov V.N., Fedotova S.N., Zh. Anal. Khim., 37, 1888 (1982). 26. Jarosz M., Chem. Anal. (Warsaw), 33, 675 (1988). 27. Ganago L.I., Ishchenko N.N., Zh. Anal. Khim., 37, 1636 (1982). 28. Armeanu V., Costinescu P., Talanta, 14, 699 (1967). 29. Kish P.P., Bukovich A.M., Ukr. Khim. Zh., 35, 1290 (1969). 30. Kish P.P., Bukovich A.M., Zh. Anal. Khim., 24, 1653 (1969). 31. Nazarenko V.A. et al., Zh. Anal. Khim., 36, 1315 (1981). 32. Akhmedli M.K., Glushchenko E.L., Gasanova Z.L., Zh. Anal. Khim., 26, 1947 (1971). 33. Shijo Y., Shimizu T., Sakai K., Bull. Chem. Soc. Jpn., 56, 105 (1983). 34. Tikhonov V.N., Bakhtina V.V., Zh. Anal. Khim., 39, 2126 (1984). 35. Evtimova B., Nonova D.,Anal. Chim. Acta, 67, 107 (1973).

References 203

36. Ganago P.I., Ishchenko N.N., Zh. Anal. Khim., 35, 1718 (1980). 37. Pyatnitskii I.V., Pinaeva S.G., Ukr. Khim. Zh., 43, 188 (1977). 38. Kazachkova P.I., Rybina G.K., Zakharova N.S., Zavod. Lab., 45, 605 (1979). 39. Nazarenko V.A., Nevskaya E.M., Zh. Anal. Khim., 24, 839 (1969). 40. Uesugi K., Shigematsu T., Talanta, 24, 391 (1977). 41. Uesugi K., Miyawaki M., Microchem. J., 26, 288 (1981). 42. Biriuk E.A., Nazarenko V.A., Ravitskaya R.V., Zh. Anal. Khim., 27, 1934 (1972). 43. Siroki M., Herak M.J., Anal. Chim. Acta, 87, 193 (1976). 44. B lanco M. et al., Talanta, 40, 261 (1993). 45. Cheng K.L., Goydish B.L.,Anal. Chim. Acta, 34, 154 (1966). 46. Langova-Hnili6kova M., Sommer L., Talanta, 16, 681 (1969). 47. Pyatnitskii I.V., Boryak A.K., Kolomiets L.L., Zh. Anal. Khim., gl, 2199 (1986). 48. Petrova T.V., Matveets I.A., Savvin S.B., Zh. Anal. Khim., 40, 280 (1985); 41, 271

(1986). 49. Turachanova N.T. et al., Zh. Anal. Khim., 43, 1042 (1988). 50. Pyatnitskii I.V., Kolomiets L.L., Sulima N.D., Zh. Anal. Khim., 40, 115 (1985). 51. Gusev S.I. et al., Zh. Anal. Khim., 38, 1274 (1983). 52. Wyganowski C., Microchem. J., 26, 45 (1981); 27, 13 (1982). 53. Al'bota L.P., Gutsulyak R.B., Al'bota N.K., Ukr. Khim. Zh., gl, 1290 (1985). 54. Sakuraba S., Talanta, 37, 637 (1990). 55. Zaki M.T., E1-Didamony A.M.,Analyst, 113, 1277 (1988). 56. de la Rosa F.J., Godoy R.E., Ariza J.L., Talanta, 35, 343 (1988). 57. Koshima H., Onishi H., Analyst, 112, 335 (1987). 58. Soljic Z., Marjanovid V., Chim. analyt., 51, 121 (1969). 59. Szfics A.I., Klug O.N., Chem. Anal. (Warsaw), 11, 665 (1966); 121, 939 (1967). 60. Sauer K.H., Nitsche M.,Arch. Eisenhiittenw., 47, 153 (1976). 61. Marczenko Z., Krasiejko M., Zawartko B., Chem. Anal. (Warsaw), 25, 275 (1980). 62. Kwapulifiska G., Chem. Anal. (Warsaw), 40, 783 (1995). 63. Agrawal Y.K., Bhatt V.J., Microchem. J., 44, 258 (1991). 64. Kakade S.M., Shinde V.M.,Analyst, 118, 1449 (1993). 65. Filik H., Tfitem E., Apak R., Mikrochim. Acta, 129, 57 (1998). 66. Hafez M. A., Kenawy M. M., Microchim. Acta, 129, 291 (1998).

Chapter 22. Germanium

Germanium (Ge, at. mass 72.59) resembles tin and arsenic in its chemical properties. It occurs in the IV and II (and-IV in GeH4) oxidation states. Germanium(H) compounds are unstable. Germanium(IV) compounds are amphoteric with the acidic properties predominating. The white sulphide, GeS2, precipitates from strongly acidic media but dissolves in alkali. Germanium(W) forms halide and oxalate complexes and heteropoly acids.


22.1.1. Extraction. Distillation

A very selective separation of germanium from other elements is achieved by extracting GeCl4 from 9 M hydrochloric acid into CC14, CHC13 or C6H6 [ 1-4]. An extraction of GeC14 with heptane from 9.4 M HC1 has been recommended [5]. Only AsC13 and traces of some other metals are also extracted. Extraction of As(III) is prevented by oxidation to As(V). Water or an oxalic acid solution strips germanium from the organic phase. Germanium can also be extracted as GeBr4 with diethyl ether.

Germanomolybdic acid has been extracted into MIBK [6], and its associate with chromopyrazole has been extracted into toluene [7].

The Ge complex with N-p-chlorophenyl-2-furohydroxamic acid is extractable from 6 M HC1 into CHC13 [8]. Ge has been also extracted from a citric acid medium into a xylene solution of Aliquat 336S [9].

Germanium can be separated quantitatively by distillation of volatile GeCl4 (b.p. 84~ from -~6 M HC1 [10,11]. Part or all of any As(m), Sn(IV), and Sb(III) distils with the GeC14. Distillation of arsenic is prevented by oxidation with KMnO4 to non-volatile As(V), or reduction to the element with powdered copper. Any fluoride in the sample solution is distilled off first (as hydrogen fluoride) at 160~ from H2SO4 medium (HCl-free). After removal of the fluoride, NaC1 is added to the still, and GeC14 is distilled.

It has been shown that the solvent extraction and the distillation of GeC14 give equally good results. Extraction is more rapid, and stripping yields germanium in aqueous solution, whereas the distillation yields germanium in moderately concentrated acid (6 M HC1). Ge can also be separated (as GeBr,) by distillation from a NaBr-phosphoric acid medium [12].


Dowex-50 cation-exchanger retains most metal ions from dilute HC1, whereas germanium is eluted as neutral GeC14 [13]. GeC14 has been separated from both cations and anions by passing the sample solution at pH 2 through a mixed-resin bed (strongly acidic cation- exchanger and weakly basic anion-exchanger) [ 14].

Strongly basic anion-exchangers retain Ge(IV) and As(V) from solution at pH 6-9. First, Ge is eluted with 0.2 M acetic acid, and then As is eluted with 0.5 M H2SO4. Traces of Ge were separated from zinc sulphate solutions with the aid of polyphenol ion-exchange resins [ 15].


Germanium has been retained on silica gel impregnated with bis(2- ethylhexyl)phosphoric acid [ 16]. Partition coefficients of germanomolybdate were studied on a Sephadex LH-20 column [ 17].

Germanium traces were separated by co-precipitation with Fe(OH)3 [5]. Germanium sulphide has also been precipitated from 6 M HC1 with Hg(II) or As(HI) as collectors.


The phenylfluorone method has been used generally for determining germanium. The sensitivity of this method is higher in the presence of surfactants. Methods based on ion associates with basic dyes deserve notice because of their high sensitivities.

22.2.1. Phenylfluorone method

Germanium(IV) reacts with phenylfluorone (formula 22.1) to form a complex which is the basis of a sensitive method for determining germanium [ 18]. The reaction proceeds slowly in acid medium (pH 0-1), but fairly rapidly (1-2 min) in acetate-buffered solution (pH 4-5). Although a greater excess of reagent accelerates the reaction rate, it is most convenient to carry out the reaction at pH 4-5 with a small excess of phenylfluorone, and then to acidify the solution to pH < 1 before measuring the absorbance. In order to stabilize the colloidal suspension of the complex, a protective colloid is added [e.g., gum arabic, gelatine, or poly(vinyl alcohol)]. The presence of methanol (--40%) helps to maintain a stable, clear pseudosolution. The excess of phenylfluorone dissolves in the acidic aqueous-methanol medium.

(22.1)

At wavelengths >500 nm the interference from the absorption of the reagent is slight. The molar absorptivity of the solution of the Ge-phenylfluorone complex, under the conditions given in the procedure below, is 5.3.104 at 510 nm (a - 0.73).

Phenylfluorone also forms coloured complexes with many metals, e.g., Sn, Sb, Ti, Fe(III), Nb, and Ta. Low concentrations of arsenic, silicon, and fluoride do not interfere in the formation of the germanium complex. Citric and oxalic acids are used to mask Mo, V, Sn, and Ti [19,20]. Preliminary separation of germanium as GeC14 by extraction or distillation renders the phenylfluorone method specific for germanium.

When the colloidal solution of germanium phenylfluoronate is shaken with CC14, the precipitate agglomerates at the interface. After removal of nearly all the CC14 and the aqueous solution (containing the excess of phenylfluorone), the precipitate is dissolved in acetone and the absorbance measured [21]. High sensitivity (~= 1.2.105 at 495 nm) is obtained, if phenylfluorone is added to a chloroform extract of Ge complex with N-p- chlorophenyl-2-furohydroxamic acid [8]. A DMF solution of phenylfluorone was also added to a toluene extract of GeC14 (~ -- 7.1.104 at 525 nm [3].An aqueous solution of Ge was also

206 22. Germanium

shaken with a solution of phenylfluorone and benzoic acid in heptanol [22].

Reagents

Phenylfluorone, 0.01% solution in methanol. Dissolve 25 mg of the reagent in methanol containing 2.5 ml of conc. HC1, and dilute the solution with methanol to volume in a 250-ml standard flask. A method for purification of phenylfluorone has been proposed [23].

Standard germanium solution: 1 mg/ml. Dissolve 0.1000 g of germanium powder in 5 ml of 1 M NaOH with 3 drops of H202 solution, acidify the solution slightly with HC1, and dilute to the mark in a 100-ml standard flask with water.

The preparation of a standard solution from germanium dioxide has also been described [24].

Acetate buffer (pH --5). Dissolve in water 450 g of sodium acetate and 240 ml of glacial acetic acid, and dilute the solution with water to 1 litre.

Procedure

Extractive separation of Ge. Add sufficient conc. HC1 to the sample solution to give a final acid concentration of at least 9 M. (The sample solution can be concentrated by evaporation after being made slightly alkaline with NaOH). Shake the solution for 2 min in a separating funnel with two portions of CC14, and wash the combined organic extracts with 9 M HC1. Strip the germanium by shaking the CC14 solution with 10 ml of water, followed by 5 ml of water containing 1 drop of 1 M NaOH; shaking time 1 min. Determination of Ge. To a 25-ml standard flask, add the solution obtained (or an aliquot thereof) containing not more than 25 gg of Ge. Add water to 10 ml, 1 ml of 1% gum arabic solution, 5 ml of phenylfluorone solution, 5 ml of methanol, and 1 ml of acetate buffer, mixing the solution after the addition of each reagent. After 5 min, dilute the solution to the mark with HC1 (1 +4), and measure the absorbance at 510 nm against a blank solution.

22.2.2. Phenylfluorone-CTA method

Cetyltrimethylammonium bromide (CTA) has been found to increase greatly the sensitivity of germanium determination with phenylfluorone [5,25].

The optimum range of HC1 concentration is 1.0-1.8 M; germanium at this acidity exists as the Ge(OH)22+ cation. The cationic surfactant, CTA, should be in large excess (--0.002 M). Under these conditions the optimal concentrations of phenylfluorone and ethanol (which is required to keep the chromogenic reagent in solution) are --8.10 -5 M and 8%, respectively. More than 12% v/v ethanol inhibits the reaction. Higher concentrations of phenylfluorone are not recommended because of the magnitude of the reagent blank. Complex formation is complete in 2-3 min, and the absorbance of the complex remains constant for at least 4 h. The molar absorptivity of the ternary complex is 1.71.105 at 507 nm (a = 2.3).

A combination of this method with separation of germanium, e.g., by extraction as GeC14, makes it specific.

Cetylpyridinium salt (CP) (e = 1.10.105) [26] and Zephiramine (e = 1.36.105 at 505 nm) [27] have been recommended for use in ternary systems with Ge and phenylfluorone. With the use of sodium lauryl sulphate as anionic surfactant the value of e = 1.18-105 at 504 nm has been obtained [28].


Reagents

Phenylfluorone, 0.001 M (-0.03%) solution. Dissolve 80 mg of the reagent in 100 ml of ethanol containing 1 ml of conc. HC1, then dilute the solution to 250 ml with ethanol. The solution is stable for one month.

CTA, 0.9% solution. Dissolve 2.25 g of the bromide salt of CTA in 150 ml of warm water. Dilute the solution to 250 ml with water. The solution is stable for one month.

Standard germanium solution: 1 mg/ml. Preparation as in Section 22.2.1.

Procedure

To a slightly acidic sample solution (about l0 ml in a 25-ml standard flask) containing not more than 8 ~tg of Ge, add 2 ml of 10% ascorbic acid solution, 3 ml of conc. HC1, and 2 ml of CTA solution. Then add 2 ml of phenylfluorone solution, dilute to volume with water, and mix. After 5 min, measure the absorbance of the solution at 507 nm v s . a reagent blank.

22 .2 .3 . O t h e r methods

In addition to phenylfluorone, a number of other 2,3,7-trihydroxy-6-fluorones have been recommended for determination of germanium. Disulphophenylfluorone (~ = 9.0. l04) reacts under the conditions similar to those of fluorone [29]. Some trihydroxyfluorones react with Ge in the presence of antipyrine and bromide (also I-, SCN-, and C104-) to form compounds extractable with CHC13 or similar solvents [30]. The sensitivity of determining Ge with various fluorones increases in the presence of cationic surfactants. The following systems have been recommended: salicylfluorone with CP (e = 1.2.105 at 530 nm) [31], tetrabromosalicylfluorone with CP (e = 1.3.105 at 520 nm) [32], and o-chlorophenylfluorone with CTA (e = 1.8.105 at 516 nm [33]). A non-ionic surfactant Triton X-305 has been used in the case of disulphophenylfluorone [34].

Germanium(W) forms heteropoly acids with molybdate and other ions. The method for determining germanium, based on yellow germanomolybdic acid [35-37] is insensitive (e = 2.0.103 at 430 nm), but reduction of the heteropoly acid to germano-molybdenum blue [38,39] considerably increases the sensitivity (e = 1.0.104 at 800 nm).

High sensitivity characterizes methods based on the formation of sparingly water- soluble ion-associates of germanomolybdate (Mo-Ge) with basic dyes. The compound with Rhodamine B can be floated and then dissolved in ethanol. The molar absorptivity is -3.7.105 [40]. The Mo-Ge compounds with Methylene Blue, Crystal Violet or Malachite Green, can be centrifuged and then dissolved in acetone. The molar absorptivities are: 4.5.105, 4.2.105, and 6.2.105, respectively [41-43]. The ion associate formed by the Mo-Ge anion (reduced with ascorbic acid) with Chrompyrazole II has been floated by shaking with toluene, then dissolved in acetone [44].

Methods based on extraction of Ge(IV) complexes with 3,5-dinitropyrocatechol or 4- nitropyrocatechol, associated with Brilliant Green (CC14, E =1.4.105), Nile Blue A (CHC13, =1.3.105), and Methylene Blue (benzene, e-1.0-105), have been recommended [45,46]. The anionic complex of Ge with Alizarin Complexone, associated with Rhodamine 6G, is the basis of a sensitive method (e - 2.9.105) [1,2]. In another very sensitive method, use has been made of tetrabromofluorescein and Rhodamine 6G [47]. Ge is determined after extraction (with chlorobenzene) with mandelic acid and Malachite Green (e = 1.33.105) [48].

Some authors recommend Pyrocatechol Violet (PV) as a reagent for germanium

208 22. Germanium

[13,49-51]. Ge is determined in the CC14 phase, to which a solution of PV in propanol is added (e = 4.3.104) [49]. This reagent has also been used in the presence of 1,10- phenanthroline [50], and of CTA [51 ].

Several anthraquinone derivatives have been suggested for the spectrophotometric determination of germanium, namely quinalizarin (extraction of the complex with chloroform in the presence of diphenylguanidine), Purpurin (1,2,4-trihydroxyanthraquinone) [52], and Alizarin Complexone [53].

Among other organic reagents for germanium, Bromopyrogallol Red is to be mentioned [54]. The ion associate of the Ge complex with Bromopyrogallol Red and Rhodamine 6G is floated with toluene, then dissolved in acetone (e = 2.6-105) [55].


The phenylfluorone method has been used for determining germanium in biological samples [56], water [57], ores and minerals [11,58], coal and coke [9,14,59], coal ashes [16], gallium [60], organogermanium compounds [61], thin germanium-niobium films [62]. The flow- injection technique has been applied for determining Ge in pharmaceutical products [63]

The phenylfluorone method involving surfactants has been used for determining Ge traces in natural waters (Zephiramine) [64], industrial waters [65,66], ores and concentrates of zinc, cobalt, nickel, lead, and copper (CTA) [5], rocks [67], alloys [68], and glass (CP) [26].

Germanium was determined in vegetables with the use of Rhodamin 6G [47], and in high purity arsenic with the use of Brilliant Green [46].

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Chem., 195, 251 (1995). 18. Tobia S.K., E1-Shahat M.F., Saad E.A., Microchem. J., 23, 525 (1978); Mikrochim.

Acta, 1979 I, 175. 19. Dranitskaya R.M., Gavrilchenko A.I., Okhitina L.A., Zh. Anal. Khim., 25, 1740 (1970). 20. Dranitskaya R.M. et al., Zh. Anal. Khim., 26, 2137 (1971).

References 209

21. Luke C.L., Chemist Analyst, 54, 109 (1965). 22. Charykov A.K. et al., Zh. Anal. Khim., 41, 1596 (1986). 23. Petrova G.S. et al., Anal. Lett., 5, 695 (1972). 24. Shimomura S., et al., Anal. Chim. Acta, 91, 421 (1977). 25. Ni Y.N., Wan C.H.,Anal. Lett., 28, 2239 (1995). 26. Amelin V.G., Andreev P.A., Zavod. Lab., 55, No 2, 16 (1989). 27. Marczenko Z., Krasiejko M., K~bek A., Microchem. J., 34, 121 (1986). 28. Burns D.T., Dadgar D., Analyst, 105, 75 (1980). 29. Suchan W.W., Nazarenko A.Yu., Lipkowska N.A., Zh. Anal. Khim., 43, 1792 (1988). 30. Nazarenko V.A., Makrynich N.I., Shustova M.B., Zh. Anal. Khim., 25, 1595 (1970). 31. Amelin V.G., Chernova R.K., Zh. Anal. Khim., 39, 1436 (1984). 32. Antonovich V.P. et al., Zh. Anal. Khim., 40, 834 (1985). 33. Shen H., Wang Z., Xu G.,Analyst, 112, 887 (1987). 34. Nazarenko A.Yu., Zh. Anal. Khim., 40, 828 (1985). 35. Chalmers R.A., Sinclair A.G., Anal. Chim. Acta, 33, 384 (1965). 36. Masson M.R., Mikrochim. Acta, 1976 I, 385. 37. Hernandis V., Maci~ L., Sala J.V., Analyst, 112, 1007 (1987). 38. Zhukovskii Yu.G., Zh. Anal. Khim., 19, 1361 (1964). 39. Paul J., Anal. Chim. Acta, 35, 200 (1966). 40. Mirzoyan F.W., Sarkisyan N.P., Petrosyan A.A., Ukr. Khim Zh., 53, 391 (1987). 41. Mirzoyan F.V., Tarayan V.M., Airiyan E.Kh., Anal. Chim. Acta, 124, 185 (1981). 42. Mirzoyan F.V. et al., Talanta, 27, 1055 (1980). 43. Mirzoyan F.W., Tarayan V.M., Airiyan E.Kh., Zh. Anal. Khim., 39, 2010 (1984). 44. Lychnikov D.S., Dorokhova E.N., Gracheva M.A., Zh. Anal. Khim., 43, 802 (1988). 45. Nazarenko V.A. et al., Ukr. Khim. Zh., 43, 1325 (1977). 46. Chwastowska J., Grzegrz6~ka E., Chem. Anal. (Warsaw), 20, 1065 (1975). 47. Zhao X. et al., Talanta, 44, 979 (1997). 48. Sato S., Tanaka H., Talanta, 36, 391 (1989). 49. Agrinskaya N.A., Golosnitskaya V.A., Kovalenko E.V., Zavod. Lab., 33, 923 (1967). 50. Ganago L.I., Semenovich I.A., Zh. Anal. Khim., 29, 1964 (1974). 51. Leong C.L., Talanta, 18, 845 (1971). 52. Nazarenko V.A., Flyantikova G.V., Tetereva A.M., Zh. Anal. Khim., 29, 284 (1974). 53. Nazarenko V.A. et al., Zh. Anal. Khim., 30, 1354 (1975). 54. Burns D.T., Dadgar D., Analyst, 105, 1082 (1980). 55. Ganago L.I., Ivanova I.F., Zh. Anal. Khim., 47, 543 (1992). 56. Schleich C., Henze G., Fresenius'J. Anal. Chem., 338, 140 (1990). 57. Kan M., Nasu T., Taga M., Anal. Sci., 5, 707 (1989). 58. Harada A., Tarutani T., Yoshimura K., Anal. Chim. Acta, 209, 333 (1988). 59. Sendul'skaya T.I. et al., Zh. Anal. Khim., 22, 445 (1967). 60. Nazarenko V.A., Chivireva A.A., Grekova I.M., Zh. Anal. Khim., 42, 360 (1987). 61. Obtemperanskaya S.I., Dudova I.V., Dikaya G.F., Zh. Anal. Khim., 23, 784 (1968). 62. Bodkin J.B., Rogowski B.A., Analyst, 102, 110 (1977). 63. Shimada K., Nakajima M., Wakabayashi H., Yamato S., Chem. Pharm. Bull., 37, 1095

(1989). 64. Nakatsuka I., Takahashi K., Ohzeki K., Ishida R., Analyst, 114, 1473 (1989). 65. Egorova 1.A., Avramenko L.I., Soy. J. Water Chem. Technol., 13, 70 (1991). 66. Kania K., Chem. Anal. (Warsaw), 36, 911 (1991). 67. Harada A., Tarutani T., Yoshimura K., Anal. Chim. Acta, 209, 333 (1988). 68. Sun Q., Wang H.T., Mou S.F., J. Chromatogr., 708, 98 (1995).

Chapter 23. Gold

Gold (Au, at. mass 196.97) occurs in compounds of Au(I) and Au(II). Gold(I) compounds resemble the corresponding Ag, Cu(I) and Hg(I) compounds. Gold(I) gives stable cyanide and iodide complexes, but gold(III) compounds are generally more stable. The hydroxide, Au(OH)3, is amphoteric. Gold(HI) yields stable halide complexes. Gold compounds are readily reduced to the metal, which dissolves in aqua regia to give AuC14-.


23.1.1. Fire assay and cupellation

Cupellation methods and fire assay are applied for separation of small and trace amounts of gold, silver, and platinum metals from their ores and concentrates [1-4].

A finely ground sample of the materials is fused with a mixture composed of metal oxide (collector), flux (soda, borax, silica), and an organic reducing substance (flour, graphite). Lead, in the form of litharge (PbO), has long been used as a metal collector. The reduction of litharge during the fusing process results in formation of tiny droplets of lead. The noble metals (especially Au, Ag, Pt, and Pd) dissolve in lead, which is collected at the bottom of the crucible in the form of a separate heavy phase. The obtained alloy, separated from the lighter slag, is placed in a porous crucible made of, e.g., bone-ash or magnesite. On ignition at high temperature, lead is oxidized, and the liquid oxide formed sinks into the porous crucible walls. The process is called cupellation. A bead of the noble metals alloy remains in the crucible.

An advantage of this method is the possibility of using large samples (15-50 g), which allows one to minimize the effect of non-uniformity of materials on the results of determination. A considerable degree of pre-concentration of noble metals is also achieved.

Silver has also been used as the collector [5]. For metals such as Rh, Ir, and Os, good results have been obtained where iron, nickel, copper, tin, and their alloys are used as the collectors [6-8]. Tin [8] and copper [6,7] have also been used, instead of lead, in the separation of gold. If noble metals are separated along with metal collectors, the obtained alloys are dissolved in acid, and the noble metals are separated by chemical methods. Copper sulphide [9] and nickel sulphide [3,10-13] have also been recommended for the separation of noble metals.

Critical evaluation of the fire assay for separation of gold [14-16] and platinum group metals [17] have been published. The problem of gold losses in the cupellation technique has been discussed [ 15,16].

23.1.2. Extraction

Gold(III) is usually separated from Pt and Pd by extraction from 4-8 M hydrochloric acid [18-20] or from HzSO4-KBr media [21 ]. Oxygen-containing solvents, such as DIPE, MIBK, mesityl oxide, or amyl acetate have been used as extractants. Iron(HI), which interferes, is masked with phosphoric acid before the extraction of gold. The chloride complex of gold can also be extracted with TOPO in MIBK [20], TBP in toluene or xylene [24]. The anionic


bromide complex of Au(III) has been extracted with ethyl acetate or with a toluene solution of TOA [24].

Small amounts of Au(III) (0.1-1,000 ~tg/ml) can be separated by extraction with DIPE from -7 M nitric acid medium [25]. Only U(VI) and Th are also extracted to some extent. Gold has also been extracted as the diethyldithiocarbamate [26] and thiooxinate [27].

Extractive separation methods for gold have been reviewed [28].

23.1.3. Other methods of separation

Gold is often precipitated by reduction to the element with tellurium as collector [27,29,30]. Tin(II) chloride, zinc, and hydrazine, which are used as reductants, also reduce Pt, Pd, Hg, and Ag. With the use of nitrite as reductant, gold can be separated from Pd and Pt, which form soluble nitrite complexes. The optimal conditions for nitrite separation of Au from Pd and Pt are: dilute HC1 medium (pH -3), and a heating time of about 1 h at 70~

2-Phenylethylenephosphonic acid and a frothing agent were added to the reaction mixture, after the reduction of gold(III) by SnCI2, and Au was separated by flotation with the use of nitrogen gas [32].

Trace amounts of gold can be separated (as chloride complex) from many other metals (including macro-amounts of copper), on strongly basic anion exchangers [33,34]. A sample solution in dilute HC1 is passed through an anionic column. Trace amounts of gold from anode slime have been pre-concentrated on Amberlite XAD-7 [35].

Gold has been separated from the platinum group metals on cation exchangers, using HBr medium [36]. Also dilute HC1 media (pH 1-1.5) have been applied in separation of Au from other metals on a cation exchanger [37].

Gold has been retained on a polyurethane sorbent from alkali cyanide media [38] and on a cellulose sorbent from dilute HC1 medium [39].

Traces of gold in high-purity mercury can be enriched by partially dissolving the sample (e.g. 10 g) in nitric acid. Almost all the gold is collected in the small Hg residue (~-100 mg) [40].


A sensitive and selective method utilizing Rhodamine B, and a simple bromide method are discussed later. Among other methods, those with other basic dyes (in addition to Rhodamine B), and with thio-Michler's ketone merit attention.


In HC1 medium, the basic dye Rhodamine B (formula 4.29) forms an ion associate with the gold(III) chloride complex, which is extractable into DIPE or benzene. The coloured extract is the basis of a sensitive and selective method of determining Au [41]. The molar absorptivity of a benzene solution of the complex is 9.7-104 (a = 0.49) a t ~max - 565 nm.

Since the efficiency of extraction of the gold(M) complex depends on the concentration of HC1 in the solution, the concentration in both sample and standard solutions must be kept constant. The optimum concentration of HC1 is 0.5-1 M.

Antimony(V) , TI(III) , Fe(III) , Ga, and Hg(II) interfere in the determination of gold with Rhodamine B. The interference is overcome by co-precipitation of the Au traces with Te. Antimony and thallium can be removed by co-precipitation with MnO2 aq. Iron(III) may

212 23.Gold

be masked with fluoride. Since the benzene extracts are sometimes turbid and rather difficult to clarify, it is preferable to extract the gold into DIPE. The ethereal extracts are stable for at least 30 min.

Reagents

Rhodamine B, 0.04% solution in 1 M HC1. Standard gold solution: 1 mg/ml. Dissolve 0.1000 g of suitably pure gold in 4 ml of

aqua regia, and evaporate the solution nearly to dryness. Add 2 ml of conc. HC1, evaporate to half the volume, and dilute the solution with water to the mark in a 100-ml standard flask. The stability of gold solutions has been discussed [42,43].

Tellurium solution, -~1 mg/ml. Dissolve 0.10 g of tellurium in 2 ml of conc. HNO3, and evaporate the solution to dryness. Dissolve the residue in 10 ml of conc. HC1, and dilute the solution with water to 100 ml.

Procedure

Separation of Au with Te as collector. To the sample solution (10-20 ml), add 1 mg of tellurium (as a solution in dilute HC1), and add hydrochloric acid until the acid concentration is 1-2 M. Heat the solution nearly to boiling, and add hydrazine sulphate in small portions (0.2 g in total). Continue heating after the appearance of a turbidity, until the precipitate coagulates. Filter off the precipitate, wash it with water, and dissolve it in a few drops of conc. HC1 and 1 or 2 drops of conc. HNO3. Evaporate the solution nearly to dryness, and dilute with water.

Determination of Au. To the sample solution containing not more than 40 g of Au (1 M HC1), add water to ---10 ml, and add 3 ml of the Rhodamine B solution. Extract the gold with two portions of DIPE (shaking time 30 s). Dilute the combined extracts with the solvent to the mark in a 25-ml or smaller standard flask (according to the amount of gold), and measure the absorbance at 565 nm against water.

23.2.2. Bromide method

Gold is readily determined from the absorbance of an organic extract of the Au(III) bromide complex [21,44].

The gold bromide complex is orange-yellow, the )Lmax being at 380 nm (e = 4.8.103 in DIPE; a = 0.024). The method is selective and simple. KBr and H2SO4 can be used instead of HBr. Ethyl acetate is a suitable alternative to DIPE as the solvent.

Large quantities of chloride should be avoided since they result in the formation of mixed chloride-bromide complexes, which are less intensely coloured. Iron(III) interferes in the determination of Au by forming an orange bromide complex, but it can be masked with phosphate or fluoride. In larger quantities, Cu, Ni, and Cr(III) also interfere.

The Au(III) bromide complex is extractable into chloroform in the presence of TOPO [45], or TOA [46].

Reagents

Hydrobromic acid, conc. (40%,-~5 M) solution. Standard gold solution: 1 mg/ml. Preparation as in Section 23.2.1.


Procedure

To 10-15 ml of the sample solution (in dilute H2SO4 or HC1) containing not more than 0.7 mg of Au, add 10 ml of conc. HBr and 1 ml of conc. H3PO4, and extract the complex of gold(III) with 2 portions of DIPE. Wash the combined extracts with 2.5 M HBr. Dilute the ethereal solution with the solvent to the mark in a 25-ml standard flask and measure its absorbance at 380 nm against the solvent.


A large group of extraction-spectrophotometric methods, similar to the Rhodamine B method, is based on extraction of ion associates of AuC14- with various basic dyes, such as Brilliant Green (toluene) [47-49], Methylene Blue (chloroform) [50-53], Nile Blue A [54], Chrompyrazole I (an antipyrine dye, formula 23.1) (toluene, e = 6.5.104 at 580 nm) [55].

+~CHz" C8H5 ,

C~C--CH3 I I O~C~N~N--CH 3

I C6H5

(23.1)

Extraction-spectrophotometric methods have also been based on other anionic complexes of Au, such as AuBr4- with Chrompyrazole I [56], Au(CN)2- with Methylene Blue (1,2-dichloroethane, e = 1.1.105) [57], azide complex with Methylene Blue (chloroform) [39].

Recently, two very sensitive flotation-spectrophotometric methods, based on adducts composed of the ion-associates Methylene Blue (MB)-AuI2- or Rhodamine 6G (R6G)- AuBr4, and the salts MB-I3- or R6G-Br3-, respectively, have been proposed. In the iodide system, the ratio Au:MB = 1:4 (e = 3.4.105) [30], and in the bromide system Au:R6G = 1:13 (e = 11.4-105) [21].

Thio-Michler's ketone (formula 46.2) reduces Au(III) and forms various complexes with Au(I) (pH 1-4), generally with participation of halide ions or solvent molecules. These complexes allow the sensitive determination of Au in water-ethanol, water-DMF, CHC13, or CHC13-butanol media. Molar absorptivity values vary, according to the medium, from 1.105 to 2.105 at 525-560 nm [58,59].

One of the more often used spectrophotometric reagents for gold is p- dimethylaminobenzylidenerhodanine (rhodanine, formula 46.3) [33,60]. The reagent reacts with gold ions in weakly acidic media (e.g., 0.1 M HC1) to form a pink-violet complex, which is either stabilized in the aqueous phase, or extracted into a mixture of chloroform and benzene (3+1) or isoamyl acetate. In aqueous-pyridine medium, e = 3.8.104 at 515 nm [60]. Rhodanine and its derivatives were applied for determination of Au and other noble metals in the presence of surfactants [61-63].

Many other organic reagents have been applied for spectrophotometric determination of gold, e.g. dithizone [64], diphenylcarbazide (~)~= 4.2.10 4 at 560 nm) [65], PAR [66], TAR

214 23.Gold

[67], Bromopyrogallol Red (e~= 3.0.10 a) [68], and 2,2'-diquinoxalyl [69]. Derivative spectrophotometry has been applied in determinations of gold in chloride

[70,71] and bromide [72] solutions containing Pt and Pd.


The Rhodamine B method has been applied for determining gold in tellurium [23] and in mercury [40].

The ion associate of gold with Methylene Blue has been used in determination of gold in ore, anode slime, and cyclone dust [53], and in solders [73]. The Nile Blue method was used for determining gold in coal dust and in ores [54].

References

1. Diamantatos A., Anal. Chim. Acta, 165, 263 (1984). 2. Kallmann S., Talanta, 34, 677 (1987). 3. Van Loon J.C., Barefoot R.R., Determination of the Precious Metals. Selected Instrumen-

tal Methods, Wiley, Chichester, 1991. 4. Yi B.Q., Analyst, 121, 139 (1996). 5. Kallmann S., Maul C., Talanta, 30, 21 (1983). 6. Banbury L.M., Beamish F.E., Z. Anal. Chem., 211, 178 (1965). 7. Diamantatos A., Talanta, 34, 736 (1987). 8. Faye G.H., Moloughney P.E., Talanta, 19, 269 (1972). 9. Kallmann S., Talanta, 33, 75 (1986). 10. Asif M., Parry S.J.,Analyst, 116, 1071 (1991). 11. Paukert T., Rube~ka I., Anal. Chim. Acta, 278, 125 (1993). 12. Juvanen R., Kallio E., Lakomaa T., Analyst, 119, 617 (1994). 13. Frimpong A. et al.,Analyst, 120, 1675 (1995). 14. Chow A., Beamish F.E., Talanta, 14, 219 (1967). 15. Trokowicz J., Chem. Anal. (Warsaw), 15, 1147 (1970). 16. Wall S.G., Chow A., Anal. Chim. Acta, 69, 439; 70, 425 (1974). 17. Shvedov V.A., Pakhomonova V.V., Chicheva V.P., Zh. Anal. Khim., 43, 1066 (1988). 18. Ichinose N., Talanta, 18, 105 (1971). 19. Haddon M.J., Pantony D.A., Analyst, 105, 371 (1980). 20. Mojski M., Kalinowski K., Chem. Anal. (Warsaw), 31,789 (1986). 21. Marczenko Z., Jankowski K.,Anal. Chim. Acta, 176, 185 (1985). 22. Shelkovnikova O.S. et al., Zh. Anal. Khim., 28, 2147 (1973). 23. Shkrobot E.P., Shebarshina N.I., Bakinovskaya L.M., Zavod. Lab., 37, 408 (1971). 24. Tsukahara I., Talanta, 24, 633 (1977). 25. Marczenko Z., Kowalski T., Anal. Chim. Acta, 156, 193 (1984). 26. Bajo S., Wyttenbach A., Anal. Chem., 49, 1771 (1977). 27. Demina L.A., Petrukhin O.M., Zolotov Yu.A., Zh. Anal. Khim., 27, 593 (1972). 28. Das N.R., Bhattacharyya S.N., Talanta, 23, 535 (1976). 29. Kozera F., Wilczewski T., Dobrowolski J., Chem. Anal. (Warsaw), 19, 577 (1974). 30. Marczenko Z., Jankowski K., Talanta, 32, 291 (1985). 31. Marczenko Z., Kasiura K., Szczygielska M., Z. Anal. Chem., 282, 47 (1976); Chem.

Anal. (Warsaw), 22, 45 (1977). 32. Dietze U., Braun J., Peter H.J., Z. Anal. Chem., 322, 17 (1985).

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33. Mizuike A., fida Y., Yamada K., Hirano S., Anal. Chim. Acta, 32, 428 (1965). 34. Koch W., Korkisch J., Mikrochim. Acta, 1973, 117. 35. Elci L., Isildar S., Dogan M.,Anal. Chim. Acta, 293, 319 (1994). 36. Dybczyfiski R., Maleszewska H., Analyst, 94, 527 (1969). 37. Pitts A.E., Beamish F.E.,Anal. Chem., 41, 1107 (1969). 38. Braun T., Forag A.B.,Anal. Chim. Acta, 153, 319 (1983). 39. Kuroda R., Yoshiknai N., Mikrochim. Acta, 1974, 653. 40. Jackwerth E., Kubok A., Z. Anal. Chem., 257, 28 (1971). 41. Onishi H., Mikrochim. Acta, 1959, 9. 42. Chow A., Talanta, 18, 453 (1971). 43. Bochkova L.P., Rykova E.A., Kazanova N.N., Zh. Anal. Khim., 39, 1521 (1984). 44. Nesterenko P.N., Ivanov V.M., Zh. Anal. Khim., 38, 1415 (1983). 45. Holbrook W.B., Rein J.E., Anal. Chem., 36, 2451 (1964). 46. Flieger A., Przeszlakowski S., Talanta, 29, 946 (1982). 47. Stanton R.E., McDonald A.J., Analyst, 89, 767 (1964). 48. Fogg A.G., Burgess C., Burns D.T.,Analyst, 95, 1012 (1970). 49. Mojski M., Lenarczyk L., Chem. Anal. (Warsaw), 23, 669 (1978). 50. Ganchev N., Atanasova B., Zh. Anal. Khim., 22, 274 (1967). 51. Ganchev N., Dimitrova A., Mikrochim. Acta, 1969, 1257. 52. Rakhmatullaev K., Giyasov A.Sh., Uzb. Khim. Zh., 3, 5 (1989). 53. Balcerzak M., Chem. Anal. (Warsaw), 37, 683 (1992). 54. Li Z., Wang J.L., Xu Q.H., Mikrochim. Acta, 116, 219 (1994). 55. Ivanov V.M., Yurzhenko N.N., Nesterenko P.N., Zh. Anal. Khim., 37, 1193 (1982). 56. Nesterenko P.N., Ivanov V.M., Zh. Anal. Khim., 37, 1977 (1982). 57. Koh T., Okazami T., Ichikawa M.,Anal. Sci., 2, 249 (1987). 58. Pilipenko A.T., Ryabushko O.P., Matsibura G.S., Zh. Anal. Khim., 34, 1088 (1979). 59. Petrov B.I. et al., Zavod. Lab., 51, No 2, 7 (1985). 60. Lichtenstein I.E., Anal. Chem., 47, 465 (1975). 61. Wu X., Liang S., Fresenius'J. Anal. Chem., 336, 120 (1990). 62. E1-Sayed A.A., Bull. Chem. Soc. Jpn., 67, 3216 (1994). 63. Gur'eva R.F., Savvin S.B., Zh. Anal. Khim., 44, 2165 (1989). 64. Zolotov Yu.A., Demina L.A., Petrukhin O.M., Zh. Anal. Khim., 25, 2315 (1970). 65. Adam J., Pribil R., Talanta, 18, 405 (1971). 66. Nagarkar S.G., Eshwar M.C., Anal. Chim. Acta, 71, 461 (1974). 67. Subrahmanyam B., Eshwar M.C., Anal. Chim. Acta, 82, 435 (1976). 68. Matougkova E., N~mcova I., Suk V., Microchem. J., 25, 403 (1980). 69. Baranowska I., Microchem. J., 26, 55 (1981). 70. Kuroda R., Hayashibe H., Yoshitsuka K., J. Trace Microprobe Tech., 7, 263 (1989). 71. Kuroda R., Hayashibe H., Yoshitsuka K., Fresenius'J. Anal. Chem., 336, 494 (1990). 72. Obarski N., Marczenko Z., Chem. Anal. (Warsaw), 40, 385 (1995). 73. Abalakina V.M., Kleimenova O.K., Dedkov Yu.M., Zavod. Lab., 58, No 10, 1 (1992).

Chapter 24. Indium

Indium (In, at. mass 114.82) is similar to gallium in its chemical properties. It occurs in aqueous solution exclusively in the III oxidation state. The hydroxide, In(OH)3, precipitates above pH 3-4 but, when freshly precipitated, redissolves in fairly concentrated alkali, thereby displaying weakly amphoteric properties. Yellow InzS3 is precipitated at pH 2-3. Indium forms halide, oxalate, tartrate, and EDTA complexes.


24.1.1. Extraction

Among the methods for the separation of small amounts of indium before its determination, the solvent extraction methods in general, and extraction of indium-iodide and -bromide complexes in particular, are of the utmost importance.

The indium iodide complex [1-3] is > 99% extracted into diethyl ether from 0.5-2.5 M HI (6-30%). Gallium is not extracted under these conditions, but it is extracted from 6 M HC1. The hydriodic acid can be replaced by 0.5-3 M H2804 containing 15-20% of KI. Chloride, bromide, fluoride, phosphate, and citrate do not interfere in the extraction of In from iodide media. Under the optimum conditions for the extraction, T1, Cd, and Sn (and some Bi, Zn, Hg, and Sb) are extracted. Aluminium and Fe(II), like Ga, are not extracted. The indium iodide complex has also been extracted into chloroform containing N- benzylaniline [4,5].

The indium bromide complex [6,7] can be extracted into diethyl ether from 4.5-5.5 M HBr (-40%). When extraction with DIPE is applied, the concentration of HBr should be -6 M. The selectivity of In extraction from a bromide medium is lower than that from iodide solutions. Ga, Fe(III), Sb(V), Au(III), TI(III), Sn, and Mo are extracted with indium. Indium can be stripped with water from the ether extract.

The indium chloride complex can be extracted from 1-4 M HC1 with TOA in chloroform or 1,2-dichloroethane [8], or with n-octylaniline in CHC13 [9]. From acidic thiocyanate media, indium has been extracted with tetra-n-hexylammonium salt in 1,2-dichloroethane [10], or with 15% TBP in kerosene [11]. Indium has been extracted also with Aliquat 336S in xylene from 0.01 M citric acid medium [12].

When a solution (pH- 9) containing fairly large amounts of cyanide and citrate is shaken with dithizone in chloroform, indium passes into the chloroform layer accompanied only by Pb, Bi, Sn(II), and T1. Bismuth may be isolated first by dithizone extraction at pH 3- 4. Dithizone in CC14 extracts In, but not T1, from a solution at pH 5-6 [13].

Indium can also be separated by extraction as the complex with oxine [14-16]. During the extraction of the In-DDTC complex at pH 3-5, extraction of Ga is prevented by the addition of oxalate.


Indium has been separated from Ga, Zn, Pb, A1, and T1 by cation exchange in HC1-H20- acetone or HBr-H20-acetone solutions [17-19]. In an ammonium carbonate medium, In


forms an anionic carbonate complex, while Zn and Cd form cationic ammine complexes, which can be separated from the indium complex on ion exchangers. Indium and tin(W) are retained from 5 M HC1 by an anion exchanger, in contrast to A1, Mn, Cu, and As. Indium is eluted from the column with 0.1 M HC1. Mixed media, such as 1 M HCI and 2- methoxyethanol or 1 M HCI and acetone, have been also used for the anion-exchange separation of In from A1 and Ga [20].

Indium may be separated from the Analytical Group III and IV metals by precipitation as the sulphide from 0.1 M HC1 medium with Sn(IV) as collector [21]. Separation from metals yielding soluble ammine complexes (Ag, Cu, Ni, Co, Zn, Cd), is achieved by precipitation as In(OH)3 with ammonia; La [13], Fe(III) [22], or A1 are suitable collectors.


Since methods for determining indium are rather unselective, the separation methods are very important. The spectrophotometric method involving 4-(2-pyridylazo)resorcinol (PAR), and the much more sensitive method based on the ternary system with Eriochrome Cyanine R and CTA (cationic surfactant) are discussed below in detail.

24.2.1. Pyridylazoresorcinol (PAR) method

4-(2-Pyridylazo)resorcinol (PAR) (formula 4.2) reacts with indium to form water-soluble complexes [23,24]. Within the pH range 6-8 a pink complex exists. The yellow colour of the reagent is constant between pH 2 and 10. The molar absorptivity e of this complex is 4.3.104 (a = 0.37) at ~,max = 510 nm (the ~max of the reagent is 425 rim).

The PAR method for determining indium at pH 6 is not very selective. At pH 3, the selectivity of the method increases, but the sensitivity decreases. When the reaction is carried out at pH 6, indium should be separated from Zn, Pb, Cr, A1, Sn(IV), Cd, Cu, and Mn. However, when 10 ~tg of In is determined at pH 3, these elements can be tolerated in the following maximum amounts: 20 ~tg of Zn, 40 lag of Pb and Cr, 300 ~tg of A1 and Sn(IV), and 1 mg of Cd, Cu, and Mn. Further interference is caused by Fe(III), Fe(II), Co, Ni, V, Zr, Bi, fluoride, oxalate, and EDTA. Tin(H) and other strong reductants decolorize the reagent irreversibly.

To determine indium in mineral concentrates (containing -~0.1% of In), the following procedure has been recommended: A weighed sample (0.1-1 g) is decomposed in a mixture of HC1 and HNO3, tin is expelled as SnBr4, and Pb is precipitated as PbSO4. The hydroxides of metals which, like indium, form sparingly soluble hydroxides, are then twice precipitated with an excess of ammonia. After the precipitated hydroxides have been dissolved in hydrochloric acid, potassium iodide is added, indium is extracted as the iodide complex into diethyl ether, and then stripped into aqueous solution and determined spectrophotometrically with PAR [23].

In the presence of antipyrine and acetate ions, the In-PAR complex can be extracted into chloroform [24]. Another method for determining In is based on a mixed complex with PAR and N-p-chlorophenyl-2-furanhydroxamic acid (CHC13) [25]. The In-PAR complex is also extracted in the presence of cetyldimethylbenzylammonium chloride (e = 6.8.104 at 510 nm [26].

218 24. Indium

Reagents

4-(2-Pyridylazo)resorcinol (PAR), 0.01% solution. Dissolve 10 mg of the reagent in 100 ml of water

Standard indium solution: 1 mg/ml. Dissolve 0.1000 g of metallic indium in 5 ml of HC1 (1 + 1), and dilute the solution to the mark with water in a 100-ml standard flask. Working solutions are obtained by suitable dilutions of the stock solution with -~0.01 M HC1.

Procedure

Extract ive separation of In. To an acidic sample solution (15-20 ml, -~1 M H2SO4), in a separating funnel containing not more than 50 ~tg of In, add -~0.1 g of sodium thiosulphate and 5 g of potassium iodide. Extract In with two 10-ml portions of MIBK (shaking time 30 s). Wash the extract by shaking with 10 ml of 1 M H2SO4 containing 2 g of KI and 20 mg of sodium thiosulphate. Determination of In. To the extract (obtained as above) in a small beaker, add 2 ml of 1 M H2SO4 and evaporate the organic solvent. Dissolve the residue in 15 ml of ~0.01 M H2SO4, add 2.5 ml of PAR solution and 2.5 ml of 40% ammonium acetate solution, then adjust the pH of the solution to 6.0-6.2. Transfer the solution to a 25-ml standard flask, dilute to the mark with water, and mix well. Measure the absorbance of the solution at 510 nm vs. a reagent blank.

24.2.2. Eriochrome Cyanine R-CTA method

Indium ions and Eriochrome Cyanine R (ECR) (formula 4.17) (in excess), in the pH range 3-7, form a complex with ~ m a x -- 565 nm [27]. In the presence of CTA (cationic surfactant) in a sufficiently large excess, an intensely coloured ternary complex is formed [28,29]. Its maximum absorption is obtained at pH 5.2_+0.1 after 5 min in the presence of acetate buffer. In the absence of acetate, the reaction proceeds more slowly. The concentrations of ECR and CTA must be in appropriate excesses (as given below in the procedure). The molar absorptivity e is 1.0.105 (a = 0.87) at 585 nm.

Beryllium, Ga, A1, Fe(III), and V(IV) interfere in the determination of In as a complex with ECR and CTA. The influence of Zr, Th and U is smaller. Anions complexing indium, viz. EDTA, citrate and tartrate, must be absent. This method for determining indium becomes highly selective when indium is first separated as the iodide complex (see Section 24.2.1).

Indium has been determined with Eriochrome Cyanine R in the presence of other surfactants, CP [29,30] and Zephiramine [29].

Reagents

Eriochrome Cyanine R (ECR), 5.10 -4 M (--0.023%) solution. Cetyltrimethylammonium bromide (or chloride) (CTA), 4.10 -3 M (0.13%) solution. Standard indium solution: 1 mg/ml. Preparation as in Section 24.2.1. Acetate buffer (pH 5.2). Add 215 ml of 0.1 M ammonia solution to 285 ml of 0.1 M

acetic acid.


Procedure

To a slightly acidic (pH 1-2) sample solution, containing not more than 25 ~tg of In, add 5 ml of ECR solution and 3 ml of CTA solution. Dilute the solution with water to about 15 ml, adjust with dilute ammonia solution to pH -5, and add 3 ml of acetate buffer. Transfer the solution to a 25-ml standard flask, dilute with water to the mark, mix well, then measure the absorbance at 585 nm vs. a reagent blank.


Besides Eriochrome Cyanine R, which has been discussed above, some other triphenylmethane chelating reagents have been proposed for determination of indium. Like ECR, Chrome Azurol S allows a sensitive determination of indium with the use of cationic surfactants, such as CTA (~ = 1.23.105) [31], or CP [30]. Indium has been determined with Pyrocatechol Violet (PV) in a binary system [32], and in a ternary one with tridodecylammonium bromide (e = 8.2.104) [33]. A considerable increase of sensitivity is obtained in the presence of surfactants, such as CTA, CP, or Zephiramine [29]. Xylenol Orange has been recommended for determination of In [34]. Methylthymol Blue has also been studied as a reagent for determination of indium [35].

Apart from PAR, many other azo reagents have been applied as spectrophotometric reagents for indium. 1-(2-Pyridylazo)-2-naphthol (PAN) has been applied widely as analytical reagent for indium [24,36-38]. The indium chelate with PAN is extractable into chloroform from solutions of pH -6 (e = 1.9.104). Indium has also been determined with PAN by derivative spectrophotometry [39]. Other azo reagents proposed for indium include 2-(2-pyridylazo)-l-hydroxynaphthalene-4-sulphonic acid [40], TAR (formula 4.7) [41], Lumogallion (e = 5.4.104 at 510 nm) (formula 21.1) [42], Sulpharsazen [43], Thoron I [44], and Eriochrome Black T (extraction into n-butanol in the presence of diphenylguanidine, e = 3.6-104) [45], Picramine M [46], 2-(2-thiazolylazo)-p-cresol (TAC) [47], and 2,4,6-tris(2- hydroxy-4-sulphonaphthylazo) 1,3,5-triazine (~ = 8.4.104) [48].

The bromide complex of indium gives extractable ion-associates with the xanthene basic dyes: Rhodamine B [49] and triphenylmethane dyes, Brilliant Green [50,51], and Crystal Violet [52], extractable into non-polar solvents. The related dye, Malachite Green, has been used to form an associate with tetra-iodoindate, extractable with benzene, hexane, and CC14 [53].

Some xanthene chelating reagents can be employed in the determination of indium, viz.

Gallein (formula 4.20) (e = 3.0.104) [3,54], Pyrogallol Red [55,56], Bromopyrogallol Red (formula 4.21) [57], and pyrogallobenzoin [58]. The latter has been used in the flotation- spectrophotometric determination of indium (e = 1-105 at 570 nm). The related reagent, phenylfluorone, used in the presence of surfactant CP, gave a highly sensitive reaction with indium (e = 1.12.105) [59]. Nitrophenylfluorone was used for determining In the presence of Ga, Pb, Te, Ba, Cu, and A1 [60].

A chelate of indium with bromooxine (e = 8.8.103 at 415 nm) [13,22] has become a basis of a rather insensitive extraction-spectrophotometric (CHC13) method of determining indium.

A number of other organic reagents have also been recommended for determination of indium, including 1-(2-pyridylmethylidenamine)-3-(salicylidenamine)thiourea (e = 6.2.104) [61], Alizarin S (e = 2.8.104, extraction with n-butyl acetate in the presence of diphenylguanidine) [62,63], HTTA [64], and haematoxylin in the presence of CTA (e =

220 24. Indium

1.3.105) [65].


The PAR method has been used for determination of indium in environmental samples [66], aluminium and its alloys [67], and in nickel and zinc alloys [26].

The following spectrophotometric reagents were applied for determining indium: 2-(2- thiazolylazo)-p-cresol (TAC), in catalysts [47]; Malachite Green, in gallium metal and in ZnGeAs2 [53]; and Pyrogallol Red, in zinc alloys [7].

Bromooxine was used for determining indium in zinc and lead ores [22] and in high- purity tin [ 13].

References

1. Gagliardi E., Tfimmler P., Talanta, 17, 93 (1970). 2. Hasegawa Y., Takeuchi H., Sekine T., Bull. Chem. Soc. Jpn, 45, 1388 (1972). 3. Minczewski J., Trybu~a Z., Krzy~anowska M., Chem. Anal. (Warsaw), 24, 9, 361 (1979). 4. Khosla M.M., Rao S.P., Anal. Chim. Acta, 58, 389 (1972). 5. Khosla M.M., Singh S.R., Rao S.P., Talanta, 21, 411 (1974). 6. Chavan M.B., Shinde V.M., Sepn. Sci., 8, 285 (1973). 7. Jadhav S.G., Murugaiyan P., Venkateswarlu C.,Anal. Chim. Acta, 82, 391 (1976). 8. Revenko V.G., Kopanskaya P.S., Bargeev V.V., Zavod. Lab., 41,267 (1975). 9. Kuchekar S.R., Chavan M.B., Talanta, 35, 357 (1988). 10. Irving H.M., Damodaran A.D., Anal. Chim. Acta, 50, 277 (1970). 11. E1-Yamani I.S., Shabana E.I., Talanta, 31,630 (1984). 12. Vibhute C.P., Khopkar S.M., Analyst, 111,435 (1986). 13. Marczenko Z., Kasiura K., Chem. Anal. (Warsaw), 10, 449 (1965). 14. Zolotov Yu. A., Lambrev V.G., Zh. Anal. Khim., 20, 1153 (1965). 15. Sramkova J., Kotrly S., Kalischova Y., Coll. Czech. Chem. Comm., 53, 3029 (1988). 16. Kotrly S., Sramkova J., Chadima R., Cermak J.,Analyst, 118, 79 (1993). 17. Strelow F.W., Victor A.H., Talanta, 19, 1019 (1972). 18. Strelow F.W., Weinert C.H., van der Walt T.N., Talanta, 21, 1183 (1974). 19. Strelow F.W., van der Walt T.N., Talanta, 34, 895 (1987). 20. Korkisch J., Hazan J., Anal. Chem., 36, 2308 (1964). 21. Rudnev N.A., Dzhumaev R.M., Zh. Anal. Khim., 19, 443 (1964). 22. Gregorowicz Z., Marczak M., Chem. Anal. (Warsaw), 14, 159 (1969). 23. Kish P.P., Orlovsky S.T., Zh. Anal. Khim., 17, 1057 (1962). 24. Biriuk E.A., Ravitskaya R.V., Zh. Anal. Khim., 26, 735 (1971). 25. Agrawal Y.K., Bhatt V.J., Analyst, 111, 57 (1985). 26. Anjaneyulu Y. et al., Analusis, 15, 106 (1987). 27. Joshi A.P., Munshi K.N., Microchem. J., 12, 447 (1967). 28. Marczenko Z., Ka~owska H., Chem. Anal. (Warsaw), 25, 555 (1980). 29. Jarosz M., Chem. Anal. (Warsaw), 33, 675 (1988). 30. Ganago L.I., Ishchenko N.N., Zh. Anal. Khim., 37, 1636 (1982). 31. Evtimova V., Nonova D.,Anal. Chim. Acta, 67, 107 (1973). 32. Tikhonov V.N., Bakhtina V.V., Zh. Anal. Khim., 39, 2126 (1984). 33. Shijo Y., Shimizu T., Sakai K., Bull. Chem. Soc. Jpn., 56, 105 (1983). 34. Kazdchkova P.I., Rybina G.K., Zakharova N.S., Zavod. Lab., 45, 605 (1979).

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35. Popova S.A. et al., Zh. Anal. Khim., 24, 682 (1969). 36. Shibata S., Anal. Chim. Acta, 23, 367, 434 (1960). 37. Cheng K.L., Goydish B.L.,Anal. Chim. Acta, 34, 154 (1966). 38. Zolotov Yu.A., Seryakova I.V., Vorobyeva G.A., Talanta, 14, 737 (1967). 39. Sharma R.L., Singh H.B., Satake M.,Analyst, 111,551 (1986). 40. Voznica P., Havel J., Sommer L., Coll. Czech. Chem. Comm., 45, 54 (1980). 41. Hnili6kova M., Sommer L., Talanta, 16, 681 (1969). 42. Pyatnitskii I.V., Boryak A.K., Kolomiets L.L., Zh. Anal. Khim., 51, 2199 (1986). 43. Pyatnitskii I.V., Kolomiets L.L., Popovich G.M., Zh. Anal. Khim., 38, 815 (1983). 44. Mottola H.A., Talanta, 11, 715 (1964). 45. Basargin N.M., Kafarova A.A., Zavod. Lab., 30, No 1, 14 (1984). 46. Petrova T.V., Sokolovskaya L.A., Savvin S.B., Zh. Anal. Khim., 42, 281 (1987). 47. Ferreira S.L., Costa A.C., Andrade H.A., Microchem. J., 44, 63 (1991). 48. Singh I., Kadyan P.S., Bull. Chem. Soc. Jpn., 61, 3689 (1988). 49. Gar6ic A., Sommer L., Coll. Czech. Chem. Comm., 35, 1047 (1970). 50. Zolotov Yu.A. et al., Zh. Anal. Khim., 29, 221 (1974). 51. Zolotov Yu.A. et al., Zh. Anal. Khim., 30, 1692 (1975). 52. Balog I.S., Kish P.P., Bagreev V.V., Zh. Anal. Khim., 44, 1213 (1989). 53. Kish P.P., Pogoyda I.I., Zh. Anal. Khim., 29, 52 (1974). 54. Minczewski J., Trybula Z., Krzy2anowska M., Chem. Anal. (Warsaw), 21, 311 (1976). 55. Krzy2anowska M., Kozifiska E., Trybula Z., Chem. Anal. (Warsaw), 24, 19 (1979). 56. Piwowarska B., Buhl F., Ciba J., Chem. Anal. (Warsaw), 31,495 (1986). 57. Pyatnitskii I.V., Kolomiets L.L., Vas'kovskaya T.A., Zh. Anal. Khim., 41,275 (1986). 58. Krzy2anowska M., Trybula Z., Chem. Anal. (Warsaw), 32, 465 (1987). 59. Al'bota P.A., Gutsulyak R.B., Ukr. Khim. Zh., 49, 385 (1983). 60. Fedin A.V., Zavod. Lab., 60, No 9, 1 (1994). 61. Rosales D., Millan I., Gomez Ariza J.L., Talanta, 33, 607 (1986). 62. Otomo M., Tonosaki K., Talanta, 18, 438 (1971). 63. Biriuk E.A., Nazarenko V.A., Zh. Anal. Khim., 30, 1720 (1975). 64. Solanke K.R., Khopkar S.M.,Anal. Chim. Acta, 66, 307 (1973). 65. Zaki M.T., E1-Didamony A.M., Analyst, 113, 1277 (1988). 66. Abbasi S.A., Anal. Lett., 21, 1705 (1988). 67. Kakade S.M., Shinde V.M.,Analyst, 118, 1449 (1993).

Chapter 25. Iodine

Iodine (I, at. mass 126.90) is a solid non-metal, which is fairly volatile at room temperature and sublimes easily. Iodine dissolves readily in aqueous KI solutions to yield the I3- complex, and is also soluble in organic solvents (CHC13, CC14, C6H6). It occurs mainly in the -I, V, and VII oxidation states. Iodide reveals reducing properties, whereas iodine, iodate, and periodate have oxidizing properties. Iodide, like chloride and bromide, forms sparingly soluble compounds and soluble complexes with some metals

25.1. Separation of iodide and iodine

Elemental iodine is volatile, and small quantities are commonly separated by distillation. In organo-iodine compounds, iodine is first oxidized to iodate with chromic acid in a concentrated H2804 medium, then reduced with phosphorous acid to iodine, which is steam- distilled and collected in an alkaline trapping solution [1 ].

Hydrogen iodide, which is volatile, can be separated by steam distillation. After oxidation of iodide in aqueous solution (e.g., with nitrite), the obtained iodine can

be extracted with non-polar organic solvents [2]. Traces of iodide can be separated as AgI by co-precipitation with silver chloride [3-5]. Trace amounts of iodide present in brines were retained on the anion-exchangers

Amberlite IRA 458 [6] or Shodex 1-524 A [7]. Methods for the separation of the halide ions are described in the Chapter on chlorine.

25.2. Determination of iodide and iodine

The colour reaction of iodine with starch is the conventional sensitive method for determining iodide. Coloured solutions of iodine in chloroform and other solvents provide a less sensitive extraction-spectrophotometric method.

25.2.1. Starch-iodine and extractive methods

Iodide can be determined spectrophotometrically (after oxidation to iodine), either as the blue iodine-starch adsorption compound, or as coloured organic extracts containing iodine. Iodide is oxidized to iodine with nitrite or iron(III).

The sensitivity of the iodide determination is enhanced six-fold if iodide ions are first oxidized to iodate ions which, in turn, are made to react with added potassium iodide in an acidic medium:

103- + 5I- + 6H + ---) 312 + 3H20

Bromine water is usually used for the oxidation of iodide to iodate [4,5]. The excess of bromine is removed by boiling, or by the addition of phenol (to form tribromophenol). Permanganate oxidizes iodide to iodate in alkaline media. The excess of permanganate is reduced with nitrite, the residual nitrite being reduced with urea.

The molar absorptivity of the starch-iodine complex (after iodate amplification) is


1.08.105 at 590 nm (a = 0.85) a t )~max -- 590 nm. In this method involving oxidation of I- to IO3-, any chloride or bromide in the sample

solution does not interfere. Iodine dissolves in benzene, CHC13, and CC14 to form violet solutions. Chloroform and

carbon tetrachloride are more convenient since they are denser than water. The sensitivity of these extraction methods is much lower than that of the starch-iodine method. The molar absorptivity of the chloroform solution of iodine (after the reaction I- + IO3- ----> 312) is --3-103.

Reagents

Starch, 1% solution. Mix 1 g of starch with 5 ml of water, and add the suspension obtained, slowly, with stirring, to 50 ml of boiling water. Then add 50 ml of glycerol, and gently boil the solution for 5 min. The solution is stable for several weeks.

Standard iodide solution: 1 mg/ml. Dissolve 1.3081 g of potassium iodide (iodate-free) in water, and dilute the solution to volume with water in a 1-1itre standard flask. Store the solution in the dark.

Phenol, 10% solution in glacial acetic acid. Potassium iodide, 0.5% solution, freshly prepared.

Procedure

Starch-iodine method. Acidify 15-20 ml of an approximately neutral sample solution, containing less than 25 gg of I (as iodide), with 1 ml of 1 M H2S04, add 2 drops of bromine water, stir well, and allow to stand for 1 min. Next add 2 drops of the phenol solution, stir well, and after 1 min add 1 ml of the KI solution and 1 ml of the starch solution, and dilute to the mark with water in a standard 25 ml flask. Measure the absorbance of the solution at 590 nm, using a reagent blank or water as reference. Extractive method. Acidify a sample solution containing not more than 1 mg of I (as iodide) with 2 ml of 1 M H2S04, add 5 drops of bromine water, stir well, and allow to stand for 1 min. An excess of free bromine should be present in the solution. Remove the bromine by heating the solution. Allow the solution to cool, and add 1 drop of the phenol reagent. After 1 min, add 2 ml of the KI solution and extract the iodine with two portions of chloroform, shaking for 1 min with each. Make the extracts up to the mark with the solvent in a 25-ml standard flask and measure the absorbance at 510 nm against chloroform.


Iodide ions (as I- or 13-) can be extracted as ion-pairs with basic dyes such as Crystal Violet [8,9], Brilliant Green [10,11], Rhodamine B (in toluene) [12], or Butylrhodamine (in benzene) [ 13]. After oxidation of iodide to iodine, the iodine is extracted into CC14 and then stripped into the aqueous phase (as I-) by shaking with thiosulphate. Finally, the I- ions associated with Methylene Blue, are extracted into 1,2-dichloroethane [ 14].

Iodide (as I-or 13-) can be extracted as ion-associates with ferroin (nitrobenzene) [15,16] or with a Cu(I) complex with neocuproine [17].

The FIA technique has also been applied for determining iodide: the I- is oxidized with bromine to 103-: the excess of bromine is reduced, the 103- is made to react with added iodide, and the absorbance is measured at 351 nm [18].

The oxidizing action of iodine on Phenol Red has also been utilized [19]. Iodine

224 25. Iodine

obtained in the reaction of I- with IO3- reacts with the leuco-form of Crystal Violet to give its oxidized form (~ = 5.6.105 at 591 nm) [ 19a].

In the indirect methods of determining iodide, use has been made of the exchange of ligands between iodide and a Hg(II) complex with diphenylcarbazone [20], the Ag complex with DDTK (in the presence of Cu ions) [21], the Hg(II) complex with dithizone [22], the Ag complex with 4-(2-quinolyl)phenol [23], and the Pd complex with 2-nitroso-5- diethylaminophenol (e = 2.3.104 at 540 nm) [24].

The catalytic effect of iodide on the oxidation of As(III) by Ce(IV) [25,26], and the oxidation of various organic substances by hydrogen peroxide [27-32] has been the basis of a number of kinetic methods for determination of iodide.

25.3. Determination of iodate and periodate

Iodate can be determined spectrophotometrically by conversion into iodine by reaction with iodide in acidic medium, and the subsequent colour reaction with starch. Nitrite, which interferes in the determination of iodate, is removed by its diazotization of added sulphanilic acid. Iodate can be determined by using it to oxidize Fe(II) to Fe(III), which then reacts with ferrocyanide to form Prussian Blue [33]. It has also been determined by means of its redox reaction with diphenylcarbazide [34]. Periodate can be determined by its redox reaction with o-dianisidine [35], benzhydrazide [36], or 2,2'-azinodi(3-ethylbenzothiazolesulphonic acid) [ 3 7 ] and phthalimidebisthiosemicarbazone ( e - 2.8.104) [38]. Cyclohexane-l,3-dione bis-thiosemicarbazone [39], 3,4-dihydroxybenzaldehyde guanylhydrazone [40], salicylaldehyde guanylhydrazone [41 ], and amyloride hydrochloride [42] allow IO4- to be determined in the presence of IO3-.


The starch-iodine method has been used for determining iodide (or iodine) in natural waters [4,5,43], milk [1], and silicate minerals [3]. The extractive method has been applied in determinations of iodine in brine [44], rocks [45], and lead telluride [46].

Extraction of iodine (from oxidation of iodide) into CC14 was applied in determination of iodide in sodium chloride [47] and in mercury(II) iodide [48]. Iodide was determined in sea- water after oxidation to iodine and flotation into benzene [49].

Iodide was determined in active carbon by the FIA method, after its reaction with IO3- [50]. A very sensitive method [19a] was applied for determining traces of iodide in tap- water, sea-water, soil, iodized salt, and pharmaceuticals.

Iodate and periodate were determined in waters after the extraction of their ion-pairs with tetramethylammonium ion [51 ].

References

1. Joerin M.M., Analyst, 100, 7 (1975). 2. Yonehara N., Yoshida M., Iwasaki I., Bull. Chem. Soc. Jpn., 43, 3797 (1970). 3. Crouch W.H., Anal. Chem., 34, 1698 (1962). 4. Matthews A.D., Riley J.P., Anal. Chim. Acta, 51, 295 (1970). 5. Tsunogai S., Anal. Chim. Acta, 55, 444 (1971). 6. Shao-Cheng Z., Lieser K.M., Z. Anal. Chem., 320, 265 (1985). 7. Bruins J., Maurer W., Z Anal. Chem., 334, 122 (1989).

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(1974). 18. A1-Wehaid, Townshend A., Anal. Chim. Acta, 198, 45 (1987). 19. Pantaler R.P., Ivkova T.I., Zh. Anal. Khim., 332, 1975 (1977). 19a. Kesari R., Rastogi R., Gupta V. K., Chem. Anal., 43, 201 (1998). 20. Okutani T., J. Chem. Soc. Jpn., Pure Chem Sect., 88, 737 (1967). 21. Komatsu S., Nomura T., Usui Y., J. Chem. Soc. Jpn., Pure Chem. Sect., 88, 1164 (1967). 22. Agterdenbos J., Jtitte B.A., Elberse P.A., Talanta, 17, 1085 (1970). 23. Barua S. et al., Analyst, 105, 996 (1980). 24. Hamada S., Motomizu S., T6ei K., Analyst, 104, 880 (1979). 25. Yaqoob M., Masoom M., Townshend A., Anal. Chim. Acta, 248, 219 (1991). 26. Jopke P., Bahadir M., Fleckenstein J., Schnug E., Commun. Soil Sci. Plant Anal., 27, 741

(1996). 27. Yonehara N., Yamane T., Tomiyasu T., Sakamoto H.,Anal. Sci., 5, 175 (1989). 28. Kamburova M., Zh. Anal. Khim., 46, 1534 (1991). 29. Liang B. et al.,Anal. Chim. Acta, 282, 87 (1993). 30. Egorova L.A., Pantaler R.P., Blank A.B., Zavod. Lab., 61, No 7, 9 (1995). 31. Mohamed A.A., Iwatsuki M., E1-Shahat M.F., Fukasawa T., Analyst, 120, 1201 (1995). 32. Tomiyasu T., Sakamoto H., Yonehara N., Anal. Chim. Acta, 320, 217 (1996). 33. Rahim S.A., Bashir W.A., Microchem. J., 29, 87 (1984). 34. Trivedi R.H. et al., Mikrochim. Acta, 1986 I, 337. 35. Guernet M., Bull. Soc. Chim. France, 1964, 478. 36. Escarrilla A.M., Maloney P.F., Maloney P.M., Anal. Chim. Acta, 45, 199 (1969). 37. Mahuzier G., Kirkacharian B.S., Harfouche-Obeika C.,Anal. Chim. Acta, 76, 79 (1975). 38. Mochon M.C., Leyva J.A., Microchem. J., 34, 83 (1986). 39. Mochon M.C., Leyva J.A., Analyst, 109, 951 (1984). 40. Kavlentis E., Analusis, 16, 253 (1988). 41. Berzas Nevado J., Valiente Gonzalez P., Analyst, 114, 243 (1989). 42. E1-Shahavi M.S., Anal. Chim. Acta, 356, 85 (1997). 43. Schnepfe M.M., Anal. Chim. Acta, 58, 83 (1972). 44. Collins A.G., Watkins J.W., Anal. Chem., 31, 1182 (1959). 45. Grimaldi F.S., Schnepfe M.M., Anal. Chim. Acta, 53, 181 (1971). 46. Silverman L., Anal. Chem., 34, 701 (1962). 47. Maleki N., Haghighi B., Safavi A., Microchem. J., 53, 147 (1996). 48. Hermon H., Schieber M., Roth M., Shamir J., Anal. Chim. Acta, 259, 181 (1992). 49. Zhiming L., Xiuyu Y., Anal. Lett., 22, 2353 (1990). 50. Monks C.D., Nacapricha D., Taylor C.G., Analyst, 118, 623 (1993). 51. E1-Shahavi M.S., A1-Hashemi F.A., Talanta, 43, 2037 (1996).

Chapter 26. Iron

Iron (Fe, at. mass 55.85) occurs in solution in the II and III oxidation states. Compounds of Fe(III) are generally the more stable. Fe(OH)2 is precipitated at above pH ---7.5, and Fe(OH)3 at above pH 2-3. Neither of the hydroxides shows acidic properties. Iron(II) exhibits properties similar to those of Ni(II) and Zn(II), forming a stable cyanide complex. Iron(II1) forms fluoride, chloride, cyanide, EDTA, tartrate, and oxalate complexes. In acid media, iron(Ill) acts as an oxidant.


26.1.1. Extraction

Iron(III) is extracted from hydrochloric acid media by diethyl ether, DIPE, 2,2'- dichlorodiethyl ether, and MIBK. For MIBK, the optimum concentration is 6-7 M HC1, while in the case of DIPE it is 7-8 M HC1. During the extraction of the iron(iiI) chloride complex, Ga, Tl(III), Au(III), Ge, As, Sb, and Mo are also extracted. Iron(III) can also be extracted from HC1 medium with TBP, TOPO, or TOA in chloroform [1 ]. Iron(III) has been extracted from bromide media with TBP [2] and from citrate media with Aliquat 336S [3]. Crown ethers in CHC13 have been also recommended for extraction of Fe(III) [4].

From weakly acid solutions, Fe(III) is extractable with acetylacetone [5], hexafluoroacetylacetone [6], and HTTA [7]. Iron(III) can be extracted with oxine in chloroform [8,9], HDEHP [10], dithiocarbamates [l 1,12], and Adogene 464 (in toluene) [13,14].

The extractive separation of Fe is often combined directly with some extraction- spectrophotometric methods of determination, e.g., with the thiocyanate- and the bathophenanthroline methods.

26.1.2. Precipitation. Ion exchange

Iron(III) is usually precipitated as the hydroxide. When it is precipitated with ammonia, aluminium can be used as collector: with an alkali-metal hydroxide as the precipitant, MnO2 aq. is a suitable collector. Ferric- and aluminium hydroxides can be co-precipitated either with ammonia from a weakly acidic medium (pH 4 -5 )o r by adding excess of ammonia (pH 9-10). Since some of the AI(OH)3 dissolves in excess of ammonia, La is a better collector. When Mn is the collector, iron can be precipitated with ammonia provided that Mn(II) and MnO4- are added before, and a few drops of ethanol are added after, the precipitation.

Traces of iron can be separated from A1, Ti, V, U, and phosphate, by precipitation as the sulphide from tartrate medium. Cadmium, lead, or any other metal which gives a sparingly soluble sulphide can be used as collector. Oxine is also useful as a precipitant for the separation of iron [8].

Iron(iiI) differs from Cu, Ni, Mn, and Mg in forming stable anionic sulphosalicylate-, tartrate-, and chloride complexes. This permits the separation of iron on cation- [15] or anion- exchange columns [16].

Separation of Fe(III) from Fe(II) can be achieved by sorption of both species on a


cation-exchanger column, and sequential elution of Fe(III) with 0.25 M oxalic acid, and Fe(II) with 2 M hydrochloric acid. Fe(II) can also be separated from Fe(III), if the latter is masked with citrate, and the former sorbed on a cation exchanger as the phenanthroline complex Fe(phen)32+ [17]. The Fe(III) retained on the cation exchange column is eluted with thiocyanate [ 18].

Fe(II) can be sorbed as its 1,10-phenanthroline complex on polyurethane foam [19]. Polyester and polyether types of polyurethane foams have been used for preconcentration of Fe(III) and Fe(II) [20].


There are numerous spectrophotometric methods for determining Fe(II) and- ( I I I ) . Three classical methods, with thiocyanate, 1,10-phenanthroline, and bathophenanthroline, are described below. One very sensitive method of determining Fe(III) with Eriochrome Cyanine R and CTA is also discussed.


Thiocyanate ions react with Fe(III) in a moderately acidic medium to yield a red colour which, for a long time, has been the basis for the determination of iron(III), or total Fe after oxidation of Fe(II) to Fe(III) [21-23]. Owing to stepwise complex formation in solution, Fe(SCN) 2+, Fe(SCN)2 +, and further complexes can be formed. The concentrations of the reagents and the pH of the medium determine which complexes are prevalent. In general, only Fe(SCN) 2+ is formed at microgram concentrations of Fe(III). The higher complexes are more intensely coloured.

Iron can be determined by the thiocyanate method in aqueous and aqueous-acetone media, or after extraction of the coloured complex with a suitable organic solvent. When iron is determined in aqueous media or in the presence of acetone, care should be taken that the concentration of thiocyanate is the same in the sample and the standard solutions. The aqueous solution must be sufficiently acid to prevent hydrolysis of Fe(III), which begins even at pH ~-3. The solution should not, however, be too acidic, otherwise the concentration of SCN- may be too small. Iron(III) thiocyanate complexes are not very stable, and can persist only at a relatively high concentration of SCN-. The optimum acidity of the solution with HC1, H2SO4, HNO3, or HC104 lies within the concentration range 0.05 - 0 . 2 M.

The colour of aqueous solutions of Fe(III) thiocyanate complexes is unstable owing to the reduction of Fe(III) by SCN-, fading by a few per cent in 30 min, and by 50% in 6 h.

Lowering the dielectric constant of the medium by addition of solvents such as acetone or dioxan intensifies the colour. The colour's intensity doubles when the acetone concentration reaches 50 % by volume. Agreement with Beer's law is not observed.

The iron(III)-thiocyanate complexes can be extracted with oxygen-containing solvents such as ethers, higher alcohols, esters, and ketones. Depending on the solvent used, different species are extracted. The 1:4 Fe:SCN complex is extracted with diethyl ether, while the 1:3 complex is extracted with TBP.

Extraction increases the sensitivity of the thiocyanate method. The molar absorptivity of the Fe(III) thiocyanate complex solution in MIBK (see procedure below) is 2.4.104 (a = 0.43) at ~max = 495 nm. The position of the absorption maximum for the complex varies between 470 and 530 nm, depending on the medium.

The anionic iron(III) thiocyanate complex forms extractable ion-associates with TOA

228 26. Iron

[24], methyltrioctylammonium ion (CHC13, e = 3.4-104 at 475 nm) [25], benzyltriethyl- ammonium ion (1,2-dichloroethane) [26].

Anions which form stable complexes with Fe(III), such as fluoride, phosphate, citrate, and oxalate, interfere in the determination of iron. Interference is also caused by other metals which form coloured thiocyanate complexes under the same conditions (Co, Mo, Bi, Ti), and by metals giving sparingly soluble compounds and coloured ions. These interferences may be avoided by selective extraction.

Reagents

Potassium thiocyanate, 20% solution. Acidify the solution with HC1 to pH --2. If colour is observed, owing to traces of Fe(III) present in the thiocyanate, extract the solution with 2-3 small portions of MIBK.

Standard iron(III) solution: 1 mg/ml. Weigh out 8.6350 g of ferric alum, FeNH4(SO4)z.12H20, dissolve it in water containing 5 ml of conc. H2SO4, and dilute to volume with water in a 1-1itre standard flask. Working solutions are obtained by suitable dilution of the stock solution with 0.01 M H2804.

Procedure

Separation of Fe (III) with a collector. If the solution contains no Ti, B i, Pb, A1 or other metals precipitated by ammonia as hydroxides, add 2 mg of La (as nitrate solution). Heat the solution to about 60~ and add excess of ammonia solution to dissolve the hydroxides of the metals, which form ammine complexes. Allow the solution to stand for a few min at 60- 70~ then filter off the precipitate on paper, and wash it with hot dilute ammonia solution. Dissolve the precipitate in a small amount of hot 2 M HC 1. Determination of Fe(III). Adjust the solution containing not more than 40 gg of Fe(III) to pH -~ 1 by adding HC1 or ammonia. Transfer the solution to a separating funnel, add 10 ml of thiocyanate solution, and extract the Fe(III) complex with two portions of MIBK. Make the combined extracts up to the mark with the solvent in a 25-ml standard flask, mix well, and measure the absorbance of the solution at 495 nm against the solvent.

Note. If the total Fe content of a solution containing both Fe(III) and Fe(II) is required, the latter must be oxidized by heating with a small amount of ammonium persulphate or hydrogen peroxide. Before the addition of thiocyanate, the solution must be cooled.

26.2.2. 1,10-Phenanthroline method

1,10-Phenanthroline (phen, formula 26.1) and 2,2'-bipyridyl (formula 26.2) are organic bases with very similar chemical properties.

5 6 t, 7 t , _ . , ~ { ' ~ . 7 t, 3 3' t, f

3 ~ ~ 8 5 ~ 5 ~

6 ,- i1 61

(26.1) (26.2)

Both reagents react rapidly with Fe 2+ ions in weakly acid media to give orange-red or


pink complexes, respectively, which are a basis for determining Fe(II) [27,28]. The absorption spectra of both complexes are shown in Fig. 26.1. The molar absorptivity of the Fe(II) complex with 1,10-phenanthroline is 1.10.104 (a =0.20) at ~max 512 nm, and that of the complex with 2,2'-bipyridyl is 8.7.103 at ~,max 522 nm. Solutions of the complexes with phenanthroline and bipyridyl are stable, and the Fe(II) bound in the complex is resistant to oxidation.

396 400 500 512 600 Wavelength, nm

Fig. 26.1. Absorption spectra of the iron complexes: iron(II)- 1,10-phenanthroline (1), iron(III)- 1,10-phenanthroline (2), and iron(II)- 2,2'-bipyridyl (3).

Iron(H) and total iron can be determined with phenanthroline or bipyridyl after reduction of Fe(III) to Fe(II). Hydroxylamine reduces Fe(III) within a few minutes in a weakly acidic medium (pH 3-4): ascorbic acid is a better reductant in a fairly acidic solution (pH 0-1) . Other reducing agents used are sulphite, dithionite, and hypophosphite [28 ].

The colour reactions are usually carried out in acetate or citrate buffers. The presence of citrate or tartrate is desirable as it prevents the precipitation of certain cations which hydrolyse in weak acid media (e.g., Ti, A1, and Bi). In the determination of Fe(II) with phenanthroline, Fe(III) can be masked with NTA [29] or fluoride [30].

The complex of 1,10-phenanthroline with Fe(II) is called ferroin and has been widely used in titrimetric analysis as a redox indicator.

1,10-Phenanthroline forms with Fe(III) a blue complex (Xmax 585 nm) which has been used for determining larger amounts of iron.

Phenanthroline and 2,2'-bipyridyl form complexes, although not intensely coloured, with Ru, Os, and Cu(I). Many metals (e.g., Zn and Cd) can form colourless complexes with phenanthroline and bipyridyl, which are more stable than the corresponding Fe(II) complexes. When determining Fe in the presence of Zn or Cd, EDTA should be used as a masking agent [31 ]. Copper can be masked with triethylenetetramine [32].

Fe(II)-complexes with phenanthroline and 2,2'-bipyridyl form extractable ion- associates, e.g.,with nitroprusside [33], tetraphenylborate [34], or picrate [35]. Fe has been determined by derivative spectrophotometry, after extraction of the ferroin associate with perchlorate anion [36].

The 1,10-phenanthroline method has been used in the automatic FIA technique [37-40].

Reagents

1,10-Phenanthroline (phen), 0.2% solution of the hydrochloride or hydrate in -0.1 M HC1. Standard iron solution: 1 mg/ml. Preparation as in Section 26.2.1. Hydroxylamine hydrochloride, 10% solution, freshly prepared.

230 26. Iron

Procedure

To a slightly acid solution, containing not more than 80 ~tg of Fe(III) or Fe(II), add 1 ml of the NHzOH solution and 10% sodium citrate solution, till the pH is 3-4. Transfer the solution to a 25-ml standard flask, add 3 ml of the phen solution, dilute to the mark with water, and mix thoroughly. After 5 min, measure the absorbance of the solution at 512 nm against water.

26.2.3. Bathophenanthroline method

Bathophenanthroline (4,7-diphenyl-l,10-phenanthroline, formula 26.3) reacts with iron(if) ions very similarly to 1,10-phenanthroline. Both methods are of similar selectivity, but the bathophenanthroline method is much more sensitive. Bathophenanthroline is soluble in ethanol [41-43]. The bathophenanthroline complex with Fe(II) is extractable into CHC13, n- hexanol, or amyl acetate. The highest distribution coefficients are obtained with n-hexanol or CHC13.

Hs sHs

(26.3)

For the quantitative extraction of Fe into CHC13, at least 10% of ethanol in the initial aqueous solution is necessary. Free bathophenanthroline is completely extracted. The chloroform extracts are diluted with ethanol in volumetric flasks.

The molar absorptivity of the Fe(II) bathophenanthroline complex in CHC13-ethanol is 2.24.104 (a = 0.40) at ~max 533 nm. Iron(III) is reduced with NHzOH, NzH4, dithionite, SnCl2, or ascorbic acid [44].

The optimum pH for the colour reaction is 4-7. Acetate buffer is usually used, and citrate, tartrate, or EDTA is added to keep readily hydrolysable metals in solution. Copper interferes in the determination of iron(if) with bathophenanthroline and should, therefore, be separated, e.g., as CuSCN, or masked with thiourea [42]. Higher concentrations of Co, Ni, Zn, and Cd interfere slightly.

Bathophenanthroline is suitable for the determination of Fe(II) in the presence of a large excess of Fe(III) which can be masked by phosphate [45].

Reagents

Bathophenanthroline, 0.2% solution in ethanol. Standard iron solution: 1 mg/ml. Preparation as in Section 26.2.1.

Procedure

To a solution (pH ~-1) containing less than 40 ~tg of Fe, add 0.1 g of NH2OH.HC1 and 2 ml of 50% sodium acetate solution. Heat to boiling, transfer the cooled solution to a separating funnel, add 5 ml of bathophenanthroline solution, and extract with two portions of CHC13. Dilute the coloured extract with ethanol in a 25-ml volumetric flask, and measure the


absorbance at 533 nm, using ethanol or water as reference.

26.2.4. Eriochrome Cyanine R-CTA method

At pH values between 2 and 10, Fe(HI) forms ternary complexes with Eriochrome Cyanine R (ECR) and cetyltrimethylammonium ions (CTA). Chrome Azurol S (CAS) and some cationic surfactants (e.g., cetylpyridinium ion, Zephiramine) give similar reactions. These ternary complexes are a basis of sensitive methods of determining Fe(III) [46-53].

The optimum pH for formation of a ternary compound Fe(IH)-ECR-CTA, suitable for Fe determinations, is 4.5___0.5. Maximal and stable absorbances (at 610 nm)a re obtained when suitable (sufficiently large) concentrations of chromogenic reagent and cationic surfactant (see Procedure) are present in the sample solutions. In the ternary system a large bathochromic shift and hyperchromic effect are observed in comparison with the binary system (without CTA).

The maximum absorbance of the ternary complex is obtained immediately after mixing the reagents. The pH is adjusted by addition of acetate buffer. And the absorbance is stable.

There is a large increase in sensitivity of the method on formation of the ternary complex (e = 1.27.105 at 610 nm; a = 2.3) as compared with the binary one, without CTA (e = 3.3.104) [49].

Numerous metals, e.g., Be, A1, Ga, In, Se, Cr(HI), Zr, U(VI), Th, V, interfere in the determination. Fe(II) does not react with ECR and CTA. Nevertheless, the colour reaction proceeds slowly as a result of oxidation of Fe(tI) to Fe(HI). This happens even in the presence of NH2OH or sulphite, but not in the presence of ascorbic acid, which keeps iron(H) in the lower oxidation state. The method with ECR and CTA becomes highly selective if the determination of Fe is preceded by extractive separation of iron as its thiocyanate complex (see Section 26.2.1).

Reagents Eriochrome Cyanine R (ECR), 5-10 -4 M (--0.024%) solution. Dissolve 58.8 mg of ECR in water and dilute with water to 250 ml.

Cetyltrimethylammonium bromide (or chloride) (CTA), 4.10 .3 M (--0.15%) solution. Dissolve 0.36 g of CTA in water and dilute to 250 ml.

Standard iron solution: 1 mg/ml.Preparation as in Section 26.2.1. Acetate buffer (pH 4.5). Add 225 ml of 0.1 M sodium acetate solution to 250 ml of 0.1

M acetic acid and dilute with water to 500 ml.

Procedure

Separation of Fe. Extract the thiocyanate complex of Fe(III) as described in Section 26.2.1. Evaporate the solvent, and mineralise the residue with conc. H2SO4 and conc. HNO3. Remove the excess of HNO3 by evaporation. To the cooled residue, add 0.1 M H2SO4 and warm to obtain a clear solution. Determination of Fe. Add 7 ml of the ECR solution and 3 ml of the CTA solution to a sample solution (pH 1-2) containing not more than 8 gg of Fe. Adjust the pH to -- 4 with dilute ammonia and add 5 ml of acetate buffer. Transfer the solution to a 25-ml standard flask, dilute with water to the mark, and mix. Measure the absorbance of the solution at 610 nm against a reagent blank.

2 3 2 26. I ron


In addition to 1,10-phenanthroline, 2,2'-bipyridyl, and bathophenanthroline, many other reagents of this type have been proposed. Triazines such as 2,4,6-tri(2'-pyridyl)-s-triazine (TPTZ, formula 26.4) (e = 2.5.104 at 595 nm) [54-57], and 3-(4-phenyl-2-pyridyl)-5,6- diphenyl-l,2,4-triazine (PPDT, formula 26.5) (e = 2.9.104 at 570 nm) [58] are often used. Ferrozine [59,60] is a reagent for Fe similar to PPDT. Other triazine derivatives have also been recommended for determination of Fe(II) [61-67]. Fe was determined in the presence of Cu [68] and in the presence of Co [69] by derivative spectrophotometry with the use of 5- phenyl-3-(4-phenyl-2-pyridyl)- 1,2,4-triazine.

F._..~e 3

2

(26.4) (26.5)

Other tr iphenylmethane reagents, besides the Eriochrome Cyanine R and Chrome Azurol S discussed above, have been applied for determination of Fe(III). A large increase in sensitivity and significant bathochromic shifts are observed when the reactions are performed in the presence of cationic surfactants, CTA, CP, Zephiramine, or dimethyllaurylbenzylammonium ions [70]. Systems of Fe(III) with Pyrocatechol Violet and CTA [71], Chromal Blue G and CTA [72], and Sulphochrome and CP [73] have been applied. The molar absorptivities of these systems are within 1.3.105-1.7.105. The methods based on Pyrogallol Red alone or with the use of surfactants are less sensitive (e within 5.2.104-7.5 �9 104) [74-76].

A number of azo reagents have been used to determine iron, namely PAN (extraction into CHC13) [77-80], PAR (e = 5.6.104 at 500 nm) [81,82], TAN [83], TAM (e = 2.7.104 at 760 nm) [84], 4-(4-methyl-2-thiazolylazo)resorcinol [85], 5-C1-PADAP [86], 5-Br-PADAP (e = 8.8.104 at 562 nm) [87-91], and Arsenazo III [92].

Several oximes have been recommended as reagents for iron, e.g., formaldoxime [93], phenyl-2-pyridylketoxime [94], 2,2'-dipyridyl-13-glyoxime (e = 1.9.104 at 558 nm; CHC13) [95], a-benzildioxime (with 4-methylpyridine) [96]. These reagents form coloured complexes with Fe(II) or Fe(III) in alkaline media.

In general, nitroso-compounds lead to more sensitive methods than oximes. Typical reagents are 1-nitroso-2-naphthol (e = 2.2.104 at 700 nm) [97], 2-nitroso-5-dimethylaminophenol (e = 4.0.104 at 750 nm) [98,99], and isonitrosobenzoyl acetone [100].

The cationic complex Fe(phen)32+ (ferroin) forms extractable ion pairs with the acid dyes: Methyl Orange (CHC13, ~ = 4.8.104 at 420 nm) [101], and Bromophenol Blue (CHC13, ~; = 6-8.104) [102,103], and eosin (CHC13, e = 1.32.105) [104]. The associates of Fe(II)- bathophenanthroline with Bromophenol Blue, and Bromophenol Red have also been applied


for the determination of iron [ 105]. Many other reagents have been used for determining iron, e.g., sulphosalicylic acid

[106-109], ferron (7-iodo-8-hydroxyquinoline-5-sulphonic acid) (e = 4.0-103 at 610 nm) [ 110], Tiron [ 111,115], HTTA [ 116,117], pyrocatechol [ 118], phenylfluorone (in presence of Triton X-100 [119], morin (in the presence of a surfactant; e = 6.3.104) [120-122], Alizarin S (extraction with Aliquat 336 in CHC13] [123], Purpurin [124,125], salicylates[126,127], 2,2'-diquinoxalyl [128], sulphanilic acid in the presence of a surfactant [129], haematoxylin [130], and hydroxamic acid derivatives [13,14,131,132].

Iron(III) was determined by the FIA technique in the presence of A1 [133] and in the presence of Cd, Co, and Cu [134].

EDTA reacts with Fe(III) to form a yellow-brown complex (~=20), which is transformed into a ternary violet complex (e=520) in the presence of H202. The method can be used for determining larger quantities of Fe [135,136]. The chloride complexes of Fe(III) have been also used as a basis for rather insensitive methods of determination [ 137,138].


The thiocyanate method has been used for determining iron in waters [139,140], in organic materials [141], nickel and its salts [142], non-ferrous metals [143], titanium tetrachloride [144], and alkalis [8].

The methods involving phen or 2,2'-bipyridyl were used in the determination of Fe, for example, in natural waters [17,28,29,145-148], in zinc and cadmium [31], in silicate minerals [149,150], in bauxites, soil, and clay [151], in aluminium oxide [152], in glass [19,153], and in the alkaline method for coal desulphurising [154]. Iron was also determined by the FIA technique with the use of 1,10-phenanthroline [108,155,156].

The bathophenanthroline method was used for determining iron in blood plasma [ 157], in plant materials [158], in waters [159], in niobium, tantalum, molybdenum, and tungsten [42,160], in molybdenum compounds [161], in cobalt [162], in cadmium and cadmium telluride [5], platinum [163], synthetic rubies and sapphires [164], silicon tetrachloride [165], and in boiler water [ 166].

The triazine reagents have been applied in the determination of iron in natural waters [55,64,167,168], in beverages [63], biological materials [63,168], plant materials [64], inorganic and organic acids [169], silicates [61,65], and metal ores [170]. Ferrozine was used for determining Fe in glass [171], sea-water [172,173], in biological samples, sewage, and bauxites [ 174]. Iron was determined in the presence of Co by derivative spectrophotometry [1751.

From among the methods mentioned above, iron has been determined with the use of Chrome Azurol S - in waters [176], Bromopyrogallol R e d - in magnetic Fe-Co-Ni films [177], sulphanilic a c id - in blood plasma [129] and in plants [109], T i ron- in geological materials [114], in aluminium alloys and copper [115], 2,2'-diquinoxalyl- in niobium oxide [128], P A N - in alloys and biological samples [79] and in waste waters [178], T A N - in geological samples [83], 5-Br-PADAP- in biological samples (by derivative spectrophotometry) [91] and in copper alloys [179], and mor in - in copper-chromium and nickel- chromium alloys [122].

The spectrophotometric methods for determining Fe in natural waters have been compared with other methods of analysis [ 180].

234 26. Iron

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Chapter 27. Lead

Lead (Pb, at. mass 207.19) occurs in its compounds in the II and IV oxidation states. Compounds of lead(iv) have acidic properties. Lead hydroxide, Pb(OH)2, is amphoteric, precipitating within the pH range 7-13. Lead(H) forms strong tartrate, acetate, thiosulphate, and EDTA complexes. The halide complexes are relatively weak.


27.1.1. Extraction

The foremost extraction method for separating lead from other elements involves extraction with dithizone (from a neutral or slightly alkaline medium), and is described below in detail. In the separation of larger quantities of lead (e.g., 1-2 mg), a chloroform solution of dithizone is more suitable, owing to the higher solubility of Pb(HDz)2 in CHC13 than in CC14 [1].

Diethyl dithiocarbamate extracts lead from slightly acidic media, from neutral media containing citrate, and from alkaline media containing cyanide and tartrate. When extracting from acid solutions the use of dibenzyl dithiocarbamate is advisable [2].

The lead-iodide complex is extractable into MIBK, methyl isopropyl ketone, or mesityl oxide [3]. The concentration of KI should be about 7%, and of HC1 ~5% [4].

The lead-chloride complex has been extracted with 30% TBP in hexane from 3 M HC1 and 2 M LiC1 medium [5]. The crown ether, dicyclohexyl-18-crown-6 has been recommended for preconcentration of Pb from HNO3 [6-8] and picric acid (pH 2-3) media [9]. 1,2-Dichloroethane, tetrachloroethane, and CHC13 are used as the solvents. Lead was also extracted, along with Hg(II) and Cu(II), with thiourea derivatives in chlorobenzene [ 10].

27.1.2. Ion exchangers and other sorbents

The anionic Pb-chloride complex can be separated from complexes of other metals on strongly basic anion exchangers [11]. From 1 M hydrochloric acid, Pb and Bi are retained on the column, whereas Ca, Ba, Cu, Fe, Sn, and T1 are eluted. Lead is then washed from the column with 0.01 M HC1 while bismuth is retained. Trace amounts of Pb have been separated from large amounts of Fe(III) using aqueous acetone (20%) medium [12].

On a strongly basic anion exchanger, Pb can be retained from sample solutions in 0.2- 0.3 M HBr, and thus it can be separated, e.g., from Zn, Ga, In, Fe(III), Cu, Co, Mn, U(IV), Ca, and Ba [13,14]. Many metals can be retained on strongly basic anion-exchangers using rather concentrated solutions of HBr. Pb was retained on the anion-exchanger, Dowex 1, from 0.15 M HBr, and then it was eluted with 6 M HC1 [ 15].

Lead sorbed on a cation exchanger can be eluted with 1 M ammonium acetate, and thus separated from A1, Ba, and Sr. Lead has been separated from Sn, Sb, Nb, Ta, Mo and W by cation exchange chromatography in tartaric-nitric acid mixtures [16]. By a similar method, lead has been separated from Bi, Sn, Cd, and In using methanol solutions of HC1 [17]. Pb was also sorbed on a cation exchanger modified with a basic dye [18].

Chelating resins are effective for the pre-concentration of lead from tap water [19], sea


water [20], and industrial waters [21]. Lead has been preconcentrated using a chelating sorbent plus iminodiacetic acid on a cellulose carrier [22], a sorbent modified with Xylenol Orange [20], or Amberlite XAD modified with Alizarin S [23] or with 8-hydroxyquinoline [24]. Tetraethyllead was adsorbed from air on active carbon [25]. Lead pyrrolidine dithiocarbamate was pre-concentrated on a C60 fullerene column [26].

Pb was sorbed on a resin [27] and polyurethane foam [28] modified with the crown ether, 18-crown-6, and its derivatives. The sorption of Pb (and other metals) on a chelating resin containing Arsenazo I was studied in water-organic solvent media [29]. A silica gel, modified with 2-mercaptobenzothiazole [30] or 2-mercaptobenzimidazole [31] was used for concentration of Pb and other heavy metals from aqueous solutions. Pb was selectively separated from A1, Mg, and Fe(III) on a cation exchanger modified with tin(W) antimonate [32].


Microgram quantities of Pb are separated from slightly acidic solutions (pH-~2) as the sparingly-soluble sulphide [33]. Ag, Cu, or Hg(II) are used as carriers, and the presence of citrate prevents precipitation of Fe(HI). Traces of lead have been isolated as the hydroxide with ammonia as precipitant and La as collector [34]. Lead has also been separated by co- precipitation with BaSO4 [35], CaCO3 [36], MnOzaq [37], or Fe(OH)3 [38].

Small amounts of lead can be isolated electrolytically as PbO2 at an anode, from ~-2 M HNO3 media [39,40].

Lead has been separated by evaporation at -1,400~ and condensation on a water- cooled quartz surface [41].


The dithizone method is the most frequently applied extractive spectrophotometric method for determining lead. The convenient PAR or Pyrocatechol Violet methods (in the presence of a surfactant) can also be recommended, after extractive separation of lead as the iodide complex.


Shaking a neutral or slightly alkaline solution of Pb with dithizone (see Section 4.5) results in the formation of the pink dithizonate, Pb(HDz)2, which is soluble in CC14, CHC13, and other non-polar solvents. The optimum pH range for the extraction of lead is 7-10 [42-44].

The molar absorptivity of the complex at ~max 520 nm is 6.86-104 (a = 0.33). Pb(HDz)2 solutions are stable if shielded from direct sunlight. Curve 3 in Fig. 4.4 shows the absorption spectrum of lead dithizonate in CC14 solution.

An alkaline aqueous solution is shaken with portions of dithizone solution in CC14. Some dithizone passes into the aqueous phase, which becomes brown. The excess of dithizone, remaining in the organic phase, is stripped with dilute potassium cyanide solution. If the KCN concentration, and accordingly the pH, are too high, some loss of lead is to be expected owing to partial decomposition of Pb(HDz)2. The lead content is calculated from the absorbance of the Pb(HDz)2 solution, or of the green dithizone solution (~max 620 nm) produced by decomposing the lead dithizonate with dilute (e.g., 0.1 M) HC1 or H2SO4.

The principal masking agent used in the determination of Pb with dithizone is cyanide,

240 27. Lead

which forms stable complexes with Ag, Hg, Pd, Au, Cu, Zn, Cd, Ni, and Co, thus preventing their reactions with dithizone. The first five of these metals can be separated from Pb by preliminary extraction with dithizone from an acidic medium (pH < 1).

In the presence of cyanide, dithizone extracts Pb together with Bi, TI(I), and In. However, indium ions are extracted by dithizone in CC14 from only weakly alkaline media. If the pH of the solution is -~ 10 (as is commonly the case in the determination of Pb), indium remains in the aqueous phase.

Thallium(I) extracted along with Pb from the ammoniacal cyanide medium is stripped from the extract, together with the free dithizone, by shaking with dilute KCN solution.

The most serious interference in the determination is caused by bismuth, which is quantitatively extracted at pH 3-10 by dithizone in CC14. The bismuth may be preliminarily extracted with dithizone at pH 3. Alternatively, the lead may be stripped with ~-0.001 M HC1 (optimum pH 3.3_+0.1) [45] from the CC14 extract containing the Bi and Pb dithizonates, and re-extracted with dithizone in CC14 after the pH of the aqueous phase has been raised.

The addition of tartrate to the aqueous solution before the extraction of lead prevents the precipitation of readily hydrolysable metals such as A1, Fe(III), and Ti. In the presence of tartrate and cyanide, small amounts of Fe do not interfere in the extraction. Larger quantities of iron(III) should be separated beforehand. Hydroxylamine is added to prevent oxidation of dithizone in the alkaline solution.

After the extraction of the Pb complex with crown ether by CHC13 from 2 M HNO3 medium, dithizone in CHC13-ethanol solutions was added to the extract [7].

In the presence of a surfactant, Pb can be determined directly in the aqueous phase [46]. Pb has been determined with dithizone, in the presence of Cu and Hg, by derivative spectrophotometry [47].

Reagents

Dithizone, H2Dz, 0.001% solution in CC14. Preparation as in Section 46.2.1. Standard lead solution: 1 mg/ml. Dissolve 1.5980 g of Pb(NO3)2 (dried at 110~ in

water containing 1 ml of conc. HNO3, and dilute the solution with water to 1 litre in a standard flask.

Potassium sodium tartrate (Seignette salt): 20% solution. Purification of the solution: add ammonia to make the pH -~8 and shake the solution in a separating funnel with small portions of dithizone in CC14 until the organic phase no longer turns pink. Store the solution in a polyethylene bottle.

Potassium cyanide: 10% solution. Prepare a 50% solution of KCN, and shake it with small portions of dithizone in CC14, until the organic phase no longer turns pink. Shake the aqueous solution with 2 or 3 portions of CHC13 to remove the free dithizone. Dilute the clear, aqueous KCN solution fivefold with water and store in a polyethylene bottle.

Hydroxylamine hydrochloride, 20% solution. Adjust aqueous NHzOH.HC1 with ammonia to pH --8, and shake with dithizone as described for the tartrate solution. The solution is unstable.

Wash solution. Dissolve 0.25 g of NH4C1 in water, add 0.5 ml of conc. ammonia solution, 1 ml of the 10% KCN solution, and dilute the solution with water to 250 ml. The solution is unstable.

Procedure

To a sample solution containing less than 50 ~tg of Pb, add 3 ml of the tartrate solution,


adjust with ammonia ( l+ l ) to pH --8, and transfer to a separating funnel. Add 1 ml of the NHzOH solution and 1-2 ml of the KCN solution. Extract with portions of the dithizone solution (1 ml of 0.001% H2Dz solution is equivalent to 4.0 gg of Pb) until the last portion of dithizone added shows no pink colour. Shake the combined extracts in the separating funnel with two portions of the wash solution. Wash the pink extract by shaking with water, transfer it to a 25-ml standard flask, and dilute to the mark with CC14. Transfer the clear solution to a cuvette and measure the absorbance at 520 nm, using CC14 as reference.

Note. To separate lead from any bismuth present in the extract, shake the CC14 extract with ~0.01 M HC1 (pH 3.0-3.5); Pb passes into the aqueous phase. Adjust the aqueous solution with ammonia to pH -8, add some KCN solution, and extract the lead with dithizone as stated above.

27.2.2. Pyridylazoresorcinol method

4-(2-Pyridylazo)resorcinol (PAR, formula 4.2) with lead(H) in weakly alkaline solution gives a red chelate that can be used in the determination of lead [4,48-50]. At the optimum pH, which is about 10, the free reagent has little influence on the absorbance of the complex (Zmax 412 nm). To obtain a maximum absorption of the complex, a 10-fold molar excess of the reagent is necessary. The reaction is rapid, and the chelate formed is stable.

The molar absorptivity of the Pb complex with PAR is 3.7.104 at ~max 520 nm (a = 0.18).

The selectivity of the Pb reaction with PAR can be increased by the use of cyanide, which efficiently masks Ag, Cd, Hg, Cu, Ni, Co, and Zn. Another way to improve the selectivity is to use a preliminary isolation of lead. Extraction as the iodide complex is a convenient method for its separation from interfering metals. Before this operation, Fe(III) and some other metals can be separated by extraction as thiocyanate complexes. Then potassium iodide is added, the HC1 concentration adjusted, and the iodide-lead complex extracted with MIBK. Besides lead, the extract should contain only cadmium (see the procedure below). However, in the preliminary extraction from thiocyanate medium, about 15% of the lead present is also extracted [4].

Reagents

4-(2-Pyridylazo)resorcinol (PAR), 0.1% aqueous solution. Standard lead solution: 1 mg/ml. Preparation as in Section 27.2.1. Potassium iodide, free from iodate. Buffer solution, pH 10. Dissolve 26 g of NH4C1 in water, add 85 ml of conc. ammonia

solution, and dilute the solution with water to 1 litre.

Procedure

Extractive separation of Pb. To the sample solution, containing not more than 100 ~tg of Pb, add enough HC1 to make its concentration 1.7 M (-~5%). Add 2 ml of 20% NH4SCN solution, and shake the solution with 10 ml of MIBK for 1 min. Transfer the aqueous phase to another separating funnel, add 1 g of solid KI, and extract the iodide-Pb complex with two 10 ml-portions of MIBK. Wash the combined extracts with 10 ml of 1.7 M HC1 containing 0.7 g of KI. Determination of Pb. To the MIBK solution in the separating funnel, add 10 ml of buffer solution of pH 10, 1 ml of 10% KCN solution, exactly 2 ml of PAR solution, and shake for

242 27. Lead

30 s. Transfer the clear aqueous solution to a 25-ml standard flask. Shake the organic phase with 5 ml of buffer solution of pH 10. Transfer the aqueous phase to the standard flask, dilute the aqueous solution with water to the mark, mix, and measure the absorbance at 520 nm against a reagent blank.

27.2.3. Pyrocatechol Violet- CP method

Lead(H) ions give with Pyrocatechol Violet (PV, formula 4.16), in the presence of a considerable excess of cetylpyridinium chloride (CP, formula 3.2) a ternary complex which has been proposed for determining Pb (Fig. 27.1) [51 ].

3

550 580 609 645 . . 750 w a v e l e n g t h , n m

Fig. 27.1. Absorption spectra of Pyrocatechol Violet (PV) vs. water (1), Pb-PV complex vs. the reagent solution (2), and Pb-PV-CP complex vs. the reagent solution (3)

The maximum and reproducible absorbance is obtained in a narrow pH range within 7.6+0.1, where the absorbance of the reagent itself is negligible. The colour reaction is fast, but the colour obtained is not ,stable - it starts to fade in about 15 min. The optimum excess is 10-fold for the reagent and 100-fold for the surfactant.

The molar absorptivity is 5.06.104 (sp. abs. 0.24) at 645 nm under the conditions specified below.

The selectivity of the method is low; a number of metals give colour reactions under similar conditions. It is advisable to separate lead before the reaction with PV and CP. The extractive method with the use of iodide and MIBK (see Section 27.2.2) gives highly selective separation of lead.

Reagents

Pyrocatechol Violet (PV), 2.10 -3 M (--0.07%) solution. Standard lead solution: 1 mg/ml. Preparation as in Section 27.2.1. Cetylpyridinium chloride (CP), 1.10 -2 M (-0.7%) solution.

Procedure

Place the sample solution (nearly neutral), containing not more than 100 ~tg of Pb in a beaker. Add 2.5 ml of the PV solution and 2.5 ml of the CP solution, and adjust the pH to 7.6+0.1 with dilute NaOH. Transfer the solution to a 25-ml volumetric flask, dilute with water up to the mark, and measure the absorbance at 645 nm using the reagent blank as a


reference.


Some other azo reagents, apart from PAR described above, have been proposed for the determination of lead, namely PAN [52] and its derivatives [53], 5-BrPADAP (~ = 4.9.104 at 575 nm) [54], TAR [55], Arsenazo III [56] (the formulae of these reagents have been given in Chapter 4), Sulpharsazen (e = 4.5.104 at 520 nm) [57].

Sensitive methods for lead include a number based on ion-associates formed by the anionic iodide-lead complex and the basic dyes, such as Malachite Green (benzene, e = 8.0-104) (58,59], Brilliant Green (59], Ethyl Violet [59], fuchsin (formula 27.1) (extraction with benzene-cyclohexane from 0.2 M H2804, E = 2.0.105 at 560 nm [60], and cyanine dyes [61]. In the method involving the antipyrine dye Chrompyrazole I (formula 23.1), the pseudo-solution formed is stabilized with the non-ionic surfactant OP-10 [62].

+

I

NH2

(27.1)

There exist methods based on extraction of the bromide-lead complex with basic dyes, such as Malachite Green [59,63], Victoria Blue 4R (59], and Butylrhodamine B (benzene, = 6.2.104] [64,65].

Pb has been also determined by the FIA technique, after conversion in a picolinic acid complex which is then associated with Malachite Green [66].

The cationic complexes of lead with 1,10-phenanthroline [25,67], cryptand(2.2.2) [68], and 18-crown-6 [69] form extractable ion-associates with the acid dyes eosin ( e - 1.1.105) [25,68], Erythrosin, Rose Bengal [25,70], Metanilic Yellow (~ = 4.5.104) [69], and Bromophenol Blue [67]. The Pb complex with Bathophenanthrolin, associated with Bromophenol Blue (e = 4.0.104), has also been applied [67].

Other organic reagents for lead include Xylenol Orange [39,71], Bromopyrogallol Red [72], Pyrogallol Red in the presence of a surfactant [73,74], CMAB-oxine [75], Bismuthiol II [76], Tyrodine ( ~ - 4.2.104) [77], 2-(o-hydroxyphenyl)benzothiazoline [78], and porphyrins [79,80].

Lead has been determined also with the use of chromophores based on crown ethers [81-85]. The catalytic effect of Pb(II) on the reduction of rezazurin with sulphide ions has also been used for its determinations [86]. Pb has been determined in mixtures with other metals with the aid of not-very-selective o-nitrophenylfluorone, after a mathematical processing of the spectra obtained [87].

Pb has been also determined in chloride solutions in the presence of Bi, Ru, and Nb, by derivative spectrophotometry [88].

244 27. Lead


The dithizone method has been applied for determining lead in biological samples [44,89,90], waters [8,15,91], soils [92],organic materials [93], plant materials [94,95], air (inorganic lead and organolead compounds) [96-98], silicate minerals [99], steel [2], molybdenum and tungsten [100], silver [34], cadmium [101], cobalt [11,13], boron [45], telluric acid [102], antimony sulphide [103], and gasoline [104].

The flow-injection technique, FIA, has been applied for continuous determination of lead with dithizone in gasoline. Lead has been also determined in sea sediments by derivative spectrophotometry [47].

Dithizone has been used in criminology in a microcrystal assay for lead traces [105]. Dithizone has also been used in the automatic determination of lead in various materials [89,96,106].

PAR has been applied for determining Pb in water [107] (by the FIA technique), in industrial waste waters [21], in environmental samples [108], steel and bronze [5,36], and copper alloys [ 109].

Lead was determined in steel and in zinc concentrates with the aid of Bromopyrogallol Red [73]. The Arsenazo III was used in the determination of lead in tin alloys [9] and aluminium [56]. Derivative spectrophotometry was used for the determination of Pb with porphyrin in clinical samples [81]. Diphenylcarbazone has been used for determining Pb in airborne dust and soil [110].

Other methods mentioned above were applied for determinations of Pb in waters [70,86], air [71 ], and various alloys [73].

References

1. Trinder N., Analyst, 91,587 (1966). 2. Stobart J.A., Analyst, 90, 278 (1965). 3. Prasada Rao T., Ramakrishna T.V., Talanta, 29, 227 (1982). 4. Dagnall R.M., West T.S., Young P., Talanta, 12, 583 (1965). 5. Yadav A.A., Khopkar S.M., Talanta, 18, 833 (1971). 6. Yahskin V.V., Korshunov M.B., Tolmacheva M.T., Zh. Anal. Khim., 50, 469 (1985). 7. Novikov J.A., Shpigun L.K., Zolotov Yu.A., Zh. Anal. Khim., 44, 422, 1305 (1989). 8. Novikov J.A., Shpigun L.K., Zolotov Yu.A.,Anal. Chim. Acta, 230, 157 (1990). 9. Vibhute R.G., Khopkar S.M., J. Indian Chem. Soc., 66, 720 (1989). 10. Ide S., Takagi M., Anal. Sci., 6, 599 (1990). 11. Kasiura K., Meus M., Chem. Anal. (Warsaw), 23, 305 (1978). 12. Kasiura K., Lech D., Chem. Anal. (Warsaw), 31, 269 (1986). 13. Uny G. et al.,Anal. Chim. Acta, 53, 109 (1971). 14. Strelow F.W., Anal. Chim. Acta, 183, 307 (1986). 15. Korkisch J., Sorio A., Talanta, 22, 273 (1975). 16. Strelow F.W., Van der Walt T.N., Anal. Chem., 47, 2272 (1975). 17. Strelow F.W., Anal. Chem., 57, 2268 (1985). 18. Shtokalo M.I., Kostenko E.E., Zhuk I.Z., Zh. Anal. Khim., 47, 1827 (1992). 19. De Mora S.J., Harrison R.M.,Anal. Chim. Acta, 153, 307 (1983). 20. Paull B., Foulkes M., Jones P.,Analyst, 119, 937 (1994). 21. Kuban V., Bulawa R., Coll. Czech. Chem. Comm., 54, 2674 (1989). 22. Gennaro M.C. et al., Anal. Chim. Acta, 174, 259 (1985).

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23. Saxena R., Singh A.K., Sambi S.S., Anal. Chim. Acta, 295, 199 (1994). 24. Lee D.W., Eum C.H., Lee I.H., Jeon S.J., Anal. Sci., 4, 505 (1988). 25. Tananaiko M.M., Bilenko N.S., Zavod. Lab., 42, 761 (1976); Zh. Anal. Khim., 34, 1899

(1979). 26. Gallego M., Petit de Pena Y., Valcarcel M., Anal. Chem., 66, 4074 (1994). 27. Horwitz E.P. et al., Anal. Chim. Acta, 292, 263 (1994). 28. Sukhan V.V., Nazarenko A.Yu., Mikhalyuk P.I., Ukr. Khim. Zh., 56, 43 (1990). 29. Chwastowska J., Jablofiska H., Chem. Anal. (Warsaw), 34, 407 (1989). 30. Dias N.L., Gushikem Y., Polito W.L., Anal. Chim. Acta, 306, 167 (1995). 31. Moreira J.C., Pavan L.C., Gushikem Y., Mikrochim. Acta, 1990 III, 107 (1990). 32. Varshney K.G., Gupta U., Bull. Chem. Soc. Jpn., 63, 1515 (1990). 33. Burridge J.C., Hewitt E.J., Analyst, 110, 795 (1985). 34. Marczenko Z., Kasiura K., Chem. Anal. (Warsaw), 9, 87 (1964). 35. Tiptsova V.G. et al., Zh. Anal. Khim., 23, 1065 (1968). 36. Dagnall R.M., West T.S., Young P., Talanta, 12, 589 (1965). 37. Blakeley S.J., Manson A., Zatka V.J.,Anal. Chem., 45, 1941 (1973). 38. Tikhonov V.N., Zh. Anal. Khim., 43, 1800 (1988). 39. Piryutko M.M., Tsvetkovskii I.B., Zh. Anal. Khim., 27, 1536 (1972). 40. Piryutko M.M., Benediktova N.V., Merezhinskii K.Yu., Zh. Anal. Khim., 37, 1648

(1982). 41. Marshall R.R., Hess D.C., Anal. Chem., 32, 960 (1960). 42. Mathre O.B., Sandell E.B., Talanta, 11, 295 (1964). 43. Jones R.A., Szutka A., Anal. Chem., 38, 779 (1966). 44. Oelschl~iger W., Schwarz E., Z. Anal. Chem., 258, 203 (1972). 45. Marczenko Z., Chem. Anal. (Warsaw), 9, 1093 (1964). 46. Paradkar R.P., Williams R.R., Anal. Chem., 66, 2752 (1994). 47. Mathew L., Rao T.P., Iyer C.S., Damodaran A.D., Mikrochim. Acta, 118, 249 (1995). 48. Geary W.J., Nickless G., Pollard F.H.,Anal. Chim. Acta, 26, 575; 27, 71 (1962). 49. Kle6kova Z., Langova M., Havel J., Coll. Czech. Chem. Comm., 43, 3163 (1978). 50. Ren S.X., Gao L., J. Autom. Chem., 17, 115 (1995). 51. Jarosz M., Swietlow A., Microchem. J., 37, 322 (1988). 52. Medina Escriche J. et al, Talanta, 32, 1058 (1985). 53. Chen H.W., Tang F.L., Gu C., Brindle I.D., Talanta, 40, 1147 (1993). 54. Gusev S.I., Nikolaeva E.M., Zh. Anal. Khim., 24, 1674 (1969). 55. Malakhova N.M., Olenovich N.L., Krainyakova M.M., Zavod. Lab., 35, 917 (1977). 56. Michaylova V., Kuleva N., Talanta, 27, 63 (1980). 57. Tikhonov V.N., Petrova T.P., Zh. Anal. Khim., 43, 274 (1988). 58. Kish P.P. et al., Zavod. Lab., 49, No 3, 8 (1983). 59. Kish P.P., Bazel' Ya.R., Balog I.S., Zh. Anal. Khim., 39, 820, 1226 (1984). 60. Kish P.P., Bazel' Ya.R., Balog I.S., Zh. Anal. Khim., 38, 2008 (1983). 61. Kish P.P., Bazel' Ya. R., Zh. Anal. Khim., 44, 313 (1989). 62. Kish P.P., Bazel' Ya.R., Zikan K.I., Zh. Anal. Khim., 41, 1061 (1986). 63. Kish P.P., Bazel' Ya.R., Balog I.S., Ukr. Khim. Zh., 49, 523 (1983). 64. Kish P.P., Bazel' Ya.R., Balog I.S., Zh. Anal. Khim., 39, 1052 (1984). 65. Kish P.P. et al., Zavod. Lab., fi0, No 8, 22 (1984). 66. Lopez T.I., Rangel A.O., Sartini R.P., Zagatto E.A., Analyst, 121, 1047 (1996). 67. Buhl F., Mikula B., Chem. Anal. (Warsaw), 32, 307 (1987). 68. Szczepaniak W., Juskowiak B., Anal. Chim. Acta, 140, 261 (1982); Mikrochim. Acta,

1987 II, 237; Chem. Anal. (Warsaw), 33, 911 (1988).

246 27. Lead

69. Pyatnitskii I.V. et al., Zh. Anal. Khim., 38, 2176 (1983). 70. Sreevalsan Nair D., Prasada Rao T., Iyer C.S., Damodaran A.D., Anal. Lett., 26, 523

(1993). 71. Amfinova L.D., Ivkova T.I., Pantaler R.P., Zavod. Lab., 52, No 11, 3 (1986). 72. Rao T.P., Ramakrishna T.V., Talanta, 27, 439 (1980). 73. Namboothiri K.K., Ramakrishna T.V., Indian J. Technol., 27, 201 (1989). 74. Sicilia D., Rubio S., Perez-Bendito D., Anal. Chim. Acta, 266, 43 (1992). 75. R6bisch G.,Anal. Chim. Acta, 47, 539 (1969). 76. Watanabe K., Imaeda Y., Kawagaki K., Bull. Chem. Soc. Jpn., 55, 3147 (1982). 77. Poeva N.N., Akimova T.G., Savvin S.B., Zh. Anal. Khim., 40, 1019 (1985). 78. Uhlemann E., Pohl V.,Anal. Chim. Acta, 65, 319 (1973). 79. Schneider J.A., Hornig J.F.,Analyst, 118, 933 (1993). 80. Igarashi S., Furukawa H., Kawakami T., Anal. Proc., 32, 107 (1995). 81. Li Z., Zhu Z., Jan T., Pan J.,Analyst, 124, 1227 (1999). 82. Petrova T.V., Sultanov A.V., Savvin S.B., Zh. Anal. Khim., 44, 603 (1989). 83. Petrova T.V., Dzherayan T.G., Savvin S.B., Zh. Anal. Khim., 45, 579 (1990). 84. Savvin S.B. et al., Fresenius' J. Anal. Chem., 340, 217 (1991). 85. Badaouni F.Z., Bourson J., Anal. Chim. Acta, 302, 341 (1995). 86. Li M.S., Pacey G.E., Talanta, 42, 1857 (1995). 87. Fedin A.V., Zh. Anal. Khim., 49, 209 (1994); Zavod Lab., 60, No 9, 1 (1994). 88. Spitsyn P.K., Korepina M.E., Antonov A.V., Zh. Anal. Khim., 50, 92 (1995). 89. Browett E.V., Moss R., Analyst, 90, 715 (1965). 90. Voloder K., Ivi6i6 N., Sviger B., Mikrochim. Acta, 1971, 341. 91. Abbott D.C., Harris J.R., Analyst, 87, 387 (1962). 92. Vackova M., Smirnova L., Chem. Listy, 84, 871 (1990). 93. Cella-Torrijos R., Perez-Bustamante J.A., Analyst, 103, 1221 (1978). 94. Riebartsch K., Gottschalk G., Z. Anal. Chem., 214, 179 (1965). 95. Kerin Z., Mikrochim. Acta, 1968, 927. 96. Moss R., Browett E.V., Analyst, 91,428 (1966). 97. Snyder L.J.,Anal. Chem., 39, 591 (1967). 98. Groffman D.M., Wood R., Analyst, 96, 140 (1971). 99. Stanton R.E., McDonald A.J., Carmichael I., Analyst, 87, 134 (1962). 100. Piischel R., Lassner E., Mikrochim. Acta, 1965, 751. 101. Krasiejko M., Marczenko Z., Mikrochim. Acta, 1975 I, 585. 102. Veale C.R., Wood R.G., Analyst, 85, 371 (1960). 103. Norwitz G., Cohen J., Everett M.E., Anal. Chem., 32, 1132 (1960). 104. Klinghoffer O., Ru2i6ka J., Hansen E.H., Talanta, 27, 169 (1980). 105. McGinnis M.D., Thornton J.I., Espinoza E.O., J. Forensic Sci., 32, 242 (1987). 106. Bano F.J., Crossland R.J., Analyst, 97, 823 (1972). 107. Novikov E.A., Shpigun L.K., Zolotov Yu. A., Zh. Anal. Khim., 43, 2213 (1988). 108. Raman B., Shinde V.M., Bull. Chem. Soc. Jpn., 62, 3679 (1989). 109. Malakhova N.M., Olenovich N.L., Kotel'naya N.I., Zh. Anal. Khim., 35, 475 (1980). 110. Thakur M., Deb M.K.,Analyst, 124, 1331 (1999).

Chapter 28. Magnesium

Magnesium (Mg, at. mass 24.31) occurs in its compounds exclusively in the II oxidation state. The hydroxide, Mg(OH)2, begins to precipitate at pH 9.6 and shows no amphoteric properties: it is less soluble than the phosphate or oxinate. Magnesium forms rather weak tartrate-, citrate-, and EDTA complexes.



It is generally necessary to separate magnesium from the Analytical Group I-III metals before its determination. These other metals are often removed by precipitation as their hydroxides, sulphides, 8-hydroxyquinolinates, or dithiocarbamates (at pH 8-9), while magnesium (along with Ca, Sr, and Ba) remains in solution.

For separation from amphoteric metals (e.g., A1, Sn, and Pb), magnesium is precipitated as Mg(OH)2 with a solution of NaOH. When Mg(OH)2 is precipitated with an alkali in the presence of tartrate, Fe, Cu, Ni, Mn, and certain other metals remain in solution. Titanium is masked as the peroxide complex in the alkaline solution. Activated carbon has been employed as gatherer of trace amounts of magnesium as the hydroxide [ 1 ].

Traces of magnesium are separated from metals that form soluble cyanide complexes (Ni, Zn, Mn, and others) by precipitation as the phosphate from an alkaline cyanide medium containing La as collector [2].

Traces of magnesium have been co-precipitated with Ca or Ba fluorides as collector [3].


Mg and Ca are retained on cation-exchange columns, from which Mg is then eluted with dilute HC1 or HC104 [4]. Magnesium and calcium have been separated from Fe, A1, Zn, Mn, and Cu based on differences in stability of the EDTA complexes. Only Mg and Ca are retained on Dowex 50 cation-exchanger from mixtures of these metals in a solution of pH 3.6 containing EDTA; Mg and Ca are then eluted successively with 0.7 M HNO3 and 2 M HNO3 [6].

Magnesium has been separated from calcium on anion-exchange resins [7]. Chloroform solutions of oxine [8], its derivatives [9], or BPHA [10], have been used for

extraction separations of Mg.


The coloured adsorption compounds formed by Mg(OH)2 with organic dyes are essential parts of the methods for determining magnesium (e.g., the Titan Yellow method). Other methods are based on soluble coloured magnesium complexes formed with some organic reagents (e.g., Eriochrome Black T) in ammoniacal media.

248 28. Magnesium

28.2.1 Titan Yellow method

When precipitated from solutions containing certain coloured high-molecular-weight organic reagents, magnesium hydroxide gives coloured adsorption compounds [11]. The compound formed with Mg(OH)2 and Titan Yellow (formula 28.1) provides the basis of a commonly used spectrophotometric method for determining magnesium [11-15]. Commercial preparations of the sodium salt of Titan Yellow (also known as Thiazole Yellow or Clayton Yellow) from different sources display differences in their absorption spectra [13,14]. The absorption maximum of a solution of Titan Yellow at pH > 12 is at 410 nm, whereas a colloidal solution of the adsorption compound formed with Mg(OH)2 and Titan Yellow is pink with )Lmax at 545 nm. At that wavelength, the absorbance of the free reagent is negligible.

S03H SOzH

(28.1)

The molar absorptivity of the pseudo-solution of the magnesium compound with Titan Yellow is 3.6.104 at 545 (a = 1.5). The intensity and reproducibility of the colour obtained are affected by the method of pH adjustment, the excess of Titan Yellow, the protective colloid used, the temperature of the solution, and the time of standing. Immediately after the start of the colour reaction, an increase in absorbance is noticed, but after 10-30 min the colour of the solution remains almost constant. After this it weakens progressively. Hydroxylamine is reported to stabilize the colour [ 12].

Solutions containing more than 2 ~tg of Mg per ml are unstable and soon become turbid as Mg(OH)2 coagulates. Protective colloids, such as poly(vinyl alcohol) prevent the coagulation. Gelatine, gum arabic, and starch are also used.

Temperature greatly affects the reproducibility of the results. Heating a coloured solution from 20 to 30~ increases the absorbance by -~20%.

A synergistic effect on the determination of magnesium is exerted by calcium. Although calcium itself gives no colour reaction with Titan Yellow, its presence with the magnesium causes increased absorbance. Since no further increase occurs above a certain concentration of calcium, the increased absorbance is exploited by adding excess of calcium to the sample and the standard solutions.

Species which decrease the quantity of Mg(OH)2 precipitated upon the addition of NaOH, such as ammonium salts, and anions which precipitate Mg (e.g., phosphate), interfere in the Titan Yellow method. A number of metal cations decrease or increase the colour intensity. Decreased absorbance in the presence of A1, Zn, or Sn(IV) is believed result from adsorption of aluminate, zincate or stannate ions on the Mg(OH)2, which reduces the amount of dye adsorbed.

A high concentration of salts in the sample solution interferes by increasing the solubility of Mg(OH)2.

Reagents

Titan Yellow, 0.01% solution. Dissolve 10 mg of the reagent in 100 ml of water. The solution is stable for at least one week.

Standard magnesium solution: 1 mg/ml. Dissolve 10.1350 g of MgSO4.7H20 (or 4.9500


g of the salt ignited at 400-500 o C) in water containing 1 ml of conc. H2SO4, and dilute the solution with water to volume in a 1-1itre standard flask.

Calcium chloride, 2% solution of CaC12.2H20, free of Mg. Add sodium hydroxide to this solution until its concentration is -~1 M . After 1 h, filter off the precipitated Mg(OH)2 and slightly acidify the solution with hydrochloric acid.

Procedure

To the sample solution, in a 25-ml standard flask, containing not more than 12 gg of Mg, add 1 ml of the calcium chloride solution, 2.5 ml of the Titan Yellow solution, 3 ml of 1% poly(vinyl alcohol) solution, and water to -17 ml. While swirling the solution vigorously, add 1 M NaOH dropwise from a burette until the solution changes colour. Then add 2.5 ml more NaOH solution with continued mixing, and dilute the solution with water to the mark. After 15 min, measure the absorbance of the solution at 545 nm against a blank solution containing the same amount of calcium.

28.2.2. Eriochrome Black T method

The reaction of a blue Eriochrome Black T (Erio T, formula 28.2) solution in an alkaline medium (pH 7.5-11.5) with magnesium ions, to form a pink complex, has been employed in the determination of magnesium [ 1,6].

OH OH

N~-'N S03H (28.2)

N0x

Since slight alteration of the pH changes the blue colour of the Eriochrome Black T, the pH of the sample and the standard solution must be controlled carefully. The sensitivity of the method depends on the quality and excess of the Erio T used. The sensitivity decreases rapidly with increasing ionic strength.

The molar absorptivity of the complex at pH 9.6 at )Lmax 520 nm is 1.8.104 (a - 0.76). Although the method has greatest sensitivity at pH 10.4 (e = 2.3.104), a pH of 9.6 is more convenient since calcium (in four-fold amount relative to magnesium) does not interfere at the lower pH value.

Larger quantities of calcium must be separated, e.g., by precipitation as the oxalate or sulphate (in 90% methanol), before the determination of magnesium. The Analytical Group I-III metals must also be separated first. Small amounts of Fe, Cu, Zn, and Ni may be masked with cyanide. Phosphate should be removed, e.g., by ion-exchange separation.

One version of the method proposed includes extraction of the Mg-Eriochrome Black T complex into butanol [16]. Extractable ion-associates of the anionic Mg-Erio T complex with TOA (CHC13, ~ = 5.5.104 at 555 nm) [17] have also been used for determining Mg. The FIA technique has also been used for the determination of Mg [ 18].

Reagents

Eriochrome Black T (Erio T), 0.2% solution in methanol; this solution is stable for about one

250 28. Magnesium

week. Standard magnesium solution: 1 mg/m. Preparation as in Section 28.2.1. Buffer solution: pH 9.6. Dissolve 60 g of NH4C1 in water, add 120 ml of conc. ammonia

solution, and dilute the solution with water to 1 litre.

Procedure

To a slightly alkaline sample solution (pH 8-9), containing not more than 25 ~tg of Mg and free from the Group I-III metals add, in a 25-ml volumetric flask, 10 mg of ascorbic acid and 10 mg of potassium cyanide (to mask possible traces of Fe, Ni, Cu, Zn, and other metals). After 5 min, add 1 ml of buffer solution and 25 ml of the Eriochrome Black T reagent. Dilute the solution with water to volume in a 25-ml standard flask and measure the absorbance at 520 nm against a reagent blank.

Note. Under these conditions a fourfold amount of calcium relative to magnesium does not interfere.


Besides Titan Yellow, several other organic dyes have been recommended which react with Mg(OH)2 to form adsorption compounds in alkaline media, e.g., Magneson II, Phenazo [11 ], and polymethine dyes [ 19-21 ].

Most of the other methods are based, as with Erio T, on reactions of magnesium ions to form coloured chelates with azo reagents in alkaline media. Among these reagents are: Calmagite (formula 28.3) (e = 2.0.104 at 540 nm) [9,22,23], PAR [24-26], and Arsenazo III [27]. High sensitivity, as in the Calmagite method, has been obtained with the use of Xylidyl Blue I (Magon Sulphonate) and Xylidyl Blue II (Magon) [28-31].

OH OH

HO, S-<L CH 3

(28.3)

Other azo reagents for Mg include Eriochrome Black B [6], Chromotrope 2R (~ = 3.7.10 4 a t

570 nm) [32], Nitrophosphonazo I (e = 3.8.104 at 580 nm) [33,34], 2-(4'-phenylazo)- chromotropic acid [35], Beryllon II [36,37], Chlorophosphonazo I [38] and Chlorophosphon- azo III [~ = 5.6.104 at 573 nm) [39-41]. The latter reagent has also been used in the FIA technique [40,41 ].

8-Hydroxyquinoline has been used as a basis for low sensitivity extractive- spectrophotometric methods for magnesium [42,43]. The Mg oxinate is extracted into chloroform in the presence of 2-butoxyethanol [42], butylamine [8,42], or Zephiramine. The molar absorptivity with butylamine is 7.0.103 at 388 nm. A more selective method for determination of Mg, using CMAB-oxine w a derivative of 8-hydroxyquinoline w has been recommended [44].

Some other organic reagents have been also proposed for determination of Mg, e.g., Eriochrome Cyanine R [45], Alizarin S [46], o-cresolphthalein [47] and its derivatives [48], 1,2,7-trihydroxyanthraquinone [49], 1,8-dihydroxyanthraquinone (e = 1.2.104 at 510 nm) [50], and leuco-quinizarin [51]. Mg has been determined also with the use of emodin


(1,3,8-trihydroxy-6-methylanthraquinone) [52-54]. Derivative spectrophotometry has been utilized for the determination of Mg in the presence of Cu and Ca [54]. Mg has been determined with the use of various hydroxyanthraquinones in media containing surface- active agents [52]. Magnesium has been determined successfully, in the presence of a 5-fold excess of Fe(III) and a 100-fold excess of Ca and A1, with the use of Congo Red [55].

Extractable ion-associates of Rhodamine B with anionic Mg complexes with bromo- oxine [56] and iodo-oxine ( ~ - 2.7.104 at 545 nm, toluene) [57] have been used for determining magnesium.


Titan Yellow has been used for determination of magnesium in nickel and its alloys [3], silicate minerals [14,58], carbonate minerals [58], soil extracts [13], plant materials [59], and blood [60].

Magnesium has been determined with the use of Eriochrome Black T in waters [61 ], soil extracts and rocks [18], non-ferrous metallurgy products [62], nickel, zinc, and manganese salts [2], and sodium chloride [1].

The azo reagents have been applied for determination of Mg in waters [24-26], soils [31 ], and rocks [29]. The FIA technique has been applied for determining Mg with the use of PAR in liquid after dialysis [26].

References

1. Marczenko Z., Mojski M., Balcerzak M., Mikrochim. Acta, 1975 I, 539. 2. Marczenko Z., Kasiura K., Chem. Anal. (Warsaw), 7, 775 (1962). 3. Cyrankowska M., Chem. Anal. (Warsaw), 19, 309 (1974). 4. Mann C.K., Anal. Chem., 32, 67 (1960). 5. Chwastowska J., Szymczak S., Chem. Anal. (Warsaw), 14, 1161 (1969). 6. Slegers G., Claeys A., Analyst, 99, 471 (1974). 7. Fritz J.S., Waki H.,Anal. Chem., 35, 1079 (1963). 8. Newman E.J., Watson C.A., Analyst, 88, 506 (1963). 9. Hofer A., Heidinger R., Z. Anal. Chem., 230, 95 (! 967). 10. Chwastowska J., R62afiska B., Chem. Anal. (Warsaw), 23, 745 (1978). 11. Babko A.K., Romanova N.V., Zavod. Lab., 34, 1435 (1968); Zh. Anal. Khim., 24, 786

(1969). 12. Van Wesemael J.C., Anal. Chim. Acta, 25, 238 (1961). 13. Hall R.J., Gray G.A., Flynn L.R., Analyst, 91, 102 (1966). 14. King H.G., Pruden G., Analyst, 92, 83 (1967). 15. King H.G., Pruden G., Janes N.F., Analyst, 92, 695 (1967). 16. Zolotov Yu.A., Bagreev V.V., Zh. Anal. Khim., 22, 1423 (1967). 17. Pyatnitskii I.V., Pinaeva S.G., Pospelova N.V., Zh. Anal. Khim., 30, 2316 (1975). 18. Zolotov Yu.A. et al., Anal. Chim. Acta, 308, 386 (1995). 19. Asmus E., Klank W., Z. Anal. Chem., 206, 88 (1964). 20. Asmus E., Kuchenbecker H., Z. Anal. Chem., 248, 291 (1969). 21. Asmus E., Ortlepp W., Z. Anal. Chem., 266, 343 (1973). 22. Harrison F.H., Metallurgia, 70, 251 (1964). 23. Ingman F., Ringbom A., Microchem. J., 10, 545 (1966). 24. Gomez E., Estela J.M., Cerda V., Anal. Chim. Acta, 212, 513 (1991).

252 28. Magnesium

25. Gomez E. et al., Analyst, 120, 1181 (1995). 26. Hernandez O. et al., Anal. Chim. Acta, 320, 177 (1996). 27. Blanco M. et al., Anal. Chim. Acta, 224, 23 (1989). 28. Apple R.F., White J.C., Talanta, 8, 419 (1961). 29. Abbey S., Maxwell J.A., Anal. Chim. Acta, 27, 233 (1962). 30. Svoboda V., Chromy V.,Anal. Chim. Acta, 54, 121 (1971). 31. Qui X.C., Zhu Y.Q., Microchem. J., 33, 364 (1986). 32. Shibata S. et al., Anal. Chim. Acta, 44, 345 (1969). 33. Qiu X.C. et al., Chem. Anal. (Warsaw), 32, 285 (1987). 34. Lin J.M. et al., Z. Anal. Chem., 326, 777 (1987). 35. Zhukova M.P., Kostina L.V., Petrova T.V., Zh. Anal. Khim., 39, 1416 (1984). 36. Zhu Y.Q., Zhang L., Li J., Analyst, 107, 957 (1982). 37. Qiu X.C., Zhu Y.Q.,Anal. Chim. Acta, 149, 375 (1983). 38. Qui X.C., Zhu Y.Q., Mikrochim. Acta, 1983 III, 1. 39. Kaneko K., Yoshida M., Ozawa T., Anal. Chim. Acta, 132, 165 (1981). 40. Yuan Y., Anal. Chim. Acta, 212, 291 (1988). 41. Novikov E.A., Shpigun L.K., Zh. Anal. Khim., 48, 1326 (1993). 42. Jankowski S.J., Freiser H., Anal. Chem., 33, 776 (1961). 43. Noriki S., Nishimura M., Anal. Chim. Acta, 72, 339 (1974). 44. R6bisch G., Rericha A., Anal. Chim. Acta, 154, 267 (1983). 45. Elenkova N.G., Popova E.S., Talanta, 22, 925 (1975); 23, 467 (1976). 46. Khalifa M.E., Chem. Anal. (Warsaw), 41,357 (1996). 47. Nogueira A.R. et al., J. Agric. Food Chem., 44, 165 (1996). 48. Yamane T., Goto E., Talanta, 38, 139 (1991). 49. Roman Ceba M., Fernandez-Gutierrez A., Palomera A., Anal. Lett., 10, 907 (1977). 50. Roman Ceba M., Fernandez-Gutierrez A., Mahedero M.C., Mikrochem. J., 27, 339

(1982). 51. Bello Lopez M.A. et al., Analyst, 111,429 (1986). 52. Pal T., Jana N.R., Talanta, 41, 1291 (1994). 53. Pal T., Jana N.R., Das P.K.,Analyst, 117, 791 (1992). 54. Pal T., Jana N.R.,Analyst, 118, 1337 (1993); Talanta, 40, 1519 (1993). 55. Putilina O.N., Makarevskaya V.V., Zh. Anal. Khim., 45, 1173 (1990). 56. Bel'tyukova S.V., Poluektov N.S., Zh. Anal. Khim., 25, 1714 (1970). 57. R6bisch G., Rericha A.,Anal. Chim. Acta, 153, 281 (1983). 58. Evans W.H., Analyst, 93, 306 (1968). 59. Chenery E.M., Analyst, 89, 365 (1964). 60. Butler E.J. et al.,Anal. Chim. Acta, 30, 524 (1964). 61. Impedovo S., Traini A., Papoff P., Talanta, 18, 97 (1971). 62. Shkrobot E.P., Shebarshina N.I., Blyakhman A.A., Zavod. Lab., 35, 539 (1969).

Chapter 29. Manganese

Manganese (Mn, at. mass 54.94) occurs in its compounds in the oxidation states n, III, IV, vI and VII. The manganese(H) hydroxide precipitates at pH --8.5 and exhibits no amphoteric properties. Manganese(H) forms complexes with EDTA, cyanide, tartrate, and ammonia. The white Mn(OH)2 darkens slowly on standing in air, as it is oxidised to hydrous manganese dioxide. Manganese(m) occurs in sulphate, cyanide, and pyrophosphate complexes. A manganese(IV) complex with formaldoxime is also known. Brown hydrous MnO2 aq. is sparingly soluble in alkalies and in non-reducing acids. Permanganate ion is a strong oxidant. When reduced with alcohol in an alkaline medium, MnO4- is converted into the green manganate ion, MnO42-. On acidification, the manganate disproportionates into MnO4- and MnOzaq.


29.1.1. Extraction

Manganese(H) forms, with DDTC, a pale precipitate which, on standing in air in the presence of excess of the reagent, changes into the brown-violet Mn(DDTC)3. The complex is soluble in CHC13, amyl acetate, and mixtures of iso-amyl alcohol with CC14. The extraction of Mn(DDTC)3 (optimum pH 6-8) is used for separation of manganese before the determination. Traces of Mn can be extracted into CHC13 as the oxinate [1,2], thio-oxinate [1], or complex with HTTA [3]. The thiocyanate complex of manganese has been extracted with trioctylmethylammonium chloride (benzene) [4], and the liquid anion-exchangers, Aliquat 336 and Alamine 336-S (toluene) [5]. The associate of the benzohydroxamate with the amine Aliquat 336 has also been extracted [6].


Sodium hydroxide quantitatively precipitates traces of Mn as MnO2aq., with Fe(II-I), La, or Mg as carrier. Manganese can also be separated as MnOzaq. by treatment with excess of ammonia in the presence of H202. This method enables one to separate manganese from Ti, V, and other metals which form soluble peroxide complexes.

The chloride complex of manganese(H) is weaker than the chlorides of other metals (e.g., Fe, Co, Cu, Zn), thus Mn can be separated from these metals by ion-exchange, using cation or anion exchangers. Ion-exchange chromatography has been used for separation of manganese from natural waters [7,8]. Manganese has been retained on the cation-exchanger Dowex 50 from a medium containing 12 M nitric acid (10%), acetone (10%), and MIBK (80%); 2 M nitric acid has been used as the eluent [9].


The well-known and often-used spectrophotometric method for determining manganese, which is based on the coloured MnO4- ion, is highly selective but rather insensitive. Greater sensitivity is achieved in the method using formaldoxime. Even more sensitive is the

254 29. Manganese

extractive photometric PAN method. The spectrophotometric methods for determination of manganese have been reviewed [10].

29.2.1. Permanganate method

Oxidation of manganese(H) by powerful oxidants in acidic solutions to yield violet MnO4- ions constitutes the basis of this method [ 11,12]. The reactions with potassium periodate and ammonium persulphate, two popular reagents for the oxidation of Mn 2+, are:

2Mn 2+ + 5IO4- + 3H20 --~ 2MnO4- + 5IO3- + 6H + 2Mn 2+ + 582082- q- 8H20 ~ 2MnO4- + 10HSO4- + 6H +

With persulphate as oxidant, it is necessary to add a small amount of Ag(I) or Co(II) ions to catalyse the reaction. The oxidation of manganese with periodate or persulphate is carried out in sulphuric acid or nitric acid, or in a mixture of the two. The concentration of the acid affects the rate of oxidation of Mn(II). With KIO4, higher concentrations of H2SO4 and HNO3 can be used than with (NH4)2S208. Periodate oxidation yields a more stable solution of MnO4- than does persulphate oxidation.

Reducing species, including chloride, present in the solution interfere in the determination of manganese. They are removed beforehand by evaporating the sample solution with H2804 until white fumes appear. Large amounts of coloured metal ions ( e.g., Ce 4+ , Ni 2+ , Co 2+ , C u 2+ , Cr2072-, and U022+) also interfere. Fe(III) ions are masked as a colourless complex by phosphoric acid. The effect of Fe(III) on the determination of manganese as MnO4-has been studied in detail [13]. Interference from coloured ions other than Ce(IV) is overcome by measuring the absorbance before and after reduction of the Mn(VII) by sodium azide (20-30 mg NAN3), hydrogen peroxide, or sodium nitrite.

Turbidity (AgC1) caused by traces of chloride present during silver-catalysed persulphate oxidation is prevented by the addition of a little mercury(H) sulphate [a stable complex, HgCI2, is formed in the presence of Hg(II)]. The mechanism and the conditions of Mn(II) oxidation by persulphate in the presence of Ag(I) ions have been studied [ 11 ].

The oxidation of manganese(H) has also been performed with the use of sodium bismuthate [14], ammonium and cerium(W) nitrate in the presence of Ag + ions [15], and sodium perxenonate Na4XeO6 which acts rapidly, even at low temperatures, in 0.1-0.2 M HNO3 medium [ 16].

The permanganate method is one of the less sensitive spectrophotometric methods. The molar absorptivity is 2.4.103 (a = 0.044) at 528 nm. The absorption spectrum of MnO4- is shown in Fig. 29.1. A double maximum at 528 nm and 548 nm is observed in the visible region. Two small inflexions occur at 514 nm and 570 nm.

Permanganate ion forms ion-pairs with the tetraphenylarsonium [17,18]-, ethylenebis(triphenylphosphonium) [19,20], trimethylenebis(triphenylphosphonium) [21,22], and benzyltributylammonium [23,24]- cations, soluble in chloroform and in other solvents. These compounds are used in the extractive variants of the permanganate method.

Reagents

Silver nitrate, -- 0.001 M solution. Dissolve 85 mg of AgNO3 in 500 ml of water. Keep the solution in an amber glass bottle.

a) Standard manganese(VII) solution: 1 mg/ml. Dissolve 2.8730 g of KMnO4 in water containing 2 ml of conc. H2SO4, and dilute the solution with water to 1 litre.

b) Standard manganese(H) solution: 1 mg/ml. Dissolve 2.7490 g of anhydrous MnSO4 in water containing 1 ml of conc. H2SO4, and dilute the solution with water to 1 litre. The


anhydrous salt is obtained from hydrated manganese(R) sulphate by drying at 150~ and subsequent ignition at about 400~ Alternatively, acidify precisely 91 ml of 0.02 M KMnO4 with 5 ml of conc. H2SO4, and add hydrogen peroxide dropwise until the solution is decolorized. Heat the solution to decompose the excess of H202, cool, and dilute with water to 1 litre in a volumetric flask.

1.00 . . . . 1 , . . . _

5 0 0 . . 6 0 0 700 wavelength, nm

Fig. 29.1. Absorption spectrum of permanganate ion in 0.5 M H2S04

Procedure

Oxidation of Mn with periodate. Dilute the sample solution containing not more than 300 ~tg of Mn, free from chloride and other reducing agents, in a beaker, with water to about 15 ml. Add 2 ml of conc. H2SO4, 0.5 ml of conc. HNO3, 1 ml of conc. H3PO4, and 0.15 g of KIO4, and stir. Heat the solution nearly to boiling, and keep at about 90~ for 10 min. Cool the solution, transfer it to a 25-ml standard flask, dilute to the mark with water, and stir well. Measure the absorbance of the solution at 528 nm against water or a reagent blank.

Oxidation of Mn with persulphate. To the sample solution add 0.5 ml of conc. H2SO4, 0.5 ml of conc. HNO3, 1 ml of conc. H3PO4, 1 ml of the AgNO3 solution, and 0.25 g of (NH4)2S208. Heat the solution nearly to boiling and keep at this temperature for about 5 min. Proceed further as described above.

29.2.2. Formaldox ime method

If a solution containing manganese and formaldoxime (formaldehyde oxime, formula 29.1) is made alkaline with ammonia or sodium hydroxide, a colourless manganese(H) complex is formed. Oxygen instantaneously transforms this complex into a brown-red complex of manganese(IV).If the initial solution contains either MnO4- ions or a suspension of MnOzaq., formaldoxime first reduces the manganese to Mn(II) and then converts it into a coloured formaldoxime complex [25]. The molar absorptivity of the complex is 1.12-104 (a = 0.20) at ~ m a x "- 455 nm.

H\ .C=NOH

H / (29.1)

Irrespective of the alkalizing agent used, and of the Mn:formaldoxime ratio, only one

256 29. Manganese

coloured complex is formed in the solution (see the absorption curve in Fig. 29.2). The Mn:CHzNOH ratio in the complex is 1:6, corresponding to Mn(CHzNO)62-. The stability of the complex is shown by the fact that the colour reaction is not affected by tartrate, oxalate, phosphate, sulphide, cyanide, or EDTA. Reductants such as ascorbic acid, sulphite, or hydroxylamine do not hinder the rapid formation of the coloured complex.

In 0.04-0.05 M NaOH medium, the coloured complex can be heated for 15 min at 90~ without any change in colour. The complex is less resistant to heat at other concentrations of NaOH and in ammoniacal solutions.

Formaldoxime gives coloured complexes also with Ce, Cu, Fe, Ni, Co, and V. The formaldehyde complexes with Ce and Cu decompose in less than 1 min at 70~ The violet complex of Fe decomposes at 70~ within 20 min. The V- complex decomposes on heating for 5 min at 90~ The formaldoxime complexes of Co and Ni are more resistant to increased temperatures.

1

3

l.O0

4

y ~ss 500 s3~ 600

wavelength, nm

Fig. 29.2. Absorption spectra of manganese (1), cerium (2), vanadium (3), nickel (4), and iron (5) formaldoximates.

Cyanide prevents the formation of the formaldoximates of Ni, Co, Cu, and Fe(II). Larger quantities of Fe(III) can be separated from Mn by precipitation with a Zn(OH)2 suspension. Use of cyanide as a masking agent, and heating the solution to 90~ (to decompose V- and Ce formaldoximates) make this method specific for manganese [25].

The reagent is added either as the acid solution obtained from formalin and hydroxylamine hydrochloride or as solid (CHzNOH)3.HC1. The solid form is more convenient for determining traces of manganese, when it is important to limit the volume of the final solution.

Reagents

Formaldoxime, 1M solution (-5%). Mix 7.9 g of formalin (38% solution of CH20) with a solution of 7.0 g of NHzOH.HC1 and dilute with water to 100 ml. The solution is acidic, 1 M in HC1, and stable.

Solid formaldoxime hydrochloride, (CHzNOH)3.HC1. Dissolve 105 g of NH2OH.HC1 in 110 ml of water. Add 45 g of paraformaldehyde, and stir. Heat the solution under reduced pressure (--25 mm Hg) at about 40~ When large quantities of crystals begin to precipitate, stop the heating, add 80 ml of anhydrous ethanol, and stir. Leave overnight, then filter off the


crystals and wash them with anhydrous ethanol. Recrystallise the product from anhydrous ethanol and dry in a vacuum drier.

Standard manganese solution: 1 mg/ml. Preparation as in Section 29.2.1. Zn(OH)2 suspension. Preparation and purification from Fe, Mn, and other metals:

dissolve 2 g of zinc nitrate in 50 ml of water, add 5 mg of La (in the form of the nitrate solution), some H202 solution, and ammonia in excess. Filter off the coagulated precipitate, evaporate off the excess of ammonia from the filtrate, and acidify the solution slightly with HC1. Precipitate Zn(OH)2 by adding dilute NaOH solution until the pH is 8-9. Filter off the precipitate and wash it with water until C1- ions are no longer present in the washings. Mix the precipitate with 100 ml of water.

Procedure

To a weakly acidic solution containing not more than 50 ~tg of Mn, add 20% potassium sodium tartrate solution (in the presence of A1, Cr, Ti, or U), about 10 mg of ascorbic acid (in the presence of Fe), and about 20 mg of KCN (in the presence of Ni, Co, Fe, or Cu). Next, add 1 ml of 1 M formaldoxime, followed immediately by 1 ml more of 1 M NaOH than is required for neutralization. Dilute the solution with water to the mark in a 25-ml standard flask. After 10 min, measure the absorbance of the solution at 455 nm, using water or a blank solution as reference.

Notes. 1. If there is more Fe than Mn present, remove the Fe(III) by extraction of its chloride or thiocyanate complex, or by precipitation (along with A1 and Ti) with the Zn(OH)2 suspension.

2. Before the addition of Zn(OH)2, adjust the solution to pH 2-3 with dilute NaOH solution. After the Zn(OH)2 is added, heat the solution for about 15 min at ~70~ filter off the coagulated precipitate, and wash it with hot water.

3. If the sample solution is likely to contain traces of V or Ce, heat the coloured solution for 15 min at 90~ Filter off any precipitate present after cooling.

29.2.3. Pyridylazonaphthol (PAN) method

1-(2-Pyridylazo)-2-naphthol (formula 4.1) reacts with Mn(II) in weakly alkaline solution (pH 8-10) to form a chelate which is sparingly soluble in water. When the suspension is shaken with chloroform, the red-violet complex and the excess of the orange reagent pass into the organic layer. The coloured extract constitutes the basis of the extractive spectrophotometric method for determining manganese [26-28].

The method is highly sensitive. The molar absorptivity is 5.8.104 (a - 1.05) a t )Lma x =

564 nm. The excess of free reagent interferes only slightly at this wavelength. The optimum pH for the reaction and extraction of the complex is 9.2+0.4, which is

obtained with an ammoniacal buffer. The aqueous solution should contain hydroxylamine to prevent oxidation of the Mn(II). In the absence of the reducing agent the obtained results are irreproducible.

Since the distribution coefficients of PAN and its manganese complex are high, it is sufficient to extract the aqueous solution with only one portion of chloroform. Carbon tetrachloride, benzene, or isoamyl alcohol can be used as extractants. Only slight changes are observed in the value of )Lmax for the complex in the different solvents.

Tartrate is used to mask hydrolysable metals (e.g., A1, Cr, Zr, In, Bi, and Fe) but does not interfere with the reaction of manganese and PAN. Cyanide does not interfere in the reaction between PAN and manganese. It is added to mask a number of metals such as Cu,

258 29. Manganese

Ni, Cd, Fe(II), Zn, and Co. Iron(l]/) is masked by fluoride. In the presence of tartrate and cyanide, 1-mg amounts of Ni, Cu, Cd, Cr, V, A1, Fe, In, Ti, Sn, and Sb can be tolerated [28]. Zn, Pb, and Co can be tolerated in quantities not larger than 50, 100, and 500 ~tg, respectively.

Extraction may be avoided, if PAN is used in the presence of the non-ionic surfactant Triton X-100 (~ = 4.4.10 4 a t 562 nm) [29].

Reagents

1-(2-Pyridylazo )-2-naphthol (PAN), 0.1% solution in ethanol. Standard manganese(H) solution: 1 mg/ml. Preparation as in Section 29.2.1. Hydroxylamine hydrochloride, 10% solution, freshly prepared. The solution (neutralized

to pH --8 with NaOH) must be purified by scrubbing with dithizone. Ammoniacal cyanide buffer. Dissolve 10.5 g of NH4C1 in water, add 60 ml of conc.

ammonia solution and 0.75 g of KCN, and dilute the solution with water to 500 ml.

Procedure

Put the weakly acidic sample solution, containing not more than 15 pg of Mn, into a separating funnel. Add 1 ml of 10% sodium-potassium tartrate solution and 1 ml of hydroxylamine solution, and dilute with water to --15 ml. Add 3 ml of buffer and 1 ml of PAN solution. After 2 min, shake the solution with chloroform for 1 min. Transfer the chloroform extract to a 25-ml standard flask and dilute to the mark with solvent. Measure the absorbance of the solution at 564 nm against a reagent blank.


Besides 1-(2-pyridylazo)2-naphthol (PAN), some other azo reagents have been proposed for determination of manganese, namely 5-Br-PADAP (formula 4.3) (e = 1.06.105 at 562 nm) [30], PAR (~ = 8.6.104 at 496 nm) [31,32], TAN [33], TAR (formula 4.7) [34], 1-(2- quinolylazo)-2,4,5-trihydroxybenzene [35,36], and nitrobenzo-azopyrocatechol (e - 5.8-104) [37]. Carboxybenzene S has been used for direct determination of manganese in an extract of Mn-DDTC in chloroform [38]. The method is characterized by high sensitivity (~ - 1.5.105) and high c o n t r a s t (~max- -- 180 nm).

Dithizone (in the presence of pyridine or phen as synergistic agents) forms an extractable (CC14) complex with manganese (e = 5.7.104 at 510 nm) [39-42]. Manganese can also be determined with HTTA (e = 6.1.103 at 420 nm) [43,44] and 8-hydroxyquinoline (extraction in the presence of quaternary bases) [45].

Manganese has been determined in the form of ion-associates: the anionic chloro-oxine complex of manganese with Rhodamine 6G, extractable into benzene (e = 7.0.104) [46], and the cationic complex of Mn(II) with phen, associated with the acid dye Erythrosin, and extractable into ethyl acetate (~ = 1.5.105) [47].

A very sensitive method for determination of Mn is based on its complex with sulphosalicylic acid, salicylfluorone, and CTA (e = 3.1.105 at 590 nm [48].

In dilute H 2 8 0 4 or HC104 containing oxidants (BrO3-, Cr2072-) manganese(II) reacts with phosphoric acid to give the violet Mn(III) pyrophosphate complex (e = 7.5.102) [49- 52]. The anionic Mn(III) pyrophosphate complex can be extracted into chloroform in the


presence of n-dodecylamine [53 ]. There are very sensitive catalytic speetrophotometric methods for the determination of

manganese, in which traces of Mn(II) catalyse the oxidation of various organic substances by another oxidant (e.g., KIO4), with the formation of coloured reaction products. In these methods, the amount of colour depends on the reaction time. The catalytic effect of Mn on oxidation of Malachite Green by periodate has been utilised in determination of manganese by the FIA method [54-57].


The permanganate method has been used for determination of manganese in wine [58], cast iron and steel [18-20,59], rare earth compounds [60], rhenium and its compounds [61], and copper selenide [62]. Larger quantities of manganese have been determined by the differential spectrophotometry technique [63]. The permanganate method has been applied also in automatic determination of manganese in environmental samples [64], and in steel and slags [65]. Manganese has been determined in steel by the FIA technique, based on the extraction of the MnO4- associate with ethylenebis(triphenylphosphonium) cation [20]. The 4 th order derivative spectra have been used in determining traces of manganese (10-3-10 -5 %),

with no pre-concentration, in nickel and its salts [66], steel [67], and cobalt compounds [68]. Formaldoxime has been applied in the determination of manganese in water [69,70],

plants [69,71], silicate and carbonate minerals [72], uranium oxide [9], tin [73], and alkalies [74]. The formaldoxime method has been automated in the determination of manganese in water [75] and in silicate minerals [76]. The FIA technique has been applied in the analysis of natural waters [77] and silicates [78].

PAN has been applied in the determination of Mn in beryllium [79], platinum and gold [80], cadmium [81], steel [82], and in niobium, tantalum, molybdenum, and tungsten [28]. In this case the macro-amounts of Nb, Ta, Mo, and W were separated by extraction as the cupferronates. The azo dyes were used in determining Mn in food [35], and in plants and chemical reagents [36].

References

1. Rane A.T., Nepali D.R., Mikrochim. Acta, 1986 III, 27. 2. Akatsuka K., Atsuya J., Anal. Chim. Acta, 202, 223 (1987). 3. Nakamura S., Imura H., Suzuki N., J. Radioanal. Chem., 82, 33 (1984). 4. Pfibil R., Adam J., Talanta, 20, 49 (1973). 5. Claassen V.P., de Jong G.J., Brinkman U.A., Z. Anal. Chem., 287, 138 (1977). 6. Menon S.K., Agrawal Y.K., Analyst, 111, 335 (1986). 7. Matsui H., Anal. Chim. Acta, 69, 216 (1974). 8. Smith R.G., Jr., Anal. Chem., 46, 607 (1974). 9. Korkisch J., Htibner H., Mikrochim. Acta, 1976 I, 25. 10. Chiswell B., Rauchle G., Pascoe M., Talanta, 37, 237 (1990). 11. Gottschalk G., Z. Anal. Chem., 212, 303 (1965). 12. Lazareva V.I., Lazarev A.I., Kharlamov I.P., Zavod. Lab., 45, 193 (1979). 13. Baraj B., et al., Analusis, 22, 408 (1994). 14. Miura Y., Koh T., Bunseki Kagaku, 35, 524 (1986). 15. Rao G.G., Murty K.S., Talanta, 11,955 (1964). 16. Bane R.W., Analyst, 90, 756 (1965).

260 29. Manganese

17. Richardson M.L., Analyst, 87, 435 (1962). ! 8. Goto H., Kakita Y., Z. Anal. Chem., 254, 18 (1971). 19. Burns D.T., Chimpalee D., Anal. Chim. Acta, 199, 241 (1987). 20. Burns D.T. et al., Anal. Chim. Acta, 217, 183 (1989). 21. Burns D.T., Chimpalee D., Chimpalee N., Z. Anal. Chem., 332, 453 (1988). 22. Burns D.T., Chimpalee D., Chimpalee N., Anal. Proc., 26, 11 (1989). 23. Burns D.T., Harriott M., Barakat S.A., Anal. Chim. Acta, 258, 325 (1992). 24. Burns D.T. et al., Anal. Chim. Acta, 270, 213 (1992). 25. Marczenko Z., Anal. Chim. Acta, 31, 224 (1964); Bull. Soc. Chim. France, 1964, 939. 26. Shibata S.,Anal. Chim. Acta, 23, 367 (1960); 25, 348 (1961). 27. Betteridge D., Fernando Q, Freiser H., Anal. Chem., 35, 294 (1963). 28. Donaldson E.M., Inman W.R., Talanta, 13, 489 (1966). 29. Goto K., Taguchi S., Fuliue Y., Ohta K., Watanabe H., Talanta, 24, 752 (1977). 30. Xing-chu Q., Yu-sheng Z., Ying-quan Z., Chem. Anal. (Warsaw), 30, 127 (1985). 31. Nonova D., Evtimova B., Talanta, 20, 1347 (1973). 32. Ahrland S., Herman R.G., Anal. Chem., 47, 2422 (1975). 33. Grzegrz6~ka E., Chem. Anal. (Warsaw), 22, 303 (1977). 34. Gaokar U.G., Eshwar M.C., Mikrochim. Acta, 1982 II, 247. 35. Singh J., Poonam Mrs., Talanta, 31, 109 (1984). 36. Laila A.A.,Anal. Chim. Acta, 204, 355 (1988). 37. Zeinalova S.A., Guseinov I.K., Rustamov N.Kh., Zh. Anal. Khim., 38, 241 (1983). 38. Savvin S.B., Petrova T.V., Dzheraian T.G., Zh. Anal. Khim., 33, 516 (1978). 39. Akaiwa H., Kawamoto H.,Anal. Chim. Acta, 40, 407 (1968). 40. Marczenko Z., Mojski M., Anal. Chim. Acta, 54, 469 (1971). 41. Akaiwa H., Kawamoto H., Kogure S., Bunseki Kagaku, 28, 498 (1979). 42. Chakraborti N., Roy S.K., Talanta, 40, 1499 (1993). 43. Onishi H., Toita Y., Talanta, 11, 1357 (1964). 44. Yoshida H., Nagai H., Onishi H., Talanta, 13, 37 (1966). 45. Noriki S., Nishimura M., Anal. Chim. Acta, 94, 57 (1977). 46. Minczewski J., Chwastowska J., Lachowicz E., Chem. Anal. (Warsaw), 18, 199 (1973). 47. Hoshi S., Inone S., Bunseki Kagaku, 32, 287 (1983). 48. Ishchenko N.N., Ganago L.I., Ivanova I.F., Zh. Anal. Khim. 53, 29 (1998). 49. Hofmann P., Stern P.,Anal. Chim. Acta, 46, 159 (1969). 50. Knoeck J., Diehl H., Talanta, 14, 1083 (1967). 51. Kroshkina A.B., Shilova L.K., Bebeshko G.I., Zh. Anal. Khim., 35, 320 (1980). 52. Zhou H., Mitamura S.,Anal. Lett., 25, 911 (1992). 53. Shevchuk I.A., Simonova T.N., Zh. Anal. Khim., 23, 1386 (1968). 54. Zhang C., Kawakubo S., Fukasawa T., Anal. Chim. Acta, 217, 23 (1989). 55. Quintero M.C. Silva M., Perez-Bendito D., Talanta, 36, 109 (1989). 55a. Maniasso N., Zagatto E.A.,Anal. Chim. Acta, 336, 87 (1998). 56. Resing J.A., Mottl M.J., Anal. Chem., 64, 2682 (1992). 57. Zhang C.L., Narusawa Y., Bull. Chem. Soc. Jpn., 67, 2994 (1994). 58. Garcia-Jahres C.M., Lage-Yusty M.A., Simal-Lozano J., Fresenius'J. Anal. Chem., 338,

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Chapter 30. Mercury

Mercury (Hg, at. mass 200.59) occurs in its compounds in the I and II oxidation states. Mercury(H) is similar in its chemical properties to copper(H) and lead (II), whereas mercury(I) resembles silver and gold(I). Mercury(II) forms stable, mostly water-soluble, halide complexes.


Various methods for separation and preconcentration of trace quantities of mercury in environmental samples have been reviewed [1].

30.1.1. Extraction

The extraction of mercury as the dithizonate is a popular separation method and is discussed in detail below. Extraction of Hg(II) from a strong acid medium (H2SO4) enables one to separate Hg from Cu, Bi, Zn, Ni, Pb, and other metals (except Au, Pt, and Pd). In the presence of chloride, Hg can also be separated from Ag, since chloride at moderate concentrations does not interfere with the extraction of Hg(HDz)2. Mercury is separated from larger quantities of copper by extracting Hg(II) with small portions of dithizone (in CC14) until the violet colour of Cu(HDz)2 appears in the extract.

Mercury can be separated by the extraction of the iodide complexes with cyclohexanone from an acid medium. Owing to the high stability of HgI2 and HgI3-, the stoichiometric ratio of iodide:mercury is sufficient [2]. Extraction of the molecular species HgX2 (X = I-, Br-, C1-) with non-polar solvents is a very selective method of separating mercury.

Chloride [3], bromide [4], iodide, and thiocyanate are useful in the extractive separation of Hg. From halide media, mercury has been extracted with tribenzylamine in CHC13 [4] and trilaurylamine in benzene [3].

Mercury has been separated efficiently from Zn and Cd by extraction with dicyclohexyl-18-crown-6 in 1,2-dichloroethane (from 2 M HNO3) [5]. Other crown ethers have also been used for selective extraction of Hg [6,7]. Mercury is separated from many metals (including Zn and Cd) by extraction with HDEHP in octane (from acidic media) [8], and with TOA in benzene (from H2804 medium) [9].


In the absence of large quantities of other metals, mercury can be separated by precipitation as the sulphide, by saturating an acidic aqueous solution with hydrogen sulphide. Cadmium, As, and Cu are used as scavengers. Traces of Hg are also precipitated quantitatively as the sulphide from a neutral or weakly alkaline medium. A suitable metal sulphide or hydroxide may be used as the collector [10].

Mercury (together with Ag, Au, and Pt) can be removed from solution by reduction to the metal with NzH4, NHzOH, or SnCI2. Mercury is also deposited by cementation with


less noble metals (Cu, Zn, Fe) or by electrolysis at a platinum cathode. Trace amounts of mercury have been deposited electrolytically on a copper wire [11]. Traces of mercury in silver have been enriched by collecting mercury, with a little silver, as the iodide [ 12].

30.1.3. Distillation and sorption

Tin(II) chloride reduces mercury to the volatile elementary form, which can then be distilled with air as carrier gas and trapped in a solution of KMnO4 [ 13]. Mercury can be distilled by igniting the sample in a quartz still [14] . The metal has been volatilized from organic and inorganic samples by mixing them with Na2CO3 and Na202 and heating at 600-800~ [15]. Traces of mercury vapour have been trapped on silver [16], or on gold [14,17,18].

Mercury, Cd, Pb, and Bi can be separated by retaining on a cationite column with subsequent sequential elution with 0.1-0.6 M HBr. Most of the other metals remain in the column [ 19].

Selective preconcentration of mercury(II) and its organomercury compounds (e.g., methylmercury, phenylmercury) has been achieved by the use of polystyrene- and other resins, or polyurethane foam impregnated with zinc dithizonate, thio-HTTA [20], or dithizone [21,22].


Two methods are presented in detail: an extractive dithizone method and the thio-Michler's ketone method. Both of them have good selectivity but the latter is more sensitive. Methods based on ion-associates formed by anionic mercury complexes with basic dyes are also noteworthy.


Mercury(II) ions in an acid medium react with excess of dithizone (formula 1.1) to form the orange-yellow dithizonate, Hg(HDz)2, which is soluble in CC 14 (or CHC 13), and is the basis of this spectrophotometric method [23]. The molar absorptivity at ~max ----- 485 nm is 7.1.104 (a = 0.35).

Mercury(II) dithizonate is readily formed even when dithizone is shaken with 5 M H2SO4 containing mercury. Concentrations of HC1 greater than 1 M impede the formation of Hg(HDz)2, owing to the formation of stable mercury(H) chloride complexes. The Hg(HDz)2 formed is resistant to the action of dilute alkalis (e.g., 2 M NaOH) which are used to wash free HzDz from the extract. A carbon tetrachloride solution of Hg(HDz)2 is sensitive to light, its colour changing to greenish-blue [24,25]. The orange-yellow colour is slowly restored when the extract is left in the dark, or when it is shaken with dilute H2SO4. When the organic phase is shaken with dilute acetic acid, which dissolves to some extent in CC14, the yellow colour of Hg(HDz)2 is stabilized.

Dithizone (in CC14) extracts, along with mercury(II), also the noble metals [Pt(II), Au, Pd, Ag], and Cu from strongly acidic solutions. At pH -~0, bismuth and other metals are not extracted, even if they are present in large amounts relative to mercury.

Copper is removed by masking the mercury as strong bromide- or iodide complexes in acid media. Only copper dithizonate is extracted from 1 M HC1 containing KI. Alternatively, the mercury and copper can be extracted from acid solution, and the mercury stripped with an acidic solution of KI or KBr. The mercury is re-extracted from the aqueous

264 30. Mercury

phase after the addition of ammonia. A convenient way of determining Hg in the presence of Cu by selective extraction takes

advantage of the fact that the rate of formation of Hg(HDz)2 is considerably higher than that of Cu(HDz)2. The aqueous solution (pH -0) is shaken with small portions of dithizone in CC 14 until the final portion has the violet colour of Cu(HDz)2.

From a slightly acidic medium (pH - 4) containing EDTA, mercury can be extracted in the presence of Cu, Bi, Ni, Zn, and Pb. Silver is removed either by masking with chloride or thiocyanate, or by shaking the CC14 extract containing Ag and Hg dithizonates with 1 M HC1 for 20 s, the silver passing into the aqueous phase while all the mercury remains in the extract [ 12].

Interference by Pd is prevented by prior separation with dimethylglyoxime, either as extractant or as precipitant. In the selective extraction method, Pd does not interfere since its dithizonate is formed rather slowly [Pd(HDz)2 is grey-green]. Before mercury is determined, gold and platinum should also be separated, e.g., by selective reduction to the elements.

Free dithizone is stripped from the extract with either dilute ammonia or 0.1 M NaOH. The absorbance of the yellow-orange Hg(HDz)2 solution, or of the corresponding green dithizone solution after the Hg(HDz)2 has been decomposed with aqueous KI, is then measured.

Organomercury(II) ions (e.g., methyl-, ethyl-, and phenylmercury) also react with dithizone and have been determined by this method [26,27]. Mercury(H) can be determined in the presence of organomercury(II) compounds [28]. Organomercury(II) compounds, isolated with dithizone, can be separated by liquid-phase chromatography [29].

Mercury has been determined with dithizone in aqueous media in the presence of the surfactants Triton X-100 [30-33] or CP [34].

Before the determination of mercury with dithizone, it can be separated with high selectivity from the acetate medium, in the presence of picric acid, by using a chloroform solution of 1,3-diaza-2-thiabenzo-15-crown-5 [35] or related compounds [36].

Reagents

Dithizone, 0.001% solution in CC14. Preparation as in Section 46.2.1. Standard mercury solution: 1 mg/ml. Dissolve 1.7130 g of Hg(NO3)z.H20 in water

containing 1 ml of conc. HNO3, and dilute the solution to volume with water in a 1-1itre standard flask. The stability of dilute solutions of mercury has been discussed [37-39].

Acetic acid, 2 M ( -10%) solution. Purify the solution by shaking with a solution of HzDz in CC14.

Potassium iodide solution, pH -4. Dissolve 15 g of KI and 5 g of potassium hydrogen phthalate, and dilute with water to 250 ml. Add 0.1 M thiosulphate solution dropwise to remove free iodine. Purify the solution by shaking with dithizone in CC 14.

Procedure

Add HC1 to the solution, which should be free from Au, Pt, and Pd, and contain not more than 40 ~tg of Hg, to make the HC1 concentration -1 M , and extract Hg in a separating funnel with small portions of the dithizone solution (1 ml of 0.001% H2Dz is equivalent to 3.9 ~tg of Hg). The last portion of dithizone should not change its green colour unless copper is present, in which case it may take on the violet colour of Cu(HDz)2.

Remove free dithizone from the combined extracts by shaking with dilute ammonia solution (5 drops of conc. NH3 solution in 25 ml of water), then shake the extract with


CH3COOH. Dilute the Hg(HDz)2 solution with CC14 in a 25-ml volumetric flask, and measure the absorbance of the solution at 485 nm against CC14.

Notes. 1. To avoid interference by possible copper contamination, measure the absorbance of the extract at 620 n m (~,max for dithizone), shake the extract with KI solution buffered at pH ---4, and re-measure the absorbance of the extract at 620 nm. The difference in the absorbance values corresponds to the mercury content.

2. Mercury is more rapidly extracted with dithizone from HNO3 or H2SO4 than from HC1 media.

30.2.2. Thio-Michler's ketone method

In the reaction of mercury(II) ions with thio-Michler's ketone (TMK, formula 46.2) the ions are reduced to Hg(I) and then complexed. This colour reaction, in various forms, has been recommended for selective and sensitive determination of Hg [40-42].

In DMF-water medium at pH 2-5 and an excess of TMK, a red-violet complex is formed. Its molar absorptivity is 1.7.105 at ~max 560 nm (a - 0.85). At this wavelength the absorbance of the free TMK (Xmax - 460 nm) is very small. The composition of the Hg(I) complex seems to be [Hg2(TMK)4) ]2+.

The reaction of mercury(H) with TMK can also be done in other media, e.g., in 40% ethanol [42] or 30% n-propanol [41], but the sensitivities are lower than in DMF-H20. For extraction of the complex into isoamyl alcohol, e - 8.8.104 [40].

The mercury-TMK complex is not formed in the presence of iodide. Many complexing agents (chloride, bromide, sulphate, acetate, citrate, tartrate, and EDTA) do not interfere. Some of these substances may be used to mask hydrolysable metals. Of the cations, only Pd(II), Pt(II), and Au(III) interfere. Reductants and oxidants should be absent.

Reagents

Thio-Michler's ketone (TMK), 0.001M solution in dimethylformamide (DMF) (28.5 mg of TMK in 100 ml). The solution should be kept in darkness.

Standard mercury solution: 1 mg/ml. Preparation as in Section 30.2.1. Acetate buffer, pH 3.9. Dissolve 5 g of CH3COONa.3H20 in 50 ml of water, add 11.5

ml of glacial acetic acid, and dilute the solution with water to 100 ml.

Procedure

To a weakly acidic sample solution (pH about 4), containing in -~ 10 ml not more than 20 gg of Hg(II), add 2 ml of acetate buffer, 2.5 ml of DMF and 2.5 ml of TMK solution. Dilute to the mark with water in a 25-ml standard flask, mix, and after 10 min measure the absorbance of the solution at 560 nm against a reagent blank.


Mercury(II) forms anionic complexes with I-, Br-, and C1- ions, which react with basic dyes to give ion-associates extractable into organic solvents. Sensitive extractive spectrophotometric methods for determining mercury are based on such reactions with the following dyes: Crystal Violet (e = 9-104-10-104) [43-46], Malachite Green and Methyl

266 30. Mercury

Violet [47], Pyronine G [48,49], Bindschedler's Green (indamine dye, formula 30.1) (e - 1.7.105) [50], and Thiazolyl Blue [51]. Benzene, toluene, chloroform, 1,2-dichloroethane, and nitrobenzene are used as extractants.

{CH3)zN N N(CH3)z (30.1)

A very selective variant of these methods has been proposed [52]. Mercury is extracted into benzene as a molecular species, HgI2 or HgBr2, which is transformed into a coloured ion-associate by equilibration of the extract with an aqueous basic dye solution also containing iodide or bromide ions. The highest sensitivities have been obtained with Brilliant Green, Crystal Violet and Butylrhodamine B.

Rhodamine 6G [53] and Rhodamine B [54] have been used for spectrophotometric determination of mercury(H) in the aqueous phase. The systems can be stabilized with gelatine or poly(vinyl alcohol).

In a sensitive flotation-spectrophotometric method (e = 3.4.105), a compound [(MB+)(HgI3-)].3[(MB+)(I3-)] (MB = Methylene Blue) is separated by flotation with cyclohexane from a 0.4 M HC1 medium, containing I- and MB; then it is washed with water and dissolved in methanol [55]. Mercury has also been determined spectrophotometrically after separation (by flotation with cyclohexane) from solutions containing the iodide complex and Brilliant Green (e = 5.96.105) [56].

Mercury(H) forms with 1,10-phenanthroline a cationic complex which gives ion- associates with the acid dyes Rose Bengal B [57] and eosin [58]: the associates have also been used for determining mercury in aqueous medium.

A number of organic sulphur compounds are recommended as reagents for mercury. The thio-Michler's ketone and the dithizone methods have been described above. Other, less sensitive reagents include, thiodibenzoylmethane [59], thiobenzoylacetone (~ = 1.7-104) [60], 1-salicylidene-5-(2-pyridylmethylidene)-isothiocarbonehydrazide (40% DMF, ~; = 1.7.104 at 400 nm) [61 ], and 1-(2-thienyl)benzothiazoline [62].

Azo dyes have often been recommended as sensitive reagents for the determination of mercury, e.g., p-phenylazo-3-aminorhodanine [63], 2-(8-quinolylazo)-4,5-diphenylimidazole (~ = 7.3.104) [64,65], Cadion A and Cadion 2B [66], and 5-Br-PADAP (formula 4.3) (e = 8.4-104) [67]. Derivative spectrophotometry has been applied in the determination of Hg with 5-(2-carboxyphenylazo)-8-hydroxyquinoline [68].

Other organic reagents used in spectrophotometric methods for mercury are diphenylcarbazone [69], Xylenol Orange [70-72], and 1-(phthal-l-azinyl)-3,5- diphenylformazan (~ = 4.2.104 at 520 nm) [73], and other formazans [74], rhodanine derivatives [75,76], and various porphyrin derivatives [77,78].

Organic reagents used in spectrophotometric determinations of Hg have been discussed in detail [79].


The dithizone method has been used in determination of mercury in natural waters [80], coal and its products [81], sulphide minerals [82], silver [12], tin [10], cadmium [83], uranium compounds [84] selenium [85], Hg-Cd-Te thin films [86], foodstuffs [87,88], and water

References 267

[89]. Derivative spectrophotometry has been applied in the determination of mercury with dithizone in the presence of Cu and Pb in sea sediments [90].

Mercury has been determined in the aqueous phase by means of derivative spectrophotometry using PAN in the presence of CTA [91]. Mercury has been determined with PAN in sewage, fish, and kidney stones, after preliminary extraction with TOPO in xylene [92], and in pharmaceuticals and sewage after extraction with tris-(2-ethylhexyl) phosphate in toluene [93].

The ion associate of the Rhodamine 6G complex with iodide has been used for determining Hg in natural waters [94], and that of the complex with thiocyanate for determining Hg in river sediments and in CdCI2 [95]. Mercury has been determined in industrial solutions with the use of the iodide complex and Brilliant Green [56].

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(1977). 53. Ramakrishna T.V. et al.,Anal. Chim. Acta, 84, 369 (1976). 54. Hernandez Cordoba M.et al., Mikrochim. Acta, 1984 III, 467. 55. Marczenko Z., Lobifiski R., Chem. Anal. (Warsaw), 34, 87 (1989). 56. Mathew L. et al., Mikrochim. Acta, 127, 125 (1997). 57. Tananaiko M.M., Bilenko N.S., Zavod. Lab., 40, 1049 (1974). 58. Mudakavi J.R.,Analyst, 109, 1577 (1984). 59. Uhlemann E., Schuknecht B., Anal. Chim. Acta, 69, 769 (1974). 60. Murti M.V., Khopkar S.M., Bull. Chem. Soc. Jpn., 50, 738 (1977). 61. Rosales D., Gomez Ariza J.L., Anal. Chem., 57, 1411 (1985). 62. Capitan G.F. et. al., Ann. Chim. (Rzym), 77, 945 (1987). 63. Savvin S.B., Roeva N.N., Zh. Anal. Khim., 40, 820 (1985). 64. Shibata S., Furukawa M., Nakashima R., Anal. Chim. Acta, 81, 131 (1976). 65. Miwa S., Furukawa M., Shibata S., Anal. Chim. Acta, 120, 405 (1980). 66. Popa Gr., D~net A.F., Popescu M., Talanta, 25, 546 (1978). 67. Nonova D., Stoyanov K., Mikrochim. Acta, 1984 I, 143. 68. Saran R., Basu Baul T.S., Talanta, 41, 1537 (1994). 69. Okutani T., Bull. Chem. Soc. Jpn., 41, 1728 (1968). 70. Cabrera-Martin A. et al., Talanta, 16, 1023 (1969). 71. Cabrera-Martin A., Peral-Fernandez J.L., Burriel-Marti F., Talanta, 22, 489 (1975). 72. Peral-Fernandez J.L. et al., Talanta, 30, 179 (1983). 73. Barbina T.M., Podchainova V.N., Zh. Anal. Khim., 38, 1222 (1983). 74. Isakova N.W., Zolotov Yu.A., Ionov V.P., Zh. Anal. Khim., 44, 1045 (1989). 75. Zaki M.T., Abdel-Rahman R.M., E1-Sayed A.Y.,Anal. Chim. Acta, 307, 127 (1995). 76. Gur'eva R.F., Savvin S.B., Zh. Anal. Khim., 44, 2165 (1989). 77. Peng X.J., Mao Q.K., Cheng J.K., Fresenius'J. Anal. Chem., 348, 644 (1994). 78. Peng X.J., Mao Q.K., Cheng J.K., Mikrochim. Acta, 113, 81 (1994). 79. Roeva N.N., Savvin S.B., Zh. Anal. Khim., 47, 1750 (1992). 80. Theraulaz F., Thomas O.P.,Mikrochim. Acta, 113, 53 (1994). 81. Gardner D., Anal. Chim. Acta, 93, 291 (1977). 82. Leong P.C., Ong H.P., Anal. Chem., 43, 940 (1971). 83. Krasiejko M., Marczenko Z., Mikrochim. Acta, 1975 I, 585.

References 269

84. Mare6ek J., Singer E., Z. Anal. Chem., 203, 336 (1964). 85. Pollock E.N., Talanta, 11, 1548 (1964). 86. Marczenko Z., Mojski M., Czarnecka I., Chem. Anal. (Warsaw), 18, 189 (1973). 87. Fujita M. et al.,Anal. Chem., 40, 2042 (1968). 88. Nabrzyski M., Anal. Chem., 45, 2438 (1973). 89. Lugowska M., Rubel S., Anal. Chim. Acta, 138, 397 (1982). 90. Mathew L., Rao T.P., Iyer C.S., Damodaran A.D., Mikrochim. Acta, 118, 249 (1995). 91. Sharma R.L., Singh H.B., Talanta, 36, 457 (1989). 92. Raman B., Shinde V.M.,Analyst, 115, 93 (1990). 93. Nambiar D.C., Shinde V.M., Fresenius'J. Anal. Chem., 350, 652 (1994). 94. Padmaja P. et al., Int. J. Environ. Anal. Chem., 63, 47 (1996). 95. Jarosz M., Lubowski P., Quim. Anal. (Barcelona), 13, 19 (1994).

Chapter 31. Molybdenum and tungsten

Molybdenum (Mo, at. mass 95.94) is an amphoteric element with predominantly acidic properties. Molybdenum occurs principally in the VI oxidation state as molybdate (MOO4 2-) ions which form condensed species in acid media. In strongly acidic solutions, molybdenyl cations MoO22+ occur. Depending on the reducing agent used, Mo(VI) is reduced to Mo(V) or Mo(III). Molybdenum(VI) gives fluoride, peroxide, tartrate, oxalate, and chloride complexes. It forms also heteropoly acids with Si, P, V, As(V), Ge, etc. White MoO3 is volatile at above 550~ Tungsten (W, at. mass 183.85) is an element similar to Mo and Cr. The basic properties of W(VI) are not so accentuated as those of Mo(VI). Tungstic acid is less soluble than molybdic acid in mineral acids. W(VI) gives tartrate, oxalate, fluoride, and chloride complexes, as well as heteropoly acids. Tungsten may occur also in the rather unstable V, IV, and III oxidation states.


31.1.1. Extraction

Extraction of the molybdenum complex with ~-benzoinoxime (formula 31.1) into CHC13, from 0.01-2 M HC1 is a selective method for separating molybdenum [1-4]. Under the conditions employed in the extraction of the molybdenum a-benzoinoxime complex Mo, MoO2(C14H1202N)2, only W, V, Cr, and Pd are also extracted. Tungsten and vanadium are not extracted in the presence of phosphoric acid and Fe(l/).

--~1 ;~NOH

-~CHOH (31.1)

Extraction of W (like Mo), in the form of its (x-benzoinoxime complex, into CHC13 enables one to separate it from Cr and V [5-7].

From 6-7 M HC1 media, molybdenum is separated by extraction of the chloride complex into diethyl ether, MIBK, or isoamyl alcohol. Fe(III), Au, As(HI), Ga, Sb(V), Sn(IV), and W are also extracted, but W is not extracted in the presence of phosphoric acid. Mo can be separated from Re(VII) by extraction with TOPO (toluene) from 2.5-3.1 M HC1 (or 3.8-4 M HBr) [8].

From phosphate solutions, Mo(VI) [and U(VI)] are extracted with Alamine 310 mixed with TBP, dipentyl sulphoxide, and Cyanex 301 in benzene solutions [9,10]: both Mo and W are extracted with Aliquat 336 [11]. Molybdenum has been separated by extraction with Aliquat 336S from ascorbic acid solutions [12].

Molybdenum and W have been separated from steel samples by extraction with tris(2- ethylhexyl) phosphate [13]. A strong synergistic effect has been observed where mixtures of phosphonic acids are used for the extraction of Mo [ 14].

31.1. Methods of separation and preconcentration 271

Small amounts of Mo are extracted from many metals as the chelates, formed with BPHA [15,16], 8-hydroxyquinoline [17], and HDEHP [18,19]. Also, Mo has been separated from tungsten oxide by extraction with DDTC [20].

Tungsten has been extracted as the chelate with 8-hydroxyquinoline and its derivatives [21]. The possibilities of the extractive separations of W [22] and both Mo and W [23] have been discussed in detail.


Trace amounts of molybdenum have been separated from natural waters by retention of the anionic Mo-thiocyanate complex on the anion-exchanger Dowex 1. Mixtures of 2 M HC104 and 1 M HC1 are used for the elution [24]. The Mo-thiocyanate complex has also been concentrated on the anion exchanger Amberlite XAD [25,26], with the use of acetone as the eluent [25]. Molybdenum has been retained on anion-exchange columns from an HC1 medium prior to being determined by the thiocyanate method [27-30]. Mo has been sorbed selectively (pH 3.5) on the anion exchanger Sephadex G-25, from which it can be eluted with EDTA [31 ].

Traces of Mo and W have been separated from other metals on anion-exchangers from mixtures of sulphuric acid with hydrogen peroxide. The tungsten retained on the column is eluted with NaOH-NaC1 solution [32].

Tungsten (and molybdenum) have been sorbed on Dowex 1 (C1-) resin from sea-water acidified to pH -1; a 0.5 M solution of NaOH and NaC1 has been used as eluent. The sorption of the thiocyanate complex on an anion-exchange column has been used for separation of W from rare earth elements [33].

Molybdenum, sorbed as MO22+ on a cation-exchange column, can be eluted by H202 solution [34], along with some metals, but most of other metals remain on the column.

A review of ion-exchange chromatography methods used for separation of Mo and W and for their isolation from other metals has been given [35].

Molybdenum and tungsten have been separated on a chelating resin with thioglycolate groups. A mixture of these metals is sorbed from an acetate solution of pH -4.3, then Mo and W are eluted successively with 2 M HC1 and a mixture of 0.1 M NaOH and 0.1 M NaC1 [36]. The sorption of Mo and W on silicate sorbents, modified with hydroxamic acids, was also studied [37,38]. Mo was also preconcentrated on dibenzo-18-crown-6 ether columns from HC1 [39] and HBr [40] media.


Hydrogen sulphide precipitates molybdenum from acid medium as MoS3; Sb, As, or Cu (2-4 mg) are suitable collectors. Molybdenum can be separated in this way from Cr, V, Ti, U, etc.

Trace amounts of molybdenum have been co-precipitated with Th [41], or A1 [42] hydroxides as collectors. During the precipitation of MnOzaq. from dilute HNO3, traces of Mo are co-precipitated, together with Sb, Sn, Bi, TI(III), and Au [43].

In order to separate tungsten from metals that form hydroxides insoluble in strongly alkaline media (e.g., Fe, Ti, Mn, Mg), the sample is fused with Na2CO3 or Na202, and the melt is leached with water. Mo, Cr, V, and As pass into the solution together with W. Tungsten has also been separated with MnOzaq as the collector [44], or with ammonium molybdophosphate [45].

In the analysis of steel, W was also separated as cinchonine tungstate [46].

272 31. Molybdenum and tungsten

31.2. Molybdenum determination methods

Three methods are discussed here in detail: the classical thiocyanate and dithiol methods, and a newer more sensitive method that utilises Bromopyrogallol Red in the presence of a cationic surfactant. Certain methods involving fluorones, and methods based on ion- associates with basic dyes, are also sensitive spectrophotometric methods for determination of molybdenum.


Molybdenum(VI) reacts in acid media (HC1, H2SO4, HC104) with thiocyanate ions, in the presence of reducing agents, to give an orange-red colour, which is a basis of determination of Mo [47]. The system may contain several molybdenum(V) complexes. Since the mechanism of the reaction is complicated, it is essential to keep the reaction conditions constant if good reproducibility is to be obtained. The concentrations of thiocyanate, reducing agent, and acid are critical.

The most commonly used reducing agent is SnCI2. With this reductant, the presence of Fe(III) in the sample solution is indispensable. In the absence of iron the results obtained are low and of poor reproducibility. Ascorbic acid, hydrazine, thiourea, and potassium iodide in the presence of Cu(I) are also used, besides Sn(II). With these weaker reducing agents, more intensely coloured solutions are obtained. Stannous chloride is a strong reducing agent and is believed to reduce some of the Mo(VI) to an oxidation state lower than Mo(V). This molybdenum does not participate in the colour reaction [48,49].

Iron(III) does not interfere in the determination of Mo since, under the reaction conditions, it is reduced to Fe(II). Tungsten is masked with citric or tartaric acid, and titanium is masked with fluoride. Larger quantities of Re, U, V, Co, Cu, and Bi interfere.

The absorbance may be measured for the coloured aqueous solutions or for the extracts in iso-amyl alcohol, butyl acetate, MIBK or DIPE [50-52]. Extraction is done most conveniently with a denser-than-water solution of isoamyl alcohol and CC14.

The molar absorptivity of the thiocyanate complex in isoamyl alcohol (absorption spectrum shown in Fig. 31.1) is 1.6.104 (specific absorptivity 0.17) at 470 nm.

The anionic thiocyanate complex of Mo forms ion-pairs with various organic cations, which are extractable from acid solutions into non-polar solvents. The most useful are: tetraphenylarsonium [53] and tetraphenylphosphonium [54] ions, CTA (in the presence of SnCI2) [55], N-octylbenzamidine [56], and thioacetanilide [57].

?

~.oo

2

t,70 500wavelength, nm 600

Fig. 31.1. Absorption spectra of thiocyanate complexes of tungsten (1) and molybdenum (2) in isoamyl alcohol solutions

31.2. Molybdenum determination methods 273

Reagents

Potassium thiocyanate, 10% solution. Standard molybdenum solution: 1 mg/ml. a) Dissolve 1.5000 g of MOO3, in 25 ml of 2 M NaOH, acidify the solution slightly with

HC1, and dilute with water to 1 litre in a standard flask. b) Dissolve 2.5220 g of NazMoO4.2H20 in water containing 1 ml of conc. HC1, and

dilute the solution with water in a standard flask to 1 litre. Tin(II) chloride, 10% SnClz.2H20 solution in HC1 (1+9). Ferric alum, FeNH4(SO4)2-12H20, 0.5% solution in 0.5 M H2SO4. t~-Benzoinoxime, 0.2% solution in CHC13.

Procedure

Separation of Mo with a-benzoinoxime. Shake the sample solution (---1 M in HC1) in a separating funnel for 2 min with two portions of ~-benzoinoxime solution. Wash the combined extracts with 1.5 M HC1. Add 0.5 ml of conc. H2SO4 to the organic extract in a beaker, and evaporate off CHC13. Mineralise the organic matter by adding conc. HNO3 dropwise and heating to fumes. Dilute the cooled residue with 10 ml of water, heat to boiling, and then cool the sample. Depending on the amount of molybdenum, either all or an aliquot of this solution is used in the subsequent determination. Determination of Mo. Add 20 ml of weakly acidic sample solution (HC1, H2SO4), containing not more than 50 ~tg of Mo, to a separating funnel. Add successively 3 ml of conc. HC1, 3 ml of 20% citric acid solution, 1 ml of Fe(III) solution, 3 ml of thiocyanate solution, and, with swirling, 3 ml of SnC12 solution. After 5 min, shake the solution with two portions of isoamyl alcohol. Make up the combined extracts to the mark with the solvent in a 25-ml standard flask, and measure the absorbance at 470 nm, using the solvent as reference.

31.2.2. Dithiol method

Dithiol (toluene-3,4-dithiol, formula 31.2) reacts with Mo(VI) in strongly acidic media (4-12 M HC1, 3-7 M H2SO4) to form a green, sparingly-soluble complex, which can be extracted with organic solvents (benzene, CC14). The presence of Fe(II) in the solution promotes the reaction between dithiol and Mo and also increases the extraction of the dithiolate.

..•SH H3C SH

(31.2)

Solid dithiol (m.p. 31 ~ and its solutions are unstable since they are readily oxidised by atmospheric oxygen. Dithiol is stored in sealed glass ampoules. Its aqueous solutions are stabilized by adding a reducing agent, e.g., thioglycolic- or ascorbic acid, and by storage in a refrigerator. Alternatively, an aqueous suspension of the stable zinc dithiolate can be used as the reagent [58].

Green solutions of Mo(VI) dithiolate in organic solvents (amyl-, isoamyl-, or butyl acetate, or MIBK) are the basis of this determination of molybdenum [59-61]. The complex exhibits two absorption maxima in the visible spectrum, at 440 nm and at 675 nm. The molar absorptivity at 675 nm is 2.2.104 (a - 0.23).

Tungsten reacts with dithiol in a similar way to molybdenum, hence it can be masked


with citric- or tartaric acid. Tin(II) dithiolate is also extracted with amyl acetate; however, this dithiolate does not interfere, as it has a different colour from Mo dithiolate.

Iron(III), which interferes in the determination of molybdenum, can be reduced to Fe(II) with iodide, the liberated iodine being reduced with thiosulphate. Most heavy metals form sparingly soluble dithiolates, but these are not co-extracted with molybdenum dithiolate [62]. Oxidants, which decompose dithiol, interfere in the determination of molybdenum.

Reagents

Dithiol, 0.2% solution. Dissolve 0.2 g of dithiol and 0.2 g of ascorbic acid in 100 ml of 0.2 M NaOH. The solution is unstable and must be used within 2 days.

Standard molybdenum solution: 1 mg/ml. Preparation as in Section 31.2.1.

Procedure

To a weakly acidic sample solution (-~10 ml) containing not more than 50 lag of Mo, add 5 ml of conc. HC1 and 2 ml of 20% KI solution. Mix, and let the solution stand for 5 min, then add 0.1 M sodium thiosulphate dropwise until the solution is decolorized. Add 2 ml of 20% citric acid solution and 2.5 ml of dithiol solution. After 10 min, shake the aqueous solution for 30 s with two 8-ml portions of amyl acetate. Dilute the green extract to volume with the solvent in a 25-ml standard flask, and measure the absorbance of the solution at 675 nm, using the solvent as reference.

31.2.3. Bromopyrogallol R e d - CTA method

The reaction of Bromopyrogallol Red (BPR) (formula 4.21) with Mo(VI) is not suitable for analytical use because the absorption maxima of the binary complex and of the reagent itself are too close. However, in the presence of cationic surfactants, bathochromic and hyperchromic effects are observed. These ternary systems allow a sensitive determination of molybdenum: the absorbance of the reagent at the absorbance maximum of the ternary complex is insignificant [63-65]. The best results are obtained with the use of CTA ions.

The molar absorptivity with this cationic surfactant (optimum pH value about 1) is 8.4-104 (a = 0.87) at 625 nm [65]. The use of CP or Zephiramine leads to lower sensitivities [63].

The reaction of Mo with BPR and CTA is fairly selective. No interference is caused by Cu (300-fold excess), Co (500), Fe(III) (600), A1 and Cr(III) (1,000), and Ni (4,000), but W(VI), V(V), Ti and Mn always interfere [65]. Molybdenum(VI) is often determined in alkaline media, after a separation involving precipitation of most metals with NaOH or ammonia. In this case, only W and V(V) accompany molybdenum in solution. The influence of V(V) can be eliminated by adding ascorbic acid.

Molybdenum has been determined by BPR in the presence of poly(vinylpyrollidone) e = rd 4 7.5-10 ) [66]. A high increase in sensitivity can be obtained by the use of 3 order derivative

spectrophotometry. A related reagent, Pyrogallol Red (with CTA, CP, etc.) has also been used in

determinations of Mo (~ -~9-104) [67-69].

Reagents

Bromopyrogallol Red (BPR), 1.10-4M solution in water-acetone (1 + 1).

31.2. Molybdenum determination methods 275

Cetyltrimethylammonium chloride (or bromide) (CTA), 1.10 .3 M (approx. 0.05%) solution.

Standard molybdenum solution, 1 mg/ml. Preparation as in Section 31.2.1.

Procedure

To an acidic sample (about 0.1 M in HC1 or H2SO4) (-15 ml) in a 25-ml standard flask, containing not more than 12 ~g of Mo, add 2 ml of BPR solution and 3 ml of CTA solution. Adjust the pH to -1, dilute to volume with water, and mix well. After 20 min, measure the absorbance at 625 nm v s . a reagent blank.


Several other compounds that, like thiocyanate and dithiol, have sulphur as the ligand atom, are used in spectrophotometric methods for Mo. Some examples are: thioglycolic acid (mercaptoacetic acid, thioacetic acid) [70,71], dithiocarbamates [72-74], rubeanic acid [75], and aminobenzenethiol [76]. Molybdenum has been determined also as the thiosulphate complex [77].

Methods for determining molybdenum with use of 2,3,7-trihydroxy-6-fluorones are highly sensitive, o-Nitrophenylfluorone is especially recommended [78-80]. In the presence of DAM (diantipyrylmethane), a chloroform-extractable ternary compound is formed with = 1.30-105 [78]. Other fluorones used include phenylfluorone (formula 22.1) [81-86], propylfluorone, salicylfluorone, and methylfluorone [87-89].

Some methods based on ion-association of thiocyanate anionic complexes with basic dyes are very sensitive. To name a few, they include Crystal Violet (c - 2.3.105) [90], Rhodamine B [91,92], and Rhodamine 6G [92,93]. The ion-associate of tetrabromocatecholmolybdic acid with Methylene Blue is extracted into CHC13 [94]. The complex of Mo with p-chloromandelic acid, associated with Malachite Green, has been extracted with chlorobenzene [95]. The ternary complex: Mo-BPR-Crystal Violet has been separated by froth flotation before the determination of Mo (e - 1.4.105) [96]. The Mo complex with 3,5-dinitropyrocatechol has been applied for determination of Mo after association with Rhodamine B and separation by solvent flotation (cyclohexane) [97].

Several azo reagents have been proposed for molybdenum, such as TAR [98], Sulphonitrophenol S [99], pyrocatechol [100] and pyrogallol [101] azo derivatives. Molybdenum has been determined in fluoride solutions with the use of Nitrosulphonazo III [102].

Other organic reagents recommended for determination of Mo include: Alizarin Complexone [103], 1-nitroso-2-naphthol [104], morin [105], rutin [106], quercetin [107], 1,5-diphenylcarbazone [108], quinalizarin in the presence of surfactant [109], tiron [12,31,39], and 2,2'-biquinoxalyl [110]. The FIA technique has been applied in the determination of Mo with the use of carminic acid [ 111 ].

A number of methods for determining Mo have been based on its catalytic effect in oxidation of iodide by hydrogen peroxide [112-118], or in reduction of some organic substances, e.g. Nile Blue [119], Toluidine Blue [120], Metanilic Yellow [121], Safranine [122], and Nile Red [123] by suitable reducing agents.

Molybdenum has been determined in the presence of W after the reduction (induced by UV) to a Mo-W heteropoly acid [124].


31.3. Tungsten determination methods

Like molybdenum, tungsten is usually determined by the thiocyanate or the dithiol method. A proper choice of experimental conditions makes it possible to determine W in the presence of Mo, and vice versa, by any of the two methods. Salicylfluorone is a basis for a more sensitive method for determination of tungsten.


Tungsten(V) forms a yellow complex with thiocyanate in acid medium (HC1, H2SO4 ), in the presence of a reducing agent. The colour reaction is rapid, if thiocyanate is added to the neutral or slightly alkaline solution followed by reducing agent and acidification. If the original solution is acidic, the tungsten exists in the form of tungstic acid polymers which are less reactive and not so rapidly reduced and complexed with SCN- ions, as the non-- polymerized molecules existing immediately after acidification of tungstate. In slightly acidic medium, in the presence of tartrate, W forms tartrate complex which reacts readily with SCN-ions [125,126].

Stannous chloride and tin amalgam have been used as reducing agents. For measurement, the complex is extracted into organic solvents, usually isoamyl

alcohol, DIPE, and MIBK; a mixture of isoamyl alcohol and CHC13 has also been used. The molar absorptivity of the complex in isoamyl alcohol is 1.56.10 4 at ) ~ m a x - - 403 nm (a = 0.09). The absorption spectrum of the complex is shown in Fig. 31.1. The existing procedures for the determination of W with thiocyanate have been discussed in detail [ 127].

In strongly acidic medium (8-9 M HC1) containing SnCI2, thiocyanate does not give a colour reaction with Mo; the absorbance should be measured after about 20 min. The colour of the tungsten complex is stable for 1-2 hr.

To determine W admixtures in molybdenum, the molybdenum thiocyanate complex is first extracted in the presence of thioglycolic acid, then the tungsten thiocyanate complex is extracted after addition of TIC13 as reducing agent 128].

Iron(III) does not interfere, if it is converted into Fe(II). Niobium is effectively masked by oxalate. Fluoride and nitrate both interfere. Before its determination by the thiocyanate method, tungsten is usually separated from the majority of metals by precipitation or extraction, often with ~-benzoinoxime [6,7].

The W thiocyanate complex forms (in rather concentrated HC1 solutions) extractable (CHC13) ion associates with tetraphenylarsonium ion [129,130], DAM [131], diethazine (e = 1.46-104 at 405 nm) [132], propericiazine (e = 1.82.104 at 410 nm) [133], and thioacetanilide [57]. The W-thiocyanate complex has been extracted (pentanol + benzene) after association with N,N-diphenylbenzamidine [134] and CTA [135].

Reagents

Potassium thiocyanate, 20% solution. Standard tungsten solution: 1 mg/ml. Dissolve 1.7900 g of Na2WO4.2H20 in water, and

dilute the solution with water to volume in a 1-1itre standard flask. Stannous chloride: 40% solution of SnClz.2H20 in conc. HC1.

31.3. Tungsten determination methods 277

Procedure

The sample solution, containing not more than 150 gg of W should be slightly alkaline or neutral. If it contains tartrate, it may also be acidic. Add 2.5 ml of thiocyanate solution, 5 ml of SnCI2 solution, and enough conc. HC1 to bring its concentration in the solution to 8-9 M.. The total volume of the solution should be -25 ml. After 15 min extract the tungsten complex with two portions of isoamyl alcohol. Dilute the extract with the solvent in a 25-ml standard flask, and measure the absorbance at 403 nm, using the solvent as reference.

31.3.2. Dithiol method

Dithiol (formula 31.2) forms with tungsten, in acid medium, in the presence of Sn(II), a blue-green complex sparingly soluble in water but extractable with amyl or butyl acetate, or CHC13. The coloured extracts have been a basis for spectrophotometric determination of W [136].

The dithiol is used either in sodium hydroxide solution or dissolved in amyl acetate. The absorption maximum for amyl acetate solutions is at 640 nm, e = 1.92.104 (a = 0.10).

Molybdenum reacts like tungsten with dithiol and must be separated before the determination of W. Molybdenum can be separated as sulphide, or extracted as the dithiolate under the conditions (acidity, temperature, reducing agent) chosen so that W does not react with dithiol.

Reagents

Dithiol, 0.5% solution in amyl acetate. Standard tungsten solution: 1 mg/ml. Preparation as in p. 31.3.1. Tin(II) chloride, 10% solution of SnClz.2H20 in HC1 (1 + 1).

Procedure

Separation of Mo. Evaporate to dryness the sample solution, containing not more than 120 ~tg of W. Dissolve the neutral residue in 1.9 M HC1. Shake the solution for 30 s with one volume of the dithiol solution. Wash the extract by shaking it for a while with 1.9 M HC1. Combine the washing solution with the original aqueous solution. The extract containing the Mo dithiolate can be used for determining molybdenum. Extraction and determination of W. To the aqueous phase from the separation of molybdenum (-15 ml in volume) add 10 ml of conc. HC1, 5 ml of SnC12 solution, and 20 ml of dithiol solution. Heat the separating funnel in a water-bath at 80~ with occasional shaking.

After cooling, wash the extract with conc. HC1, dilute it to volume with amyl acetate in a 25-ml standard flask, and measure the absorbance at 640 nm against the solvent.

Note. For accurate separation of Mo from W, strict observance of working conditions is required, especially the 1.9 M HC1 concentration in the solution from which molybdenum dithiolate is extracted,


The best derivative of 2,3,7-trihydroxy-6-fluorene, for determination of W, is


salicylfluorone (formula 4.21) [137]. The reactions of W with salicylfluorone and other fluorones (phenylfluorone, dibromophenylfluorone, etc.) were studied in the presence of cationic (CP, CTA) and non-ionic (Triton X-100, syntanol, OC-20) surfactants in acidic media (0.1-1 M H 2 8 0 4 o r HC1) [138-140]. A considerable increase in sensitivity is observed in the salicylfluorone method with the use of cationic or non-ionic surfactants, where values in the range 1.24.105 - 1.36.105 have been obtained. The determination of W with the aid of o-nitrophenylfluorone and DAM has been recommended [140].

Xanthene reagents, such as Bromopyrogallol Red (BPR) (formula 4.19) [141-143] and Gallein (formula 4.18) have been used for determination of W in the presence of surfactants. The W complex with BPR and diphenylguanidine was separated by flotation with petroleum ether and was dissolved in butanol (e = 8.4.104 at 610 nm) [143].

Pyroeateehol Violet (PV) is an interesting triphenylmethane reagent for determination of tungsten [146,147]. No better results were obtained in experiments with its analogues, such as ECR, CAS, or Xylenol Orange [ 146].

Flotation-spectrophotometric methods based on ion associates of Brilliant Green and W complex with 3,5-dinitropyrocatechol (e = 1.3.105) [148], and of Crystal Violet and thiocyanate complex of W (e = 2.1.105) [149] are very sensitive.

Other organic reagents used for determination of W include: rutin (e = 5.0-104) [ 150,151], 8-hydroxyquinoline (CHC13) [ 152], bromo-oxine (e = 4.7.103) [ 153], thioglycolic acid [ 154], and thiodibenzoylmethane [ 155].

Tungsten has been determined also basing on its catalytic effect on oxidation of iodide [115,116,118,156] and chloropromazine [157] by hydrogen peroxide [157].


31.4.1. Separation and determination of Mo

The thiocyanate method (in various modifications) has been used in determinations of molybdenum in plant material [158], natural waters [28,41], cast iron and steel [25,56,159,160], tantalum, niobium, and tungsten [2], vanadium and its compounds [27] corundum and lithium niobate [55], ores and minerals [161,162], uranium concentrates [29,163] lithium fluoride [164], platinum chloride [42], and fertilisers [56]. A review of applications of the thiocyanate method for determining Mo has been given [165].

The dithiol method has been applied for determining molybdenum in plant material [166], milk [167], sea water [168], biological materials [3,43,169], foodstuffs 169,170], silicate rocks [32,43,169,171], geological samples [172], tungsten and its compounds [173], niobium and tantalum [174], copper [175], and steel 169].

Molybdenum was determined in natural waters with the aid of CBP and Zephiramine, after selective preconcentration on a Sephadex gel column [63].

The BPR-CTA method was used for determining Mo in sea-water and in steel [65]. Molybdenum was determined in steel in the presence of CP and a non-ionic surfactant OP-1 [176].

Phenylfluorone was applied in determination of Mo in natural waters [84,86] and in alloys of non-ferrous metals [87]. Molybdenum was determined in steel with the use of nitrophenylfluorone [ 177].

From among other reagents mentioned above, the following were used for Mo determinations: quercet in- in sewage and in silicate rocks [107], Rhodamine B - in biological materials [97], quinizarin- in alloys and steel [109], and T A R - in copper [98].

References 279

Tiron has been applied for determination of Mo in an ore and in biological materials [12], alloys [40], water and rocks [31 ], steel, and nickel alloys [39].

The catalytic methods have been used for determining Mo in plants [ 112, 113,117,123], soil extracts [114], and steel [119,123].

31.4.2. Separation and determination of W

The thiocyanate method has been utilised for determination of tungsten in ores and concentrates [7], molybdenum and its compounds [128], steels [6,129,131,134,178], refractory alloys [131] titanium, zirconium, and their alloys [136], nickel alloys [179], and ferromolybdenum [ 135].

The dithiol method was used for determining tungsten in soils [180], silicates and minerals [32,171,181,182], titanium [136,183], zirconium [183], tantalum [174], niobium [ 184], and copper [ 175].

The salicylfluorone was used in determination of tungsten in geological samples [139], steel [185], alloys, alkali metal halides, and ammonium perrhenate [186].

The catalytic method was used for determining W in waters [157].

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Chapter 32. Nickel

Nickel (Ni, at. mass 58.71) usually occurs in the II oxidation state, but some complexes contain nickel in higher oxidation states (III and IV). Nickel(II) sulphide is precipitated at pH -~4. Nickel(II) hydroxide (precipitated at pH -~7) dissolves in ammonia owing to formation of ammine complexes, but is insoluble in excess of NaOH. Nickel(H) also forms stable cyanide, oxalate, and EDTA complexes.


32.1.1. Extraction

Nickel dimethylglyoximate is sparingly soluble in aqueous media but readily soluble in chloroform. This permits the specific separation of small quantities of nickel by extraction, usually from weakly ammoniacal medium containing tartrate to prevent the precipitation of hydrolyzable ions such as Fe(III) and A1 [1,2]. Larger quantities of Mn interfere with the extraction of Ni(HDm)2 since manganese(if) is readily oxidized, and can subsequently oxidize the Ni(II) in the dimethylglyoxime complex, thus preventing its extraction into CHC13. This interference is prevented by adding hydroxylamine. Cu and Co are extracted on a limited scale, but are removed when the extract is shaken with dilute ammonia.

Extraction with dithizone in CC14 or CHC13 results in the separation of nickel along with traces of a number of other heavy metals [3]. By taking advantage of the differences in resistance of dithizonates to dilute hydrochloric acid, it is possible to separate nickel from Zn and Cd [4].

32.1.2. Precipitation. Ion exchange

The separation of nickel by precipitation as the dimethylglyoximate is almost specific [5]. For greater precision in isolating Ni from Co and Cu, the precipitation of Ni(HDm)2 is done twice by introducing H2Dm into an acidic solution, and subsequently adding ammonia. For trace amounts of Ni, Pd can be used as collector.

When the sample is fused with sodium carbonate and then leached with water, nickel remains quantitatively in the undissolved residue [6].

In the presence of KCN in alkaline medium, nickel is precipitated as Ni(OH)2, whereas cobalt remains in the solution as the cyanide complex. Suitable collectors for nickel are Mg(OH)2 or La(OH)3.

Since the chloride complex of Ni is much weaker than those of Co, Cu, Zn, Fe(III), and certain other metals, it is possible to separate nickel from these metals on ion-exchange columns [7]. When a solution in 8-9 M HC1 is passed through a strongly basic anion- exchanger, nickel alone is eluted. Nickel has been separated from other metals using aqueous acetone or aqueous methanol solutions [8].

After the sorption of Ni and other metals (Zn, Co, Cu, Mn, Fe, U etc.) on a cation- exchange column, these metals are first eluted with 0.5 M HC1 in acetone solution, and then Ni is eluted with a HzDm solution in 0.5 M HC1 and acetone [9].

Trace amounts of Ni have been sorbed on a chelating resin with 8-hydroxyquinoline [10,11], and on polyurethane foam [12].



The most important spectrophotometric reagents for determining nickel are dioximes, which give specific and fairly sensitive methods. An example of a very sensitive method is one using the azo reagent 5-Br-PADAP.

33.2.1. Dimethylg lyoxime method

Dimethylglyoxime (H2Dm, diacetyldioxime, Chugaev's reagent) reacts with Ni ions in a neutral or ammoniacal medium to form a pink, flocculent precipitate which has been the basis of the well known gravimetric method for determining nickel. The nickel dimethylglyoximate chelate is soluble in CHCI3 and other non-polar organic solvents. The extraction of Ni(HDm)2 is primarily important in the separation of nickel, but relatively large quantities of nickel have been determined by means of the pale yellow chloroform solution of Ni(HDm)2.

Curve 1 in Fig. 32.1 represents the absorption spectrum of Ni(HDm)2 in CHC13. The molar absoq)tivity at ~.,,a~ = 360 nm is 3.4.103, and at 400 nm is 1.8.103.

,H rJ, H3C~C"NOH H3C~C~N N~C~CH3

2 ] + N"* ----~ t[ \'Uf~, \ ,I + ,U+ (32.1)

ONXH.,,"O Ill an alkaline medium and in the presence of oxidants, nickel forms a brown-red, water- soluble dimethylglyoxime complex which is the basis of the very popular method for determining nickel. In this complex, nickel is in the IV oxidation state, and the anionic complex has the formula Ni(Dm)3 2-. The usual oxidants are bromine, persulphate, or iodine, bu! the oxidant used has no effect on the colour obtained. Since the formation of the complex is rather slow, it is advisable to wait several minutes betore measuring the absorbance. The coloured solutions are unstable. It is most important to add the reagents in the following sequence: HaDm, oxidant, ammonia.

Curve 2 in Fig. 32.1 shows the absorption spectrum of the complex Ni(Dm)f- The molar absorptivity at Z,nax = 445 nm is 1.5.104 (a = 0.26).

Metal ions (e.g., Fe 2+, Co 2+, Cu >) which form coloured, water-soluble complexes with H2Dm interfere. However, the complexes of these metals are decomposed by EDTA, and a preliminary extraction as Ni(Hl)m)2 allows nickel to be isolated from Cu, Co, Fe, Cr, AI, and Mn. The presence of hydroxylamine ensures the quantitative extraction of nickel and prevents interference from Cu and Mn. In the presence of large quantities of Co and Fe the use of triethanolamine as masking agent is recommended [ 13].

When the chloroform extract is shaken with 0.5 M HC 1, Ni(HDm)2 is decomposed and the nickel is stripped into the aqueous phase. After the CHC13 has been removed, this aqueous solution is treated with H2Dm, oxidant, and ammonia (or NaOH solution) to forln Ni(Dm)32- . The dimethylglyoxime is added as an alcoholic solution or as a solution in 0.2 M NaOH.

286 32. Nickel

r,.)

�9 r~

,.cb c~

360 /400 436 /./,5 500 600 wavelength, nm

Fig. 32.1. Absorption spectra of nickel(ll) with dimethylglyoxime (H2Dm) in CHCla (1), nickel(IV) with H2Dm in alkaline solution (2), and nickel(ll) with ct-furyldioxime in CHCla (3).

The anionic nickel complex with H2Dm, formed in NaOH medium in the presence of an oxidant can be extracted into organic solvents in the presence of diphenylguanidine [14].

Nickel has been determined by the dimethylglyoxime method in the presence of iodine, by the flow injection technique (FIA) [15].

Reagents

Dimethylglyoxime (H2Dm), 1% solution in ethanol. Standard nickel solution: 1 mg/ml. Dissolve 6.7300 g of (NH4)2Ni(SO4)2.6H20 in water

containing 2 ml of conc. H2SO4, and dilute the solution with water to 1 litre. It is also possible to prepare a more concentrated solution of a nickel salt, determine the concentration of nickel gravimetrically, and then dilute the solution with water until it contains exactly 1 mg of Ni per ml.

Bromine water, saturated aqueous solution. Potassium persulphate, 4% solution, freshly prepared.

Procedure

Extractive separation of Ni. To the solution containing not more than 50 gg of Ni, add 1-3 ml of 20% sodium potassium tartrate solution, 1 ml of 10% NH2OH.HC1 solution, 2 ml of the H2Dm solution, and ammonia to pH 9-10. Shake the solution for about half a minute in a separating funnel with two portions of CHC13. Wash the combined extracts by shaking with dilute ammonia solution (1+50), then strip the nickel from the organic phase by shaking for 1 min with 0.5 M HC1. Discard the chloroform layer. Determination of Ni. Quantitatively transfer the solution obtained into a 25 ml standard flask and add, successively, 1 ml of the HzDm solution, 1 ml of bromine water (or persulphate solution), and 2.5 ml of conc. NH3 solution. Dilute the solution to the mark with water, and mix well. After 10 min, measure the absorbance at 445 nm against water.

32.2.2. tx-Furildioxime method

ct-Furildioxime sometimes called Neonickelone, formula 32.2) reacts with nickel ions in a similar way to dimethylglyoxime and other dioximes, forming a chelate which is sparingly


water-soluble, but is extractable into chloroform and similar solvents. The yellow colour of the organic extract provides the basis of a specific spectrophotometric method for determining nickel [ 16]. This method is more convenient than the method discussed above with HzDm and oxidant. The sensitivities of the dimethylglyoxime (+ oxidant) and the ~-furildioxime methods are similar, but the latter method is simpler since it enables one to extract Ni from the aqueous phase to a smaller volume of an organic solvent.

The nickel c~-furildioxime complex is formed and extracted quantitatively in the rather narrow pH range from 7.5 to 9.0. The pH can be adjusted most conveniently by adding small portions of sodium bicarbonate to the slightly acidic sample solution. Alternatively, a drop of 1% solution of phenolphthalein can be added to the solution, and ammonia added carefully until the indicator just turns pink.

(32.2)

Chloroform, CC14, o-dichlorobenzene, and ethyl acetate have been used as solvents. The solubility of the complex in CC14 is rather low and hence this solvent can be used only for extracting limited amounts of nickel (<1 ~tg Ni/ml CC14).

The absorbance maximum of the nickel ~-furildioxime chelate in CC14 is at 435 nm (the molar absorptivity is 2.0.104; a = 0.34) (see Fig. 32.1). The value of e increases slightly when the complex is extracted from a solution in 25% ethanol.

Low and erratic results obtained after the extraction of nickel c~-furildioximate with certain chloroform samples are probably due to the presence of trace oxidants from the decomposition of CHC13. Preliminary shaking of the solvent with thiosulphate solution before the extraction prevents such interferences.

Copper, which interferes by forming a coloured extractable complex with a- furildioxime, is effectively masked with thiourea or thiosulphate [16]. Addition of Na-K tartrate prevents the hydrolysable metals precipitating under the conditions of this reaction.

Reagents

~-Furildioxime, 0.5% solution in ethanol, ff the solution is coloured, shake it with activated carbon.

Standard nickel solution: 1 mg/ml. Preparation as in Section 32.2.1. Sodium bicarbonate, saturated solution (--10%). Chloroform. Shake the solvent with 0.1 M Na2S203, and then with water.

Procedure

Place a weakly acidic sample solution (pH --1), containing not more than 50 ~tg of Ni, in a separating funnel, and add 20% potassium sodium tartrate solution (0.2-1 ml) and 1 ml of the ~-furildioxime solution. Add the NaHCO3 solution in small portions with stirring until CO2 is no longer evolved. Allow to stand for 10 min, and then extract the Ni complex with two portions of CHC13. Dilute the extract to the mark with the solvent in a 25-ml standard flask, and measure the absorbance at 435 nm, using the solvent as a reference.

288 32. Nickel

Note. Small amounts of copper in the sample should be masked with 1-2 ml of 5% thiourea solution before addition of the ~z-furildioxime.

32.2.3. 5 - B r o m o - P A D A P m e t h o d

2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) (formula 4.3) with Ni(II) ions forms a complex which is the basis of a very sensitive method for determining nickel [2,17-20].

The solution should be kept for --30 min at room temperature for complete colour development; the colour intensity then remains virtually constant for 24 h. The usable pH range is 4-10, but in order to reduce the effect of foreign ions, a pH of--5.5 is recommended. The absorbance values at this pH of water-ethanol (2:1) solution are maximal when the reagent:Ni molar ratio lies between 4:1 and 12:1.

The absorption maximum of 5-Br-PADAP is at 443 nm in 30% ethanolic medium, and the nickel complex has two maxima, at 523 and 558 nm. The molar absorptivity at 558 nm is 1.26.105 (a = 2.1).

The method is not very selective. Among interfering species are Co, Cd, Zn, Mn, Cu, Pb, Hg(II), Ag, Fe(III), A1, and Zr. The last three can be masked by addition of thiosulphate, metaphosphate, or fluoride. The final determination of Ni with 5-Br-PADAP should be combined with a preliminary isolation of Ni by extraction with dimethylglyoxime.

Reagents

5-Br-PADAP, 0.02% (--7.10 -4 M) ethanolic solution. Standard nickel solution: 1 mg/ml. Preparation as in Section 32.2.1. Acetate buffer (pH 5.5). Dissolve 100 g of sodium acetate trihydrate in water, add 4.5 ml

of glacial acetic acid and dilute with water to 500 ml.

Procedure

Transfer a sample solution, containing not more than 10 ~tg of Ni, into a 25-ml standard flask. Add, with swirling, 1 ml of acetate buffer, 8 ml of ethanol, and 2 ml of 5-Br-PADAP solution. Dilute to volume with water and mix well. After 30 min, measure the absorbance of the solution at 558 nm against a reagent blank.

32.2.4. O t h e r m e t h o d s

A large group of reagents for Ni is that of the dioximes. Apart from H2Dm and o~- furildioxime, the following reagents have been recommended for the spectrophotometric determination of Ni: 4-butylnioxime [21], carboxynioxime [22], heptoxime (1,2-cyclo- heptanedionedioxime) [23], ~-benzildioxime (diphenylglyoxime, Nickelone) [24], and naphthaquinonedioxime [25].

Formaldoxime, which is commonly used to determine Mn, can also serve as a reagent for nickel since it forms a brown nickel complex (e = 1.84-104 at 473 nm) [26]. Other oxime reagents for determining Ni include 2-hydroxyacetophenone oxime [27], diacetylmonoxime glycinimine (e = 1.7-104 at 450 nm) [28], and 3-hydroxy-2-methyl-l,4-naphthaquinone monoxime [29].


The use of the a z o reagent 5-Br-PADAP has been discussed. Many other azo compounds are useful spectrophotometric reagents for nickel, namely: PAR (~ = 7.6.104 at

495 nm) [19,30-32], PAN [6,33,34], TAR [19], TAN [35], TAM (~ = 4.6.104 at 575 nm; CHC13) [36], 2-(2-benzothiazolylazo)-5-dimethylaminobenzoic acid (~ = 1.17-105 at 635 nm; CHC13) [37], 2-(3,5-dibromo-2-pyridylazo)-5-(N-ethyl-N-sulphopropylamino)benzoic acid (e - 1.37-105 at 620 nm) [38], 2-[2-(3,5-dibromopyridyl)azo]-5-dimethylaminobenzoic acid (3,5-diBr-PAMB) (e - 1.45-105. CHC13 [39]. It has been shown that Ni can be determined, with the use of TAN, in the presence of many other metals, by means of derivative spectrophotometry using 3rd--4 th order spectra [40].

Many spectrophotometric methods for Ni are based on organic reagents containing sulphur as a ligand atom, e.g., quinoxaline-2,3-dithiol (~; = 1.8.104 at 660 nm) [41] (in the presence of Zephiramine [42]), 3-(4-methoxyphenyl)-2-mercaptopropenoic acid (~ = 1.9.104 at 415 nm) [43], biacetyl bis(4-phenyl-3-thiosemicarbazone) [44], thiazole-2-carbaldehyde- 2-quinolylhydrazone (~ = 7.2.104 at 522 nm) [45], and 2,2'-dipyridyl-2-benzothiazolylhydrazone (~ = 5 .4 - l04 ) [46]. A considerable increase of sensitivity of this method has been attained by using 2 nd order derivative spectrophotometry. DDTC has been used in the presence of a surfactant for determination of Ni (~ = 3.2.104 at 324 nm) in the presence of Cu(II) and Co(II) [47].

A sensitive spectrophotometric method is based on the extraction of the Ni complex with pyridine-2-aldehyde-2-quinolylhydrazone (~ = 6.7.104 at 515 nm) [48-50]. The following similar reagents have also been recommended: 5-methylfurfural-l-phthalazinehydrazone [51], 2-pyridinecarbaldehyde-2-(5-nitro)pyridylhydrazone (~ - 1.0.105) [52], 2- pyridinecarbaldehyde-3,5-dinitro-2-pyridylhydrazone [53], and 1,5-bis(di-2-pyridylmethylene)thiocarbonylhydrazide [54].

The dyes Rhodamine 6G [55] and Crystal Violet [56] form extractable ion-associates with the anionic complexes of nickel with chloro-oxine [55] and 4-chloro-2-nitroso-1- naphthol [56], (~; = 7.7.104 - 8 .2 .104) . The ion associate of Rose Bengal with the cationic nickel complex with 1,10-phenanthroline, extractable into nitrobenzene, is also the basis of a sensitive method (E > 1.105) [57]. Hydroxynaphthol Blue has been applied for determination of Ni in the presence of Cu by the derivative spectrophotometry method [58].

In a highly sensitive indirect method (~ > 3.0.105) Ni is separated as the precipitate formed with H2Dm and diphenylboric acid, Ni(HDm)2.2(C6Hs)2BOH. After mineralization of the organic moiety, boron is determined by the curcumin method (see Section 11.2.1) [59].

Other organic reagents which have been recommended as reagents for Ni include: Xylenol Orange [60,61], Zincon (e - 2.4.104) [62], 3-hydroxypicolinealdehyde azine (e = 4.2.104 at 480 nm) [63], 2-benzoyl-4-(4-nitrophenyl)-acetohydrazine [64,65], 3-(2-pyridyl)- 5,6-diphenyl-l,2,4-triazine (PDT) [66], and 2-(2-pyridylmethyleneamino)phenol [67].

A review of spectrophotometric reagents for determination of nickel has been presented [68].


The dimethylglyoxime method has been used for determination of Ni in foodstuffs [10], platinum-group metals [69], iron ores [70], niobium, tantalum, molybdenum, and tungsten [71], steel [72], sewage [73], and aluminium alloys [74]. Dimethylglyoxime in the presence of oxidant was used for determining Ni in foodstuff [75], sea-water [76], plants [77], steel [5], lead and antimony [78], copper alloys [79], zinc and cadmium [80], and tungsten and its

290 32. Nickel

alloys [81,82]. The ~-furildioxime method has been used for determining nickel in silicate and sulphide

minerals [16], tungsten selenide [83], steel [16], indium and aluminium [84], cadmium [85], alkalis [3], beryllium [86], rhenium [87], compounds of rare-earth elements [88,89], petroleum products [90], and boiler water [91 ].

The 5-Br-PADAP method was applied for the determination of Ni in cobalt and iron salts [2] and in copper alloys [92].

PAR was applied in the determination of Ni in crude oil [30], and steel [93,94]. PAN was used for determining Ni in drinking water [95] and in sewage [96]. Nickel in the presence of Zn and Cu was determined by derivative spectrophotometry [97]. DDTC was used for determining Ni in natural waters [98]. Nickel was determined in iron and aluminium alloys by the derivative spectrophotometry with the use of the cyanide complexes [99]. The thiocyanate complex was applied in determination of Ni by the FIA method [ 100].

2-[2-(5-Methylbenzothiazolyl)azo]-5-dimethylbenzoic acid was applied for determining Ni in aluminium alloys [101] and alloy steels [102].

Small amounts of nickel were separated from nickel oxide by selective dissolving nickel metal in a solution of thiocyanate silver complex (pH -5) [103].

References

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292 32. Nickel

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Chapter 33. Niobium and tantalum

Niobium (Nb, at. mass 92.91) hydrolyses (in the absence of complexing anions) over the pH range 0-14. Polymerized forms of Nb(V) give pseudo-solutions or they separate as a white precipitate. When fused with NaOH, Nb205 forms the niobate, which is soluble in NaOH solutions. Niobium(V) forms stable fluoride, tartrate, oxalate, and peroxide complexes. The niobium complexes are more stable than the corresponding Ta complexes. A niobium chloride complex is formed in >5 M HC1 solutions. Niobium(V) can be reduced to coloured species of Nb(III) and Nb(IV). In an acid medium, zinc metal reduces Nb(V), but not Ta(V).

Tantalum (Ta, at. mass 180.95) is, in its chemical properties, similar to Nb. The Ta(V) complexes are less stable than the corresponding Nb complexes. Tantalum(V) is harder to reduce to the lower, coloured, oxidation states, than is Nb(V).



On being heated in acid solutions (or pseudo-solutions), niobium and tantalum hydrolyse and coagulate to form hydrous oxides. The following compounds may be used as collectors for traces of niobium or tantalum: Zr(OH)4 and MnOzaq. in acid solutions, and Fe(OH)3 and Mg(OH)2 in alkaline solutions. When an alkaline melt (NazCO3, NaOH) is leached, Nb and Ta remain in the solid phase, while W, Mo, V, and Re pass into the aqueous solution [1 ].

From solutions containing not too much oxalic or tartaric acid, or EDTA, Nb and Ta may be precipitated by cupferron [2,3] or phenylarsonic acid [4]; zirconium is often used as collector.

When boric acid is added to a solution of Nb-, Ta-, W-, Mo-, or Zr fluoride complexes, the boron displaces the fluoride (formation of BF4-) from the Nb, Ta, and W complexes, causing the precipitation of these metals, while Zr and Mo remain in solution, since their fluoride complexes are more stable [5].

33.1.2. Extraction

Niobium and tantalum form stable fluoride complexes which are extractable with oxygen- containing solvents and can thus be separated from many metals, such as Ti, Zr, Sn, Mo, W, U, and Fe [6,7]. By suitable choice of concentrations of HF, HC1, and H2SO4, tantalum can be separated from niobium and v ice v e r s a [6]. MIBK is most often used as the solvent. Niobium and Ta can be stripped from the organic phase with a H202 solution.

Niobium can be separated from Ta and other metals in HF or HC1 media by extracting into CHC13, CC14, cyclohexane, 1,2-dichloroethane, or xylene the ion-associates, which are formed by Nb (and Ta) complexes with tetraphenylarsonium cation, TOA [8,9], DAM [10], or Aliquat 336 [11-13]. The thiocyanate complexes of Nb with triphenylphosphonium cation [14] or with amides [ 15] have also been extracted.

Highly selective separation of Nb has been based on extraction from 5 M HC1 by means of a-benzoinoxime in CHC13 [ 16,17].

294 33. Niobium and tantalum

Ion-associates of niobium anionic complexes with pyrocatechol [18], or oxalate [19], and quaternary ammonium ions have been used for separating Nb from other refractory metals (Ti, W, Mo, V). Niobium has been extracted from solutions containing Fe(III), Ti(IV), and V(IV) with the use of 4,5-dibromopyrocatechol [20]. The crown-ether dibenzo- 18-crown-6 has also been used for the extraction of Nb [21]. Tantalum has been separated from V (in 6 M HC1) by extraction into triphenylphosphine oxide in toluene [22].

33.1.3. Ion exchange and sorption

Many methods for isolating and separating Nb and Ta are based on their fluoride complexes [23,24]. The complexes of Nb, Ta, and some other metals are retained in a polyethylene column filled with an anion exchanger, from which they are then washed out with suitable eluents. If a 6 M HC1 and 1 M HF medium is applied, Nb is retained in the column and Zr passes into the eluate [25]. Nb and Ta have been separated on the anion-exchanger Dowex 1 from a medium containing HC1 and H2C204 [26]. Either Ta is first eluted with a mixture of 0.1 M H2C204 and 2 M HC1, or Nb is eluted first with 0.5 M H2C204 and 1 M HC1. The procedure can be used for determination of Nb in tantalum and vice versa . Niobium and tantalum have been also sorbed on the anion-exchanger Dowex 1 from mixtures of HF- H2SO4 and HF-HC1 [27,28]. Cation exchangers were also used for separation of Nb, Ta, and other metals, using H202 mixtures with HC1, H2SO4, or HNO3 [29].

The use of liquid ion exchangers for the separation of Nb and Ta (and other metals) has been reviewed [30].

Tantalum has been separated from niobium with the aid of chelating ion-exchangers [31,32]. Polyurethane foam saturated with DAM, TBP, or MIBK has also been used in the separations [33].

33.2. Determination of niobium

A description is given below of the classical thiocyanate method and of the more sensitive method based on Bromopyrogallol Red.


Niobium(V) reacts with thiocyanate in HC1 solutions to form a yellow complex, which has been a basis of determining Nb. The niobium is determined spectrophotometrically either after extraction of the complex [34-36] or in an aqueous acetone medium. The sensitivities in both cases are similar, but the extraction method is less subject to interference by other metals. Diethyl ether is commonly used as the solvent, but ketones, esters, and higher alcohols are also suitable.

The absorption maximum of an ethereal solution of the Nb thiocyanate complex is at 385 nm. The molar absorptivity of the complex is 3.5.104 (a = 0.38).

Niobium is extracted in the presence of SnCI2 which reduces Fe(III) and other interfering oxidants. The concentrations of SnCI2, HC1, and thiocyanate greatly affect the intensity of the colour and the reproducibility of the results. These concentrations should not be lower than 4% SnCI2, 2 M HC1, and 10% KSCN. Since some thiocyanic acid is also extracted by ether, one is recommended to use ether saturated with thiocyanic acid. A mixture of diethyl ether and CC14 (1+1) makes a useful extractant since it is denser than water.

33.3. Determination of niobium 295

Niobium is kept in solution by the addition of tartaric acid, whose presence prevents the hydrolysis of tantalum and the consequent occlusion of niobium in the precipitate.

Tungsten, molybdenum, and vanadium interfere in the determination of niobium. In contrast to the corresponding tungsten complex, the niobium-thiocyanate complex is decomposed by oxalic acid. Fe([II), U, Ti, and Ta do not interfere if they are present in no greater than hundred-fold amounts relative to niobium. Phosphate and fluoride interfere, but the latter can be masked with aluminium ions [37].

The thiocyanate complex of niobium may be extracted in the presence of ethylenebis(triphenylphosphonium) cation [38], promazine [39], triphenylguanidine [40], and dibenzo- 18-crown-6 ether [41 ]. Chloroform, trichloroethylene, toluene and benzene are used as the solvents.

Reagents

Potassium thiocyanate, 30% solution. Standard niobium solution: 1 mg/ml. Fuse 0.1430 g of Nb205 with 4 g of K2S207 in a

quartz or platinum crucible. Dissolve the melt in a hot 5% tartaric acid solution, allow to cool, and dilute with the tartaric acid solution to 100 ml in a volumetric flask. Working solutions are obtained by suitable dilution of the stock solution with 2% tartaric acid.

Stannous chloride: 20% solution in 2 M HC1.

Procedure

To the sample solution containing not more than 50 ~tg of Nb ( complexed with tartrate), add concentrated HC1 and the SnCI2 and thiocyanate solutions until the solution is --3 M in HC1, 5% in SnCI2, and 12% in KSCN. After 5 min, extract the niobium thiocyanate complex with 2 portions of diethyl ether. Transfer the extracts to a 25-ml standard flask, make up to the mark with ether, and measure the absorbance at 385 nm against a reagent blank solution.

Note. The ratio of the volumes of extractant to the volume of the aqueous phase must be identical for both the sample and the standard solutions.

33.2.2. Bromopyrogallol Red method

In a mixed EDTA-tartrate medium at pH --6, Bromopyrogallol Red (BPR) (formula 4.21) reacts with niobium to form a blue complex used as a basis for Nb determination [42,43]. At 610 nm, the molar absorptivity is 4.75-104 (specific absorptivity 0.51). The absorption maximum of the reagent is at 560 nm.

Since the niobium-BPR complex is insoluble in water, gelatine is added to give a stable colloidal dispersion. The colour reaction proceeds so slowly that maximum absorbance is attained only after 90 min, after which it remains constant. The greatest sensitivity is obtained in solutions buffered between pH 5.8 and 6.6 with ammonium acetate.

The Nb complex with BPR is very stable. Its formation is not affected by 1,000-fold excess of oxalate, fluoride, or phosphate.

Most interfering cations are masked with EDTA. Tartrate masks milligram quantities of Ta, Ti, W, Mo, Sb(V), and Sn(IV); U(VI) and Zr can be masked with phosphate; fluoride masks A1 and Th; cyanide masks any silver present. The interfering effect of Ce(IV) and V(V) can be eliminated by addition of ascorbic acid which reduces them to Ce(III) and V(IV), respectively. Thus, the BPR method can be regarded as specific, provided that


suitable masking agents are used. If the sample solution contains Nb separated from other metals, no masking agents,

including EDTA and tartrate, are necessary. In this case the sensitivity of the Nb reaction with BPR is higher (~ = 6.0-104) [42].

Niobium has also been determined in 1 M HC1 in the presence of CP (15% DMF) [44].

Reagents

Bromopyrogallol Red (BPR), 0.02% solution. Dissolve 20 mg of the reagent in 50 ml of ethanol, and dilute the solution to volume with water in a 100-ml standard flask. The solution should not be used if over a week old.

Standard niobium solution: 1 mg/ml. Preparation as in Section 33.2.1. Acetate buffer, pH 6.0. Dissolve 80 g of ammonium acetate in water, add 6 ml of glacial

acetic acid, and dilute the solution with water to 1 litre.

Procedure

Add the sample solution (--5 ml) containing not more than 30 pg of Nb (complexed with oxalate or tartrate) to a 25-ml standard flask. Neutralise to pH -~6, and add 3 ml of 20% potassium sodium tartrate solution, 3 ml of 5 % EDTA solution, 2 ml of the BPR solution, 3 ml of the acetate buffer, and 0.5 ml of the gelatine solution. Mix the solution and set aside for 90 min. Then dilute with water to the mark, and measure the absorbance at 610 nm against a reagent blank solution.

Note. If A1 or Th is present, add NaF; if U or Zr is present, add Na3PO4; if Ag is present, add KCN (Caution!).


Several other azo compounds have been employed as spectrophotometric reagents for niobium. The reactions are carried out in the presence of complexants (tartrate, oxalate, H202). The coloured species produced are generally ternary niobium complexes. PAR (e = 3.6.104) [45-53] and Sulphochlorophenol S ( e - 3.3.104) [54-61] are often employed.

Higher sensitivity is obtained in reactions of Nb with Sulphonitrophenol M (formula 33.1) (~ - 5.3-104) [55,62,63]. Other reagents recommended for Nb determination include Sulphonitrazo E [64], TAR [21,65], arsonophenylazochromotropic acid [66], an azo derivative of 8-hydroxyquinoline [67], and 2-(5-chloro-2-pyridylazo)-5-dimethylaminophenol (MIBK) [68].

HO3S , OH HO OH ; 03H

0 2 N HO 3s" g v "SO3H (33.1)

Among 2,3,7-trihydroxy-6-fluorones, the best results in the determination of Nb were obtained with phenylfluorone and o-nitrophenylfluorone (~ - 1.3.105 - 1.7-105) [69-71 ]. A very sensitive method has been based on a mixed complex of Nb with o-nitrophenylfluorone and DAM (~ = 1.9-105) [72]. Even higher sensitivity (~ = 2.1.105) has been obtained with the

33.3. Determination of tantalum 297

use of salicylfluorone and surfactants [73,74]. A high sensitivity (e = 2.1.105) is also characteristic for the flotation method, based on

an ion-associate formed by the anionic oxalate complex of Nb(V) with 3,5- dinitropyrocatechol and Rhodamine B [16].

Other organic spectrophotometric reagents for Nb include 8-hydroxyquinoline-5- sulphonic acid [75], 5,7-dichloro-8-hydroxyquinoline (e =1.3.104 at 400 nm) [76], 5-chloro- 7-iodo-8-hydroxyquinoline [77], Lumogallion [78], and thioglycolic acid [79]. Nb has also been determined after extraction of its complex with 3-hydroxyflavone [80] and N-m- phenylstyrylacrylhydroxamic acids [81 ].

The yellow niobium peroxide complex gives a less sensitive method (e = 1.0.103) suitable for determining larger quantities of Nb [82]. The method is suitable for determination of Nb in the presence of Ta, Ti, and other metals. Nb has been determined in the presence of Ti by derivative spectrophotometry, as a complex with H202 and 5-Br- PADAP [83].

The ion-associate of the anionic Nb complex with tetrabromopyrocatechol and Brilliant Green has been used in the extractive method (toluene) [84,85]. Niobium has been determined in the presence of Zr with the use of Xylenol Orange [86].

33.3. Determination of tantalum

The pyrogallol method for determining Ta is quite selective but rather insensitive. For traces, the extractive spectrophotometric methods using Methyl Violet or other basic dyes are recommended.

33.3.1. Pyrogallol method

In an acid medium (HC1, H2SO4), pyrogallol reacts with Ta(V) to form a yellow complex, whose absorption maximum occurs in the near-ultraviolet. This reaction has long been the basis of a simple and selective method for determining Ta [87,88].

The absorption spectrum of the Ta complex and the interferences owing to other elements (particularly Nb and Ti) vary according to the reaction conditions. A solution of 4 M HC1 and 0.02 M (NH4)2C204 is a suitable medium, since the colour from pyrogallol complexes with niobium and titanium is insignificant. In this medium, a ternary tantalum complex with pyrogallol and oxalic acid, and the colourless Nb oxalate complex are formed. To reduce the interference by Nb, tartrate is sometimes added.

The molar absorptivity of the tantalum-pyrogallol complex in 4 M HC1 and 0.02 M (NH4)2C204 solution is 2.4.103 (a = 0.013) at ~max = 335 nm. It is advisable, however, to measure the absorbance at longer wavelengths to avoid interference by the excess of pyrogallol, whose absorption maximum occurs at 315 nm.

The colour reaction of Ta should be carried out in a reducing medium, as pyrogallol is readily oxidized in contact with air to give dark-coloured products. The excess of pyrogallol influences the intensity of the colour obtained.

Mo, W, Sb, U, and fluoride interfere seriously in the determination of Ta with pyrogallol. Fluoride can be masked with boric acid. The tantalum-pyrogallol complex may be extracted in the presence of quaternary ammonium bases [88,89].


Reagents

Pyrogallol, 20% solution. Dissolve 20 g of resublimed pyrogallol in water, add 10 ml of conc. HC1 and 2 g of SnClz.2H20 (dissolved in 5 ml of conc. HC1), and dilute the solution with 0.1 M HC1 to 100 ml.

Standard tantalum solution: 1 mg/ml. Fuse 0.1220 g of Ta2Os, with 4 g of K2S207 in a silica or platinum crucible. Dissolve the melt in 4% (NH4)2C204 solution and dilute to the mark with the same reagent in a 100-ml standard flask. Working solutions are obtained by suitable dilution of the stock solution with 2% solution of ammonium oxalate.

Ammonium oxalate and hydrochloric acid solution containing 15 g of (NH4)2C204 and 760 ml of conc. HC1 per litre.

Procedure

Place the sample solution (7-8 ml) containing not more than 1.5 mg of Ta in a 25-ml standard flask. Add 10 ml of the (NH4)2C204 and HC1 solution, and 5 ml of the pyrogallol solution. Dilute the solution with water to the mark. After 30 min, measure the absorbance of the solution at 350 nm against a reagent blank.

33.3.2. Methyl Violet method

In dilute HF medium, tantalum forms a complex, TaF6-, which combines with the basic dye Methyl Violet to form an ion-pair, which can be extracted with benzene. The coloured extract has been the basis for a sensitive method of determining Ta [90].

Maximum absorbance of the coloured extract is obtained when the pH of the reaction medium is 2.1-2.3, and the HF concentration is 0.2-0.3 M. When the volumes of the aqueous phase and benzene are 30 ml and 10 ml, respectively, 80% of the tantalum is extracted. Extraction of free Methyl Violet is only slight.

The molar absorptivity of the benzene extract obtained under the conditions specified in the procedure below is 7.5.104 (specific absorptivity 0.42) at ~max 605 nm. Low concentrations of HC1 and H2804 do not interfere with the extraction of Ta. In the presence of HNO3, however, more free Methyl Violet is extracted.

Concentrations of niobium up to 0.2 mg/ml can be tolerated in this method. Rhenium at concentrations >5 ~tg/ml causes high results for tantalum. High concentrations of Mo and A1 cause low results since they mask the hydrofluoric acid as stable fluoride complexes. Moderate amounts of Zr, Ti, W, Fe, and Cu do not interfere.

Reagents

Methyl Violet, 0.1% solution in 0.2 M HF. Store the solution in a polyethylene bottle. Standard tantalum solution: 1 mg/ml. Dissolve 0.1000 g of tantalum in 5 ml of conc. HF

and a few drops of conc. HNO3. Evaporate the solution (in a platinum or Teflon vessel) to dryness, add a few drops of conc. HC1 and 2 ml of conc. HF, and evaporate to dryness again. Dissolve the residue in 1 ml of conc. HF and dilute the solution with water (with stirring) to 100 ml. Store the solution in a polyethylene bottle. Working solutions are obtained by suitable dilution of this stock solution with 0.2 M HF.

33.3. Determination of tantalum 299

Procedure

To the sample solution (in a polyethylene separating funnel) containing not more than 20 ~tg of Ta, add 2 ml of the Methyl Violet solution and sufficient hydrofluoric acid to make its concentration 0.2-0.3 M in a volume of 30 ml (pH 2.1-2.3). Add 10 ml of benzene and shake for 1 min. Measure the absorbance of the benzene extract at 605 nm against benzene.

Note. When there are small amounts of tantalum in the sample a reagent blank should be used as reference because of the small solubility of Methyl Violet in benzene.


Many other basic dyes besides Methyl Violet have been used in sensitive extraction- spectrophotometric methods for the determination of Ta as the anionic complex TaF6- [92]. Mention may be made of Crystal Violet (formula 4.27) (e = 8 .5 .10 4) [91-93], Brilliant Green (~ = 1.2.105) [94,95], Malachite Green [96,97], Methyl Green (~ = 1.2.105) [98], Rhodamine 6G and butylrhodamine B [99], Methylene Blue (~ = 9.1.104) [98], Nile Blue A [100], Capri Blue (~ = 1.1.105) [101], and Victoria Blue B [102]. Ion-associates with these dyes are extractable from acid solutions into benzene, toluene, CHC13, xylene, or dichloroethane.

Some 2,3,7-trihydroxy-6-fluorones are sensitive reagents for Ta, e.g., phenylfluorone [103], salicylfluorone [104,105], and 9-(2'-hydroxyphenyl)-2,3,7-trihydroxy-6-fluorone (~ -- 2.1.105 at 505 nm) [ 106]. High sensitivity (~ = 1.6-105-1.8 �9 105) has been obtained in the presence of surfactants [ 105]. This method has been used for the determination of tantalum in niobium metal. Tantalum has been determined with the use of 4,5-dibromo-o- nitrophenylfluorone [ 107].

From the group of azo reagents the following have been recommended for determination of Ta: PAR [22,108-110], Arsenazo I [111], and 2-(2-thiazolylazo)-5- dimethylaminophenol (TAM)(~ = 4.1-104) [112].

Other organic reagents for determination of Ta include pyrogallolsulphonic acid [ 113] and dibromogallic acid [ 114].

Tantalum has also been determined as tantalomolybdenum blue [115], and after the extraction (CHC13) of its thiocyanate complex, associated with triphenylguanidine [ 116].

Tantalum has been determined in the presence of Nb by derivative spectrophotometry with the use of Picramine-epsilon [ 117].


33.4.1. Separation and determination of Nb

The thiocyanate method has been used for determining niobium in steels [35,118], tantalum and its compounds [6,7,119], cobalt alloys [37], uranium [120], rocks and minerals [1,121], sodium metal [122], and thin Nb-Ti films [123]. Niobium has been determined in various metals and alloys with the use of Bromopyrogallol Red [ 124].

PAR has been applied in determinations of niobium in silicate rocks [29], steels [48,49,125], magnetic and electric alloys [22,50], zirconium and titanium alloys [51], copper alloys [52], thin Nb-Ge films [53], and high-phosphorus optical materials [ 126].

Niobium has been determined with the use of Sulphochlorophenol S in rocks and minerals [60,127-129], steels [57,58], tungsten, uranium, and beryllium [59], and titanium


dioxide and ilmenite [61]. Sulphonitrophenol M was applied in determinations of niobium in minerals [63]. Trace amounts of niobium (--1-10 -4 %) in geological materials were determined by the flotation method with the use of Rhodamine B [ 16].

Other methods mentioned above were used in determining Nb standard samples of alloys and steels [81 ], in lithium fluoride [78], and volcano dusts [32].

33.4.2. Separation and determination of Ta

The pyrogallol method has been used for determining tantalum in ores and minerals [130], steels [ 131 ], niobium [ 132], zirconium alloys [ 133], and beryllium and its oxide [ 134].

Tantalum has been determined using Methyl Violet in concentrates [135]. Larger amounts of tantalum were determined by differential spectrophotometry [ 136]. Other basic dyes were used in determination of tantalum in ores [94], niobium and its compounds [97], and uranium and zirconium [96].

Tantalum was determined with the use of PAR in ores [110], steels [108], alloys [22], and compounds of rare earth elements [ 109].

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Chapter 34. Nitrogen

Nitrogen (N, at. mass 14.01) is a chemically inert gas (N2) at room temperature. It occurs in its compounds in the following oxidation states: -III in ammonia, -II in hydrazine, -I in hydroxylamine, III in nitrite, and V in nitrate. From other elements it is usually separated as ammonia.

34.1. Separation of nitrogen as ammonia

The separation of small amounts of nitrogen as ammonia consists in distillation from alkaline medium [1-4]. Ammonia escapes quantitatively when a strong base is added in excess and the solution is subsequently heated. The ammonia and steam driven off are condensed and collected in dilute acid. To prevent the liquid from being superheated, porous porcelain chunks are added to the still. Ammonia has also been separated by vacuum distillation and by isothermal diffusion.

Nitrate and nitrite nitrogen are reduced to ammonia which is then distilled off [5]. Powdered Devarda's alloy (50% Cu, 45% A1, 5% Zn) in a cold alkaline medium is used as the reducing agent. The reaction is carried out in the same flask from which ammonia is afterwards distilled. The flask is cooled with water if the reduction is too rapid.

In Kjeldahl 's method, organic nitrogen is converted into ammonia which is separated by distillation. The method entails boiling a sample with concentrated H2804 in the presence of a catalyst (Hg, Cu, or Se) [1,6]. If the nitrogen is present in a group which contains oxygen, a reduction step is also necessary. Amine- and imine nitrogen forms are converted into ammonia, which is retained in the solution as the ammonium ion and can be titrated with hydrochloric acid. The method is used for determination of nitrogen in organic compounds [7,8]. Nitrogen has also been determined in an automatic system [9]. The results of determining nitrogen in water by the FIA technique and by the classic method have been compared [10]. Optimum conditions for sample decomposition by microwaves, before the separation of nitrogen by the Kjeldahl method, have been studied [ 11 ].

34.2. Methods for ammonia determination

Two methods of similar sensitivity are commonly used for determining ammonia. These are the classical Nessler method and the indophenol method. The indophenol method is more convenient since the blue reaction product, which is the basis for the spectrophotometric measurement, is soluble in water.

34.2.1. Indophenol method

The reaction of ammonia with hypochlorite and phenol in an alkaline medium (Berthelot's reaction) yields a blue product which is the basis of a sensitive and specific spectrophotometric method for determining nitrogen as ammonia [12-19]. The probable mechanism of the reaction is:

34.3. Methods for nitrite determinat ion 305

'•• CIO-+NHa OH ~ 0 N--CI + @ O H .-~

0,...._2_ H §

(34.1)

Chloramine, which is formed as the first step of the reaction, reacts with phenol to yield quinonechloramine. This reacts with another phenol molecule to give indophenol. The blue colour is due to the indophenol anion, formed in alkaline medium. The intensity of the blue colour is greatly increased by adding a little acetone (-0.2 ml of acetone per 25 ml of solution). The molar absorptivity a t )~max "- 625 nm is 4.5.103 (a = 0.32).

The indophenol can be extracted with isobutyl- or isoamyl alcohol after adding a considerable amount of sodium chloride to the aqueous solution as a salting-out agent. The organic extract is, however, less intensely coloured than a corresponding solution in aqueous acetone [20]. The reaction is similar if thymol is used instead of phenol [21,22].

Ammonia is usually determined after being separated from a strongly alkaline medium and then absorbed in dilute H2SO4 or HC1. It is sometimes possible to carry out the indophenol reaction without separating the ammonia (e.g., in natural waters). In the presence of EDTA, moderate quantities (0.1-0.5 mg) of Ca, Mg, and A1 do not interfere. The addition of tartrate prevents the precipitation of hydrolysable metals. Phosphate interferes in the colour reaction [23].

Since the sensitivity of the method is often limited by the high blank values caused by the presence of traces of ammonia in the reagents, purification of the reagents by distilling ammonia from their alkaline solutions may be necessary.

Reagents

Phenol-acetone solution. Dissolve 70 g of phenol in 15 ml of ethanol, add 20 ml of acetone, and dilute the solution with ethanol to 100 ml.

Sodium phenolate solution. Immediately before use, mix 10 ml of the phenol-acetone solution with 10 ml of 30% aqueous NaOH solution, and dilute with water to 50 ml.

Sodium hypochlorite, 2% solution. (Check the content of C10- in the solution iodometrically).

Standard ammonia solution: 1 mg NH3/ml. Dissolve 3.1410 g of ammonium chloride (previously dried at --100~ in water, and dilute the solution with water to 1 litre in a volumetric flask.

Standard ammonia solution: 1 mg N/ml. Prepare as above but with 3.8190 g of dried NH4C1.

Sodium hydroxide: 30% aqueous solution. Boil the solution for 10-15 min in an open vessel to remove traces of ammonia.

Procedure

Distillation of NH3. Place the sample solution containing ammonia in a 70-150 ml still. Immerse the condenser outlet in a receiver containing 5 ml of water and 5 drops of 0.1 M

306 34. Nitrogen

H2SO4. Pour 10-20 ml of 30% NaOH into the still, and dilute with water to 40-80 ml. Add a few fragments of porous porcelain to promote regular ebullition, and distil a quarter of the liquid volume from the still into the receiver.

Determination of NH3. Place all or part of the distillate, containing not more than 40 gg of NH3, in a 25-ml standard flask. Add 2.5 ml of the sodium phenolate solution and 1 ml of the sodium hypochlorite solution. Dilute the solution to the mark with water, and mix thoroughly. After 30 min, measure the absorbance at 625 nm against water (or a reagent blank solution when traces of ammonia are being determined).

Note. For determination of traces of ammonia, the sample solution should be made slightly acidic with 0.5 ml of 0.1 M H2S04, and concentrated by evaporation, before the distillation.

34.2.2. Nessler's method

In 1856, Nessler recommended an alkaline solution of mercury(II) iodide and potassium iodide as a reagent for the determination of ammonia [24]. Nessler's reagent reacts with ammonia in an alkaline medium, to give a brown-orange, sparingly soluble product, according to the following equation:

2HgI42- + NH3 + 3OH- ---> NH2Hg2IO$ + 7I- + 2H20

Since the product forms a stable dispersion only at very low concentrations, protective colloids such as gum arabic, gelatine, or poly(vinyl alcohol) are added.

The molar absorptivity at e~max = 370 nm is 6.8.103; at 400 nm, it is 5.1.103 (specific absorptivity 0.36).

The determination of ammonia is usually preceded by a distillation from strongly alkaline solution. Nessler's method is commonly used for determining ammonia in natural waters. Since Ca and Mg present in water interfere, they are masked with tartrate.

Reagents

Nessler's reagent. Dissolve 2.5 g of KI in l0 ml of water. Add saturated HgC12 solution until a permanent precipitate forms. Add 25 ml of 30% NaOH solution, dilute to 200 ml with water, and mix well. Decant the clear solution from the precipitate, and store the solution in an amber-glass bottle.

Standard ammonia solution. Preparation as in Section 34.2.1. Potassium sodium tartrate (Seignette salt), 20% solution. Remove traces of ammonia by

making the solution alkaline with sodium hydroxide, and boiling for 10-15 min.

Procedure

Place the clear, colourless, neutral solution, containing not more than 50 ~tg of NH3, in a 25- ml standard flask, add 1 ml of the tartrate solution, 1 ml of 1% gum arabic solution, and 1 ml of Nessler's reagent, and dilute the solution to the mark with water. After 10 min, measure the absorbance of the pseudo-solution at 400 nm, using a reagent blank solution as reference.

34.3. Methods for nitrite determination 307


There are methods, similar to the indophenol method, in which coloured products of ammonia reaction are formed in alkaline media with the use of salicylic and dichlorocyanuric acids [25], phenol, hypochlorite and nitroprusside [26], or salicylic acid, hypochlorite, and nitroprusside (e = 1.5.104 at 698 nm) [27,28]. The last method has been used for determining ammonia in soils [29].

Ammonia may also be oxidized to nitrite, which is then determined by a suitable method [30].


The indophenol method has been applied for determination of nitrogen (as ammonia) in biological materials [31,32], plant materials [33,34], foods [1,2,35], air [36], boiler water [37], and other waters [38-40], organic substances [17,41], refractory alloys [42], tantalum alloys [43], vanadium, titanium, and uranium [21], alkali and alkaline earth metals [22].

The indophenol method has been applied also in a fully automated version (including distillation of ammonia) [2,3,13,44] and with the FIA technique [45,46]. The two versions were applied for determining nitrogen in soil extracts [13,47], plants [45], natural waters [3,44,46,48], and crude oil [49].

Nessler's method has been used for the determination of nitrogen (as ammonia) in biological materials [32], plant materials [50], air [51], waters [39,52,53], fodder [54], and tungsten [55].

Nitrogen has been determined in soils and plants by the FIA technique with the use of salicylic acid, nitroprusside, and dichloroisocyanurate [56]. Nessler's method has been applied in the FIA technique [50,53,57].

34.3. Methods for nitrite determination

The classical Griess method for nitrite determination is very well known. However, some of the more recent methods which are also based upon the formation of azo dyes may be better. A critical review of the nitrite determination methods has been published [58].

34.3.1. Modified Griess method

In an acid medium, nitrite reacts with primary aromatic amines to form a diazonium salt. The salt is then coupled with a suitable aromatic compound containing an-NH2 or -OH group to yield an azo dye which is the basis of the spectrophotometric method [59,60].

In the Griess method (1879), nitrite, sulphanilic acid, and 1-naphthylamine are reacted as follows:

H O 3 S - - ~ N H z "•+ + NOz + 2H + ~ HO~S N~N + 2Hz0

~ NH z _• ~ H+(34"2) HO3S N"'~--~ N NHz +

308 34. Nitrogen

The Griess method is highly sensitive. The molar absorptivity at )~max = 520 nm is 4.0-104, specific absorptivity 2.8). The method is specific, but only of moderate precision.

The solution in which nitrite is determined must not contain oxidants, reductants, or coloured substances. Neither should urea or aliphatic amines be present since they may react with nitrite to liberate free nitrogen. Copper(II) ions catalyse the decomposition of the diazonium salt, thereby causing low results.

Since 1-naphthylamine is a well-known carcinogen [61], it is recommended to replace it by 1-naphthylamine-7-sulphonic acid. This reagent is used in the procedure given below.

Reagents

Sulphanilic acid solution. Dissolve 0.50 g of sulphanilic acid in 120 ml of water and 30 ml of glacial acetic acid. Store in a brown bottle.

1-Naphthylamine-7-sulphonic acid solution. Dissolve 0.50 g of the reagent in 120 ml of hot water. Filter, cool, and add 30 ml of glacial acetic acid. Store in a brown bottle.

Standard nitrite solution: 1 mg NOz/ml. Dissolve 1.5000 g of anhydrous NaNO2 in water, add 1 ml of CHC13 to stabilize the solution, and 0.2 g of NaOH, and dilute with water to 1 litre in a volumetric flask.

Procedure

To the neutral solution (-~ 15 ml), containing not more than 20 btg of nitrite (NO2-), add 1.0 ml of the sulphanilic acid solution, mix, and allow to stand for 10 min. Add 1.0 ml of the 1- naphthylamine-7-sulphonic acid solution, dilute to the mark in a 25-ml standard flask, and mix. After 20 min, measure the absorbance at 520 nm, using a reagent blank solution as reference.


A number of other organic compounds is suitable for the diazotization and coupling reactions [62-69]. For example, the reaction with o-nitroaniline and N-(1- naphthyl)ethylenediamine leads to a method with e = 6.0.104 at 545 nm [68]. In some cases, 8-hydroxyquinoline is used for the coupling [70,71]. Many azo dyes formed by the use of various reagents are extractable [72-74]. In one of the methods [74], with hexanol as extractant, e = 5.2.104 at 610 nm.

Some authors [75-77] have proposed using flow-injection analysis for determining nitrite, through the formation of azo dyes.

Other reagents used for determining nitrites have been reviewed [78-80].


The Griess method has been applied widely for determining nitrite in foods and waters. The method has also been used for automatic determination of nitrite (and NO3- after reduction to NO2-) in waters and in soil extracts [81], in living cell fluids [82], and in blood [83].

The methods based on azo dyes have found application in determinations of nitrite in soil extracts [67,68], blood [73,84], waters [63,65,77], waste-water effluents [84a], and fruits [84b].

34.4. Methods for nitrate determination 309

34.4. Methods for nitrate determination

Most spectrophotometric methods for determining nitrate are based either on: 1), nitration or oxidation of appropriate organic reagents to form coloured compounds or on, 2), reduction of NO3- to NO2- or NH3 with subsequent determination of this species. Prevalent among the methods belonging to the first group are those using phenoldisulphonic acid, and xylenols.

34.4.1. Phenoldisulphonic acid method

Reaction between 1-phenol-2,4-disulphonic acid (1-hydroxybenzene-2,4-disulphonic acid) and HNO3 occurs when a dry sample (or dry residue from the evaporation of the sample solution) containing nitrate is mixed with a solution of the reagent in concentrated ti2SO4. The reaction product, nitrophenoldisulphonic acid, is pale yellow, but when the solution is made alkaline, the intensely coloured anion which is the basis of this spectrophotometric method is formed [85].

The reaction is specific for nitrate. The absorption maximum of the nitrophenoldisulphonic acid is at 410 nm. The molar absorptivity is 9.4.103 (a = 0~ Neither the nature nor the excess of the reagent used to raise the pH (NH3, NaOH, KOH) affects the colour.

Chloride causes low results owing to the reaction between HCI and HNO3 when the phenoldisulphonic acid in concentrated sulphuric acid is added to the sample (3C1-~ NO3 ~ + 4H+ --~ C12 + NOCI + 2H20). This effect is, however, negligible if the amount of chloride present is less than twice that of nitrate. Larger amounts of chloride should be separated beforehand by precipitation as AgCI. Since the presence of silver ions in the solution after removal of AgC1 is detrimental, the excess silver is precipitated with sodium phosphate. If the concentration of chloride in the solution is known exactly, it is better to use a stoichiometric (or slightly lesser) quantity of silver sulphate as precipitant.

Nitrite interferes, since it may be partially converted into nitrate under the conditions of the determination. When the concentration of nitrite is not higher than that of nitrate, the effect is negligible. Large quantities of nitrite must be removed, e.g., by reduction with sodium azide, urea, or hydrazine. Ammonium ions cause low results when nitrate is determined. Preliminary expulsion of ammonia by heating the solution after it has been made alkaline with sodium hydroxide is recommended.

EDTA is added before the evaporation when larger quantities of Ca and Mg are present, thus preventing precipitation when the solution is finally made alkaline. To prevent loss of HNO3 during evaporation to dryness, the solution is neutralized with NaOH or CaCOs.

Reagents

Phenoldisulphonic acid, solution in conc. H2SO4. Dissolve 12.5 g of phenol in 75 ml of conc. H2SO4, add 37.5 ml of 13% oleum, and stir well. Heat the solution in a 250-ml conical flask for 2 hr on a boiling water-bath with occasional stirring.

Standard nitrate solution: 1 mg NO3-/ml. Dissolve in water 1.6310 g of KNO3 (previously dried at 110~ and dilute the solution with water to 1 litre in a volumetric flask.

Silver sulphate solution. Dissolve in water 1.10 mg of Ag2SO4 and dilute the solution with water to 250 ml in a volumetric flask. One ml of solution is equivalent to 1 mg of CI-.

Calcium carbonate suspension. Mix 1 g of CaCO3 with 100 ml of water.

310 34. Nitrogen

Procedure

Neutralize the solution (containing not more than 100 gg of NO3-) in a small evaporating dish, add 1 ml of the CaCO3 suspension, and evaporate to dryness on a water-bath. Treat the cooled residue with 1 ml of phenoldisulphonic acid reagent and stir well. After 5 min, dilute with 10 ml of water, and transfer the solution quantitatively to a 25-ml standard flask. Add conc. ammonia solution until the solution becomes intensely yellow, then add 5 ml more, and dilute the solution with water to the mark. Measure the absorbance of the yellow solution at 410 nm against water or a reagent blank solution.

Notes. 1) If, after being made alkaline, the solution becomes turbid or a precipitate is formed, the solution should be filtered before the absorbance is measured.

2) If the amount of C1- present exceeds twice that of NO3-, the former should be separated by adding a slightly less than stoichiometric quantity of AgzSO4 to the acid solution. After 30 rain, the precipitated AgC1 is filtered off, and the filter paper is washed with a small volume of dilute NazSO4 solution. The combined filtrate and washings are neutralized, and analysed for nitrate as described above.


The following dimethylphenols yield coloured nitration products in sulphuric acid media: 2,6-xylenol (e = 7.9.103 at 330 nm) [86], 2,4-xylenol [87,88], and 3,4-xylenol [89,90]. Other organic reagents nitrated include: phenol [91,92], 2-butylphenol [93], 4,5-dihydroxy- coumarin [94], 2,7-diaminofluorene [95], and resorcinol [96].

Some sensitive spectrophotometric methods for determining nitrate utilize extractable ion-associates of the nitrate ion with the basic dyes: Crystal Violet (chlorobenzene, pH 6) [97], Nile Blue A [98], and Methylene Blue (1,2-dichloroethane) [99]. Nitrogen has been determined also by the FIA technique with the use of Malachite Green [ 100].

Nitrate is often determined, after reduction to nitrite by cadmium [64,76,101-103], hydrazine [104], or titanium trichloride [105] as an azo dye. The flow-injection technique has been often applied [75,76,104,106,107].

Nitrate can also be determined after reduction to ammonia [5].


Nitrate has been determined, after the reduction to nitrite, in soil extracts [81,103], waters [48,64,81,101,103-105,107], and plants [106].

Nitrite has been determined in soil extracts also after the reduction to ammonia [28]. The reactions of nitration have been applied in the determination of nitrate in waters, plants, vegetables, and soil extracts [89,91 ].

Nitrate has been determined by other methods in waters [86,90] and vegetables [93].

34.5. Determination of other nitrogen compounds

Hydrazine (NH2.NH2) can be determined with the use of 2-hydroxy-l-naphthaldehyde (e = 2.7-104 at 412 nm) [108], and pyridylpyridinium chloride [109]. A yellow Schiff's base is obtained in condensation of hydrazine with vanillin in acid media (e = 5.3-10 4 at 400 nm) [110]. The reduction of Ag + by hydrazine yields a red sol of silver (~ = 4.2.10 4 at 415 nm)

34.5. Determination of other nitrogen compound 311

[111]. In an indirect method hydrazine is determined with the use of Fe(III) and ferrozine [112].

Hydroxylamine (NH2OH) can be determined by its colour reaction with di(2-pyridyl) ketone guanylhydrazone [113]. It can also be determined in an indirect method by its reaction with biacetyl to give dimethylglyoxime, which then reacts with Ni(II) [114]. The FIA technique has been applied in the determination of NH2OH with the use of ferrozine [115]. A review has been given of methods used for the determination of hydrazine [116].

Nitrogen dioxide absorbed in alkali can react to form azo dyes [117-120]. These diazotization reactions are suitable for continuous automatic monitoring of atmospheric NO 2 [117]. Oxides of nitrogen in cigarette smoke have been determined with brucine [121]. The amount of NO in liquid N204 (0~ can be determined from the green colour of the N203 [122]. NO2 in the atmospheric air has been absorbed in Cu(DDTC)2 in toluene and determined from the decrease in the initial absorbance at 437 nm [123]. In a proposed indirect method, NO reduces Cu(II) to Cu(I), which is then determined with the use of cuproine [ 124].

Azide (N3-) forms a coloured complex with Cu(II), which has ~max at 375 nm [125]. Azide reduces Ce(1V) to Ce(III) which gives a coloured complex with Arsenazo III [126]. Azide can also by determined from the decrease of yellow colour of Ce(IV) which is reduced by azide [127].

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74. Baveja A.K., Nair J., Gupta V.K., Analyst, 106, 955 (1981). 75. Anderson L., Anal. Chim. Acta, 110, 123 (1979). 76. Gin6 M.F. et al.,Anal. Chim. Acta, 114, 191 (1980). 77. Nakashima S. et al., Anal. Chim. Acta, 155, 263 (1983). 78. Raman V., Dabras M.S., Microchem. J., 40, 242 (1989). 79. Nikonorov V.V., Moskvin L.N., Zh. Anal. Khim., 51, 737 (1996). 80. Kawakami T., Igarashi S., Anal. Chim. Acta, 354, 159 (1997). 81. Henriksen A., Selmer-Olsen A.R., Analyst, 95, 514 (1970). 82. Boudard F., Vallot N., Cabaner C., Bastide M., J. Immunol. Methods, 174, 259 (1994). 83. Litchfield M.H., Analyst, 92, 132 (1967). 84. Shechter H., Gruener N., Shuval H.J., Anal. Chim. Acta, 60, 93 (1972). 84a. Staden F.J., Merwe T.A., Microchim. Acta, 129, 35 (1998). 84b. Wang G.F., Satake M., Horita K., Talanta, 46, 671 (1998). 85. Mubarak A., Howald R.A., WoodriffR.,Anal. Chem., 49, 857 (1977). 86. Andrews D.W., Analyst, 89, 730 (1964). 87. Norwitz G., Keliher P.N., Anal. Chim. Acta, 98, 323 (1978); 109, 373 (1979). 88. Norwitz G., Farino J., Keliher P.N., Anal. Chim. Acta, 105, 335 (1979). 89. Elton-Bott R.R., Anal. Chim. Acta, 90, 215 (1977). 90. Osibanjo O., Ajayi S.O., Analyst, 105, 908 (1980). 91. Elton-Bott R.R.,Anal. Chim. Acta, 108, 285 (1979). 92. Velghe N., Claeys A.,Analyst, 108, 1018 (1983). 93. Tanaka A., Nose N., Iwasaki H., Analyst, 107, 190 (1982). 94. Nakamura M., Analyst, 106, 483 (1981); Mikrochim. Acta, 1983 II, 69. 95. Hainberger L., Nozaki J., Mikrochim. Acta, 1979 I, 75; 1979 II, 187. 96. Velgh N., Claeys A.,Analyst, 110, 313 (1985). 97. Baca P., Freiser H., Anal. Chem., 49, 2249 (1977). 98. Pokorny G., Likussar W., Anal. Chim. Acta, 42, 253 (1968). 99. Ciesielski H., Soignet G. et al., Analusis, 6, 38 (1978). 100. Aoyagi M., Yasumasa Y., Nishida A.,Anal. Sci., 5, 235 (1988). 101. Davison W., Woof C.,Analyst, 104, 385 (1979). 102. Elliott R.J., Porter A.G.,Analyst, 96, 522 (1971). 103. Pandurangappa M., Balasubramanian N., Mikrochim. Acta, 124, 137 (1996). 104. Madsen B.C.,AnaL Chim. Acta, 124, 437 (1981). 105. A1-Wehaid A., Townshend A.,Anal. Chim. Acta, 186, 289 (1986). 106. Gin6 M.F. et al., Anal. Chim. Acta, 155, 131 (1983). 107. Nakashima S. et al., Z. Anal. Chem., 319, 506 (1984). 108. Mafies J., Campillos P., Font G., Martre H., Prognon P., Analyst, 112, 1183 (1987). 109. Asmus E., Ganzke J., Schwarz W., Z. Anal. Chem., 253, 102 (1971). 110. Amlathe S., Gupta V.K., Analyst, 113, 1481 (1988). 111. Pal T., Maity D.S., Ganguly A., Analyst, 111, 1413 (1986). 112. Dias F., Olojola A.S., Jaselskis B., Talanta, 26, 47 (1979). 113. Kavlentis E., Microchem. J., 37, 22 (1988). 114. Pittwell L.R., Mikrochim. Acta, 1975 II, 425. 115. Bourke G.C., Stedman G., Wade A.P.,Anal. Chim. Acta, 153, 277 (1983). 116. Kolasa T., Wardencki W., Talanta, 21, 845 (1974). 117. H~intzsch S., Nietruch F., Prescher K.E., Mikrochim. Acta, 1969, 550. 118. Fisher G.E., Becknell D.E. Anal. Chem., 44, 863 (1972). 119. Bultez A.,Analusis, 2, 190 (1973). 120. Ishii K., Aoki K., Anal. Chem., 55, 604 (1983).

314 34. Nitrogen

121. Smith G.A., Sullivan P.J., Irvine W.J., Analyst, 92, 456 (1967). 122. Wright C.M., Orr A.A., Bailing W.J.,Anal. Chem., 40, 29 (1968). 123. Zhelyazkova B.G., Yardev P.B., Yordanov N.D., Talanta, 30, 185 (1983). 124. Kinoshita S., Wakita H., Masuda I., Anal. Chim. Acta, 169, 373 (1985). 125. Neves E.A., Oliveira E., Sant'Agostino L., Anal. Chim. Acta, 87, 243 (1976). 126. Kubaszewski E., Kurzawa Z., Chem. Anal. (Warsaw), 30, 609 (1985). 127. Terpinski E.A., Analyst, 110, 1403 (1985).

Chapter 35. Oxygen

Oxygen (O, at. mass 16.00) is a gas, 0 2 ( 0 3 in ozone). It occurs in most compounds in the -II oxidation state, and in the -I state in peroxides. With other elements, it forms numerous

�9 2- 2- 2+ 2+ oxide complexes such as CrO4 , M o O 4 , V O , U O 2 , 8042- , NO3-. Volatile oxides include OsO4 and CO2. Oxygen compounds of great importance in analysis are hydrogen peroxide and the peroxide complexes of a number of metals, e.g., Ti, V, Nb, U, and Zr.

35.1. Determination of oxygen

The well-known titrimetric method for determining oxygen in water is based on oxidizing the Mn(II) in Mn(OH)2 with oxygen. After the addition of KI and acidification of the solution with H2SO4, an equivalent amount of iodine is liberated. In the spectrophotometric modification of the method the iodine is determined either as a blue complex with starch, or after the iodine has been extracted into chloroform.

Oxygen impurities (0.1-0.0001%) in various gases can be determined by the colour reaction with anthraquinone-2-sulphonate in alkaline solution. The red solution of the reagent (reduced with zinc amalgam) is decolorized when oxidized by oxygen. This reaction is suitable for the continuous spectrophotometric determination of oxygen [ 1 ].

The colour reaction with Indigo Carmine is useful for determining oxygen dissolved in water [2-4]. The yellow reduced form of the reagent (leucobase) turns red 0~max = 555 nm) under the influence of oxygen. Leuco forms of other dyes have also been proposed for determining oxygen. These include Methylene Blue [4-7], Berbelin Blue (formula 35.1) [8], Methyl Red [9], and Safranine T [4].

~ eHs ~6Hs I +

, ( 3 5 . 1 ) if" "}'Oz = 4" OH-

H3C" ~ "N- v -CH z H~C" v -'Nr, -" ~- " "C 3 I

H

In other methods for determining oxygen in water, 3,3-dimethylnaphthidine [10], DCTA [+ Mn(II)] [11,12], and EDTA [+ Mn(II)] [13] have been used. The determination of oxygen in water has also been based on the reduction of absorbance by a sol of gold in the presence of cyanide (formation of a colourless complex Au(CN)2- ) [14] and a silver cyanide complex [15]. The oxidation of Fe(II) to Fe(III) [determination of Fe(II) with 1,10- phenanthroline] has been used for determining oxygen in semiconducting oxides [16]. The colour reduction of the blue Fe(II) complex with Cacotheline Blue has also been used for the determination of oxygen dissolved in water [17].

35.2. Determination of ozone

Since ozone is a stronger oxidant than oxygen, it is determined on the basis of colour redox reactions with reagents which are not oxidized by oxygen.

316 35. Oxygen

The iodine liberated by ozone from slightly acidic iodide solutions can be determined by the iodine-starch method [18]. This method has been used for determining ozone in ozonised air [18a]. Organic reagents which have been used for spectrophotometric determination of ozone include 1,1-diphenylethylene [19,20], 1,2-di-(4-pyridyl)ethylene [21], indigo disulphonate (c = 2.2.104) [22], and indigo trisulphonate [23].

Determination of ozone in air is based on its reaction with bis(terpyridyl)iron(II) [24]. In another method ozone oxidizes Mn(II) in phosphoric acid medium to Mn(III), which is then made to react with o-tolidine [25]~

The data on ozone determination methods and continuous automatic determination of Os have been collected [26].

35.3. Deternfination of hydrogen peroxide

Spectrophotometric methods for determining hydrogen peroxide are based on its capacity to form stable peroxide complexes as well as on its oxidizing and reducing properties.

A widely known, but relatively insensitive, method is the t i tanium method [27-29], based on the orange-yellow titanium peroxide complex formed in acid ([-12SO4) medium. The titanium peroxide- 8-hydroxyquinolinate method is more sensitive (c = 3~ :~ at 45(1i nm) [30]. Mixed complexes of Ti with H202 and oxalate [31], Xylenol Orange, or Chrome Azurol S [32] also provide a basis for determining hydrogen peroxide. The Xylenol ()range complex with V(V) is discoloured in the presence of HeO2 [33,134].

Some sensitive methods for determining H202 in aqueous media are based on the reactions of H202 with 4-aminoantipyrine and phenol, or with N-ethyl-N-(sulpho~ propyl)aniline and 4-aminoantipyrene [35]. The FIA technique has also been applied [35].

Hydrogen peroxide can reduce Cu(II) in the presence of Neocuproin in excess. The resulting coioured complex of Neocuproin with Cu(I) is equivalent to the amount of tteO~ present [36]~ From an iodide solution, containing some Mo(VI) as a catalyst, He0::, iil~m'ates an equivalent amount ot iodine, which is then determined [37]. A sensitive method for determining hydrogen peroxide depends on the oxidation of the colourless leucobase of phenolphthalein in alkaline medium containing copper(II), to form the familiar red colour [37,38]. The alkaline phenolphthalein solution is converted into the leucobase by heating with zinc dust. Oxo-peroxo-pyridine-2,6-dicarboxylato-vanadate(V) complex is used for detmmining hydrogen peroxide in rainwater [39].

Hydrogen peroxide can also be determined [40] from the decrease in absorbance (al 418 nm) of an alkaline i>rricyanide solution as a result of the reaction;

2Fe(CN)e, > + |t202 4- 2OH- ~ 2Fe(CN)64- + 2H20 + ()2

In a similar method, [-1202 decolorizes (reduces) a green alkaline solution of manganate (mnO4 2- ) [41 ].

References

1. Waclawik J., Waszak S., Chem. Anal. (Warsaw), 4, 343 (1959); 8, 633 (1963). 2. Meyling A.[t., Frank G.H., Analyst, 87, 60 (1962). 3. John P.A., Winefordner J.D., Silver W.S., Anal. Chim. Acta, 30, 49 (1964). 4. Hamlin P.A., Lambert J.L., Anal. Chem., 43, 618 (1971). 5. Goodfellow G.|., Webber H.M., Analyst, 104, I 105, 1119 (1979).

References 317

6. Kuznetsov V.V., Murasheva M.V., Zh. Anal. Khim., 51, 1087 (1996). 7. Martinez S.A., Rios A., Valcarcel M.,Anal. Chim. Acta, 284, 189 (1993). 8. Altmann H.J., Z. Anal. Chem., 262, 97 (1972). 9. Turanov A.N., Zavod. Lab., 56, No 8, 41 (1990). 10. Fadrus H., Mal~ J.,Analyst, 96, 591 (1971). 11. Sastry G.S., Hamm R.E., Pool K.H., Anal. Chem., 41, 857 (1969). 12. Malaiyandi M., Sastri V.S., Talanta, 30, 983 (1983). 13. Rahim S.A., Mohamed S.H., Talanta, 25, 519 (1978). 14. Pal T., Jana N.R., Das P.K., Analyst, 116, 321 (1991). 15. Pal T., Das P.K.,Analyst, 113, 1601 (1988). 16. Malkova Z. et al., Fresenius'J. Anal. Chem., 347, 478 (1993). 17. Murthy K.N., Rao L.N., Sarma D.R., Fresenius'J. Anal. Chem., 351, 586 (1995). 18. Cohen I.C., Smith A.F., Wood R.,Analyst, 93, 507 (1968). 18a. Machado E.L. et al.,Anal. Chim. Acta, 380, 93 (1999). 19. Collard R.S., Pryor W.A.,Anal. Chim. Acta, 108, 255 (1979). 20. Dechaux J.C., Analusis, 13, 30 (1985). 21. Hauser T.R., Bradley D.W.,Anal. Chem., 38, 1529 (1966); 39, 1184 (1967). 22. Bergshoeff G. et al., Analyst, 109, 1165 (1984). 23. Straka M.R., Pacey G.E., Gordon G.,Anal. Chem., 56, 1973 (1984). 24. Tomiyasu H., Gordon G.,Anal. Chem., 56, 752 (1984). 25. Hofmann P., Stern P.,Anal. Chim. Acta, 45, 149 (1969); 47, 113 (1969). 26. Lachowicz E., R62afiska B., Chem. Anal. (Warsaw), 32, 433 (1987). 27. Wolfe W.C., Anal. Chem., 34, 1328 (1962). 28. Csanyi L.J., Anal. Chem., 42, 680 (1970). 29. Clapp P.A., Evans D.F., Sheriff T.S., Anal. Chim. Acta, 218, 331 (1989). 30. Cohen I.R., Purcell T.C., Anal. Chem., 39, 131 (1967). 31. Sellers R.M., Analyst, 105, 950 (1980). 32. Matsubara C., Takamura K., Microchem. J., 22, 505 (1977). 33. Matsubara C., Takamura K., Microchem. J., 24, 341 (1979). 34. Csanyi L.J., Microchem. J., 26, 10 (1981). 35. Johnson K.S. et al.,Anal. Chim. Acta, 201, 83 (1987). 36. Baga A.N. et al., Anal. Chim. Acta, 204, 349 (1988). 37. Frew J.E., Jones P., Scholes G.,Anal. Chim. Acta, 155, 139 (1983). 38. Dukes E.K., Hyder M.L., Anal. Chem., 36, 1689 (1964). 39. Tanner P.A.,Wong A.Y.,Anal. Chim. Acta, 370, 279 (1998). 40. Aziz F., Mirza G.A., Talanta, 11, 889 (1964). 41. Bubyreva N.S. et al., Zavod. Lab., 35, 1044 (1969).

Chapter 36. Palladium

Palladium (Pd, at. mass 106.42) is a platinum-group metal, and occurs in the II and IV oxidation states. Palladium(II) compounds are the more stable. Unlike the other platinum metals, palladium is soluble in conc. HNO3. Brown-red Pd(OH)2 precipitates at pH ~-4, but dissolves in excess of an alkali-metal hydroxide. Palladium(U) gives stable nitrite, ammine, cyanide, chloride, bromide, and iodide complexes. Palladium(II)- and (IV) are reduced to the metal by SO2, Fe(II), and ethanol.


Methods for separating the platinum metals (including palladium) are discussed in the Chapter 38 on platinum.

36.1.1. Extraction

Palladium(II) dioximates are specifically extracted from dilute acid with chloroform [1,2]. Dithizone can be used to separate Pd from Pt [3,4].

Extraction of the iodide [5,6]-, bromide [7,8]-, chloride [6,8-10]-, and thiocyanate [11] complexes of palladium gives convenient separations from a number of metals. Chloropyridine [5], di-n-octyl sulphide [6], TBP [7], and TOPO [8] have been used in these extractions.


The precipitation of palladium dimethylglyoximate from an acid medium is an excellent separation method. In the separation of microgram quantities of Pd, nickel has been used as a collector [12]. The optimum pH for the precipitation is 6.5 (acetate medium). If Au(III) is separated beforehand by reduction with oxalic acid, and if copper is masked with EDTA, the separation of traces of palladium (with Ni as collector) is specific.

Palladium metal can be co-precipitated with tellurium (SnCI2 is used as the reducing agent) [ 13]. Traces of palladium can also be precipitated as the sulphide with Pb as collector, or as the hydroxide with Fe(III) as collector. Traces of palladium have been separated with AgCN as collector [ 14,15].


Ion-exchange separation methods are based on the retention of the palladium chloride complex on strongly basic anion-exchangers, and of the cationic ammine complex on cation-exchangers [ 16]. A cellulose anion-exchanger has been used to separate Pd from Pt and Ir [17], and from Ir [16].

Traces of Pd have been preconcentrated from water with silica gel impregnated with thionalide [ 18], PAN [ 19,20], or 2- [2-(triethoxysilyl)ethylthio] aniline [21 ].

Like other platinum metals, Pd is separated from ores and concentrates by fire assay and eupellation methods with lead, tin, copper; iron, nickel, and copper alloys [22,23], or


nickel- or cuprous sulphides [24] as collectors. The cupellation methods are combined with ion-exchange [22,23].

36.2. Methods of determinat ion

A dithizone method, a very sensitive thio-Michler's ketone method, and a less sensitive iodide method are discussed below. A simple modification of the iodide method gives a very sensitive method involving the formation of the blue starch-iodine complex.

36.2.1. Iodide methods

In an acid medium (HC1, H2SO4) containing excess of iodide, palladium forms a brown-red complex, PdI42-, which provides the basis for a moderately sensitive spectrophotometric method of determining Pd [5,14,25,26]. The concentration of HC1 or H2SO4 (up to 10 M) does not affect the colour. A reductant (e.g., ascorbic acid) is added to reduce the iodine liberated by atmospheric oxygen. The molar absorptivity of the complex at )~max = 410 nm is 1.02.104 (a - 0.10). The PdI42- complex may be extracted as the ion-association complex with DAM [27].

In the presence of a small excess of iodide, Pd gives PdI2 which is sparingly soluble in acidic media. When shaken with benzene or DIPE, PdI2 passes into the organic phase; it has been the basis for a sensitive indirect determination method. The suspension of PdI2 in the organic solvent is stripped by dilute ammonia. The aqueous solution is acidified, then the iodide is oxidized by bromine to IO3-. This iodate reacts with added I- to liberate iodine, which is determined as its coloured complex with starch (see Section 25.2.1). One Pd atom in PdI2 is equivalent to twelve atoms of iodine.

In the above-given method, e - 2.2.105 (spec. Abs. 2.1) at 590 nm as ~,max of the blue starch--iodine complex.

The combination of the iodide method with the extractive separation of Pd as its dimethylglyoximate makes the determination specific for palladium.

Reagents

Potassium iodide, 20% and 0.01% solution (iodine-free), freshly prepared. Standard palladium solution: 1 mg/ml. Dissolve 0.1000 g of metallic palladium in aqua

regia. Evaporate the solution nearly to dryness, add 3 ml of conc. HC1, and evaporate to half volume. Dilute the solution with water to volume in a 100-ml standard flask.

Dimethylglyoxime (H2Dm), 1% solution in ethanol. Starch, 1% solution. Preparation as in p.25.2.1.

Procedure

Extractive separation of Pd. Acidify the sample solution with hydrochloric acid (to --0.2 M in HC1), add 2 ml of the HzDm solution and 5 ml of 0.1 M EDTA, mix well, and allow to stand for 10 min. Extract the Pd(HDm)2 with two portions of CHC13, shaking for 1 min. Wash the combined chloroform extracts with two portions of 0.2 M HC1, and evaporate the organic phase to dryness on a water-bath. Mineralize the residue by heating with a few drops of conc. H2SO4 and conc. HNO3. Expel the nitric acid, allow the residual solution to cool, dilute it with water, and heat it until it clears.

320 36. Palladium

Determination of Pd (as PdI42). To the sample solution containing not more than 0.2 mg of Pd, add 2 ml of HC1 (1 + 1), 5 ml of 20% KI solution, and 1 ml of 1% ascorbic acid solution. Dilute the solution to the mark with water in a 25-ml standard flask, and measure the absorbance at 410 nm against water. Determination of Pd (by the indirect starch-iodine method). To the sample solution containing not more than 10 gg of Pd, add 1 ml of 0 .01% KI solution and 2 ml of H2SO4 (1+3), and dilute with water to --15 ml. Shake the solution with two 10-ml portions of benzene for 1 min. Wash the benzene phase with two portions of 1 M H2SO4. Strip the palladium iodide from the benzene phase by shaking for 15 s with 10 ml of ammonia solution (1+9). Place the ammoniacal solution in a 25-ml standard flask, acidify with dil. H2SO4, add 2 drops of bromine water, mix, and after 1 min add 2 drops of 1% phenol solution in glacial acetic acid. After 1 min, add 1 ml of 0.5% KI solution and 1 ml of the starch. Dilute the solution with water to the mark, and measure the absorbance at 590 nm against a reagent blank solution or water.


When an acidic solution of Pd(II) is shaken with an excess of dithizone (H2Dz) (formula 1.1) in CC14 the grey-green dithizonate, Pd(HDz)2, soluble in CC14 and CHC13, is formed. With a deficiency of dithizone, the red dithizonate PdDz, readily soluble in CHC13, is formed [28]. The grey-green Pd(HDz)2 is useful for spectrophotometric determination of palladium [3,4,12,28]. It is resistant both to acids (e.g., 3 M H2SO4) and ammonia solutions (up to 3 M NH3). This enables free dithizone to be stripped from the CC14 phase with dilute ammonia.

Figure 38.1 shows the absorption spectrum of Pd(HDz)2 in CC14. The compound has two absorption maxima in the visible spectrum. The molar absorptivity is 3.55.10 4 at ~max - "

635 nm (a = 0.33). Unlike other metal-dithizone systems, for palladium there is only a slight difference

between the colours of the complex and the free reagent. Pd(HDz)2 is formed rather slowly. The extraction rate increases with increasing excess of dithizone and in the presence of SnC12 (in small amounts) or I-.

In acid medium, other noble metals [Au, Pt(II), Hg, Ag] and Cu also react with dithizone. Palladium and platinum can be determined successively with dithizone in SnC12 [3] or iodide [4] media. Silver is masked by chloride. Iodide masks interfering Au and Hg.

Traces of palladium may be isolated, before determination with dithizone, by co- precipitation with Ni(HDm)2 from an acetate medium (pH --6.5) [ 12]. Copper can be masked with EDTA, and Au(III) is removed after reduction to the element with oxalic acid.

The separated Pd(HDm)2 precipitate dissolves slowly in 6 M HC1. However, when a suspension of Pd(HDm)2 in 2 M HC1 is shaken with a solution of dithizone in CC14, Pd(HDz)2 is formed. For determination of traces of Pd in platinum, it is advisable to use an introductory isolation of Pd as PdI2 floated with benzene [4].

Simultaneous determination of Pd and Pt as dithizonates can be made with the use of 5 th order derivative spectrophotometry [29]

Reagents

Dithizone, 0.01% solution in CC14. Preparation as in Section 46.2.1. Standard palladium solution: 1 mg/ml. Preparation as in Section 36.2.1. Dimethylglyoxime (H2Dm), 1% solution in ethanol. Nickel solution: -- 1 mg/ml. Dissolve 0.67 g of (NH4)2Ni(SO4)2.6H20 in 100 ml of water


Procedure

Separation of Pd with a collector. To the sample solution (-~100 ml) in 0.1 M HC1, containing not more than 50 gg of Pd and heated to --80~ add a macerated filter paper and 1 ml of 5% oxalic acid solution. Keep the solution at -~80~ for 1 h, then allow it to cool. Filter off the precipitate of elemental gold and silver chloride together with the paper. To the filtrate add successively 2 mg of nickel (as its sulphate solution), 2 ml of 20% potassium sodium tartrate solution [to mask Fe(III), A1, Ti, etc.], 2 g of sodium acetate, 1 ml of 0.1 M EDTA, and 2 ml of the H2Dm solution (pH -~6.5). After 30 min, filter off the precipitate of nickel- and palladium dimethylglyoximates. Wash the precipitate from the filter paper into a beaker, add 1 ml of conc. HC1, and evaporate to 5-10 ml, depending on the quantity of Pd in the solution.

Determination of Pd. Shake the solution containing the Pd(HDm)2 suspension for 5 min with 5 ml of 0.01% dithizone solution (1 ml of this solution corresponds to 21 ~tg of Pd). Wash the organic extract by shaking with two portions of 2 M HC1, and strip the residual dithizone with dilute ammonia solution (1+50). Transfer the organic phase to a standard flask of suitable capacity, dilute to the mark with CC14, and measure the absorbance at 450 nm, using CC14 as the reference.


Palladium and thio-Michler's ketone (TMK) (formula 46.2) form red mixed ligand complexes Pd(TMK)2X2 (X = CI-, Br-, I-, or SCN-) on extraction with CHC13, in the presence of suitable halide ions in weakly acidic aqueous solution. The complexes [Pd(TMK)4(Sol)2] 2+ are formed when the extraction is done with mixtures of CHC13 with a polar solvent (Sol) such as ethanol or DMF, or with only a polar solvent, e.g., amyl alcohol. Similar complexes (Pd:TMK = 1:4) form in mixed organic-aqueous solutions containing 30-40% of DMF or 40-50% of ethanol.

The thio-Michler's ketone has been proposed for a very sensitive spectrophotometric determination of palladium [30,31]. Depending on the conditions, the value of e for the method is 1.105 - 3-105.

The use of the mixed medium (water-DMF) is recommended because the molar absorptivity is 3.0-105 (a = 2.8) at 530 nm. TMK in DMF solution has its maximum absorbance at 450 nm.

The optimal acidity corresponds to pH 3.0_+0.2. Only Au, Hg, and Pt interfere in determination of palladium. The reaction of TMK with Hg(II) is slower (about 2 h). Oxidants also interfere; the reagent behaves as a reductor. Chloride masks silver ions. EDTA can be used to prevent hydrolysis of some metal ions. Palladium can be separated, before its determination with TMK, by extraction of its bromo- complex with TOA in toluene [32].

Reagents

Thio-Michler's ketone (TMK), 0.001 M solution in DMF (28.5 mg of TMK in 100 ml). The solution is not very stable and should be kept in darkness.

Standard palladium solution: 1 mg/ml. Preparation as in Section 36.2.1. Acetate buffer, pH 3.0. Mix 50 g of CH3COONa.4H20 in 100 ml of water with 350 ml

of glacial acetic acid, adjust to pH 3.0, and dilute to 500 ml with water.

322 36. Palladium

Procedure

To a weakly acidic solution containing in l0 ml not more than 8 pg of Pd, add 2 ml of acetate buffer, 7 ml of DMF and 2.5 ml of TMK solution. Dilute to the mark with water in a 25-ml standard flask, and mix thoroughly. After 10 min, measure the absorbance at 530 nm against a reagent blank.


Many organic spectrophotometric reagents for Pd incorporate sulphur as a ligand atom. Apart from dithizone and thio-Michler's ketone, which have been discussed above, examples include the thiourea derivatives [33-36]: p-dimethylaminobenzylidene-rhodanine (rhodanine) (e = 4.9.104) [37] and its derivatives [38], thiodibenzoylmethane [39,40], and thiosemicarbazone derivatives [41-47].

Oxime reagents give highly selective extraction methods for determining palladium. The commonest are: ct-furildioxime (e = 2.25.104 at 380 nm, in CHC13) [1], dimethylglyoxime (HzDm) (e = 1.7.104 at 380 nm) [26], t~-benziloxime [48], and heptanone oxime [49,50].

Certain nitroso compounds are sensitive spectrophotometric reagents for Pd. Of particular value is p-nitrosodimethylaniline (formula 38.1), which reacts with palladium in the cold at pH 2-2.5 to form a red complex (e = 8.6.104 at 535 nm). The corresponding platinum complex is not formed unless the solution is heated. Further reagents of this group are p-nitrosodiphenylamine [13], nitrosodibenzylaniline [51], and 2-nitroso-5-diethylaminophenol [52].

Many azo compounds have been suggested as reagents for Pd, e.g., PAR (e = 1.8.104 at 510 nm) [53-58], PAR in the presence of Zephiramine [59] and in the presence of diphenylguanidine [60], p-Cl-phenylazo-R-acid [61], thiazolylazo derivatives [62,63], 5-Br- PADAP (e = 8.4.104) [64,65], other bromo- and chloro-pyridylazo derivatives [66-68], Sulphonitrophenol M (formula 33.1) [69,70], 5-phenylazo-8-aminoquinoline (extraction with MIBK, e = 7.9.104 at 620 nm) [71], sulphochlorophenol-azorhodanine (e = 1.2.105) [72,73], Arsenazo III [74,75], and Palladiazo (formula 36.1) (e = 5.7.104 at 640 nm) [76-78].

HO OH

(36.1)

A considerable increase in the sensitivity of methods for palladium determination has been found with triphenylmethane reagents in ternary systems that include cationic surfactants, e.g., Eriochrome Cyanine R with CP [79], Chromal Blue G with CTA (e = 1.0-105) [80], Chrome Azurol S-CTA (or -CP) [81], Eriochrome Azurol B-CTA (e = 1.15.105) [82], and Eriochrome Azurol G-CTA [83].

Methods for Pd determination based on ion-associates with basic dyes are often very sensitive. In extraction-spectrophotometric methods, thiocyanate [84-89], chloride, and bromide [84] anionic complexes of palladium are associated with Brilliant Green [84], Malachite Green [85,88], Rhodamine B (e = 9.0.104) (86), Rhodamine 6G [89], and Methylene Blue [87]. In flotation-spectrophotometric methods, ion-associates formed by the


bromide complex of palladium and Rhodamine 6G (e = 3.0.105) [90], the thiocyanate complex and Methylene Blue [91], and Pd complex with SnC13- and Rhodamine 6G (e = 2.8.105 ) [92] have been described. The solvents used in the flotation methods include benzene (DMF for dissolving the compound), benzene and acetone [91 ], and DIPE [92].

Methods based on coloured Pd complexes formed in acid-chloride (~ =2.8-103) [93-97] or -bromide [93,98] media, often in the presence of Sn(II), are not very sensitive. These complexes may be extracted in the presence of TOA (benzene) [ 1 ], tetraphenylarsonium ion (CHC13) [94], DAM (CHC13, ~ = 1.5.104) [98], triphenylphosphine (~; = 1.5.104 at 346 nm) [951.

Derivative spectrophotometry has been used for the determination of Pd in the presence of Pt and Au in bromide solutions [ 100], and in the presence of Pt in iodide solutions [ 101 ].


The methods described in detail in Section 36.2, or only mentioned, have been used as follows for spectrophotometric determination of palladium: the thio-Michler's ketone - - in silver, copper, and anodic slime [32], in catalysts [31]; with thiosemicarbazide derivatives m in water [44] and alloys [46]; with palladium-carbon powder with o~-benzilmonoxime [48]; with PAR in catalysts and ores [58]; with thiazolylazo derivatives m in Ni-AI catalysts [63]; with 5-Br-PADAP m in titanium alloys; with pyridylazo derivatives - in nickel alloys [68]; with sulphonitrophenol- in silver alloys [70]; with Arsenazo Ill ~ in iron and meteorites; and with Palladiazo in catalysts, minerals, silica gel, and calcium carbonate [78].

References

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324 36. Palladium

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249 (1972). 79. Duchkova H., Malat M., (~ermakova L, Anal. Lett., 9, 487 (1976). 80. Uesugi K., Shigematsu T., Anal. Chim. Acta, 84, 377 (1976). 81. Kant R., Srivastava R., Prakash O., Croat. Chem. Acta, 54, 465 (1981). 82. Gregorowicz Z. et al., Mikrochim. Acta, 1983 II, 181. 83. Uesugi K., Miyawaki M., Microchem. J., 41, 78 (1990). 84. Khvatkova Z.M., Schpeg M.A., Golovina V.V., Zh. Anal. Khim., 36, 2367 (1981). 85. Pilipenko A.T., Khvatkova Z.M., Zh. Anal. Khim., 41, 2045 (1986). 86. Lopez Garcia I., Martinez Avilles, Cordoba M.M., Talanta, 33, 411 (1986). 87. Mafiko R.W., Chem. Anal. (Warsaw), 36, 763 (1991). 88. Mafiko R.W., Chem. Anal. (Warsaw), 36, 17 (1991). 89. Ramalingom P.A. et al., Chem. Anal. (Warsaw), 42, 75 (1997). 90. Marczenko Z., Jarosz M., Talanta, 28, 561 (1981). 91. Marczenko Z., Jarosz M.,Analyst, 106, 751 (1981). 92. Kalinowski K., Marczenko Z., Anal. Chim. Acta, 186, 331 (1986). 93. Dalziel J.A., Donaldson J.D., Woodget B.W., Talanta, 16, 1477 (1969). 94. Nasouri F.G., Witwit A.S., Talanta, 16, 1492 (1969). 95. Mojski M., Plesifiska M., Microchem. J., 24, 117 (1979). 96. Khalonin A.S. et al., Zavod. Lab., 52, 7 (1986). 97. Koch K.R., Auer D., Talanta, 40, 1975 (1993). 98. Danilova V.N., Lisitchenok S.L., Zh. Anal. Khim., 26, 1157 (1971). 99. Kuroda R., Hayashibe Y., Yoshitsuka K., Fresenius'J. Anal. Chem., 336, 494 (1990). 100. Obarski N., Marczenko Z., Chem. Anal. (Warsaw), 40, 385 (1995). 101. Marczenko Z., Obarski N., Chem. Anal. (Warsaw), 39, 347 (1994).

Chapter 37. Phosphorus

Phosphorus (P, at. mass 30.97) occurs mainly in the V oxidation state as phosphate (derived from orthophosphoric acid) and the condensed forms, pyro-, meta-, and polyphosphate. Phosphoric acid gives stable heteropoly acids with Mo(VI), W(VI), V(V) etc. Phosphorus occurs also in the III, I and -HI oxidation states in phosphite, hypophosphite and phosphine (PH3), respectively.

37.1. Methods of separating phosphate

When a sample is dissolved, the phosphorus usually passes into solution as P(V). Rather than isolate the phosphate, it is often better to isolate the interfering elements, leaving the phosphate to be determined in the mother liquor. Examples of such separations include distillation of Si, As, and Ge as volatile halides [1] or of boron as trimethyl borate [2], precipitation of heavy metals as sulphides from an acid medium, retention of cations on a strongly acidic cation exchanger, and electrolytic separation of metals.

The separation of P(V) from other elements, in particular from Si, is often achieved by extracting phosphorus as a heteropoly acid from a slightly acidic solution of pH --1.4. Higher alcohols, esters, and ethers are suitable extractants [3-5]. By extraction with a mixture of butanol and chloroform, molybdophosphoric acid can be separated from molybdoarsenic acid [6]. Isobutyl acetate extracts molybdophosphoric acid, but not molybdosilicic acid, from a solution at pH 0.3-1.0.

In the determination of traces of P in silicon tetrachloride, shaking the sample with concentrated sulphuric acid causes the phosphorus to pass into the acid layer [7].

Trace amounts of phosphate can be co-precipitated with AI(OH)3, Fe(OH)3, or Be(OH)2 as collector [8-10]. Traces of phosphate, co-precipitated with AI(OH)3 at pH 8.5, have been floated with Na oleate, by passing a stream of nitrogen [ 11 ].

Traces of phosphate were concentrated on a cation-exchanger impregnated with Fe(III) [12,13] or with barium chloranilate [14]. Mixtures of P(V), As(V), and Si(IV) were separated on an anion-exchange column [15].

37.2. Methods of determining phosphate

Microgram quantities of phosphorus are conventionally determined by the phosphomolybdenum blue method. The molybdovanadophosphoric acid method is suitable for determining relatively large quantities of P(V). Sensitive spectrophotometric methods based on ion-associates with basic dyes deserve attention.

37.2.1. Phosphomolybdenum blue method

In an acid medium containing excess of molybdate, orthophosphate forms pale yellow molybdophosphoric acid ("Mo-P"). This reaction is useful for determining phosphate at fairly high concentrations, but the sensitivity can be increased considerably by the use of aqueous acetone [ 16-18].

A sensitive spectrophotometric method for determining phosphorus is based on the

37.2. Methods of determining phosphate 327

reduction of molybdophosphoric acid to phosphomolybdenum blue ("P-Mo") under mild conditions to prevent reduction of the free molybdic acid [19-24]. Reductants employed include hydrazine, SnC12, ascorbic acid, sulphite, and other reducing agents [25,26]. In the preparation of "P-Mo" blue, a single reagent consisting of ammonium molybdate, hydrazine sulphate, and H2SO4 is convenient (see procedure below).

Molybdophosphoric acid is reduced either in aqueous medium (-~0.5 M H2SO4) or in the organic phase (usually n-butanol) after molybdophosphoric acid has been extracted. Alternatively, the phosphomolybdenum blue may be formed in the aqueous phase, then extracted into n-butanol [27]. Aqueous acetone medium is also used [28]. The absorbance of the phosphomolybdenum blue depends on the medium, its acidity, and the kind of the reducing agent used.

The molar absorptivity of the blue solution in butanol after hydrazine reduction is 2.5.104 (a = 0.81) at ~max 780 nm. Extraction of phosphomolybdenum blue displaces the absorption maximum slightly towards shorter wavelengths.

Interfering species in the determination of phosphorus by the phosphomolybdenum blue method are As(V), Si, and Ge, which also react with molybdate to form the corresponding acids which are reduced to the respective heteropoly blues. Arsenic(V) does not interfere when reduced to As(III) using sulphite or thiourea. In the presence of vanadium(V), molybdovanadophosphoric acid is produced. Large amounts of vanadium(V) are reduced with Mohr's salt to V(IV) before the molybdate is added. The difference in the rates of formation of the phosphomolybdenum- and silicomolybdenum- blues has been utilized for the determination of phosphorus in the presence of silicon [29]. The interference of silicon can be prevented by the use of a sufficiently acidic medium [30].

Oxalic-, tartaric-, and citric acids, and EDTA affect the completeness of reduction of "Mo-P" acid [31 ]. Before the determination of phosphate, any nitrate must first be reduced to ammonia, which is then distilled from the alkaline medium [32].

Phosphomolybdenum blue may be extracted with CHC13 in the presence of dioctylamine, trioctylamine, or propylene carbonate [33].

The FIA technique has also been used in the determination of phosphorus(V) [15,34].

Reagents

Molybdenum reagent. Solution (1). Dissolve 1.0 g of ammonium molybdate in 100 ml of 2 M H2804. Solution (2). Dissolve 0.10 g of hydrazine sulphate in 100 ml of water. Immediately before use, mix 10 ml of solution (1) with 10 ml of solution (2), and dilute to 100 ml with water. Solutions (1) and (2) should not be stored longer than 4 days.

Standard phosphorus(V) solution: 1 mg/ml. Dissolve in water 4.3900 g of potassium dihydrogen phosphate (KH2PO4) dried at l l0~ add 1 ml of CHC13 (to prevent the formation of mould), and dilute with water to 1 litre.

Ammonium molybdate, 10% solution adjusted with ammonia to pH 7.4_+0.2.

Procedure

Extractive separation of P. Evaporate the sample solution freed from As (e.g., by extraction as AsC13, cf. Section 8.1.1) nearly to dryness, dilute with 20 ml of water, add 2 ml of the ammonium molybdate solution and adjust the pH to 1.4_+0.1 with 0.5 M H2SO4. After 5 min, transfer the solution to a separating funnel and extract the "Mo-P" acid with two 10-ml portions of butanol. Wash the alcoholic extract with 0.05 M H2SO4.

Determination of P. Evaporate an aliquot of the extract (or an aqueous sample solution

328 37. Phosphorus

freed from As, Ge, and Si), containing not more than 30 ~tg of P, to dryness in a beaker with nitric acid. Add 12 ml of the molybdenum reagent, and place the beaker on a boiling water- bath for 10 min. Transfer the cooled solution to a separating funnel, and extract the phosphomolybdenum blue with two portions of n-butanol. Dilute the extract to the mark with the solvent in a 25-ml standard flask, and measure the absorbance at 720 nm, using the solvent as reference.

Notes. 1. The absorbance of the P-Mo blue may be measured in the aqueous solution. In this case, the coloured aqueous solution is diluted to the mark in the volumetric flask with the molybdenum reagent.

2. Molybdophosphoric acid may be extracted, then stripped with dilute ammonia solution (1 +50), the solution acidified with nitric acid and evaporated, and the complex reduced to the heteropoly blue.

37.2.2. Molybdovanadophosphoric acid method

Addition of molybdate to an acidic solution containing orthophosphate and vanadate, results in the formation of the yellow-orange molybdovanadophosphoric acid having the Mo:V:P ratio of 11:1:1 [35,36].

The absorption maximum of the compound is at 314 nm (e = 2.0-104). At 400 nm, e = 2.5.103 (specific absorptivity 0.08). In the molybdovanadophosphoric acid method, the absorbance is measured either at 315 nm (sensitivity as high as that in the "P-Mo" blue method), or at 400-470 nm (much lower sensitivity).

The colour depends on the acidity of the solution and on the concentrations of the reagents used. The optimum acid concentration is 0.5-1.0 M HNO3 (H2SO4, HC104, or HC1). In insufficiently acid solutions, the yellow colour is produced even in the absence of orthophosphate; in excessively acid solutions, the formation of molybdovanadophosphoric acid proceeds too slowly. The concentration of V(V) and Mo(VI) in the final solution should be --0.002M and --0.01M, respectively. Since the reagents also produce a slight colour in the absence of phosphate, the absorbance must be measured against a reagent blank solution.

In --0.8 M HNO3 silicon does not interfere provided it is not present in greater amount than phosphorus. In more acidic media, even more silicon can be tolerated. At higher concentrations, silicic acid can be converted into the inert polymeric form by heating the sample solution to fumes with conc. HC104.

Large amounts of Fe(III) interfere, but may be masked with fluoride, the excess of which is complexed with boric acid. Reductants and certain coloured metal ions [e.g., Cr(VI), Ni, Co, Cu, and U(VI)] also interfere. Molybdovanadophosphoric acid may be separated from many coloured ions by extraction with oxygen-containing organic solvents [35]. Reducing agents must be absent.

Reagents

Ammonium metavanadate, 0.25% solution. Dissolve 1.25 g of NH4VO3 in 250 ml of hot water. Cool the solution and add 10 ml of conc. HNO3. Allow the solution to stand overnight, filter if necessary, and dilute with water to 500 ml. Store the solution in a polyethylene container.

Ammonium molybdate, 5% solution. Dissolve 2.5 g of the reagent in 250 ml of water (at --50~ Allow the solution to stand overnight, filter if necessary, dilute with water to 500 ml, and store in a polyethylene container.

37.2. Methods of determining phosphate 329

Standard phosphorus(V) solution: 1 mg/ml. Preparation as in Section 37.2.1.

Procedure

To the slightly acidic sample solution containing not more than 0.3 mg of P(V), add successively 2.5 ml of HNO3 (1+1), 2.5 ml of the vanadate solution, and 2.5 ml of the molybdate solution, mixing the solution after the addition of each reagent. Dilute the solution to volume with water in a 25-ml standard flask. After 30 min, measure the absorbance at 400 nm against a reagent blank solution.

37.2.3 Other methods With basic dyes, molybdophosphoric acid forms ion-associates which are the basis of sensitive extraction methods for determining P(V). Malachite Green [37--48], Crystal Violet [49-52], Brilliant Green [53,54], Ethyl Violet [55], Methylene Blue [56,57], Rhodamine B [58,59], and Rhodamine 6G [60,61] are among the most widely used. The ion-associates, which are sparingly soluble in water, can be extracted, and the absorbance of the extracts measured [39,40,55,56]. The associates may also be separated by flotation [44,49] or centrifugation [53,58] before being dissolved in polar solvents. Aqueous pseudo-solutions are often stabilized with, e.g., poly(vinyl alcohol) [38,41,50], Syntanol DS-10 [37], or Zephiramine [57]. The most sensitive methods include those utilizing Malachite Green (e = 3.3.105) [44] and Brilliant Green (e = 2.9.105) [53]. The derivative spectrophotometry technique has been applied in the determination of phosphate with Rhodamine 6G [61 ].

The molybdophosphate anion has been associated, in a HC1 medium, with a cationic complex of Co with 5-C1-PADAP. The sparingly soluble associate has been separated by flotation with butyl acetate and dissolved in methanol (~ - 3.4.105 at 560 nm) [62].

A method based on phosphoantimonylmolybdenum blue has also been proposed [62a]. Numerous indirect amplification methods have been devised for the determination of

phosphate. The molybdenum in an extract of molybdophosphoric acid (Mo:P = 12:1) has been determined with thiocyanate [63], phenylfluorone [64], dithiol (e = 1.7-105), Sulphonitrophenol S (e - 4.6-105) [65], or 2,2'-diquinoxalyl [66]. The indirect method that involves the Fe(II)-ferrozine complex [67] is unusually sensitive (~ = 9.7.105). In another method involving the complex of Ce(III) and Arsenazo III, the phosphate gives a sparingly soluble CePO4 and liberates an equivalent quantity of Arsenazo III [68].


The phosphomolybdenum blue method has been used for determining phosphorus in biological materials [69-71], milk [72], vegetables [73], wine and blood serum [74], sewage [75,76], waters [5,11,77-87], soils and plants [88-94], rocks and minerals [95], geological deposits (sediments) [96], organic compounds [97], fertilisers [98], cast iron and steel [8,26,99,100], nickel and its alloys [26,100,101], copper alloys [101], aluminium alloys [102], platinum and gold [6], gallium and its compounds [103], iron ores [104], silicon [1], boron [2], tungsten materials [9], arsenic and its oxide [105], niobium- and tantalum oxides [106], neodymium and yttrium oxides [107], silicates [108-110], glass [111], coke [112], coal dust [91], silumins [ 10], and concentrated chloride solutions [ 113]. Optimum conditions for determining P in soils, waters, and plants were studied [81]. Phosphorus was determined by the FIA technique in soils [91] and waters [87].

The method has been applied for determination of phosphate in the presence of

330 37. Phosphorus

phosphate esters [114], pyrophosphate, and polyphosphate [115]. The condensed phosphates are converted into phosphates by boiling for 15 min in 2.5 M H2804 medium.

When trace amounts of phosphates were determined in water, they were preconcentrated (as the P-Mo blue) in a column packed with silane-coated glass beads; DMF was used as the eluent [116]. The P-Mo blue may also be concentrated on a nitro- or acetyl- cellulose membrane in the presence of dodecyltrimethylammonium bromide [ 117].

The P-Mo blue method has been applied for automatic determination of P(V) in silicate rocks [118], soils [119], sea deposits [120], and water [121]. This method has been applied also in the FIA technique [122-126].

The method involving the Mo-V-P acid has been used in determinations of phosphorus in biological tissues [127], plant material [128], fruits [129], fish products [130], foodstuffs [131], phosphate minerals [132], cast iron and steel [133,134], niobium, zirconium and its alloys, titanium and tungsten, aluminium, copper, and white metal [135], nickel alloys [134,135], metallurgy products [136], molybdenum concentrates [137], silicon tetrachloride [7], cement [138], and lubricants[139]. The flow injection technique has been applied for determining phosphate in minerals [140] and in plant materials [141 ].

The methods involving basic dyes have been used for determining phosphate in natural waters [38,39,41,43,48,55,59,142], biological materials [47,50,52], soils [45,51], uranium [37], iron [46,49], and nickel, cobalt, copper, and zinc [46]. The FIA technique has also been applied [40,43,48].

Various methods were applied for determining phosphorus in calcium chloride extracts [143].

The spectrophotometric methods for the determination (and speciation) of phosphorus in natural waters have been compared with other analytical techniques [144].

37.4. Determination of other phosphorus compounds

Hypophosphite can be determined, in the presence of phosphate, from the colour it gives with ammonium molybdate in H2SO4 medium [145]. Hypophosphite may also be determined by its bleaching of the colour of the Fe(III)-SCN- complex [ 146 ].

Pyrophosphate is determined by using its effects on the colour reactions of Fe with SCN- or 1,10-phenanthroline [147].

A method for separation of hypophosphite, phosphite, phosphate, pyrophosphate, and triphosphate by ion exchange chromatography, with HC1 and KC1 solutions as eluents, has been proposed [148].

References

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Chapter 38. Platinum

Platinum (Pt, at. mass 195.09) occurs in its compounds in the II and IV oxidation states, compounds of Pt(IV) being the more stable. The hydroxide Pt(OH)4 dissolves in excess of NaOH. Platinum(W) forms chloride, iodide, cyanide, and nitrite complexes. Platinum(H) and -(IV) are more difficult to reduce to the metal than is gold(HI). Zinc and aluminium in acid solution, and formaldehyde in an alkaline medium, are suitable reductants. Of the other platinum metals, palladium resembles platinum most closely, and osmium and ruthenium resemble it least.

38.1. Methods of separation and preconcentration metals

of platinum

38.1.1. Fire assay and cupellation

These methods, used to isolate small amounts of the noble metals (platinum metals, Au, Ag) from ores and concentrates, have been discussed in Section 23.1.1.

38.1.2. Extraction

A comprehensive review of the solution chemistry and of solvent extraction and ion- exchange methods for separating platinum metals has been published [1].

Thioeyanate [2] and chloride [3-8] complexes play an important role in the extractive separation of platinum metals. They are extracted in the presence of pyridine (MIBK), TBP, phosphonium salts (ethyl acetate), DAM, DAPM (CHC13), and TOPO (1,2-dichloroethane). Often, the extraction is carried out in hydrochloric acid media in the presence of SnCI2 (ligand SnC13-) which plays the role of labilizing agent [8-11]. Solvent extraction is often applied in separation of platinum from other metals [3,6-8].

Bromide complexes of Pt and other metals of this group are extracted from acid media with TBP, MIBK, amyl acetate [ 12] or TOPO [6]. Pt and Pd can be separated from Rh and Ir by extraction (with TBP) from iodide solution [13]. The same method has been used for separating platinum from copper [14].

Dithizone allows Pt and Pd to be separated from each other and from other metals [15- 18]. Pt has been separated from Pd in the presence of SnCI2 [ 16] or iodide [ 17].

Other organic reagents used for extractive separation and preconcentration of platinum group metals include thiourea derivatives [19] and organic sulphides [20].

Integrated schemes for extractive separation of all platinum metals and gold have been developed [21,22].


Platinum metals form stable anionic chloride complexes which allow their separation from a number of common metals on both anion- and cation-exchangers [1,23-25]. Platinum metals can also be separated as the bromide [26,27]. Columns with chelating resins have


also found use in the separation and preconcentration of platinum metals [28-32]. Platinum, as well as Pd, Au, and Rh, can be precipitated by the following reducing

agents: formic acid, hydrazine, tin(II) chloride, calomel, and sodium tetrahydroborate [33]. Tellurium and selenium are used as collectors [34,35]. When an acid solution is boiled with powdered tellurium, Pt and Pd are precipitated, whereas Rh and Ir remain in the solution [34].

From mixtures of platinum metals, Ru and Os can be separated by distillation as volatile tetroxides, and a double precipitation of Pd, Rh, and Ir hydroxides (with Fe used as collector) at pH 8 enables them to be separated from platinum.

Ion-flotation has been used to separate and preconcentrate platinum metals as chloride complexes in the presence of cationic surfactants [36].


Two methods for determining platinum are described here in detail: the well-known insensitive stannous chloride method, and a sensitive method based on an ion-associate of Pt(II) with SnC13- and Rhodamine 6G.

38.2.1. Tin(II) chloride method

Stannous chloride reacts with Pt(IV) in dilute HC1 acid to yield a yellow-orange complex which is the basis of the spectrophotometric method for determining Pt [37]. The molar absorptivity is 1.3-104 a t ~max "- 403 nm (a --- 0.067).

The most suitable concentration of HC1 is 0.8-1.5 M: the concentration of SnCI2 is not critical for the colour reaction.

The coloured complex is extractable into esters and ethers, amyl acetate being the most recommended. The molar absorptivity in amyl acetate is similar to that in aqueous medium, but the absorption maximum is shifted slightly towards shorter wavelengths. The high distribution coefficient facilitates the concentration of platinum in a small volume of the organic solvent.

Palladium and platinum react similarly with SnCI2. If, however, the sample solution containing palladium and platinum is first made alkaline with ammonia, then acidified with hydrochloric acid to -~ 1 M in HC1, and finally treated with SnCI2, only the platinum complex is extracted. Other platinum metals also form coloured complexes with SnCI2 [ 11 ].

Higher sensitivity is obtained by extracting the anionic Pt complex with chloroform or benzene in the presence of TOA or another long-chain high molecular-weight amine [9]. Platinum has been extracted with TOA before the determination in gold [38].

A related method for platinum is based on the orange-red complex obtained in acidic bromide solution in the presence of tin(II). The complex has been extracted with chloroform in the presence of DAM [39], or with benzene in the presence of DAPM [40].

Reagents

Tin(II) chloride, 25% solution in HC1 (1+3). When covered with a layer of xylene, the solution is protected from atmospheric oxygen and may be stored for a long time.

Standard platinum(IV) solution: 1 mg/ml. Dissolve 0.1000 g of platinum in 4 ml of aqua regia and evaporate the solution nearly to dryness. Add 5 ml of conc. HC1 and 0.1 g of NaC1, and evaporate the solution to dryness. Dissolve the solid in 20 ml of HC1 (1 + 1), and dilute

336 38. Platinum

the solution to volume with water in a 100-ml standard flask. Working solutions are obtained by appropriate dilutions of the stock solution with dilute HC1 (e.g., 0.2 M).

Procedure

Place the sample solution, containing not more than 0.3 mg of Pt, in a 25-ml standard flask. Add 3 ml of conc. HC1 and 5 ml of the SnC12 solution, dilute the solution to the mark with 0.01 M HC1, mix well, and measure the absorbance at 403 nm against water.

In the extraction modification of the method, add to the acidified sample solution (see above) 5 ml of 25% ammonium chloride solution, 5 ml of the SnCI2 solution, and 0.01 M HC1 to -20 ml. Extract the solution with two portions of amyl acetate (containing 1% of resorcinol). Dilute the combined extracts with the solvent to the mark in a standard flask of suitable capacity, and measure the absorbance at 400 nm against the solvent.

38.2.2. Rhodamine 6G method

This very sensitive flotation-spectrophotometric method is based on the ion associate formed by the anionic chlorostannate(II) complex of platinum(H) with the xanthene basic dye Rhodamine 6G (formula 4.30) [41]. When the aqueous phase is shaken with DIPE, the sparingly soluble compound collects on the wall of the separating funnel. The solid associate is washed, dissolved in acetone, and its absorbance is measured.

Hydrochloric acid (-1 M) is a suitable medium for formation of the associate. The concentration of tin(II) should be 0.02-0.04 M, and Rhodamine 6G should be in -50-fold excess with respect to Pt. At higher SnCI2 and dye concentrations the absorbance of the reagent blank increases. The amount of DIPE used in flotation has no effect on the Pt separation efficiency. The time of shaking should not be shorter than 1 min. After the washing with 2 portions of 1 M HC1 the absorbance of the reagent blank is not higher than 0.05.

Under the conditions of this procedure e = 2.8.105 (sp. abs. 1.4) at 530 nm. Beer's law is obeyed in the Pt concentration range 0.08-0.6 lag/ml. The high sensitivity of the method is accounted for by the fact that three cations of the dye are equivalent to one Pt atom [41 ].

Pd and Rh affect the determination of Pt. The smaller effect of other platinum group elements is due to the lower rate of formation of their complexes with SnC13-. A convenient method of separating Pt from Pd consists in extraction of Pt with triphenylphosphine oxide solution in 1,2-dichloroethane [7] (see the procedure below).

Reagents

Rhodamine 6G, 0.001 M solution (about 0.05% ). Tin(II) chloride: 10% solution of SnC12.2H20 in 2 M HC1. Triphenylphosphine oxide (TPPO), 0.1 M solution in 1,2-dichloroethane. Standard platinum(IV) solution: 1 mg/ml. Preparation as in Section 38.2.1.

Procedure

Separation of Pt from Pd. Evaporate an acid sample solution (in HC1) to dryness on a water-bath. Dissolve the residue in 10 ml of 7-8 M HC1. Extract the platinum by shaking with three 5-ml portions of TPPO solution (each portion for 10 min). Wash the combined dichloroethane extracts by shaking with 10 ml of 7-8 M HC1 for 10 min.


Before determining Pt, evaporate the organic solvent and mineralize the residue with a few drops of conc. H2804 and conc. HNO3. Heat the residue (after evaporation) with some drops of aqua regia, and evaporate almost to dryness with several drops of conc. HC1 added. Determination of Pt. Evaporate to dryness the acidic (HC1) sample solution, containing not more than 15 gg of Pt, on a boiling water-bath. Add 2 ml of the SnCI2 solution. Transfer the solution to a separating funnel. Add appropriate amounts of conc. hydrochloric acid and water to obtain 20 ml of solution that is 0.8-1.2 M in HC1. Then add 1 ml of the Rhodamine 6G solution and 5 ml of DIPE, and shake for 1 min. Open the funnel at the top, allow the phases to separate, and slowly discard the aqueous layer. Wash the ether phase and the precipitate by shaking with two successive 20-ml portions of 1 M HC1 for 30 s each. Carefully remove the aqueous and organic layers and dissolve the isolated solid in acetone. Transfer the solution to a 25-ml standard flask and dilute to the mark with acetone. Measure the absorbance of the solution at 530 nm against a reagent blank prepared in the same way.


Besides the Rhodamine 6G-SnC12 flotation-spectrophotometric method described above, similar methods using other basle dyes, e.g., Victoria Blue B, Victoria Blue 4R, Capri Blue [41], Crystal Violet (~ = 2.1.105) [42], or Nile Blue A [43], have been proposed. An ion- associate of the chloride platinum complex with Methyl Green has been extracted with a mixture of 1,2-dichloroethane and CC14 (E = 1.45.105) [44]. The ion-associate of the Pt- thiocyanate complex with Malachite Green has been extracted with benzene [45], and a thiocyanate- or iodide- Pt complex associated with Crystal Violet has been extracted into xylene or toluene [46].

The cationic hexa-ammine complex of Pt(IV) gives an ion-pair with the acid dye, ethyl eosin, and this is also used in determinations of platinum (e = 8.0.104) [47,48].

A sensitive but unselective method is based on the reaction with p-nitrosodimethylaniline (formula 38.1) or p-nitrosodiethylaniline [49].

'CH3'2N-- N0 (38.~)

When a Pt(IV) solution (pH 2-3) is heated with excess of the reagent, an orange-red complex slowly forms (e = 5.7.104 at 525 nm). Because palladium reacts rapidly with the reagent, even in the cold, it should be separated before the determination of platinum. Alternatively, platinum can be determined from the difference between the absorbances before and after the solution is heated. The other platinum metals must be separated beforehand.

Dithizone reacts with platinum(II) in an acid medium (1-4 M HC1 or H2804) containing tin(II), to form a brown-yellow dithizonate Pt(HDz)2, which is soluble in CC14 or CHC13.

338 38. Platinum

2 3 1 2 3

i

_/.90 4,00 z,,50 500 600 635 700

wavelength, nm

Fig. 38.1. Absorption spectra of dithizone (H2Dz) (1), Pd(HDz)2 (2), and Pt(HDz)2 in 0014

The molar absorptivity is 3.8.10 4 at 490 nm and 3.5.104 at 710 nm. The absorption spectrum is shown in Fig. 38.1. Pt and Pd can be separated and determined one after the other by using dithizone [ 16,17].

Traces of Pt in palladium were determined by the dithizone method after separation of the macro-amounts of Pd as the sparingly soluble Pd(NH3)I2 [50]. Simultaneous determination of Pt and Pd is possible in the 5 th order derivative spectrophotometry technique [51 ]. Trace amounts of Pt in larger amounts of Pd have been determined by means of the 2 nd order derivative of Pt(HDz)2 absorption curve [52].

Azo dyes have also been used in determining Pt, e.g., PAR [53,54], Sulphonitrophenol M [55], and azo compounds based on Rhodanine and Thiorhodanine, such as Tyrodine and Sulphochlorophenolrhodanine (e = 1.0.105) [55-59].

Derivative spectrophotometry has been applied for determining Pt in the presence of Au and Pd in chloride [60]-, bromide [61 ]-, and iodide [62] solutions.


The tin(II) method has been used for determining platinum in catalysts [63,64] and ores [65]. Differential spectrophotometry was used for determining platinum in its alloy with ruthenium [66] and in catalysts [67]. The contents of platinum in catalysts have also been determined by derivative spectrophotometry [68].

The Rhodamine 6G method has been applied for determining 5.10 -3 % concentrations of Pt in palladium [52] and in catalysts [69].

Platinum present in amounts 0.01-5% in catalysts has been determined as the ion- associate of the thiocyanate-Pt(IV) complex with Crystal Violet [46,70].

References

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340 38. Platinum

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Chapter 39. Rare-earth elements

The lanthanides are a group of elements with atomic numbers 57-71, which exhibit very similar chemical properties. They occur naturally in two slightly different groups - - the cerium group, (elements 57-63), and the yttrium group (elements 63-71). Yttrium has very similar chemical properties to the lanthanides in the second group and, although it is not itself a lanthanide (having no f- electrons), it is generally included with the lanthanides in the group of elements known as the rare-earth elements (REE).

The cerium group includes lanthanum La, cerium Ce, praseodymium Pr, neodymium Nd, promethium Pm, samarium Sm, and europium Eu. The yttrium group consists of yttrium Y, gadolinium Gd, terbium Tb, dysprosium Dy, holmium Ho, erbium Er, thulium Tm, ytterbium Yb, and lutetium Lu.

All the rare-earth elements occur in the III oxidation state in compounds, and can be separated and determined in this form to provide what is known as the total REE. Samarium, europium, and ytterbium also occur in the unstable II oxidation state, whereas cerium, praseodymium, and terbium can be found in the IV oxidation state.

Rare-earth element hydroxides, M(OH)3, precipitate from nitrate solution at pH values above 6.3-7.8 and reveal no amphoteric properties. Like thorium, the rare-earth elements yield acid-insoluble fluorides and oxalates, and soluble EDTA-, tartrate-, and citrate- complexes.

Cerium (Ce, at. mass 140.12), which is the most popular rare-earth element, occurs also in the IV oxidation state. In chemical properties, cerium(IV) resembles Th and U(IV). The yellow Ce(OH)4 precipitates at pH --1. Cerium(III) is oxidized to Ce(IV) in acidic media by bismuthate, silver(II) oxide, or persulphate (in the presence of Ag+).


39.1.1. Ion-exchange. Chromatography

The most important methods for separating the rare-earth elements are based on ion- exchange.

Rare-earth elements have been separated on strongly basic anion-exchangers by taking advantage of the small differences in the stabilities of the EDTA complexes [1]. Trace amounts of REE have been separated from uranium on Dowex 1 anion-exchanger, using mixtures of methanol with HC1 or acetic acid [2]. Anion-exchange separation of the rare- earth metals with mixtures of HNO3 and water-miscible alcohols is also feasible [3,4].

EDTA has also found application in the cation-exchange separations. From an EDTA solution at pH 2.1, Dowex 50 cation-exchanger retains the REE elements, whereas Th is eluted as the more stable EDTA complex. The lighter lanthanides have been separated by eluting them from a cation-exchanger with EDTA or NTA solution.

The REE have been separated on strongly acid cation-exchangers with citrate media [5,6]. REE have been separated from other elements on cation-exchangers using hydrochloric acid [7,8], HC1 + ethanol [9], and aq. HNO3 [10] media. The REE have been preconcentrated on a chelating ion-exchanger [ 11 ].

The chromatographic methods for separation and isolation of REE have been reviewed [12,13].

342 39. Rare-earth elements


Rare-earth elements may be isolated by precipitation as their oxalates, fluorides, or hydroxides. When the oxalates are precipitated from a weakly acidic medium (pH 1-4), Ca is used as a collector [14,15].

If the medium is too acidic for precipitation of the oxalates, the REE can be separated as fluorides [16,17]. Thorium is co-precipitated, and calcium is a suitable collector. Both methods give good separation of REE from Fe, A1, Ti, Zr, U(VI), Nb, Ta, and certain other metals.

In 0.2 M NazCO3, the light lanthanides (cerium group) are precipitated quantitatively, while the remaining lanthanides and scandium are only partly precipitated [18]. Separation of Ce(IV) as the hydroxide (pH --1) enables the separation of Ce from other REE. Ti, Zr, or Fe(III) can be used as carriers. Ce(IV) may also be precipitated as iodate.

39.1.3. Extraction

Rare-earth elements can be separated from other metals by extraction methods using the complexes with TOPO (MIBK) [19], HTTA with TOPO (toluene) [20,21], HDEHP [22-24], DDTC [25], 8-hydroxyquinoline with phen [26] or with tetra-n-heptylammonium ion (CHC13) [27], high molecular-weight amine [28], EDTA in the presence of Capriquat [29], a-diketones [30], and 2-thenoyltrifluoroacetone [31-34].

Crown ethers have been used for extractive separation of lanthanides [35-40]. The effect of solvent on extraction of REE with 18-crown-6 ether (and trifluoroacetone) has been discussed [41 ].

Ce(IV) has been separated from other lanthanides in HNO3 by extraction with Aliquat 336 solution in xylene [42] or TOA in CHC13 [43]. The lanthanides were also extracted from thiocyanate media using TBP [44], or as ion-associates of complexes with 2,3- naphthalenediol and surfactants [45]. Extraction of REE from multi-component mixtures by phosphoro-organic compounds and trialkylammonium nitrates has also been studied [46].


The Arsenazo III method presented below can be used to determine either the total REE or the individual lanthanides and yttrium. The 8-hydroxyquinoline method gives a possibility of determining cerium as Ce(IV) in the presence of all the remaining REE.

The determination of lanthanides in geological samples has been reviewed [47].

39.2.1. Arsenazo III method

In weakly acidic media the lanthanides and yttrium react with Arsenazo III (formula 4.10) to form coloured complexes which are the basis of the sensitive method for determining the total of REE or any element from this group [48-51 ]. In weakly acidic solution, the reagent is violet, and its complexes with the rare-earth elements are green. The maximum absorbance for Ce(III) is obtained at pH 2.3-2.7. The optimum pH values for the various REE differ slightly.

The reagent does not absorb at the wavelength of the absorption maximum of the complex (650 nm). The molar absorptivity of the Arsenazo III complex with cerium is 5.6.104 (specific absorptivity 0.40). The other REE complexes have similar molar


absorptivities. Chloride, sulphate, and phosphate do not interfere in the determination of REE. Neither

do small amounts (less than 1 mg per 50 ml of solution) of Ti, A1, Ca, and Fe [reduced to Fe(II) with ascorbic acid], but larger quantities of these metals and certain other metals (Th, Zr, U, Bi, Cu) should be removed. Many interfering metals can be masked with EDTA [52- 54].

The REE are separated from the calcium used as the collector in the precipitation of the oxalates or fluorides, by reprecipitation as the hydroxides with ammonia. In this instance, Fe(III) is used as the collector.

In aqueous organic media, the sensitivity in determining REE with Arsenazo III is increased significantly [55].

The reaction with Arsenazo III has been used in the study of extraction equilibria in the systems of REE and phosphonic acid derivatives [56].

Reagents

Arsenazo III, 0.05% solution. Dissolve 50 mg of the reagent in 100 ml of water. Standard lanthanum solution: 1 mg/ml. Dissolve 0.1170 g of La203, dehydrated by

ignition, in 5 ml of hot HC1 (1+1). Dilute the solution to volume with water in a 100-ml standard flask.

Standard cerium solution: 1 mg/ml. Dissolve 0.1228 g of cerium(W) oxide, CeO2 (dehydrated by ignition), in 5 ml of hot HC1 (1 + 1), and add 0.1 g of NHzOH.HC1. Dilute the solution to volume with water in a 100-ml standard flask.

Standard yttrium solution: 1 mg/ml. Dissolve 0.1270 g of Y203 (dehydrated by ignition), in 5 ml of hot HC1 (1+1). Dilute the solution to volume with water in a 100-ml standard flask.

Formate buffer, pH 3.5. Dissolve 60 ml of formic acid and 28 g of NaOH in water, and dilute the solution with water to 1 litre.

Procedure

Separation of R E E as oxalates. To the acid solution (50-100 ml) add 5-10 ml of 8% oxalic acid solution. Adjust the pH of the solution to 2.0-2.5, heat to -~80~ and add 5 mg of calcium (as a chloride solution) dropwise with stirring. Heat the solution for 1 h, but do not boil. After several hours (or on the next day), filter off the precipitate and wash it thoroughly with 1% oxalic acid solution and water. Ignite the precipitate to the oxides, and dissolve it in a small amount of hot 4 M HC1.

To separate the REE from calcium, dilute this solution with water to 10-15 ml, add 3 mg of A1 (as a salt solution), and co-precipitate REE hydroxides and aluminium hydroxide with ammonia (pH -~9). Dissolve the precipitate in a small amount of hot 2 M HC1.

Determination of REE. To the acid solution (pH -~ 1) containing not more than 40 gg of REE, add 1 ml of 1% ascorbic acid solution. After several min, add 1 ml of the buffer and 2 ml of the Arsenazo III solution. Add water to 20 ml and mix well. Adjust pH to 2.6___0.1 with 0.1 M NaOH. Transfer the solution to a 25-ml standard flask, add water up to the mark, mix well and measure the absorbance at 650 nm, using the reagent blank solution as the reference.



8-Hydroxyquinoline (oxine) (formula 4.42) reacts in ammoniacal media with cerium ions to give a sparingly soluble chelate, extractable into CHC13. The brown-red colour of the extract is used for determining cerium.

It has been shown that the coloured extract contains cerium(IV) oxinate, which in the presence of reducing agents gives pale-yellow Ce(III) oxinate. In the absence of reductants the yellow Ce(III) oxinate is readily oxidized by atmospheric oxygen to the intensely coloured Ce(IV) oxinate.

In CHC13 solutions ~max of this compound is 495 nm, and e = 6.7.103 (sp. abs. 0.048). All the other REE and Th give, like Ce(III), pale yellow oxinates which exhibit a small

absorption at 495 nm. Thus, in order to avoid the interference of REE and Th on the determination of cerium, one is recommended to measure the absorbance of Ce(IV) oxinate at 530 nm (although the sensitivity is slightly lower than at 495 nm).

The extraction of Ce(IV) oxinate is complete when the pH of the ammoniacal solution is 9.9-10.6. The absorption spectra of Ce(IV) oxinate solutions in 1,2-dichloroethane, CC14, and benzene, differ slightly from that in the chloroform solutions.

The presence of citrate in the aqueous solution interferes in the quantitative extraction of cerium oxinate. The solution should also not contain other Group 1-IV metals besides REE and Th. In the presence of EDTA, titanium is not extracted from solutions of pH 9.9-10.6 [57].

Reagents

8-Hydroxyquinoline (oxine), 1% solution in ethanol. Standard cerium solution: 1 mg/ml. Dissolve 3.1000 g of Ce(NO3)3.6H20 in water, add

2 ml of conc. HNO3, and dilute to 1 litre in a standard flask. Oxalic acid, 8% solution.

Separation of Ce as oxalate. To the acid sample solution add 5-20 ml of oxalic acid solution and 3-5 mg of lanthanum (as a salt). Adjust the pH of the solution to 2-3, heat to 70-80~ and keep at this temperature for 1 h. After 2-3 h, filter off the precipitate and wash it thoroughly with 1% oxalic acid solution and water. Ignite the precipitate to the oxide, and dissolve it in a small volume of hot 4 M HC1.

Determination of Ce. Dilute the obtained acid solution, containing not more than 0.3 mg of Ce, with water to 10 ml, add 1 ml of oxine solution, 1 drop of 1% phenolphthalein solution in ethanol, and ammonia (1+1) until the solution is rose coloured. Add 1 ml of ammonia (1+1) (the pH should be within 9.9-10.6) and transfer the solution to a separating funnel. Shake the solution with 2 portions of CHC13 (5 min shaking with each portion). Dilute the combined extracts with chloroform to 25 ml in a standard flask, and measure the absorbance at 530 nm, using the solvent as reference.


In addition to Arsenazo HI, the following azo dyes containing the arsonic or phosphonic acid group have also been recommended as spectrophotometric reagents for either individual lanthanides or the total REE: Arsenazo I [58,59], p-Acetylarsenazo [60], Carboxyarsenazo (e = 5.2.104 for La) [61], Dicarboxyarsenazo III [62], Chlorophosphonazo III [63,64], Chlorophosphonazo [65], Chlorophosphonazo DAL [66], p-Acetylchlorophosphonazo


[67,68], p-Nitrochlorophosphonazo [69]. In the extractive method for La with the use of Chlorophosphonazo III, diphenylguanidine and butanol, ~ = 1.6.105 [70].

Other azo reagents include PAR [71-75], TAR [76] and its derivatives [77], 5-Br- PADAP [78-82], Carboxynitrazo (~ = 1.4.105) [83], Sulphonazo III [84], Calmagite [85], and Hydroxynaphthol Blue [86]. High sensitivity in determination of La and Ce was obtained in the presence of PAN [87]. Praseodymium and erbium were determined with PAN and its derivative in the presence of a non-ionic surfactant [88,89]. Neodymium was determined in glasses, based on the 4 th order derivative spectrum of its complex with PAN [90].

Xylenol Orange has been applied for determination of REE [91-95]. High sensitivity is obtained in the presence of CP (e = 9.2.104 at 625 nm for La) [96-99]. Other tr iphenylmethane reagents proposed are Methylthymol Blue [100], Chrome Azurol S [101-103], and Pyrocatechol Violet [104]. The sensitivity of these methods increases in the presence of CP [ 105-107].

From among the xanthene chelating reagents, Bromopyrogallol Red [108-110], and Gallein [111] have found application in the spectrophotometric determination of REE.

Rhodamine B, and the anionic complexes of REE with oxine [112], salicylic acid derivatives [ll3,114], and 3,5-dinitrocatechol [115] have been the basis for sensitive methods of determining REE (e = 9.104 at 550 nm). Praseodymium has been determined by a sensitive flotation-spectrophotometric method (e = 1.8.105) with the use of 5,7-dichlorooxine and Rhodamine 6G [116].

REE ions with 1,10-phenanthroline form cationic complexes such as [La(phen)33+], which give ion-associates with xanthene acid dyes, eosin or Erythrosin [117,118]. Molar absorptivities are of the order of 1-105. Europium(II) has been determined by an indirect method, after reduction of Fe(III) and reaction of the resulting Fe(II) with 1,10- phenanthroline [ 119].

Trivalent cations of REE in aqueous solutions, acidified with HC1, HNO3, or HC104, absorb in the UV or VIS. The absorption bands are narrow, with sharp, non-overlapping peaks, but the molar absorptivities are rather small (1-10), and individual species of REE can be determined at concentrations of the order of 1 mg/ml [120]. Higher sensitivities are obtained after the ions have been converted into EDTA complexes [121 ]. The determination can be made more selective and sensitive by the use of the derivative spectrophotometry techniques [ 122-124]. Neodymium and erbium have been determined in the mixtures of REE by the derivative spectrophotometry technique using ferron and diethylamine [ 125].

Europium(III) has been selectively reduced in the Jones column to Eu(II) which can reduce Methylene Blue to the colourless leuco- form [126]. Europium(II) also reduces phosphomolybdic acid to molybdenum blue [127] and gives a colour reaction in solutions containing 1,10-phenanthroline and Fe(III). 2,2'-Diquinoxalyl has also been used for determining Eu [ 128].

Cerium(IV) ions give a yellow colour in H 2 8 0 4 and HNO3 solutions, with maximum absorption in the UV region and e = 5.6.103 at 320 nm.

Certain determination methods are based on coloured complexes of Ce(IV) with organic reagents, such as formaldoxime [129], N-p-tolylbenzohydroxamic acid [130], or 3-thianaphthenoyltrifluoroacetone (e = 5.5.103 at 424 nm) [ 131 ].

Some methods are based on the colour effects occurring in the oxidizing action of Ce(IV) on, e.g., o-tolidine and tetron (N,N'-tetramethyl-o-tolidine) (e = 2.5-104 at 470 nm) [132].



The Arsenazo III has been used for determining the total REE and individual metals of this group in natural waters [133], air [134], ores, rocks, and minerals [ 135-138], steels 19,139,140], chromium and its alloys [141 ], copper-nickel alloys [54], and fertilizers [ 142]. Continuous FIA has been applied [78,138].

The Arsenazo I method was applied for the determination of REE in steel [58] and in minerals [59].

The 8-hydroxyquinoline method was used for determining cerium in cast iron and steel [5]. Cerium was determined with PAR in rocks [8], with Xylenol Orange- in apatites [98], with p-Acetylchlorophosphonazo - in minerals [67], and with Chlorophosphonazo in plant materials [65].

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Chapter 40. Rhenium

Rhenium (Re, at. mass 186.21) occurs mainly in oxidation states VII, V, and IV. Rhenium(VII) compounds are the most stable. The colourless perrhenate, ReO4- ion exhibits weak oxidizing properties. The chemical properties of rhenium resemble those of Mo and Mn. Rhenium(V) and (IV) form halide, oxalate, and tartrate complexes.


The main requirement in connection with the determination of small quantities of Re is its separation from Mo, which is usually the major interfering element. Small amounts of rhenium are normally found with Mo in natural materials, such as molybdenite.

40.1.1. Extraction

Traces of rhenium can be separated from larger amounts of Mo by shaking a chloroform solution of tetraphenylarsonium chloride with an aqueous solution at pH 8-9 containing ReO4-, thereby forming the CHC13-soluble ion associate [(C6Hs)4As+][ReO4-]. Molybdate ions remain in the aqueous solution. The distribution coefficient for rhenium is -~1,000. At chloride concentrations greater than 0.1 M, the efficiency of Re extraction is worse. Tetraphenylarsonium perrhenate is extracted over a wide pH range, but a medium at pH 8-9 is most suitable for the separation from molybdenum. EDTA is added to prevent the precipitation of hydroxides of other metals [1,2]. AuC14-, MnO4-, C104-, and SCN- are extracted together with ReO4-. Tetraphenylphosphonium chloride may be substituted for tetraphenylarsonium chloride.

From acid media, ReO4- can be extracted as its ion-pair with tribenzylammonium ion (dichloromethane) [3], tetraoctylammonium ion (CHC13) [4], tributylammonium ion [5], N-benzylaniline (CHC13) [5], or amiloride (4-methyl-2-pentanone) [6]. ReO4- can be extracted from 6 M NaOH with pyridine [7]. Order of citation changed: please confirm.

Re(VII) has been separated by extraction with cyclohexanone from various inorganic acids [8]. Rhenium can be extracted from bromide media into CHC13 as ReOBr4 with dithiol [9]. Nitrotetrazolium chloride has been used for extraction (dichloroethane) of Re from mixed chloride and iodide media [10].

Rhenium has been extracted also with the crown ether, dibenzo-18-crown-6 (formula 1.11)[11].

Problems connected with the extractive separation of Re from Mo and other metals have been studied in detail [12-14].


Rhenium can be separated from many metals by precipitation with H2S in 5-6 M HC1 (or 2.5-3 M H2SO4). Arsenic(III) is a suitable collector since it does not interfere in the subsequent determination of rhenium by the thiocyanate method.

At pH < 7.5 the MoO42- ion can be co-precipitated with Fe(III) used as collector, and ReO4-remains in the solution [15].


The sample containing Re is sintered at 600-700~ for 2-3 h with CaO and Ca(NO3)2, then the sinter is leached with hot dilute bromine water. Most metals (including molybdenum, which is then present as CaMoO4) remain in the solid residue while ReO4- ions pass into solution [ 16].

Perrhenate is also leached with water from a sample melted with NazCO3 or NaOH. Rhenium heptoxide, Re207, is volatile and can be separated by distillation at 260-

280~ from a concentrated sulphuric acid medium [1,17,18]. It may be accompanied by As(HI), Hg, Se, and to a lesser degree by Sb, Te and Mo. In the presence of hydrochloric acid, rhenium distils as the oxychloride at a lower temperature, but As, Ge, Hg, Sn, Se, Mo, Te, and T1 are wholly or partly co-distilled. In view of the partial co-distillation of Mo, Re cannot be directly determined in the distillate. In the distillation of Re, carbon dioxide or air is used as the carrier gas.

When a slightly acidic solution is run through a cation-exchange column, most metals (Fe, Cu, Ni, Mn, A1, etc.) are retained as cations, while rhenium is eluted as the ReO4- anion.

Small amounts of ReO4- can be conveniently retained on an anionite column [19,20]. By using suitable eluents it is possible to separate Re from Mo and W [21 ].


A common spectrophotometric method for the determination of rhenium involves extraction of the thiocyanate complex into isoamyl alcohol. The ~-furildioxime method has similar sensitivity, the rhenium complex being measured either in the aqueous medium or in the CHC13 extract. Methods based on ion-pairs of ReO4- with basic dyes are the most sensitive.


The red-orange colour formed when SnC12 is added to an HC1 solution containing rhenium(VII) and thiocyanate is the basis of the spectrophotometric method for rhenium [5,22,23]. The reaction is complex, and the colour obtained depends on the concentration of the reducing agent and thiocyanate, and on the reaction time.

It has been shown that two complexes can be formed: a greenish-yellow and a red- orange complex with absorption maxima at 350 and 430 nm, respectively. The formula [ReOz(SCN)4] 3- has been proposed for the red-orange complex. Complexes of Re(IV) have also been reported [24].

Reproducible results in the determination of Re are obtained only when the reaction conditions are kept constant. Maximum colour intensity requires the presence of a small excess of SnCI2. The most suitable acid concentration is 2 M HC1, and the thiocyanate concentration should not be lower than 1%. Ascorbic acid has also been used instead of SnCI2 in Re determinations by the thiocyanate method [25,26].

Extraction of the thiocyanate complex with higher alcohols, diethyl ether, or DIPE enhances the sensitivity of the method, because the high distribution coefficients facilitate concentration of the complex in a small volume of organic extractant [27].

The molar absorptivity of the rhenium-thiocyanate complex in isoamyl alcohol is 3.8.104 (a = 0.21) at 430 nm. The absorption spectrum is shown in Fig. 40.1. The following species interfere in the determination of rhenium by the thiocyanate method; Mo, W, V, and Cr, as well as oxidizing and reducing agents.

The thiocyanate complex of Re has been extracted with a thioacetanilide solution in benzene [28], DAPM [29], or hexamethylphosphoramide (or other amides) in CHC13 [30,31 ].

352 40. Rhenium

A mixed Re complex with thiocyanate and thiourea has been extracted into acetophenone [32]. The sensitivity of determining Re by the thiocyanate method increases in the presence of Fe(III) and NO3-[33].

1 2

~oo c~o ' soo "s3oL L" 6oo wavelengtla, nm

Fig. 40.1. Absorption spectra of the rhenium thiocyanate complex in isoamyl alcohol (1) and the rhenium a-furildioxime complex in aqueous acetone (2)

Reagents

Potassium thiocyanate, 20% solution. Standard rhenium solution: 1 mg/ml. Dissolve 0.1550 g of KReO4 in water and dilute

the solution to the mark with water in a 100-ml standard flask. Tin(II) chloride: fresh 0.1% solution prepared by dissolving 0.1 g of SnClz.2H20 in 100

ml of HC1 (1 +3). Tetraphenylarsonium chloride, 1% solution.

Procedure

Extractive separation of Re. Adjust -~30 ml of the perchlorate-free sample solution to pH 8-8.5 with NaHCO3. If hydrolysable metals are present, add EDTA first. Transfer the solution to a separating funnel, add 1 ml of the tetraphenylarsonium chloride solution, and extract Re by shaking for 2 min with two 5-ml portions of CHC13. Before draining the extract, allow the phases to separate completely by letting them stand for 30 min. Wash the combined extracts with 1 ml of 1% NaHCO3 solution, and again allow the phases to stand for 30 min before separating them. Carefully separate the chloroform phase and evaporate it to dryness in a platinum crucible containing a little NazCO3 dissolved in a few drops of water. Fuse the residue with 0.5 g of NazCO3, dissolve the cooled melt in water, and neutralize the solution with 1 ml of H2S04 (1+3). Determination of Re. Acidify ~20 ml of solution containing not more than 100 gg of Re with 5 ml of conc. HC1, and add 3 ml of the thiocyanate solution. Heat the solution to 50~ in a beaker on a water-bath, add 0.5 ml of the SnC12 solution with vigorous stirring, and keep the solution at 50~ for 20 min on the water-bath. After cooling to room temperature, transfer the solution to a separating funnel and extract with two portions of isoamyl alcohol. Dilute the extracts to the mark with the solvent in a 25-ml standard flask, and measure the absorbance at 430 nm, using isoamyl alcohol or water as the reference.


40.2.2. ~-Furildioxime method

In acid medium (HC1, H2804) containing SnC12 as a reducing agent, c~-furildioxime (formula 32.2) reacts with rhenium to form a red complex [34-36].

The coloured complex is sparingly soluble in aqueous medium, but dissolves in solutions containing 25% of acetone, which also accelerates the reaction. The reaction rate decreases, however, at acetone concentrations above 25%.

Heating the solution (to not higher than 60~ since volatile acetone is present) also increases the reaction rate. The optimum acidity in the medium corresponds to 0.8-1 M HC1. The reaction with rhenium requires a considerable excess of ~-furildioxime, the quality of which affects the absorbance [34]. The minimum amount of stannous chloride necessary is 2.5 ml of 10% SnCI2 solution per 25 ml of the final solution.

The molar absorptivity of the complex in aqueous acetone at ~max 530 nm is 4.1.10 4 (a = 0.22).

The rhenium cz-furildioxime complex can be extracted into CHC13. The distribution coefficient is 150. Addition of isoamyl alcohol clears the chloroform extract. In CHC13 medium e = 3.7.104.

Thiocyanate, nitrate, and fluoride interfere in this method for determining rhenium. Palladium and copper in acid media sparingly soluble ~-furildioximates. Normally, rhenium should be separated from Mo before the determination.

Reagents

a-Furildioxime, 0.5% solution in acetone. If coloured, purify the solution by shaking it with active carbon.

Standard rhenium solution: 1 mg/ml. Preparation as in Section 40.2.1. Tin(H) chloride: a freshly prepared 10% solution in HC1 (1 +9).

Procedure

Place -~ 15 ml of the slightly acidic sample solution, containing not more than 100 ~tg of Re, in a 25-ml standard flask. Add 2 ml of conc. HC1 and 6 ml of the c~-furildioxime solution, and mix well. Add 2.5 ml of the SnCI2 solution and dilute to the mark. Heat the flask in a water- bath at 60~ for 20 min with occasional shaking. Cool the solution in a cold water-bath to room temperature, and measure the absorbance at 530 nm against a reagent blank or water.

Extraction into chloroform. After heating at 60~ as above and cooling to room temperature, transfer the coloured solution to a separating funnel and extract the rhenium complex by shaking for 1 min with two portions of CHC13. Dilute the combined extracts in a standard flask with chloroform and isoamyl alcohol (4+1). Measure the absorbance of the extract at 530 nm, using the solvent as reference.


The most sensitive spectrophotometric methods for the determination of Re involve the extraction of ion-associates formed by ReO4- with basic dyes. From among the triarylmethane dyes, use has been made of Methyl Green (benzene, e - 1.2.105 at 640 nm) [37], fuchsine (formula 27.1) [1] Brilliant Green (benzene, e = 1.0.105) [38], Crystal Violet [39], Victoria Blue 4R (formula 4.28) [40], Rhodamine B [41], Safranine T [42], Nile Blue

354 40. Rhenium

(formula 4.32) (e = 7.2.104) [43], and Chrompyrazole II [44]. In another method [45] the associate of ReO4- with Brilliant Green is adsorbed on microcrystalline benzophenone which is then dissolved in benzene. A method based on the reaction of perrhenate with Nitrotetrazolium Blue and SnCI2 in CHC13 medium is very sensitive [46]. Trace amounts of Re were separated from large quantities of Mo by extraction into benzene solution of Brilliant Green [47].

In addition to a-furildioxime, other oximes are used as spectrophotometric reagents for rhenium, such as dimethylglyoxime [7,48,49], c~-pyridyldioxime [50], o~-benzoinoxime [51], methyl-2-pyridylketoxime [52], and phenyl-2-pyridylketoxime [53]. c~-Benzoyldioxime gives with Re, in thiocyanate medium, a complex ()~max = 430 nm) extractable into isoamyl acetate [54]. In each case the colour reaction occurs in the presence of SnCI2.

In 3-5 M HC1 containing SnCI2, thiourea forms a yellow cationic rhenium complex which is soluble in water, thus affording the basis of a less sensitive method for determining Re (e -~ 6.103) [55-58]. The colour reaction is slow, and heating to 70~ is advantageous. Above 75~ browning of the solution can be observed as the thiourea complex decomposes. Several complexes can be formed in the rhenium-thiourea system. Which species predominates, depends on the amount of the excess of reagent and the acidity. The thiourea method is suitable for the differential spectrophotometric determination of larger quantities of rhenium [59].

Phenylthiourea (e - 9.5.103) [60] and 2-pyridylthiourea (e - 1.6.104) [61] have been proposed as spectrophotometric reagents for rhenium. Other thiourea derivatives recommended are 1-phenylthiosemicarbazide and 1,4-diphenylthiosemicarbazide (formula 42.1) [62].

Other organosulphur reagents for rhenium include dithiol [63], Bismuthiol II (formula 49.1) (~ = 2.2.104) [64], rubeanic acid [65], mercaptopropionic acid [66], o-hydroxythiobenzhydrazide [67], and thiobenzhydrazide [68].

A molybdo-rhenium heteropoly acid has also been applied for determination of rhenium [69,70].


The thiocyanate method has been used for determining rhenium in dusts and in molybdenite [51], copper ores and concentrates [28,71,72], molybdenum, tungsten, and vanadium [73], and coal [ 16].

o~-Furyldioxime has been applied in determinations of Re in copper concentrates [71], molybdenite [36], molybdenum concentrate [4], and molybdenum and tungsten alloys [74]. Dimethylglyoxime was used for determining Re in molybdenite, tungsten, and minerals [7].

Rhenium has been determined in copper concentrates and rhenium-tungsten alloys with the use of Nitrotetrazolium Blue [46].

The thiourea method has been applied for determining rhenium in used catalysts [75]. The results of Re determination in ores by the spectrophotometric method and by other methods have been compared [76].

References

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(1994). 11. Koshima H., Onishi H., Anal. Chim. Acta, 232, 287 (1990). 12. Yatirajam V., Kakkar L.R.,Anal. Chim. Acta, 47, 568 (1969); 54, 152 (1971). 13. Yatirajam V., Kakkar L.R., Talanta, 17, 759 (1970). 14. Yatirajam V., Kakkar L.R., Mikrochim. Acta, 1970, 708; 1971, 479. 15. Novikov A.I., Zh. Anal. Khim., 16, 588 (1961). 16. Kuznetsova V.V., Zh. Anal. Khim., 16, 736 (1961). 17. Basifiska M., Rutkowski W., Chem. Anal. (Warsaw), 13, 799 (1968). 18. Solt M.W., Wahlberg J.S., Myers A.T., Talanta, 16, 37 (1969). 19. Hamaguchi H., Kawabuchi K., Kuroda R., Anal. Chem., 36, 1654 (1964). 20. Morgan J.W. Golightly D.W., Dorrzapt A.T. Jr., Talanta, 38, 259 (1991). 21. Korkisch J., Feik F., Anal. Chim. Acta, 37, 364 (1967). 22. Ko~licka M., W6jtowicz M., Adamiec J., Chem. Anal. (Warsaw), 15, 247 (1970). 23. Wahi A., Kakkar L.R., Fresenius' J. Anal. Chem., 338, 298 (1990). 24. Jordanov N., Pavlova M., Zh. Anal. Khim., 19, 221 (1964); 20, 591 (1965); 22, 212

(1967). 25. Wahi A., Kakkar L.R., Fresenius' J. Anal. Chem., 343, 904 (1992). 26. Wahi A., Kakkar L.R., Anal. Sci., 10, 509 (1994). 27. Skiba H., W6jtowicz M., Chem. Anal. (Warsaw), 10, 183 (1965). 28. Mishra N. et al., Anal. Sci., 6, 407 (1990). 29. Akimov V.K., Kliot L.Ya., Busev A.I., Zh. Anal. Khim., 28, 118 (1973). 30. Mitra M., Mitra B.K., Talanta, 25, 597 (1978). 31. Mishra N., Sinha S.K., Patel K.S., Mishra R.K., Bull. Chem. Soc. Jpn., 62, 3400 (1989). 32. Savariar C.P., Hariharan T.R., Mikrochim. Acta, 1975 I, 477. 33. Kuznetsova N.M., Monakhova N.G., Zh. Anal. Khim., 45, 683 (1990). 34. Fryer F.A., Galliford D.J., Yardley J.T., Analyst, 88, 188, 191 (1963). 35. Peshkova V.M., Ignat'eva N.G., Zh. Anal. Khim., 22, 757 (1967). 36. Dor2 D., Dobrowolski J., Chem. Anal. (Warsaw), 24, 465 (1979). 37. Taraian V.M., Mirzoian F.V., Sarkisian Zh.V., Zavod. Lab., 45, 409 (1979). 38. Fogg A.G., Burgess C., Burns D.T.,Analyst, 95, 1012 (1970). 39. Gajbakian A.G. et al., Zh. Anal. Khim., 42, 2093 (1987). 40. Pilipenko A.T., Kish P.P., Zheltvai I.I., Ukr. Khim. Zh., 37, 477 (1971). 41. Lebedeva S.P., Taraian V.M., Zh. Anal. Khim., 30, 1403 (1975). 42. Pilipenko A.T., Nguen Mong Shin', Ukr. Khim. Zh., 32, 1211 (1966). 43. Gagliardi E., Ftisselberger E., Mikrochim. Acta, 1972, 385. 44. Busev A.I. et al., Zh. Anal. Khim., 21,574 (1966); 22, 205 (1967). 45. Burns D.T., Tungkananuruk N.,Anal. Chim. Acta, 204, 359 (1988). 46. Plastinina E.I., Borisova L.V., Zh. Anal. Khim., 44, 2211 (1989). 47. Lyam Ngok Tkhu, Ukr. Khim. Zh., 57, 67 (1991). 48. Ko21icka M., W6jtowicz M., Adamiec J., Chem. Anal. (Warsaw), 15, 701 (1970). 49. Wahi A., Kakkar L.R.,Anal. Sci., 9, 409 (1993). 50. Trussell F., Thompson R.J., Anal. Chem., 36, 1870 (1964).

356 40. Rhenium

51. Khaira S., Kakkar L.R., Fresenius'Z. Anal. Chem., 335, 404 (1989). 52. Thompson R.J., Gore R.H., Anal. Chim. Acta, 31,590 (1964). 53. Guyon J., Murmann R.K.,Anal. Chem., 36, 1058 (1964). 54. Wahi A., Kakkar L.R., Fresenius'J. Anal. Chem., 352, 387 (1995). 55. Borisova L.V., Zh. Anal. Khim., 24, 1361 (1969). 56. Nemodruk A.A., Bezrogova E.V., Zh. Anal. Khim., 24, 1534 (1969). 57. Lazarev A.I., Lazareva V.I., Komarnaya V.D., Zavod. Lab., 42, 1304 (1976). 58. Bahr H., J~dras C., Chem. Anal. (Warsaw), 23, 1031 (1978). 59. Shanina T.M., Mikhailovskaya V.S., Gelman N.E., Zh. Anal. Khim., 29, 2059 (1974). 60. Pollock E.N., Anal. Chim. Acta, 32, 418 (1965); 47, 367 (1969). 61. Dutta G., Sur B., Mikrochim. Acta, 1986 I, 359. 62. Borisova L.V., Plastinina E.I., Ermakov A.N., Zh. Anal. Khim., 29, 743 (1974). 63. Koyama M. et al., Chem. Anal. (Warsaw), 17, 679 (1972). 64. Lazarev A.I., Lazareva V.I., Zh. Anal. Khim., 32, 751 (1977). 65. Bozhkov O.D., Jordanov N., Borisova L.V., Talanta, 35, 62 (1988). 66. Talipova L.L., Lapin S.B., Parpiev N.A., Zh. Anal. Khim., 31, 106 (1976). 67. Gangopadhyay P.K., Shome S.C., Anal. Chim. Acta, 75, 235 (1975). 68. Gangopadhyay S., Gangopadhyay P.K., Shome S.C., Anal. Chim. Acta, 83, 409 (1976). 69. Semenovskaya E.N., Basova E.M., Zh. Anal. Khim., 37, 2014 (1982). 70. Basova E.M., Semenovskaya E.N., Zh. Anal. Khim., 39, 1834 (1984). 71. Stefanov S., Jordanov N., Pavlova M., Mikrochim. Acta, 1976 II, 449. 72. Ghosh A., Patel K.S., Mishra R.K., Analyst, 115, 969 (1990). 73. Pavlova M., Kadieva S., Jordanov N., Z. Anal. Chem., 285, 271 (1977). 74. Cotton T.M., Woolf A.A., Anal. Chem., 36, 248 (1964). 75. Bochkova L.P., Borisova L.V., Korchemnaya E.K., Zh. Anal. Khim., 47, 809 (1992). 76. Kolosova L.P., Karchevskaya G.Ya. et al., Zavod. Lab., 56, No 12, 19 (1990).

Chapter 41. Rhodium and iridium

Rhodium (Rh, at. mass 102.91) and iridium (Ir, at. mass 192.22) occur in the HI, IV and VI oxidation states, the compounds of rhodium(III) and iridium(IV) being the most stable. Powdered Rh and Ir, when fused with Na202, give RhO42- and IrO42-, respectively. Strong reducing agents (e.g., zinc, magnesium) reduce rhodium(III) (in solutions) to the metal. Iridium(III) and iridium(W) are not so readily reduced, although complete reduction may be achieved in the presence of Pd and Pt. Rhodium(III) and iridium(IV) give stable halide, cyanide, and ammine complexes.


41.1.1. Extraction

From 1-6 M HC1, containing SnC12, rhodium(III) can be separated from Ir, Pd, and Pt by extraction with benzene in the presence of DAPM [1], TOA [2,3], or 2-octylaminopyridine [4]. Rhodium is also extracted with isoamyl alcohol from an acid medium containing bromide and Sn(II) after heating at -~90~ [5].

From a 0.06 M HC1 medium hexachloroiridate(IV) can be extracted as an ion-pair with tetraphenylarsonium cation [6]. This method enables one to separate Ir from Rh and Pd, but not from Pt. Ir has been separated from Rh, also, by extraction with tetra-n-butylammonium ion [7].

Rh (also Ir, Pd, and Pt) can be extracted with bis-(2-ethylhexyl)dithiophosphoric acid [8]. Ir can be extracted with mercaptobenzothiazole [9] or thiobenzanilide [10]. Rhodium has been separated from Ir by the extraction (CHC13) of its complex with 2- mercaptobenzimidazole [11 ]. HDEHP has been used for extraction separation of Rh from Ru and Ir [12]. Extraction of Rh (and other noble metals) by thiourea and its derivatives was studied [ 13].


Rhodium(HI) can exist as a cationic complex in hydrochloric acid medium, whereas Ir(IV), Pt(IV), and Pd(II) exist as anionic complexes. These properties enable their separation with the use of ion exchangers [ 14-16].

Chelate complexes of Rh with TAR [17,18], TAN [19], PAN [20-22], and 8- hydroxyquinoline [23] and its derivatives [24] have been used for separation of Rh by the liquid chromatography.

The sorption of Rh and Ir on polyurethane foam from thiocyanate medium has been applied for the isolation of Rh and Ir and for their separation [25-27].

Iridium can be separated from Rh and Pt owing to the fact that the anionic Ir chloride complex is precipitated with Ag + [7]. Precipitation methods have been proposed also for separation of Rh from Ir and Pt [28]. Rhodium(III) may be separated from iridium(W) by reduction to the metal [iridium being reduced only to Ir(III)] with Cu powder in 1 M HC1 or sodium borohydride (NaBH4)] [29,30]. Iridium can also be separated from Rh and Ru by flotation of the ion-associate formed by the chloride complex of Ir(IV) with a cationic

3 5 8 41. R h o d i u m and i r idium

surfactant [31,32]. Iridium can be separated by distillation from boiling HC104 in a stream of chlorine.

This method is specific for Ir in the absence of Ru and Os [33]. Fire-assay methods have been applied for separation of Rh and Ir [34-37]. These methods have been discussed in Section 23.1.1.

41.2. Methods of rhodium determination

The determination of rhodium with the use of tin(II) in chloride or iodide medium, and a sensitive method based on the ion-associate of Rh-SnC13- complex with Rhodamine 6G, have been presented.

41.2 .1 . T i n ( I I ) chloride (or iodide) method

When SnC12 is added to a solution of rhodium(III) in HC1 and the system heated, the solution turns red. The red complex has been used for spectrophotometric determination of Rh [38,39].

The absorption maximum of the Rh complex with SnC13- is at 470 nm. The molar absorptivity, e = 4.2.103 (a = 0.041). If rhodium is present in solution as the sulphate complex, the solution should be heated for some time after the addition of HC1 to allow the rhodium sulphate complex to be converted into the chloride complex. The other platinum metals interfere in the determination.

The SnC12 method has been used to determine rhodium in platinum concentrate [40]. A considerable increase in sensitivity is obtained when bromide [41] or iodide [42] is

used instead of chloride. In the bromide method, a yellow-orange complex is obtained (~; = 2.9.104 at )~max 427 nm; a = 0.29). The rhodium-tin(II) iodide complex is red and has an absorption maximum at 460 nm (e = 3.9.104; a = 0.34). In the iodide method, the optimum concentration of HC1 is 1 M. The concentration of KI should not be lower than 4% in the final coloured solution. The quantity of SnC12 only slightly affects the absorbance. The iodine liberated by air in the initial stage of the procedure is reduced when the SnC12 is added.

Reagents

Tin(II) chloride, 10% solution in 2 M HC1. Potassium iodide, 20% solution. Standard rhodium solution: 1 mg/ml. Fuse 0.1000 g of metallic rhodium powder with 2

g of potassium pyrosulphate in a silica crucible. Dissolve the melt in hot 1 M HC1 and make the solution up to volume with this acid in a 100-ml standard flask.

Procedure

Tin(II) chloride method. To the sample solution containing not more than 0.4 mg of Rh, add 5 ml of the SnC12 solution. Heat the solution for 15 min nearly at boiling point. Cool the solution, dilute to the mark with 2 M HC1 in a 25-ml standard flask, and measure the absorbance at 470 nm against water as the reference.

Tin(II) iodide method. To the sample solution containing not more than 50 ~tg of Rh, add 5 ml of the KI solution. Mix well, and heat for 15 min in a boiling water-bath. To the

41.2. Methods of rhodium determination 359

cooled solution, add 5 ml of the SnC12 solution. Dilute the solution to volume in a 25-ml standard flask with dilute HC1 so that the final HC1 concentration is 1 M. Place the unstoppered flask in the boiling water-bath for 2 min. Cool the solution and measure its absorbance at 460 nm against a reagent blank solution.


In -2 M HC1 medium rhodium forms anionic complexes with SnC13- and C1- ions. With a large excess of tin(H), and at room temperature, a stable red complex is obtained after 1 h. The same effect is reached with 10-15 min heating on a boiling water-bath. The complex gives with Rhodamine 6G (R6G) (formula 4.30) a sparingly soluble compound that collects on the inside surface of the separating funnel after shaking (for 45 s) of an aqueous phase with DIPE.

The optimum SnC12 concentration during flotation is 0.01-0.02 M. At higher SnC12 concentration, the absorbance of the blank increases. The excess of basic dye should be at least 50-fold with respect to Rh. The precipitate in the separating funnel is washed with 2 M HC1, then dissolved in acetone. This solution is the basis of a very sensitive and precise method for spectrophotometric determination of rhodium [43]. The molar absorptivity is 4.0.105 (a = 3.9) at 530 nm.

A 5-fold excess of Pd and Ir causes about 15% increase in the absorbance. The influence of Pt is more serious. These three metals should be separated from Rh before its determination. Ruthenium and osmium, when present, are readily volatilized as the tetroxides.

Reagents

Rhodamine 6G (R6G), 1.10 -3 M (---0.05 %) solution. Tin(II) chloride, SnC12.2H20, 10% solution in 2 M HC1. Rhodium standard solution: mg/ml. Preparation as in Section 41.2.1. Rhodium(III)

chloride can also be used to prepare a standard solution [43].

Procedure

Evaporate to dryness the acid (HC1) sample solution (containing not more than 5 gg of Rh) on a boiling water-bath. Add 0.5 ml of SnCI2 solution and 2 M HC1 to about 15 ml. Heat the solution on a boiling water-bath for 15 min. Transfer the cooled solution to a separating funnel. Add 0.5 ml of R6G solution and 5 ml of DIPE, then shake for 40 s. Allow the phases to separate, and slowly discard the aqueous layer. Wash the ether phase and the precipitate by shaking with 10 ml of 2 M HC1 for 30 s. Remove the aqueous and the organic layers carefully, dissolve the precipitate in acetone, transfer the solution to a 25-ml standard flask and dilute to the mark with acetone. Measure the absorbance of the solution at 530 nm against a reagent blank.


The Malachite Green ion-associate with the Rh-SnC13- complex in HC1 medium has been used for flotation-spectrophotometric determination of rhodium [44].

Numerous azo reagents have been proposed for spectrophotometric determination of rhodium: PAN (e = 1.15.104) [20,45,46], PAR (in the presence of surfactant CP, e - 6.2.104)

360 41. Rhodium and iridium

[47], TAN [19], TAR [17,18], 4-(4-methyl-2-thiazolylazo)resorcinol in aqueous-organic medium [48] and other resorcinol derivatives [49], 2-(5-bromopyridylazo)-5-propyl-N- sulphopropylaminophenol [50], 5-(2-pyridylazo)-p-cresol (e = 8.5.104) [51 ], 2-thiazolylazo- p-cresol [52], and sulphochlorophenolazorhodanine [53].

Organic reagents for rhodium with sulphur as a ligand atom include sulphoallthiox [54,55]. Dithizone reacts with rhodium(III) at pH 8.5 on heating. The dithizonate formed ()~max = 560 nm) can be extracted, after cooling, into CHC13 [56]. Various other organic reagents have been proposed for determining Rh, viz. nitroso-R salt [57,58], 1,5-diphenylcarbazide [59], Chrome Azurol S (in the presence of CP) (e = 6.2-104) [60], and Chrompyrazole I in the presence of SnCI2 [61]. Rhodanine derivatives have been used for determination of Rh in the presence of Pd [62] and heavy metals [63].

Rhodium has been determined in the presence of Ru by the derivative spectrophotometry method as a complex with octadecyldithiocarbamate [64]. The 3 rd order derivative spectrophotometry has been used for determining rhodium as a complex with o- hydroxy-hydroquinonephthalein [65].

A number of spectrophotometric methods for determining Rh has been based on its catalytic effect on redox reactions, such as the oxidation of Methyl Red [66], Methyl Orange [67], or copper [68] with periodate. The catalytic reactions have been the basis for Rh determinations in technological samples [67] and in copper and nickel alloys [68].

41.3. Methods of iridium determination

The old tin(II) bromide method and a new, very sensitive method involving the basic dye Rhodamine 6G are described. A critical review of Ir determination methods has been published [69].

41.3.1. Tin(II) bromide method

When tin(II) in HBr solution is added to an acidic solution of the iridium bromide complex, a yellow complex is produced on heating. The absorbance of this complex is the basis of the spectrophotometric determination of iridium [70-72]. Maximum colour intensity is obtained by heating the solution for 1 min on a boiling water-bath. If the heating is continued for longer than 2 min the colour intensity decreases.

The molar absorptivity of the complex a t )Lmax -- 402 nm is 5.10 4 (a = 0.26). Before its determination by this method, iridium must be separated from Rh, Pt, and Pd.

Other metals (e.g., Co, Ni, Cu, Fe, Sb) interfere slightly. Low concentrations of HC1 (<0.5 M) and higher ones of H2SO4 in the sample solution may be tolerated.

The analogous method for determining iridium with tin(H) chloride in HC1 medium [73] is less sensitive than the tin(H) bromide method. It has been used for determining Ir in the presence of Pt (in catalysts), by derivative spectrophotometry [74].

Also, the tin(II) iodide method is less sensitive than the Sn(II) bromide method [75]. The iridium- (and rhodium-) halide complexes may be extracted as ion-pairs with

DAM, diphenylguanidine or tribenzylamine [76]. The process has been used as a basis for extraction-spectrophotometric methods of Ir determination.

Reagents

Stannous bromide, 25% solution, obtained by dissolving tin in 40% solution of HBr.

41.3. Methods of iridium determination 361

Standard iridium solution: 1 mg/ml. Fuse 0.1000 g of iridium powder with 2 g of sodium peroxide in a silver crucible. Dissolve the melt in water, acidify the solution with nitric acid, and heat to boiling. Neutralize the solution with NaHCO3. Filter off the precipitated IrO2 aq. and wash with water. Dissolve the precipitate in 10 ml of HC1 (1+3). After dilution with water, filter off any AgC1 present, and wash the filter with dilute HC1. Dilute the Ir solution to volume with water in a 100-ml standard flask.

Procedure

Place ~ 10 ml of sample solution, the HC1 concentration of which is not higher than 0.5 M, and which contains not more than 70 ~tg of Ir, in a 25-ml standard flask. Add 5 ml of conc. HBr, and heat for 10 min in a boiling water-bath. Add 5 ml of the SnBr2 solution, and mix the solution well. After 2 min cool the solution quickly under cold water. Dilute the solution to the mark with 0.01 M HC1, and measure the absorbance at 402 nm against water.


In hydrochloric acid medium, iridium reacts with SnC13- ions to form an anionic complex which reacts with the basic dye, Rhodamine 6G (R6G), yielding a sparingly soluble ion- associate. The compound precipitates at the phase boundary when the solution is shaken (for about 1 min) with DIPE. The precipitate is readily soluble in acetone. This solution has been used as a basis for a very sensitive flotation-spectrophotometric method for determining Ir [6].

The maximum and stable absorbance of the acetone solution is obtained when the solution containing Ir, after addition of SnC12, is heated on a boiling water-bath for 10-15 min, the tin(II) concentration is in the range 0.15-0.3 M, and the acidity corresponds to 2.5- 3.5 M HC1. At least a 50-fold molar excess of Rhodamine 6G is necessary, but very large excesses cause high results and an increased blank value. Reproducible results are obtained when the HC1 concentration during flotation is 2.3-2.7 M.

The ion-associate of SnC13- with Rhodamine 6G precipitates together with the iridium compound, but it is decomposed when the precipitate is washed with 2.5 M HC1. The absorbance of the blank does not exceed 0.06. The molar absorptivity is 3.6.105 (a = 1.9) at 530 nm. The high sensitivity of the method results from the fact that the molar ratio of R6G to Ir in the floated and washed compound is 4:1. It has been suggested that the compound is not a simple ion-associate but an adduct of the iridium(Ill) ion-associate and of the SnC13- ion-pair with the dye, of the following formula: [(R6G+)3IrC12(SnC13-)43-] [(R6G+)(SnC13-)].

Serious interference in the determination of iridium is caused by Rh and Pt. The selectivity of the method can be improved by preliminary separation of Ir(IV) by extraction with tetraphenylarsonium chloride (CHC13). If large amounts of Pt, Pd, Ru and Os are present, it is advisable to separate these metals, e.g., by selective reduction of Pt to the metal, extraction of Pd as its dimethylglyoximate, and distillation of Ru and Os as the tetroxides.

Reagents

Rhodamine 6G (R6G), 2-10 .3 M (--0.1%) solution. Iridium standard solution: 1 mg/ml. Preparation as in Section 41.3.1. The method for

preparation of the solution from Ir(III) chloride is given in ref. [4]. Tetraphenylarsonium chloride (TPA), 0.2% solution in CHC13.


Cerium(IV) sulphate, 1% solution in 0.05 M H2SO4.

Procedure

Separation of iridium. Evaporate the sample solution, containing not more than 10 ~tg of Ir(III) or Ir(IV), to dryness on a water-bath. Add 2 drops of the Ce(IV) solution and dissolve the residue in 10 ml of 0.06 M HC1. Transfer the solution to a separating funnel, add 10 ml of the TPA solution, and shake for 45 s. Wash the organic phase with 10 ml of 0.06 M HC1 to which 2 drops of Ce(IV) solution have just been added (shake for 45 s). Transfer the organic phase to a beaker, evaporate to dryness on a water bath, add a drop of conc. HNO3, followed by 3 ml of conc. HC1, and heat to expel nitrogen oxides. Add 0.5 ml of conc. HC1 and evaporate to fumes. Determination of iridium. To the residue, add 3 ml of 10% SnCI2 solution in 2 M HC1, and then enough conc. HC1 to give 10 ml of solution with an HC1 concentration in the range 2.5- 3.5 M. Heat the solution on a boiling water bath for 15 min. Allow to cool to room temperature and transfer the contents of the beaker to a separating funnel. Add HC1 and water to give 20 ml of solution, 2.4-2.8 M in HC 1. Then add 1 ml of R6G solution, 5 ml of DIPE, and shake for 1 min. Discard the aqueous layer. Wash the ether phase and the precipitate by shaking with three 5-ml portions of 2.5 M HC1 for 30 s each. Carefully remove the aqueous and the organic layers and dissolve the isolated precipitate in acetone. Transfer the solution to a 25-ml standard flask, dilute to the mark with acetone, and measure the absorbance of the solution at 530 nm against a reagent blank prepared in the same way.


Besides Rhodamine 6G (see above), Malachite Green and Crystal Violet form ion-associates with the anionic iridium chloride-SnC 13- complexes and these can also be used for flotation- spectrophotometric determination of iridium. The molar absorptivity e with Malachite Green is 1.55.105 [77,78].

Several azo reagents have been recommended for spectrophotometric determination of Ir: PAR [79], TAR [80], 2-(2-thiazolylazo)-5-diethylaminophenol (e = 4.8.104) [79-81], 5- (2-thiazolylazo)-2-ethylamino-p-cresol [80], and 2-pyridylazo-p-cresol [52]. Owing to kinetic inertness, the formation of Ir, unlike those of other platinum metals, often requires heating of the solution.

Among other organic reagents used for determining Ir, are ~-benzilmonoxime [82-84] and 1,5-diphenylcarbazide [59]. Diphenylamine sulphonate has been used for determination of various nitrite-chloride complexes of iridium [85].

The catalytic effect of Ir on the oxidation of N-methyldiphenylamine-4-sulphonic acid with KIO3 has also been applied for determination of Ir [86].

References

1. Busev A.I., Akimov V.A., Zh. Anal. Khim., 18, 610 (1963). 2. Khattak M.A., Magee R.J., Anal. Chim. Acta, 45, 297 (1969). 3. Kanert G.A., Chow A.,Anal. Chim. Acta, 69, 355 (1974). 4. Borshch N.A., Petrukhin O.M., Zh. Anal. Khim., 33, 2181 (1978). 5. Lee A.S., Beamish F.E., Bapat M.G., Mikrochim. Acta, 1969, 329. 6. Marczenko Z., Kalinowski K.,Anal. Chim. Acta, 144, 173 (1982).

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Chapter 42. Ruthenium and osmium

Ruthenium (Ru, at. mass 101.07) and osmium (Os, at. mass 190.2) occur mainly in oxidation states III, IV, VI and VIII. Ruthenium and osmium compounds can be oxidized to the volatile tetroxides RuO4 and OsO4, osmium being easier to oxidize. When fused with Na202, Ru and Os give RuO42- and 0s042-. These are stable in alkaline media, but when acidified they disproportionate to give the tetroxide and Ru(IV) or Os(IV), respectively. Ruthenium- (IV) and-(III) and osmium(III) give halide complexes.



Ruthenium and osmium can be isolated as the volatile RuO4 (b.p. 108~ and 0s04 (b.p. 130~ from sulphuric acid media [1-6]. Osmium is easier to oxidize than ruthenium, and this property may serve as a basis for their separation. Nitric acid oxidizes Os to OsO4 but does not oxidize Ru if the HNO3 is <40%. Both tetroxides are formed and can be distilled off in the presence of hot conc. HC104, KMnO4, NaBiO3, or KIO4, but only 0s04 is formed and can be distilled off in the presence of H202. Once OsO4 has been removed, KMnO4 is added and RuO4 can be distilled off. The distillate containing Ru and Os tetroxides can be absorbed in water [7], hydrochloric acid [8], or in solutions containing reducing agents. In alkaline media, ruthenate and osmate are formed.

RuO4 can be isolated by heating a dry sample mixed with NaBiO3 in a quartz boat placed in a tube flushed with moist oxygen [9].

42.1.2. Extraction and other methods

Ruthenium and osmium, when oxidized to tetroxides, are extractable into CC14 or CHC13, from dilute H2SO4 or HNO3. The oxidants are listed above in Section 42.1.1. Before the extraction of OsO4, RuO4 is reduced with Fe(II). Since the distribution coefficients are not high, a several-fold extraction of OsO4 (RuO4) is usually necessary [10,11]. Ruthenium (osmium) is stripped from the organic solvent with 1 M H2SO4 in the presence

of NaSCN, and the absorbance of the resulting ruthenium- (or osmium-) thiocyanate complex measured directly.

The chloride-, bromide-, and thiocyanate complexes of ruthenium(III) and osmium(IV) can be extracted from acid solutions by oxygen-containing solvents, also in the presence of TBP or amines [10,12-15]. Osmium has been separated from Ru after conversion into OSI62- and extraction with TOA [16]. From mixtures of thiocyanate complexes of Ru and Os, only the Os complex can be extracted into diethyl ether containing a small amount of peroxide [14]. Polyurethane foam has also been used for separating Ru and Os as their thiocyanate complexes [ 17,18].

Ruthenium can be separated from osmium by extraction as the ion-pair of perruthenate [Ru(VIII)] with tetraphenylarsonium ion [19] or with benzyltributylammonium chloride [2O].

Ruthenium has been extracted from semiconducting materials using bis-(2-

366 42. Ruthenium and osmium

ethylhexyl)amine [21]. The extraction of Ru (also Pt, Pd, and Rh) with thiourea has been studied [22]. The OsC16 2- complex was extracted with dibenzo-18-crown-6 [23].

Ruthenium and osmium have been separated from other metals with the use of ion- exchange resins [24,25]. The chelate complexes of Ru and Os with PAR [26,27], 8- hydroxyquinoline and its derivatives [28-30], as well as with 2-(2-thiazolyl)-5- diethylaminophenol [31] have been used for concentration of the metals by liquid chromatography. A polyurethane foam has been used for concentration of ruthenium in natural waters [32].

Cupellation methods and fire assays were used for the isolation of Ru and Os [33-36]; see also Section 23.1.1.

42.2. Methods of ruthenium determination

The spectrophotometric method for determination of Ru with the use of 1,4- diphenylthiosemicarbazide is rather insensitive, as compared with the very sensitive method based on an ion-associate of the anionic Ru(III)-SCN- complex with the basic dye, Capri Blue.

42.2.1. Diphenylthiosemicarbazide method

1,4-Diphenylthiosemicarbazide (formula 42.1) has been used by Hara and Sandell [37] for the spectrophotometric determination of Ru. Heating a solution of the reagent and ruthenium in the presence of SnCI2 as reductant results in the formation of a red-violet CH3Cl-soluble complex in which the ruthenium is probably tervalent.

/ N ~ - ~ (42.1)

The molar absorptivity of the complex in CHC13 is 1.01-104 (a = 0.01) a t )~max -- 560 nm. Under the conditions of the Ru determination the absorbance of the reagent is negligible. The optimum acidity is between 4.0 and 5.5 M HC1. At HC1 concentrations >6 M the complex decomposes. The maximum absorbance is obtained after heating the solution for 10 min at 100~ The optimum quantity of SnCI2 is 0.3-0.4 ml of 5% solution per 25 ml of the coloured solution. A tenfold amount of osmium relative to ruthenium does not interfere in this method. The presence of Re causes slightly high results.

Before the determination with 1,4-diphenylthiosemicarbazide, the ruthenium should be separated as RuO4 by distillation or extraction. The solution from which RuO4 is distilled must contain no HC1. The acid is expelled from the sample solution by evaporation with H2SO4 to white fumes.

1,4-Diphenylthiosemicarbazide has been used to determine ruthenium in meteorites [38], Pt-Ru(4) alloys [5], and by the differential technique in Pt-Ru(20) alloy [39].

2,4-Diphenylthiosemicarbazide has also been employed in Ru determination [40], but the colour of its ruthenium complex is less stable.

42.3. Methods of osmium determination 367

Reagents

1,4-Diphenylthiosemicarbazide, 0.2% solution in ethanol. Standard ruthenium solution: 1 mg/ml. Fuse 0.1000 g of powdered ruthenium with 2 g

of Na202 in a silver crucible. Dissolve the melt in water. Neutralize the solution, then acidify it with HC1 to a final concentration 1 M, and filter off any AgC1 precipitate. Dilute the solution to the mark with water in a 100-ml volumetric flask.

Tin(II) chloride, a freshly prepared 5% solution in HC1 (1 + 1).

Procedure

Distillative separation of Ru. Place -~25 ml of sample solution (0.5 M in H2SO4 and containing not more than 150 ~tg of Ru) in a 50-ml distillation flask. Add 1.5 ml of 1% KMnO4 solution (the solution should not be decolorized). Connect the still with a simple condenser and a receiver containing 0.4 ml of the SnCI2 solution in 15 ml of HC1 (1 + 1). Heat the still, while bubbling air through the liquid contents at a rate of 2 bubbles / sec, and boil the solution for 30 min. Determination of Ru. Dilute the distillate to ---30 ml with 5 M HC1, add 3 ml of the 1,4- diphenylthiosemicarbazide solution, mix well, and heat in a boiling water-bath for 10 rain. Transfer the cooled solution to a separating funnel, and shake with two portions of CHC13 for 1 min. Place the combined extracts in a 25-ml standard flask, dilute to the mark with the solvent, and measure the absorbance at 560 nm against a reagent blank solution.

42.2.2. Capri Blue method

The anionic thiocyanate complex of ruthenium(HI), Ru(SCN)6 3-, reacts with Capri Blue (basic oxazine dye, formula 4.31) to form a sparingly soluble ion-associate which precipitates at the phase boundary and on the wall of the separating funnel during shaking of the aqueous phase with DIPE. After separation, the ion-associate is dissolved in methanol and the absorbance of the solution is measured [41].

The absorbance depends on the concentration of thiocyanate and the pH, and also on the time and temperature of heating of the ruthenium solution with thiocyanate. The ruthenium complex is formed quantitatively when the solution is heated for at least 10 min, with a thiocyanate concentration greater than 0.15 M and pH 0.5-3.5. At least a 15-fold molar excess of Capri Blue is necessary.

The ion-associate of SCN- with Capri Blue separates together with the ion-associate of ruthenium. The former decomposes during washing with water. If the precipitate and the DIPE are shaken with three portions of water the excess of the dye is nearly completely removed; the absorbance of the blank does not exceed 0.07.

The molar absorptivity in methanol solution is 2.7.105 (a = 2.6) at 630 nm. The method is not selective. Other platinum metals also form floatable ion-associates

with SCN- and Capri Blue. Certain other metals, such as Au, Ag, Cu, Hg, and W, also interfere, and hence a preliminary separation of Ru is necessary. For example, distillation of the tetroxides allows Ru and Os to be isolated from practically all other metals. Then, extraction from thiocyanate medium [14] separates Os and Ru, and one after the other can be determined, e.g., with Capri Blue.


Reagents

Capri Blue, 1.10 .3 M (--0.035%) solution. Ammonium thiocyanate, 5% (0.7 M) solution. Ruthenium standard solution: 1 mg/ml. Preparation as in Section 42.2.1. Peroxide-containing diethyl ether. Shake the commercial preparation with 2%

ammonium ferrous sulphate solution in 2 M H2SO4 and then distil. To the freshly purified ether add 1 drop of 30% H202 per 15 ml.

Procedure

Separation of Ru and Os by distillation. Place the sample solution containing Ru and Os in a still. Add 6 ml of conc. H2804 and 2 ml of conc. HC104. Attach the still to two receivers in series. Place 5 ml of 0.2 M NH4SCN solution in 0.4 M HC1 in each of the receivers, pass nitrogen continuously (3 bubbles / sec) and heat the flask carefully until fumes of HC104 appear. Stop the heating, and continue to pass nitrogen until the still has cooled.

Extractive separation of Os. Transfer the contents of the receivers to a conical flask and wash the receivers with a few ml of the absorption solution (NH4SCN+HC1). Heat the solution for 5 min in a boiling water-bath, allow to cool, and transfer to a separating funnel. Extract the Os complex with two 10-ml portions of peroxide-containing diethyl ether.

Determination of Ru. To the sample solution (--15 ml) (e.g., the solution left after separation of Os as above), containing not more than 8 pg of Ru, add thiocyanate to give 0.2 M SCN- concentration, adjust the pH to 2-3 (by means of ammonia and dilute H2SO4), and heat on a boiling water-bath for 15 min. Allow the solution to cool and transfer it to a separating funnel. Add 1 ml of the Capri Blue solution and 5 ml of DIPE, and shake for 30 s. Remove the aqueous phase and wash the ether phase by shaking it with three portions of water (shaking time 15 s). Carefully discard the DIPE and dissolve the precipitate in methanol. Transfer the solution to a 25ml standard flask, and dilute to the mark with methanol. Measure the absorbance of the solution at 630 nm against a reagent blank.


Thiourea reacts with Ru(III) in a hot acidic medium to form a water-soluble blue complex Ru(NH2.CS.NH2)63+. The method based on this reaction [42,43] is rather insensitive (e = 2.5.103 at 620 nm). Ref. 42a is cited later, but not here. Please adjust or correct.

Many other sulphur-organic reagents have found use in the determination of Ru, e.g., rubeanic acid [44] and unithiol [45]. 2-Thiobarbituric acid has been used in a method based on derivative spectrophotometry [46]. Dithizone (at 85~ forms with Ru a complex extractable into CHC13 [47].

A group of methods for determining Ru is based on nitroso compounds such as 1- nitroso-2-naphthol and 2-nitroso- 1-naphthol (e = 1.8.104) [9,48], 3-nitroso-2,6-pyridinediol (49], and isonitrosoacetophenone [50].

The methods based on ion-associates of Ru complexes with basic dyes are very sensitive. The method based on the ion-associate with Capri Blue has already been presented. The thiocyanate-Ru complex has also been associated with Crystal Violet and Rhodamine 6G (in the presence of gelatine) (~ = 1.4.105) [15,51]. The ion-associate of the Ru complex with SnC13- and Crystal Violet is a basis of a flotation-spectrophotometric method of Ru determination (e = 2.1.105 at 600 nm) [52]. More sensitive methods are based on flotation of


the compound of the ruthenium chloride complex with Rhodamine 6G (E - 5.1.105) [53] and on flotation of complex ion-associate of Rhodamine 6G with the anionic complex of Ru(II) with C1- and SnC13-ions (e -- 5.105) [54]. The methods of flotation separation and spectrophotometric determination of Ru (and Os) with basic dyes have been discussed in detail [55].

Other organic reagents recommended for Ru determination include, TPTZ (formula 26.4) [56], Sulphochlorophenolazorhodanine [57,58], o~-furyldioxime [59], 8-hydroxyquinoline (~ = 1.2-104 at 430 nm, CHC13) [60], and Chrome Azurol S [61 ].

The thiocyanate complex of Ru(III) is a basis of rather insensitive method (~ = 5.5.103 at 570 nm [15,62-64]. The complex is extractable into MIBK [14], chloroform in the presence of 1,10-phenanthroline [65], and hexamethyl phosphoric triamide (HMPA) in MIBK [66].

Suitable methods for determining larger amounts of ruthenium are based on the orange ruthenate ion (RHO42-) (E = 1.74.103 at 465 nm) [67-69], and on the greenish-yellow perruthenate ion (RuO4-) [19]. Ruthenate is formed when RuO4 is absorbed in NaOH solutions, or when RuO4 is stripped from an organic phase with alkali. Perruthenate is formed in an alkaline medium containing hypochlorite or periodate.

Ruthenium, in the presence of osmium, can be determined after reduction of RuO4 in 6-10 M HC1 (RUG162- complex) (Os does not interfere in the determination of Ru) [8]. Ruthenium can be determined also as various chloride complexes combined with SnC13- ions [54,70]. Such a complex has been used for determining Ru in the presence of Os, by derivative spectrophotometry [71 ]. This method has also been used for determining Ru in the presence of Rh after the reaction with octadecyldithiocarbamate [72] and triazine derivatives (in picrate medium) [73]. Simultaneous determination of Ru and Os by derivative spectrophotometry can be performed after absorption of volatile RuO4 and OsO4 in water [7].

Ruthenium can be determined by spectrophotometric methods, based on its catalytic effect on various redox reactions, e.g., ferrozine with KIO4 [74], Ce(IV) with As(III) [75,76], haematoxylin with H202 [77], or Thymol Blue with BrO3- [78].

42.3. Methods of osmium determination

The classical diphenylcarbazide method and a new, sensitive flotation method, based on the anionic complex of Os with SnC13-, associated with Rhodamine B, are presented in detail below.

42.3.1 Diphenylcarbazide method

1,5-Diphenylcarbazide, a well-known reagent for Cr(VI), has been applied for the spectrophotometric determination of Os [79]. Diphenylcarbazide (in ethanol) reacts with OsO4 (in CHC13) to form a blue-green complex having ~max 560 rim. A freshly prepared solution of 1,5-diphenylcarbazide does not absorb between 400 and 700 nm. The molar absorptivity of the osmium complex in CHC13 is 3.1-104. The sensitivity has been increased as a result of certain modifications [80,81]. Now, osmium tetroxide is made to react with 1,5-diphenylcarbazide in HC104 medium (0.7-1.5 M) and acetic acid (--2.8 M) at 60~ The complex formed is extracted into CHC13. Other extraction solvents are less efficient. The diphenylcarbazide should be in considerable excess. The presence of ethanol is unfavourable.

The sensitivity is very dependent on the quality of 1,5-diphenylcarbazide used. With six varied preparations, molar absorptivities between 1.4.105 and 2.2.105 (a = 0.7-1.1) have


been obtained at 560 nm [80]. The method is specific for Os when used in conjunction with separation of 0S04 by

distillation. Ruthenium interferes to some extent, but osmium can be separated from it by using a suitable oxidant such as H202, which oxidizes Ru to RuO4.

The method has been used to determine trace amounts of Os (6.10 -4 %) in pla t inum- ruthenium alloy [5].

Reagents

1,5-Diphenylcarbazide, fresh 0.4% solution in ethanol. Standard osmium solution: 1 mg/ml. Carefully break an accurately weighed glass

ampoule containing --0.5 g of OsO4 in a beaker containing -100 ml of twice distilled water. Wash, dry, and weigh the glass fragments of the ampoule, and calculate the weight of OsO4 by difference. Dilute the osmium solution with water until 1 ml contains precisely 1 mg of Os. Perform all these operations in a fume cupboard, and keep the osmium solution in a bottle with a precision-ground stopper on account of the toxic properties and offensive odour of the tetroxide.

Procedure

Extractive separation of Os. Adjust the sample solution containing Os (in an oxidation state lower than VIII) with sulphuric acid so that the volume is -~5 ml and the concentration of H2SO4 is --2 M. Oxidize the osmium by adding 5% KMnO4 solution dropwise until the sample solution has a stable pink colour. Decolorize the solution by adding dropwise 2% ammonium ferrous sulphate solution. Add 3 ml of conc. HNO3 and 2 ml of water, and extract OsO4 with two portions of CHC13. Wash the combined extracts with 10 ml of 0.1 M H2804.

Distillative separation of Os. Add sulphuric acid to the sample solution containing Os until the solution is -~2 M in H2S04. Place the solution (--25 ml) in a 50-ml distillation flask, add 1 ml of 0.3% H202 solution, and connect a simple condenser and a receiver containing cold twice-distilled water. Distil for about 30 min, keeping the contents of the still gently boiling, and passing nitrogen at 2-3 bubbles / sec. After stopping the heating, continue to pass nitrogen until the still has cooled.

Determination of Os. Add in succession to a 50-ml flask, 3 ml of 60% HC104, 4 ml of glacial acetic acid, 2 ml of 1,5-diphenylcarbazide solution, and a known volume of sample solution containing not more than 20 ~tg of OsO4. Dilute to 25 ml with water and heat in a water-bath at 60~ for 30 min. Place the cooled solution in a separating funnel and extract the Os complex with two 10-ml portions of CHC13 (shaking time 10 s). Transfer the extracts to a 25-ml standard flask, dilute to the mark with CHC13, mix, and measure the absorbance of the solution at 560 nm against a reagent blank.


In -~2 M solutions of HC1, osmium gives with SnC13- (0.4 M SnCI2, heating for 1.5 h at 100~ the anionic complex OsClz(SnC13)22- which can react with Rhodamine B (xanthene azo dye) to give a sparingly soluble ion-associate. While this aqueous phase is shaken with cyclohexane (flotation) the associate collects on the wall of the separating funnel. The optimum HC1 concentration during the flotation should be within 0.4-0.6 M, at about 100- fold molar excess of the dye with respect to Os. The floating Os ion-associate is


accompanied by the sparingly soluble salt formed by the dye cations with SnC13-. This salt can easily be removed by shaking the precipitate and the organic phase with 0.5 M HC1 (shaking with three 10-ml portions of HC1). The reagent blank value is small (-0.10) if the dye concentration in the aqueous phase is 2.5-10 -4 M, and SnCI2 concentration -~ 1 M.

The Os compound isolated by flotation and washed, is dissolved in acetone or methanol, and the obtained coloured solution serves as a basis for a very sensitive method of determining Os [82] (e = 4.1-105 at 560 nm, sp. abs. =2.2).

The floatable Os compound, used as the basis of the above method, is an adduct of Os(II) complex with C1- and SnC13- ions associated with Rhodamine B (RB +) and of an associate of the dye with SnC13-, with general formula: [(RB+)zOsClz(SnC13)22- ].2[RB+)(SnC13-)]. The compound is stable under the conditions of washing-off the uncombined residue of the dye salt.

Ruthenium interferes in the determination of osmium. Preliminary distillation of Os from 5 M HNO3 medium, as the volatile OsO4' enables one to separate it from Ru.

Reagents

Rhodamine B, 2.10 .3 M (-~0.1%) solution. Standard osmium solution: 1 mg/ml. Preparation as in Section 42.3.1. Tin(II) chloride, 1 M (--20%) solution in 2 M HC1.

Procedure Place the sample solution, containing not more than 10 gg of Os (free of Ru and other metals), in a small conical flask, add 2 ml of HC1 (1+1) and 4 ml of the SnCI2 solution. Dilute the solution with 0.01 M HC1 to a volume of-~ 10 ml and heat on a boiling water bath for 1.5 h. After cooling, transfer the solution to a separating funnel, add 25 ml of 0.01 M HC1 and 5 ml of the Rhodamine B solution. Shake the solution (for 1 min) with 5 ml of cyclohexanone. Carefully drain the aqueous layer and wash the precipitate and the organic layer by shaking with three 10-ml portions of 0.5 M HC1 (time of shaking with each portion, 1 min). Carefully drain the solvent from the separating funnel. Dissolve in acetone the precipitate collected on the walls. Transfer the obtained solution to a 25-ml volumetric flask, dilute with acetone up to the mark, and mix well. Measure the absorbance at 560 nm against the reagent blank.


The sensitive methods of determining Os are based on the anionic complex of Os(II) with C1- and SnC13- ions. This complex forms with Rhodamine B an ion-associate, which is floatable by shaking the aqueous phase with cyclohexanone (the method described above) or toluene (e = 6.2.105) [83]. Osmium has also been separated by flotation and determined (e = 2.105) as an ion-associate of the ionic complex with SnC13- and Crystal Violet [84]. A high sensitivity (~ = 4.0.105) has also been obtained when the complex of OsC162- with Rhodamine 6G (without SnCl2) is floated with toluene [85]. A mixture of C6H5C1 with CC14 (3+1) can be used for extraction of the OsC16 2- complex with Brilliant Green (~ = 2.0.105) [86]. The thiocyanate complex of Os has been the basis of flotation methods for determining Os with the use of Capri Blue (see the determination of Ru) and Methylene Blue (e = 2.2.105) [87]. The ion-associate of the Os-thiocyanate complex with Rhodamine 6G serves as a basis for determining Os in an aqueous pseudo-solution stabilized with gelatine [88].

A well-known, less sensitive method for the determination of osmium is based on the


reaction with thiourea in H2SO4 or HC1 medium to give a red complex of formula Os(NHz.CS.NH2)63+ (~max = 480 nm) [42].

Other sulphur-containing organic reagents for determination of osmium are, 2,3- quinoxalinedithiol (c = 1.7.104) [89], Bismuthiol II [90], rubeanic acid [91], and dithizone (complex formed at 80~ in alkaline solution, then extracted into CHC13, c = 3.1.104 at 360 nm) [92]. The second-order derivative spectrum of the Os-thiourea complex has been used for determining Os in the presence of Ru [42a].

Among other organic reagents proposed for spectrophotometric determination of osmium the following are worth mentioning: PAR (c = 2.5.104) [93-96], 3-nitroso-2,6- pyridinediol [97], acenaphthenequinone monoxime (c = 8.8.104)[98], and promazine [99].

The thiocyanate complex forms the basis of a less sensitive method for determining osmium [14,100,101] . The molar absorptivity is 1.7.104 at 620 nm (in diethyl ether). The anionic thiocyanate complex has been extracted with hexamethylphosphoramide in CHC13 (c = 2.1.10 4) [101].

Larger amounts of osmium can be determined as osmate, OsO42- (c = 2.75.103 at 340 nm) [69], complex OsC162- (c = 8.4.103) [85], and complex with SnC13-(~; = 2.4.103) [82]. The complex with Sn(II) chloride has been used for determining Os in the presence of Ru by using derivative spectrophotometry [71 ].

The catalytic effect of Os on various redox reactions has been used for its determination [102-106]. The FIA technique has also been applied in Os determinations [105,106].

References

1. Chung K.S., Beamish F.E., Talanta, 15, 823 (1968). 2. Kiba T. et al., Talanta, 19, 451 (1972). 3. Ionov V.P., Potapova S.A., Dubronina Z.I., Zh. Anal. Khim., 29, 1393 (1974). 4. Alimarin I.P., Khvostova V.P., Kadyrova G.I., Zh. Anal. Khim., 30, 2007 (1975). 5. Marczenko Z., Balcerzak M., Chem. Anal. (Warsaw), 24, 867 (1979). 6. Morgan J.W., Golighthy D.W., Dorrzapf A.F. Jr., Talanta, 38, 259 (1991). 7. Balcerzak M., Swi~cicka E., Anal. Chim. Acta, 349, 53 (1997). 8. Balcerzak M., Swi~cicka E., Talanta, 43, 471 (1996). 9. Menis O., Powell R.H., Anal. Chem., 34, 166 (1962). 10. Berg E.W., Moseley H.E., Anal. Chim. Acta, 47, 360 (1969). 11. Epperson C.E., Landolt R.R., Kessler W.V., Anal. Chem., 48, 979 (1976). 12. Meier H. et al., Mikrochim. Acta, 1969, 557, 573, 826, 836, 1083, 1107. 13. Kalyanaraman S., Khopkar S.M., Anal. Chim. Acta, 78, 231 (1975). 14. Marczenko Z., Balcerzak M.,Anal. Chim. Acta, 109, 123 (1979). 15. Jaya S., Rambarishna T.V., Bull. Chem. Soc. Jpn., 57, 267 (1984). 16. Balcerzak M., Komar K., Swi~cicka E., Kasiura K.,Anal. Sci., 13, 33 (1997). 17. A1-Bazi S.J., Chow A., Anal. Chim. Acta, 157, 83 (1984). 18. A1-Bazi S.J., Chow A., Talanta, 31, 189 (1984). 19. Dinstl G., Hecht F., Mikrochim. Acta, 1963, 895. 20. E1-Shahavi M.S., Abu-Zuhri A.Z., A1-Daheri S.M., Fresenius'J. Anal. Chem., 350, 674

(1994). 21. Bakurdhieva V., Ivanov N., Zh. Anal. Khim., 44, 899 (1989). 22. Vest P., Schuster M., Koenig K.H., Z. Anal. Chem., 335, 759 (1989). 23. Beklemishev M.K., Kuz'min N.M., Zolotov Yu.A., Zh. Anal. Khim., 44, 356 (1989). 24. Van Loon J.C., Beamish F.E., Anal. Chem., 36, 1771 (1964).

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Chapter 43. Scandium

Scandium (Sc, at. mass 44.96) occurs in its compounds exclusively in the III oxidation state. Some of its chemical properties resemble those of the lanthanides and yttrium. Scandium hydroxide Sc(OH)3 precipitates at a pH as low as 4.8 and dissolves in alkaline medium: in this respect scandium resembles aluminium.



The anionic scandium-sulphate complex is sorbed by the strongly basic anion-exchanger Dowex 1 [1]. Under the conditions employed, rare-earth elements, A1, Be, and some other elements are eluted, but Th, Zr, and U are retained along with scandium. Scandium can also be sorbed on anion-exchangers as the thiocyanate complex [2]. Scandium and rare-earth elements have been retained on a strongly basic anion-exchanger from a medium of 90% glacial acetic acid and 10% 3 M HC1. First, yttrium and the lanthanides are run off the column, then scandium is eluted [3].

Scandium is selectively eluted from a strongly acidic cation-exchange column with dilute H2804 or acidic (NH4)2804 solution [4]. Rare-earth and other metals (e.g., A1, Ca, Cd, Co, Cu, In, Mg, Ni, and Zn), which do not form anionic sulphate complexes, remain in the column. A thiocyanate solution, acidified with hydrochloric acid, also elutes scandium from cation exchangers [5].


All Sc may be extracted with diethyl ether from an acidic solution (0.1-0.2 M HC1) containing 50% NH4SCN. Rare-earth elements remain in the aqueous solution [6].

From 9-11 M HC1 scandium can be extracted into TBP, while rare-earth elements, A1, Be, U, and Cr remain in the aqueous phase [7].

Scandium can also be separated from rare-earth and other metals by extraction as a complex of HTTA [8,9], or salicylate [10]. Xylene [8], CHC13 [9], and mesityl oxide [10] have also been used for extraction of Sc. Scandium has been extracted from ascorbic acid medium with Aliquat 336S [ 11 ], or with acetylacetone in the presence of 3,5-dichlorophenol [12]. Macrocyclic ethers have also been used for extraction of Sc [13].

Along with the rare-earth elements, Sc may be separated as the sparingly soluble oxalate or fluoride. Suitable collectors in the oxalate and the fluoride methods are lanthanum and calcium, respectively [14].

Scandium can be isolated from a number of elements (A1, Sn, W, Mo, Nb, Ta, V, Ti) by precipitating the hydroxide with KOH in the presence of H202, with Fe(III) as collector.

Precipitation of scandium with ammonium tartrate in neutral medium separates it from the rare-earth elements [15,16]. Yttrium is employed as a collector for microgram quantities of Sc. Larger amounts of other elements (>20 mg of A1, 20 mg of Fe, 2 mg of Zr, or 2 mg of Th) prevent the quantitative separation of scandium as the tartrate. Scandium is separated from yttrium by extraction as the thiocyanate complex.

376 43. Scandium

Trace amounts of scandium have been enriched from aqueous solutions by means of flotation with the use of Fe(OH)3, CTA, and a stream of nitrogen [17].


The Xylenol Orange method has been recommended for spectrophotometric determination of Sc. Numerous azo reagents have also been proposed for scandium. Some methods enable scandium to be determined in the presence of rare-earth elements.


Xylenol Orange (XO) (formula 4.19) reacts in a slightly acidic medium with scandium to form a red-violet complex which is the basis of a sensitive spectrophotometric method for determining scandium [14,18-21]. The reagent is yellow-orange in acidic solution (pH 1-5), but turns red-violet above pH 5.

Maximum absorbance of the scandium-XO complex is obtained between pH 2.5 and 2.7. As the acidity rises, the absorbance rapidly drops; with increase in pH, the absorbance slowly decreases. The absorption maximum of the complex occurs at 560 nm. At this wavelength the reagent absorbs imperceptibly. The molar absorptivity of the complex is 2.9.104 at ~max 560 nm (a = 0.65). Solutions of the complex are stable with respect to time.

At pH 2.6, Th, Zr, Ti, Fe(III), Bi, In, A1, and Y (but not the lanthanides) interfere. Reducing the pH minimizes these interferences and even masks yttrium completely. Iron(III) and cerium(IV) are masked by reduction with ascorbic acid. Oxalate, sulphate, fluoride, and phosphate interfere in the determination of scandium,

Reagents

Xylenol Orange (XO), 0.05% aqueous solution. Standard scandium solution: 1 mg/ml. Dissolve 0.1530 g of Sc203 in l0 ml of hot 2 M

HC1, and dilute the solution to volume with water in a 100-ml standard flask. Working solutions are obtained by suitable dilutions of the stock with 0.01 M HC1.

Chloroacetate buffer, pH ~3. Dissolve in water 3 g of NaOH, add 15 ml of chloroacetic acid, and dilute the solution with water to 250 ml.

Procedure

To the slightly acidic (pH "- 1) sulphate-free sample solution containing not more than 30 gg of Sc, add 1 ml of 1% ascorbic acid solution, 1 ml of the chloroacetate buffer and 2.5 ml of the XO solution. Dilute the solution with water to --20 ml, and adjust its pH to 2.0__0.1). Transfer the solution to a 25-ml standard flask, and make up to the mark with water. After 10 min, measure the absorbance of the solution at 560 nm against a reagent blank solution.

Note. If yttrium is known to be present in the sample solution [introduced, for example, as a collector (2-5 mg) during the precipitation of Sc as tartrate], the same amount of yttrium should be added to the standard solutions used to prepare the calibration curve.



Many azo dyes have been recommended as spectrophotometric reagents for scandium. Arsenazo HI (formula 4.10) [22-24] gives similar sensitivity to Xylenol Orange. The optimum pH is between 1.5 and 3.0" the molar absorptivity is 2.9.10 4 a t 640 nm. Arsenazo I [8,10] is less sensitive and less selective. Other azo reagents used for determining scandium include Chlorophosphonazo HI [25], Chlorophosphonazo-p-C1 [26,27], p- acetylchlorophosphonazo in the presence of Ce(III) [28], p-nitrochlorophosphonazo [29], PAR [30,31], and TAR (~ = 5.0.104 at 540 nm) [32].

The following triphenylmethane dyes have been employed for determination of Sc similarly to Xylenol Orange: Methylthymol Blue [33], Chrome Azurol S [34,35], Chromal Blue G [36], and Eriochrome Brilliant Violet B [37]. Much higher sensitivities have been obtained in the presence of some cationic surfactants [38-40]. In the method with Chrome Azurol S and Zephiramine, the e value is 1.5.105 at 610 nm, and in the method with Eriochrome Cyanine R and CP, e = 9.2.104 at 600 nm [40]. When o-hydroxy- quinonephthalein and CP are used, the molar absorptivity is 1.1.105 at 555 nm [41]. Scandium has been determined with the use of Nile Blue in a poly(vinyl alcohol) medium [42].

Further organic reagents for scandium are Bromopyrogallol Red [43-45], o-chlorophenylfluorone and CTA (e = 1.31.105 at 569 nm) [46], indoferron [47], and 2-pyridylidene- 2-aminophenol [48].


Xylenol Orange has been applied for the determination of scandium in minerals and coal ashes [16], copper- and nickel alloys [49], and iron-rich materials [50].

The Arsenazo III method has been utilized for determining Sc in minerals [51]. Scandium in mixtures with rare earth elements was determined by derivative spectrophotometry with the use of Chlorophosphonazo-p-C1 [27]. p-Acetyl- chlorophosphonazo with Ce(III) has been used for determining Sc in copper, aluminium, manganese, and magnesium alloys [28]. Traces of scandium in silicate rocks and sediments were determined with the use of Bromopyrogallol Red [43].

References

1. Hamaguchi H., et al., J. Chromatogr., 16, 396 (1964); Anal. Chem., 36, 2305 (1964). 2. Hamaguchi H., et al., Talanta, 11,495 (1964). 3. Kuroda R., Hikawa I., J. Chromatogr., 25, 408 (1966). 4. Strelow F.W., Bothma C.J.,Anal. Chem., 36, 1217 (1964). 5. Hamaguchi H., et al., Talanta, 10, 153 (1963); J. Chromatogr., 22, 143 (1966). 6. Kalyanaraman S., Khopkar S.M., Anal. Chem., 49, 1192 (1977). 7. McDonald J.C., Yoe J.H., Anal. Chim. Acta, 28, 264 (1963). 8. Ohishi H., Banks C.V.,Anal. Chim. Acta, 29, 240 (1963). 9. Zolotov Yu.A., Shakhova N.V., Alimarin I.P., Zh. Anal. Khim., 23, 1321 (1968). 10. Langade A.D., Shinde V.M., Anal. Chem., 52, 2031 (1980). 11. Karve M.A., Khopkar S.M., Bull. Chem. Soc. Jpn., 64, 655 (1991). 12. Katsuta S., Imura H., Suzuki N., Anal. Sci., 7, 661 (1991). 13. Deorokar N.V., Khopkar S.M., Bull. Chem. Soc. Jpn., 64, 1962 (1991).

378 43. Scandium

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Chapter 44. Selenium

Selenium (Se, at. mass 78.96) forms selenide, selenite and selenate ions, in the oxidation states -II, IV, and VI, respectively. Selenium(IV) compounds are the most stable. Selenium dioxide sublimes readily (unlike TeO2). On dissolution in nitric acid, selenium is oxidized to Se(IV). Strong oxidizing agents (e.g., aqua regia) oxidize Se to Se(VI). Moderate reducing agents reduce Se(IV) and Se(VI) to the element. Selenium compounds are more easily reduced and less easily oxidized than the corresponding tellurium compounds.



Selenium is usually separated by distillation as the volatile selenium bromide (SeBr4) or chloride (SeCI4) [1,2]. Selenium is distilled from concentrated HBr medium [in the presence of bromine to prevent the reduction of Se(IV)] and from concentrated HC1. Perchloric or sulphuric acid is added to the still, and distillation is continued until white fumes of H2SO4 or HC104 appear. Passage of nitrogen through the liquid promotes the distillation. During the distillation, Te remains quantitatively in the still, but As, Ge, and Sb are distilled with the selenium.

Selenium may be separated from various non-volatile materials as the volatile SeO2, which forms when a stream of oxygen is passed over the sample in a tube heated to 1,000~ The SeO2 sublimed onto the cold part of the tube is dissolved and determined [3,4]. A special apparatus has been proposed for the separation of selenium as SeO2 [5].

The volatile hydrogen selenide, HzSe, is also used for separation of selenium [6].


Selenium is readily separated by reduction to the element with SnC12, $02, hypophosphite, or hydrazine [7]. Arsenic [8,9] and tellurium [10] are suitable collectors for traces of Se. When selenium is precipitated from 1-8 M HC1 with SO2, the following are wholly or partly reduced to the element: Te, Au, Pt, Pd, Hg, Bi, Sb, Sn, and Cu.

Traces of selenium can be co-precipitated with Fe(III), La, Mn, or aluminium hydroxide [11-13]. Se(VI) has been co-precipitated as PbSeO4 together with PbSO4 [14].

When samples mixed with NazCO3 and MgO are ignited at 800~ a sinter is formed, from which water leaches Se(VI), while sparingly soluble MgTeO4 remains in the solid residue [ 15].

44.1.3. Extraction and ion-exchange

The selenium(W) chloride complex in 6-7 M HC1 reacts with methyl ethyl ketone to form a compound which can be extracted with chloroform. Such extraction separates Se from Te [16].

Complexes of selenium with xanthate [17], 4-nitro-o-phenylenediamine (toluene) [18], and DDTC [19,20] have also been used in extractive separation of Se from many elements.

380 44. Selenium

Selenium(IV) has been selectively retained on an anion-exchange column [21 ], and on a cation-exchanger modified with Bismuthiol II [22] or with Bismuthiol(II)-sulphonic acid [23]. Sorption of Se (along with Te) on a polyurethane foam has also been applied [24].

The ion-exchange methods for separating Se and Te are discussed in Chapter 49.


The sensitive method based on 3,3'-diaminobenzidine is widely used. Selenium is determined either in the aqueous medium or after extraction with toluene. The method based on the coloured sol of elemental selenium is much less sensitive.

Spectrophotometric methods of determining selenium have been reviewed [25,26].

44.2.1.3,3'-Diaminobenzidine method

Selenium(W) reacts with 3,3'-diaminobenzidine (DAB) in acid medium to form the yellow piazselenol, which is sparingly soluble in water and which is utilized for spectrophotometric determination of Se [27-29] (formula 44.1).

HzN NH~ H~N ~ S e

The colour reaction is carried out in 0.1 M HC1, and the time necessary for colour development in the aqueous pseudo-solution is 50 min. In the extractive spectrophotometric method [36], the time for reaction at pH 2-3 (in the presence of formic acid) is 30 min, after which the solution is neutralized to pH 6-7, and the piazselenol is extracted into toluene. The colour reaction may be accelerated by heating the solution. Within the pH range 5-10, the distribution coefficient of piazselenol between toluene and water is high, and one portion of toluene extracts practically all the selenium complex into the organic phase. The free reagent (DAB) is also extracted. Related solvents such as benzene and xylene may be substituted for toluene.

The two absorption maxima of piazselenol occur at 340 and 420 nm. Since DAB absorbs strongly at 340 nm but negligibly at 420 nm, absorbances are measured at 420 nm.

The molar absorptivity of the toluene solution of piazselenol at 420 nm is 1.02.104 (sp. abs. 0.13). This method is specific for selenium. Tellurium does not react with DAB, but V(V) and Fe(III) oxidize DAB to give coloured oxidation products. Iron(III) can be masked with fluoride or phosphate. EDTA is used as masking agent to prevent the precipitation of metals in the neutral medium. Substances capable of reducing selenium to the element interfere in the determination of selenium by the 3,3'-diaminobenzidine method.

Reagents

3,3'-Diaminobenzidine hydrochloride (DAB), 0.5% solution in boiled and cooled water. The solution is stable for a few hours, then turns brown under the influence of atmospheric oxygen.

Standard selenium solution: 1 mg/ml. Dissolve in water 1.4050 g of SeO2 (resublimed and stored over P205), and dilute the solution with water in a volumetric flask to 1 litre.


Procedure

To the sample solution containing not more than 100 ~tg of Se in a volume o f - 2 0 ml, add 2 ml of 10% formic acid solution and 2.5 ml of DAB, and then adjust the pH to 2.0-2.5. Let the solution stand for 30 min, then neutralize with ammonia to pH 6-7. Extract the piazselenol by shaking for 1 min with two portions of toluene. Dilute the extract with toluene to the mark in a 25-ml standard flask, and measure the absorbance at 420 nm against water or a reagent blank solution.

Note. If metal ions are present (A1, Bi, Cu, Ni, etc.), add at the start of the procedure 1-5 ml of 5% EDTA solution as masking agent. To mask Fe(III), add also NaF (0.05-0.2 g, depending on the amount of Fe).

44.2.2. Selenium sol method

Reduction to a brown-yellow sol of elemental selenium in acid medium containing a protective colloid has been made the basis of a simple but rather insensitive method for determining selenium. Suitable protective colloids for preventing coagulation of the selenium sol are gum arabic, gelatine, and poly(vinyl alcohol).

Tin(II) chloride, ascorbic acid, thiourea, or hydrazine are used to reduce selenium(W). In 3-4 M HC1 solutions, SnCI2 rapidly reduces Se(IV) in the cold. Depending on the reducing agent and acid strength, pseudo-solutions of different colour are obtained.

The molar absorptivity of the selenium sol obtained with SnCI2 in 3 M HC1 containing poly(vinyl alcohol) is 1.7.103 at 400 nm (a = 0.022). Towards longer wavelengths, the absorptivity of the sol decreases; in the ultraviolet it increases. At 325 nm the absorptivity is twice as great, and at 450 nm half as great, as that at 400 nm.

Interference in this method for determining selenium comes from Te, Hg, Au, and platinum metals, all of which are easily reduced to the element.

Reagents

Tin(H) chloride, SnC12.2H20, 20% solution in 3 M HC1. Standard selenium solution: 1 mg/ml. Preparation as in Section 44.2.1. Poly(vinyl alcohol), 2% solution.

Procedure

Distillation separation of Se. To the sample solution of selenium(iv) in a 50-ml still, add just enough conc. HC1 to bring its concentration to 7 M. Add 10 ml of H2SO4 (1+1) and a few fragments of porous porcelain, and connect the still to a condenser, the end of which is immersed in a small amount of 2 M HC1 in the receiver. Distil until white fumes of H2804 appear in the still. To the cooled still, add 5 ml of conc. HC1 and distil again until white fumes appear. The receiver should be cooled with ice-water.

Determination of Se. To the sample solution (in -3 M HC1) containing not more than 0.5 mg of Se in a volume of--15 ml, add 3 ml of poly(vinyl alcohol) solution, and mix well. Add 1.0 ml of the SnCI2 solution with stirring. Measure the absorbance of the coloured pseudo-solution at 400 nm, vs. water as the reference.

382 44. Selenium


Similarly to 3,3'-diaminobenzidine, other aromatic o-diamines also react with selenium(IV) in HC1 medium. The o-phenylenediamine method [30,31] is more sensitive than the DAB method. The following reagents have been proposed for selenium: N-methyl-o-- phenylenediamine (e = 1.9-104 at 346 rim) [32], 2-aminodiphenylenediamine [33], 4-nitro- 1,2-diaminobenzene [34], 4,5-diamino-2,6-dimercaptopyrimidine [35], 1,2-diamino-4- chlorobenzene [36], 4,5,6-triaminopyrimidine [37], 3,4-diaminobenzoic acid [38], and 2,3- diaminonaphthalene [39,40].

Among the organosulphur compounds proposed for the spectrophotometric determination of selenium, are: Bismuthiol II (known as a reagent for Te) [41], 1,4- diphenylthiosemicarbazide [42], 2-mercaptoethanol [43], dithio-oxamide (rubeanic acid) [44], and DDTC [45]. Dithizone has been a basis for a sensitive method of determining Se (~ - 7.4.104) [46]. The mechanism of the reaction between Se(IV) and dithizone is still the subject of contradictory opinions [47,48].

In a sensitive, indirect method Se(IV, VI) is reduced by Cr(II) to hydrogen selenide which, on passing in a stream of nitrogen through an alkaline solution of Fe(CN)63-, reduces the latter to Fe(CN)64- [Se(-II) --+ Se(IV)]. The ferrocyanide formed reacts with the 1,10- phenanthroline complex of Fe(III) to give an equivalent amount of the complex Fe(phen)32+. The absorbance of this complex is measured at 508 nm (e = 6.8.104) [49]. The molar absorptivity increases to ~ - 1.4.105 0Vmax = 535), if bathophenanthroline is used instead of 1,10-phenanthroline. In another indirect method Se(IV) oxidizes ferrocene to the ferricenium ion which is oxidized to Fe(III), then reduced to Fe(II), to be determined finally by the colour reaction with 1,10-phenanthroline (~ = 4.2-104) [50].

Selenium has been determined with 5,5-dimethyl-l,3-cyclohexanedione [51], 6-amino- 1-hydroxynaphthalenesulphonic acid [52,53], and 1-aminonaphthalene-7-sulphonic acid [54]. Determinations of Se involved also the following dyes: Rhodamine B [55], Methylene Blue [56], Xylenol Orange [57], and Rhodamine 6G (by the amplification method, in iodide medium, after oxidation of iodide to iodine, and reaction of the IO3- with the dye) [58].

Some spectrophotometric methods for Se determination are based on its catalytic effect on the redox reactions of various organic compounds [59-65].


The 3,3'-diaminobenzidine method has been applied for determination of Se in biological materials [28,66], soils [67], air [68], silicates [11], sulphide ores [1], copper [8,14,18], organic substances [69], lead [8,14], steel [29], antimony and bismuth tellurides [70], thin Cd-Se films [71], silver chloride and uranium oxide [12].

Aromatic diamines were used for determining selenium in biological materials [35,36], environmental samples [72], sewage [73], copper [38], non-ferrous metals alloys [31], semiconductors [35], sulphur [38], commercial sulphuric acid [30].

Selenium has been determined in tellurium with the aid of dithizone [46]. Selenium present in anodic slimes has been determined after the reaction with iodide and extraction of the liberated iodine [74]. In pharmaceutical products selenium was determined with the use of 1-aminonaphthalene-7-sulphonic acid [54].

References 383

References

1. Barcza L., Zsindely S., Z. Anal. Chem., 199, 10,117 (1964). 2. Rybakov A.A., Ostroumov E.A., Zh. Anal. Khim., 38, 446 (1983). 3. R62afiska B., R62owski J., Mikrochim. Acta, 1984 II, 481. 4. Meyer A., Hofer Ch., T61g G., Z. Anal. Chem., 290, 292 (1978). 5. Han H.B., Kaiser G., T61g G., Anal. Chim. Acta, 128, 9 (1981). 6. Remer D.C., Veillon C., Tokonsbalides P.T., Anal. Chem., 53, 245 (1981). 7. Bye R., Talanta, 30, 993 (1983). 8. Luke C.L., Anal. Chem., 31, 572 (1959). 9. Kujirai O. et al., Talanta, 30, 9 (1983). 10. Voronkova M.A., Sidorenko G.A., Zh. Anal. Khim., 22, 1085 (1967). 11. Chau Y.K., Riley J.P., Anal. Chim. Acta, 33, 36 (1965). 12. Russell B.G. et al., Talanta, 14, 957 (1967). 13. Reichel W., Bleakley B.G.,Anal. Chem., 46, 59 (1974). 14. Jackwerth E., Z. Anal. Chem., 235, 235 (1968). 15. Kniazeva R.N., Kleiman V.Ya., Zavod. Lab., 31,410 (1965). 16. Jordanov N., Futekov L., Talanta, 12, 371 (1965); 13, 163 (1966); 15, 850 (1968). 17. Donaldson E.M., Talanta, 24, 441 (1977). 18. Donaldson E.M., Talanta, 35, 633 (1988). 19. Desai G.R., Paul J., Microchem. J., 22, 176 (1977). 20. Lo J.M., Lin C.C., Yeh S.J.,Anal. Chim. Acta, 272, 169 (1993). 21. Nakayama M., Chikuma M., Tanaka H., Talanta, 30, 455 (1983). 22. Nakayama M. et al., Talanta, 31, 269 (1984). 23. Nakayama M. et al., Talanta, 34, 435 (1987). 24. Stewart I.I., Chow A., Talanta, 40, 1345 (1993). 25. Murashova V.I., Sushkova S.G., Zh. Anal. Khim., 24, 729 (1969). 26. Raptis S.E., Kaiser G., T61g G., Z. Anal. Chem., 316, 105 (1983). 27. Barcza L., Mikrochim. Acta, 1964, 967. 28. Cummins L.M., Martin J.L., Maag D.D., Anal. Chem., 37, 430 (1965). 29. Nivibre P., Chim. Anal., 47, 125 (1965). 30. T6ei K., Ito K., Talanta, 12, 773 (1965). 31. Shkrobot E.P., Shebarshina N.I., Zavod. Lab., 35, 417 (1969). 32. Kasterka B., Chem. Anal. (Warsaw), 25, 215 (1980). 33. Kasterka B., Mikrochim. Acta, 1989 I, 337. 34. Stibilj V., Dermelj M., Franko M., Byrne A.R., Anal. Sci., 10, 789 (1994). 35. Izquierdo A., Prat M.D., Aragones L., Analyst, 106, 720 (1981). 36. Nbve J., Hanocq M., Molle L., Mikrochim. Acta, 1980 I, 41,259. 37. Bodini M.E., Alzamora O.E., Talanta, 30, 409 (1983). 38. Kasterka B., Dobrowolski J., Chem. Anal. (Warsaw), 32, 749 (1987). 39. Huang X.R. et al., Fresenius'J. Anal. Chem., 354, 195 (1996). 40. Ramachandran K.N., Kumar G.S., Talanta, 43, 1711 (1996). 41. Navratil O., Sorfa J., Coil. Czech. Chem. Comm., 34, 975 (1969). 42. Sushkova S.G., Murashova V.I., Zh. Anal. Khim., 21, 1475 (1966). 43. Afsar H., Apak R., Tor I., Analyst, 114, 1319 (1989). 44. Lebed' N.B., Pantaler R.P., Zh. Anal. Khim., 41, 2224 (1986). 45. Warner D.A., Paul J., Microchem. J., 20, 292 (1975). 46. Kasterka B., Dobrowolski J., Chem. Anal. (Warsaw), 15, 303 (1970). 47. Stary J. et al., Anal. Chim. Acta, 57, 393 (1971).

384 44. Selenium

48. Campbell A.D., Yahaya A.H., Anal. Chim. Acta, 119, 171 (1980). 49. Bode H., Schulze K., Z. Anal. Chem., 327, 154 (1987). 50. Kamaya M., Murakami T., Ishii E., Talanta, 34, 664 (1987). 51. Bodini M.E. et al., Talanta, 37, 439 (1990). 52. Ramachandran K.N., Kaveeshwar R., Gupta V.K., Talanta, 40, 781 (1993). 53. Manish R., Ramachandran K.N., Gupta V.K., Talanta, 41, 1623 (1994). 54. Pyrzyfiska K., Anal. Sci., 13, 629 (1997). 55. Liu S., Zhou G., Huang Z., Talanta, 37, 749 (1990). 56. Bernal J.L. et al., Talanta, 37, 931 (1990). 57. Amin A.S., Zareh M.N., Anal. Lett., 29, 2177 (1996). 58. Ramesh A., Ramakrishna T.V., Subramanian M.S., Bull. Chem. Soc. Jpn., 67, 2121

(1994). 59. Ensafi A.A., Dehaghi G.B., Anal. Lett., 28, 335 (1995). 60. Ensafi A.A., Afldaami A., Massoumi A., Anal. Chim. Acta, 232, 351 (1990). 61. Safavi A., Afldaami A., Anal. Lett., 28, 1095 (1995). 61a. Afkhami A., Mosaed F., Zh. Anal. Khim., 54, 1271 (1999). 62. Shiundu P.M., Wade A.P., Anal. Chem., 63, 692 (1991). 63. Sanchez-Pedreno C., Albero M.I., Garcia M.S., Saez A., Talanta, 38, 677 (1991). 64. Parham H., Shamsipur M., Bull. Chem. Soc. Jpn., 64, 3067 (1991). 65. Gokmen I.G., Abdelqader E., Analyst, 119, 703 (1994). 66. Cummins L.M. et al., Anal. Chem., 36, 382 (1964). 67. Stanton R.E., McDonald A.J., Analyst, 90, 497 (1965). 68. West P.W., Cimeoman C., Anal. Chem., 36, 2013 (1964). 69. Domenech R., Chim. Anal., 51,440 (1969). 70. Cheng K.L., Goydish B.L.,Anal. Chem., 35, 1965 (1963). 71. Marczenko Z., Mojski M., Czarnecka I., Chem. Anal. (Warsaw), 18, 189 (1973). 72. Ramachandran K.N., Kumar G.S., Talanta, 43, 1711 (1996). 73. Kasterka B., Chem. Anal. (Warsaw), 37, 361 (1992). 74. Somer G., Ekmekci G., Anal. Sci., 13, 205 (1997).

Chapter 45. Silicon

Silicon (Si, at. mass 28.09) occurs in its compounds in the IV oxidation state, e.g., in silica (SiO2) or in silicic acids. A characteristic feature of silicon(W) is its ability to form the tetrafluoride (SiF4) and heteropoly acids, e.g., with molybdic acid.


45.1.1. Distillation. Extraction

Volatile SiF4 distils from a hot HC104 or H2SO4 medium, containing excess of HF, in a closed apparatus made of platinum, silver, or Teflon [1]. The SiF4 liberated is absorbed in NaOH or H3BO3 solution.

Separation of SiF4 by the Conway microdiffusion method is a slow process [2,3]. The sample to be analysed (e.g., copper, aluminium, iron alloys, dolomite, titanium dioxide) is decomposed with acids in a polystyrene Petri dish to which hydrofluoric acid is subsequently added. The SiF4 evolved is trapped in NaOH solution, e.g., in another Petri dish placed beside. Both vessels are placed inside another, tightly closed, polystyrene vessel. To achieve quantitative separation of silicon, the reaction is carried out at 70~ for-~ 18 h.

Trace amounts of silicon can be evolved as SiF4 when a solid sample is heated with ammonium fluoride at 200-380~ [4]. The volatile SiF4 is carried by a nitrogen stream into an alkaline absorbing solution.

Molybdosilicic acid can be extracted from a 1 M H2SO4 (or HNO3) with oxygen- containing organic solvents [5], silicon being determined directly as the yellow heteropoly molybdosilicic acid or as silicomolybdenum blue. The molybdosilicic acid must be formed at lower acidity ([H +] < 0.7 M) and the acidity raised just before extraction. In the presence of high molecular-weight amines, molybdosilicic acid or its reduced form (heteropoly blue) may be extracted with toluene, CHC13, or a mixture of CHC 13 and isoamyl alcohol.


Ion exchangers are applicable both for separating small amounts of silicon and for removing various anions and cations from silicon [6,7]. Silicon has been retained as SiF62- on a strongly basic anion-exchanger and then displaced from the fluoride complex (and the column) by elution with H3BO3 solution. Silicon traces have been also retained on strongly basic anion exchanger, and then eluted with dilute ammonia [8]. Weakly basic anion- exchangers have been used to separate silicate from other anions [6]. Mixtures of silicon, P(V), and As(V) have been separated on anion-exchange columns [9].

Microgram amounts of silica can be co-precipitated with niobium, which precipitates as niobic acid. This method enables silicon to be quantitatively separated from major quantities of P(V), As(V), Fe(III), and A1 [10]. Trace amounts of silicon have been co- precipitated with molybdophosphoric acid from 8 M HNO3 [ 11 ].

Milligram amounts of silica can be precipitated without any collector by evaporation with HC104 to white fumes.

When samples are fused with alkalies (NazCO3, NaOH) and the melt is leached with

386 45. Silicon

water, silicon passes into solution, while many elements (e.g., Fe, Ti, Cu, Ni, and Zr) remain in the solid residue.


Silicon is determined spectrophotometrically as the yellow heteropoly molybdosilicic acid (less sensitive method) or, after reduction, as silicomolybdenum blue. Very sensitive methods, based on ion-association complexes with basic dyes, are becoming increasingly important.

45.2.1. Molybdosilicic acid method

In alkaline solutions, silica exists in the form of silicate ions (e.g., SIO32-). In dilute solutions (up to 0.1 mg of Si per ml) between pH 1 and 8, water-soluble monomeric silicic acid is the stable form. In more concentrated solutions of the same acidity, monosilicic acid condenses to disilicic acid and polysilicic acids which can be transformed into colloidal species.

Soluble monosilicic acid reacts with molybdic acid at pH 1-2, in the presence of at least a 0.05 M excess of molybdenum, to form the yellow soluble 13-molybdosilicic acid. The yellow colour is the basis of a rather insensitive spectrophotometric method for silicon [12- 15]. The absorption maximum of the complex is in the ultraviolet. At 400 nm, the molar absorptivity is 2.2.103 (a = 0.08) (in the presence of acetone).

At lower acidity, the reaction product is a-molybdosilicic acid, which is more stable than the 13-form but which absorbs only half as intensely at 400 nm [16,17]. Silicon is often determined from the colour of the a-form of molybdosilicic acid [18-20]. Besides acidity, the temperature, Si:Mo concentration ratio, and the degree of condensation of the molybdate ions influence the concentrations of the two heteropoly acid forms. At pH > 7, MoO42- ions are stable, but they condense when acidified.

Precise results are obtained in this method only if the conditions are kept rigorously the same for both samples and standards.

Silica is converted into monosilicic acid by fusing with sodium carbonate and acidifying the alkaline solution produced when the melt is dissolved in water. Alternatively, an acidic sample solution may be made alkaline with sodium hydroxide and heated to convert colloidal silica into silicate. Soluble monosilicic acid is formed after appropriate dilution and acidification.

On heating with dilute hydrofluoric acid, silica is transformed into the soluble hexafluorosilicic acid, HzSiF6. Aluminium chloride or boric acid introduced subsequently mask the excess of HF and decompose HzSiF6 to monosilicic acid (the more stable A1F63- or BF4 complexes being produced).

Phosphorus(V), Ge(IV), and As(V) which give yellow heteropoly acids, interfere. Before silicon is determined, Ge and As may be separated by volatilization or extraction of GeC14 and AsC13. Molybdophosphoric and molybdoarsenic acids are separated from molybdosilicic acid by extraction with butyl acetate at pH 0.3-1.0. The extractive separation of silicon from P(V) and As(V) has been discussed [5,21] . Molybdosilicic acid has been extracted with TOA in toluene [22]. Ion-associates with basic dyes, e.g., Chrompyrazole have been floated with a mixture of toluene and acetone [23].

Ferric ions interfere in the determination of silicon. Large quantities are separated by extraction, and smaller ones are masked with phosphoric acid.


Reagents

Ammonium molybdate, 10% solution, at pH adjusted to 7.4_+0.2 by means of ammonia. Standard silicon solution: 0.1 mg/ml. Fuse 0.2140 g of ignited and comminuted silica,

SiO2, with 2 g of NazCO3 in a platinum crucible. Dissolve the melt in water, dilute the solution with water to -~900 ml, acidify with 1 M H2SO4 to pH -1.5, and make up the solution to volume in a 1-1itre standard flask with water.

Procedure

In a 25-ml standard flask place 5 ml of a 1:1 mixture of ammonium molybdate solution and 1 M sulphuric acid, and add 3 ml of acetone and an aliquot of sample solution (e.g., neutralized solution from sodium carbonate fusion) containing not more than 0.2 mg of Si. Dilute to the mark with water, mix well, let stand for 15 min, and measure the absorbance at 400 nm against a reagent blank.

45.2.2. Silicomolybdenum Blue method

Molybdosilicic acid reacts with suitable reducing agents to yield the intensely coloured silicomolybdenum blue, upon which a sensitive method for determining silicon is based [24- 26]. The reaction conditions are adjusted so that only molybdosilicic acid, and not unreacted molybdic acid, is reduced.

Tin(II) chloride or oxalate, Fe(II) (Mohr's salt), ascorbic acid, sodium sulphite, and other reagents have been used as reductants [27,28]. To prevent partial reduction of molybdic acid, molybdosilicic acid is reduced in sufficiently acidic medium. Molybdosilicic acid is produced in a slightly acidic medium but, once formed, it does not decompose if the acidity is strongly increased (up to 1.5 M H2SO4). The most suitable acidity for the reduction depends on the reducing agent used. The various forms of molybdosilicic acid (a, [~) and the various reductants yield products which differ in absorption spectra, absorption maxima, and stability [27].

Molybdosilicic acid can also be reduced after extraction into an oxygen-containing organic solvent (e.g., amyl alcohol) [29,30]. Alternatively, silicomolybdenum blue may be formed in the aqueous phase and then extracted. The heteropoly blue exhibits similar molar absorptivities in both the organic phase and in the aqueous solution, but the absorption maximum is shifted slightly towards shorter wavelengths in organic solvents.

Silicomolybdenum blue produced by extraction of molybdosilicic acid into amyl alcohol and reduction with SnCI2 has its )~max at 750 nm. The molar absorptivity is 1.7.104, sp. abs. 0.60.

P(V), Ge, and As(V), which form corresponding heteropoly acids and heteropoly blues, must be separated or masked before the determination of Si. It is possible to separate silicomolybdenum and phosphomolybdenum blues by extraction [31 ].

The silicon traces present in the reagents and water used interfere in the determination of microgram amounts of silicon. Analytical grade HC1, H2SO4, and HF, and distilled water contain 2.10 -5 %, 7.10 -5 %, 4.10 -2 %, and 2-10 -6 % of Si, respectively [ 10]. These reagents may be considerably purified by distillation in quartz or platinum apparatus. Platinum, Teflon, and polyethylene vessels should be used and the silicon in a reagent-blank solution should be taken into account when traces of silicon are determined. The interfering effect of various substances on the determination of Si as silicomolybdenum blue was studied after decomposition of the samples with HF [32].

388 45. Silicon

Reduced molybdosilicic acid may be extracted with CHC13 in the presence of 1,2- propanediol carbonate [33] or with toluene in the presence of TOA [34]. Silicomolybdenum blue can be associated with dodecyltrimethylammonium ion and retained on a nitrocellulose membrane; the compound, along with the membrane, is then dissolved in DMF and its absorbance measured [35].

Reagents

Ammonium molybdate, 10% solution adjusted with ammonia to pH 7.4_+0.2. Standard silicon solution: 0.1 mg/ml. Preparation as in Section 45.2.1. Tin(H) chloride, SnClz.2H20, 50% solution in HC1 (1 + 1). Niobium solution, --1 mg of Nb in 1 ml. Heat 0.145 g of Nb205 in a platinum crucible

with 5 ml of conc. HF until the oxide dissolves. Evaporate the solution to --1 ml, add 2 ml of H2504 (1 + 1), and heat to white fumes. Let cool, rinse the walls of the crucible with water, and heat until white fumes appear. Let cool, rinse the walls of the crucible with water and heat to fumes again. Repeat the operation once more to remove HF completely. Pour the niobium solution in conc. H2504 into 30 ml of 5% aqueous ammonium oxalate solution, dilute the clear solution of niobium oxalate complex to 100 ml, with water, and mix well.

Procedure

Separation of Si with a collector. To the sample solution, containing not more than 25 ~tg of Si, in a Teflon or platinum vessel, add 2 ml of the niobium solution and 5 ml of conc. HC104 and evaporate the solution to fumes, expelling most of the perchloric acid. Dilute the residue with 10-20 ml of HC104 (1+50), stir until the salts dissolve, add some macerated filter paper, filter off the precipitate, and wash it with very dilute perchloric acid. Ignite the filter paper and precipitate in a platinum crucible. Add 1 ml of 5% HF to the cooled crucible and heat in a sealed vessel in a water-bath at --70~ for 30 min. Transfer the solution from the crucible to a polyethylene beaker, dilute with water to --10 ml, and add 5 ml of 3 % boric acid solution.

Determination o f Si. To the solution obtained as above, add 1 ml of the molybdate solution and adjust the pH to 1.4_+0.1 with 0.5 M H2SO4. After 5 min, transfer the solution to a separating funnel, add 5 ml of H2504 (1+4), and extract molybdosilicic acid with two 15- ml portions of isoamyl alcohol. Wash the extract by shaking with 10 ml of 0.5 M H2SO4. Transfer the organic phase to a 25-ml standard flask, and add one drop of the SnC 12 solution, -~1 ml of diethyl ether (to clarify the solution), and isoamyl alcohol to the mark. Mix well, and after 5 min measure the absorbance of the blue solution at 750 nm, vs. a reagent blank as reference.

Note. Because of the traces of Si present in the reagents the use of a reagent blank in absorbance measurement is recommended.


Sensitive methods for determining silicon are based on the formation of molybdosilicate ion- associates with certain basic dyes. The ion-associate with Crystal Violet (formula 4.26) can be extracted with a 3:2 mixture of cyclohexanol and isoamyl alcohol [36]. The sparingly soluble compound is centrifuged and then dissolved in acetone (e = 4.2-105 at 590 nm) [37].

The ion-associate with Rhodamine B is separated by flotation with DIPE and dissolved in ethanol (e= 5.0.105 at 555 nm) [38]. Other basic dyes have also been proposed, namely


Chrompyrazole II (antipyrine dye) [23,39], Brilliant Green [40-42] , Safranine T (e = 1.5.105) [43], Methylene Blue, and Methylene Green [44]. In the case of Methylene Blue, the compound with the Mo-Si is centrifuged and dissolved in acetone (e = 4.3-105 at 660 nm).

There exist several indirect methods for the spectrophotometric determination of silicon. After extraction of molybdosilicic acid, the Mo has been determined with phenylfluorone [45], or 2-amino-4-chlorobenzenethiol (e = 2.0.105) [46]. When silicic acid is added to a solution which contains hydrofluorotitanic acid and H202, a yellow titanium peroxide complex is formed. Chloranilic acid has also been used for determination of silicon [47].


Silicon has "been determined as molybdosilicic acid in sewage [48], organic compounds [49], rocks and minerals [19,50,51], bauxites [52], cast iron and steel [53], high purity copper [23], copper alloys [21], various metals [54], refractory materials [55], vanadium pentoxide [22], and semiconductors [56]. The method has been applied also in the differential spectrophotometry [57] and flow injection [58,59] techniques.

The silicomolybdenum blue method has been used for determining silicon in biological materials [60], waters [25,61-64], air [65,66], soil extracts [67], industrial waste waters [27], organic materials [68,69], various inorganic reagents [10], phosphoric acid [3], cast iron and steel [70-74], copper and its alloys [3,75,76], nickel and its alloys [71], aluminium [3,71], uranium and its compounds [71], titanium and its compounds [3,71,77], platinum and gold [5], molybdenum and its compounds [78], tungsten trioxide [79], refractory metals [1], gallium arsenide [80], gallium phosphide [6], and ferrophosphorus [81].

The phosphomolybdenum blue has been applied in automatic determination of Si in natural waters [82], industrial solutions [83], as well as in the flow injection technique for determination of silicon in sea water [84], soil extracts [67,85], and glass [86].

The ion associates with Malachite Green have been used for determining Si in water [42] and glass [41 ].

References

1. Stobart J.A.,Analyst, 94, 1142 (1969). 2. Alon A., Bernas B., Frenkel M., Anal. Chim. Acta, 31, 279 (1964). 3. Householder R., Russell R.G., Anal. Chem., 36, 2279 (1964). 4. Szabo Z.G., Zapp E.E., Perczel S., Mikrochim. Acta, 1974, 167. 5. Marczenko Z., Lenarczyk L., Chem. Anal. (Warsaw), 19, 679 (1974). 6. Luke C.L., Anal. Chem., 36, 2036 (1964). 7. Duce F.A., Yamamura S.S., Talanta, 17, 143 (1970). 8. Bazzi A., Boltz D.F., Microchem. J., 20, 462 (1975). 9. Narusawa Y., Anal. Chim. Acta, 204, 53 (1988). 10. Marczenko Z., Kasiura K., Chem. Anal. (Warsaw), 9, 321 (1964). 11. Barkovskii V.F., Radovska T.L., Zaporozhets A.S., Zh. Anal. Khim., 25, 1853 (1968). 12. Govett G.J., Anal. Chim. Acta, 25, 69 (1961). 13. Chalmers R.A., Sinclair A.G., Anal. Chim. Acta, 33, 384 (1965); 34, 412 (1966). 14. Truesdale V.W., Smith C.J., Smith P.J., Analyst, 102, 73 (1977). 15. Kircher C.C., Crouch S.R., Anal. Chem., 54, 2303 (1982). 16. Truesdale V.W., Smith C.J., Analyst, 100, 203,797 (1975).

390 45. Silicon

17. Truesdale V.W., Smith P.J., Smith C.J., Analyst, 104, 897 (1979). 18. Kato K., Anal. Chim. Acta, 82, 401 (1976). 19. Sarkar R.S., Das M.S., Anal. Chim. Acta, 134, 401 (1982). 20. Lamathe J., Hautbout R., Analusis, 13, 420 (1985). 21. Shkavavskii Yu.F., Lynchak K.A., Chernogorenko V.B., Zavod. Lab., 51, No 12, 5

(1985). 22. Dorokhova E.N. et al., Zh. Anal. Khim., 29, 2014 (1974). 23. Gurentsova O.I., Prokhorova G.V., Osipova E.A., Dorokhova E.N., Zh. Anal. Khim., 48,

332(1993). 24. Hargis L.G., Anal. Chem., 42, 1494, 1497 (1970). 25. Tarasova N.S., Dorokhova E.N., Alimarin I.P., Zh. Anal. Khim., 35, 1520 (1980). 26. Smith J.D., Milne P.J., Anal. Chim. Acta, 123, 263 (1981). 27. Morrison I.R., Wilson A.L., Analyst, 88, 88, 100 (1963). 28. Nag6rski B., Chem. Anal. (Warsaw), 20, 903 (1975). 29. Pakalns P., Flynn W.W., Anal. Chim. Acta, 38, 403 (1967). 30. Kakita Y., Goto H., Talanta, 14, 543 (1967). 31. Tarasova N.S., Dorokhova E.N., Alimarin I.P., Zh. Anal. Khim., 36, 459 (1981). 32. Sadiq M., A1-Muhanna H., Alam I., Commun. Soil Sci. Plant Anal., 19, 1693 (1988). 33. Trudell L.A., Boltz D.F., Anal. Chim. Acta, 52, 343 (1970). 34. Zhukova L.B., Dorokhova E.N., Tikhomirova T.I., Zh. Anal. Khim., 33, 710 (1978). 35. Kasahara I. et al., Anal. Chem., 59, 787 (1987). 36. Babko A.K., Shkaravskii Yu.F., Go~kowska A., Chem. Anal. (Warsaw), 11, 1091

(1966). 37. Mirzoyan F.V. et al., Zh. Anal. Khim., 34, 1515 (1979). 38. Go~kowska A., Pszonicki L., Talanta, 20, 749 (1973). 39. Dorokhova E.N., Gracheva N.A., Dracheva L.V., Zh. Anal. Khim., 43, 265 (1988). 40. Motomizu S., Oshima M., Ojima Y., Anal. Sci., 5, 85 (1989). 41. Motomizu S. et al., Analyst, 114, 1679 (1989). 42. Susanto J.P., Oshima M., Motomizu S., Analyst, 120, 2605 (1995). 43. Pilipenko A.T., Shkaravskii Yu.F., Ukr. Khim. Zh., 42, 1183 (1976). 44. Mirzoyan F.V., Tarayan V.M., Zh. Anal. Khim., 35, 1293 (1980). 45. Halasz A., Polyak K., Pungor E., Talanta, 18, 691 (1971). 46. Trudell L.A., Boltz D.F., Talanta, 19, 37 (1972). 47. Mazuranic K. et al., Microchem. J., 46, 374 (1992). 48. Sager M., Puxbaum H., Mikrochim. Acta, 1984 I, 361. 49. Debal E., Talanta, 19, 15 (1972). 50. Uchida T. et al., Anal. Sci., 4, 523 (1988). 51. Foner H.A., Gal I., Analyst, 106, 521 (1981). 52. Fresenius W., Schneider W., Z. Anal. Chem., 214, 341 (1965). 53. Macher F., Glasz M., Mikrochim. Acta, 1964, 104. 54. Bill J., Mikrochim. Acta, 1966, 1047. 55. Privalova M.M., Makhova G.P., Tulina M.D., Zh. Anal. Khim., 29, 279 (1974). 56. Kitazume E., Usami K., Mater. Trans., 30, 184 (1989). 57. Beshikdashyan M.T., Vasireva M.G., Zh. Anal. Khim., 36, 1082 (1981). 58. Yokoyama T. et al., Bull. Chem. Soc. Jpn., 55, 3477 (1982). 59. Kuroda R., Ida I., Kimura H., Talanta, 32, 353 (1985). 60. Tfima J., Mikrochim. Acta, 1962, 513. 61. Morrison I.R., Wilson A.L., Analyst, 94, 54 (1969). 62. Fanning K.A., Prison M.E., Anal. Chem., 45, 136 (1973).

References 391

63. Ramachandran R., Gupta P.K., Anal. Chim. Acta, 172, 307 (1985). 64. Gamo T. et al., Anal. Sci., 10, 843 (1994). 65. Chu-Fang Wang et al., Anal. Chim. Acta, 342, 239 (1997). 66. Wang C.F., Tu F.H., Jeng S.L., Anal. Chim. Acta, 342, 239 (1997). 67. Raben-Lange B., Broe Bendtsen A., Jorgensen S.S., Commun. Soil Sci. Plant Anal., 25,

3241 (1994). 68. Christopher A.J., Fennell T.R., Webb J.R., Talanta, 11, 1323 (1964). 69. Bradley A., Altebrando D., Anal. Chem., 46, 2061 (1974). 70. Sauer K.H., Keller H.,Arch. Eisenhiittenw., 41, 961 (1970). 71. Pakalns P., Anal. Chim. Acta, 54, 281 (1971). 72. Tarasova N.S., Dorokhova E.N., Alimarin I.P., Zh. Anal. Khim., 38, 63 (1983). 73. Ptushkina M.N., Tikhonova L.I., Shelyapina T.D., Zavod. Lab., 57, No 1, 73 (1991). 74. Basak A.C. et al., Microchem. J., 54, 48 (1996). 75. Sturton J.M.,Anal. Chim. Acta, 32, 394 (1965). 76. Pakalns P., Anal. Chim. Acta, 40, 327 (1968). 77. Barkovskii V.F., Radovskaya T.L., Zavod. Lab., 35, 160 (1969). 78. Fukker K., Hegedfis A.J., Mikrochim. Acta, 1961, 227. 79. Chkanikova O.K., Dorokhova E.N., Zh. Anal. Khim., 34, 944 (1979). 80. Lin R.S., Yang M.H., Z. Anal. Chem., 325, 272 (1986). 81. Brooking K.A., Belcher C.B., Talanta, 22, 777 (1975). 82. Truesdale V.W., Smith C.J., Analyst, 101, 19 (1976). 83. Kowalski Z., Migdalski J., Kolder E., Chem. Anal. (Warsaw), 21, 655 (1976). 84. Thomsen J., Johnson K.S., Petty R.L., Anal. Chem., 55, 2378 (1983). 85. Borggaard O.K., JCrgensen S.S., Analyst, 110, 177 (1985). 86. Archer F.A., Street K.W. Jr., Anal. Chim. Acta, 262, 243 (1992).

Chapter 46. Silver

Silver (Ag, at. mass 107.87) occurs in its compounds in the (I)- oxidation state. So far, silver(II) is only of limited value in spectrophotometry. Silver(I) -sulphide and-halides are sparingly soluble. Ammine, cyanide, and thiosulphate complexes of silver are formed. In the presence of excess of C1- or SCN-, traces of silver form soluble complexes.


46.1.1. Extraction

Silver is often separated from other metals by extraction with dithizone. The details of this separation are discussed below in the dithizone method for determining silver�9

Other methods for separating silver involve extraction of the diethyldithiocarbamate into chloroform [pH 4-11 (EDTA as masking agent)] [ 1]. The thiocyanate complex of silver can be extracted from 0.1-1 M solutions of H2SO4, HC1, or HC104 into TBP [2].

Selective separation and preconcentration of microgram quantities of silver are often based on the use of macrocyclic compounds. Thus, silver has been extracted from nitrate medium by means of a crown ether, dicyclohexyl-18-crown-6 [3], and macrocyclic compounds containing sulphur or nitrogen atoms, in 1,2-dichloroethane solutions [4-8]. Among several 15- and 19-membered macrocyclic compounds [9,10] containing nitrogen, oxygen, or sulphur as the electron-donor atoms, the most favourable properties have been found for 2,3:8,9-dibenzo-4,7,13-trithia- l, 10-diazacyclopentadecane (formula 46. l) used as a solution in 1,2-dichloroethane or chloroform, with dipicrylammate or picrate as the counterions [ 11 ].

(46.1)


�9 Silver ions can be retained on a strongly basic anion-exchange column in the chloride form, and subsequently eluted with ammonia [12,13].

Silver may be separated from Ce, Zr, Th, Be, and Fe(III), on strongly acidic cation- exchangers, by converting these metals into anionic complexes, or separated from Cu, U, A1, and Zn by selective elution with nitric acid [14]. After retention of Pb, Ag, and Hg on Dowex 50, lead is eluted first with 0.25 M ammonium acetate, then silver with 0.5 M ammonia solution�9 Silver has been separated on a cation-exchanger from Hg, Co, Ni, and Zn on the basis of the differing stabilities of their EDTA complexes at pH 4.6 [15]. Silver retained in a column with a macroporous cation-exchange resin bed has been eluted with 2 M HNO3 or 0.5 M HBr in aqueous acetone solution [ 16].


In convenient precipitation methods, traces of silver are separated with the use of collectors; tellurium has been recommended as a collector, and SnC12 as a reducing agent.

Good separation of silver is also obtained by precipitation of the sulphide with Hg, Cu, or Pb as scavenger. It is possible to co-precipitate silver chloride or bromide with TI(I) as collector. Traces of silver have been collected on powdered dithizone [17,18], or rhodanine [19].

Selective sorption of Ag from solutions containing mixtures of various metals has been obtained with the use of a polyacrylamide sorbent, impregnated with a compound of a crown ether, dibenzo-18-crown-6 with molybdophosphoric acid [20].

Microgram amounts of silver are separated on shaking an aqueous solution (about 1 M in HNO3) with mercury (redox exchange). Less noble metals (e.g., Cu, Bi, Pb) remain in solution [21 ].

Silver can be isolated along with the platinum metals by fire assay, with copper(I) sulphide as collector [22]. Silver (and gold) present in the sample are collected in copper on melting at -~1,200~ After dissolving the copper alloy and diluting the obtained solution, silver (and gold) are reduced to the metals with formic acid or hydroquinone [23].


Among the many spectrophotometric methods for determining silver, the dithizone method is particularly important. Some newer methods, such as that using thio-Michler's ketone, and methods based on ion-associates with dyes, can be recommended.


Dithizone (formula 4.37) reacts with silver ions in acid medium (H2SO4, HNO3, HC104) to form the orange-yellow dithizonate, which is soluble in CC14 and other inert solvents. A solution of HzDz in carbon tetrachloride extracts silver rapidly, even from a 4 M H2804 medium.

Silver dithizonate exhibits a molar absorptivity of 3.05-104 (a - 0.28), at ~max = 462 nm (the absorption spectrum of AgHDz is shown in Fig. 4.4) [24,25].

At higher pH values and in the presence of excess of Ag, the purple dithizonate AgzDz is formed, but this compound is readily transformed into AgHDz in acidic media in the presence of an excess of dithizone. AgHDz is stable even in 5% NaOH solutions.

From a strongly acidic chloride-free medium, noble metals [Au, Pt(II), Pd, Hg] and Cu are extracted together with the silver. The presence of chloride in the acid solution prevents the formation of silver dithizonate [26]. Trace amounts (10-20 gg) of chloride do not interfere in the extraction.

The AgHDz. can be decomposed by shaking the carbon tetrachloride extract with 1 M HC1, thereby separating the silver from the co-extracted metals. Similarly, an acidified thiocyanate solution strips silver from the extract containing Ag, Hg(II), and Cu(II) dithizonates.

Dithizone extracts Ag from solutions containing chloride but at pH values of 2-5. In the presence of EDTA (pH 4-5) Ag (and also Hg and Au) can be extracted from a solution containing considerable amounts of Cu, Bi, Cd, Zn, Ni, and Pb. When a solution containing Ag, Au, and EDTA at pH 4.7 is heated to boiling, the gold is reduced by EDTA to the element. Mercury can be volatilized by igniting the sample before the determination of silver.

394 46. Silver

Since silver dithizonate is more stable than copper dithizonate, a solution of violet Cu(HDz)2 in CC14 may be used in the extractive titration technique instead of dithizone. The colour change from orange-yellow to violet is then more easily observed.

Reagents

Dithizone (H2Dz), 0.01% solution in CC14. Dissolve enough reagent to provide 50 mg of "active" (non-oxidized) dithizone in 100 ml of CCla. Filter the solution through a filter paper into a 500-ml separating funnel. Shake the green solution with 100 ml of aqueous ammonia (1+50). Discard the brown CC14 layer containing oxidation products of HzDz. Acidify the orange ammoniacal dithizone solution with 1 M HC1, and shake it with 200 ml of CC14 to decolorization of the aqueous phase. Dilute the green CC14 solution of HzDz to 500 ml with the solvent, and keep in an amber glass bottle under a layer of 1 M H2SO4. Prepare working solutions (e.g., 0.001% HzDz) by suitable dilution of the stock solution (0.01%).

Determine the concentration of dithizone in the CC14 solution either by measuring the absorbance of the green solution, or by extractive titration with a standard silver solution, as follows. Place in a separating funnel 100 ~tg of Ag (10 ml of a 0.01 mg/ml solution), add -~0.001% dithizone solution in portions from an amber-glass burette, and shake. Drain the resulting orange silver dithizonate solution from the separating funnel, and continue shaking with successive dithizone portions. Towards the end-point, add 0.5-0.2 ml portions of the H2Dz solution. The titration of 100 gg of Ag requires 23.75 ml of exactly 0.001% HzDz solution.

Standard silver solution: 1 mg/ml. Dissolve 1.5750 g of dried (110~ silver nitrate in water containing 1 ml of conc. HNO3, and dilute the solution to the mark with water in a 1-1itre standard flask.

Carbon tetrachloride. Regeneration of the solvent used or purification of the commercial product is carried out as follows. Shake a portion of the solvent (about 400 ml) in a 500-ml separating funnel successively with H2SO4 (1+2), 10% NaOH solution, water, 1% KMnOa solution in H2804 (1+9), and 5% Na2SO3 solution. Dehydrate the clear CC14 layer by mixing it in a conical flask with 50 g of anhydrous K2804. Pour the solvent into a still, add 1 g of Na2S203.5H20, and distil the CC14, collecting the fraction boiling at 76- 78~

It is often sufficient to purify commercial grade (or old) carbon tetrachloride by shaking with dilute (e.g., 0.1 M) Na2S203 and then with water.

Procedure

Adjust the acidity of the chloride-free solution, containing not more than 50 ~tg of Ag, with H2804 or HNO3 until the concentration is 1-2 M, with respect to acid, and extract Ag (along with Au, Pd, and Pt) with small portions of dithizone in CC14 (1 ml of 0.001% HzDz solution corresponds to 4.2 gg of Ag). The last portion of dithizone added should not change from green to yellow, although it may turn violet [owing to Cu(HDz)2].

Shake the combined orange-yellow CC14 extracts for 20 s with 1 M HC1. Separate the aqueous layer (containing Ag), add a little EDTA, adjust to pH 4-5 with ammonia, and xtract Ag with portions of 0.001% dithizone solution in CC14. Remove free dithizone from the extract with dilute ammonia solution (2 drops of conc. NH3 solution in 25 ml of water). Dilute the AgHDz solution with carbon tetrachloride in a 25-ml standard flask, and measure the absorbance at 462 nm, using CC14 as reference.

References 395


Thio-Michler's ketone (TMK), 4,4'-bis(dimethylamino)thiobenzophenone has been proposed for a very sensitive spectrophotometric determination of silver (formula 46.2) [27]. The yellow reagent, soluble in many organic solvents, reacts with silver ions over a wide pH range (2-8) giving a red-violet complex, soluble in water-organic solvent media, and extractable into some organic solvents (e .g. , butanol, or a mixture of butanol and CHC13). The complex is solvated by oxygen-containing solvents. The presence of anionic surfactants promotes the co-ordination of a greater number of the reagent molecules [28].

S S-

, , e / " ~ ~ "~c. , ~ (46.2) H3 H3

The molar absorptivity depends on the medium. It is 1.40.105 (a = 1.3) at 530 nm in DMF (40-50%) (pH -~3), 1.06.105 in ethanol (50-60%), 1.12.105 at 520 nm in a mixture CHC13-butanol, and 0.96.105 in butanol.

Under the conditions suitable for determination of silver, TMK reacts with Au, Pd, Pt(II), Hg, and Cu. When they are present, a preliminary separation of Ag may be necessary.

All halides, except fluoride, interfere in the determination of silver. An approximately equimolar amount of chloride can be tolerated. In certain cases, large amounts of chloride or bromide may be used to mask silver in the determination of other noble metals with TMK. Thiosulphate, thiocyanate, sulphide, phosphate, as well as reducing and oxidizing agents interfere in the silver determination.

Tartaric acid and EDTA may be used for masking hydrolysable metal ions at pH about 3.

Reagents

Thio-Michler's ketone (TMK), 0.001 M solution in dimethylformamide (DMF) (28.5 mg of TMK in 100 ml). The solution should be kept in darkness.

Standard silver solution: 1 mg/ml. Preparation as in Section 46.2.1. Acetate buffer, pH 3.0. Mix 50 g of sodium acetate trihydrate in 100 ml of water with

350 ml of glacial acetic acid, adjust to pH 3.0, and dilute to 500 ml with water.

Procedure

To an acid solution (pH -~3), containing in -10 ml not more than 20 gg of Ag, add 2 ml of acetate buffer, 6 ml of DMF and 2.5 ml of TMK solution. Make up the solution to volume with DMF in a 25-ml standard flask, and mix thoroughly. After 10 min, measure the absorbance at 530 nm vs. a reagent blank solution as reference.


p-Dimethylaminobenzylidenerhodanine (rhodanine, formula 46.3) reacts with silver ions in an acid medium to form a compound which is sparingly soluble in water. The method for determining silver is based on the red pseudo-solution of the complex in the presence of an excess of yellow rhodanine [29,30]. The molar absorptivity is 2.0.104 at 450 nm.

396 46. Silver

HN--------C=O . S~C~s" (46.3)

Protective colloids (gum arabic or gelatine) can be added as stabilizing agents. Au, Pt, Pd, and Hg react similarly to silver in an acid medium, but copper does not interfere. Anions that form sparingly soluble silver salts do interfere.

The cationic complex of silver and 1,10-phenanthroline (phen) has been found to react with Bromopyrogallol Red to yield an ion-associate which can be extracted into nitrobenzene (~ = 3.2.10 4 at 590 nm) [31,32]. The cationic complex, Ag(phen)2 + gives ion- associates also with acid dyes, such as Rose Bengal (formula 4.35) and eosin (formula 4.34) (nitrobenzene) [33]. Extractable ion-associates of cationic silver complex with 1,4,8,11- tetrathiacyclotetradecane (crown ether), and various chromogenic anions [34] should also be mentioned.

Sensitive extraction-spectrophotometric methods are based on the extractable (into CHC13, 1,2-dichloroethane, benzene, or toluene) ion-associates of basic dyes and anionic Ag complexes with cyanide [35,36], iodide [37,38], and bromide [39]. In these methods, use has been made of such dyes as Crystal Violet [35,39], Brilliant Green [38,39], Malachite Green [39], Methylene Blue [36], and Nile Blue A [37]. In some of these methods the molar absorptivities are close to 1-105 [36,39]. A flotation method has been proposed, based on the addition compound [R6G+][Ag(SCN-)z]-[R6G+][SCN -] which is formed by silver ions (at pH 2-5) in the presence of thiocyanate and Rhodamine 6G (flotation with DIPE, the precipitated compound is washed and dissolved in acetone, e = 1.5-105) [40]. The complex Ag(CN)2-, associated with Crystal Violet, has been utilized in another flotation-spectrophotometric method of determining silver [41]. Silver has been determined also in a system comprising thiocyanate and Rhodamine B, as an aqueous pseudo-solution, in the presence of poly(vinyl alcohol) [42].

Other spectrophotometric organic reagents used for the determination of silver include 2-(3,5-dibromo-2-pyridylazo)-5-diethylaminophenol (3,5-diBr-PADAP) in the presence of the anionic surfactant lauryl sulphate (~ = 7.7.104) [43] and dodecyl sulphate (E = 6-104) [44], 4-(3,5-dibromo-2 pyridylazo)-N,N-diethylaniline in the presence of dodecyl sulphate [45], Cadion 2B in the presence of Triton X-100 (~ = 1.0-105) [46], 4-(2-quinolylazo)phenol [47], 4-(p-nitrophenylazo)-l-amino-3-pyridynol (~ = 1.07-105 at 605 rim) [48], and thyrodine (after extraction of silver with the use of macrocyclic compounds) [5,49].

In a rather insensitive method of determining silver (e -~1-104) use has been made of a coloured sol which is produced when silver ions are reduced to the element by means of ascorbic acid in the presence of gelatine (pH ~8) [50].


The dithizone method has been used for determining silver in sewage [12], aluminium and its compounds [51], uranium compounds [52], tin [26], lead [53], gold [54], metal tellurides [55,56], and glass and ceramic materials [57].

A related compound, di-2-naphthylthiocarbazone (dinaphthizone) has been used for determining Ag in selenium and tellurium (e = 4.7-104 at 505 rim) [58].

The thio-Michler's ketone was applied for the determination of silver in mineral waters [59,60], sewage [61], and lead [59,60].

References 397

Rhodanine and its derivatives have been used for separation and determination of silver in waters and in silicate rocks [62], ores [63,64], and copper, tin, and lead alloys [65].

Traces of silver in copper amalgam [32] and in tellurium [66] have been determined by the method involving 1,10-phenanthroline and Bromopyrogallol Red.

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255 (1988). 12. Pierce T.B., Analyst, 85, 166 (1960). 13. Lukashenkova N.V., Tolmatseva N.S., Shkrobot E.L., Zavod. Lab., 39, 541 (1973). 14. Rangnekar A.V., Khopkar S.M., Mikrochim. Acta, 1965, 642. 15. Shrimal R.L., Talanta, 18, 1235 (1971). 16. Strelow F.W., Talanta, 32, 953 (I 985). 17. Fukuda K., Mizuike A., Anal. Chim. Acta, 51, 77 (1970). 18. Mizuike A., Hiraide M., Kawakubo S., Mikrochim. Acta, 1979 II, 487. 19. Mizuike A., Fukuda K.,Anal. Chim. Acre, 44, 193 (1969). 20. Der-Liang T., Jeng-Shang S., Yu-Chai Y.,Analyst, 112, 1413 (1987). 21. Mizuike A., Sakamoto T., Sugishima K., Mikrochim. Acre, 1973, 291. 22. Kallmann S., Talanta, 33, 75 (1986). 23. Diamantatos A., Talanm, 34, 736 (1987). 24. Dyer F.F., Schweitzer G.K., Anal. Chim. Acta, 23, 1 (1960). 25. Miller A.D., Grosse Yu.I., Zh. Anal. Khim., 30, 913 (1975). 26. Marczenko Z., Kasiura K., Chem. Anal. (Warsaw), 10, 449 (1965). 27. Cheng K.L., Mikrochim. Acta, 1967, 820. 28. Pilipenko A.T., Matsibura G.S., Terletskaya A.V., Zh. Anal. Khim., 41,829 (1986). 29. Stephen W.I., Townshend A., J. Chem. Soc., 1965, 3738. 30. Borisova R., Koeva M., Topalova E., Talanta, 22, 791 (1975). 31. Dagnall R.M., West T.S., Talanta, 8, 711 (1961); 11, 1533 (1964). 32. Yaroshenko O.P., Gavrilova V.N., Sumskaya N.R., Zavod. Lab., 57, No 9, 4 (1991). 33. E1-Ghamry M.T., Frei R.W., Anal. Chem., 40, 1986 (1968). 34. Saito K., Masuda Y., Sekido E., Bull. Chem. Soc. Jpn., 57, 189 (1984). 35. Markham J.J.,Anal. Chem., 39, 241 (1967). 36. Koh T., Katoh M.,Anal. Chim. Acta, 109, 107 (1979). 37. Likussar W., Raber H., Anal. Chim. Acta, 50, 173 (1970).

398 46. Silver

38. Busev A.I., Shestidesyatnaya N.L., Zh. Anal. Khim., 29, 1138 (1974). 39. Shestidesyatnaya N.L., Kotelyanskaya L.I., Chuchulina I.A., Zh. Anal. Khim., 30, 1303

(1975). 40. Kalinowski K., Marczenko Z., Chem. Anal. (Warsaw), 32, 941 (1987). 41. Ishchenko N.N., Ganago L.I., Ivanova I.F., Zh. Anal. Khim., 52, 848 (1997). 42. Lopez Garcia I., Hernandez Cordoba M., Sanchez-Pedreno C., Analyst, 109, 1573

(1984). 43. Shui-Chien H., Chang-Ling Qu., Shiu-Sheng W., Talanta, 29, 85 (1982). 44. Jarosz M., Oszwa~dowski S., Marczenko Z., Chem. Anal. (Warsaw), 37, 335 (1992). 45. Ohohita K., Wada H., Nakagawa G., Anal. Chim. Acta, 182, 157 (1986). 46. Fu-sheng W., Fang Y., Talanta, 30, 190 (1983). 47. Barua S., Garg B.S., Singh R.P., Singh I.,Analyst, 105, 996 (1980). 48. Tarin P., Figuerola E., Blanco M., Mikrochim. Acta, 1986 I, 97. 49. Morosanova E.I., Kosyrieva O.A., Kuz'min N.M., Zolotov Yu.A., Zh. Anal. Khim., 43,

1614(1988). 50. Pal T., Maity D.S., Analyst, 111, 49 (1986). 51. Beinrohr E., Hofbauerova H., Mikrochim. Acta, 1989 II, 119. 52. Mare6ek J., Singer E., Z. Anal. Chem., 203, 336 (1964). 53. Jones P.D., Newman E.J.,Analyst, 87, 66 (1962). 54. Marczenko Z., Kasiura K., Krasiejko M., Chem. Anal. (Warsaw), 14, 1277 (1969). 55. Fano V., Zanotti L., Anal. Chim. Acta, 72, 419 (1974). 56. Marczenko Z., Podsiad~o E., Mikrochim. Acta, 1976 II, 317. 57. Roy S.K., Kundu D., Anal. Lett., 24, 139 (1991). 58. Tiptsova V.G., Andreichuk A.M., Bazhanova L.A., Zh. Anal. Khim., 21, 1179 (1966). 59. Pilipenko A.T., Ryabushko O.L., Matsibura G.S., Ukr. Khim. Zh., 47, 751 (1981). 60. Pilipenko A.T., Ryabushko O.L., Matsibura G.S., Zavod. Lab., 48, No 5, 7 (1982). 61. Stryjewska E., Rubel S., Chem. Anal. (Warsaw), 26, 615 (1981). 62. E1-Sayed A.A., Bull. Chem. Soc. Jpn., 67, 3216 (1994). 63. Wu X., Liang S., Fresenius'J. Anal. Chem., 336, 120 (1990). 64. E1-Zawawy F.M., E1-Shahat M.F., Mohamed A.A., Zaki M.T., Analyst, 120, 549 (1995). 65. Zhou N. et al., Talanta, 37, 531 (1990). 66. Dobrowolski J., Szwabski S., Chem. Anal. (Warsaw), 15, 1033 (1970).

Chapter 47. Strontium and barium

Strontium (Sr, at. mass 87.62) and barium (Ba, at. mass 137.33) occur in solution exclusively in the II oxidation state. The basicity and solubility in water increase from Ca(OH)2 to Ba(OH)2. Barium chromate and -sulphate are less soluble than the corresponding strontium compounds. The stability of the relatively weak complexes (e.g., with EDTA or tartrate) diminishes in the sequence Ca, Sr, Ba.


Before spectrophotometric determination, it is usually necessary to separate strontium or barium from the Analytical Group I, II, and III metals. Suitable methods are discussed in Chapter 14.


Mixtures of alkaline-earth metals are separated on strongly acidic cation-exchangers. The cations are retained on a cationite column, and then they are eluted selectively with appropriate complexants, based on the differences in stability of complexes formed by the alkaline-earth metals with suitable complexing eluents.

Complexones such as EDTA (complexone III) [1-3] and DCTA (complexone IV) [4,5] are suitable eluents, but other complexing agents, such as citrate [3,6] and sulphate [7] are also applied. Barium has been separated from strontium and other metals by cation-exchange chromatography using mixed HCl-organic solvent eluents [8]. Strontium has been enriched and determined in sea water [5] and in milk [2].

Calcium, strontium, and barium have been separated by elution from a strongly basic anion-exchanger with citrate. Calcium and strontium have been separated by using a mixed medium, comprising 0.25 M HNO3 and methanol [9]. Barium is quantitatively retained on the chromate form of an anion- exchanger, while calcium passes through [10]. If barium and calcium are passed through an anion-exchanger column, in a medium containing HNO3 and methanol, only Ba is sorbed. It can then be eluted with 0.5 M HNO3 [ 11 ].

Strontium was sorbed from aqueous-organic media on a resin impregnated with Arsenazo I [12] and on a polyurethane foam [13].


Calcium, strontium, and barium have been separated by extraction with Azo-azoxy BN (formula 14.1) in CC14 + TBP [14]. First, Ca is extracted from 0.05 M NaOH, then strontium from 0.8 M NaOH solution; barium remains in the aqueous phase. Strontium is 95% extracted from 0.8-2 M NaOH, in the presence of a tenfold excess of Azo-azoxy BN.

Strontium may be separated from other metals by extraction with HTTA in MIBK [15,16] or with HTTA and TBP in CC14 [17]. Strontium and barium can be extracted with BPHA in CHC13 [ 18].

Macrocyclic reagents, such as crown ethers, 18-crown-6 (formula 1.15), 15-crown-5 [19-21], and [cryptand-2.2.2] [20,22] give cationic complexes with Sr and Ba. When

400 47. Strontium and barium

associated with picrate ion, these complexes are a basis for separation of Sr and Ba [ 19,23], Sr (and Ca) [24], Sr [20,25], and Ba [21,26], using CHC13, 1,2-dichloroethane, and nitrobenzene as solvents. The ion associate with Erythrosin B has been used also for determination of Sr [24]. Strontium has been extracted with 18-crown-6 ether in the presence of trichloroacetate [27]. The effect of surfactant on Sr extraction with 18-crown-6 ether has been studied [28].

Benzoyltrifluoroacetone (in CHC13) has also been used for extraction of Sr and Ba (as well as Ca) [29].

In the separation of Ca(NO3)2 from Sr(NO3)2 by extraction (leaching) better results were obtained when acetone was used as the solvent instead of a (1+1) mixture of ethanol and diethyl ether [30].

Traces of Sr have been co-precipitated as the chromate with Ba [31] and as the oxalate with Ca [32]. Enrichment of strontium on MnOzaq. has also been utilized [33].

Barium has been precipitated as BaCrO4 from solutions containing Sr and Pb, by lowering slowly the pH of the solution containing the DCTA complexes of these metals. As much as 99.5% Ba precipitated, as the pH reduced from 10.3 to 6.7 when ammonia was expelled by heating the solution. The amounts of Sr and Pb co-precipitating with the Ba were <0.1% [34]. Barium has been co-precipitated with PbCrO4 [35].

Strontium impurities have been isolated from calcium salts by precipitating calcium as Ca(OH)2 with NaOH solution. After double precipitation of Ca(OH)2, the amount of strontium co-precipitated had decreased to 3 ppm [36].

47.2. Determination of strontium

Of the bisazo derivatives of chromotropic acid, Nitchromazo (Nitro-orthanilic S) (formula 47.1) is particularly suitable for determination of strontium [37,38].

S03H HO OH HO3S

(47.1)

The colour reaction ()Lmax = 650 nm), which is the basis for the determination of strontium, is carried out in aqueous acetone medium at pH 2.8. Strontium has been determined in barium salts by this method [37]. DMF or acetonitrile may be used instead of acetone [38].

The related reagents such as Chlorophosphonazo HI [39], Sulphonazo III [40], Carboxy- nitrazo [41], and Arsenazo III (e = 4.0.104 at 650 nm) [42] have also been used for the determination of strontium.

The Chlorophosphonazo III method has been used for determination of Sr in electroluminescent thin films [43] and Ba in mixed Y-Ba-Cu oxides [44].

47.3. Determination of barium

One of the foremost spectrophotometric reagents for barium is Sulphonazo IIl (Orthanilic S, formula 47.2) [21,40,45,46]. The colour reaction is carried out in weakly acid or neutral medium (pH 2-8) in 60% acetone (or ethanol) or in aqueous solution. The molar absorptivity

References 401

depends on the pH, the medium, and the excess of reagent (e = 4.0-10 4 a t 640 nm, pH 8, 60% acetone; e = 1.1.104, pH 2.4, water). In aqueous solution, 100 ~tg of Sr or 300 ~tg of Ca can be tolerated.

The following bisazo derivatives of chromotropic acid have been proposed as related reagents for barium: Nitchromazo [38], Chlorophosphonazo lJI [47], Carboxyarsenazo (in 50% ethanol, pH 5.6, e = 4.1.104 at 640 nm) [48], and Arsenazo III [42].

O,H HO OH HO3S N = N ~ ~ ~ N = N ~ HO~S SO3H

(47.2)

Barium can be determined indirectly by precipitation as BaCrO4, dissolution of the precipitate in acid, and measurement of the absorbance of the Cr2072- ions or of the Cr(VI)- 1,5-diphenylcarbazide complex [49].

Barium has been determined, with the use of Nitchromazo, in Y-Ba-Cu mixed oxides [51 ]. The Sulphonazo III has been applied for determination of Ba in waters [52].

References

1. Bouquiaux J.J., Gillard J.H., Anal. Chim. Acta, 30, 273 (1964). 2. Brandt P.J., Van't Riet B., Anal. Chem., 38, 1790 (1966). 3. Strelow F.W., Weinert C.H., Talanta, 17, 1 (1970). 4. Noshkin V.E., Mott N.S., Talanta, 14, 45 (1967). 5. Andersen N.R., Hume D.N., Anal. Chim. Acta, 40, 207 (1968). 6. Ibbett R.D., Analyst, 92, 417 (1967). 7. Christova R., Kruschevska A., Anal. Chim. Acta, 36, 392 (1966). 8. Strelow F.W., Anal. Chem., 40, 928 (1968). 9. Fritz J.S., Waki H., Garralda B.B., Anal. Chem., 36, 900 (1964). 10. Winowski Z., Chem. Anal. (Warsaw), 13, 583 (1968). 11. Kasiura K., Zawartko-Kosifiska B., Chem. Anal. (Warsaw), 26, 697 (1981). 12. Chwastowska J., Jablofiska H., Chem. Anal. (Warsaw), 34, 407 (1989). 13. Nemeth C., Somlai J., Toth J., J. Radioanal. Nucl. Chem., 204, 285 (1996). 14. Gorbenko F.P., Lapitskaya E.V., Zh. Anal. Khim., 23, 1139 (1968); Zavod. Lab., 34,

1051 (1968). 15. Johnson W.C. Jr., Anal. Chem., 38, 954 (1966). 16. Akaza I., Bull. Chem. Soc. Jpn., 39, 971 (1966). 17. Sekine T., Dyrssen D.,Anal. Chim. Acta, 37, 217 (1967). 18. Chwastowska J., R62afiska B., Chem. Anal. (Warsaw), 21, 85 (1976). 19. Maeda T., Kimura K., Shono T., Z. Anal. Chem., 313, 407 (1983). 20. Suzuki N., Fukaya T., Imura H., Anal. Chim. Acta, 194, 261 (1987). 21. Mohite B.S., Khopkar S.M., Anal. Chim. Acta, 206, 363 (1988). 22. Juskowiak B., Chem. Anal. (Warsaw), 37, 471,479 (1992). 23. Mackova J., Mikulaj V., Rajec P., J. Radioanal. Nucl. Chem., 183, 85 (1994). 24. Mikulaj V., Vasekova L., J. Radioanal. Nucl. Chem., 150, 281 (1991). 25. Horwitz E.P., Dietz M.L., Fisher D.E., Anal. Chem., 63, 522 (1991). 26. Mohite B.S., Jadage C.D., Pratap S.R., Analyst, 115, 1367 (1990).

402 47. Strontium and barium

27. Sukhan V.V., Kronikovskii O.I., Nazarenko A.Yu., Zh. Anal. Khim., 43, 1953 (1988). 28. Rajec P., Mikulaj V., Mackova J., J. Radioanal. Nucl. Chem., 150, 315 (1991). 29. Sekine T., Takagi K., Nguyen T.K., Bull. Chem. Soc. Jpn., 66, 2558 (1993). 30. Baranov V.I., Bilenskii V.D., Zh. Anal. Khim., 17, 295 (1962). 31. Bene~ J., Kry~ M., Coll. Czech. Chem. Comm., 33, 2822 (1968). 32. Bene~ J., Coll. Czech. Chem. Comm., 35, 591 (1970). 33. Shipman W.H.,Anal. Chem., 38, 1175 (1966). 34. Firsching F.H., Werner P.H., Talanta, 19, 790 (1972). 35. Zolotovitskaya E.S. et al., Zh. Anal. Khim., 36, 1518 (1981). 36. Patti F., Hernandez J.A., Anal. Chim. Acta, 55, 325 (1971). 37. Kreshkov A.P., Kuznetsov V.V., Zavod. Lab., 34, 134 (1968). 38. Kreshkov A.P., Kuznetsov V.V., Zh. Anal. Khim., 25, 874 (1970). 39. Lukin A.M., Zelichenok S.L., Chernysheva T.V., Zh. Anal. Khim., 19, 1513 (1964). 40. Kemp P.J., Williams M.B.,Anal. Chem., 45, 124 (1973). 41. Petrova T.V., Savvin S.B., Khakimkhodzhaev N., Zh. Anal. Khim., 25, 2110 (1970). 42. Mikhailova V, Kouleva N., Talanta, 21,523 (1974). 43. Oliferenko G.L., Evdokimova T.V., Azmetova G.V., Zavod. Lab., 58, No 8, 14 (1992). 44. Turanov A.N., Zavod. Lab., 56, No 8, 9 (1990). 45. Bude~insky B., Vrzalova D., Z. Anal. Chem., 210, 161 (1965). 46. Slovak Z., Fischer J., Borak J., Talanta, 15, 831 (1968). 47. Lukin A.M., Smirnova K.A., Chernysheva T.V., Zh. Anal. Khim., 21, 1300 (1966). 48. Basargin N.N., Nogina A.A., Zh. Anal. Khim., 25, 2320 (1970). 49. Anand K.S., Dayal P., Anand O.N., Z. Anal. Chem., 247, 310 (1969). 50. Shkadauskene O.P., Shkadauskas Yu.S., Blazhis I.K., Zavod. Lab., 60, No 6, 11 (1994). 51. Kuznetsov V.V., Samorukova O.L., Zakharov E.K., Zavod. Lab., 56, No 8, 3 (1990). 52. Manna F., Chimenti F., Bolasco A., Fulvi A., Talanta, 39, 875 (1992).

Chapter 48. Sulphur

Sulphur (S, at. mass 32.06) occurs in the oxidation states -II (in sulphide), IV (in 802 and sulphite), and VI (in H2804 and sulphate). Sulphur dissolves in certain organic solvents (e.g., CS2 and C6H6); when dissolved in an alkali metal sulphide solution, it yields yellow polysulphides (S 2- + nS ~ Sn+12-). When heated with an alkaline solution of sulphite, sulphur forms thiosulphate. Sulphide, sulphite, and thiosulphate are reducing agents. Persulphate has strongly oxidizing properties. Complexes are formed by sulphide (e.g., with As, Sb, Mo), and also by thiosulphate [e.g., with Ag, Cu, and Fe(III)]. The most stable sulphur species is sulphate, which forms sparingly soluble compounds, for example with Ba and Pb, and stable soluble complexes with Zr and Th.


Many methods for separating sulphur (in various forms) from other elements are associated with methods for its determination and are discussed with them below.

Of the methods for separating sulphur, those based on distillation are the most important. Oxy-compounds of sulphur are reduced to hydrogen sulphide, which is carried in an inert gas stream (e.g., nitrogen) to a receiver containing Zn or Cd ions [1-5] (see the Methylene Blue method). When S(-II) is determined, the HzS is distilled after acidification of the sample.

On ignition of the sample in air or oxygen, sulphur is converted into SO2, which is subsequently absorbed in a suitable medium and determined spectrophotometrically [6-8] (see the pararosaniline method).

A very selective method for separating sulphur is to precipitate sulphate as BaSO4 from a dilute HC1 medium. Chromate is a suitable collector for the precipitation of traces.

Sulphur is separated from metals when the solution is passed through a strongly acidic cation exchanger [9-11]. Hydrogen sulphide in air can be concentrated by using an anion- exchanger [ 12].

Elemental sulphur can be separated from other elements by leaching with acetone, pyridine, or carbon disulphide.


Various forms of sulphur have been determined. Two particularly sensitive methods are discussed below. In the Methylene Blue method sulphur is first converted into hydrogen sulphide, and in the pararosaniline method SO2 is the reacting species. The classical turbidimetric (BaSO4) method is also discussed. All these methods are highly selective.

Spectrophotometric (and other) methods for S determination have been reviewed [13].

48.2.1. Methylene Blue method

Sulphur in the form of sulphate (or in any other oxy-compound), is reduced by a mixture of HI and hypophosphite to give H2S, which is carried in a nitrogen stream to a zinc acetate solution where it is trapped as ZnS. Then, in an acid medium, the sulphide reacts with p-

aminodimethylaniline and Fe(III) to yield Methylene Blue (MB, formula 48.1) [4,14-18]. A related dye, Ethylene Blue, is obtained, if p-aminodiethylaniline is made to react with HzS [19,20].

+ (C~]= N ~ S ~ = N [ CH3| 2

N (48.1)

E 1

The Methylene Blue is formed in a rapid reaction. The sulphide-containing sample is mixed with an acidified solution of the amine, then Fe(III) solution is added with stirring, and the solution thoroughly mixed again. The most suitable acidity for the reaction is the concentration of 0.6 M HC1 or 0.3 M H2SO4 [ 15]. The colour reaction converts only 65-70% of the hydrogen sulphide into Methylene Blue.

Methylene Blue can be extracted into CHC13 in the presence of C104- [14]. Methylene Blue is a basic dye (MB+), which forms the [MB+][C104 -] ion-pair (soluble in CHC13) in the presence of an excess of perchlorate.

The quantitative extraction of MB from the aqueous phase requires 103-10a-fold excess of perchlorate and shaking with two or three portions of CHC13. Equilibrium is attained after shaking for 20-30 s. The optimum acidity for the Methylene Blue formation is also suitable for the extraction with CHC13. The absorption spectra of Methylene Blue in chloroform and in aqueous solution are shown in Fig. 48.1. The absorption maximum in CHC13 (~max -- 650 nm) is the sharper and the more intense (e = 3.5.104; a = 1.1). The absorption maximum in aqueous solution is at a slightly longer wavelength (~,max 662 nm).

[ . . . .

600 650 662 700 wavelength, nm

404 48. Sulphur

Fig. 48.1. Absorption spectra of ion associate of Methylene Blue with CIO4 in chloroform (1) and Methylene Blue in aqueous solution (2)

If the determination of only sulphide-sulphur is required, the weighed sample is treated in the distillation flask with H2SO4 (1 + 2) (instead of the reducing agent), and the HzS is distilled off in the nitrogen stream. The rest of the procedure is as given below.

Reagents

p-Aminodimethylaniline, -~0.005 M solution. Dissolve 0.50 g of (CH3)2NC6H4NH2.0.5H2SO4 in HC1 (1 + 1), and dilute the solution with the same acid to 500 ml.


Standard sulphur solution (as sulphate): 1 mg/ml. Dissolve in water 5.4370 g of K2SO4 (previously ignited at 400-500~ and dilute the solution with water to 1 litre in a standard flask.

Reducing agent. Dissolve 4.0 g of sodium hypophosphite in 25 ml of glacial acetic acid and 100 ml of conc. HI. Reflux the solution in a 250-ml round-bottomed flask for 1 h with nitrogen passed through at a rate of 3 or 4 bubbles/sec. Allow the solution to cool (still with nitrogen passing), transfer the reagent to a bottle with a ground-glass stopper, and store in the dark.

Zinc acetate, 0.25 M solution. Dissolve 27.5 g of zinc acetate and 7.0 g of sodium acetate in 500 ml of water.

Iron(III),-~0.25 M solution. Dissolve 30 g of Fe(NH4)(SO4)z.12H20 (ferric alum) in 1 M HC1, and make up the solution to 250 ml with the same acid.

Nitrogen-wash solution. Dissolve 2 g of KMnO4 and 4 g of Hg(NO3)2 in 100 ml of dilute HNO3 (1 +99); boil the solution in an open vessel for 20 min, then cool.

Sodium perchlorate, ~ 1 M solution. Dissolve 31 g of NaC104 in 250 ml of water.

Procedure

Place the sample, containing not more than 15 gg of S, in a flask (1) in the distillation apparatus. Make the sample alkaline and evaporate it to dryness in the flask, while bubbling nitrogen through. Change the water in the washer (3) every few determinations. Add to the receiver (4) 2.5 ml of the zinc acetate solution and 15 ml of water, and connect to the apparatus as shown in Fig. 48.2.

Pipette 5 ml of the reducing reagent into the flask (1) containing the sample. Quickly connect the flask to the rest of the apparatus, and simultaneously connect the supply of nitrogen (purified by passage through the wash solution at a rate of 1 or 2 bubbles / sec) to the side-tube (2). Bring the solution in the flask (1) to the boil within 1-2 min, and continue the boiling for 15-20 min.

?

HZ 0 ~ r . - - -

Fig. 48.2. Apparatus for distillation and absorption of hydrogen sulphide

Disconnect the receiver, together with the delivery tube (5), and add 2.5 ml of the NaC104 solution. Stir well, then add through the delivery tube (5) to the bottom of the receiver 2.5 ml of the p-aminodimethylaniline solution and 0.5 ml of the iron(III) solution.

406 48. Sulphur

Stopper the receiver and mix the solution. After 5 min, transfer the coloured solution to a separating funnel and extract with 2- or 3 portions of CHC13. Place the combined extracts in a 25-ml standard flask, dilute to the mark with chloroform, and measure the absorbance at 650 nm against a reagent blank solution.

Note. If the final extraction is not applied, addition of NaC104 solution is not necessary. In this case, after the appearance of colour in the receiver, the solution is transferred to a standard flask, diluted to the mark with water, and the absorbance is measured at 662 nm.

48.2.2. Pararosaniline method

Sulphur dioxide reacts with a 0.1 M solution of tetrachloromercurate to form a stable dichlorosulphitomercurate complex:

HgC142- + 8 0 2 -k- H20 ~ HgC12SO32- + 2C1- + 2H +

Combined in this complex, 8 0 2 is resistant to oxidation. The complex reacts with excess of formaldehyde to give hydroxymethanesulphonic acid:

HgC12SO32- + HCHO + 2H + + HO.CH2.SO3H + HgC12

In strongly acidic media, pararosaniline (hydrochloride) exists as a colourless species. The reaction of colourless pararosaniline with hydroxymethanesulphonic acid gives a purple compound which is the basis of this sensitive spectrophotometric method for determining SO2, sulphite, or any other form of sulphur converted into SO2 [21-24]. The coloured reaction product has the formula:

H2 N ~ I /

CH2"SO3 H (48.2)

The molar absorptivity of the product at )gmax -- 560 nm is 3.0.104 (sp. abs. 0.47). The sensitivity of the reaction is increased threefold in the presence of DMF. It is probable that in the presence of DMF the reaction of pararosaniline with formaldehyde and SO2 gives a tri- substituted product [21 ].

Total sulphur is determined in metals and other solids by combustion in a stream of oxygen to give SO2, which is subsequently absorbed in tetrachloromercurate solution [6,8,25].

Reagents

Pararosaniline hydrochloride, 1% solution. Pararosaniline (bleached), 0.04% solution. To 4 ml of the 1% pararosaniline solution

add 6 ml of conc. HC1, and dilute the solution with water to 100 ml, with stirring. Formaldehyde, 0.2% solution, freshly prepared. Dilute 5 ml of 40% HCHO solution

with water to 1 litre.


Sodium tetrachloromercurate Na2HgC14, 0.1 M solution�9 Dissolve 27.2 g of HgC12 and 11.7 g of NaC1 in water and dilute the solution with water to 1 litre.

Standard sulphite solution: 0.1 mg SO2/ml. Dissolve 0�9 g of NaHSO3 in a 0.1 M solution of NazHgI4, and dilute with the same reagent to 1 litre. The solution is unstable�9

Procedure

Place 10-15 ml of the sodium tetrachloromercurate solution containing not more than 25 ~tg of SO2 (after absorption of SO2 from a gas mixture, or separation by distillation from an acidified solution) in a 25-ml standard flask. Add 2.5 ml of the bleached pararosaniline solution and 2.5 ml of formaldehyde solution. Dilute to the mark with NazHgC14 solution, mix, and allow to stand for 30 min. Measure the absorbance of the solution at 560 nm, using a reagent blank solution as reference.

48.2.3. Turbidimetric (BaSO4) method

When the concentration of sulphate in the solution is sufficiently low, the barium sulphate formed after the addition of barium chloride does not coagulate to form a precipitate, but remains as a fine suspension producing a turbidity�9 The determination of small quantities of sulphate (or any other sulphur compound after oxidation to 8042-) is based either on comparison of the turbidity with a set of standards or on its photoelectric estimation [26-28]. This turbidimetric method for determining sulphate is simple and fast, but of rather low precision and sensitivity.

The BaSO4 suspension is formed in dilute hydrochloric acid medium. The turbidity varies with time, but is almost stable for a certain period after 10-15 min. Since the BaSO4 crystals are liable to age on prolonged standing, the standards should be prepared simultaneously, or the absorbance should be measured at a definite time after the addition of barium chloride, e.g., after 15 min. The determination is carried out at room temperature; rise in temperature increases the rate of ageing of the precipitate. The addition of 20-30% of ethanol reduces the solubility of barium sulphate and increases the degree of dispersion. It is recommended to add protective colloids to stabilize the suspension [29,30]. Stabilization is also obtained by adding glycol or glycerol.

Reagents

Barium chloride, 2% solution�9 Standard sulphur solution (as sulphate): 1 mg S/ml. Preparation as in p. 48.2.1.

Procedure

Place the sample solution containing not more than 100 ~tg of S (as a sulphate) in a Nessler cylinder. Acidify with 2 ml of 2 M HC1, add 10 ml of ethanol and water to -~40 ml, pour in quickly 5 ml of BaCI2 solution, and stir well. After 15 min, compare the turbidity with a series of standards prepared simultaneously. The cylinders should be observed from above, against black paper, in a uniformly brightly illuminated place. �9 The absorption of the turbid solution may also be measured photoelectrically at an appropriate wavelength.

408 48. Sulphur


Sulphate can be determined indirectly by using a suspension of barium chloranilate. In the formation of BaSO4 at pH -4, sulphate ions displace an equivalent amount of coloured chloranilate ions [9,31-33]. The absorbance of the solution is measured at 530 nm. The reaction is usually carried out in aqueous alcohol, since alcohol reduces the solubility of barium chloranilate. The method has been applied for determining sulphate by the FIA technique [34].

Many complexes of various organic reagents with Ba have been used as reagents for indirect determination of sulphate, such as Chlorophosphonazo III [35-37], Dimethylsulphonazo III [11,38-40], Sulphonazo III [40], Nitchromazo [41], Orthanilic K [42], Thoron I [43], Methylthymol Blue [44,45].

2-Aminopyrimidine has been proposed for indirect determination of sulphate [46-48]. Sulphate has been determined also by the turbidimetric method [49].

Sensitive indirect methods are based on precipitation of sulphate ions with benzidine or 4-amino-4'-chlorobiphenyl (e - 5.5-104 at 480 nm), and obtaining suitable azo dyes [50,51 ].

Many methods for sulphide and H2S are based on the reducing properties of S(-II). Hydrogen sulphide reduces molybdate in acid medium to molybdenum blue, and the molybdophosphate to phosphomolybdenum blue [52]. Iron(III) reduced by HzS in the presence of 1,10-phenanthroline gives the orange Fe(phen)32+ complex [2,53]. Hydrogen sulphide may be determined after conversion into thiocyanate by the reaction with Fe(III) [54]. Sulphide has been determined also by a colour redox reaction with nitroprusside [55- 57]. In another sensitive reaction the sulphide ions decompose the Ag complex with Cadion 2B and Triton X-100 (e - 2.5.105) [58]. In another indirect method sulphide releases the chloranilate ion from the Hg(II) chloranilate [59]. Sulphide has also been determined by a method based on its reaction with bromate, followed by bromination of 2',7'- dichlorofluorescein by the bromine released [60].

Sulphite and sulphur dioxide can be determined from the changes in colour, produced by their reducing effect on iron(III) in the presence of 1,10-phenanthroline [61,62], TPTZ [63], or ferrozine [64] (for formulae see Ch. 26). Sulphite may also be determined indirectly with Hg(II) chloranilate [59] or thiocyanate [65]. Other reagents for sulphite include 2,6- dichlorophenolindophenol [7] and 5,5'-dithio-bis(2-nitrobenzoic acid) [66-68]. Reaction of $02 with IO3- in the presence of excess of chloride yields IC14-, which forms an ion-pair with Pyronine G [69].

The bleaching of Methylene Blue by $2032- affords a sensitive method for determination of thiosulphate [70]. It is possible to determine thiosulphate after extraction of its ion- associates with some basic dyes, e.g., Rhodamine B, Rhodamine 6G, or Crystal Violet [71]. Thiosulphate present in concentrations of the order of 10 -6 M have been determined after the oxidation with iodine by measuring the absorbance due to I3- [72]. Thiosulphate can also be determined in an indirect reaction, in which $2032- reacts with Hg(II) thiocyanate to release SCN- which gives a colour reaction with Fe(I~) [73]. A method for simultaneous determination of thiosulphate, sulphite, and sulphide has been proposed [74].

Polythionates can be determined spectrophotometrically after isolation from other sulphur anions and separation of individual polythionates by chromatographic methods and cyanolysis [75-84].

Analytical properties of thiosulphate and polythionate anions have been reviewed [85].



The Methylene (and Ethylene) Blue method has been applied in determinations of sulphur in plants [86], biological materials [87], waters [12,88], air [5,12,16,20], hydrocarbons [89], iron alloys [90,91], cobalt and zirconium [91], titanium [92], thallium and its halides [93], arsenic [94], selenium [95], and various reagents (including barium chloride) [14]. Flow- injection analysis has been applied in the determination of sulphur by the Methylene Blue method [96].

The pararosaniline method has found numerous applications owing to its sensitivity and selectivity. It has been used in determinations of sulphur (or SO2) in air [22,97,98], plants [99,100], wine [101], soils [102], organic compounds [25], selenium [103], and copper [6,8]. This method has also been applied for continuous determination of SO2 in wine [104,105].

Sulphate has been determined by the turbidimetric method in plant materials [29,30,106,107], soil [29], and natural waters [108,109].

Sulphur dioxide was determined in wine by the FIA technique with the use of 5,5'- dithiobis(2-nitrobenzoic acid) [61 ].

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1. Norwitz G., Analyst, 96, 494 (1971). 2. Davis J.B., Lindstrom F., Anal. Chem., 44, 524 (1972). 3. Norwitz G., Gordon H.,Anal. Chim. Acta, 77, 239 (1975). 4. Siemer D.D.,Anal. Chem., 52, 1971 (1980). 5. Vasireddy S., Street K.W. Jr., Mark H.B. Jr., Anal. Chem., 53, 868 (1981). 6. Barabas S., Kaminski J., Anal. Chem., 35, 1702 (1963). 7. Pfleiderer G., Stock A., Otting F., Z. Anal. Chem., 239, 225 (1968). 8. Pugh H., Waterman W.R., Anal. Chim. Acta, 55, 97 (1971). 9. Stoffyn P., Keane W., Anal. Chem., 36, 397 (1964). 10. Dedkova V.M., Akimova T.G., Savvin S.B., Zh. Anal. Khim., 36, 1358 (1981). 11. Bartels U., Pham T.T., Z. Anal. Chem., 310, 13 (1982). 12. Pacz D.M., Guagnini O.A., Mikrochim. Acta, 1971, 220. 13. Szekeres L., Talanta, 21, 1 (1974). 14. Marczenko Z., Lenarczyk L., Chem. Anal. (Warsaw), 10, 729 (1965). 15. Hofmann K., Hamm R., Z. Anal. Chem., 232, 167 (1967). 16. Zutshi P.K., Mohadevan T.N., Talanta, 17, 1014 (1970). 17. Matheson N.A.,Analyst, 99, 577 (1974). 18. Krichevskaya A.M., Fedorov A.A., Zh. Anal. Khim., 32, 1149 (1977). 19. Kirsten W.J., Mikrochim. Acta, 1978 II, 403. 20. Wood C.F., Marr I.L.,Analyst, 113, 1635 (1988). 21. Huitt H.A., Lodge J.P., Anal. Chem., 36, 1305 (1964). 22. Scaringelli F.P., Saltzman B.E., Frey S.A., Anal. Chem., 39, 1709 (1967). 23. Arikawa Y., Ozawa T., Iwasaki I., Bull. Chem. Soc. Jpn., 41, 1454 (1968). 24. King H.G., Pruden G., Analyst, 94, 43 (1969). 25. Dokl~dalov~ J., Mikrochim. Acta, 1965, 344; Z. Anal Chem., 208, 92 (1965). 26. Wimberley J.W.,Anal. Chim. Acta, 42, 327 (1968). 27. Ryaguzov A.I., Palkin A.A., Pyatibratov S.A., Zavod. Lab., 40, 1329 (1974). 28. Voulgaropoulos A. et al., Mikrochim. Acta, 1985 III, 385. 29. Chaudhry I.A., Cornfield A.H., Analyst, 91, 528 (1966).

410 48. Sulphur

30. Basson W.D., B6hmer R.G., Analyst, 97, 266 (1972). 31. Carlson R.M., Rosell R.A., Vallejos W., Anal. Chem., 39, 688 (1967). 32. Schafer H.N., Anal. Chem., 39, 1719 (1967). 33. Gales M.E., Kaylor W.H., Longbottom J.E., Analyst, 93, 97 (1968). 34. Kamaya M., Nagashima K., Ishii E., Fresenius'J. Anal. Chem., 347, 409 (1993). 35. Evseeva T.I., Keruchen'ko T.A., Malikova T.Yu. Zh. Anal. Khim., 35, 1942 (1980). 36. Boiko A.I. et al., Zavod. Lab., 48, No. 10, 20 (1982). 37. Qiu X.C., Zhu Y.Q., Chem. Anal. (Warsaw), 30, 263 (1985). 38. Reijnders H.F., Van Staden J.J., Griepink B., Z. Anal. Chem., 295, 122 (1979); 300, 273

(1980). 39. Kondo O., Miyata H., T6ei K.,Anal. Chim. Acta, 134, 353 (1982). 40. Fernandez T., Luis A.G., Montelongo F.G., Analyst, 105, 317 (1980). 41. Oliferenko G.L., Evdokimova T.V., Azmetova G.V., Zavod. Lab., 58, No. 8, 14 (1992). 42. Eremina I.D., Shpigun L.K., Zolotov Yu.A., Z. Anal. Chem., 42, 1631 (1987). 43. Bruno P. et al., Talanta, 31,479 (1984). 44. Colovos G., Panesar M.R., Parry E.P., Anal. Chem., 48, 1693 (1976). 45. Madsen B.C., Murphy R.J., Anal. Chem., 53, 1924 (1981). 46. Archer A.W.,Analyst, 100, 755 (1975). 47. Suzuki K.Y., Ohzeki K., Kambara T., Anal. Chim. Acta, 136, 435 (1982). 48. Mroczkowski W., Cygafiski A., Chem. Anal. (Warsaw), 29, 205 (1984). 49. Stephen W.I., Anal. Chim. Acta, 50, 413 (1970). 50. Mroczkowski W., Sykut H., Cygafiski A., Chem. Anal. (Warsaw), 26, 861 (1981). 51. Mroczkowski W., Cygafiski A., Chem. Anal. (Warsaw), 27, 307 (1982). 52. Grishko V.P., Grishko V.I., Yudelevich I.G., Zh. Anal. Khim., 44, 434 (1989). 53. Rahim S.A., Salim A.Y., Shereef S.,Analyst, 98, 851 (1973). 54. Koh T., Yamamuro N., Miura Y., Anal. Sci., 6, 917 (1990). 55. Casapieri P., Scott R., Simpson E.A., Anal. Chim. Acta, 45, 547 (1969). 56. Bethea N.J., Bethea R.M.,Anal. Chim. Acta, 61, 311 (1972). 57. Sunita G., Gupta V.K., Chem. Anal. (Warsaw), 41, 313 (1996). 58. Wei F.S., Teng E.J., Rui K.S., Talanta, 31, 1024 (1984). 59. Humphrey R.E., Hinze W., Anal. Chem., 43, 1100 (1971). 60. Shanthi K., Balasubramanian N., Fresenius'J. Anal. Chem., 351,685 (1995); Analyst,

121, 647 (1996). 61. Maquieira A. et al.,Anal. Chim. Acta, 283, 401 (1993). 62. Stephens B.G., Lindstrom F., Anal. Chem., 36, 1308 (1964). 63. Attari A., Igielski T.P., Jaselskis B., Anal. Chem., 42, 1282 (1970). 64. Stephens B.G., Suddeth H.A., Analyst, 95, 70 (1970). 65. Attari A., Jaselskis B.,Anal. Chem., 44, 1515 (1972). 66. Hinze W.L., Elliott J., Humphrey R.E., Anal. Chem., 44, 1511 (1972). 67. Humphrey R.E., Ward M.H., Hinze W., Anal. Chem., 42, 698 (1970). 68. Brown D.S., Jenke D.R., Analyst, 112, 899 (1987). 69. Selvapathy P.,Analyst, 112, 1139 (1987). 70. Quentin K.E., Pachmayr F., Z. Anal. Chem., 200, 250 (1964). 71. Grigonene K.M., Ramanauskas E.I., Butkyavichius Yu.P., Zh. Anal. Khim., 27, 2028

(1972). 72. Koh T. et al., Talanta, 44, 577 (1997). 73. Miura Y., Koh T.,Anal. Chim. Acta, 173, 33 (1985). 74. Koh T., Miura Y., Anal. Sci., 3, 543 (1987). 75. Koh T., Wagai A., Miura Y.,Anal. Chim. Acta, 71,367 (1974).

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76. Koh T., Taniguchi K., Anal. Chem., 46, 1679 (1974). 77. Koh T., Taniguchi K., Iwasaki I., Bull. Chem. Soc. Jpn., 51, 164 (1978). 78. Koh T., Aoki Y., Iwasaki I.,Analyst, 104, 41 (1979). 79. Miura Y., Koh T.,Anal. Chem., 52, 1855 (1980). 80. Miura Y., Koh T., Bull. Chem. Soc. Jpn., 59, 3057 (1986). 81. Badri B.,Analyst, 113, 351 (1988). 82. Koh T., Miura Y., Suzuki M.,Analyst, 113, 949 (1988). 83. Koh T., Miura Y., Suzuki M.,Anal. Sci., 4, 267 (1988). 84. Koh T. et al., Anal. Sci., 5, 79 (1989). 85. Koh T., Anal. Sci., 6, 3 (1990). 86. Steinberg A. et al.,Anal. Chim. Acta, 27, 158 (1962). 87. Patel S.S., Spencer C.P., Anal. Chim. Acta, 27, 278 (1962). 88. Grasshoff K.M., Chan K.M., Anal. Chim. Acta, 53, 442 (1971). 89. Farley L.L., Winkler R.A., Anal. Chem., 40, 962 (1968). 90. Kriege O.H., Wolfe A.L., Talanta, 9, 673 (1962). 91. Fedorov A.A., Krichevskaya A.M., Zavod. Lab., 36, 1433 (1970). 92. Mizuike A., Kondo A., Mikrochim. Acta, 19"71, 841. 93. Goryushina V.G., Biryukova E.Ya., Zavod. Lab., 35, 1163 (1969). 94. R62ycki C., Suszczewski W., Chem. Anal. (Warsaw), 19, 1231 (1974). 95. Sj6borg B.L., Talanta, 14, 693 (1967). 96. Leggett D.J., Chen N.H., Mahadevappa D.S., Anal. Chim. Acta, 128, 163 (1981). 97. Dasgupta P.K., Delesare K., Ullrey J.C., Anal. Chem., 52, 1912 (1980). 98. Rohme K.A., Scaringelli F.P., Anal. Chem., 47, 2474 (1975). 99. Mroczkowski W., Cygafiski A., Chem. Anal. (Warsaw), 31, 653 (1986); 40, 641 (1995). 100. Mroczkowski W., Cygafiski A., Chem. Anal. (Warsaw), 35, 675 (1990). 101. Richter P., Luque de Castro M.D., Anal. Chim. Acta, 283, 408 (1993). 102. Mroczkowski W., Cygafiski A., Chem. Anal. (Warsaw), 32, 477 (1987). 103. Acs L., Barabas S., Anal. Chem., 36, 1825 (1964). 104. Hieke E., Kreisel A., Z. Anal. Chem., 285, 39 (1977). 105. Zhi Z., Rios A., Valcarcel M., Analyst, 120, 2013 (1995). 106. Novozamsky I., Van Eck R., Z. Anal. Chem., 286, 367 (1977). 107. Hunt J.,Analyst, 105, 83 (1980). 108. Krug F.J. et al.,Anal. Chim. Acta, 145, 179 (1983). 109. Van Staden J.F., Taljaard R.E., Anal. Chim. Acta, 331, 271 (1996).

Chapter 49. Tellurium

Tellurium (Te, at. mass 127.60) occurs in the -II, IV, and VI oxidation states in telluride, tellurite, and tellurate, respectively. In acidic non-complexing media (e.g., H2SO4), Te 4+ cations exist. The oxide, TeO2, is only slightly volatile (it escapes when ignited in a crucible at 800-900~ and is not readily soluble in water. Reducing agents reduce tellurium(W) to the element. Tellurium can be oxidized to Te(IV) only by powerful oxidants. Tellurium(W) forms chloride and bromide compounds which are not as volatile as the corresponding selenium compounds.


49.1.1 Precipitation

A frequently used method for separating tellurium from most elements consists in its reduction in acid medium to the element [ 1], usually by SnCI2, SO2, or hypophosphite. Se, As, Hg, Au, Ag, Pd, and Pt are precipitated together with Te. Selenium and arsenic are suitable collectors for traces of tellurium [ 1 ].

Selenium is quantitatively separated from tellurium by evaporating to dryness a solution of both elements in HC1 or HBr (in the presence of a small amount of Br2). Selenium is volatilized while tellurium remains in the vessel.

Tellurium has been separated from Se after sintering the sample with Eschka mixture (1 part by weight of NazCO3 + 2 parts of MgO) for 40 min at 800~ [2]. When the sinter is leached with water, the readily soluble magnesium selenate passes into solution while sparingly soluble magnesium tellurate remains in the precipitate.

It has been recommended that one should co-precipitate traces of Te with Fe(OH)3 as collector (pH 6-9) [3]. Traces of Te(IV) have been separated from sea-water by co- precipitation with Fe(OH)3 and flotation [4]. The tellurium compounds with trifluoroethyl xanthate [5], methylpiperazine dithiocarbamate [6], and 2-mercapto-4-methyl-5-phenylazopyrimidine [7] can be co-precipitated with microcrystalline naphthalene.

Trace amounts of Te were separated with Se as carrier (after the reduction to the element) by flotation with xylene [8].

49.1.2. Extraction

A comprehensive review of extraction separation methods for tellurium has been given [9]. Tellurium(W) can be extracted from hydrochloric acid (also in the presence of bromide

or iodide) media with MIBK [10,11] and other ketones [12]. Acetophenone has been recommended for extraction of Te from H2SO4-KI media [13].

In many cases, tellurium has been extracted with non-polar solvents from strongly acidic solutions (HC1, HBr) in the presence of TBP or TOPO [14], DAM [15], or quaternary ammonium salts [16,17]. Tellurium can be separated by extraction of its complexes with DDTC (pH 8.5, CC14) [18,19], or xanthate (12 M HC1, CHC13) [3]. Te(IV) has been extracted from the HC1 or HBr media by means of tris(2-ethylhexyl) phosphate [20]. The TeI6- complex has been extracted (CHC13) with hexadecylpyridine [21 ].


49.1.3. Ion-exchange and other methods

Tellurium(W) has been separated from large amounts of Se(IV) by sorption of the Te(IV) complex in 6 M HC1 on a strongly basic anion-exchanger [22]. Tellurium can be separated from many metals on a strongly basic anion- exchanger by using a conc. aqueous LiC1 medium [23].

Strongly acid cation-exchangers retain Te from a 0.3 M HC1 medium, whereas Se is not sorbed. Mixtures of Te and Se were separated on Dowex 50W-X8 cation-exchanger from a mixed medium containing formic acid, methyl ethyl ketone, and 50% methanol [24].

Tellurium(iv) can be separated from Te(VI), Se(IV) and Se(VI) on a diethylaminocellulose column, using 1 M hydrochloric a c i d - glacial acetic acid (1+9) mixture as eluent [25]. Tellurium (and Se) has also sorbed on a polyurethane foam [26].

Tellurium has been separated as bromide by distillation from conc. phosphoric acid at about 300~ [27].


The extractive spectrophotometric method with Bismuthiol II is sensitive and highly selective. A method based on the coloured sol of elementary tellurium is simple, but not very sensitive. The methods for determining tellurium have been reviewed [28].

49.2.1. Bismuthiol II method

Bismuthiol II, a reagent used for the gravimetric determination of bismuth, has been adopted as a spectrophotometric reagent for tellurium [15, 29-33]. Bismuthiol II reacts in acid medium with tellurium(W) to form a neutral 1:4 complex which is extractable into CHC13. The course of the reaction is as follows:

SH

+ Te 4 + ~ S ~ ~S / / e ~ q- /~H + (49.1) / S S~ N = C \ / /C=N S S

Free Bismuthiol II, which absorbs in the same region as the tellurium(iv) complex, is extracted into chloroform together with the complex, but is then stripped with an aqueous buffer solution at pH -8.

The absorption maximum of the Te-Bismuthiol II complex occurs at 330 nm (e = 3.6.104, a = 0.31). At 400 nm, the molar absorptivity is - 8 .0 .10 3. More than 99% of the complex is extracted into chloroform from 3 M HC1. Extraction from a weakly acid medium (pH 3.5) is about 95%.

In the determination of tellurium by extracting the Bismuthiol II complex from 3 M HC1 into CHC13, Se, Cu, As, Pd, and Hg interfere. The effect of selenium is considerable: with equal quantities (by weight) of selenium and tellurium present, the absorbance is more than

414 49. Tellurium

doubled. In the case of copper, the increase in absorbance is 80%, and in the case of mercury, it is 15%. Oxidizing substances, which decompose Bismuthiol II, also interfere. Nitric acid must not be present even in trace amounts. The complex of Te with Bismuthiol II is extractable into benzene at pH -4. In this case, selenium interference is less. Many metals, which interfere at this pH, can be masked with EDTA [29].

The colour reaction can be carried out also in a weakly acid medium, at pH -2, which is then raised to 6.5 before extraction of the complex into chloroform. At this pH, extraction of free Bismuthiol II is negligible. EDTA is again used as masking agent. A 20% increase in sensitivity is achieved by adding ammonium sulphate to the aqueous solution [30].

Naphthylbismuthiol, which is more selective and more sensitive than Bismuthiol II, is also used for determination of tellurium [ 11,34].

Reagents

Bismuthiol II, 0.25% solution in cold water which has been previously boiled. The solution is usable for one day.

Standard tellurium solution: 1 mg/ml. Place in a beaker 1.0000 g of powdered tellurium. Add 50 ml of conc. HC1, and introduce conc. HNO3 in small portions with heating until the tellurium dissolves. Then add 100 ml of water, and boil the solution for 5 min. Add 20 ml of conc. HC1, and dilute the solution to volume with water in a 1-1itre standard flask.

Buffer solution (pH 7.5). Dissolve 7 g of NaHzPO4, 7 g of borax, and 5 g of EDTA in 500 ml of water, and adjust the solution to pH 7.5_+0.1 by adding NaOH solution.

Procedure

Add 6 M HC1 (previously boiled and cooled) to the sample solution, containing not more than 60 ~tg of Te, until the concentration of HC1 is 3 M. Add 4 ml of Bismuthiol II solution, and stir thoroughly. After 1 min, extract Te by shaking with two 10-ml portions of chloroform for 1 min. Wash the extract with the buffer solution. Dilute the extract to the mark with chloroform in a 25-ml standard flask. Measure the absorbance of the yellow solution at 330 nm, using a reagent blank solution as reference.

49.2.2. Tellurium sol method

Tellurium(W) is reduced to the element in acid medium, and the resulting coloured sol is a suitable basis for the spectrophotometric determination of Te [22,35] . To stabilize the pseudosolutions, the reduction is carried out in the presence of protective colloids, e.g., gum arabic, gelatine, or poly(vinyl alcohol).

When tin(H) chloride is used [22], 1.5-3 M HC1 is the most suitable medium. With hypophosphite as reducing agent, the acidity of the solution should be kept within the limits 0.1-0.4 M HC1 or H2SO4. Since the colour (brown, blue, red) of the sol depends on the acidity and the kind and concentration of the reducing agent, the reaction conditions must be precisely the same for the sample solution and for the standard solutions.

The molar absorptivity of the tellurium sol formed by reduction with SnCI2, under the conditions described in the procedure below, is 5.4.103 (a = 0.043) at 400 nm. At 340 nm the absorbance is 15% higher, and at 500 nm it is 20% lower, than the absorbance at 400 nm. Interference in the determination of tellurium with SnCI2 as reductant comes from other elements also precipitated by the reducing agent, namely Se, Hg, Au, and the platinum metals. Selenium is readily removed from tellurium by double evaporation to dryness in


hydrochloric or hydrobromic acid.

Reagents

Tin(if) chloride, SnC12.2H20, 20% solution in 2 M HC1. Standard tellurium solution: 1 mg/ml. Preparation as in Section 49.2.1. Poly(vinyl alcohol), 2% solution. Selenium(IV) solution,- 1 mg/ml. Preparation as in Section 44.2.1.

Procedure

Precipitation separation of Te. Heat a clear sample solution (-~100 ml, in -1 M HC1) to boiling in a beaker. Add 2 mg of selenium [as selenium(iv) solution], then the SnC12 solution dropwise until the yellow colour disappears [if the sample solution contains Fe(III)], and 2 ml more of SnC12 solution. Stand the beaker for --3 h in a hot water-bath, and allow it to cool overnight. Filter off the precipitate of Se and Te on a sintered glass crucible, and wash with dilute SnC12 solution and water. Dissolve the precipitate in 5 ml of conc. HC1 containing 3 or 4 drops of hydrogen peroxide. Evaporate the solution to dryness in the beaker. Add 2 ml of conc. HC1 and evaporate to dryness again (to eliminate volatile selenium). Dissolve the residue in the beaker in dilute HC1.

Determination of Te. Acidify -~15 ml of sample solution, containing not more than 0.3 mg Te, to -2 M in HC1. Add 2 ml of 2% poly(vinyl alcohol) solution and stir well. Add 2 ml of SnC12 solution with stirring, and make up the solution to the mark with water in a 25-ml standard flask. Mix thoroughly and measure the absorbance at 400 nm, using water as reference.


Tellurium(IV) reacts with thiourea, S=C(NH2)2, (whose concentration in the final solution should be -10%) in 1 M H2SO4, HNO3, or H3PO4 to form a yellow cationic complex suitable for spectrophotometry [36]. The absorption maximum of the complex occurs at 320 nm. The cationic complex of tellurium with thiourea can be extracted with TBP as the ion-associate with thiocyanate ions. The following thiourea derivatives have been proposed for the spectrophotometric determination of tellurium: 1,4diphenylthiosemicarbazide [37], diphenylthiocarbazide, dinaphthylthiourea, and tetramethylthiourea [38].

Other organosulphur reagents used for determining tellurium include morpholine dithiocarbamate [39], dithiopyrylmethane (13 = 5.2-104 at 360 nm) [40], and isobutyl- dithiopyrylmethane (13 = 5.1.104 at 358 nm) [41].

The iodide complex of Te(IV), associated with CTA, has been the basis of a sensitive method for determining tellurium (extraction into CHC13; 13 = 5.9.104 at 360 nm) [42,43].

The methods of determining Te with the use of basic dyes are very sensitive. The tellurium bromide complex has been extracted as the ion-associate with Butylrhodamine B [44,45] or Victoria Blue 4R (13 = 8.0.10 4) [46]. The halide complexes give also ion- associates, extractable into toluene (13 = 3.75-104) with Violet Red [47--49]. The TeBr6 e- associate with Rhodamine 6G can be floated with benzene, and the separated compound is then dissolved in a mixture of benzene with ethanol (13 = 1.7.105) [50]. The ion-associate of the iodide complex of Te(IV) with Nile Blue A has been floated with cyclohexane, and the separated compound dissolved in methanol (13 = 1.4.105 at 640 nm) [51]. High sensitivity has

416 49. Tellurium

been attained in the system Te-KI-Rhodamine B (e = 2.8-105) in aqueous poly(vinyl alcohol) medium [52].

Tellurium has been determined in mixtures with Ga, Pb, In, Ba, Cu, and A�94 by using a mathematical treatment of the absorption spectra of complexes with nitrophenylfluorone [53,54].

Tellurium has been determined from the absorbance measurements (within the range 190-300 nm) of its hydride, obtained by reduction with NaBH4 [55].

Use has also been made of the catalytic effect of Te on the reduction of some organic compounds [56,57].


The Bismuthiol II method has been used for determining tellurium in ores [58], thin Hg-Cd- Te films [33,59], and sulphide ores [29].

The coloured sol has been utilized in the determination of tellurium in steel and cast iron [20,31 ], alloyed steels [35,60], and selenium [61 ].

Basic dyes were applied for the determination of Te in thin films [48,49] and steel [52]. Tellurium was determined in synthetic rubber by measurement of absorbance of the hydride [55]. The cyanine dye, N,N'-di(acetoxyethyl)indocarbocyanine, associated with the chloride complex of Te(IV), has been used for the determination of Te in semiconductor thin films [62].

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10. Jordanov N., Havezov I., Z. Anal. Chem., 248, 296 (1969). 11. Nazarenko I.I., Kislova I.V., Rabinovich B.S., Zh. Anal. Khim., 30, 1389 (1975). 12. Havezov I. et al., Z. Anal. Chem., 262, 179 (1972). 13. Havezov I. Stoeppler M., Z. Anal. Chem., 258, 189 (1972). 14. Markl P., Jettmar A.A., Mikrochim. Acta, 1978 II, 285. 15. Pollock E.N.,Anal. Chim. Acta, 40, 285 (1968). 16. Torocheshnikova I.I. et al., Zh. Anal. Khim., 36, 478 (1981). 17. Tsukahara I., Yamamoto T., Talanta, 28, 585 (1981). 18. Kamada T., Sugita N., Yamamoto Y., Talanta, 26, 337 (1979). 19. Lo J.M., Lin C.C., Yeh S.J.,Anal. Chim. Acta, 272, 169 (1993). 20. Desai G.S., Shinde V.M., Talanta, 39, 405 (1992). 21. Ishchenko N.N. et al., Zh. Anal. Khim., 45, 1557 (1990). 22. Simek M., Chem. Listy, 60, 817 (1966). 23. Busev A.I. et al., Zh. Anal. Khim., 25, 1374 (1970). 24. Husain S.W., Marageh M.G., Khanchi A.R., App. Radiat. Isot., 43, 859 (1992).

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25. Kuroda R., Yoshikumi N., Talanta, 22, 81 (1975). 26. Stewart I.I., Chow A., Talanta, 40, 1345 (1993). 27. Rybakov A.A., Ostroumov E.A., Zh. Anal. Khim., 42, 1842 (1987). 28. Murashova V.I., Sushkova S.G., Zh. Anal. Khim., 24, 729 (1969). 29. Jankovsky J., K~ir O., Talanta, 5, 238 (1960). 30. Cheng K.L., Goydish B.L., Talanta, 13, 1210 (1966). 31. Yoshida H., Taga M., Hikime S., Talanta, 13, 185 (1966). 32. Navrfitil O., Sorfa J., Coll. Czech. Chem. Comm., 34, 975 (1969). 33. Marczenko Z., Krasiejko M., Czarnecka I., Chem. Anal. (Warsaw), 22, 275 (1977). 34. Busev A.I. Simonova L.N., Zh. Anal. Khim., 22, 1850 (1967). 35. Maneschi S., Gallazzi C.,Anal. Chim. Acta, 54, 461 (1971). 36. Hikime S., Bull. Chem. Soc. Jpn., 33, 761 (1960). 37. Mel'chekova Z.E., Murashova V.I., Zh. Anal. Khim., 25, 556 (1970). 38. Terpinski E.A., Analyst, 113, 1473 (1988). 39. Seithi C.L. et al., Talanta, 31, 848 (1984). 40. Dolgorev A.V. et al., Zh. Anal. Khim., 34, 2192 (1979). 41. Dolgorev A.V., Zibarova Yu.F., Zavod. Lab., 46, 17 (1980). 42. Vijayakumar M., Ramakrishna T.V., Aravamudan G., Talanta, 26, 323 (1979). 43. Marczenko Z., Krasiejko M., Stec J., Chem. Anal. (Warsaw), 26, 1051 (1981). 44. Murashova V.I., Skripchuk V.G., Zh. Anal. Khim., 27, 340 (1972). 45. Skripchuk V.G. et al., Zavod. Lab., 49, No. 5, 1 (1983). 46. Kish P.P., Kremeneva S.G., Zh. Anal. Khim., 25, 2200 (1970). 47. Kish P.P., Balog I.S., Andrukh V.A., Bogdanova A.V., Zavod. Lab., 56, No. 7, 22

(1990). 48. Kish P.P., Balog I.S., Andrukh V.A., Zh. Anal. Khim., 45, 915 (1990). 49. Kish P.P., Andrukh V.A., Balog I.S., Zh. Anal. Khim., 46, 2328 (1991). 50. Skripchuk V.G., Murashova V.I., Zh. Anal. Khim., 29, 1823 (1974); 32, 452 (1977). 51. Kowalski T., Chem. Anal. (Warsaw), 32, 379 (1987). 52. Luo H., Mikrochim. Acta, 106, 21 (1992). 53. Fedin A.V., Zavod. Lab., 60, No. 9, 1 (1994). 54. Fedin A.V., Zh. Anal. Khim., 49, 209 (1994). 55. Asensio J.S. et al., Anal. Lett., 28, 121 (1995). 56. Safavi A., Afldaami A., Massoumi A., Microchem. J., 52, 3 (1995). 57. Mousavi M.F., Almasian M.R., Anal. Lett., 29, 1851 (1996). 58. Dang Ch.T., Simonova L.N., Busev A.I., Zavod. Lab., 37, 408 (1971). 59. Marczenko Z., Mojski M., Czarnecka I., Chem. Anal. (Warsaw), 18, 189 (1973). 60. Koch O.G., Anal. Chim. Acta, 63, 156 (1973). 61. Futekov L., Atanasova B., Talanta, 19, 817 (1972). 62. Balogh I.S., Andruch V., Anal. Chim. Acta, 386, 161 (1999).

Chapter 50. Thallium

Thallium (T1, at. mass 204.37) occurs in its compounds in the I and III oxidation states. Thallium(I) compounds are the more stable. Thallium(I) forms sparingly soluble compounds, white T1C1, yellow TII, and black TlzS, as well as thiosulphate and ammine complexes of low stability. Colourless T13+ ions can exist only in strongly acidic media since the brown hydroxide, TI(OH)3, which has no amphoteric properties, is precipitated at pH values as low as 0.3. Thallium(III) yields halide-, oxalate-, and tartrate complexes. TI(I) is oxidized to TI(III) only by powerful oxidants, e.g., MnO4-, aqueous C12, and aqueous Br2.


50.1.1. Extraction

Of the thallium halide complexes, the bromide complex is the most important in extraction separations. TI(III) and Au(III) are extracted quantitatively from 1-3 M HBr with diethyl ether or DIPE, or MIBK. At that concentration of HBr, Fe(III), Ga, In, Hg, and Te are extracted in small amounts. After extraction with MIBK, Ga, In, Fe(III), and Sn are stripped with a 1.5 M HBr formamide solution, leaving only TI(III) in the ketone phase [1]. The thallium(III) bromide complex has been extracted with TOA in benzene [2], or n- octylaniline in CHC13 [3].

Extraction of the chloride complex of thallium(Ill) from HC1 media is less selective [4-6]. For diethyl ether and DIPE, the most suitable media are 4 M HC1 and 6 M HC1, respectively. Au(III), Fe(III), Sb(V), Sn, Hg, Mo, As, Ge, and Ga are partly extracted along with the thallium(Ill) from an HC1 medium. Extraction of TI(III) from less concentrated HC1 media by using CHC13 solutions of n-octylaniline has been proposed [7].

Thallium(I) can be extracted into a chloroform solution of dithizone from an ammoniacal citrate-cyanide medium (pH 11). Lead, bismuth, and tin(H) are also extracted [8,9]. Thallium chelates with DDTC, oxine, thio-oxine, and HTTA are applicable for the extraction [10,11]. Thallium(Ill) has also been extracted with Aliquat 336S in xylene from citrate media (pH 2-6) [12]. Extraction of TI(III) with dichloroethane from I- - H2SO4 media in the presence of crown ethers has been applied [13-15].


Traces of thallium(III) can be precipitated from alkaline solutions as TI(OH)3 with Fe(III) or La hydroxide [ 16] as a collector. Thallium is separated quantitatively from a dilute HNO3 medium with MnO2 aq. [ 17].

The volatility of thallium causes losses during ignition or fusion with sodium carbonate, borax, or sodium hydroxide. Traces of thallium have been separated as T1C1 by heating to 1300~ a mixture of the pulverized sample and magnesium chloride in a quartz tube [ 18].

A strongly acidic cation-exchanger retains TI(I), but not Bi, Cu, Fe, Pb, or Zn, from an EDTA solution at pH 4. The T1 may be eluted with 2 M HC1. Thallium(I) is also retained from tartrate, citrate, or pyrophosphate solutions (pH 3-5), while Fe, Cu, Zn, Cd, Pb, and Sb pass to the eluate. In this case, thallium can be eluted with 6 M HC1. Mixed HCl-acetone

50.2. Methods of determinat ion 419

eluents have been used to separate TI(III) from A1, Ga, and In [19]. From 0.1 M HC1 medium, Tl(llI) is retained as the anionic chloride complex on a strongly basic anion- exchanger [20].


The spectrophotometric methods for determining thallium which are discussed in detail are the sensitive Rhodamine B- and Brilliant Green- extraction methods, and an indirect, less sensitive, starch-iodine method. Methods involving other basic dyes are also noteworthy.


Thallium(III), in the form of the T1C14- or T1Br4- complex ions, reacts in acid media (1-2 M HC1) with the basic xanthene dye, Rhodamine B (formula 4.29), to form a red-violet, slightly fluorescent ion-associate complex, which is soluble in benzene, DIPE, and isoamyl alcohol. These extracts have been used for the spectrophotometric determination of thallium [8]. A mixture of C6H6 and CC14 (2+1) is a convenient extractant since it is denser than water. The molar absorptivity of a (C6H6 + CC14) solution of the ion-pair is 9.7.104 (a = 0.48) at ~max -" 560 n m .

Bromine water or Ce(IV) sulphate is used to oxidize TI(I) to TI(III). Excess of bromine is distilled off or reacted with phenol. Excess of Ce(IV) is reduced with NHzOH [21 ].

Under the reaction conditions employed for thallium(llI), Rhodamine B also reacts with Au(III), Fe(III), Sb(V), Hg(II), and Ga. Hence, thallium is usually first separated from interfering metals, e.g., by extraction with dithizone in CHC13. In this separation, the metals which are extracted with thallium do not interfere in the Rhodamine B reaction.

From the dithizone-CHC13 medium T1 is stripped with dilute HNO3, or the solvent is volatilized, and dithizone mineralized with a mixture of H2SO4 and HNO3. Sometimes, thallium is separated before the determination, as the bromide complex, by extraction with diethyl ether.

Gold and mercury are removed from aqueous solution by cementation on metallic copper, while antimony is reduced partly to the metal and partly to Sb(III), and thallium(III) is reduced to TI(I). Iron(HI) may be masked with phosphoric acid.

Thallium(HI) bromide extract in DIPE can be directly shaken with Rhodamine B in 1 M H2804 [22].

Reagents

Rhodamine B, 0.1% solution in 2 M HC1. Standard thallium (I) solution: 1 mg/ml. a) Dissolve 1.3030 g of T1NO3, (dried at 110~ in water containing 2 ml of conc.

HNO3, and dilute the solution to volume with water in a 1-1itre standard flask. b) Dissolve 1.0360 g of TI(I) oxide, T120, in 15 ml of hot HNO3 (1+1). Dilute the

solution with water to the mark in a 1-1itre standard flask. Dithizone, 0.01% solution in chloroform. Preparation as in Section 46.2.1.

420 50. Thallium

Procedure

Separation of T1 with dithizone. To the acidic sample solution, containing not more than 40 gg of T1, add 1 ml of 2% ascorbic acid solution. After 3 min, add 3 ml of 10% sodium citrate solution, ammonia to make the pH 9-10, 2 ml of 10% KCN solution, and water to 20-25 ml. Extract T1 with three portions of the dithizone solution, shaking for 2 min with each portion. Add 5 drops of conc. H2504 to the combined chloroform extracts in a beaker, and evaporate off the chloroform. Then heat more intensely and add conc. HNO3 dropwise to mineralize the organic residue. Evaporate some of the sulphuric acid. Cool the residue and dissolve it in 10-15 ml of 2 M HC1.

Determination of T1. To the solution in 2 M HC1, add 1 ml of saturated bromine water and heat the solution (without boiling) until the yellow colour of the free bromine in the solution disappears. Cool the solution, transfer it to a separating funnel, add 2 ml of Rhodamine B solution, and shake the solution for 1 min with two portions of a mixture of C6H6 and CC14 (2+ 1). Make up the combined extracts to the mark with the solvent in a 25-ml standard flask. Measure the absorbance of the solution at 560 nm, using the solvent or a reagent blank solution as reference.

50.2.2. Brilliant Green method

Brilliant Green which is a basic triphenylmethane dye (formula 4.26), gives ion-associates with T1C14- or T1Br4-. These associates are extractable into organic solvents, such as amyl acetate, DIPE, or benzene. The intensely coloured extract constitutes the basis of a sensitive method for determining T1 [5,23,24].

Thallium is extracted after oxidation to TI(I]I) with bromine, the excess of which is removed either by boiling or by reaction with phenol or sulphosalicylic acid. The thallium- Brilliant Green ion-associate is then formed by shaking the organic extract with a solution of Brilliant Green in 0.1-0.2 M HC1. The molar absorptivity of the complex in DIPE is 1.05.105 at 630 nm (a = 0.51).

Complex anions such as SbC16-, GaC14-, FeC14-, HgBr4-, W042-, C1042-, CrzO72-, SCN-, and other anions capable of forming extractable associates with Brilliant Green, interfere with the determination of T1.

A very selective method for determining T1 [5] consists in the extraction of thallium(III) into DIPE from 6 M HC1 and shaking the extract with a solution of Brilliant Green in 0.15 M HC1. Under such conditions, only Sb and Au interfere.

Reagents

Brilliant Green, 0.02% solution in 0.15 M HC1. Standard thallium solution: 1 mg/ml. Preparation as in Section 50.2.1. Phenol, 10% solution in glacial acetic acid.

Procedure

Extractive separation of T1. To the sample solution, containing not less than 50 gg of T1, add 5 drops of saturated bromine water and conc. HC1 until the HC1 concentration is 6 M. Transfer the solution to a separating funnel and add 5 drops of the phenol solution. After 5 min, extract thallium by shaking for 1 min with two portions of DIPE. Wash the combined ethereal extracts by shaking with 3 ml of 6 M HC1.


Determination of T1. Shake the clear ethereal extract for 30 s with 5 ml of Brilliant Green solution. Wash the aqueous solution by shaking it with 5 ml of DIPE. Place the combined ethereal solutions in a 25-ml standard flask, make up to the mark with ether, mix well, and measure the absorbance at 630 nm against a reagent blank solution.

50.2.3. Starch-iodine method

In a hydrochloric acid medium (optimum concentration-0.5 M HC1), thallium(I) is oxidized with bromine to thallium(III). Thallium(llI) oxidizes iodide to iodine, which gives a blue complex with starch. The molar absorptivity of the starch-iodine complex is 3.9.10 4 ( a --

0.19) at 590 nm. The oxidation of thallium(I) with bromine occurs quantitatively in the cold. The excess

of bromine is conveniently removed by reaction with phenol. The formation of a suspension of bromophenol (which interferes in the absorbance measurement) can be avoided, if the excess of bromine is small. The aqueous glycerol solution of starch is stable and gives a good reproducibility of results. The excess of starch does not affect the colour. The amount of the added KI should always be the same. Atmospheric oxygen slowly liberates iodine from the iodide, but the resulting increase in absorbance amounts to scarcely 7% in one hour.

The iodine liberated by thallium(lIl) may be extracted into CHC13 o r CC14 and the absorbance of the extract measured, but this method is much less sensitive than the starch- iodine method.

The sample solution to be reacted with I- must not contain other oxidants capable of oxidizing iodide to iodine. Small amounts of Fe(III) may be masked with phosphoric acid.

Reagents

Potassium iodide: 0.5% solution, freshly prepared. Starch, 1% solution. Preparation as in Section 25.2.1. Standard thallium solution: 1 mg/ml. Preparation as in Section 50.2.1.

Procedure

To 15-20 ml of acidic (pH -~1) sample solution containing not more than 100 gg of T1, add 1 ml of conc. HC1 and 5 drops of bromine water, and stir. After 1 min, add 5 drops of phenol solution (1% in glacial acetic acid) and mix well. After 1 min, add 1 ml of KI solution and 1 ml of starch solution, and dilute the solution to the mark with water in a 25-ml standard flask. Mix the solution, and measure its absorbance at 590 nm against a reagent blank solution.


Apart from the methods already described, using Rhodamine B and Brilliant Green, there are many spectrophotometric methods based on extraction of the ion-associates formed by basic dyes and T1C14- or T1Br4- [25,26]. Of particular importance are: Crystal Violet (DIPE, e = 1.0-105) [26-30], Methyl Violet (toluene, e = 6.4.104) [31,32], Methyl Green (benzene, E = 1.0-105) [26,33], Victoria Blue 4R [34], Methylene Blue (CHC13, e = 1.1.105) [26,35], Meldola's Blue [36], Safranine T (~ = 4.4.104 at 510 nm) [37], Cationic Rose 2S (cyanine dye) (e - 8.4.104 at 550 nm) [38,39] and other cyanine dyes [40,41] (for the formulae of the

422 50. Thallium

dyes, see Chapter 4). Dithizone has been utilized not only for the separation of thallium but also for its

determination [16,42]. The molar absorptivity of T1HDz in CHC13 is 3.7-104 at ~max 505 nm. Interfering species include Pb, Bi, and Sn(II). Other sulphur-organic reagents proposed for determining T1 include thiodibenzoylmethane [43] and thio-HTTA [44].

A number of azo reagents has also found application for determination of thallium, viz. PAR and TAR [45,46], PAN [47-49], TAN [47], 2-(2-pyridylazo)-5-diethylaminophenol (e - 6.4.104) and its bromo derivatives [50], and 7-(4,5-dimethylthiazolylazo)-8-hydroxyquinoline-5-sulphonic acid (in the presence of CP, e - 1.3.105) [51].

Some other organic reagents for determining thallium include Bromopyrogallol Red [52-54] (in the presence of CP, e = 3.6.104) [52]. Thallium(III) has been determined as the yellow iodide complex [2,55], which may be extracted into benzene in the presence of DAM or DAPM (e = 1.2.10 4 a t 400 nm). The thiocyanate complex of TI(III) in the presence of pyridine has also been used for determining thallium [56].


The Rhodamine B method has been used for determining thallium in foods and minerals [22], zinc and cadmium [57] and lead [17]. The Brilliant Green has been utilized for determining thallium in sea water [58], urine [23], waters, sewage, and ores [59], cadmium [27,60], antimony [27], and indium [5].

Other basic dyes have been used in the determination of thallium in soils [61 ], antimony and cadmium [27], lead and its alloys [29,31], zinc and its alloys [28], and tungsten [32].

Thallium has been determined in aluminium and its alloys with the use of PAR [46] and PAN [49].

References

1. Dean J.A., Eskew J.B.,Anal. Lett., 4, 737 (1971). 2. Tsukahara I., Sakakibara M., Yamamoto T., Anal. Chim. Acta, 83, 251 (1976). 3. Khosla M.M., Rao S.P., Anal. Chim. Acta, 68, 470 (1974). 4. Zolotov Yu.A., Alimarin I.P., Sukhanovskaya A.I., Zh. Anal. Khim., 20, 165 (1965). 5. Marczenko Z., Ka~owska H., Mojski M., Talanta, 21, 93 (1974). 6. Chavan M.B., Shinde V.M., Chem. Anal. (Warsaw), 19, 1183 (1974). 7. Kuchekar S.R., Chavan M.B., Talanta, 35, 357 (1988). 8. Onishi H., Bull. Chem. Soc. Jpn., 30, 567 (1957). 9. Irving H.M. et al., Anal. Chim. Acta, 181, 125 (1986).

10. Bagreev V.V., Zolotov Yu.A., Zh. Anal. Khim., 17, 852 (1962). 11. Wachter C., Weisweiler W., Mikrochim. Acta, 1982 I, 307. 12. Vibhute C.P., Khopkar S.M., Analyst, 111,435 (1986). 13. Vibhute C.P., Khopkar S.M.,Anal. Chim. Acta, 222, 215 (1989). 14. Beklemishev M.K. et al., Zh. Anal. Khim., 44, 1058 (1989). 15. Gandhi M.N., Khopkar S.M., Anal. Chim. Acta, 270, 87 (1992). 16. Marczenko Z., Kasiura K., Chem. Anal. (Warsaw), 10, 449 (1965). 17. Luke C.L., Anal. Chem., 31, 1680 (1959). 18. Sager M., Mikrochim. Acta, 1984 I, 461. 19. Strelow F.W., Victor A.H., Talanta, 19, 1019 (1972). 20. Matthews A.D., Riley J.P., Anal. Chim. Acta, 48, 25 (1969).

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Chapter 51. Thorium

Thorium (Th, at. mass 232.04) occurs in solutions exclusively in the IV oxidation state. In its chemical properties it resembles Zr and Ti, as well as the rare-earth elements. In aqueous solutions at pH < 1, it exists as colourless Th 4+ ions. It is less readily hydrolysed than Ti or Zr. The hydroxide Th(OH)4 (precipitating at pH 3.5-4) has no amphoteric properties. Thorium forms stable complexes with tartrate, citrate, and EDTA and less stable complexes with sulphate, nitrate and carbonate.



Traces of Th can be precipitated as thorium hydroxide by ammonia (pH > 4), with Fe(III), A1 or La being suitable collectors [1-2]. Thorium is separated from rare-earth elements by double precipitation (at pH --5) of the hydroxide.

The precipitation of thorium oxalate with calcium as collector is a much favoured separation method [3]. In a weakly acid medium (pH 1-4) it can be precipitated with oxalic acid. Rare-earth metals and U(IV) are also precipitated, but many metals (e.g., Fe, A1, Ti, Zr, Nb, and Mo) remain in solution as soluble oxalate complexes.

The separation of Th as the sparingly soluble thorium fluoride is equally selective [4]. The solubility of ThF4 is lower than that of the oxalate. Usually, the sample solution in hydrofluoric acid is evaporated to a small volume and diluted with water to precipitate Th, U(IV), and rare-earth metals. La, Ce, or Ca are used as collectors for traces of Th. Since the fluoride precipitate is difficult to filter off, centrifugation is advisable.

Thorium can be separated from rare-earth- and other metals by precipitation of the iodate, Th(IO3)4, from --1 M HNO3 in the presence of tartaric acid and H202 [4,5]. The iodate precipitate also contains any Zr and Ce(IV) present. Mercury(H) and cerium(W) have been used as collectors.

Traces of thorium have been co-precipitated from strongly acidic medium with barium sulphate [6]. After isolation with bismuth phosphate, thorium can be leached from the precipitate with (NH4)2CO3 solution [7].

51.1.2. Ion exchange. Extraction

Strongly acid cation-exchangers have been used for the separation of Th from rare-earth elements and other metals [8,9]. From the metal cations retained on the column 3-5 M HC1 elutes rare-earth- and most other metals except thorium. Thorium is eluted with 10 M HC1 or 3 M H2SO4, as well as with 5 M HNO3 [10], ammonium oxalate [ 11 ], ammonium carbonate [ 1] or ammonium sulphate solutions. Cation-exchange chromatography has also been used to separate thorium with the use of media such as HBr [ 12], formic acid + dimethyl sulphoxide [13], and nitric acid + methanol +TOPO [14].

Thorium has been sorbed on strongly basic anion-exchangers as the nitrate complex; most other metals are not retained [15-19]. Other media employed include chloride [20], sulphate [2], thiocyanate [21], and citrate [22]. In numerous cases, mixed aqueous-organic


solvent media are used for separating thorium [15,18,20,21]. Thorium has been separated from Eu, U(VI), and Pu(IV) from non-aqueous media [thenoyltrifluoroacetone + TBP (or TOPO) in benzene] [23]. Mixtures of Th and U have been separated on cellulose ion exchangers [24]. Thorium has been eluted with 2 M HC1 or 1 M H2SO4.

Thorium is one of the few multivalent metals [others are Au(III), Ce(IV), U(VI), and Cr(VI)] which are extractable as nitrate complexes from nitric acid solutions [25-28]. The extractants used include TBP in CC14 [26,29] , TOPO in cyclohexane, toluene or xylene [25,30,31], and triphenylarsine oxide in CHC13 [27]. Other reagents used for extraction of the nitrate complex of thorium include dibutyl dithiophosphate in various organic solvents [32], dibutyl sulphoxide in xylene [28], and bis(2-butoxyethyl) ether [33].The liquid anion- exchanger Aliquat 336 in xylene [34] and a solution of tertiary ammonium salt (Hyamine) in dichloroethane [35] have been also proposed for extraction of Th. The presence of Li, Na, or A1 nitrate improves the extraction of thorium. Sulphate, phosphate, and tartrate do not interfere, but fluoride must be masked, e.g., by aluminium. Thorium has been separated from U and Pu with the use of Alamine 336 and TOPO (in xylene or cyclohexane) [36].

Thorium can be extracted from H2SO4 medium with N-butylaniline in CHC13 [37], from salicylate medium (pH 3.5-5) with mesityl oxide [38] and tris(2-ethylhexyl phosphate) [39], and from succinate solution (pH 7-8) with Aliquat 336 in benzene [40].

Thorium is also separated by extraction with HDEHP in CC14 or cyclohexane (from 0.1 M HC1) [41,42], and with HTTA in benzene, xylene or CC14 from dilute acid medium (HNO3 or HC1) [43,44]. A synergistic effect of heterocyclic amines (1,10-phenanthroline and 4,7-diphenyl-l,10-phenanthroline) on the extraction of Th with HTTA has been discovered [45].

The crown ether 18-crown-6 (as the picrate associate) [46] and calixarenehydroxamic acids have also been used for extraction of Th [47].


Among the many spectrophotometric methods for determining thorium, those based on azo compounds containing arsonic acid groups are of considerable importance. Although the Thoron I method is still employed, it is much less sensitive and less selective than the newer Arsenazo HI method.

51.2.1. Thoron I method

Thoron I (formula 4.9) reacts with thorium ions in acid medium to yield a red, water-soluble complex, which forms the basis for the spectrophotometric determination [48,49]. Thoron I in acid solution is orange.

Between pH 0.4 and 1.0, the absorbance of the complex is constant. A 0.2-0.3 M HC1 medium is optimum. The absorbance reduces with increasing acidity. The molar absorptivity of the complex in 0.25 - HC1 is 1.7.104 (a = 0.07) at 540 nm.

Fluoride, oxalate, phosphate, and (to a lesser degree) sulphate interfere. In a 0.01 M sulphate solution, the absorbance of the Th complex with Thoron I reduces to 60%.

Zirconium and hafnium also give coloured Thoron I complexes, but these metals can be masked with tartaric acid (especially meso-tartaric acid): 1 ml of 5% tartaric acid solution in 25 ml of solution effectively masks 1 mg of zirconium. Uranium(IV) interferes in the determination of thorium, but does not interfere significantly when oxidized to U(VI). Up to 5 mg of A1, 5 mg of Fe(II), 5 mg of Ce, and 2 mg of Ti can be tolerated. Ascorbic acid is used to reduce iron(III).

426 51. Thorium

Reagents

Thoron I, 0.1% aqueous solution. Dissolve 100 mg of the reagent in 100 ml of water. Standard thorium solution: 1 mg/ml. Dissolve 2.5 g of Th(NO3)4.H20 in water

containing 5 ml of conc. HNO3, and dilute the solution to the mark with water in a 1-1itre standard flask. Determine the concentration of thorium in the solution by evaporating an aliquot to dryness and igniting the residue in a platinum crucible at 1,000~ to ThO2. Dilute the solution with water till the thorium concentration is exactly 1 mg/ml. Working solutions are obtained by appropriate dilution of the standard solution with -~0.1 M HNO3.

Procedure

Place the sulphate-free sample solution, containing not more than 150 ~tg of Th, in dilute HC1 in a 25-ml standard flask. Add 1 ml of 1% ascorbic acid solution, 1 ml of 5% tartaric acid solution, 2 ml of the Thoron I solution, and sufficient hydrochloric acid to make its concentration 0.25 M after dilution to the mark with water. Measure the absorbance of the solution at 540 nm, using a reagent blank solution as reference.

51.2.2 Arsenazo III method

Arsenazo III (formula 4.10) reacts with thorium in strongly acidic solution to give a grey- green water-soluble complex, which has been used for the determination of thorium [50-54]. The method is very sensitive, and the absorbance varies only slightly with change in HC1 concentration between 1 and 10 M. The maximum absorbance is obtained in 8 M HC1.

The molar absorptivity of the complex in 3 M HC1 is 1.15.105 (specific absorptivity 0.50) at ~max = 655 nm. The absorption spectra of Arsenazo III and its thorium complex are similar to the Zr spectra shown in Fig. 57.1.

The Arsenazo III method for determining thorium is highly selective. In the presence of oxalic acid as a masking agent, thorium can be determined in 2.5-3.5 M HC1 in the presence of Zr, Hf, and Nb. Iron(HI), which interferes, is reduced to Fe(II) by ascorbic acid, and U(IV) is oxidized to U(VI) by adding some KMnO4 (followed by ascorbic acid to decolorize the solution). At high concentrations of chloride (HC1 + LiC1) Th can be determined in the presence of a 100-fold amount of uranium(VI). The effect of Ti on the determination of Th has been discussed [55,56].

Under the conditions given in the procedure below, 5 mg of Zr , 5 mg of Ti, and 5 mg of Fe can be tolerated. Aluminium and REE do not interfere. In many materials Th can be determined without preliminary separation.

Fluoride, phosphate, and (to a lesser degree) sulphate interfere in the reaction with Arsenazo III. The presence of 0.02-0.05 M H2SO4 in the 3 M HC1 medium reduces the absorbance of the thorium complex by 15-20%.

Thorium-Arsenazo III complex can be extracted with butanol from 1 M HC1 (e = 6.4.104 at 670 nm) [57]. Thorium has been determined in the organic phase after extraction with HDEHP in cyclohexane and addition of aqueous Arsenazo III and isopropyl alcohol (e = 8.8.104 at 660 nm) [41]. In another method Arsenazo III was added to the xylene extract of the ion-associate of the nitrate complex of thorium with Aliquat 336 [34]. Thorium has been determined after froth flotation separation of the ternary compound of Th with Arsenazo III and surfactants [58,59].

The ion-exchanger Chelex-100 sorbed --80% of Th from Arsenazo III solutions [60]. Thorium has been also concentrated on an ion-exchanger soaked with dibromo-o-


nitroarsenazo [61 ]. Simultaneous determination of Th, U, and La has been performed by measuring the

absorbance of their complexes with Arsenazo III at three different wavelengths [62]. Derivative spectrophotometry has been used in determinations of Th in the presence of Zr [63], U [64], U and Zr [65], and La [66].

Reagents

Arsenazo HI, 0.05% solution. Dissolve 50 mg of the reagent in 100 ml of water. Standard thorium solution: 1 mg/ml. Preparation as in Section 51.2.1.

Procedure

Separation of Th as oxalate. To 50-150 ml of acid sample solution, add 5-20 ml (depending on the amount of other metals present) of 8% oxalic acid solution. Adjust the pH to -~3, heat the solution to --80~ and add dropwise, while stirring, 5 mg of calcium (as CaCI2 solution). Keep the solution at 80-90~ for 1 hr. After 3-4 hr, filter off the precipitate, and wash it with 1% oxalic acid solution and water. Dissolve the precipitate in a small quantity of hot 2 M HC1. Determination of Th. Place the sulphate-free acid sample solution, containing not more than 30 lag of Th, in a 25-ml standard flask. Add 6 ml of conc. HC1 and 3 ml of 8% oxalic acid solution, and mix well. Add 2 ml of the Arsenazo III solution, dilute to the mark with water, mix well, and measure the absorbance at 655 nm against a reagent blank solution.


From among azo reagents containing the arsonic acid group, Arsenazo I (Neothoron) [67], Arsenazo DAL [68], and p-Dimethylarsenazo HI (~ = 1.3-105, 5 M HC1) [69] have been applied for determining thorium. Reagents with phosphonic groups, such as Chlorophosphonazo HI (e = 1.2-105 at 620 nm) [70-72] and m-Carboxychlorophosphonazo (e = 1.0.105 at 676 nm [73,74] have been proposed

Other azo reagents for thorium include TAR [75], 5-Br-PADAP (e = 1.66-105) [76], Alizarin Black SN [77], and 2,3,3'-trihydroxy-4'-sulphoazobenzene [78].

Thorium has been also determined spectrophotometrically with triphenylmethane reagents, such as Xylenol Orange [79,80], Methylthymol Blue (e = 5.0-104 at 580 nm) [5,81], and Chrome Azurol S (CAS) [82]. A considerable increase in sensitivity and bathochromic shift of ~max have been observed in the presence of cationic surfactants [80,83,84]. In the determination of thorium with CAS in the presence of CTA (or CP), the molar absorptivity is 1.4.105 at 630 nm (pH 5.5-6), and with the use of Zephiramine e = 1.3.105 at pH 6.5-6.7).

Other organic spectrophotometric reagents, recommended for determination of thorium are: Bromopyrogallol Red [85], morin (e = 3.3.104 at 422 nm), quercetin [86,87], phenylfluorone [87], and 8-hydroxyquinoline [88]. The anionic complex of 5,7-dibromo- oxine with Th forms an ion-associate with Rhodamine B which can be extracted into benzene (~ = 8.8.104 at 552 nm) [89].

Thorium has been determined in the presence of U(VI) by derivative spectrophotometry with the use of carminic acid [90,91] and 4-(2-thiazolyl-azo)rezacetophenone [92,93].

Methods based on thoromolybdophosphoric heteropoly acid [94] and its reduced form

428 51. Thorium

[95,96] should also be mentioned.


The Thoron method has been used for determining Th in plants [97], monazite sands and concentrates [9,30,31,98,99], silicate minerals [4,8,100], ores [101], zirconium minerals [49], and lanthanide compounds [42,44,102]. The froth flotation technique has been applied in the separation of Th from monazite sands [99].

Arsenazo III was applied in the determination of thorium in biological materials [103,104], natural waters [34,105,106], fertilizers [107], glass [108], silicate minerals [2,10,27,55,109], niobium and tantalum minerals [110], uranium minerals [3,18], manganese ores [19], lanthanide compounds [26,44], zirconium minerals [111], titanium concentrates [ 111], ilmenite and rutile [ 112]. Thorium was determined in waters with the use of the FIA technique [ 106].

The m-Carboxychlorophosphonazo has been applied for determining Th in industrial waters [74] and 2-hydroxy-5-sulphonic-arsenazo was used for determining Th in REE-Mg- Si alloy.

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Chapter 52. Tin

Tin (Sn, at. mass 118.69) occurs in its compounds in the II and IV oxidation states. Tin(H) is unstable. The hydroxide, Sn(OH)2, precipitates at pH --2 but is amphoteric, redissolving in NaOH (pH 13) to form stannite. Tin(II) forms oxalate and chloride complexes. Tin(W) hydrous oxide (metastannic acid) precipitates even at pH 0.5 and redissolves at pH -9 as stannate. Tin(W) gives stable halide-, oxalate-, and tartrate complexes.


52.1.1. Extraction

In the most often used method the tin(IV) iodide complex is extracted from acid medium (-4 M H2SO4 or HC104) containing iodide, with benzene, toluene, cyclohexane, or n-hexane; Sb and In remain in the aqueous phase. The iodide concentration should not be too high, to avoid formation of anionic tin complexes. Chloride and fluoride interfere [1-5]. The method has been used for separation of nanogram quantities of tin from steel and alloys [6].

Tin(W) can be extracted from HC1 medium with TOPO or TEHPO in cyclohexane or TBP in octanol [7]. The ion-associate of SnC162- with trioctylammonium ions has been extracted into butyl acetate [8], and with Amberlite LA-2 into xylene [9].

The bromide complex of tin with N,N'-diphenylbenzamidine has been extracted with CHC13 [ 10].

Chelates of tin with DDTC [11], BPHA [12], and 8-hydroxyquinoline [13] have been used for extractive separation of tin. Because tin(W) oxinate is extracted into chloroform at pH 0.85 only in the presence of chloride, tin can be separated specifically by first extracting other metal oxinates at pH 0.85, then adding NH4C1 and extracting tin oxinate [13].

52.1.2. Precipitation. Distillation

Microgram quantities of tin are often co-precipitated as stannic acid with MnO2 aq. (see procedure p.52.2.1). Antimony is also precipitated quantitatively, and Au, T1, Bi, and W are wholly or partly precipitated [ 14,15].

Tin can be separated from molybdenum by co-precipitating tin(IV) with Fe(III) by adding ammonia to make the pH 6-8 [16]. Trace amounts of tin have been isolated by oxine with A1 as a carrier [ 17]. Tin can also be precipitated from acid medium (HC1, H2SO4) as the sulphide. Copper, Mo, or Cd may be used as scavengers [ 11 ].

Distillation of SnBr4 is a convenient method for separating small amounts of tin [18- 20]. In a procedure given below, first As(HI), Ge(IV), and Sb(III) are distilled off as chlorides from a medium comprising H2SO4, HC1, and H3PO4. Tin does not distil in the presence of phosphoric acid. Hydrobromic acid is then added to the still, and SnBr4 is distilled off.

Tin has been separated from a silicate material by mixing the sample with NH4I and subliming SnI4 at dull red heat. From metal samples tin was distilled off as mixed halides [21].

Tin has been isolated also as volatile SnH4, generated by adding sodium borohydride

432 52. Tin

NaBH4 to an acidic sample solution. The liberated tin hydride is absorbed in KMnO4 solution at pH 1.6 [22].


Two methods for determining tin are presented below: the phenylfluorone method and the method based on Pyrocatechol Violet. Besides, separation of tin by co-precipitation with MnOzaq., by distillation, and extraction are also described. A review of sensitive methods for determining tin based on ternary systems has been given [23].

52.2.1. Phenylfluorone method

Tin(IV) reacts with phenylfluorone (formula 22.1) in acidic medium to form a sparingly soluble complex. At low concentrations of tin, this complex occurs in solution as a sol suitable for the spectrophotometric determination of tin [11,24,25]. The reagent solution is yellow, whereas the pseudo-solution of the Sn-phenylfluorone complex is orange-red.

A pH of 1.0-1.2 is the most suitable for the reaction. When the solution is too acid, the reaction between tin(IV) and phenylfluorone does not proceed to completion, and maximum absorbance is not attained. Conversely, when the solution is insufficiently acid, the free reagent precipitates, causing a pink turbidity. Gum arabic or poly(vinyl alcohol) are used as the protective colloid.

The molar absorptivity of the pseudo-solution is 7.7.10 4 at 510 nm (sp. abs. 0.65). The method is of low selectivity. Numerous multivalent metals (e.g., Sb, Ge, Zr, Ga, Fe, Mo, and Ti) interfere. Small amounts of Ti, and Mo can be masked with hydrogen peroxide. Antimony, which often accompanies tin, can be masked with citric acid.

The use of a protective colloid is not necessary at very small concentrations of Sn in the sample solution, but the reaction mixture should be left standing for a longer time before the absorbance measurement.

The sensitivity of the method may be enhanced considerably by the use of surfactants, such as CTA [26].

Reagents

Phenylfluorone, 0.01% solution. Dissolve 10 mg of reagent in methanol, add 1 ml of 2 M HC1, and dilute the solution with methanol to 100 ml.

Standard tin(W) solution: 1 mg/ml. Dissolve 1.0000 g of tin in 50 ml of conc. H2SO4. After dissolution of the metal heat the solution to fuming. Dilute the cooled solution to volume with -~0.5 M H2SO4 in a 1-1itre standard flask. Working solutions are obtained by suitable dilution of the stock solution with 0.2 M H2SO4.

Citrate solution. Dissolve 147 g of trisodium citrate and 105 g of citric acid in water, and dilute the solution with water to 1 litre.

Acetate buffer. Dissolve 450 g of sodium acetate and 240 ml of glacial acetic acid in water and dilute with water to 1 litre.

Procedure

Separation of Sn by precipitation with Mn02 aq. Heat the chloride-free sample solution, containing 3 ml of conc. HNO3 per 100 ml, to almost boiling. Add 1 ml of 1% KMnO4 solution and 2 ml of 1% Mn(NO3)2 solution, and heat the solution for 30 min without


boiling. Filter off the precipitate on a paper and wash with hot HNO3 (1 + 100). Dry and burn off the filter paper, and fuse the precipitate in a nickel crucible with 0.3 g of Na202 and a pellet of NaOH. Heat the melt until dark red. Leach the cooled fusion cake with hot water, transfer the content of the crucible to a beaker, and add 5 ml of H2SO4 (1 +4) and 2 drops of 3% H202 solution. Evaporate the solution to reduce its volume.

Distillative separation of Sn. To a 100-ml still containing the sample solution, add 25 ml of H2SO4 (1 + 1), 3 ml of conc. H3PO4, 20 ml of HC1 (1 + 1), and 1 g of hydrazine sulphate. Dip the end of the condenser into a beaker of water. Bubble a slow stream of CO2 through the solution in the still, and heat to boiling. When the temperature in the still has reached 160~ add dropwise 20 ml of HC1 (1 + 1) at a rate of 1 drop every 4 s, while maintaining the temperature between 155 and 165~ Stop the heating and rinse the condenser with water. Discard the distillate, which contains AsC13, GeC14, and SbC13.

Dip the condenser of the distillation apparatus into 10 ml of water in the beaker. Add dropwise to the still a mixture of 15 ml of HC1 (1+1) and 7 ml of 48% HBr. Bubble CO2 through slowly, heating the still to maintain the temperature between 145 and 160~ The distillation should take 15-20 min. Stop heating, and rinse the condenser with a small amount of water. Add to the distillate 2 ml of H2SO4 (1+1) and 5 ml of conc. HNO3, and cover the beaker with a watch glass. When the vigorous reaction (decomposition of HBr) has ceased, remove the cover glass and evaporate the solution to white fumes.

Determination of Sn. To the solution containing not more than 25 gg of Sn, add 1 ml of the citrate solution, 1 ml of 3% H202 solution, 1 ml of 1% gum arabic solution, 2.5 ml of the phenylfluorone solution, and 2 ml of acetate buffer. Adjust the pH of the solution to 1.1_+0.1 and make up the solution to volume with water in a 25-ml standard flask. After 30 min measure the absorbance at 510 nm, using a reagent blank as reference.

52.2.2. Pyrocatechol Violet method

Pyrocatechol Violet (PV) (formula 4.16) reacts with Sn(IV) in weakly acidic medium (pH 2- 4.5) to give a red water-soluble complex, which is the basis of a method for determining tin [27-29].

Within the pH range suitable for tin-determination, many metals react with PV [e.g., A1, Ga, In, Fe(HI), Sb, Bi, Th, Zr, and Mo]. A preliminary separation of tin by extraction of SnI4 from a H2SO4 medium (see the procedure below) makes the method specific for Sn [2,3,4,30].

The molar absorptivity is 6.8-104 (sp. abs. 0.57) at 552 nm. The absorbance of the free reagent is insignificant at pH 3.8. The formation of the complex is slow, and the reaction mixture should be left standing for 15-30 min before the absorbance is measured.

The tin(IV)-PV complex has been extracted into some alcohols, as an ion-associate with diphenylguanidine [31].

Tin has been determined with PV in the presence of CTA at pH 2.2, whereby a higher sensitivity of the method is achieved (e - 9.6.104 at 662 nm) [32-34]" CP has also been applied [35,36].

Reagents

Pyrocatechol Violet (PV), 0.05% aqueous solution. The solution is stable for a week. Standard tin(IV) solution: 1 mg/ml. Preparation as in Section 52.2.1.

434 52. Tin

Procedure

Separation of Sn by extraction. To the solution containing Sn, add H2SO4 until its concentration is 4.5 M. Transfer the cooled solution to a separating funnel, add 2 g of KI, mix well, and shake for 2 min with 10-15 ml of toluene. Discard the aqueous phase and wash the toluene extract with 5 ml of 4.5 M H2804 containing 0.5 g of KI. (The toluene layer may be coloured with extracted iodine). Strip the tin by shaking the extract for -~30 s with two portions of 0.2 M NaOH.

Determination of Sn. Acidify the alkaline solution, containing not more than 30 ~tg of Sn, with 3 ml of HC1 (1+1) and decolorize (reduce iodine) by adding dropwise 2% ascorbic acid solution. Add 2.5 ml of the PV solution and 5 ml of 20% sodium acetate solution. Adjust the pH to 3.8+_0.1 with ammonia. Transfer the solution to a 25-ml standard flask, dilute to the mark with water, and mix thoroughly. After 30 min, measure the absorbance of the solution at 552 nm against a reagent blank.


Many other fluorones, in addition to phenylfluorone, have been applied for the determination of Sn [20,37-42]. High sensitivity (13 = 1.1.105 at 545 nm) has been attained for the method based on 3'-pyridylfluorone (formula 52.1) [37-41].

(52.1)

Among other fluorones used are: propylfluorone [43], 5-bromosalicylfluorone [44], salicylfluorone (formula 4.24) and dibromophenylfluorone (formula 57.2) in the presence of CP [45], o-nitrophenylfluorone in the presence of a cationic surfactant (13 = 1.85.105 at 545 nm) [46], and disulphophenylfluorone (formula 4.25) in the presence of non-ionic surfactants (13 -- 1.5.105) [47].

Tin(H) can be determined with the use of the ion-associate of SnC13- with Crystal Violet (formula 4.27) (4-heptanone, 13 = 8.5.104) [48]. The associates of SnC13-with Malachite Green [49] and Butylrhodamine B [50] has been extracted into benzene. Usually, TIC13 is used to keep tin as Sn(II). The flotation of the ion-associate of SnC13- with Rhodamine 6G by means of DIPE enables the separation and determination of traces of Sn(II) in tin(IV) chloride [51 ]. The anionic complex of tin(W) with 3,5-dinitrocatechol can be associated with Brilliant Green (CC14+C6H6, t3 = 1.75-105) and Nile Blue A (13 - 1.3.105) [52]. The ion- associate of the Ti(IV)-3-nitroalizarin complex with Brilliant Green (CHC13, 13 = 2.0.105) [53], and the complex with thiocarbamide and Xylenol Orange [54] have also been used for Sn determination.

Among sulphur- containing reagents, dithiol (formula 31.2) [55,56], thio-oxine [57,58], and dithizone [59] should be mentioned.

Gallein [60,61] and Bromopyrogallol Red [62,63] (xanthene chelating reagents, related to fluorones) have been recommended for determination of tin. In the presence of the


cationic surfactants (CP, CTA) the sensitivity of the methods increases markedly [63,64] �9 The molar absorptivity with Pyrogallol Red and CTA is 6.5.104 at 480 nm [65].

Several azo chelating reagents have been applied for spectrophotometric determination of tin, e.g., PAN [66], PAR (e = 5.2.104) [66,67], Rezarson [68], 5-(2-pyridylazo)-2- monoethylamino-p-cresol [69], and pyrimidyl derivatives [70].

Flavone reagents; quercetin [71,72] and morin (formulae 57.4 and 57.3) [73] have been recommended for determining tin. In the presence of antipyrine, the Sn(IV) compounds can be extracted into chloroform [73].

Other organic reagents used for determining tin include haematoxylin [71,74-76], chloro-oxine [77-79], and 2,2'-diquinoxalyl [80]. Micro-quantities of Sn(II) in the presence of Sn(IV) have been determined with the use of ferrozine [81]. Ferron has been applied for extractive separation (CHC13, tribenzylamine) and determination of Sn [82].


The phenylfluorone method has been applied for determination of tin in biological materials [83], waters and in fruit juices [84,85], rocks, minerals and soils [86], cast iron and steel [11,15,22,24,87], non-ferrous metals [88], lead and its alloys [9], and zinc [16]

3'-Pyridylfluorone was used for determining tin in zinc and copper alloys [37], steels [38,39], hydrogen peroxide solutions [40], and organotin compounds [41 ].

The Pyrocatechol Violet method has been used for determining tin in biological materials [5], foods [5,89], organic substances [2], rocks and ores [12], copper alloys [90], and steel [33]. Traces of tin in water and in lake sediments were determined with the aid of PV and CTA after preconcentration on polyurethane foam impregnated with dithiol [91 ]. Tin was also determined by the PV method in alloys after extractive isolation with the aid of tris(2-ethylhexyl)phosphate [92].

Haematoxylin was used for determining tin in cast iron and steel [74] and in lead- antimony alloys [75].

References

1. Gilbert D.D., Sandell E.B., Microchem. J., 4, 491 (1960). 2. Analytical Methods Committee, Analyst, 92, 320 (1967). 3. Corbin H.B.,Anal. Chem., 45, 534 (1973). 4. Donaldson E.M., Talanta, 27, 499 (1980). 5. Coles L.E., Schuller P.L., Vaessen H.A., Pure Appl. Chem., 54, 1737 (1982). 6. Nalini S., Balasubramanian N., Ramakrishna T.V., Bull. Chem. Soc. Jpn., 68, 1145

(1995). 7. Woidich H., Pfannhauser W., Mikrochim. Acta, 1973, 279, 665. 8. Tsukahara I., Yamamoto T.,Anal. Chim. Acta, 135, 235 (1982). 9. Hofer A., Landl B., Z. Anal. Chem., 244, 103 (1969). 10. Chakraborty A.R., Patel K.S., Mishra R.K., Int. J. Environ. Anal. Chem., 45, 229 (1991). 11. Luke C.L., Anal. Chim. Acta, 37, 97 (1967). 12. Koeva M., Mareva S., Jordanov N., Anal. Chim. Acta, 75, 464 (1975). 13. Pollock E.N., Zopatti L.P., Anal. Chem., 37, 290 (1965). 14. Reynolds G.F., Tyler F.S.,Analyst, 89, 579 (1964). 15. Amsheev A.A., Zavod. Lab., 34, 789 (1968). 16. Mizuike A., Hiraide M.,Anal. Chim. Acta, 69, 231 (1974).

436 52. Tin

17. Burridge J.C., Hewitt I.J.,Analyst, 110, 795 (1985). 18. De Bruyne P., Hoste J., Bull. Soc. Chim. Belg., 70, 221 (1961). 19. Terada K., Takahata M., Z. Anal. Chem., 321,760 (1985). 20. Flyantikova G.V., Lasovskaya O.N., Zavod. Lab., 56, No. 4, 10 (1990). 21. Zhou N., Mikrochim. Acta, 1991 III, 159. 22. Hosoya M., Konno H., Takeyama S., Bunseki Kagaku, 32, 444 (1983). 23. Sumina E.G., Chernova R.K., Zh. Anal. Khim., 38, 1048 (1983). 24. Leblond A.M., Boulin R., Chim. Anal., 50, 171 (1968). 25. Dymov A.M., Ivanov I.G., Romantseva T.I., Zh. Anal. Khim., 26, 2360 (1971). 26. Kulkarni V.H., Good M.L., Anal. Chem., 50, 973 (1978). 27. Yakovlev P.Y., Razumova G.P., Zavod. Lab., 31, 1307 (1965). 28. Wakley W.D., Varga L.P., Anal. Chem., 44, 169 (1972). 29. Kasiura K., Boroch E., Chem. Anal. (Warsaw), 24, 181 (1979). 30. Newman E.J., Jones P.D., Analyst, 91,406 (1966). 31. Shestidesyatnaya N.L., Kotelyanskaya L.I., Yanik M.I., Zh. Anal. Khim., 31, 67 (1976) 32. Dagnall R.M., West T.S., Young P., Analyst, 92, 27 (1967). 33. Ashton A., Fogg A.G., Burns D.T., Analyst, 98, 202 (1973); Z.Anal. Chem., 264, 133

(1973). 34. Kasiura K., Chem. Anal. (Warsaw), 23, 625 (1978). 35. Chernova R.K. et al., Zh. Anal. Khim., 33, 858 (1978). 36. Rudometkina T.F., Chernova I.B., Orlov V.V., Zavod. Lab., 55, No. 2, 14 (1989). 37. Asmus E., Kraetsch J., Z. Anal. Chem., 223, 401 (1966). 38. Asmus E., Kossmann U., Z. Anal. Chem., 245, 137 (1969). 39. Asmus E., Weinert H., Z. Anal. Chem., 249, 179 (1970). 40. Asmus E., Jahny J., Z. Anal. Chem., 255, 186 (1971). 41. Asmus E., Kropp B., Moczko F.M., Z. Anal. Chem., 256, 276 (1971). 42. Wang D., Xie Z., Wu Q., Song Y., Jin S.,Analyst, 116, 1189 (1991). 43. Olenovich N.L., Savenko G.I., Zavod. Lab., 41, 658 (1975). 44. Toporov S.V., Olenovich N.L., Zh. Anal. Khim., 39, 1641 (1984). 45. Antonovich V.P., Suvorova E.N., Shelikhina E.I., Zh. Anal. Khim., 37, 429 (1982). 46. Liu Y.M., Yu R.Q.,Analyst, 112, 1135 (1987). 47. Nazarenko A.J., Zh. Anal. Khim., 43, 269 (1988). 48. Ducret L., Maurel H., Anal. Chim. Acta, 21, 79 (1959). 49. Ackermann G., K6the J., Chem. Anal. (Warsaw), 17, 445 (1972). 50. Shumova T.I., Blum I.A., Zavod. Lab., 34, 659 (1968). 51. Kalinowski K., Marczenko Z., Microchem. J., 39, 198 (1989). 52. Nazarenko V.A. et al., Zh. Anal. Khim., 28, 1100 (1973); 30, 617 (1975). 53. Flyantikova T.V., et al., Zh. Anal. Khim., 37, 1043 (1982). 54. Yaroshenko O.P., Gavrilova V.N., Sumskaya N.R., Zavod. Lab., 58, No. 8, 11 (1992). 55. Analytical Methods Committee, Analyst, 93, 414 (1968). 56. Pyare R., Nath P.,Analyst, 110, 1321 (1985). 57. Vashchenko S.T., Suprunovich V.I., Zavod. Lab., 48, No. 3, 14 (1982). 58. Uvarova K.A., Marova S.F., Zh. Anal. Khim., 50, 1141 (1995). 59. Janitsch A., Mauterer R., Z. Anal. Chem., 314, 681 (1983). 60. Jones J.C., Analyst, 93, 214 (1968). 61. Popov V.A., Rudenko E.I., Pal'chun T.A., Zh. Anal. Khim., 30, 1965 (1975). 62. Thierig D., Umland F., Z. Anal. Chem., 221,229 (1966). 63. Huang X. et al., Talanta, 44, 817 (1997). 64. Tran Hong Con, N6mcova J., N6mec J., Suk V.,Anal. Chim. Acta, 115, 279 (1980).

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65. Wyganowski C., Mikrochim. Acta, 1979 I, 399. 66. Rakhmatullaev K., Tashmamatov Kh., Zh. Anal. Khim., 29, 2402 (1974). 67. Kasiura K., Olesiak K., Chem. Anal. (Warsaw), 14, 139 (1969). 68. Kaslina N.A. et al., Zavod. Lab., 44, 520 (1978). 69. Rakhmatullaev K., Zakirov B.G., Zh. Anal. Khim., 38, 666 (1983). 70. Singh I., Saini R., Ann. Chim. (Rome), 85, 193 (1995). 71. Babko A.K., Karnaukhova N.N., Zh. Anal. Khim., 22, 868 (1967). 72. Manolov K., Stamatova V., Matschev A., Mikrochim. Acta, 1976 II, 343. 73. Olenovich N.L., Savenko G.I., Zh. Anal. Khim., 30, 2158 (1975). 74. Specker H., Graffmann G., Z. Anal. Chem., 228, 401 (1967). 75. Shirodker R., Schibilla E., Z. Anal. Chem., 248, 173 (1969). 76. Leong C.L., Analyst, 102, 837 (1977). 77. Sanz-Medel A., Gutierrez A.M., Analyst, 103, 1037 (1978). 78. Gutierrez A.M. et al., Talanta, 32, 927 (1985). 79. Gutierrez A.M., Laorden M.V., Sanz-Medel A., Nieto J.L., Anal. Chim. Acta, 184, 317

(1986). 80. Baranowski R. et al., Chem. Anal. (Warsaw), 19, 997 (1974). 81. Bajic S.J., Jaselskis B.,Analyst, 116, 1059 (1991). 82. Arya S.P., Bhatia S.C., Bansal A., Fresenius'J. Anal. Chem., 345, 679 (1993). 83. Oelschl~iger W., Z. Anal. Chem., 174, 241 (1960). 84. Valencia M.C., Gimeno D., Capitan-Vallvey L.F.,Anal. Lett., 26, 1211 (1993). 85. Capitan-Vallvey L.F., Valencia M.C., Miron G., Anal. Chim. Acta, 289, 365 (1994). 86. Agterdenbos J., Vlogtman J., Talanta, 19, 1295 (1972). 87. Mareva C., Iordanov N., Kadeva S., Zavod. Lab., 41, 660 (1975). 88. Toporov S.V., Olenovich N.L., Zavod. Lab., 57, No. 12, 5 (1991). 89. Adcock L.H., Hope W.G., Analyst, 95, 868 (1970). 90. Spinola Costa A.C., Teixeira L.S., Ferreira S.L., Talanta, 42, 1973 (1995). 91. Omar M., Bowen H.J., Analyst, 107, 654 (1982). 92. Gaudh J.S., Shinde V.M., Mikrochim. Acta, 124, 123 (1996).

Chapter 53. Titanium

Titanium (Ti, at. mass 47.88) occurs in acid solutions as the titanic ion Ti 4+ or the titanyl ion TiO 2+ (in a less acidic medium). At pH ~-1 basic salts precipitate, followed by the hydroxide, which displays very weak amphoteric properties. Titanium(W) forms stable fluoride, peroxide, tartrate, oxalate, and EDTA complexes, and weak sulphate, thiocyanate and chloride complexes. Titanium(m) compounds [violet Ti 3+ ions obtained by reduction of titanium(W) with zinc] have no importance for spectrophotometric determination of titanium.



Precipitation of titanium as the hydroxide Ti(OH)4 with an excess of NaOH solution enables the separation of Ti from V(V), Cr(VI), Mo(VI), W(VI), P(V), and A1 [1,2]. Double precipitation gives the best separation. Traces of Ti are separated in the presence of Fe, A1, Mn, Mg, or La as collector. Precipitation of Ti with ammonia in the presence of EDTA separates this metal from Fe, Mn, A1, and other metals.

When a NazCO3 fusion cake is leached with water, Ti hydrolyses and remains in the undissolved residue.

Mention should also be made of the precipitation of titanium as the cupferronate [3] or oxinate with A1 as a carrier [4].

Titanium is extracted as the cupferronate (EDTA-tartrate medium, pH-~8) [5,6] and separated from other metals, including Nb, Ta, and V [5,6]. Traces of titanium [along with Fe(III), A1, and Mn] can be extracted as the oxinate into CHC13 [4].

Titanium has been extracted from hydrochloric acid medium by HTTA (mixed with TBP) [7]. Titanium has been extracted from salicylate medium by using Aliquat 336 in xylene [8]. Solutions of HDEHP and other alkylphosphoric acids in benzene or other solvents have also been used for separation of Ti [9].

In a number of methods discussed below, the extractive separation of titanium is connected directly with the determination.


Anionic titanium fluoride [10] and thiocyanate [11] complexes are retained on a column of strongly basic anion-exchanger. The titanium can be eluted with 3-4 M HC1, 1 M H2SO4, 1 M HC104, or H202 in 0.05 M H2SO4. Ti, Zr, Nb, Ta, Mo, and W can be separated by ion- exchange and successive elution with solutions of HF, NH4F, and NH4C1 [12,13]. Titanium, Zr, and Mo have been separated by virtue of the differences in stability of their oxalate complexes as a function of the concentration of H2SO4 [ 14].

Cation-exchangers are used for the separation of Ti from other metals [15,16]. Titanium and zirconium have been separated by sorbing both of them on a Dowex-50 column and then eluting first Ti with 25% HC104, and subsequently Zr, with a mixture of NHaF and HC104 [17].


The titanium complex with H202 can be sorbed on silica gel and thus separated from Mo, W, and V [18].


At higher concentrations titanium is determined by the hydrogen peroxide method. Higher sensitivities are attained with the methods using chromotropic acid. The thiocyanate (with extraction) and fluorone methods are the most sensitive.

53.2.1. Hydrogen peroxide method

In acid media titanium ions give with hydrogen peroxide a yellow-orange complex [TiO.H202] 2+ which forms the basis of the well-known method for determining titanium [19]. The method is simple and fairly selective, but of rather low sensitivity.

The molar absorptivity at )Lmax 410 nm (see Fig. 53.1) is 7.102 (a = 0.015).

(D

c~

C~ cr

c~

~3

z.O0 410 t,60 500 600 wavelength,nm

Fig. 53.1. Absorption spectra of the hydrogen peroxide complexes of titanium (1), vanadium (2) and molybdenum (3)

The H202 concentration should not be less than 0.1 M; the nature and concentration of the acid used (H2SO4, HC1, HNO3, HC104) are not important. Sulphuric acid (0.7-1.8 M) is most often used as the medium for the reaction Hydrochloric acid is not recommended because of the yellow colour of the chloride complexes with Fe(III), which usually accompanies titanium. Iron(m) may be masked with phosphoric acid, but as a consequence of its complexing effect on titanium, phosphoric acid diminishes the colour intensity of the titanium complex. The presence of fluoride, which combines strongly with titanium, makes it impossible to use the method. Boric acid can serve as a masking agent for F- ions. Oxalate and tartrate ions also interfere. Coloured metal ions and those which give complexes with hydrogen peroxide (V, Mo, U, Nb, Cr) interfere, but only vanadium exerts an appreciable effect. Titanium can be determined in the presence of V by measuring the reduction in absorbance when fluoride is added.

Titanium has been determined in the presence of V and Mo by derivative spectrophotometry [20]. The hydrogen peroxide method has been used in determining Ti by the FIA technique in the presence of V [21].

440 53. Titanium

Reagents

Hydrogen peroxide: 3% H202 aqueous solution. Standard titanium solution: 1 mg/ml. a) Weigh 0.8350 g of TiO2 ignited at --900~ and fuse it with 8 g of K28207 in a silica or

platinum crucible. Dissolve the melt in 150 ml of hot H2804 (1 +2). Dilute the solution to the mark in a 500-ml standard flask with H2SO4 (1 +5).

b) Weigh 3 g of KzTiF6.H20 in a platinum evaporating dish, add 100 ml of H2804 (1 + 1) and evaporate the solution to white fumes. After cooling, rinse the walls of the vessel with water and evaporate to fumes again. Repeat the operation once more to completely remove HF. Dilute the cooled solution to volume in a 500-ml standard flask with sulphuric acid (1 +5) and mix well. Determine Ti gravimetrically as TiO2 in an aliquot of the solution. Add enough H2804 (1+5) to the titanium solution to make the titanium concentration exactly 1 mg/ml. Prepare working solutions by suitable dilution with 0.5 M H2804.

Procedure

Acidify the sample solution, containing not more than 0.8 mg of Ti, in a 25-ml standard flask with sulphuric acid so as to obtain --1 M H2804 concentration in the final solution. Add 1 ml of conc. phosphoric acid, 3 ml of the H202 solution, make up to the mark with water, and mix well. Measure the absorbance at 410 nm, using water as reference.

53.2.2. Chromotropic acid method Chromotropic acid (1,8-dihydroxynaphthalene-3,6-disulphonic acid, formula 53.1) gives water-soluble, brown-red titanium complexes which differ in composition and colour according to the acidity of the medium [22]. The pH at which the colour is developed is therefore critical. The solutions are usually buffered with formate (pH 3-3.5) or acetate (pH 4-5).

H~ OH

s ~ ~ s (53.1) HOz 03H

The molar absorptivity of the titanium chromotropic acid complex solution at pH 3.5 and ~max 460 nm is 1.7-104 (a = 0.36).

The chief interference is from Fe(III), which forms a green complex with chromotropic acid. Before the determination of Ti, larger quantities of iron should be separated or smaller ones reduced with ascorbic acid or sulphite. Vanadium in quantities not exceeding those of titanium has no appreciable effect on the determination of Ti. Molybdenum at concentrations below 50 ~tg/ml does not interfere. Fluoride interferes by masking titanium, but can be removed by fuming with H2804. Oxidants (e.g. HNO3) must be absent because chromotropic acid is fairly easily oxidized.

Solutions of chromotropic acid darken on standing in contact with atmospheric oxygen. A related reagent, 2,7-dichlorochromotropic acid, is resistant to oxidation. It gives a coloured complex with titanium at pH --2. The complex has ~max 490 nm (~ = 1.12.104). A (1:1:1) complex is formed in the presence of DAM in 0.8-2 M HC1 [23].


Reagents

Chromotropic acid, 1% solution. Dissolve 1 g of chromotropic acid disodium salt in water, add 20 ml of 2% ascorbic acid solution, and dilute with water to 100 ml. Store in an amber- glass bottle.

Standard titanium solution: 1 mg/ml. Preparation as in Section 53.2.1. Formate buffer, pH 3.5. Dissolve in water 60 ml of formic acid and 28 g of NaOH and

dilute the solution to 1 litre with water.

Procedure

To the acid sample solution containing not more than 50 gg of Ti, add 1 ml of 2% ascorbic acid solution and heat the solution. Add to the cooled solution 1 ml of chromotropic acid solution, adjust the pH to --2 with ammonia, and add 5 ml of the buffer. Dilute to volume with water in a 25-ml standard flask and stir well. After 10 min, measure the absorbance at 460 nm, using a reagent blank as reference.


In a solution containing high concentrations of thiocyanate and HC1, titanium forms a yellow thiocyanate complex, which has been a basis of a sensitive method for its determination [24,25]. The absorbance can be measured for either an acetone-water solution or an MIBK extract. From a solution of 5.8 M NaSCN and 0.8 M HC1, MIBK extracts 95% of the titanium present, along with most of the reagent (as HSCN). The degree of extraction diminishes if the thiocyanate and HC1 concentrations are decreased in the aqueous phase.

The molar absorptivity of the Ti complex in MIBK is 8.5.104 at 417 nm (in acetone- water solution it is 7.8.104). Interfering species include Fe(III), Nb, and Mo. Thiocyanates of those metals may be extracted preliminarily from 4-5 M HC1, the extraction of titanium being only negligible [25].

The ternary complex of Ti with thiocyanate and diantipyrylmethane (DAM) is extracted into chloroform. The molar absorptivity of the complex in CHC13 is 8.0.104 (a = 1.6) at ~max 420 nm. The distribution coefficient of the complex is so high that a single extraction is adequate, but the absorbance of the extract is affected by the purity of the chloroform used. The optimum acidity of the aqueous phase is 2-3 M HC1.

Coloured extractable thiocyanate complexes with DAM are also formed by Fe(III), Cu, Co, W, Mo and Nb. Ni and V give only weakly coloured complexes and do not interfere in amounts of 3-5 mg. Fluoride, phosphate, and EDTA (if less than 10 mg) do not affect the extraction of the ternary Ti complex. Iron(m) and Cu(II) can be reduced with thiosulphate or SnCI2 [26].

In other modifications of the thiocyanate method for determining Ti the complex is extracted with cyclohexane in the presence of TOPO [3], with 1,2-dichloroethane or xylene (triethanolamine) [27,28], or chloroform (tetraphenylarsonium ion, e = 7.5.104) [29], and dimedrol (benzene, e = 8.2.104) [30].

Reagents

Potassium thiocyanate, 30% solution. Diantipyrylmethane (DAM), 5% solution in 2 M HC1. Standard titanium solution: 1 mg/ml. Preparation as in Section 53.2.1.

442 53. Titanium

Tin(H) chloride, 10% solution in 2 M HC1.

Procedure

To the acid sample solution (-10 ml in volume) containing not more than 12 ~tg of Ti, add 5 ml of the thiocyanate solution, 3 ml of the DAM solution, 3 ml of conc. HC1, and 1 ml of the SnCI2 solution. Extract the Ti complex with two portions of CHC13. Dilute the extracts to volume with chloroform in a 25-ml volumetric flask, mix well, and measure the absorbance at 420 nm, using a reagent blank as reference.

53.2.4. Other methods The sensitive method for determining Ti, based on the ternary Ti(IV)-SCN-DAM complex, was described above. Diantipyrylmethane (formula 1.8) gives in dilute HC1 a yellow [Ti(DAM)34+ complex which can be used for Ti determination [31-33]. The molar absorptivity is 1.45.104 at 390 nm. Chloride (to 4 M), sulphate (0.6 M) and nitrate (0.2 M) do not interfere in 0.5 M HC1 medium [34]. In the presence of SnCI2 a coloured complex is formed with DAM, extractable into chloroform [35].

Among the most sensitive methods for determining Ti are those based on the use of fluorones, such as phenylfluorone (formula 4.23) [36-39], salicylfluorone (formula 4.24) [40], disulphophenylfluorone (formula 4.25) [41-44], and 2,6,7-trihydroxyphenylfluorone [45]. Often a second ligand takes part in the colour reactions of Ti with fluorones (in acid media), e.g., BPHA [36], N-phenyllaurohydroxamic acid [37], antipyrine [40], and surfactants (CP, CTA, Triton X-305) [38,41-43]. Most of these mixed-ligand complexes are extractable into CHC13. The sensitivity of some methods is very high, and the molar absorptivities attain such values as 1.63-105 [38], and 1.71-105 [39].

A group of less sensitive Ti determination methods have been based on hydroxylamine derivatives, such as BPHA (formula 55.1) [46], benzohydroxamic acid [47,48], salicylhydroxamic acid (extraction into TOA-CC14; e = 1.2.104 at 370 nm) [47,49,50].

Of the triphenylmethane and xanthene reagents for Ti, mention should be made of Pyrocatechol Violet [51], Chrome Azurol S and Eriochrome Cyanine R [52], Eriochrome Azurol G [53], Pyrogallol Red [54], Bromopyrogallol Red [55,56], and o-hydroxy- hydroquinonephthaleine [57,58]. The use of surfactants markedly enhances the sensitivity of these methods [52,55,57]. In the method involving Chrome Azurol S and CTA e = 7.3.104 at 565 nm; with Pyrocatechol Violet and CP ~ = 7.5-104 (high contrast, )Lmax. : 295 nm).

The azo dyes, such as PAN [59,60], thiazolylazopyrocatechol (extraction into CHC13 with diphenylguanidine, e = 6.8.104) [61], and 5-Br-PADAP [62] are also used as spectrophotometric reagents for Ti.

Some o-diphenols, such as Tiron [63-67] and pyrocatechol [68-70] have also been used for determining Ti. These reagents are used in the presence of diphenylguanidine and antipyrine bases, thus enabling extraction of the Ti complexes.

High sensitivity is characteristic of the extractive spectrophotometric methods for determining Ti, based on ion-associates formed by the anionic complexes of Ti with 3,5- dinitropyrocatechol, tetrabromopyrocatechol [71 ], and basic dyes such as Malachite Green (e = 1.3.105) [72], Brilliant Green (~ - 2.0.105), or Methylene Blue (e - 1.7.105) [71]. The associates can be extracted into CHC13, CC14, or chlorobenzene.

Among other organic reagents used for Ti determinations mention should be made of salicylic acid and its derivatives [73,74], haematoxylin (with CTA) ( ~ - 3.5.104) [75], benzoylacetone [76], Alizarin (with fluoride) (e = 7.0.104 at 513 nm) [77], tetrahydroxy-


phenazine [78], and 2,2'-diquinoxalyl (e = 3.2-10 4) [79-82].


The hydrogen peroxide method has been used for determining titanium in uranium alloys [83], ilmenite ore [84], and silicon-based catalysts [85]. Titanium was determined in steel by derivative spectrophotometry [86].

Titanium has been determined with the use of chromotropic acid in sea-water [87], rocks [88], bauxites [89], steels [90], magnetic alloys [91], and alkalies [4]. Differential spectrophotometry has been used in the determination of titanium in the tobacco industry products [92].

Dichlorochromotropic acid was used for determining Ti in the presence of excess of uranium [93] and beryllium [94], in aluminium alloys [95], and in minerals and rocks [96].

The thiocyanate methods were applied in determinations of titanium in silica [97], glass and quartz [98], steel and aluminium alloys [30], and platinum chloride [2].

The method based on the use of DAM has been applied for determining titanium in biological materials [11], silicate rocks [32,99,100], cast iron and steel [33], molybdenum and tungsten [6,18], vanadium [ 18], zirconium, hafnium, and niobium [ 101 ], lithium fluoride [102], nickel, aluminium, and molybdenum alloys [11], and ferrotitanium [103]. Titanium was determined in aluminium alloys with the use of DAM in the presence of SnC12 [35,104].

The fluorones have been utilized for determining titanium in steel [41,105,106] and bronzes [41]. The Tiron method was used for determining titanium in chromites [107], molybdenum and its alloys [14,16], aluminium alloys [ 108], and copper alloys [ 109].

References

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444 53. Titanium

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(1978). 42. Savvin S.B., Chernova R.K., Lobacheva I.V., Zh. Anal. Khim., 36, 9 (1981). 43. Nazarenko A.J., Zh. Anal. Khim., 40, 828 (1985). 44. Pilipenko L.A., Kolomiets L.L., Gavrilova E.F., Zaruba L.N., Zh. Anal. Khim., 47, 1635

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(1987). 54. Sicilia D., Rubio S., Perez-Bendito D., Anal. Chim. Acta, 284, 149, 453 (1994). 55. Hausenblasova Z., N6mcova I., Suk V., Microchem. J., 26, 262 (1981). 56. Koch S., Ackermann G., Mosler H., Talanta, 31, 667 (1984). 57. Mori I., Fujita Y., Sakaguchi K., Bull. Chem. Soc. Jpn., 55, 3649 (1982). 58. Mori I. et al.,Anal. Sci., 1, 429 (1986). 59. Betteridge D., John D., Snape F.,Analyst, 98, 520 (1973). 60. Llobat-Estelles M. et al., Analyst, 111, 53 (1986). 61. Ivanov V.M., Ngyen X.V., Zh. Anal. Khim., 35, 903, 2124 (1980). 62. Jarosz M., Oszwa~dowski S., Ku~ S.,Analusis, 22, 141 (1994). 63. Barton A.F., McConnell S.R.,Anal. Chem., 48, 363 (1976). 64. Szczepaniak W., Juskowiak B., Chem. Anal. (Warsaw), 25, 719 (1980). 65. Koch S., Ackermann G., Scholze V., Talanta, 28, 915 (1981). 66. Shida J., Tsujikawa Y., Anal. Sci., 10, 775 (1994). 67. Hoshi S. et al., Anal. Sci., 13, 863 (1997).

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(1986). 81. Zolotajkin M., Baranowska I., Chem. Anal. (Warsaw), 38, 653 (1993). 82. Baranowski R., Gregorowicz Z., Pieszko C., Talanta, 41, 1161 (1994). 83. Evans H.B., Hallcock R.R., Anal. Chem., 39, 842 (1967). 84. Lunina G.E., Romanenko E.G., Zavod. Lab., 34, 538 (1968). 85. Haukka S., Saastamoinen A., Analyst, 117, 1381 (1992). 86. Ku~ S., Marczenko Z., Obarski N., Chem. Anal. (Warsaw), 37, 569 (1992). 87. Capitan-Vallvey L.F., Valencia M.C., De Orbe I., Microchem. J., 40, 166 (1989). 88. Santelli R.E., De Casia dos Santos Araujo R.,Analyst, 117, 1519 (1992). 89. Purohit R., Devi S.,Analyst, 117, 1175 (1992). 90. Sommer L., Coll. Czech. Chem. Comm., , 27, 2212 (1962). 91. Bagdasarov K.N., Ocnanov X.A., Zavod. Lab., 34, 1044 (1968). 92. Tikhonov V.N., Zh. Anal. Khim., 22, 525 (1967). 93. Kuznetsov V.I., Basargin N.N., Kokisheva T.N., Zh. Anal. Khim., 17, 457 (1962). 94. Basargin N.N., Kokisheva T.N., Solov'eva N.V., Zh. Anal. Khim., 19, 553 (1964). 95. Budanova L.M.,Pinaeva S.N., Zavod. Lab., 29, 149 (1963). 96. Klassova N.S., Leonova L.L., Zh. Anal. Khim., 19, 131 (1964). 97. Sugawara K.F., Yao-Sin Su.,Anal. Chim. Acta, 80, 143 (1975). 98. Chakrabarti N., Roy S.K., Indian J. Chem., Sect. A., 28A, 1130 (1989). 99. Mochizuki T., Kuroda R., Analyst, 107, 1255 (1982). 100. Chan-Huan Chung, Anal. Chim. Acta, 154, 259 (1983). 101. Wood D.F., Jones J.T.,Anal. Chim. Acta, 47, 215 (1969). 102. Egorova L.A. et al., Zavod. Lab., 59, No. 6, 13 (1993). 103. Karaseva L.V., Zavod. Lab., 58, 58 (1992). 104. Polyak L.Ya., Zh. Anal. Khim., 29, 1338 (1974). 105. Ronghuan H., Jianhua W., Analusis, 23, 180 (1995). 106. Wang J.H., He R.H., Tong Y.H., Chem. Anal. (Warsaw), 41, 577 (1996). 107. Easton A.J.,Anal. Chim. Acta, 78, 224 (1975). 108. Otto M., Wegscheider W.,Anal. Chem., 61, 1847 (1989). 109. Sukhan V.V., Simonenko V.I., Kuznetsova Zh.A., Zavod. Lab., 55, 24 (1989).

Chapter 54. Uranium

Uranium (U, at. mass 238.04) occurs in the III, IV, V, and VI oxidation states, compounds of U(VI) and U(IV) being of major importance. The yellow uranyl cation precipitates as the hydroxide UO2(OH)2 at pH -~4. Uranium(VI) is amphoteric, the sparingly soluble uranate and diuranate (NazUO4 and NazU207) being formed in NaOH medium. Uranium(VI) gives peroxide, fluoride, tartrate, carbonate (pH 7-12), and nitrate complexes. Uranium(IV) and thorium have similar properties. The hydroxide U(OH)4 has no amphoteric properties. In acid medium sparingly soluble UF4 and U(C204)2 precipitate. Uranium(W) is oxidized slowly to U(VI) by atmospheric oxygen and, more rapidly, by iodine and iron(HI).


54.1.1. Extraction

Uranium(VI) can be extracted selectively from nitrate medium with oxygen-containing solvents. The extracted species is solvated undissociated uranyl nitrate. The concentration of free nitric acid must not be too high (0.1-1 M), because at higher acidities the degree of extraction of other metals ICe(IV), Th, Zr, Au(HI)] is increased. To prevent the dissociation of uranyl nitrate, and to enhance the distribution coefficient of uranium, considerable quantities of aluminium, calcium, or ammonium nitrate (salting-out agents) are added to the aqueous solution. Sulphate, and small amounts of phosphate and fluoride, do not interfere. In the presence of chloride, Fe(III) and other metals giving chloride complexes can be extracted. The solvents used include MIBK [ 1,2], TBP in heptane [3], TBP in amyl acetate [4], TOPO in cyclohexane [5,6] TOPO in benzophenone [7], and triphenylarsine oxide in chloroform [8]. Uranium has been extracted from nitrate media with quaternary ammonium salts [9].

Uranium(VI) is often extracted from hydrochloric acid media (2-6 M HC1) by means of such solvents, as MIB K [ 10], 3-methyl- 1-butanol [ 11 ], TOA in xylene [ 12,13 ], DAM in 1,2- dichloroethane [14], Aliquat 336 in xylene [15], in mixtures with TOPO, Alamine 310 and its mixtures with TBP, TOPO, etc. [ 16].

Solvents used for extraction of U(VI) from dilute H2SO4 medium include TOA in benzene, CC14, or CHC13 [17,18], Alamine 310 in admixture with TBP and Cyanex 301 [19], and Aliquat-336 with TOPO (medium HNO3+HzSO4) [20]. Uranium has also been extracted as its complex with HDEHP [21-24], mixtures of phosphor-organic acids [25], quinonylphenylphosphoric acid [26], salicylic acid (mesityl oxide) [27], malonic acid (Amberlite LA-1 in xylene) [28], high molecular-weight aliphatic amides [29,30], and Tropolone in mixtures with TBP, DOSO, TOPO [31]. Uranium was also extracted into ethylacetyloctane with pyridine [32] and di(2-ethylhexyl) sulphoxide [33].

The crown ether, 18-crown-6 [34-36], [2.2.2]-cryptand [371, and calixarene derivatives of hydroxamic acids have also been used for extraction of U(VI) [38].

54.1.2. Ion exchange. Sorption

Uranium(VI) may occur in solution as cations or complex anions and so can be separated


from other elements by using of either cation- or anion- exchangers [39]. Unlike most metals, uranium(VI) does not form stable complexes with EDTA. On

passage of an EDTA solution through a column of strongly acid cation-exchanger, U (and also Be and Ti) is retained in the column, the other metals passing to the eluate as anionic complexes. Uranium has also been separated from other elements on cation-exchangers in chloride [40], bromide [40], and nitrate [41] media. Water-organic solvent mixed media containing, e.g., acetone, dimethyl sulphoxide, or tetrahydrofuran, are used in these methods.

Anionic sulphate [42,43], thiocyanate [44], nitrate [45,46], phosphate [47], and azide [48] complexes have been used for the separation of uranium by means of anion- exchangers. Uranium is usually eluted from the column with hydrochloric acid (0.2-1 M).

Numerous methods have been developed for the separation of uranium from other metals on anion-exchangers, using acid media containing methanol, ethanol, acetone, diethyl ether, or dioxan [49-52].

Polyacrylonitrile fibre sorbent impregnated with Polyarsenazo N has been applied for preconcentration of trace amounts of U in natural waters [53]. Trace quantities of U in sea- water were concentrated on silica gel columns [54] and on cellulose sorbent impregnated with Arsenazo HI [55]. Chelate sorbents for preconcentration of uranium have been treated in detail [56,57].

Uranium has been sorbed on polyurethane foams with HDEHP [58], phenylphosphonic acid [59], Adogen [60], and dibenzoylmethane [61].


When sodium carbonate is present in excess, the uranyl ion forms a soluble carbonate complex, whereas most metals separate as carbonates, basic carbonates, or hydroxides [62]

with the exception of V, Be, and Th, which remain partly in solution together with uranium. If the precipitate contains several metals, double precipitation is necessary. The amount of uranium retained by the precipitate does not exceed a few percent. When a carbonate fusion is used for decomposition of the sample, and the melt is leached with water, uranium passes into solution.

Hydroxylamine forms a soluble complex with U(VI), and thus prevents the precipitation of U(VI) from alkaline media [63].

Ammonia precipitates U(VI) as the sparingly soluble diuranate (NH4)2U207. Aluminium or Fe(III) can be used as a carrier [42]. In the presence of EDTA, only U, Ti, Be, Sn(IV), Sb, Nb, and Ta are precipitated by ammonia. Titanium is a suitable collector. Traces of uranium have also been co-precipitated as the phosphate with A1 as collector [64].


A sensitive and selective method based on the use of Arsenazo III, a moderately sensitive method involving benzoylmethane, and a low-sensitivity thiocyanate method are described below.


Arsenazo III (formula 4.10) reacts with uranium(W) in strongly acidic medium, to give (as with Th and Zr ions) a green-blue complex. The method based on this reaction is far superior in sensitivity and selectivity to the other spectrophotometric methods for

448 54. Uranium

determining U [65-67]. The molar absorptivity of the complex in 6-8 M HC1 in the presence of an excess of

Arsenazo III is 1.27.105 (a = 0.50) at ~max 665 nm. Granular zinc or bismuth is used in hydrochloric acid medium to reduce U(VI) to U(IV).

In 4 - HC1 medium, if oxalic acid is added to mask Zr (and Hf), only Th interferes with the determination of U(IV). Of the common anions, only fluoride interferes.

Arsenazo HI gives with U(VI) a coloured complex which is also used for determination of uranium [65,68,69]. The colour reaction is usually carried out either in slightly acidic (pH 2-3) or strongly acidic (5-6 M HNO3, HC1 or HC104) medium. In the latter case, a considerable excess of Arsenazo III is needed. The method based on this reaction is convenient, since it eliminates the reduction step, but its sensitivity is much lower. The molar absorptivity is 6.0.104 (specific absorptivity 0.25) at 655 nm.

The selectivity of the method is improved by the use of masking agents, such as diethylenetriaminepenta-acetic acid (DTPA), EDTA, or DCTA [70].

The reaction of U(VI) with Arsenazo III can also be carried out in an organic solvent. Uranium(VI) is extracted from aqueous solution (HNO3, HC1) with a solution of TBP in MIBK [71], TOPO in benzene, or DAM in 1,2-dichloroethane [14]. In this way, uranium is separated from Th, Zr, Fe(III), RRE, and other metals. The extract is shaken with the Arsenazo HI solution.

The anionic uranium(VI)-Arsenazo HI complex has been extracted from aqueous solution (pH 1-3) with CHC13 in the presence of Zephiramine [72]. In another method U(VI) is extracted with triphenylarsine oxide in CHC13, back-extracted with 5.5 M HC104 containing oxalic acid, and converted into the Arsenazo III complex (e = 7.3.104) [8].

Uranium, thorium, and lanthanum were determined in one and the same solution by measuring the absorbance of their complexes with Arsenazo III at three different wavelengths [73]. Derivative spectrophotometry was applied for determining U in the presence of Th [74] or Th and Zr [75].

Reagents

Arsenazo III, 0.25% solution. Dissolve 0.25 g of Arsenazo III and 0.5 g of sodium acetate in 100 ml of water. Store the solution in an amber-glass bottle.

Standard uranium(VI) solution: 1 mg/ml. Dissolve 2.1080 g of uranyl nitrate, UO2(NO3)z.6H20, in water containing 1 ml of conc. HNO3. Dilute the solution with water to volume in a 1-1itre standard flask. Standardize by precipitating ammonium diuranate and igniting to U308. Prepare working solutions by dilution with --0.01 M HNO3.

Concentrated nitric acid, saturated with urea. To 500 ml of conc. HNO3 add 5 g of urea, stir, and allow to stand overnight. Filter off undissolved urea on a sintered-glass crucible.

Tributyl phosphate (TBP), (20% v/v solution) in toluene. Mix 20 ml of TBP with 80 ml of toluene and wash the solution twice with 40-ml portions of 5% Na2CO3 solution, then twice with 40-ml portions of water.

Wash solution. Dissolve 65 g of ammonium nitrate and 0.5 g of EDTA in 40 ml of water, adjust the pH to -~2, dilute with water to 100 ml, mix well, and filter.

Procedure

Extraction of U(VI) with TBP (in toluene). To the sample solution (-12 ml) containing not more than 80 gg of U, add 10 g of NH4NO3 and 0.1 g of EDTA. Heat the solution to --70~ cool it to room temperature, and adjust the pH of the solution to 1.0 with HNO3 or ammonia.


Extract uranium by shaking with 10 ml of TBP solution in toluene for 3 min. After separation of the phases, discard the aqueous solution and wash the extract by shaking with two portions (5 ml each) of wash solution for -~5 s each.

Determination of U. To the washed extract add 5 ml of Arsenazo III solution, 2 ml of water, and 0.5 ml of conc. ammonia solution, and shake for 3 min. After separation of the phases transfer the aqueous phase to a 25-ml standard flask. Wash the organic phase with two portions of water (shaking time 2 min). Add the two washes to the standard flask. Add 10 ml of conc. HNO3, mix, allow to cool, make up with water to the mark, and after 5-10 min measure the absorbance at 655 nm against a reagent blank.

54.2.2. Dibenzoylmethane method

At pH 6.5-8.5 the enol form of dibenzoylmethane (formula 54.1), a reagent of the [~- diketone group, forms a yellow uranyl chelate complex which has been used for determination of uranium [76,77]. It dissolves in aqueous ethanol medium containing pyridine, or may be extracted with ethyl acetate, butyl acetate, or CHC13 [78].

o o OH 0

(54.1)

The absorption-maximum of the complex in ethyl acetate is at 395 nm, molar absorptivity e = 2.0-104 (a = 0.088). Dibenzoylmethane itself absorbs at wavelengths shorter than 400 nm.

Phosphate, citrate, and most metals forming dibenzoylmethane complexes interfere, but EDTA is an excellent masking agent for most of the metals.

The dibenzoylmethane method is often used after uranium has been extractively separated from other metals as the nitrate complex [78,79], or by the carbonate method (see procedure below).

Reagents

Dibenzoylmethane, 0.2% solution in ethanol. Standard uranium(VI) solution: 1 mg/ml. Preparation as in Section 54.2.1. Sodium carbonate, 10% solution.

Procedure

Separation of U by the carbonate method. To 20-40 ml of acidic solution (HNO3, H2SO4, HC1) of U(VI), add 1 ml of ethanol and NaeCO3 solution in excess (--5 ml more than the amount necessary for neutralization of the sample solution). Heat the solution for 15 min at 80-90~ Filter off the precipitate on a filter paper and wash it with hot 1% NaeCO3 solution. Dissolve the precipitate in hot 2 M HC1 and re-precipitate with Na2CO3. Acidify the combined filtrates slightly with HC1 and evaporate to the desired volume.

Determination of U. To the slightly acidic solution containing not more than 150 ~tg of U, add 1 ml of 2% EDTA solution, 1 ml of dibenzoylmethane solution, and ammonia to pH 7-8. Extract the uranium complex with two portions of ethyl acetate. Dilute the extracts to volume with solvent in a 25-ml standard flask and measure the absorbance of the solution at 395 nm, using as reference the solvent or a reagent blank solution.

450 54. Uranium

Note. If titanium is present, add ammonium oxalate to the sample solution before applying the colour reaction.


Thiocyanate reacts with uranyl ions in acid medium to form a series of yellow complexes such as UOzSCN +, UOz(SCN)2, and UOz(SCN)3-. Higher concentrations of thiocyanate displace the equilibrium towards the last-mentioned and more intensely coloured complex. The absorption maximum of this complex lies in the near ultraviolet, at -~350 nm. At wavelengths shorter than 360 nm thiocyanate ions begin to absorb.

Uranium may be determined in aqueous or aqueous acetone medium (50-60% acetone), or after extraction of the complex into an organic solvent [80]. The uranium-thiocyanate complex may be extracted with diethyl ether, amyl alcohol, TBP in CC14, di-(2- ethylhexyl)methyl phosphonate in benzene [81], and Septonex in CHC13 [82]. The use of extraction increases the selectivity of the method.

The molar absorptivity of the uranium(VI)-thiocyanate complex in a mixture of TBP and CC14 is 2.9.103 (a = 0.012) at 380 nm.

Extraction of the complex from a weakly acidic medium in the presence of EDTA prevents interference by Fe(III) and other metals. The interference of Fe(III) can also be eliminated by the addition of a reducing agent (SnC12 or ascorbic acid).

Uranium(VI) can be separated from an HNO3 and Al(NO3)3 medium, and thiocyanate is added to the extract to obtain the colour reaction. In another modification of the method, U is extracted from an HNO3 or NH4SCN medium into ethyl methyl ketone, to which an acetone solution of NH4SCN is then added.

The thiocyanate method is of rather low sensitivity and is applicable for determination of larger contents of uranium in ores and concentrates. Uranium was also determined in the presence of fluoride.

Reagents

Potassium thiocyanate, 40% solution. Standard uranium(VI) solution: 1 mg/ml. Preparation as in Section 54.2.1.

Procedure

To the sample solution (in HNO3, H2SO4, or HC104) containing not more than 1 mg of U(VI) in 10 ml, add 2.5 ml of 10% EDTA solution and ammonia to give pH 3.5-4.0. Transfer the solution to a separating funnel, add 5 ml of thiocyanate solution, and shake with two portions of a mixture of TBP+CC14 (1 +4). Dilute the combined extracts to volume with the solvent in a 25-ml standard flask, mix well, and measure the absorbance at 380 nm v s .

the solvent as a reference.


Besides Arsenazo III, other azo dyes with arsonic or phosphonic groups have been recommended, namely Arsenazo I (pH 7-9, e = 2.3.104 at 596 nm) [43,83], Phosphonazo III [84], Chlorophosphonazo III [pH -~1, e = 8.1.104) [70,84-87], and p-Carboxychloro- phosphonazo [88].

PAN (formula 4.1) reacts with U(VI) in ammoniacal medium to yield a red-violet


complex, extractable into CHC13 (e = 2.3.104 at 560 nm) [7,89,90]. A higher sensitivity is obtained in the method with the related reagent, PAR, (~ = 3.9.104) [91-94].

5-Br-PADAP (formula 4.3) has become a basis for a sensitive method (pH --8, ~ = 7.6.104 at 575 nm) [95-98]. This reagent has been also used in the FIA technique for determining U [3,99,100]. Among other reagents worthy of mention are: 3,5-diBr-PADAP (pH 8.5-10, ~ = 9.1.104) [101,102], 5-Br-PAPS [103], 2-(3,5-dibromo-2-pyridylazo)-5-[N- ethyl-N-(sulphopropyl)amino]phenol [ 104], 5-(2-pyridylazo)monoethylamine-p-cresol [ 105], 2-(2-thiazolylazo)-5-dimethylaminophenol (TAM) [ 106], TAR [ 107], 2-(2-thiazolylazo)-4,6- dimethylphenol [ 108], and 4-(2-thiazolylazo)-6-chlororesorcinol [ 109].

Triphenylmethane reagents have gained considerable importance in the determination of uranium [110]. In this group it is worth mentioning Chrome Azurol S, especially if used in the presence of cationic surfactants (CP, CTA, Septonex, etc.) (pH 4-5, e = 1.0.105- 1.2.105 at -~630 nm) [111-115]. Other reagents suggested include Eriochrome Cyanine R (with CP) [116], Eriochrome Azurol B (pH 4-7; with CTA or CP, e = 1.35.105) [114], and Chromal Blue G (with CTA) [117]. Some sensitive methods for determining uranium are based on ion-associates consisting of anionic uranium(VI) complexes and basic dyes such as Brilliant Green and Crystal Violet [ 118], Rhodamine B [ 119], Butylrhodamine B [ 120], Malachite Green [ 121], Brilliant Green [122,123]. In the methods based on anthranilic acid with Rhodamine 6G [124], or thiocyanate with Rhodamine B [125] the associates formed are not extracted, but the absorbance is measured for aqueous pseudo-solutions, protected with gelatine or poly(vinyl alcohol).

Other organic reagents proposed for determination of uranium include octadecyl dithiocarbamate [126], Bromopyrogallol Red [18,127], Pyrogallol Red (~ = 3.6.104 in the presence of CP) [128], benzoyltrifluoroacetone [129], sulphosalicylic acid [130], 2,4- dinitrosoresorcinol [131], and various hydrazones [132-134]. Uranium and thorium have been determined simultaneously by the second derivative method using 4-(2'-thiazolylazo- rezacetophenone) oxime [135-137]. Uranium was determined in the presence of Th by derivative spectrophotometry with the use of carminic acid [138]. N-Hydroxy-N,N'- diphenylbenzamidine has been applied for determination of U (and Th) [ 139].

The peroxide method for determining U is of rather low sensitivity [140]. A yellow peruranate is formed in the reaction of H202 with U(VI) in alkaline media. Hydrogen peroxide can be added to the carbonate complex of U, or an acidic solution containing U and H202 is made alkaline with NaOH. The absorbance is measured at 370-400 nm.

Derivative spectrophotometry has been used for determining U in the presence of Pu [141].


Arsenazo III has been used for determining, in strongly acid media, uranium in natural waters [ 142], and in ores and rocks [66], after reduction of U(VI) to U(IV).

Arsenazo HI has been applied also for determining uranium as U(VI) in plants [143], natural waters [15,44,48,53,54,64,144-146], in rocks, minerals and soils [24,34,42,51,147], ores [62,148], organic substances [149], phosphorus compounds [26], thorium oxide [10], fertilizers [ 150], and glass [ 151 ].

Arsenazo III was used in determining uranium by the FIA technique in geological materials [152,153] and in sea-water [154].

The dibenzoylmethane method was applied for determining U in zirconium and hafnium

452 54. Uranium

[155]. Uranium was determined in phosphoric acid [156] and in ores [157,158] with the use of

PAN. The PAR method was applied for determining uranium in natural waters [92,94], sewage [94], rocks and waters [159], geological samples and biological materials [22,36], monazite sands [160], tin [161 ], and phosphoric acid [96].

The 5-Br-PADAP method was used in the determination of uranium in waters [96], sewage [97,98], ores [3,95], and phosphoric acid [96].

Other methods mentioned above were also used for determining uranium: Chrome Azurol S - in waters [147], Chlorophosphonazo I I I - in sea water [87], p-Carboxychloro- phosphonazo - in waters [88], Brilliant Green- in waters and ores [118]. The thiocyanate method was applied in the determination of U in environmental samples [38].

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Chapter 55. Vanadium

Vanadium (V, at. mass 50.94) occurs in the V, IV, III, and II oxidation states, vanadium(V) compounds being the most stable. In alkaline medium, the colourless vanadate VO3- ions exist, whereas in strongly acidic media, the yellow VO2 + cations are present. Within the intermediate pH range polymerized orange-yellow anionic forms occur. Vanadium(V) forms heteropoly acids with P(V), Mo(VI), and W(VI), and also peroxide complexes. Vanadium(IV) occurs as the blue vanadyl ion VO2 +, stable in acid solutions and readily oxidized to vanadium(V) in alkaline solution. The VO 2+ cation is amphoteric. At pH -4, VO(OH)2 precipitates and at pH -9 it dissolves. Vanadium(W) forms fluoride-, oxalate-, and EDTA complexes. The green V 3+ and violet V 2+ ions are strong reducers.



Highly selective extractive separation of vanadium is possible with BPHA or oxine. The procedure is described in detail below.

Extraction of V(V) a-benzoinoximate into CHC13 from a solution at pH 2-4 enables V to be separated from Fe and other metals [1,2]. Vanadium(W) is extracted as a mixed-ligand complex, VO(TTA)2.TOPO [3].

Vanadium(V) is reduced to V(II) in -1 M H2804 by zinc amalgam. After addition of thiocyanate and pyridine, and adjustment of the pH to 5.2-5.5, the ternary complex is extracted with CHC13. Hydrogen peroxide is used to strip vanadium [4].

Traces of V(V) are retained quantitatively by Fe(OH)3 as collector in precipitation of Fe(III) (2-5 mg) with ammonia, final pH 6-7. From alkaline medium, V(V) is co- precipitated with Mg(OH)2 as collector [5].

Vanadium is often isolated from a large number of elements by fusing the sample either in a platinum crucible with NazCO3 and a small amount of KNO3, or in a nickel crucible with Na202, the melt being leached in either case with cold water. The precipitate contains Fe, Cu, Ti, Ni, Co, Mn, and some A1. Besides V, the filtrate contains As, Cr, P, Mo, W, the rest of the A1, and also Mn. In leaching of the melt, the addition of a small amount of ethanol or H202 reduces Mn(VI) and Mn(VII) and leaves all the Mn in the precipitate.

Good separation of V from Cr and Cu can be attained by precipitating V(V) with cupferron [along with Fe(III) as collector] from an acid solution (pH <1). Besides the Fe, Ti and Zr are also precipitated with the vanadium.

Vanadium can be separated from metals which are precipitated as their hydroxides by ammonia, by conversion into a peroxide complex. If the precipitation and separation are performed twice, the losses of vanadium by retention on the precipitate do not exceed 10%.


Vanadium and molybdenum can be separated by means of a strongly basic anion- exchanger. From thioglycolic acid medium (pH 1-1.5), Mo is retained in the column while V passes into the eluate. For determination of vanadium in steel, an acetate solution of pH 2.5-3 was passed through a column with a strongly basic anion-exchanger. V(V), Cr(VI), and Mo(VI) were then washed out from the column with 0.6 M NaOH, 8 M HC1, and 1 M


HC1, respectively [6]. A procedure has been developed for the separation of V, Zr, Ti, Mo, W and Nb, with a

strongly basic anion-exchange resin [7]. To avoid interference from niobium, HF is used to elute vanadium and other metals. Vanadium, Mo and W are retained by a column containing a liquid anion-exchanger (Aliquat 336 on a solid support). The metals are selectively eluted with an aqueous solution containing H202 and various concentrations of H2804 [8] .

Cation-exchangers retain V(IV) and V(V) from strongly acid solutions. Vanadium is eluted selectively with 0.01 M H2804 or HC104 containing 1% of H202 from a column which retains many other metals [9]. Vanadium is eluted from a Dowex 50 cation-exchange column by 0.5 M HC1, whereas Fe, Ni, Mn, Th, U, A1, etc., are not [10]. Citric acid was used for elution of V from a cation exchange column [ 11 ].

Methods for vanadium separation and determination have been reviewed [ 12].


The following review presents the selective, but rather insensitive methods based on the use of BPHA and 8-hydroxyquinoline, a moderately sensitive method involving 4-(2- pyridylazo)resorcinol (PAR), and a very sensitive method based on the ion-associate of a vanadium chelate complex with Brilliant Green

55.2.1. BPHA method

N-Benzoyl-N-phenylhydroxylamine (BPHA, formula 55.1) gives with V(V) in strongly acid media (2-10 M HC1) a sparingly soluble, violet chelate, extractable into CHC13, which forms the basis of a selective method for determining V [13].

~ C-~-O

~ N - - O H

(55.~)

The optimum acidity of the reaction medium is 3-5 M HC1. Maximum colour intensity is attained with at least 10-fold excess of BPHA. Chloroform is the best extractant, but the complex is soluble also in CC14, benzene, or diethyl ether. The molar absorptivity is 5.1.103 (a = 0,10) at ~max 525 nm. The chloroform solution of BPHA does not absorb at 525 nm, hence CHC13 is used as reference in the absorbance measurement.

Very few metals give complexes with BPHA in acid media. Even considerable quantities (20-40 mg) of A1, Co, Cr, Cu, Fe(III), Mn, Ni, Th, and Zn do not interfere in the determination of V. Interfering species are: Mo(VI), Ti, and Zr which form, in strongly acid media, coloured (yellow, red) complexes with BPHA, soluble in CHC13. The concentration of HNO3 in the solution should not exceed 1 M. Strong oxidants, capable of reacting with BPHA, as well as substances reducing V(V), interfere.

Reagents

Benzoylphenylhydroxylamine (BPHA), 0.1% solution in chloroform, free from ethanol. The solution is stable when stored in an amber-glass bottle.

458 55. Vanadium

Standard vanadium solution: 1 mg/ml. a) Dissolve in dilute NaOH solution 1.7850 g of V205 ignited previously at 500~

acidify the obtained solution with H2804, and dilute with water to volume in a 1-1itre standard flask.

b) Dissolve 2.295 g of ammonium vanadate, NH4VO3, in water containing 5 ml of conc. ammonia solution. Add 10 ml of conc. nitric acid and dilute with water to 1 litre in a standard flask.

Procedure

Acidify the sample solution containing not more than 150 ~tg of V(V) with HC1 so that the HC1 concentration in the solution is --4 M. Transfer the solution to a separating funnel and shake with two portions of BPHA solution (shaking time, 1 min). Place the extracts in a 25- ml standard flask, make up with chloroform to the mark, mix, and measure the absorbance at 525 nm against CHC13 or a reagent blank.


8-Hydroxyquinoline (oxine, formula 4.42) gives with vanadium(V) in weakly acid media (pH 2-5) a chelate soluble in CHC13 and in some other solvents. The coloured oxinate extract forms the basis of an extractive-spectrophotometric method for V determination [ 14- 17].

The complex absorbs over a wide visible radiation range. The absorption maximum at 550 nm is generally used. The molar absorptivity of a chloroform solution of vanadium oxinate at 550 nm is 3.0.103 (a = 0.06). Chloroform solutions of the vanadium complex are stable in colour if the chloroform used is free from ethanol.

To eliminate the effect of other metals which form oxinates in slightly acidic medium, the following procedure has been recommended. The chloroform extract which is obtained at pH 4, and contains V and Fe(III), and partly also A1, Co, Zn, Ni, Mo, W, Cu, Ti, and Bi oxinates, is shaken with an aqueous alkaline solution (pH 9.4), thereby stripping vanadium into the aqueous phase and leaving behind Fe together with the other metals in the CHC13 solution. From the acidified aqueous phase, V is re-extracted with a CHC13 solution of oxine and the absorbance is measured. In this manner the oxine method becomes specific for V(V).

It is convenient to increase the acidity of the solution from pH 4.0 to pH 2.6-3.0 before the oxinate is extracted. The extraction rate and distribution coefficient remain the same as at pH 4, and Fe(III) is also extracted completely, but the extraction of aluminium and of other partially extractable metals is reduced 3-4-fold.

Vanadium(V) can be extracted with oxine in CHC13 at pH 5.5, in the presence of Ca- EDTA. Under these conditions there is interference only from Sn, Ti, and W, which are not masked by EDTA. A synergistic action has been observed during extraction of vanadium in the presence of phenols [ 18].

Vanadium(III) can be determined with oxine after reduction of vanadium(V) with dithionite and extraction of the obtained chelate into CC14 [19].

Reagents

8-Hydroxyquinoline (oxine): 0.5% and 0.1% solutions in chloroform. Standard vanadium solution: 1 mg/ml. Preparation as in Section 55.2.1. Buffer solution (pH 9.4). Add 40 ml of conc. ammonia solution and 20 ml of conc.


HNO3 to 800 ml of water. Adjust the solution with ammonia or acid to pH 9.4 and dilute with water to 1 litre.

Chloroform, free from ethanol. Wash the commercial product five or six times with water, dry over anhydrous CaCI2, and distil.

Procedure

Extractive separation of V. Adjust the pH of the sample solution containing not more than 50 ~tg of V(V) to 2.8+0.2, transfer the solution to a separating funnel and extract with 2 portions of 0.5% oxine solution (shaking-time, 2 min). Wash the extracts with water acidified with HC1 to pH -~3. Strip the V with two portions of the buffer solution (shaking- time 5 rain).

Determination of V. Adjust the alkaline solution containing V with 4 M HC1 and dilute ammonium solution to pH 2.8+0.2 and extract the vanadium with 2 portions of 0.1% oxine solution (shaking time 2 min). Dilute the extracts with chloroform to the mark in a 25-ml standard flask and measure the absorbance at 380 nm against a reagent blank.

55.2.3. Pyridylazoresorcinol method

4-(2-Pyridylazo)resorcinol (PAR, formula 4.2) reacts with vanadium [(W) or (V)] ions in the pH range 1-7, giving water-soluble complexes. At pH 1-4.5, complexes are formed with ~max 525 nm and e values 1.7.104; and at pH 4.5-7, Xmax = 545 nm and e = 3.6.104 (a = 0.77) (with VO2 +) and ~ = 3.3.104 (with VO2+). In the two ranges of acidity, differently dissociated forms of PAR react with V [20]. Vanadium(IV) is oxidized by aerial oxygen during its reaction with PAR, and a complex of vanadium(V) forms gradually, whatever the original oxidation state [21 ].

The V-PAR chelate have been proposed as the basis of the spectrophotometric method for determining vanadium. The excess of free reagent does not interfere much at the ~,max of the vanadium complex [22,23].

In the presence of DCTA, only Nb, U and Ti interfere in the determination of V. In the presence of DCTA the reaction proceeds slowly; the maximum absorbance is attained after 30 min. EDTA breaks down the V-PAR complex, but tartrate, oxalate, phosphate and fluoride have little influence. Interference by titanium decreases in the presence of propanol [24]. Titanium and Fe(III) can be masked by fluoride.

It has been shown that the complex formed at pH 5-6 is anionic, and is extractable by CHC13 in the presence of organic bases. In extractive modifications of the method, nitron [25], Zephiramine [26,27], tetrazolium [28] and xylomethazolium (nitrobenzene, ~ = 4.6.104 at 540 rim) [29] ions have been used.

In the presence of PAR and hydroxylamine, V forms a ternary 1:1:1 complex, which has been proposed for determining V [30]. The selectivity of the method (~ - 3.0.104) is better in acid (pH 1.5-3) than in alkaline media (pH 8-11). A mixed-ligand complex of V(V) with PAR and H202 has been also proposed for determining vanadium [31,32].

Reagents

4-(2-Pyridylazo)resorcinol (PAR): 0.02% solution. Standard vanadium solution: 1 mg/ml. Preparation as in Section 55.2.1. 1,2-Diaminocyclohexanetetra-acetic acid (DCTA): 0.1 M solution. Ammonium acetate: 2 M solution, adjusted with acetic acid to pH 5.5.

460 55. Vanadium

Procedure

To a slightly acid (pH 3-6) sample solution in a 25-ml standard flask, containing not more than 20 gg of V(V), add 2.5 ml of DCTA solution, 1 ml of ammonium acetate solution, and exactly 2.5 ml of PAR solution. Dilute with water to the mark, mix, and measure the absorbance of the solution after 30 min at 545 nm against a reagent blank.

55.2.4. Brilliant Green method

In acid medium (0.025 M H2804, pH 1.3_+0.1) V(V) reacts as VO(OH) 2+ with 3,5- dinitropyrocatechol (DNP) to give an anionic complex (formula 55.2), which forms an ion associate with the cationic dye, Brilliant Green. This compound is sparingly soluble in water, but can be extracted into CHC13. The coloured extract has been the basis of a very sensitive method for determining V [33].

NO2 N.02 z-

(55.2)

The best concentration of 3,5-dinitropyrocatechol is -~8-10 -5 M (15-fold molar excess relative to 5 gg of V), and the molar concentration of the Brilliant Green should be about half as much. The fading of the CC14 extract may be prevented by a small addition of slightly acidified ethanol.

The molar absorptivity (see the procedure below) is 1.70.105 at 630 nm (sp. abs. 3.3). The value for the blank is -0.08.

The interfering elements are Ti, Mo, W, Ga, and In. Also Fe(III), A1, and Cr(III) interfere slightly. The interfering anions are thiocyanate, EDTA and, to a lesser degree, fluoride, iodide, perchlorate, oxalate, and tartrate.

The method is specific for vanadium if the vanadium is first extracted as the BPHA- complex from a medium containing HF and H2SO4 (see the procedure).

Reagents

3,5-Dinitropyrocatechol (DNP), 8-10 -4 M (-~0.016%) solution in 20% ethanol. Brilliant Green, 8.10 -4 M (--0.04%) solution in ethanol. Standard vanadium solution: 1 mg/ml. Preparation as in Section 55.2.1. N-Benzoyl-N-phenylhydroxylamine (BPHA), 0.1% solution in CHC13.

Procedure

Extractive separation of V. Place an acidic (H2SO4) sample solution, containing not more than 7 gg of V(V), in a polypropylene separating funnel, and add 2.5 ml of conc. HF and water to about 25 ml. Mix the solution and shake with 3 portions (5-, 5-, and 2.5 ml) of BPHA in CHC13 (shaking time, 2 min). Evaporate the extract to dryness in a platinum evaporating dish, and ignite the solid residue. After cooling, add to the ashes 1 ml of 0.1 M NaOH, dilute the solution with water, heat to boiling, and cool.

Determination of V. Adjust the solution to pH 5-7, transfer it to a separating funnel,


add 2.5 ml of the DNP solution and 2.5 ml of 0.25 M H2804. Dilute with water to --20 ml, add 10 ml of CC14 and 1.2 ml of the Brilliant Green solution, and shake for 1 min. After the separation of phases, transfer the organic layer to a 25-ml volumetric flask containing 2.5 ml of ethanol and 2 drops of 0.01 M H2804. Make up to the mark with CC14, mix, and measure the absorbance at 630 nm against a reagent blank.


Besides BPHA, many other hydroxylamine derivatives have been proposed for determining vanadium, including N-cinnamoyl-N-phenylhydroxylamine (CHC13, e = 6.3.104) [34], N-benzoyl-N-(o-tolyl)hydroxylamine [35], and N-(m-tolyl)-N-phenylhydroxylamine [361.

In the group of hydroxamic acids used for determining vanadium are: benzohydroxamic acid [37], N-salicylhydroxamic acid [37,38], N-benzylbenzohydroxamic acid [39], phenylbenzohydroxamic acid [40], N-benzyl-2-naphthohydroxamic acid [41], and 5-bromosalicylhydroxamic acid [42,43].

Apart from the PAR method described above, many others are based on azo reagents. Some of these methods are very sensitive for vanadium. In this group are the pyridylazophenol derivatives [44], 2-(3,5-dibromo-2-pyridylazo)-5-dimethylaminobenzoic acid (3,5-di-Br-PAMB) (e = 5.95.104) [45], 5-Br-PADAP (e = 5.48.104) [46,46a], 2-(5- chloro-2-pyridylazo)-5-dimethylaminophenol (~ = 5.55.104) [47], N-methylanabasine-a'- azo-p-cresol (in the presence of H202) [48], 5-(8-quinolylazo)-2-monoethylamine-p-cresol [49], and Sulphonitrophenol K in the presence of NHzOH (e = 5.5.104) [50].

High sensitivity characterizes the methods based on ion-associates formed by anionic complexes of V(V) with basic dyes. The Brilliant Green method has been described above [33]. The vanadium complex with PAR associated with Crystal Violet is extracted into a mixture of benzene with MIBK (3+2) (e = 1.1.105) [51]. In a proposed flotation- spectrophotometric method, the V complex with 3,5-dinitropyrocatechol, associated with Rhodamine B, is separated by shaking the solution with cyclohexane; the separated compound is washed and dissolved in acetone (e = 2.1-105 at 555 nm) [52]. A similar sensitivity is achieved in the method using 5,7-dichloro-oxine and Rhodamine 6G [53]. Another flotation-spectrophotometric method for determining V has been based on 3,5- dinitrosalicylic acid and Rhodamine B [54].

Triphenylmethane reagents recommended for determining V include Pyrocatechol Violet (e = 3.7.104) [55-57], Xylenol Orange [58], Chrome Azurol S (CAS) [59,60], and Eriochrome Cyanine R (ECR) [60-62]. A considerable increase in sensitivity [e = (6-8). 104] has been attained by using CAS and ECR in the presence of surfactants (CTA, CP, Zephiramine) [60,61 ].

Methods with the use of fluorones also have high sensitivity. Among the reagents proposed are: phenylfluorone [63], 3'-pyridylfluorone (e = 1.25.105) [64], and salicylfluorone (in the presence of CTA, ~ = 4.8.104) [65]. The V complex with salicylfluorone has been extracted into a mixture of n-butanol with CHC13 (e = 1.2.105) [66].

Various organic reagents have found application in the determination of vanadium, namely Pyrogallol Red in the presence of surfactant [67], Bromopyrogallol Red (e = 7.104 at 580 nm) [68], Alizarin S (in the presence of CP) [69], formaldoxime [70], 2-nitroso-5- dimethylaminophenol [71], 2,2'-dipyridyl-2-quinolylhydrazone [72], pyrogallol [73-75], and carminic acid in the presence of surfactant [76].

Many photometric methods for V are based on redox reactions, and the colour resulting

462 55. Vanadium

from the oxidation of various organic compounds by V(V). Mention may be made of 3,3'- diaminobenzidine (DAB), the well-known reagent for selenium [77], 3,3'dimethylnapthidine [78], and o-phenylenediamine [79,80]. A basis of a sensitive indirect method is reduction of Fe(III) by V(II) (obtained with the Jones reductor) in the presence of ferrozine [81]. Vanadium(V) has been determined on the basis of its catalytic effect on certain reactions [82-86].

Thioeyanate gives coloured complexes with V(IV) and V(III) [87-89]. The molar absorptivity of the V(III) complex is 7.2.103 at 400 nm. Vanadium(III) may be determined in the presence of V(IV). The vanadium(W) complex can be extracted with chloroform in the presence of pyridine [87] or 4-pyridone [89].

A low-sensitivity method for determining V(V) has been based on the greenish-yellow colour of the tungstenphosphovanadic heteropoly acid, which is formed by V(V) in the presence of H3PO4 in -0.5 M H2SO4 [90-93]. The molar absorptivity of the coloured compound is 1.4.103 at 400 nm. The compound can be extracted into n-hexanol or MIBK.

A review of spectrophotometric methods for simultaneous determining V(IV) and V(V) has been given [94].


The extraction-spectrophotometric method with the use of BPHA has been applied for determining vanadium in air [95], waters [5], iron ores [96], silicates [97], ilmenite [98], and steel and cast iron [99].

The method based on the use of PAR was used for determining vanadium in biological materials [27,100], sewage [24], natural waters [26,101,102], silicate rocks [103], petroleum [104], titanium tetrachloride [25], cerium dioxide [105], steel [29], uranium alloys [31], and titanium alloys [29,31 ].

Good precision and accuracy were obtained in determinations of trace amounts (10 -5 %) of vanadium in ferri-ammonium and aluminium-ammonium alums by the method with Brilliant Green [33].

Hydroxamic acids were utilized in determinations of vanadium in steel and rocks [106,107], nickel, cast iron, and titanium alloys [40], natural waters, and petroleum [42]. The crown hydroxamic acid has been applied for determining V in ores, steel, blood, and environmental samples [ 108].

Vanadium has been determined in soils and sewage with the use of pyrogallol [75]. Phenylfluorone was used for determining V in steel [63], 1,5-diphenylcarbazide for determining V in industrial, biological, and soil samples [109], and 6-chloro-3-hydroxy-7- methyl-2-(2-thienyl)-4H-chromen-4-one for determining V in steel, flue dust, and waters Ill0].

The use of derivative spectra of the peroxide complexes of V, Ti, and Mo enables the determination of vanadium in mixtures of these metals [ 111 ].

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1. Luke C.L., Anal. Chim. Acta, 37, 267 (1967). 2. Bock R., Jost B., Z. Anal. Chem., 250, 358 (1970). 3. Suzuki N., Takahashi M., Imura H.,Anal. Chim. Acta, 160, 79 (1984). 4. Yatirajam V., Arya S.P., Talanta, 22, 861 (1975). 5. Rtiter J., Schwedt G., Z. Anal. Chem., 311, 112 (1982).

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(1980). 17. Antonovich V.P. et al., Zh. Anal. Khim., 38, 1808 (1983). 18. Kojima I., Tanaka M.,Anal. Chim. Acta, 75, 367 (1975). 19. Yatirajam V., Arya S.P., Talanta, 23, 596 (1976). 20. Karpova O.I., Lukachina V.V., Pilipenko A.T., Ukr. Khim. Zh., 39, 195 (1973). 21. Pogranichnaya R.M., Goloborod'ko U.F., Reznik B.E., Zh. Anal. Khim., 34, 917 (1979). 22. Gagliardi E., Ilmaier B., Mikrochim. Acta, 1967, 180. 23. Mushran S.P., Prakash O., Verma T.R., Bull. Chem. Soc. Jpn., 45, 1709 (1972). 24. Morgen E.A., Dimova L.M., Zh. Anal. Khim., 38, 2181 (1983). 25. Pogranichnaya R.M. et al., Zh. Anal. Khim., 30, 180 (1975). 26. Nishimura M. et al.,Anal. Chim. Acta, 65, 466 (1973). 27. Chwastowska J., Kosiarska E., Chem. Anal. (Warsaw), 30, 395 (1985). 28. Dimova L.M., Morgen Z.A., Zavod. Lab., 50, No. 10, 7 (1984). 29. Yerramilli A. et al., Anal. Chem., 58, 1451 (1986). 30. Lukanina V.V., Pilipenko A.T., Karpova O.I., Zh. Anal. Khim., 38, 86 (1973). 31. He X.,Tubino M., Rossi A. V., Anal. Chim. Acta, 389, 275 (1999). 32. Schneider J., Csanyi L.J., Mikrochim. Acta, 1976 II, 271. 33. Marczenko Z., Lobifiski R., Talanta, 35, 1001 (1988). 34. Gusakova N.N. et al., Zh. Anal. Khim., 30, 721 (1975). 35. Jegffery P.G., Kerr G.O.,Analyst, 92, 763 (1967). 36. Inoue S., Hoshi S., Matsubara M., Talanta, 33, 611 (1986). 37. Grigor'eva M.F. et al., Zh. Anal. Khim., 34, 2171 (1979). 38. Chatterjee A.B. et al., Mikrochim. Acta, 1983 III, 307. 39. Nanewar R.R., Tandon U., Talanta, 25, 352 (1978). 40. Chivireva N.A., Chukhrii Yu.P., Antonovich V.P., Zh. Anal. Khim., 45, 1909 (1990). 41. Sahu B., Tandon U., Talanta, 34, 653 (1987). 42. Pascual-Reguera M.I., Molina-Diaz A., Ramos-Martos N., Anal. Lett., 24, 2245 (1991). 43. Capitan-Vallvey L.F. et al., Analusis, 17, 280 (1989). 44. Kiss E., Anal. Chim. Acta, 77, 205 (1975). 45. Katami T. et al.,Analyst, 109, 461 (1984). 46. Krasiejko M., Marczenko Z., Mikrochim. Acta, 1986 III, 89. 46a. Spimola Costa A. C. et al., Mikrochim. Acta, 130, 41 (1999). 47. Olsina R.A., Marchevsky E.J., Kamachi E., Microchem. J., 36, 348 (1987). 48. Krukovskaya E.L., Talipov S.T., Rybikova V.V., Zh. Anal. Khim., 37, 446 (1982). 49. Dimova L.M., Morgen E.A., Zh. Anal. Khim., 41, 1832 (1986). 50. Savvin S.B. et al., Zh. Anal. Khim., 26, 2364 (1971); 27, 1972 (1972). 51. Minczewski J., Chwastowska J., Pham Hong Mai, Analyst, 100, 708 (1975). 52. Lobifiski R., Marczenko Z., Anal. Sci., 4, 629 (1988).

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Chapter 56. Zinc

Zinc (Zn, at. mass 65.37) occurs exclusively in the II oxidation state. The hydroxide, Zn(OH)2 is precipitated at pH 7; in excess of alkali it dissolves to form tetrahydroxozincate anions. It is readily soluble in ammonia, yielding the ammine complex. White zinc sulphide begins to precipitate at pH 1.2. Zinc forms stable complexes with cyanide and EDTA; its chloride and thiocyanate complexes are rather weak.


56.1.1. Extraction

Among numerous extractive methods for separation of zinc, the best seems to be that in which zinc, along with the traces of other heavy metals, is extracted with dithizone [1-3] (see below for details).

The thioeyanate complex of zinc can be extracted by isoamyl alcohol, MIBK, or other oxygen-containing solvents, from acidic solutions [4-6]. Zinc can also be separated as the anionic thiocyanate complex by non-polar solvents (benzene, CHC13) in the presence of TBP, pyridine, tribenzylamine, Aliquat 336, or Amberlite LA-1 [7-9]. It is possible to separate Zn from Cd. The extraction efficiency increases in the presence of DMF or DMSO [10].

The chloride complex of Zn can be extracted from 1-5 M HC1 media, in the presence of such amines as TOA in xylene [11], trichloroethylene [12], CC14 [13], benzene [14] or dioctylmethylamine in xylene [ 15]. Extraction with trilaurylammonium chloride solution in toluene has also been applied [16].

Zinc can be separated from Cd and Pb in the iodide system [17,18]. It can also be separated from other metals as the chelate complex with oxine (CHC13, 1,2-dichloroethane) [19-22], diphenylcarbazone [23] HTTA [24], and N-phenylcinnamoylhydroxamic acid (chloroform in the presence of stearylamine) [25]. Zn has been separated from the excess of Cd with the use of 1-phenyl-3-methyl-4-stearoyl-5-pyrazolone in benzene solutions [26].


In ion-exchange methods, use is made of differences in the stability of the chloride complexes of zinc and of other metals [27,28]. Strongly basic anion-exchangers sorb Zn from 2 M HC1 medium. Zinc is eluted from the column with 0.001 M HC1.

Zinc and cadmium have been retained on the anion-exchanger Amberlite IRA-400 by passing through the column a solution in 2 M HC1. Cadmium was fixed on the column with hydriodic acid and Zn was eluted with 0.25 M HNO3. In acidic media containing iodide and sulphate, the anion-exchanger retains only Cd, and Zn passes to the eluate. When Zn and Cd are retained, as the chloride complexes, on a Dowex 1 column, Zn can be eluted with 2% NaC1 in 2 M NaOH, and then Cd can be washed out with 1 M HNO3. Zn has also been retained on a strongly basic anion exchanger (from acid thiocyanate solution) and then eluted with 0.15 M HBr [30].

Small amounts of zinc can be separated from solution as the sulphide, in a slightly


acidic tartrate medium. As the collector, Cd, Cu, and Hg are most often used. Mercury is easily removed by igniting the precipitate. Traces of zinc are separated from cadmium by complexing the Cd with iodide, while the Zn is precipitated at pH 8.3-8.5 as the hydroxide together with lanthanum as the collector [31 ]. For isolating zinc as the hydroxide from a not- too-alkaline medium, Mg(OH)2 has been used as collector. In this way, zinc can be separated from larger quantities of Sb and Sn. Zinc has also been coprecipitated with Fe(OH)2 as collector.


The sensitive and very selective dithizone method is most often used for spectrophotometric determination of zinc. Recently, the importance of azo reagents for the determination has increased. Comparative investigations have been made of the more important methods [32,33].


The dithizone method is very sensitive. The molar absorptivity of zinc dithizonate in CC14 solution (at)Vma~ 538 nm) is 9.26.104 (a = 1.42). If the correct pH and masking agents are used, the dithizone method is specific for zinc [34,35].

On shaking an aqueous zinc solution (pH 4-11) with a CC14 solution of dithizone (formula 4.37), zinc dithizonate, Zn(HDz)2, is formed and the organic layer changes colour from green to pink. The extraction is relatively slow.

Thiosulphate is most commonly used as a masking agent. At pH 4.0-5.5 (acetate buffer), thiosulphate forms stable complexes with Cu, Ag, Hg, Bi, Pb and Cd, thus preventing the reactions of these metal ions with dithizone. Thiosulphate also masks small quantities of nickel and cobalt. At higher concentrations of those metals, it is advisable to add small amounts of cyanide as masking agent. In the dithizone method for zinc, iodide, thiourea, and dithiocarbamates can also be used for masking interfering metals [36-38].

When there is more cadmium than zinc present in the solution analysed, the former is slightly extracted in spite of the masking effect of thiosulphate. Cadmium can be scrubbed from the extract by shaking it with a dilute Na2S solution, cadmium sulphide being more stable than cadmium dithizonate. In the presence of Fe, A1, Ti, and other easily hydrolysed metals, the extraction of zinc from acetate medium should be preceded by the addition of tartrate or citrate.

The zinc can be stripped from the CC14 layer by shaking with 0.01-0.02 M mineral acid. The accurate determination of zinc requires a double extraction to be applied. The first extract is stripped with dilute acid, the resulting aqueous solution is treated with acetate buffer and thiosulphate, and the extraction with dithizone is repeated.

The excess of dithizone is removed from the extract by shaking it with very dilute ammonia solution. However, when determining trace amounts of zinc, it is important to remember that traces of this metal can often be found in ammonia solutions. For the elimination of free dithizone, the Zn(HDz)2 extract is washed with thiosulphate solution to decompose the dithizonates of other metals present in the extract.

From the carbon tetrachloride extract containing Zn, Cd, Ni, and Co (all of which form soluble ammine complexes), Zn and Cd can be separated quantitatively by utilizing the differences in resistance of the four metal dithizonates to acid treatment. Ni and Co dithizonates begin to form at pH--4, but once formed are rather hard to decompose even

468 56. Zinc

with quite concentrated mineral acids. This is not the case with Zn and Cd dithizonates, which are easily decomposed by dilute HC1. Shaking the CC14 extract with two portions of dilute HC1 (pH 1.5) for 30 s transfers Zn and Cd quantitatively to the aqueous phase, leaving Ni and Co dithizonates in the CC14 phase. After the reaction with the first portion of dilute HC1, the dithizone from the decomposition of Zn and Cd dithizonates should be removed from the CC14 phase with dilute ammonia solution [2,3].

The determination of Cd with dithizone enables one to avoid the extraction with the use of surfactants [39-41 ].

Reagents

Dithizone, 0.002% solution in CC14. Preparation as in Section 46.2.1. Standard zinc solution: 1 mg/ml. Dissolve 1.0000 g of zinc metal in 15 ml of HC1 (1 + 1)

and dilute the solution with water to 1 litre. Acetate buffer, pH 5. Dissolve in water 50 g of anhydrous sodium acetate and 30 g of

glacial acetic acid and dilute with water to 250 ml. Purify the solution from traces of metals by shaking with portions of dithizone solution in CC14. Keep the solution in a polyethylene bottle.

Sodium thiosulphate, 10% solution. Purify the solution from traces of metals by shaking with dithizone solution in CC14. Keep the solution in a polyethylene bottle.

Wash solution: 10 ml of solution of the acetate buffer and 10 ml of thiosulphate solution diluted with water to 100 ml. The solution should be prepared just before use.

Ammonia solution. Saturate water, distilled twice in quartz apparatus, with ammonia gas from a cylinder. Use a polyethylene bottle and cool the water in an ice-water bath.

Procedure

Place the slightly acid (pH 2-3) solution containing not more than 10 ~tg of Zn in a volume not larger than 25 ml, in a separating funnel, add 5 ml of acetate buffer and 5 ml of thiosulphate solution, and shake with portions of the dithizone solution in CC14 (1 ml of 0.002% HzDz solution corresponds to 2.6 lag of Zn) until the green CC14 layer no longer changes colour. Each shaking should last not less than 2 min. Shake the combined extracts with two 5-ml portions of wash solution. Wash out free dithizone from the CC14 layer with dilute ammonia (1 drop of conc. NH3 solution in 25 ml of water). Dilute the pink solution of Zn(HDz)2 with CC14 in a 25-ml standard flask and mix well. If the solution is turbid, filter it through a paper filter previously washed with the dilute dithizone solution and CC14. Measure the absorbance at 538 nm, using the solvent as reference.

Note. In the determination of traces of zinc it is essential to take into account a blank test of the content of zinc in the reagents, water, and vessels.

56.2.2. Pyridylazonaphthol (PAN) method

1-(2-Pyridylazo)-2-naphthol (formula 4.1) forms with zinc ions (at pH 5-11) a red chelate which constitutes the basis of the spectrophotometric method for determining zinc. The chelate can be extracted with chloroform or other solvents. The reagent, also soluble in non- polar solvents, is yellow and absorbs only a little at the ~max of the zinc complex [ 15,42,43].

The molar absorptivity of the chloroform solution of the zinc-PAN complex is 5.2.10 4 (a = 0.79) at 560 nm.


The selectivity of the method can be increased by prior separation of the zinc, e.g., by its extraction (with benzene or xylene) as the chloride complex in the presence of high molecular-weight amines [44].

Interfering metals [e.g., Fe(III), Cd, Cu, Pb, Hg, Mo] can be masked with iodide, thiosulphate, or tartrate. Iron(HI), which interferes, is reduced to Fe(II) by means of ascorbic acid.

If Zn is extracted with TOA in benzene, PAN can be added directly to the extract [43]. In another method [25], the solution of PAN in ethanol is added to the chloroform extract of Zn combined with hydroxamic acid and stearylamine (e = 5.8-104 at 550 nm). More often, Zn is stripped from the extract with alkaline solution, then PAN is added, and the complex is extracted with chloroform (see procedure). The colour of the chloroform extract is stable.

The distribution coefficients for the extraction of Zn with TOA in benzene from HC1 medium, and for the extraction of the zinc-PAN chelate with chloroform from alkaline solution (pH 10-11) are so high that a single extraction is usually sufficient.

Zinc has been determined with PAN in the presence of the non-ionic surfactant, polyoxyethylene nonyl phenyl ether [45].

Derivative spectrophotometry has been used for determining Zn in the presence of Ni and Cu with the use of PAN [46].

Reagents

1-(2-Pyridylazo)-2-naphthol (PAN), 0.1% solution in ethanol. Standard zinc solution: 1 mg/ml. Preparation as in Section 56.2.1. Tri-n-octylamine (TOA), 2% solution in benzene.

Procedure

Extractive separation of Zn. To the sample solution add 1-50 mg of ascorbic acid [according to the amount of Fe(III) present in solution] and hydrochloric acid to reach a final concentration of 2 M HC1 in solution. Shake the solution with two portions of TOA in benzene for 1 min each time. Wash the extract by shaking with 2 M HC1. Strip the zinc with 10 ml of 0.5 M NaOH solution (shaking time 1 min).

Determination of Zn. To the alkaline solution (obtained as above) containing not more than 25 ~g of Zn, add 0.5 g of ammonium chloride, 2 ml of PAN solution, and shake with 2 portions of chloroform (shaking time 30 s). Transfer the extract to a 25-ml standard flask, and dilute to the mark with CHC13. Mix well and measure the absorbance of the solution at 560 nm against a reagent blank.


Besides PAN, numerous other pyridylazo reagents have been proposed for spectrophotometric determination of Zn. This group comprises the related reagent PAR [14,47-50]. The complex of Zn with PAR is extracted with chloroform in the presence of cetyldimethylbenzylammonium chloride (e = 9.1-104) [51]. Zinc (and other heavy metals) are preconcentrated on a cation-exchanger, modified with PAR [52]. Bromo- and chloro- derivatives of pyridylazo compounds have become a basis for more sensitive methods, e.g., 5-Br-PADAP, (formula 4.3) [53,54], 2-(3,5-dibromo-2-pyridylazo)-5-diethylaminophenol (3,5-diBr-PADAP) (e = 1.3-102 at 570 nm) [55], and other bromine derivatives [56]. Application of the chlorine derivatives of pyridylazo reagents has been reviewed [53,57,58].

470 56. Zinc

Other azo reagents proposed for the determination of zinc include TAN [59,60], TAR [61,62], Arsenazo III (~ = 3.8-104) [63], Sulpharsazen [64], and Cadion 2B (in the presence of the non-ionic surfactant Triton X-100) (E = 1.0.105 at 524 nm) [65].

Zincon (2-carboxy-2'-hydroxy-5'-sulphoformazylbenzene, formula 56.1) is a well known reagent for zinc [66,67], which forms in slightly alkaline solution a blue complex with zinc (Xmax 625 nm, ~ = 2.0.104) with zinc. Many metals interfere in the colour reaction, making it necessary to separate the zinc first.

OH HOOC

S~H

(56.1)

The anionic zinc-thiocyanate complex forms ion-pairs with basic dyes, extractable by e.g., benzene or CC14 [68]. Use has been made of Rhodamine B [69], Victoria Blue B [70], or Malachite Green [89], with ~ values within (5-12).104 [71].

Use has been made of ion-pairs formed by the cationic complex of zinc with 1,10- phenanthroline or 2,2'-dipyridyl and acid dyes, such as eosin, Erythrosin, Rose Bengal, dibromofluorescein (xanthene dyes) [72-74], Bromophenol Blue, Bromophenol Red (triphenylmethane dyes) [75,76]. In some of these methods, molar absorptivities are ~-105. Chloroform is the usual extraction solvent.

Among sulphur-containing organic reagents proposed for determination of Zn are: 1-(2-pyridylmethylidene)-5-(salicylidene)thiocarbohydrazone (e = 6.8.104) [77], methyl- glyoxalbis(4-phenyl-3-semicarbazone) [78], and 1-(2-pyridylmethylidene)-5-salicylidene- thiocarbonhydrazide (pH 5.6; 30% DMF; ~ = 6.1.104 at 417 nm) [79]. Zn was determined in the presence of Cd and Cu with the use of 1,5-bis(di-2-pyridylmethylene)-thiocarbonhydrazide [80].

Other organic reagents include phenylfluorone [81-83], 1,5-diphenylcarbazone (~ = 5.8.104 at 520 nm) [84], 2,2'-dipyridylbis(2-quinolylhydrazone) [29], tetrakis(1- methylpyridinium-3-yl)porphyrin [85], and tetrakis(5-suphophenyl)porphyrin (~ = 4.4.105) (a large increase in sensitivity has been obtained by the use of the 2 nd order derivative) [86]. Traces of Zn were determined in the presence of large amounts of Pb(II) with the use of tetrakis(4-sulphophenyl)porphyrin [87]. Zinc can be determined in the presence of copper at pH 4.2 with the use of tetrakis(3-chloro-4-sulphophenyl)porphyrin [88].


The dithizone method has been applied in the determination of zinc in sewage [89], cadmium and its compounds [31], aluminium [90,91], nickel [11], cobalt [12], gold [92], germanium compounds [37], and tellurium [93]. Zinc was determined with dithizone in aqueous media in the presence of dodecyl sulphate using the FIA technique [94].

The PAN method was used for determining zinc in blood serum [95], environmental samples [96,97], sewage [98], pharmaceutical products [99], aluminosilicates and iron ores [43], nickel and its alloys [15,42,100], and copper [42,101].

Zincon was applied for determination of zinc in blood serum [102,103], plants [104],

References 471

hair [ 102,105], natural waters [28,66,67], and alloys [67]. This reagent has also been used in the flow injection technique [67,102-105].

Zinc metal can be determined in zinc oxide by means of indirect methods. Zinc is oxidized with dichromate (in an H2SO4 or H3PO4 medium), and then the unreacted part of Cr(VI) is determined with 1,5-diphenylcarbazide [106]. In another version, the sample is dissolved in dil. sulphuric acid in the presence of Fe(III) and 1,10-phenanthroline. The red Fe(II)-phen complex is equivalent to the content of Zn in the zinc oxide studied [ 107].

Zinc was determined in environmental samples [108] and in copper selenide [ 109] with the use of PAR. Zn was determined in brass (in the presence of Ni) by derivative spectrophotometry with the use of 2-(2-pyridylmethylene-amino)phenol [110]. The thiocyanate complex of Zn associated with Tetrazolium Violet has been applied for determining Zn in cadmium [ 111 ].

References

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35. Komar N.P., Manzhelii L.S., Gorodilova L.I., Zh. Anal. Khim., 29, 2391 (1974). 36. Hulanicki A., Minczewska M., Talanta, 14, 677 (1967). 37. Galik A., Talanta, 14, 731 (1967). 38. Budevsky O., Russeva E., Stoytcheva R., Talanta, 19, 937 (1972). 39. Privalova N.V. et al., Zavod. Lab., 54, No. 10, 10 (1988). 40. Paradkar R.P., Williams R.R., Anal. Chem., 66, 2752 (1994). 41. Abdel-Latif M.S., Anal. Lett., 27, 2341 (1994). 42. Berger W., Elvers H., Z. Anal. Chem., 171,255 (1959); 199, 166 (1964). 43. Bykhovtsova T.T., Zavod. Lab., 40, 512 (1974). 44. Galban J. et al., Microchem. J., 40, 94 (1989). 45. Watanabe H., Tanaka H., Talanta, 25, 585 (1978). 46. Gallardo Melgarejo A. et al.,Analyst, 114, 109 (1989). 47. Goldstein G., Maddox W.L., Kelley M.T., Anal. Chem., 46, 485 (1974). 48. Ahrland S., Herman R.G., Anal. Chem., 47, 2422 (1975). 49. Kolomiets L.L., Pyatnitskii I.V., Zh. Anal. Khim., 35, 633 (1980). 50. Lampugnani L. et al., Anal. Lett., 23, 1665 (1990). 51. Nonova D., Nenov V., Lihareva N., Talanta, 23, 679 (1976). 52. Pilipenko A.T., Safranova V.G., Zakrevskaya L.V., Zh. Anal. Khim., 44, 1594 (1989). 53. Macka M., Kubafi V., Coll. Czech. Chim. Comm., 47, 2676 (1982). 54. Nonova D., Stoyanov K., Mikrochim. Acta, 1984 I, 143. 55. Tan Z., Shui-Sheng W., Talanta, 31, 624 (1984). 56. Horiguchi D., Saito M., Noda K., Kina K., Anal. Sci., 1, 461 (1985). 57. Shibata S., Furukawa M., Sasaki S.,Anal. Chim. Acta, 51,271 (1970). 58. Chauhan O.S. et al., Talanta, 28, 399 (1981). 59. Kawase A., Talanta, 12, 195 (1965). 60. Ferreira S.L.C. et al., Mikrochim. Acta, 118, 123 (1995). 61. Marshall B.S., Telford J., Wood R., Analyst, 96, 569 (1971). 62. Evans W.H., Sayers G.S.,Analyst, 97, 453 (1972). 63. Michaylova V., Yuronkova 1., Anal. Chim. Acta, 68, 73 (1974). 64. Tananayko M.M., Popovich N.V., Ozerova N.L., Ukr. Khim. Zh., 48, 505 (1982). 65. Nai-kui Shen et al.,Analyst, 112, 301 (1987). 66. Matsui H., Anal. Chim. Acta, 66, 143 (1973). 67. Koupparis M.A., Anagnostopoulon P.I.,Analyst, 111, 1311 (1986). 68. Kish P.P., Bazel' Ya.R., Studeniak Ya.I., Zh. Anal. Khim., 47, 1240 (1992). 69. Tvaroha B., Malfi O., Mikrochim. Acta, 1962, 634. 70. Pilipenko A.T., Kish P.P., Zinomrya I.I., Ukr. Khim. Zh., 37, 186 (1971). 71. Kish P.P., Zinomrya I.I., Zolotov Yu.A., Zh. Anal. Khim., 28, 252 (1973). 72. Tananayko M.M., Bilenko N.S., Zh. Anal. Khim., 30, 689 (1975); Zavod. Lab., 42, 1161

(1976). 73. Stolyarov K.M., Firyulina V.V., Zh. Anal. Khim., 33, 2102 (1978). 74. Ishak C.F., Pflaum R.T., Analyst, 113, 941 (1988). 75. Shestidesyatnaya N.L., Voronich O.G., Motyl' V.A., Zh. Anal. Khim., 32, 260 (1977). 76. Buhl F., Mikula B., Chem. Anal. (Warsaw), 26, 571 (1981). 77. Rosales D., Gomez-Ariza J.L.,Anal. Chim. Acta, 169, 367 (1985). 78. Herrador M.A., Jimenez A.M., Asuero A.G., Analyst, 112, 1237 (1987). 79. Morales T., Montana T., Galan G., Gomez Ariza J.L.,Analyst, 112, 467 (1987). 80. Garcia Rodriguez A.M. et al., Talanta, 40, 1861 (1993). 81. Sakuraba S., Talanta, 31, 840 (1984). 82. Winkler W., Arenh6vel-Pacula A., Chem. Anal. (Warsaw), 44, 725 (1999).

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(1978). 101. Koshturyak A., Ukr. Khim. Zh., 47, 983 (1981). 102. Liu R., Liu D., Sun A., Talanta, 40, 381, 511 (1993). 103. Hernandez O., Jimenez A.I., Jimenez F., Arias J.J., Anal. Chim. Acta, 310, 53 (1995). 104. Ferreira J.R., Zagatto E.A., Arruda M.A., Brienza S.M., Analyst, 115, 779 (1990). 105. Liu R.M., Liu D.J., Sun A.L., Liu G.H., Analyst, 120, 569 (1995). 106. Norman V.J., Analyst, 89, 261 (1964). 107. Kruse J.M., Anal. Chem., 43, 771 (1971). 108. Chakravarty S., Mishra R.K., Fresenius'J. Anal. Chem., 345, 613 (1993). 109. Ermakova L.V., Podgornaya V.A., Zavod. Lab., 54, 30 (1988). 110. Grabaric Z., Lazarevic Z., Koprivanac N.,Analyst, 119, 1099 (1994). 111. Kamburova M., Aleksandrov A., Zh. Anal. Khim., 46, 80 (1991).

Chapter 57. Zirconium and hafnium

Zirconium (Zr, at. mass 91.22) occurs in its compounds exclusively in the IV oxidation state; it is similar to titanium in its properties. In HNO3 and HC104 solutions, zirconium Zr 4+ and zirconyl ZrO 2+ ions occur. They tend to polymerize as the concentration increases. Hydrolysis of zirconium ions begins at pH 1-1.5. Zr(OH)4 has no amphoteric properties. Zirconium forms stable complexes with fluoride, EDTA, and hydroxy acids. Hafnium (Hf, at. mass 178.49) is much the same as zirconium in chemical properties. It usually accompanies zirconium to the extent of 1.5-2%. The methods for separation and determination of zirconium outlined below, also apply for hafnium.

57.1. Methods for separation and preconcentration

Methods of separation of Zr and Hf and methods for determination of small amounts of hafnium in zirconium, and conversely, have been reviewed [1 ].


For precipitation of zirconium as the hydroxide, Fe(III) or Ti is used as scavenger [2]. Depending on the elements from which the zirconium is to be separated, Zr(OH)4 is precipitated with either ammonia or NaOH solution. Zirconium is separated from titanium by precipitation in the presence of H202. If zirconium phosphate is dissolved in oxalic acid and the solution made alkaline with NaOH solution, Zr(OH)4 is precipitated (more sparingly soluble than the phosphate) [3].

After fusion of the sample or of the ignited hydroxide or phosphate precipitate with Na2CO3 or NaOH + Na202, followed by leaching of the melt with water, zirconium is in the precipitate, whereas phosphate, fluoride, and sulphate, as well as As, V, Cr, Mo, W, and A1 remain in anionic forms in the solution.

Zirconium can be co-precipitated with various metals as the arsenate [4]. Cupferron [with Fe(III) as a carrier] [5], oxine [6] and p-bromomandelic acid [7] have been used for selective separation of Zr (Hf).

Extraction of zirconium with thenoyltrifluoroacetone (HTTA) (formula 1.7) in xylene or benzene from 4-6 M HC1 or 3-4 M HC104 medium enables it to be separated from many ions, including Ti, REE, Th, A1, U, and Fe(III) [8].

Zirconium and hafnium can be extracted selectively from highly acidic media (HC1, HC104) with BPHA in CHC13 or benzene [9,10].

Zirconium (hafnium) have been separated from other metals with the use of TOPO (in cyclohexane or toluene) [11,12], TBP (in benzene) [13], or alkylphosphoric acids (toluene, benzene) [14,15] from HNO3, HC1, or H2SO4 solutions.

Zr (and Hf) has been extracted as the salicylate with the use of the amines TOA and Aliquat 336 in xylene [16]. Zr and Hf were extracted from various media with TOA in cyclohexane [17] and Aliquat 336 (from 9 M HC1) in cyclohexane (in the presence of 3% octanol) [18]. A mixture of Aliquat 336 and Alamine 336 with TBP was used for extracting Zr from chloride media [19]. Hafnium has been separated from zirconium with the use of Amberlite LA-1 in xylene in citrate medium (pH 4.5) [20].


Zr has been separated from Hf with the use of TBP in 4 M HNO3 [21 ]. These metals can also be separated in thiocyanate medium with the use hexanone as extractant. Ion-pairs of thiocyanate complexes of Zr and Hf with antipyrine and DAM were separated by extraction (isoamyl alcohol, 1,2-dichloroethane) from other metals [22]. Zirconium can be extracted first with mesityl oxide from a 4 M solution (in HNO3 and NaNO3), then hafnium is extracted from 0.4 M HNO3 and 2 M NH4SCN medium [23].

N-Phenylbenzylhydroxamic acid [24] and crown ethers [25] were applied for the extraction of Zr. Zirconium has been extracted from transition metals with the use of [2.2.2]- cryptand in CHC13 [26]. Zr (Hf) has also been extracted with tris(2-ethylhexyl)phosphate [27] and dibutyl sulphoxide (in xylene) [28].


Anion-exchangers are used to separate Zr (and Hf) as anionic complexes with fluoride [29], oxalate [30], sulphate [2], and chloride [31 ]. Most metals are separated from Zr and pass into the eluate. Those that are retained by anion-exchanger together with Zr can be separated from zirconium by use of the appropriate eluents [29]. To separate zirconium and titanium from molybdenum, advantage is taken of the higher stability of the molybdenum oxalate complex, which remains on the column while Zr and Ti are eluted with dilute H2SO4 [30]. Micro-amounts of Hf were separated as the fluoride complex from the elements of groups I, II, and III by means of the anion-exchanger, Dowex l-X8. Hafnium was eluted with 1 M acetic acid and 2 M HC1 [32]. Micro-amounts of Hf and Ta were also retained on anion- exchanger columns and separated by elution with HC1 and HF solutions of different concentrations [33].

Zirconium (and hafnium) have been separated from other metals by means of strongly acidic cation-exchangers, use being made of HC1 [34], HC104 [35], and oxalic acid [36] media. In formic acid medium, metal ions form positive ions, except for Zr which produces anionic complexes [37]. Zirconium has been separated from hafnium on cation-exchangers by virtue of the differences in stability of their formate [38] and sulphate [39] complexes. Chelating resins have also been applied for separation of Zr and Hf [39].

Trace amounts of Zr and Hf have been concentrated on silica impregnated with an organic compound with sulphonic groups [40]. Polyurethane foam has also been used for preconcentration of Zr and Hf as thiocyanate complexes [41 ].

Ion-exchangers used for separation of Zr, Nb, Mo, Hf, Ta, and W [42], and sorbents Polyorgs XXV used for selective preconcentration of Zr and Hf [43] have been reviewed.


The old Alizarin S method of low sensitivity is still of use. High sensitivity methods are those based on Xylenol Orange and Arsenazo III. The latter is also highly selective.

57.2.1. Alizarin S method

Alizarin S (Alizarin Red S, formula 57.1) reacts with Zr (Hf) ions in acid medium (pH 0.5- 1.0) to form a purple-red compound which is sparingly soluble in water. In the presence of a protective colloid, it forms a pseudo-solution. The reagent is soluble in water to give a yellow colour. In the spectrophotometric method for determining Zr [44-46] the colour reaction is carried out in 0.1-0.2 M HC1, HNO3 or HC104. In such media, the effect of other

476 57. Zirconium and hafnium

metals is fairly insignificant and the method exhibits considerable selectivity.

0 OH OH

S03H 0

(57.1)

The molar absorptivity at ~max : 520 nm is 7.0.103 (a = 0.08). The colour develops in 15 min and is stable for at least 1 h.

Moderate quantities (1-10 mg) of A1, Fe(III), Ti and Th do not interfere. Large amounts of Fe(III) should be reduced to Fe(II), best with ascorbic acid. Fluoride, phosphate, and large quantities of sulphate prevent the colour reaction.

Reagents

Alizarin S: 0.05 % aqueous solution. Dissolve 50 mg of the reagent in 100 ml of water. Standard zirconium solution: 1 mg/ml. 1. Dissolve 3.9 g of ZrOClz.8H20 in 2 M HC1 and dilute the solution with the acid to 1

litre. Standardize gravimetrically an aliquot sample by precipitation with ammonia and ignition to ZrO2. Dilute the solution with 2 M HC1 to a Zr concentration of exactly 1 mg/ml.

2. Dissolve 0.1000 g of metallic zirconium by heating with 10 ml of conc. H2804 and 1 ml of conc. HF to fumes in a platinum vessel. Allow to cool, rinse the walls of the vessel with water and evaporate again to white fumes. Cool the residue, add 25 ml of water, heat until clear, and dilute the solution with water to volume in a 100-ml standard flask.

Obtain working solutions by suitable dilutions of the stock solution with 1 M HC1 or HNO3.

Procedure

To the sample solution (0.1 M with respect to HC1) in a 25-ml standard flask, containing not more than 0.3 mg of Zr, add 1 ml of 1% gum arabic solution, 2.5 ml of Alizarin S solution, and 0.1 M HCI to the mark. Mix and let the solution stand for 15 min. Measure the absorbance at 520 nm, using a reagent blank as reference.


Xylenol Orange (XO) (formula 4.19) reacts in acid medium with Zr(Hf) ions to form a purple-red water-soluble complex, which has been recommended as a basis for determination of Zr and Hf [20,47-50]. The intensity of the colour depends on the type and concentration of the acid in the solution. The most intense colour is obtained in 0.5-1 M HC104 and 0.5-0.8 M HC1 media.

The molar absorptivity of the Zr-XO complex in 0.8 M HC104 solution is 3.5.104 (a = 0.38) at 535 nm. The absorption maximum of XO in this medium is at 440 nm. In the determination of zirconium with XO, fluoride, phosphate, and oxalate interfere. Larger amounts of sulphate reduces the colour intensity of the complex. Under the conditions given below, the following amounts of the species shown do not interfere: 5 mg of Fe(II), 5 mg of A1, 1 mg of Ti, and 0.5 mg of Th. Iron(III) should be reduced before the colour development. Larger amounts of Bi, Sn(H), Mo, and Nb interfere. XO does not react with Zr in the


presence of H202. Hafnium is incompletely masked with H202, which enables Zr to be distinguished from Hf in the Xylenol Orange method [47,48].

The sensitivity of the method can be increased by addition of gelatine (e = 7.5.104) [51 ]. Hafnium has been extracted with CHC13 from HC1 medium as the chelate complex with N- tolyl-p-methoxybenzohydroxamic acid, and the XO added has given a mixed-ligand complex (E = 8.4-104) [52]. In a similar way Zr was determined with the use of tri-n- butylacetohydroxamic acid [53].

Simultaneous determination of Zr and Hf with XO has been described. The absorbance measurements are carried out at three different acidities, chosen such that both the Hf and Zr complexes are dissociated to various degrees [54].

Reagents

Xylenol Orange (XO), 0.05% solution. Dissolve 50 mg of the reagent in 100 ml of water. Standard zirconium solution: 1 mg/ml. Preparation as in Section 57.2.1.

Procedure

Dissolve in hot --2 M HC104 or HC1 the precipitate containing Zr(OH)4 (either the precipitate which remains undissolved on leaching the sodium carbonate melt with water, or the precipitate obtained from the sample solution with sodium hydroxide). Dilute the cooled solution so as to make the concentration of HC104 within 0.8_+0.1 M, or of HC1 within 0.6_+0.1 M.

Place the solution containing not more than 30 gg of Zr in a 25-ml standard flask, add 1 ml of 1% ascorbic acid solution, and 1 ml of XO solution, dilute the solution with 0.8 M HC104 or 0.6 M HC1 to the mark, and mix well. After 10 min measure the absorbance at 535 nm, using a reagent blank solution as reference.


Zirconium (like Hf) reacts with Arsenazo HI (formula 4.10) in 2-10 M HC1 medium to form an emerald-green water-soluble complex. At this acidity hydrolysis and polymerization of zirconium ions no longer occur, which secures good reproducibility.

The sensitivity of the method depends very much on the acidity of the medium [55-57]. The maximum colour intensity can be obtained in 9-10 M HC1 (with an excess of Arsenazo III). As the HC1 concentration is reduced, the colour intensity diminishes, in 2-4 M HC1 being only 1/3 to 1/2 of that obtained in 8-10 M HC1. In the media of higher HC1 or HC104 concentration both Zr and Hf give the complexes of a metal: reagent ratio of 1:2.

The molar absorptivity of the Zr-Arsenazo III complex in 9 M HC1 medium at ~max --

665 nm is --1.2.105 (specific absorptivity 1.30). At this wavelength the excess of Arsenazo III does not absorb (see Fig. 57.1).

Over the acidity range 2-10 M HC1 only Th and U(IV) interfere; other metals do not react with Arsenazo III in strongly acid medium. In most materials, therefore, Zr can be determined in the solution immediately after the sample has been dissolved. If there is a need to separate the Zr, Ti can be used as the collector in precipitation since it does not interfere in the colour reaction.

Sulphate and phosphate have little effect on the colour reaction, but fluoride and oxalate must be absent.


500 530 wavele6~Oh,nm 665 700

Fig. 57.1. Absorption spectra of Arsenazo III (i) and its complex with Zr in 9 M HCI

Zirconium (hafnium) can be determined with Arsenazo III directly in the extract of the thiocyanate complex with antipyrine in isoamyl alcohol [22]. The fluoride complex of zi rconium (ZrF62-) has been extracted with TOA in benzene, then the extract has been shaken with Arsenazo III solution [58]. Zirconium has also been determined after froth-flotation of the Zr compound with Arsenazo HI and Zephiramine [59].

Zirconium can be determined as a ternary complex Zr-Mo-Arsenazo HI formed in the presence of an excess of molybdate at pH ~-3.1 (e = 6.4.104 at 658 nm). The above- mentioned complex has also been used in the method based on the 4 th order absorption spectrum, which gives a considerable increase of sensitivity [60].

Zirconium has been determined in the presence of Th (as the complex with Arsenazo HI) by using 2 nd order derivative spectrophotometry [61 ].

Reagents

Arsenazo III, 0.05% solution. Dissolve 50 mg of the reagent in 100 ml of water. Standard zirconium solution: 1 mg/ml. Preparation as in Section 57.2.1.

Procedure

To the acid sample solution in a 25-ml standard flask, containing not more than 20 gg of Zr, add 2 ml of Arsenazo III solution and 20 ml of conc. HC1, dilute with water to the mark, and mix well. Measure the absorbance at 665 nm, using a reagent blank as reference.

Notes. 1. On account of the high concentration of HC1 in the sample solution the cuvettes should be suitably covered before being placed in the spectrophotometric apparatus.

2. If the highest sensitivity is not needed, 6 M HC1 medium can be used.


A large group of reagents for spectrophotometric determination of Zr (Hf) are the azo reagents, but they are all inferior to Arsenazo HI in both sensitivity and selectivity. Mention may be made of Thoron [31], PAN [11,14,62], PAR (e = 6.6.104) [63,64], chloro-derivatives of PAR [65], TAN (E = 5.3.104) [66,67], 5-Br-PADAP (formula 4.3) (e = 1.5.105) [68], 2-(6- bromo-2-benzothiazolylazo)-5-diethylaminophenol (with lauryl sulphate) [69], and 3,4- dihydroxyazobenzene [70].

Besides Xylenol Orange (discussed above), other triphenylmethane reagents have been proposed for determining Zr (HI), such as Pyrocatechol Violet [71,72], Methylthymol


Blue [73,74], and Chrome Azurol S [75]. 2,3,7-Trihydroxyfluorones have given a basis for sensitive methods of determining Zr

and Hf. To mention a few, these are: o-nitrophenylfluorone (in the presence of CTA) [76,77], 2'-quinolylfluorone (~;- 1.65.105) [78], phenylfluorone and 4,5-dibromophenylfluorone (formula 57.2) (in the presence of surfactants) [79].

ar

H0

Br

(57.2)

The ion-associate (the anionic Zr-Picramine-epsilon complex and the basic dye Ethyl Rhodamine B) is floated, and dissolved in acetone (~ = 3.2-105) [80].

Flavone dyes, such as morin (formula 57.3) (e =5.1.104 at 420 nm) [81,81] and quercetin (formula 57.4) [83] are used for determination of zirconium. The 3- hydroxyflavone complex has been applied for extraction (benzene) and determination of Zr [84].

OH

Ho. _oH oH OH 0

OH

HO"~O ~"~OH (57.3) ~ "ff "OH (57.4)

OH O

Among other organic compounds worthy of mention are: Bromopyrogallol Red (~ = 7 . 0 . 1 0 4 at 670 nm) [85], 8-hydroxyquinoline (CHC13 or xylene, ~ = 1.5.104 at 386 nm [26,30,86], 3,5-dinitropyrocatechol (extraction with CHC13 and antipyrine) [87].

In the methods developed for determining hafnium, use has been made of the Mo-Hf heteropoly acid, reduced with tin(H) oxalate (e = 7.7.103 at 750 nm in n-butanol) [88-90]. Hafnium has also been determined by means of its reaction with molybdophosphoric acid and as reduced molybdosulphatohafnic acid [91 ].


Zirconium has been determined by the Alizarin S method in uranium alloys [45], titanium alloys [92], magnesium alloys [27], and rutile [3]. Determination of Zr by the differential technique has also been applied [44].

The Xylenol Orange method has been used for the determination of Zr in silicates [36], table salt [93], steel [94], nickel alloys [47,54], and refractory alloys [7,47]. A sorbent modified with Xylenol Orange has been used for preconcentration of Zr in mineral waters [95], and in basalt and granite rocks [96].

The Arsenazo III method has been applied for determination of zirconium (and hafnium) in silicate rocks [2,97], titanium dioxide [98], phosphate rocks [99], cast iron and steel


[5,100-102], nickel alloys [59,103,104], copper alloys [ 105], molybdenum alloys [ 106], and uranium [9]. Zirconium was determined in aluminium, copper, and magnesium with the use of tribromocarboxyarsenazo [ 107].

Zirconium has been determined in niobium by using Pyrocatechol Violet [71 ]. Hafnium has been determined in uranium alloys with the use of PAN [27]. 2-(2- Pyridylmethylenamino)phenol has been applied for determination of Zr in the presence of Cr (in bronzes) by derivative spectrophotometry [ 108].

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Appendix

Chapter 1. Separation and preconcentration of elements

The combination of extraction with spectrophotometry is still the most popular among separation and preconcentration procedures used. Liquid-liquid and solid-phase extractions of the suitable complexes make the basis of most recently developed methods. Spectrophotometric detection of the analyte can be directly accomplished in the obtained extracts. Stripping of the analytes from the examined extract followed by the conversion into the complex making a basis of spectrophotometric measurement is also used.

Extraction of chelate complexes using e.g. 4,7-diphenyl-l,10-phenanthroline (1,2- dichloroethane) [1], thenoyltrifluoroacetone (MIBK) [2], V-benzoinoxime (CHC13) [3] provides the examples of extraction-spectrophotometric methods recently developed.

The application of high molecular weight amines, trioctylamine (1,2-dichloroethane) [4], N-octylaniline (xylene) [5,6], diantipyrylmethane [4,7] and Cyanex 302 (toluene) [8] to separation of the elements has recently been reported. Extraction of ion pairs of anionic complexes of the analytes with the other counter ions, e.g. triphenylarsine oxide [9] or various dyes, e.g. Rhodamine B [ 10] and polymethine dyes [ 11 ], has recently been investigated.

The use of macrocyclic compounds for extraction of various analyte cations in the presence of suitable anions is still growing. Extraction-spectrophotometric methods for the determination of alkali metals (with picrate counter ion) using 18-crown-6 and its derivatives [12-15] and calix[4]arene crown ethers [16] have recently been developed. The application of hexa-acetatocalix[6]arene for extraction of Fe [17] and Pb [ 18] have been described.

The application of membrane extraction of analytes prior to spectrophotometric determination has been examined [ 19-22].

Solid-phase extraction of various analyte complexes on e.g. microcrystalline naphthalene (Ni [23] and Cu [24] with nitroso-R salt and tetradecyldimethylbenzylammonium chloride), ammonium tetraphenylborate-naphthalene (U with 2-(5-bromo-2-pyridylazo)-5- diethylaminophenol) [25], chitin column (Cr(VI) with 1,5-diphenylcarbazide) [26], strong anion-exchange cartridge (Cr(VI) at pH 8) [27], C18 cartridge (Cl2-azo dye) [28] and minicolumn (As with ammonium diethyl dithiophosphate) [29], silica modified chemically with N-allyl- or N-phenyl-N'-propylthiourea (OsO4) [30] and Sephadex DEAE A-25 (chloride form) (V with Eriochrome Cyanine R) [31 ] prior to the spectrophotometric determination has recently been reported. The application of solid-phase spectrophotometry to determine nitrite and nitrate in water samples has recently been described [32].

Separation of elements from interfering ions by solid phase extraction using polyurethane foam provides the examples of enhancement in selectivity of spectrophotometric methods [33-35]. Sorption of thiocyanate complexes of the analytes on polyurethane foam is most often applied.

Recent applications of ion-exchange and chelating resins for separation and preconcentration of elements prior to the spectrophotometric detection have been described [36-41].

Sorption on membranes immobilized with azorhodanine reagents and sulfonitrophenol M [42], fibres of poly(acrylonitrile)-carboxylated polyethylene-polyamine [43] and a filter- paper containing chemically attached hexamethylenediamino groups [44] have recently been employed for separation and preconcentration of elements.

Prepared by Maria Balcerzak

484 Appendix

Separation and preconcentration procedures recently employed for spectrophotometric determination of particular elements in various materials are discussed in Appendices to Chapters 5-57.

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Khim.), 54, 240 (1999). 44. Beklemishev M.K., Petrova Yu.Yu., Dolmanova I.F., Analyst, 124, 1523 (1999).

Chapter 2. Principles of Spectrophotometry

The principal characteristics of instruments for UV-visible spectrophotometry have recently been reviewed [ 1,2]. An interactive CD-ROM, entitled "Spectrophotometry", introducing the basis of UV-visible spectrophotometry has recently been recommended [3].

Progress in inhancement in the selectivity of spectrophotometric methods by applying derivative spectrophotometry can be emphasized. Derivative spectrophotometric methods allowing direct determination of neodymium and erbium in mixtures with other lanthanides [4], platinum and ruthenium in Pt-Ru carbon supported catalysts [5], tungsten in niobium- tantalum, vanadium and molybdenum bearing geological samples [6], uranium in standard alloys [7], nickel in standard alloy, steel and biological samples [8], zinc in standard alloys, environmental and pharmaceutical samples [9], beryllium in alloys [ 10], cobalt in alloys and biological materials [ 11], chromium in tap water [12] and palladium in activated charcoal [13] have recently been developed. Applications of derivative spectrophotometric methods to the determination of various elements preconcentrated by solid-phase extraction on microcrystalline naphthalene have been reported [7-9,11,14,15].

The use of FIA system combined with spectrophotometry becomes popular [16-18]. Applications of FIA for the determination of inorganic ions in terms of sample volume, sampling frequency, detection limit, linear range and other features have been discussed [ 17].

References 1. Ferraro J.R., Martin K., Jarnutowski R.J., Spectroscopy, 12, 18 (1997). 2. Analytical Methods Committee, Analyst, 125, 367 (2000). 3. Pringle D., J. Chem. Educ., 75, 978 (1998). 4. Wang N.X., Si Z.K., Yang J.H., Jiang W., Liang W.A., Li Z.D., Du G.Y., Zhang G.,

Mikrochim. Acta, 127, 71 (1997). 5. Balcerzak M., Swiecicka E., Balukiewicz E., Talanta, 48, 39 (1999). 6. Padmasubashini V., Ganguly M.K., Satyanarayana K., Malhotra R.K., Talanta, 50, 669

(1999). 7. Pancras J. P., Puri B.K., Mikrochim. Acta, 130, 203 (1999). 8. Taher M.A., Talanta, 50, 559 (1999). 9. Bhalotra A., Puri B.K., Talanta, 49, 485 (1999). 10. Singh H.B., Agnihotri N.K., Singh V.K., Talanta, 47, 1287 (1998).

486 Appendix

11. Pancras J.P., Puff B.K., Taher M.A., Mostafavi Dehzoei A., Sheibani A., Talanta, 46, 1107 (1998).

12. Wr6bel K., Wr6bel K., Lopez-de-Alba P.L., Lopez-Martinez L., Talanta, 44, 2129 (1997). 13. E1-Sayed A.Y., Abu-Shanab F.A., Mikrochim. Acta, 129, 225 (1998). 14. Taher M.A.,Anal. Lett., 31, 2115 (1998). 15. Pancras J.P., Purl B.K., Anal. Sci., 15, 575 (1999). 16. Ishii D., J. Flow Inj. Anal., 16, 151 (1999). 17. Prasada Rao T., Sita N.M., Iyer C.S.P., Rev. Anal. Chem., 18, 157 (1999). 18. Ruzicka J., Hansen E.H., Anal. Chem., 72, 212A (2000).

Chapter 4. Spectrophotometric Reagents

Progress in the application of Arsenazo III reagent in analytical chemistry has recently been discussed [ 1]. A review of systems based on ternary and multicomponent complexes recently developed for spectrophotometric determination of elements have been presented [2]. The use of mixed ligand, surfactant sensitized, ion-association, flotation, derivative and FIA techniques has been discussed.

Attempts to improve selectivity and sensitivity of spectrophotometric measurements by using new chromogenic reagents, e.g. of 2-(5-nitro-2-pyridylazo)-5-(N-propyl-N- sulfopropylamino)phenol for flow-injection-spectrophotometric determination of trace V in river water [3], N-butyl-N'-(sodium p-amino benzenesulfonate) thiourea to determine Pd in minerals and catalysts [4], benzeneacetaldehyde-4-hydroxy-oxo-aldoxime to determine Co in pharmaceuticals, biological materials and steels [5] , 2,6-dichloroarsenazo for the determination of Bi in copper alloys [6], N-undecyl-N-(sodium p-aminobenzenesulfonate)- thiourea to identify and determine Cu in alloy, liver and wheat [7], dimethoxyhydroxyphenylflurone for the determination of trace amounts of Mo in steel and pure iron [8] and 5-(6-methoxy-2-benzothiazole-azo)-8-aminoquinoline for the detection of Co in drainage sediment and Ni in A1 alloy [9] have recently been reported.

References 1. Basargin N.N., Ivanov V.M., Kuznetsov V.V., Mikhailova A.V., J. Anal. Chem. (Transl. of

Zh. Anal. Khim.), 55, 204 (2000). 2. Rao T.P., Reddy M.L.P., Pillai A.R., Talanta, 46, 765 (1998). 3. Yamane T., Yamaguchi Y., Mikrochim. Acta, 130, 111 (1998). 4. Hou F.L., Ma D.L., Li J.P., Wang Y.L., Anal. Lett., 31, 1929 (1998). 5. Jadhav S.B., Utekar S.S., Kulkarni A., Varadarajan A., Malve S.P., Talanta, 46, 1425

(1998). 6. Zhang H.S., Zhang J.F., Wang H., Li X.Y., Anal. Chim. Acta, 380, 101 (1999). 7. Ma D.L., Xia D.S., Cui F.L., Li J.P., Wang Y.L., Talanta, 48, 9 (1999). 8. Lui S.M., Pan J.M., Anal. Lett., 32, 1225 (1999). 9. Zhao S.L., Xia X.Q., Hu Q.F., Anal. Chim. Acta, 391,365 (1999).

Chapter 5. Alkali metals

The extraction of alkali-metal ions from H20 into 18-crown-6 solutions in different organic solvents in the presence of picrate [1] and into dibenzo-18-crown-6 solution in 1,2-

Appendix 487

dichloroethane in the presence of picrate or 2,4-dinitrophenolate [2] as counter ions has been examined. The extraction selectivity decreased in the order K+> Rb + > Cs + > Na + > Li +.

Studies of seventeen crown-formazans with 14- and 15-membered tings (structures given) as reagents for the selective spectrophotometric determination of Li have been described [3]. Lithium in serum and sea-water was determined by spectrophotometric method using a water-soluble octabromoporphyrin [4].

References

1. Takeda Y., Kawarabayashi A., Endo K., Yahata T., Kudo Y., Katsuta S., Anal. Sci., 14, 215 (1998).

2. Iglesias R., Dassie S.A., Yudi L. M., Baruzzi A.M.,Anal. Sci., 14, 231 (1998). 3. Ibrahim Y.A., Elwaby A.H.M., Barsoum B.N., Abbas A.A., Khella S. K., Talanta, 47,

1199 (1998). 4. Tabata M., Nishimoto J., Kusano T., Talanta, 46, 703 (1998).

Chapter 6. Aluminium

Chrome azurol S was used to determine A1 in tap water, dialysis fluids and alkali metal salts [1] and in mine well water [2]. The preliminary preconcentration of the complex on a polyethylene powder was applied [1]. Aluminium species (AI(III) ions and Al1304(OH)24 (H20)127+) in aqueous soil extracts and humic waters were determined by FIA method using Chrome azurol S after preliminary retaining of the analyte on a column reactor containing 8- quinolinol immobilized onto Fractogel and selective elution with different elluents [3].

Detection limit of 2.1 g 1-1 A1 in water samples was reached using Eriochrome Cyanine R and CP [4]. Interferences from Fe(III) and Cu were masked by ascorbic acid and thiourea, respectively. Eriochrome Cyanine R was also applied to the determination of A1 (at 0.02- 0.04%) in polyaryl ether ketones [5].

Aluminon was used as a chromogenic reagent for the determination of A1 in multicomponent mixtures and ores, alloys and pharmaceuticals [6]. Interference from Cu(II) was removed by prior extraction with Cyanex 302 at pH 1. The extraction from the solutions of pH 4 followed by stripping with 2 M HC1 allows the determination of A1 in the presence of Fe.

A FIA method using Methyl Thymol Blue (in acetate buffer of pH 4.25) was applied to the determination of A1 in rock and ore CRM [7]. The detection limit achieved was 30 ng m1-1.

Spectrophotometric methods that can be used for aluminium speciation in natural waters have been discussed [8].

References

1. do Nascimento D.B., Schwedt G., Mikrochim. Acta, 126, 159 (1997). 2. Gao H.W., Shi H.L., J. Anal. Chem. (Transl. of Zh. Anal. Khim.), 52, 1045 (1997). 3. Simpson S.L., Powell K.J., Nilsson N.H.S.,Anal. Chim. Acta, 343, 19 (1997). 4. Soylak M., Sahin U., Ulgen A., Elci L., Dogan M., Anal. Sci., 13, 287 (1997). 5. Buyanovskaya A.G., Strel'tsova E.D., Gumileva L.V., Martsenitsena E.L., J. Anal. Chem.

(Transl. of Zh. Anal. Khim.), 53, 952 (1998). 6. Ajgaonkar H.S.; Dhadke P. M., Talanta, 44, 563 (1997).

488 Appendix

7. Cassella R.J., Santelli R. E., Branco A.G., Lemos V.A., Ferreira S.L.C., Souza de Carvalho M.,Analyst, 124, 805 (1999).

8. Pyrzyfiska K., Bulska E., Gucer S., Hulanicki A., Chem. Anal. (Warsaw), 44, 1 (1999).

Chapter 7. Antimony

The reaction of antimony with bromopyrogallol red can make a basis of the determination of Sb(III) and Sb(V) after the preliminary reduction to Sb(III) with iodide [1]. The method was used to the determination of antimony in antileishmanial drugs.

Spectrophotometric methods for antimony speciation in waters using cationic dyes were reviewed [2] . Possible methods for preconcentration were suggested. N,N'- diphenylbenzamidine and Brilliant Green in an acidic non-ionic micellar media were used in chemical speciation and determination of Sb in fresh ground water and waste water [3].

Gas-phase spectrophotometry was employed to the determination of antimony (As, Se, and Ti) in tap water in the form of hydrides generated by treatment with NaBH4, collected in a trap cooled by liquid N2 and further evaporation at 80~ in a flow of N2 [4]. Recovery of 88.4-92.3% Sb was reported.

References

1. Rath S., Jardim W.F., Dorea J.G., Fresenius'J. Anal. Chem., 358, 548 (1997). 2. Russeva E., Havezov I.,Anal. Lab., 7, 115 (1998). 3. Deb M.K., Agnihotri P.K., Thakur M., Mishra R.K., J. Indian Chem. Soc., 76, 145 (1999). 4. Cabredo S., Galban J., Sanz J., Talanta, 46, 631 (1998).

Chapter 8. Arsenic

Preconcentration of As(III) diethyldithiocarbamate on silica chemically modified with hexadecyl groups was examined [1]. Emulsion liquid membrane (made up of L l l3A surfactant, liquid paraffin as stabilizer and kerosene as solvent with HC1 and KOH as external and internal phases) separation of As(llI) and As(V) was applied prior to detection with silver diethyldithiocarbamate 0~= 510 nm) [2]. The method was applied to Cu ore and slagged ash.

Arsenomolybdenum blue method was used to the determination of As after preliminary retention of the complexes of As(m) and ammonium diethyl dithiophosphate on a C18 sorbent [3]. The eluted complexes were merged with NaBH4 and the resulting solution was injected into the hydride generator/gas-liquid separator. The arsine generated was carried out by a stream of N2 and trapped in an alkaline iodine solution in which the colour reaction was developed.

Gas-phase spectrophotometric method was proposed for the determination of arsenic in tap water after the conversion into hydride, its collection in a trap cooled by liquid N2 and subsequent evaporation at 80~ in a flow of N2. The recovery of 73-86.7% As was reported. Simultaneous determination of arsenic, antimony, selenium and tin was accomplished.

References 1. Tikhomirova T. I., Fadeeva V. I., J. Anal. Chem. (Transl. of Zh. Anal. Khim.), 52, 203

(1997).

Appendix 489

2. Li L.Q., Yan P., Gao W.Z., Li Y. D., Fresenius'J. Anal. Chem., 363, 317 (1999). 3. Gomes Neto J.A., Montes R., Cardoso A.A., Talanta, 50, 959 (1999). 4. Cabredo S., Galban J., Sanz, J., Talanta, 46, 631 (1998).

Chapter 9. Beryllium

Beryllium in tap water, dialysis fluids and alkali-metal salts was determined with Chrome Azurol S after preconcentration of the complex on a column packed with polyethylene powder [1]. Sorption preconcentration of Be on a fibrous sorbent (poly(acrylonitrile)- carboxylated polyethylene-polyamine) prior to the determination of metal with Arsenazo I in sea water was described [2].

Trace amounts of Be in alloys were determined with 1,4-dihydroxy-9,10- anthracenedione and Triton X-100 by first-derivative spectrophotometry [3]. The detection limit was 0.23 ng m1-1.

References 1. do Nascimento D.B., Schwedt G., Mikrochim. Acta, 126, 159 (1997). 2. Moskvin L.N., Drogobuzhskaya S.V., Moskvin A.L., J. Anal. Chem. (Transl. of Zh. Anal.

Khim.), 54, 240 (1999). 3. Singh H.B., Agnihotri N.K., Singh V.K., Talanta, 47, 1287 (1998).

Chapter 10. Bismuth

2,6-Dichloroarsenazo was proposed as the chromogenic reagent for bismuth [1]. Absorbance of the blue complex was measured at 631 nm. The reagent was used to determine Bi in copper alloys.

References 1. Zhang H.S., Zhang J.F., Wang H., Li, X.Y., Anal. Chim. Acta, 380, 101 (1999).

Chapter 11. Boron

Boron in water was determined by the Azomethine-H method [1]. The detection limit was 0.015 mg 1-1. FIA method employing Azomethine-H as a chromogenic reagent was used to the determination of B in plants [2].

References 1. Harp D.L., Anal. Chim. Acta, 346, 373 (1997). 2. De Azevedo Tumang C., Caseri de Luca G., Nunes Fernandes R., Reis B.F., Krug F.J.,

Anal. Chim. Acta, 374, 53 (1998).

490 Appendix

Chapter 12. Bromine

Trace amounts of bromide in water samples were determined by FIA method using its catalytic effect on the 4,4'-bis(dimethylamino)diphenylmethane-chloramine T reaction [1]. Detection limit of 1 :g 1-1 was reported.

The reaction of bromate with fuchsin following its separation with the use of Dowex- 50 cation-exchange resin (Na + form) was a basis of the determination of bromate in drinking water [2].

Spectrophotometry was employed to study the reaction of bromine (and iodine) with two dilactam macrocyclic polyethers [3].

References

1. Yonehara N., Chaen S., Tomiyasu T., Sakamoto H., Anal. Sci., 15, 277 (1999). 2. Romele L., Achilli M.,Analyst, 123, 291 (1998). 3. Sharghi H., Massah A.R., Abedi M., Talanta, 49, 531 (1999).

Chapter 13. Cadmium

A chromogenic crown ether (NN'-bis-(2-hydroxy-5-nitrobenzyl)4,13-diazadibenzo-18-crown- 6) in a mixed micellar medium was proposed for the determination of Cd in water (the detection limit was 6 ppb) [1]. The preconcentration of Cd on a chelating resin prepared by coupling Pyrocatechol Violet to Amberlite XAD-2 was studied [2]. Quantitative adsorption was achieved at pH 5-7. Nitric acid (4 M) was used as elluent. The effects of various anions (fluoride, chloride, nitrate, sulfate and phosphate) on the adsorption of Cd (Zn, Pb(II) and Ni) were investigated.

Amido black diazoaminoazobenzene was proposed to determine Cd in waste water and metal materials [3]. An orange-red 0~max = 520 nm) chelate complex of Cd with the reagent used (1:2) makes a basis of spectrophotometric measurement. Beer's law was obeyed in the range of 0-8 g/25 ml Cd.

References 1. Vaidya B., Porter M.D., Utterback M.D., Bartsch R.A., Anal. Chem., 69, 2688 (1997). 2. Saxena R., Singh A.K., Anal. Chim. Acta, 340, 285 (1997). 3. Zhang P.F., Gao H.W., Li Y., Talanta, 47, 355 (1998).

Chapter 14. Calcium

The method employing chlorophosphonazo(HI) was applied to determine Ca in water, food and pharmaceuticals [1]. The detection limit was 0.03 mg 1-1 Ca. No interferences from elements, excess vitamins, amino acids or citrate and acetate ions were observed. The spectrophotometric method based on amino G acid chlorophosphonazo was proposed to determine Ca in human serum and cerebrospinal fluids [2]. The reagent reacts with Ca with the formation of greenish blue complex 0~ax at 670 nm; e = 7.2 x 105 1 mol-lcm-1). Beer's law is obeyed over the range 0.02-0.8 g m1-1 Ca. No interferences were observed from the commonly coexisting species present in the examined materials.

Appendix 491

Calcium and Mg in mineral water were simultaneously determined with Methylthymol Blue by both batch and flow-injection techniques [3]. FIA method using murexide as a colour agent was employed to the determination of Ca in ores and Ca-containing pharmaceutical formulations [4].

A chemometric method for simultaneous determination of calcium and magnesium in natural waters using Arsenazo III and FIA system was described [5]. The concentrations of the analytes were calculated by the H-point standard addition method for ternary mixtures.

References 1. Gao H.W., J. Anal. Chem. (Transl. of Zh. Anal. Khim.), 52, 141 (1997). 2. Ma H.M., Anal. Lett., 32, 799 (1999). 3. Blasco F., Medina-Hernandez M.J., Sagrado S., Fernandez F.M., Analyst, 122, 639 (1997). 4. Grudpan K., Jakmunee J., Vaneesorn Y., Watanesk S., Maung U.A., Sookasmiti P.,

Talanta, 46, 1245 (1998). 5. Blasco Gomez F., Bosch Reig F., Campins Falco P., Talanta, 49, 155 (1999).

Chapter 15. Carbon

Indirect micellar spectrophotometric determination of cyanide employing Triton X-100 and N-(2'-thiazolyl)-2-hydroxybenzamide was investigated [1 ].

The method for simultaneous determination of trace amounts of free cyanide and thiocyanate using 1,3-dimethylbarbituric acid/isonicotinic acid as a colour reagent was proposed [ 2]. ,

A manifold system for the simultaneous determination of CO2 and SO2 in wine, employing evaporation of the examined solution, diffusion through a hydrochloric membrane and reaction with p-rosaniline hydrochloride in the presence of formaldehyde has been described [3].

References 1. Garg B.S., Bhojak N., Dwivedi P., Bist J.S., Aggarwal M.B., Indian J. Chem., Sect. A, 38,

395 (1999). 2. Sun B.T., Noller B.N., Water Research, 32, 3698 (1998). 3. Mataix E., Luque de Castro M.D., Fresenius'J. Anal. Chem., 365, 377 (1999).

Chapter 16. Chlorine

Benzidine immobilized on cellulose and silica gel can be used for the determination of free chlorine (0.05-0.5 mg 1-1) in waters [1]. Free and combined chlorine in water and chlorine in air were determined using solid-phase extraction and azo dye formation [2]. Combined chlorine in water could be determined after masking the free chlorine with acetone.

Test kits based on immobilized Bindschedler's green and N,N-diethyl-p- phenylenediamine were proposed for the rapid determination of active chlorine in water [3].

Chlorine dioxide in water samples has been determined by FIA method employing preliminary separation (using a purge-trap system with N2 purge gas) and 4-aminoantipyrine and phenol [4]. Detection limit was 5 ppb.

492 Appendix

Extraction (1,2-dichloroethane) using leucomethylene blue was applied to determine chlorine dioxide in drinking water and waste water [5]. Detection limit of 0.02 mg 1-1 was achieved. Chlorine and hypochlorite ions were masked by oxalic acid.

References

1. OstrovskayaV.M., Zolotov Yu.A., Morosanova E.I., Marchenko D.Yu., Fresenius'J. Anal. Chem., 361, 300 (1998).

2. Pandey M., Gosain S., Sahasrabuddhey B., Verma K.K., Analyst, 123, 2319 (1998). 3. Marchenko D.Y., Morosanova E.I., Kuz'min N.M., Zolotov Y.A., J. Anal. Chem. (Transl.

of Zh. Anal. Khim.), 54, 455 (1999). 4. Watanabe T., Ishii T., Yoshimura Y., Nakazawa H., Anal. Chim. Acta, 341, 257 (1997). 5. Chen H., Wang G.Z., Yuan L., Anal. Lett., 30, 1415 (1997).

Chapter 17. Chromium

Chromium(VI) in water samples [1-4], in air particulates [5] and in a welding dust [6] was determined using 1,5-diphenylcarbazide. The method was applied to the determination of total Cr after preliminary oxidation of Cr(III) by KMnO4 [4]. Supported liquid membrane (a porous PTFE impregnated with 5% Aliquat in dihexyl ether) extraction followed ultrasonic extraction of Cr(VI) from solid samples with ammonium buffer of pH 8 or phosphate buffer of pH 7 and a flow system was used to detect the analyte in a welding dust by UV spectrophotometry [6]. Ultrasonication and strong anion-exchange solid-phase extraction was applied to the determination of Cr in air particulates [5]. Micro amounts of Cr(VI) and total Cr in natural water were determined by solid phase spectrophotometry [2]. The detection limits of 70 pg Cr(VI) and 120 pg total Cr were reported. The preconcentration of Cr-l,5- diphenylcarbazide complex on the chitin column was employed prior to the determination of Cr(VI) in rain, fiver and spring waters [3].

The derivative spectrophotometric method allowing simultaneous determination of Cr(III) and Cu(II) using methylethylenediaminetetraacetic acid was described [7]. The method was applied in analysis of various aqueous solutions.

Trace levels of Cr(VI) were determined by FIA method employing microwave- accelerated reactions of Cr2072- with dibromocarboxyarsenazo or dibromo-o- carborylchlorophonazo and spectrophotometric detection at 535 and 556 nm, respectively [8]. The detection limits were 0.087 and 0.1 g ml -I, respectively. The methods were applied to determine Cr in steels and electroplating solutions.

Colour reaction of Cr(VI) with NN-diethyl-l,4-phenylenediamine in the presence of ethanol and cyclohexyldiaminetetraacetic acid can be used in speciation analysis [9]. The method was applied to analysis of industrial waste water.

Chromotropic acid was applied as a reagent for ultraviolet spectrophotometric determination of hexavalent chromium in water [10].

References

1. Wr6bel K., Wr6bel K., Lopez-de-Alba P.L., Lopez-Martinez L., Talanta, 44, 2129 (1997). 2. Matsuoka S., Tennichi Y., Takehara K., Yoshimura K., Analyst, 124, 787 (1999). 3. Hoshi S., Konuma K., Sugawara K., Uto M., Akatsuka K., Talanta, 47, 659 (1998). 4. Balasubramanian S., Pugalenthi V., Talanta, 50, 457 (1999).

Appendix 493

5. Wang J., Ashley K., Marlow D., England E.C., Carlton G., Anal. Chem., 71, 1027 (1999). 6. Ndung'u K., Djane N.K., Malcus F., Mathiasson L., Analyst, 124, 1367 (1999). 7. Seco-Lago H., Perez-Iglesias J., Fernandez-Solis J.M., Castro-Romero J.M., Gonzalez-

Rodriguez V., Fresenius'J. Anal. Chem., 357, 464 (1997). 8. Wu X.Y., Zhao H., Chen X.G., Hu Z., Zhao Z.F., Hooper M., Anal. Chim. Acta, 374, 61

(1998). 9. Zhang M. H., Zhang Q.R., Fang Z., Lei Z.C., Talanta, 48, 369 (1999). 10. Zhao Z.Q., Gao R.M., Li J.T., Liu S. R., Liu H., Microchem. J., 58, 1 (1998).

Chapter 18. Cobalt

Cobalt in pharmaceuticals, alloys and steel was determined by non-extractive derivative spectrophotometric method using 1-nitroso-2-naphthol and Triton X-100 [1]. The detection limit was 1.68 ng m1-1. The determination of Co in the presence of Cu and Fe can be carried out.

The application of derivative spectrophotometry allows the determination of Co in the presence of Fe [2] and Fe, Ni and Cu [3]. 4-(2-Pyridylazo)resorcinol and 1,5-bis(di-2- pyridylmethylene) thiocarbonohydrazide were used as chromogenic reagents. The method [3] was applied to the determination of Co in alloys and biological materials. Cobalt(H) in steels has been determined with the use of 4-(2-pyridylazo)resorcinol and xylometazoline hydrochloride [4].

Preconcentration of trace cobalt with the ion pair of 2-(5-bromo-2-pyridylazo)-5- diethylaminophenol and tetraphenylborate onto microcrystalline naphthalene or column method and its determination by derivative spectrophotometry has been described [5]. The method was applied to the determination of trace Co in alloys and biological materials.

Methods for the determination of cobalt in soils using thiazolylblue tetrazolium bromide [6] and nitrotetrazolium blue chloride [7] were described. The examined samples were subjected to preliminary separation of the analyte with dithizone. The interference from Fe(III) could be masked with ascorbic acid [7].

The system Co-NH4SCN-malachite green in the presence of surfactants CPC and Triton X-100 makes a basis of the determination of cobalt in beverages, biological, environmental and pharmaceutical samples by FIA technique [8]. Detection limit of 20 ppb was obtained.

2-Hydroxybenzaldehyde-5-nitro-pyridylhydrazone in the presence cetyltrimethylammonium bromide was used as a reagent for the determination of trace amounts of cobalt [9]. The interference from Pb 2+, Ni 2+ and C u 2+ could be eliminated after separation of the metal ions on Amberlite IRC-718 resin.

Cobalt in soil and a vitamin B~2 injection was determined using the chromogenic reagent 4,4'-diazobenzenediazoaminoazobenzene in a micellar surfactant medium [ 10].

References 1. Singh H.B., Agnihotri N.K., Singh V.K., Talanta, 48, 623 (1999). 2. Kolomiets L.L., Pilipenko L.A., Zhmud' I.M., Panfilova I.P., J. Anal. Chem. (Transl. of

Zh. Anal. Khim.), 54, 28 (1999). 3. Garcia Rodriguez A.M., Garcia de Torres A., Cano Pavon J.M., Bosch Ojeda C., Talanta,

47, 463 (1998). 4. Bhadani S.N., Tewari M., Agrawal A., Sekhar K.C., J. Indian Chem. Soc., 75, 176 (1998).

494 Appendix

5. Pancras J.P., Puri B.K., Taher M.A., Mostafavi Dehzoei A., Sheibani A., Talanta, 46, 1107 (1998).

6. Kamburowa M., Alexandrov A., Chem. Anal. (Warsaw), 43, 1021 (1998). 7. Kamburova M., Aleksandrov A., J. Anal. Chem. (Transl. of Zh. Anal. Khim.), 54, 244

(1999). 8. Aggarwal S.G., Patel K.S., Fresenius'J. Anal. Chem., 362, 571 (1998). 9. Park C.I., Cha K.W., Talanta, 46, 1515 (1998). 10. Jin G., Zhu Y.R., Jiang W.Q., Xie B.P., Cheng B.,Analyst, 122, 263 (1997).

Chapter 19. Copper

Copper in water and plant foods was determined with the use of diethyldithiocarbamate in the presence of EDTA and citrate [1]. The detection limit was 0.2 mg 1-1. 1-(2-Pyridylazo)-2- naphthol in the presence of Triton X-100 and NN'-diphenylbenz-amidine 0~ = 520 nm, e = 1.14" 105 1 mo1-1 cm -~, detection limit of 2 ng m1-1) was applied for the determination of copper in dust and soil [2]. Earlier, the effect of an anionic surfactant on the sensitivity of the determination of Cu with PAN has been examined [3].

Extraction and stripping of copper(I) as a neocuproine complex in a surfactant medium was used to determine Cu in steel [4]. Solid-phase extraction based on the preconcentration of the complex with neocuproine on a preconditioned membrane disc containing octadecyl- bonded silica followed by extraction of Cu and elution with isopentyl alcohol was applied to the determination of copper in distilled, tap and spring waters [5].

The Cu(I)-bathocuproine complex retained on the solid-phase extractant can be used for copper speciation in fresh water [6]. Second derivative spectrophotometry using mixtures of ligands, 5-phenyl-3-(4-phenyl-2-pyridinyl)-l,2,4-triazine and bathocuproine, was used for simultaneous determination of copper and iron in tap and fiver water [7].

Copper in serum and natural waters was determined with bathocuproinedisulfonic acid using bromophenol blue as internal standard [8]. Extraction (CHC13) with Aliquat 336 preceded the spectrophotometric measurement. The ratio of the absorbances at 471 nm (copper complex) and 602 nm (internal standard) was taken as the analytical parameter. Detection limit was 0.4 g 1-1.

Derivative spectrophotometry using PAR can make a basis of selective determination of Cu in binary mixtures with Ni and Co [9]. Simultaneous determination of Cu(II) and Hg(II) employing methylenediaminetetraacetic acid (pH 7) [10] and of Cu(II) and Cr(III) with methylethylenediaminetetraacetic acid [ 11] by derivative technique was reported. The use of zero- and first-order derivative spectrophotometry for individual and simultaneous determinations of Cu and Pd using an oxazolidine derivative as a new reagent has been described [ 12].

Copper in mixtures with Fe, Co and Ni was determined using 1,5-bis(di-2- pyridylmethylene) thiocarbonohydrazide in DMF and derivative technique [ 13]. The method was applied to the determination of metal ions in alloys and biological materials.

FIA system employing the preliminary retention of Cu (and Zn) on a Chelex-100 column, elution of the metal ions with 0.1 M HNO3 and reaction with zincon was applied in analysis of brass and tap water [14]. PAR was used to the determination of Cu in industrial effluents [ 15]. The application of the complex with Michler's thioketone to the determination of Cu (the detection limit of 0.8 g 1-1) has been reported [ 16].

4,5-Dimercapto-1,3-dithiol-2-thionate was proposed for the determination of copper in metallic alloy [17]. The absorbance was measured at 430 n m ( e = 9.06"105 1 mo1-1 cm-1). A comparative study with a method using cuproin showed good agreement.

Appendix 495

The application of 1,10-phenanthroline and principal-component regression procedure allows the determination of Cu in mixtures with Co and Fe [ 18]. The method was applied to the analysis of two cobalt magnetic alloys.

FIA system employing 4-methylpiperidinedithiocarbamic acid as a reagent and a cation-exchange column packed with A650 W was applied to determine Cu (5-100 g 1-1) in river and sea water, ore and copper processing water samples [ 19].

First-derivative spectrometry and pyridoxal-4-phenylthiosemicarbazone as a reagent (in aqueous 30% DMF) was used to determine Cu and Co in steel [20].

References

1. van Staden J.F., Botha A., Talanta, 49, 1099 (1999). 2. Thakur M., Deb M.K., Talanta, 49, 561 (1999). 3. Mukhovikova N.P., Potapova E.P., J. Anal. Chem. (Transl. of Zh. Anal. Khim.), 52, 842

(1997). 4. Tagashira S., Murakami Y., Yano M., Sasaki Y., Bull. Chem. Soc. Jpn., 71, 2137 (1998). 5. Yamini Y., Tamaddon A., Talanta, 49, 119 (1999). 6. Bjorklund L.B., Morrison G M., Anal. Chim. Acta, 343, 259 (1997). 7. Toral M. I., Richter P., Rodriguez C., Talanta, 45, 147 (1997). 8. Wr6bel K., Wr6bel K., Cruz-Jimenez G., Angulo-Romero F., Anal. Chim. Acta, 387, 217

(1999). 9. Kolomiets L.L., Pilipenko L.A., Zhmud I.M., Panfilova I.P., J. Anal. Chem. (Transl. of Zh.

Anal. Khim.), 54, 28 (1999). 10. Seco-Lago H.M., Perez-Iglesias J., Fernandez-Solis J.M., Castro-Romero J.M., Gonzalez-

Rodriguez V., Anal. Lett., 31, 2747 (1998). 11. Seco-Lago H., Perez-Iglesias J., Fernandez-Solis J.M., Castro-Romero J.M., Gonzalez-

Rodriguez V., Fresenius'J. Anal. Chem., 357, 464 (1997). 12. E1-Sayed A.A.Y., Rahem M.A.A., Omran A.A.,Anal. Sci., 14, 577 (1998). 13. Garcia Rodriguez A.M., Garcia de Torres A., Cano Pavon J.M., Bosch Ojeda C., Talanta,

47, 463 (1998). 14. Richter P., Toral M.I., Tapia A.E., Fuenzalida E, Analyst, 122, 1045 (1997). 15. Breuil P., Di Benedetto D., Poyet J.P., Analusis, 26, M63 (1998). 16. Kalinichenko I.E., Ryabushko V.O., Falendysh N.F., Matsibura G.S., J. Anal. Chem.

(Transl. of Zh. Anal. Khim.), 54, 31 (1999). 17. Barreto S.R.G., Nozaki J., Barreto W.J., Microchem. J., 62, 223 ((1999). 18. Kompany-Zareh M., Massoumi A., Fresenius'J. Anal. Chem., 363, 219 (1999). 19. Isildak I., Asan A., Andac M., Talanta 48, 219 (1999). 20. Vereda E., Rios A., Valcarcel M., Analyst, 122, 85 (1997).

Chapter 20. Fluorine

The use of alizarin-lanthanum complexonate has been described for the determination of fluorine in soil [1] and plant samples [2]. The method for fluoride determination in natural waters based on the decrease in colour intensity of the Th: bromocresol orange complex in the presence of F- was presented [3].

References

1. Gaiduk O.V., Filippovich L.I., Zavod. Lab., 63(13), 11 (1997).

496 Appendix

2. Li H.B., Xu X.R., Talanta, 48, 57 (1999). 3. Khalifa M.E., Hafez M.A.H., Talanta, 47, 547 (1998).

Chapter 21. Gallium

The complex of Ga with Semiethylthymol Blue was used to determine Ga in minerals and ores [ 1]. Gallium in aluminium alloy was determined in the form of tungsten-molybdenum complexes [2].

Selective determination of Ga in waste water with the use of 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol in the presence of sodium dodecylsulfate and Brij 35 has been described [3]. The extraction of the Rhodamine B-tetrachlorogallate(III) association complex with various quaternary ammonium ions was examined [4].

References

1. Hafez M.A.H., Kenawy I.M.M., Mikrochim. Acta, 129, 291 (1998). 2. Kol'tsova E.G., Vishnikin A.B., Tsyganok L.P., Zavod. Lab., 64(10), 8 (1998). 3. Mori I., Kawakatsu T., Fujita Y., Matsuo T., Anal. Lett., 32, 613 (1999). 4. Yamamoto K., Katoh N., Anal. Sci., 15, 1013 (1999).

Chapter 22. Germanium

The use of phenylfluorone in the medium of poly(vinyl alcohol), benzalkonium chloride or octadecyltrimethylammonium methylsulfate to determine Ge has been described [ 1 ].

References

1. Liva M., Montero M., Dominguez P., Pina G., Diaz-Garcia M E., Quim. Anal. (Barcelona), 16, 55 (1997).

Chapter 23. Gold

Separation of gold from copper, platinum, palladium, nickel, zinc, cadmium and mercury using triphenylarsine oxide as an extractant has been described [ 1]. The reaction of NN- dimethyl-p-phenylenediamine with Au in the presence of potassium persulfate made a basis of sensitive method for the determination of Au (e= 1.9" 10 6 1 mol-lcm -1) [2].

The reaction of Au (Ag, Cu and Hg) with Michler's thioketone was studied [3].

References

1. Patil N.N., Shinde V.M., Mikrochim. Acta, 129, 299 (1998). 2. Mori I., Tominaga H., Fujita Y., Matsuo T., Anal. Lett., 30, 953 (1997). 3. Kalinichenko I.E., Ryabushko V.O., Falendysh N.F., Matsibura G.S., J. Anal. Chem.

(Transl. of Zh. Anal. Khim.), 54, 31 (1999).

Appendix 497

Chapter 25. Iodine

Iodide in table salt and laver was determined through the reaction of iodate with 3,5-Br2- PADAP and thiocyanate using FIA system [ 1]. A sensitive method for the determination of iodine using leucocrystal violet has recently been described [2]. The method was used to the determination of iodine in sea water.

References

1. Sun J.Y., Chen X.G., Hu Z., Fresenius'J. Anal. Chem., 357, 1002 (1997). 2. Agrawal O., Sunita G., Gupta V.K., Talanta, 49, 923 (1999).

Chapter 26. Iron

Solvent extraction of Fe(III) with calix(6)arene carboxylate derivative followed by spectrophotometric determination with thiocyanate was examined [ 1 ]. The developed method was recommended for determination of Fe(III) in samples containing A1, Mn(II), Mo(VI) and V(V). The extraction of Fe(III) with 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester followed by the determination with thiocyanate was employed to determine Fe in pharmaceuticals, metal and ores [2]. Solvent extraction separation of iron(iiI) with Cyanex 302 prior to the detection as the thiocyanate complex was applied to determine Fe(III) in multicomponent mixtures and ores, alloys and pharmaceuticals [3].

Separation and preconcentration of Fe(II) and Fe(III) from natural water on a melamine-formaldehyde resin was used prior to the detection with 1,10- phenanthroline [4]. 1,10-Phenanthroline and extraction of the formed complex into 1,2-dichloroethane was used to determine iron in phosphate luminophores [5]. The method using 1,10-phenanthroline and FIA system has been described [6]. The use of 4,7-diphenyl-l,10-phenanthroline as chromogenic reagent allows simultaneous determination of iron (and ruthenium) as ternary complexes by extractive (1,2-dichloroethane) zero-order and second derivative spectrophotometry [7]. Iron (Co and Cu) in cobalt magnetic alloys were determined by using 1,10-phenanthroline and a principal-components regression procedure [8]. The reagent was employed to determine traces of Fe in polyaryl ether ketones [9]. Solid-phase adsorption of 1,10-phenanthroline complex of Fe by silica gel was used to determine Fe in natural waters (detection limits of 0.08 ppm per 20 ml sample solution) [10].

The method based on iron catalysed oxidation of o-phenylenediamine with H202 has been developed for speciation of iron in river and tap waters [11]. Xylenol Orange was applied to simultaneous determination of iron and nickel in alloys and industrial waste water [12]. The chelate complexes of Fe and Co with PAR made a basis of simultaneous determination of both metals by the second derivative spectrophotometry [ 13].

Collection of Fe(II) on a nitrocellulose membrane filter in the form of ion associate with ferrozine and a cation surfactant was applied to the determination of Fe(II) in fiver and sea water samples [ 14]. Solid-phase spectrophotometry using ascorbic acid and ferrozine was employed to determine Fe in water, waste water, wine, soil extract, digested vegetal tissues, drugs and human hair [15].

Iron in silicate rocks was determined with the use of 2,3-dihydroxynaphthalene [16,17]. 1,5-Bis(di-2-pyridylmethylene) thiocarbonohydrazide in DMF was used as a reagent to determine Fe (Co, Ni and Cu) by calculation of first- and second derivatives [18]. The method was applied to the determination of the metal ions in alloys and biological materials.

498 Appendix

Fe in multivitamin tablets and Fe-rich ground water was determined by the use of tiron (a detection limit of 0.67 mg 1-1) [19]. Iron(H) and Fe(llI) can be determined on the basis of differential reaction kinetics of Fe(II) and Fe(III) with Tiron in a FIA system [20].

A FIA method using 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol was developed for the simultaneous determination of Fe and Zn in human hair [21]. The metod using sulfosalicyclic acid was developed for the determination of Fe in oil [22]. Iron(II) and total Fe in natural waters was determined with 3-(2-pyridyl)-5,6-diphenyl-l,2,4-triazine [23].

References

1. Khandwe R.M., Khopkar S.M., Talanta, 46, 521 (1998). 2. Jayachandran J., Dhadke P.M., Talanta, 44, 1285 (1997). 3. Ajgaonkar H.S., Dhadke P.M., Talanta, 44, 563 (1997). 4. Filik H., Ozturk B.D., Dogutan M., Gumus G., Apak R., Talanta, 44, 877 (1997). 5. Kostadinova L.,Anal. Lab., 7, 207 (1998). 6. Sato K., Tokeshi M., Kitamori T., Sawada T., Anal. Sci., 15, 641 (1999). 7. Toral M.I., Richter P., Tapia A.E., Hernandez J., Talanta, 50, 183 (1999). 8. Kompany-Zareh M., Massoumi A., Fresenius'J. Anal. Chem., 363, 219 (1999). 9. Buyanovskaya A.G., Strel'tsova E.D., Gumileva L.V., Martsenitsena E.L., J. Anal. Chem.

(Transl. of Zh. Anal. Khim.), 53, 952 (1998). 10. Zaporozhets O., Gawer O., Sukhan V., Talanta, 46, 1387 (1998). 11. Kawakubo S., Hagihara Y., Honda Y., Iwatsuki M., Anal. Chim. Acta, 388, 35 (1999). 12. Kompany-Zareh M., Massoumi A., Pezeshk-Zadeh S., Talanta, 48, 283 (1999). 13. Kolomiets L.L., Pilipenko L.A., Zhmud I.M., Panfilova I.P., J. Anal. Chem. (Transl. of

Zh. Anal. Khim.), 54, 28 (1999). 14. Chen Y., Ding C.M., Zhou T.Z., Qi D.Y., Fresenius' J. Anal. Chem., 363, 119 (1999). 15. Fernandez de Cordova M.L., Ruiz-Medina A., Molina Diaz A., Fresenius'J. Anal. Chem.,

357, 44 (1997). 16. Tarafder P.K., Balasubramanian N., Chem. Anal. (Warsaw), 44, 731 (1999). 17. Tarafder P.K., Sardana A.K., Chem. Anal. (Warsaw), 51, 145 (2000). 18. Garcia Rodriguez A.M., Garcia de Torres A., Cano Pavon J.M., Bosch Ojeda C., Talanta,

47, 463 (1998). 19. van Staden J.F., Kluever L.G., Fresenius'J. Anal. Chem., 362, 319 (1998). 20. Endo M., Abe S., Fresenius'J. Anal. Chem., 358, 546 (1997). 21. Zhao S.L., Xia X.Q., Yu G., Yang B., Talanta, 46, 845 (1998). 22. Anikina N.A., Zavod. Lab., 64 ( 8 ),12 (1998). 23. Croot P.L., Hunter K.A., Anal. Chim. Acta, 406, 289 (2000).

Chapter 27. Lead

Extraction of Pb from iodide solutions into MIBK followed by the determination with dibromo-p-methyl-methylsulfonazo 0~ = 642 nm; e = 1.02"105 1 mol-lcm -1) makes a basis of the determination of the metal in biological samples [1]. The application of meso-tetrakis- (3,5-dibromo-4-hydroxyphenyl)porphyrin (in the presence of 8-hydroxyquinoline and Triton X-100) to the determination of lead in clinical samples has recently been described [2]. The detection limit of 0.21 ng ml -~ has been achieved.

N-hydroxy-N,N'-diphenylbenzamidine and diphenylcarbazone were used to determine Pb in airborne dust particulates and soil [3]. The detection limit was 4 ng m1-1.

Appendix 499

Cyanex 302 in toluene was applied to extract Pb from non-ferrous alloys and waste water prior to the determination with PAR [4].

References

1. Li Z.J., Zhu Z.Z., Chen Y.P., Hsu C.G., Pan J.M., Talanta, 48, 511 (1999). 2. Li Z.J., Zhu Z.Z., Jan T., Pan J.M.,Analyst, 124, 1227 (1999). 3. Thakur M., Deb M.K.,Analyst, 124, 1331 (1999). 4. Argekar A.P., Shetty A.K., Talanta, 45, 909 (1998).

Chapter 28. Magnesium

Arsenazo III [1] and Methylthymol Blue [2] were used as chromogenic reagents to determine magnesium in the presence of calcium in natural waters. The determination can be carried out in FIA system.

References 1. Blasco Gomez F., Bosch Reig F., Campins Falco P., Talanta, 49, 155 (1999). 2. Blasco F., Medina-Hernandez M.J., Sagrado S., Fernandez F.M., Analyst, 122, 639 (1997).

Chapter 29. Manganese

Formaldoxime has been used to determine Mn in alloys and pharmaceuticals [1]. Extraction of Mn with bis-(2,4,4-trimethylpentyl)monothiophosphinic acid preceded the spectrophotometric measurement.

A complex formed with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol included on cyclodextrin polymer makes a basis of the determination of trace amounts of manganese in black rice and tea by solid-phase spectrophotometry [2].

Manganese in steel and soils was determined by the use of neotetrazolium chloride [3] and nitro blue tetrazolium [4]. Extraction (CH2C12) of Mn as dimethyldistearylammonium permanganate (after preliminary oxidation of Mn with peroxidisulfate) was used prior to the detection of the metal in steel [5].

Manganese in thin magnetooptical films of the Cd-Mn-Te system was determined by the use of sulfosalicylic acid, salicylfluorone and cetylpyridinium (e = 3.14"105 1 mol-lcm -1) [6]. The detection limit of 1.68" 10 -7 M has been reported.

References

1. Argekar A.P., Shetty A.K., Anal. Sci., 13, 131 (1997). 2. Jiang Z.T., Li R., Xi J.B., Yi B.Q., Anal. Chim. Acta, 392, 247 (1999). 3. Kamburova M., Talanta, 46, 1073 (1998). 4. Kamburova M.A., Nikitova D.D., J. Anal. Chem. (Transl. of Zh. Anal. Khim.), 53, 852

(1998). 5. Barakat S. A., Anal. Chim. Acta, 393, 223 (1999).

500 Appendix

6. Ishchenko N.N., Ganago L.I., Ivanova I.F., J. Anal. Chem. (Transl. of Zh. Anal. Khim.), 53, 23 (1998).

Chapter 30. Mercury

The application of 1,10-phenanthroline and Thymol Blue makes a basis of extraction (CHC13)-spectrophotometric method (e = 1.27" 105 1 mol-lcm -1) for the determination of total Hg in soils, waste water and sewage effluents [1]. Derivative spectrophotometric methods allowing simultaneous determination of Hg and Ni with diethylenetriaminepentaacetic acid [2] and Hg and Cu with methylenediaminetetraacetic acid [3] have been presented.

References

1. Zaki M.T.M., Esmaile M.A., Anal. Lett., 30, 1579 (1997). 2. Perez-Iglesias J., Seco-Lago H., Fernandez-Solis J.M., Castro-Romero J.M., Gonzalez-

Rodriguez V., Anal. Lett., 30, 317 (1997). 3. Seco-Lago H.M., Perez-Iglesias J., Fernandez-Solis J.M., Castro-Romero J.M., Gonzalez-

Rodriguez V.,Anal. Lett., 31, 2747 (1998).

Chapter 31. Molybdenum and tungsten

Extraction of Mo from hydrochloric acid medium with n-octylaniline followed by the detection using the thiocyanate complex was examined [1]. a-Benzoinoxime [2] and OO'- bis(2-ethylhexyl)dithiophosphoric acid [3] were employed to determine Mo in steel.

Dimethoxyhydroxyphenylflurone has been proposed as a chromogenic reagent for the spectrophotometric determination (e = 1.36" 105 1 mol-lcm -1) of trace amounts of Mo in steel and pure iron [4]. Thiazolylazo compounds were used as chromogenic reagents to determine Mo in human urine [5]. Thiazolyl blue tetrazolium bromide and 3,4,5-trihydroxybenzoic acid and nitrobluetetrazolium chloride were employed to determine Mo in soil [6,7].

Molybdenum can be selectively determined in the presence of W in mixed micellar medium using bromopyrolgallol red [8]. The method was applied to the determination of Mo in steel CRM and sea water.

The determination of Mo in soil using thiocyanate and mixed surfactants (CPC and Triton X-100) was described [9]. The method was applied to the determination of Mo in hot water leachates and acid digested soil. Thiocyanate complex was used to determine Mo in oil [10].

Catalytic effect of Mo on the oxidation of KI by H202 makes a basis of the determination of Mo in white wine [ 11 ].

References

1. Sawant S.S., Anuse M.A., Chavan M.B., J. Radioanal. Nucl. Chem., 218, 147 (1997). 2. de Andrade J.C., Cuelbas C.J., de Paula Eiras S., Talanta, 47, 719 (1998). 3. Sasaki Y., Tagashira S., Murakami Y., Ichikawa M.,Anal. Sci., 14, 603 (1998). 4. Lui S.M., Pan J.M., Anal. Lett., 32, 1225 (1999). 5. Amin A.S.,Anal. Lett., 32, 1575 (1999). 6. Kamburova M., Anal. Lett., 31, 2255 (1998).

Appendix 501

7. Kamburova M., Alexandrov A., Chem. Anal. (Warsaw), 44, 745 (1999). 8. Huang X.R., Zhang W.J., Xu G.Y., Han S.H., Li Y., Li C.P., Talanta, 47, 869 (1998). 9. Aggarwal S.G., Patel K.S., Mikrochim. Acta, 129, 265 (1998). 10. Anikina N.A., Zavod. Lab., 64 (8), 12 (1998). 11. Bejan D.,Anal. Chim. Acta, 390, 255 (1999).

Chapter 32. Nickel

Preconcentration of trace nickel with the ion pair of disodium 1-nitroso-2-naphthol-3,6- disulfonate and tetradecyldimethylbenzylammonium chloride on microcrystalline naphthalene (or by the column method) was applied prior to the determination of Ni in standard alloy, steel and biological samples by derivative spectrophotometry [1]. The detection limit of 10 ppb was reported.

The use of chelate complexes with PAR and derivative spectrophotometry allows selective determination of Ni in mixture with Co and Cu [2]. Flow-injection spectrophotometry and PAR were used to determine Ni in silicates and alloys [3]. The detection limits of 77 ng m1-1 was achieved. An online solid-phase extraction system with polyurethane foam was used for separation of Ni from interfering metals (Fe, Cu, Zn and Co). Online analysis of Ni and Cu in industrial effluents using the complexes with PAR has been described [4].

FIA system employing PAN immobilized on a cation-exchange resin (Dowex 50 W) was applied to determine Ni in steel, alloys, petroleum, mineral oil and waste water [5]. The method employing Xylenol Orange allows simultaneous determination of Ni and Fe in alloys and industrial waste water [6]. Ni (Fe, Co and Cu) in alloys and biological materials were simultaneously determined with the use of 1,5-bis(di-2-pyridylmethylene) thiocarbonohydrazide (DMF) [7]. 1,10-phenanthroline and eosine were employed to determine Ni in aluminium bronze [8]. The complex with dimethylglyoxime makes a basis of the determination of Ni in oil [9].

Synthesis of new benzoic acid type thiazolylazo reagents and their application to the determination of Ni has been reported [10,11]. The developed methods were applied to the determination of Ni in aluminium alloys and alloy steels.

References

1. Taher M.A., Talanta, 50, 559 (1999). 2. Kolomiets L.L., Pilipenko L.A., Zhmud' I.M., Panfilova I.P., J. Anal. Chem. (Transl. of Zh.

Anal. Khim.), 54, 28 (1999). 3. Ferreira S.L.C., de Jesus D.S., Cassella R.J., Costa A.C.S., de Carvalho M.S., Santelli R.

E., Anal. Chim. Acta, 378, 287 (1999). 4. Breuil P., Di Benedetto D., Poyet J.P., Analusis, 26, M63 (1998). 5. Ayora Canada M.J., Pascual Reguera M.I., Molina-Diaz A., Fresenius'J. Anal. Chem., 363,

59(1999). 6. Kompany-Zareh M., Massoumi A., Pezeshk-Zadeh S., Talanta, 48, 283 (1999). 7. Garcia Rodriguez A. M., Garcia de Torres A., Cano Pavon J.M., Bosch Ojeda C., Talanta,

47, 463 (1998). 8. Asha K., Tony K.A., Prasada Rao T., Lyer C.S.P., Indian J. Chem., Sect. A: 37, 1144

(1998). 9. Anikina N.A., Zavod. Lab., 64, 12 (1998).

502 Appendix

10. Fan X., Zhang G., Zhu C.,Analyst, 123, 109 (1998). 11. Fan X., Zhu C., Microchem. J., 59, 284 (1998).

Chapter 34. Nitrogen

The use of microwaves in the acceleration of digestion and colour development in the determination of total Kjeldahl nitrogen in soil has been examined [1]. Visual method based on the colour development of indothymol blue formed between ammonia and thymol has recently been developed for the determination of ammonia-nitrogen in environmental waters [2]. UV photo-oxidation of reduced forms of nitrogen to nitrate by peroxodisulfate can make a basis for the determination of total nitrogen in urban and industrial waste waters [3].

Nitrogen dioxide in air and nitrate and/or nitrite in water, soil, analytical-grade KBr, potassium sulfate, ammonium sulfate, potassium nitrate and ammonium nitrate and toothpaste were determined by means of the decrease in absorbance of Neutral Red caused by diazotization with nitrite from the sample [4]. A mixture of oxalic acid, p-aminoacetophenone and N-1-naphthyl ethylenediamine has been proposed as an efficient absorbing system for in situ spectrophotometric determination of nitrogen dioxide in environmental samples [5].

The method for nitrite determination based on the diazotization-coupling reaction by column preconcentration and on the reduction of nitrate to nitrite using the Cd-Cu reductor column has been proposed for the determination of nitrate and nitrite in water and some fruit samples [6]. On-line monitoring of nitrite in fertilizer process streams, natural and waste water effluents based on the diazotization of nitrite in the sequential injection system with N- (1-naphthyl)etylenediammonium dichloride and the formation of a highly coloured dye has been described [7].

References

1. Mason C.J., Coe G., Edwards M., Riby P.G., Analyst, 124, 1719 (1999). 2. Okumura M., Fujinaga K., Seike Y., Honda S., Fresenius'. J. Anal. Chem., 365, 467 (1999). 3. Roig B., Gonzalez C., Thomas O., Anal. Chim. Acta, 389, 267 (1999). 4. Gayathri N., Balasubramanian N., Analusis, 27, 174 (1999). 5. Sunita G., Gupta V.K., Chem. Anal. (Warsaw), 42, 117 (1997). 6. Wang G.F., Satake M., Horita K., Talanta, 46, 671 (1998). 7. Staden F.J., Merwe T.A., Mikrochim. Acta, 129, 33 (1998).

Chapter 35. Oxygen

The spectrophotometric method for the determination of ozone in ozonized air current by measurement of its corresponding iodide-starch complex at 580 nm using a FIA system with chemical gas-liquid transfer microreactor has been developed [ 1 ].

Hydrogen peroxide in rain water has been determined after the formation of the stable oxo-peroxo-pyridine-2,6-dicarboxylatovanadate complex in acid media [2]. The detection limit of 5.8 nmol H202 for 20 ml of rain water sample was reported.

Appendix 503

References

1. Machado E.L., Rosa M.B., Flores E.M.M., Paniz J.N.G., Martins A.F., Anal. Chim. Acta, 380, 93 (1999).

2. Tanner P.A., Wong A.Y.,Anal. Chim. Acta, 370, 279 (1998).

Chapter 36. Palladium

Preconcentration of Pd in the form of the complex with 2-(5-bromo-2-pyridylazo)-5- diethylaminophenol on a column packed with ammonium tetraphenylborate-naphthalene sorbent was described [1,2]. The method has been applied to the determination of Pd in catalysts, alloys and biological materials [1]. The detection limit of 15 ppb was achieved using third derivative spectrophotometric measurement [2].

Sorption of Pd on resin containing imidazolylazo functional groups was applied prior to the determination of the metal with PAN in geological and medicinal samples [3]. Solid (naphthalene)-liquid extraction of the complex with PAN has been described [4].

The method based on the complex of Pd with dibromo-o-carborylchlorophosphonazo was developed for the determination of Pd in catalysts and anode mud samples [5]. Derivative spectrophotometry of the complexes with oxazolidine derivative makes a basis of the simultaneous determination of Pd and Cu in alloys and activated charcoal [6].

The application of thiocyanate and Rhodamine 6G to the determination of Pd in activated charcoal has been presented [7].

Highly selective liquid-liquid extraction of Pd from sulphuric acid medium followed by the determination of the metal with a-benzoilmonoxime has been described [8]. New azo chromophore reagent, para-Cl-phenylazo-R-acid, has been proposed for the determination of Pd(II) [9]. Molar absorptivity (e) equals to 7.7"10 4 1 mol-lcm -I (~nax at 560 nm).

References

1. Pancras J.P., Puri B.K., Ann. Chim. (Rome), 89, 427 (1999). 2. Taher M.A.,Anal. Lett., 31, 2115 (1998). 3. Das D., Das A.K., Sinha C., Anal. Lett., 32, 567 (1999). 4. Gao J.Z., Peng B., Fan H.Y., Kang J.W., Wang X.D., Talanta, 44, 837 (1997). 5. Huang J., Chen X.G., Hu Z.D., Zhao Z.F., Hooper M., Fresenius' J. Anal. Chem., 363, 117

(1999). 6. E1-Sayed A.A.Y., Rahem M.A.A., Omran A.A., Anal. Sci., 14, 577 (1998). 7. Pillai A.R., Ouseph P.P., Ramachandran K.K., Rao T.P., Chem. Anal. (Warsaw), 42, 75

(1997). 8. Ensafi A.A., Eskandari H., Microchem. J., 63, 266 (1999). 9. Hanna W.G., Talanta, 50, 809 (1999).

Chapter 37. Phosphorus

On-line monitoring of phosphate in natural water and effluent stream by the molybdenum blue method using sequential injection analysis has been described [1]. The detection limit of 0.5 mg 1-1 PO4 3- was reported.

504 Appendix

Interferences from chromate, germanate, tungstate and vanadate when determining phosphate by phosphoantimonylmolybdenum blue method have been studied [2].

References

1. Staden J.F., Taljaard R.E., Mikrochim. Acta, 128, 223 (1998). 2. Blomquist S., Westin S.,Anal. Chim. Acta, 358, 245 (1998).

Chapter 38. Platinum

Extraction of Pt(IV) with N-n-octylaniline followed by the determination of the metal by the stannous chloride method has been investigated [1]. The 99.6-99.8% recoveries of Pt from electrical contact, solder and oakay alloys were reported.

The stannous chloride method has been applied to the determination of platinum in Pt and Pt-Ru catalysts with carbon support by direct and derivative spectrophotometry [2]. The calculation of the first-derivative spectra allows the determination of Pt in the presence of Ru.

References

1. Lokhande T.N., Anuse M.A., Chavan M.B., Talanta, 47, 823 (1998). 2. Balcerzak M., Swiqcicka E., Balukiewicz E., Talanta, 48, 39 (1999).

Chapter 39. Rare-earth elements

Sensitive method for the determination of Er(III) (e = 1.36"105 1 mol-lcm -1) in geological materials and glasses with the use of 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol has recently been developed [ 1 ].

References 1. Angelo D., Fernandez J., Martinez L., Marchevsky E., J. Anal. Chem. (Transl. of Zh. Anal.

Khim.), 54, 586 (1999).

Chapter 40. Rhenium

Extraction of ReO4- with basic polymethine dyes from aqueous and aqueous-organic solutions has been investigated [1]. The method based on the complex with rhodamine B and thiocyanate can be used for the determination of Re in the 1-3.63 ppm concentration range [2].

References 1. Kormosh Z.A., Bazel' Y.R., J. Anal. Chem. (Transl. of Zh. Anal. Khim.), 54, 607 (1999). 2. Wahi A., Kakkar L.R., Anal. Sci., 13, 657 (1997).

Appendix 505

Chapter 41. Rhodium and iridium

The preconcentration of chelate complex of Rh with 2-(5-bromo-2-pyridylazo)-5- diethylaminophenol on microcrystalline naphthalene prior to the spectrophotometric determination has been studied [1,2]. The method allowing simultaneous determination of Rh and Ir using derivative spectrophotometry has been developed [2]. Limits of detection were 72 ng Rh and 133 ng Ir.

Attempts to the use spectrophotometry to speciation of Ir(IV) in hydrochloric acid medium by capillary zone electrophoresis have been described [3].

References

1. Taher A.M.,Anal. Chim. Acta, 382, 339 (1999). 2. Pancras J.P., Puri B.K.,Anal. Sci., 15, 575 (1999). 3. Sanchez J.M., Salvado V., Havel J., J. Chromatogr., A, 834, 329 (1999).

Chapter 42. Ruthenium and osmium

The method for the determination of ruthenium in the presence of Pt using the complexes with tin(if) chloride and second derivative spectrophotometry has been developed [1]. The determination of Ru in Pt-Ru catalysts with carbon support was described. Ruthenium in carbon-supported Pt-Ru-Ge catalyst was determined by simple selective spectrophotometric method using the complex with thiourea [2]. A 18-fold excess of Pd or Rh did not interfere with the determination of Ru.

Attempts to use spectrophotometry for investigation of the oxidation states and forms of Ru(IV) in hydrochloric acid solutions were described [3]. Silica chemically modified with thiourea derivatives was used to determine Os in the slime of wet dust collection and in wash acid [4].

References 1. Balcerzak M., Swiqcicka E., Balukiewicz E., Talanta, 48, 39 (1999). 2. Balcerzak M., Swi~cicka E., Bystroflska D., Anal. Lett., 32, 1799 (1999). 3. Dong S., Yang X.Y., Anal. Chim. Acta, 345, 243 (1997). 4. Losev V.N., Bakhtina M.P., Bakhvalova I.P., Trofimchuk A.K., Runov V.K., J. Anal.

Chem. (Transl. of Zh. Anal. Khim.), 53, 1014 (1998).

Chapter 43. Scandium

Extraction of Sc salicylate with triphenylphosphine oxide prior to the determination using Alizarin Red S has been investigated [1]. Separation of Sc from Ti(V), V(V), Cr(VI), Fe(III), Y(III), La(III), Ce(III), Nd(III) and Sm(III) was reported.

References

1. Bhilare N.G., Shinde V.M., Fresenius'J. Anal. Chem., 357, 462 (1997).

506 Appendix

Chapter 44. Selenium

1-Naphthylamine-7-sulfonic acid was used as a reagent to determine Se in pharmaceutical vitamin preparations [1]. The oxidation of iodide by Se(IV) in acid medium, followed by the formation of an ion-association complex between triiodide and Rhodamine B fixed on a lipophilic dextran gel makes a basis of the determination of Se by solid-phase spectrophotometry [2]. The method based on the catalytic effect of Se(IV) on the reduction of thionine with sulfide ion was used to determine Se in Kjeldahl tablets and shampoo [3]. The detection limit was 5 ng m1-1 of Se. Selenium(IV) in the presence of Se(VI) and total Se in water and in a health care product can be determined on the basis of the catalytic effect of Se(IV) on the reaction of Methyl Violet with sulphide (pH 8) [4].

References 1. Pyrzyflska K., Anal. Sci., 13, 629 (1997). 2. Valencia M.C., Arana Nicolas E., Capitan-Vallvey L.F., Talanta, 49, 915 (1999). 3. Mousavi M.F., Ghiasvand A.R., Jahanshahi A.R., Talanta, 46, 1011 (1998). 4. Afldaami A., Mosaed F., J. Anal. Chem. (Transl. of Zh. Anal. Khim.), 54, 1123 (1999).

Chapter 45. Silicon

Total and soluble Si in rain and atmospheric aerosols were determined using FIA system and ammonium molybdate as a reagent merged with oxalic acid and 1-amino-2-naphthol-4- sulfonic acid as a reductant [1 ]. The molybdosilicic acid makes a basis of the determination of Si in airborne particulate matter [2].

References

1. Giacomelli M.C., Largiuni O., Piccardi G., Anal. Chim. Acta, 396, 285 (1999). 2. Wang C.F., Tu F.H., Jeng S.L., Anal. Chim. Acta, 342, 239 (1997).

Chapter 46. Silver

FIA system employing the complex with 1,10-phenanthroline and gallocyanine were applied to determine Ag in fiver water [1 ]. Sorption-photometric method for the determination of Ag (Au, Cu, Hg, Pd and Pt) using immobilized azorhodanines and sulfonitrophenol M has been developed [2].

Diphenylamine in aqueous Triton X-100 medium was applied to determine Ag in photographic print paper [3]. The catalytic effect of Ag on the peroxodisulfate oxidation of Brilliant Cresyl Blue with 1,10-phenanthroline as activator makes the basis of the determination of Ag in river water [4].

References

1. Ensafi A.A., Zarei K., Fresenius'J. Anal. Chem., 358, 475 (1997).

Appendix 507

2. Gur'eva R.F., Savvin S.B., J. Anal. Chem. (Transl. of Zh. Anal. Khim. ), 52, 217 (1997). 3. Pal A., Chem. Anal. (Warsaw), 43, 853 (1998). 4. Ensafi A.A., Abbasi S., Anal. Lett., 30, 327 (1997).

Chapter 47. Strontium and barium

Strontium in high-temperature superconductors and in ground water was determined using Arsenazo III as a chromogenic agent [1]. 10-Fold amounts of Bi, Zn, Cd or Si, 50-fold amounts of Pb or 90-fold amounts of Cu did not interfere with the determination.

References

1. Shkadauskene O.P., Shkadauskas Yu.S., Zavod. Lab., 63 (11), 9 (1997).

Chapter 48. Sulphur

Free and total sulfite in wines were determined using the induced oxidation of manganese(II) and FIA system [ 1]. Sulfur dioxide as a product of the reaction of the examined sample with sulfuric acid diffused through a PTFE membrane to a solution of Mn(II) in acetate buffer of pH 5.5. The oxidized Mn formed reacted with iodide to form iodine that was detected at 352 nm. The DL of 1 mg 1-1 was reported.

A manifold system for the simultaneous determination of SO2 and CO2 in wine has been described [2]. The reaction with p-rosaniline hydrochloride/formaldehyde makes a basis of spectrophotometric measurement (~ = 578 nm).

Derivative spectrophotometry (>>200 nm) was employed to determine SO2 (NH3, NO and NO2) in waste gases [3].

References

1. Silva R.L.G.N.P., Silva C.S., Nobrega J.A., Neves E.A.,Anal. Lett., 31, 2195 (1998). 2. Mataix E., Luque de Castro M.D., Fresenius'J. Anal. Chem., 365, 377 (1999). 3. Vogt F., Klocke U., Rebstock K., Schmidtke G., Wander V., Tacke M., Appl. Spectrosc.,

53, 1352 (1999).

Chapter 49. Tellurium

A method for extraction and spectrophotometric determination of traces of Te(IV) from hydrochloric acid media with 1-(4-bromophenyl)-4,6,6-trimethyl-l,4-dihydro-pyrimidine-2- thiol (4-bromoPTPT) in CHC13 has been described [1]. Tellurium in alloys and in synthetic mixtures with Se(IV) was determined.

Tellurium in semiconductor thin films was determined after complexation and extraction with NN'-di(acetoxyethyl)indocarbocyanine (DAIC) in the presence of various halides (chloride, bromide or iodide) ()~ = 560 nm) [2].

A review of spectrophotometric methods for tellurium determination has been presented [3]. The systems employing inorganic compounds forming coloured products, N-

508 Appendix

and S-containing reagents, and cationic dyes forming with Te anions associates for extraction- spectrophotometry were discussed.

References

1. Kolekar G.B., Anuse M.A., Bull. Chem. Soc. Jpn., 71, 859 (1998). 2. Balogh I.S., Andruch V., Anal. Chim. Acta, 386, 161 (1999). 4. Andruch V., Matherny M., Chem. Listy, 92, 521 (1998).

Chapter 50. Thallium

Preconcentration of T1 in the form of ion associate with rhodamine B on polyethylene powder was applied prior to the determination in tap water, dialysis fluids and alkali-metal salts [ 1 ].

The reaction between TI(HI) bromide and cis-syn-cis-dicyclohexano-18-crown-6 immobilized on silica gel was studied [2]. Sorption-spectrophotometric method for the determination of T1 (3 0.05 mg 1 -~) the presence of heavy metal ions has been proposed.

References

1. do Nascimento D.B., Schwedt G., Mikrochim. Acta, 126, 159 (1997). 2. Zaporozhets O.A., Ivan'ko L.S., Sukhan V.V., J. Anal. Chem. (Transl. of Zh. Anal. Khim.),

55, 130 (2000).

Chapter 51. Thorium

Separation of thorium and uranium from ore samples using anion-exchange column (Dowex l-X8) has been investigated [1 ]. Thorium was eluted with 6 M HC1.

The use of silk fibroin for separating Th from aqueous solutions was studied [2]. Arsenazo III method was employed to determine Th in the filtrate.

The application of liquid emulsion membrane based on trioctylphosphine oxide extractant for the separation of thorium and uranium was described [3]. Thoron I was used to determine Th in aqueous phase.

Trace amounts of Th in geological samples were determined after conversion into molybdothoric acid and reaction with basic dyes [4]. Thorium in ores and minerals was determined using the ternary purple-coloured complex formed between Th(IV), bromocresol orange and cetylpyridinium bromide (~ = 560 nm; ~ = 9.2 x 10 4 1 mo1-1 cm -1) [5].

The application of 2-hydroxy-3-carboxy-5-sulfoarsenazo to the determination of Th in alloys and waste water was described [6].

References

1. Altas Y., Tel H., Eral M., J. Radioanal. Nucl. Chem., 241,637 (1999). 2. Aslani M.A.A., Eral M., Akyil S., J. Radioanal. Nucl. Chem., 238, 123 (1998). 3. E1-Reefy S.A., Selim Y.T., Aly H.F., Anal. Sci., 13, 333 (1997). 4. Wang J.L., Li Z.B., Cheng G.X., Xu Q.H., Mikrochim. Acta, 126, 313 (1997). 5. Khalifa M.E., Hafez M.A.H., Talanta, 47, 547 (1998). 6. Chen Y., Li Z.J., Zhu Z.Z., Pan J.M., Analyst, 124, 1839 (1999).

Appendix 509

Chapter 52. Tin

The method using Stilbazo in the presence of poly(sulfonylpiperidinylmethylene hydroxide) has been developed [1]. The best complexing conditions for the determination of Sn have been determined. The proposed method was tested in analysis of standard reference samples of bronzes.

Gas-phase spectrophotometry with the use of a diode-array spectrophotometer was employed to the determination of Sn (As, Sb and Se) in tap water after preliminary conversion of the analytes into hydrides by treatment with NaBH4 [2].

References

1. Chmilenko F.A., Zhuk L.P., Chmilenko T.S., Kharun M.V., J. Anal. Chem. (Transl. of Zh. Anal. Khim.), 55, 146 (2000).

2. Cabredo S., Galbfin J., Sanz J., Talanta, 46, 631 (1998).

Chapter 53. Titanium

The complex of Ti with H202 makes a basis of the determination of Ti in ilmenites [1] and minerals and alloys [2]. Extraction of the titanium salicylate complex with triphenylarsine oxide in CHC13 preceded the spectrophotometric measurement [2].

Extraction-photometric determination of titanium in alloys using naphthalene-2- sulfonic acid and diantipyrylmethane was investigated [3]. No interference from large quantities of many common metals was reported.

References

1. de Andrade J.B., Nunes G.S., Veiga M.P., Costa A.C.S., Ferreira S.L.C., Amorim A.M.M., Reis S.T., Talanta, 44, 165 (1997).

2. Bhilare N.G., Nambiar D.C., Shinde V.M., Anal. Lett., 30, 173 (1997). 3. Denisova S.A., Lesnov A.E., Petrov B.I., Zavod. Lab., 64 (8), 6 (1998).

Chapter 54. Uranium

The liquid-liquid extraction of uranium(VI) from acetate aqueous solution employing the ion- pair phase separation of perfluorooctanoate ion with tetrabutylammonium ion has recently been reported [ 1 ]. Arsenazo III was used to determine U in the obtained extracts.

A liquid emulsion membrane based on trioctylphosphine oxide extractant was applied for separation of uranium and thorium from Ce, Cu and Cd [2]. Highly selective U022+ imprinted polymer (synthesized using uranium vinylbenzoate, divinylbenzene in styrene and 2,2'-azobisisobutyronitrile) was proposed for purification, preconcentration and determination of the uranyl ion [3]. Dibenzoylmethane was used as a colorimetric reagent. The detection limit of 160 ppt was achieved.

Ammonium tetraphenylborate-naphthalene adsorbent was proposed for the preconcentration and determination of traces of uranium in standard alloys using 2-(5-bromo-

510 Appendix

2-pyridylazo)-5-diethylaminophenol and fourth-derivative spectrophotometry [4] . The method was applied to the analysis of U in coal fly ash and alloys.

The UV absorption spectrum of the complex uranium(VI) with tributyl phosphate in supercritical carbon dioxide has been examined [5]. The spectra from supercritical fluid extraction were similar to those obtained after extraction of the complex into hexane.

Arsenazo III was used to determine natural uranium in urine [6] and simultaneous determination of uranium and plutonium at trace levels in process streams by derivative spectrophotometry [7].

Extractive-spectrophotometric method for the determination of uranium with bromopyrogallol red and dibenzyldimethylammonium chloride was proposed [8]. The determination of U with bromopyrogallol red using nonionic surfactant reagent without preliminary extraction has been described [9].

References

1. Takahashi A., Ueki Y., Igarashi S., Anal. Chim. Acta, 387, 71 (1999). 2. E1-Reefy S.A., Selim Y.T., Aly H.F.,Anal. Sci., 13, 333 (1997). 3. Bae S.Y., Southard G.L., Murray G.M., Anal. Chim. Acta, 397, 173 (1999). 4. Pancras J.P., Puri B.K., Mikrochim. Acta, 130, 203 (1999). 5. Sasaki T., Meguro Y., Yoshida Z., Talanta, 46, 689 (1998). 6. Kalaiselvan S., Jeevanram R.K., J. Radioanal. Nucl. Chem., 240, 277 (1999). 7. Dubey A.N., Relan G.R., Vaidyanathan S., J. Radioanal. Nucl. Chem., 240, 741 (1999). 8. Pathak K.B.B., Das H.K., J. Indian Chem. Soc., 75, 180 (1998). 9. Gorenshtein L.I., Sukhan V.V., J. Anal. Chem. (Transl. of Zh. Anal. Khim.), 54, 418 (1999).

Chapter 55. Vanadium

The use of the complex with N-substituted-phenylstyrylacrylohydroxamic acid and Aliquat 336 for the determination of V in alloys, rock, environmental and clinical samples has recently been reported [1]. Studies on the complexation of V(V) with PAR on the solid phase of a fibrous sorbent filled with an anion exchanger were described [2]. The DL of of 3-4 ng ml -~ V was achieved. The effect of modification of a solid support with 8-hydroxyquinoline and 8-hydroxyquinoline-5-sulfonic acid on analytical characteristics of the reaction of vanadium(V) with PAR on a solid phase was studied [3]. Extraction of V with the use of 3- hydroxy-2-(4-methoxyphenyl)-6-methyl-4H-chromen-4-one was employed to detect V in flue dust and steel [4].

Various chromogenic reagents, such as PAR and 2,2',5,5'-tetraphenyl-3,3'-(p- biphenyl)ditetrazolium chloride (CHC13) [5], 4-nitrocatechol and iodonitrotetrazolium chloride (CHC13) [6], N-phenylcinnamohydroxamic acid (CHC13) [7] and 2'- hydroxyacetophenonebenzoylhydrazone (CHC13) [8] were used to determine V in steels. Vanadium in steel was also determined directly in aqueous solution after conversion into the complex with hydrogen peroxide and PAR [9].

Diphenylcarbohydrazide was used to determine vanadium in alloys, steel, natural and potable water, waste water, blood, urine and soil [10]. The DL of was 20 ng m1-1 was achieved. 2-Hydroxy-l-naphthaldehyde benzoylhydrazone was proposed for simultaneous spectrophotometric determination of V, Cu and Fe [ 11 ].

Spectrophotometric methods applicable to speciation analysis for V in different oxidation states have been reviewed [12]. Attempts to speciation of V(IV) and V(V) with Eriochrome Cyanine R in natural waters by solid-phase spectrophotometry has been described

Appendix 511

[13]. The method allowing the determination of V(IV) in the presence of V(V) using 2-(5- bromo-2-pyridylazo)-5-diethylaminophenol has been presented [14].

References 1. Patel K.R., Menon S.K., Agrawal Y.K., Mikrochim. Acta, 130, 219 (1999). 2. Shvoeva O.P., Dedkova V.P., Savvin S.B., J. Anal. Chem. (Transl. of Zh. Anal. Khim., 54,

712 (1999). 3. Shvoeva O.P., Dedkova V.P., Savvin S.B., J. Anal. Chem. (Transl. of Zh. Anal. Khim.), 55,

21 (2000). 4. Agnihotri N., Dass R., Mehta J.R., Chem. Anal. (Warsaw), 42, 397 (1997). 5. Gavazov K., Simeonova Zh., Alexandrov A.,Anal. Lab., 7, 127 (1998). 6. Simeonova Zh., Gavazov K., Alexandrov A., Anal. Lab., 7, 184 (1998). 7. Abebaw A., Chandravanshi B.S., Chem. Anal. (Warsaw), 43, 33 (1998). 8. Agnihotri N., Dass R., Mehta J.R., J. Indian Chem. Soc., 75,486 (1998). 9. He X., Tubino M., Rossi A.V.,Anal. Chim. Acta, 389, 275 (1999). 10. Jamaluddin Ahmed M., Banoo S., Talanta, 48, 1085 (1999). 11. Reddy V.K., Reddy S.M., Reddy P.R., Reddy T.S., J. Anal. Chem. (Transl. of Zh. Anal.

Khim.), 55, 435 (2000). 12. Antonovich V.P., Chivireva N.A., Presnyak I.S., J. Anal. Chem. (Transl. of Zh. Anal.

Khim., 52, 502 (1997). 13. Bosque-Sendar J.M., Valencia M.C., Boudra S., Fresenius'J. Anal. Chem., 360, 31

(1998). 14. Costa A.C.S., Teixeira L.S.G., Jaeger H.V., Ferreira S.L.C., Mikrochim. Acta, 130, 41

(1998).

Chapter 56. Zinc

A chelating resin prepared by coupling Pyrocatechol Violet to Amberlite XAD-2 was applied to preconcentrate Zn (Ni and Pb) from well water [1]. 4M HNO3 was used to recover the metal ions. Solid-phase extraction of thiocyanate complexes with polyurethane foam makes a basis of quantitative separation of zinc traces from cadmium matrices [2]. PAR was used to determine Zn in aqueous extract.

Zinc (and Cu) in brass and tap water were determined with the use of zincon after preliminary preconcentration on Chelex-100 column [3]. The application of N-hydroxy-NN'- diphenylbenzamidine and diphenylcarbazone to the determination of zinc in airborne dust particulates has been described [4]. Trace amounts of zinc in standard alloys, environmental and pharmaceutical samples were determined by fourth derivative spectrophotometry using PAN as a reagent and ammonium tetraphenylborate supported on naphthalene as an adsorbent [5]. The detection limit was 9.5 ng m1-1 Zn. Submicrogram amounts of zinc in water and rock samples were determined with 5-(2'-carbomethoxyphenyl)azo-8-quinolinol in the anionic micellar medium of sodium dodecyl sulfate [6].

FIA system using thiocyanate and malachite green in the presence of surfactants CPC and Triton X-100 was applied to determine Zn (and Co) in beverages, biological, environmental and pharmaceutical samples [7]. Detection limit of 15 ppb was obtained. Phenylfluorone in the presence of Triton X-100 and cetylpyridinium chloride was used for the determination of Zn in insulin ()~ = 573 nm, ~ = 1.09"105 1 mol-lcm -~) [8]. Zinc in industrial effluents was determined by the use of the complex with PAR [9]. Simultaneous

512 Appendix

determination of Zn and Mn in mixtures using 5,8-dihydroxy-l,4-naphthoquinone (naphthazarin, NAZA) and derivative spectrophotometry was presented [10].

Complexation with PAN immobilized on a Dowex 50Wx4 cation exchanger makes a basis of the determination of Zn in human hair, natural water and pharmaceutical and cosmetic formulations [11]. Flow-injection solid-phase spectrophotometry using TAN immobilized on silica gel was proposed for the determination of Zn in pharmaceutical preparations [12]. Adsorption of the complex with PAR on Sephadex anion exchanger makes a basis of solid phase spectrophotometric method for the determination of Zn in environmental samples [ 13].

References

1. Saxena R., Singh A.K., Anal. Chim. Acta, 340, 285 (1997). 2. Santiago de Jesus D., Souza de Carvalho M., Spinola Costa A.C., Costa Ferreira S.L.,

Talanta, 46, 1525 (1998). 3. Richter P., Toral M.I., Tapia A.E., Fuenzalida E., Analyst, 122, 1045 (1997). 4. Thakur M., Kanti Deb M., Mishra R.K., Chem. Anal. (Warsaw), 43, 843 (1998). 5. Bhalotra A., Puri B.K., Talanta, 49, 485 (1999). 6. Saran R., Baishya N.K., J. Indian Chem. Soc., 76, 416 (1999). 7. Aggarwal S.G., Patel K.S., Fresenius'J. Anal. Chem., 362, 571 (1998). 8. Winkler W., Arenhovel-Pacula A., Chem. Anal. (Warsaw), 44, 725 (1999). 9. Breuil P., Di Benedetto D., Poyet J. P., Analusis, 26, M63 (1998). 10. Sedaira H., Talanta, 51, 39 (2000). 11. Ayora-Canada M.J., Pascual-Reguera M.I., Molina-Diaz A., Anal. Chim. Acta, 375, 71

(1998). 12. Teixeira L.S.G., Rocha F.R.P., Korn M., Reis B.F., Ferreira S.L.C., Costa A.C.S., Anal.

Chim. Acta, 383, 309 (1999). 13. Molina M.F., Nechar M., Bosque-Sendra J.M., Anal. Sci., 14, 791 (1998).

Chapter 57. Zirconium and hafnium

Extraction with triphenylarsine oxide in CHC13 followed by the reaction with Arsenazo III (after stripping of the analytes with 4 M HC1) makes a basis of the determination of Zr and Hf (and Ti) in minerals and alloys [1 ]. Sequential liquid-liquid extraction and spectrophotometric determination of zirconium(IV) with calixarene hydroxamic acid and thiocyanate has been described [2]. Colorimetric determination of Zr and Hf with Xylenol Orange and a liquid- liquid extraction of thenoyltrifluoroacetone complexes using H20-IBMK has been studied [3].

Preconcentration of nanogram amounts of zirconium by chelating ion exchange (containing 8-hydroxyquinoline, resorcinol or hydroquinone and formaldehyde, furfuraldehyde or benzaldehyde) was proposed for the determination of Zr in the presence of Cu(II), Ni(II), Pb(II), Zn(II), Cd(II), Fe(II), Mn(II), AI(III), Cr(VI), Mo(VI), Th(IV), V(V), Ce(IV), V022+ and Ti(IV) [4]. The determination of Zr was performed using Xylenol Orange. FIA system using Xylenol Orange was applied to determine Zr in an eluate from a 99roTe generator [5]. The detection limit of 0.08 ppm has been reported.

References

1. Bhilare N.G., Nambiar D.C., Shinde V.M., Anal. Lett., 30, 173 (1997). 2. Agrawal Y.K., Sanyal M., Shrivastav P., Menon S.K., Talanta, 46, 1041 (1998).

Appendix 513

3. Peralta-Zamora P., Martins J.W., Talanta, 49, 937 (1999). 4. Purohit R., Devi S., Talanta, 44, 319 (1997). 5. Grundpan K., Utamong M., Taylor C.G.,Anal. Comm., 35, 107 (1998).

Index

1,10-phenanthroline 4, 45, 46, 63, 83, 115, 133, 135, 136, 142, 182, 183, 209, 227-234, 244, 268, 291,317, 332, 348, 372, 386, 400, 401, 412. 429, 475, 476, 488, 501, 503, 504, 506, 508, 514

1,1-diphenylethylene 318 1,2,4-trihydroxyanthraquinone 209 1,2-di(4-pyridyl)ethylene 318 1,2-diamino-4-chlorobenzene 386 1,3,4-trihydroxyanthraquinone-2-carboxylic acid

130 1,3,8-trihydroksy-6-methylanthraquinone see:

emodine 1,3-diaza-2-thiadibenzo-15-crown-5 ether 266 1,4,8,11-tetrathiacyclotetradecane 400 1,4-diphenylthiosemicarbazide 357,369, 370, 386,

419 1,5-bis(di-2-pyridylmethylene)thiocarbon-

hydrazide 475 1,8-dihydroxyanthraquinone 253 1,8-dihydroxynaphthalene-3,6-disulphonic acid see:

chromotropic acid 1,1'-diantrimide 124, 125 1,5-diphenylcarbazide 66, 159, 163,214, 225,361,

363,370, 483,494 1-(2-pyridylazo)-2-naphtho153,220, 259, 473,500 1-(2-pyridylazo)-4-cyclopentiresorcinol 184 1 -(2-pyridylmethylidene)-5-salicylidenethiocarbon-

hydrazide 475 1-(2-pyridylmethylideneamino)-3-(salicylideneami-

no)thiourea 221 1 -(2-quinolylazo)-2,4,5-trihydroxybenzene 260 1-(2-quinolylazo)-m-aminopheno1202 1 -(2-thiazolylazo)-2-naphthol 55 1 -(phthal-l-azinyl)-3,5-diphenylformazane 268 14-crown-4 ether 77 15-crown-5 ether 78,403 18-crown-6 ether 78,345,403,404 1-amino-8-naphthol-3,6-disulphonic acid 124 1-aminonaphthalene-7-sulphonic acid 386 1-hydroxy-2-carboxyanthraquinone 109 1-hydroxybenzene-2,4-disulphonic acid 310 1-nitroso-2-naphthol 167, 169, 170, 174, 234, 277,

371 1-nitroso-2-naphthol-6-sulphonic acid 1-phenol-2,4-disulphonic acid 310 1-phenyl-3-methyl-4-stearoyl-5-pyrazolone 471 1-phenyl-3-thiobenzoylthiocarbamide 184

1-phenylthiosemicarbazide 357 1 -salicylidene-5-(2-pyridylmethylidene)izotio-

carbonhydrazide 267 1-tolyl-3-methyl-4-perfluoroacyl-5-pyrazolone 77 2,2'-azinodi(3-ethylbenzothiazolesulphonic acid)

225 2,2'-bipyridyl 107,229, 230, 233,234 2,2'-bipyridyl-[3-glyoxime 233 2,2'-biquinolyl 182, 277 2,2'-dipyridyl-2-quinolylhydrazone 463 2,2'-dipyridyl-2-benzothiazolylhydrazone 143, 291 2,2'-dipyridyl-2-pyridylhydrazone 173 2,2'-dipyridylbis(2-quinolylhydrazone) 475 2,2'-dipyridylketoxime 173 2,2'-diquinolyl-2-quinolylhydrazone 136, 467 2,2'-diquinolylketoxime 173 2,2'-diquinoxalyl 95,163, 215, 234, 235, 331, 348,

439,447 2,3,3'-trihydroxy-4'-sulphoazobenzene 431 2,3,4-trihydroxy-4'-sulphoazobenzene 116 2,3:8,9-dibenzo-4,7,13-trithia- 1,10-diazacyclo-

pentadecane 396 2,3-diaminonaphthalene 386 2,3-dihydroxynaphthalene 504 2,4,6-tri(2'-pyridyl)-s-triazine 4, 233 2,4,6-tris(2-hydroxy-4- sulphonaphthylazo) 1,3,5-

triazine 220 2,4-dinitro-l,8-naphthalenediol 124 2,4-dinitroresorcinol 452 2,6,7-trihydroxyphenylfluorone 446 2,6-dichlorophenolindophenol 412 2,7-dichlorochromotropic acid 444 2,9-dimethyl-4,7-diphenyl- 1,10-phenanthroline 182 2,9-dimetyl- 1,10-phenanthroline (see: neocuproine) 2-(2- thiazolylazo) p-cresol 220, 221 2-(2- thi azolylazo )-4,6-dimethylphenol 452 2-(2- thiazolylazo)-5-diethylaminophenol 365 2-(2- thiazolylazo)-5-dimetylaminophenol 301,456 2-(2-benzothiazolylazo)-5-dimethylaminobenzoic

acid 291 2-(2-pyridylazo)-l-hydroxynaphthalene-4-

sulphonic acid 220 2-(2-pyridylazo)-5-diethylaminophenol 426 2-(2-pyridylmethyleneamino )phenol 291 2-(2-thenyl)benzothiazoline 267 2-(2-thiazolylazo)-4-rnethyl-5-(sulphomethylami-

no)benzoic acid 173

Index 515

2-(3,5-dibromo-2-pyridylazo)-5-(N-ethyl-N- sulphopropylamino)benzoic acid 291

2-(3,5-dibromo-2-pyridylazo)-5-[N-ethyl-N-(3- sulphopropyl)amino]pheno1456

2-(3,5-dibromo-2-pyridylazo)-5-diethylaminophenol 55,400, 475

2-(3,5-dibromo-2-pyridylazo)-5-dimethylaminobenzoic acid 173,466

2-(3,5-dichloro-2-pyridylazo)-5-dimethylaminophenol 172

2-(4'-phenylazo)chromotropic acid 251 2-(4-sulphophenylazo)chromotropic acid 4, 57 2-(5-bromo-2-pyridylazo)- 1,5-diaminobenzene 172 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol

289 2-(5-bromo-2-pyridylazo)-5-propyl-N-sulphopro-

pylaminophenol 360 2-(5-chloro-2-pyridylazo)-5-diethylaminophenol

54, 172 2-(5-chloro-2-pyridylazo)-5-dimethylaminophenol

135,298 2-(6-bromo-2-benzothiazolylazo)-5-diethylamino-

phenol 483 2-(8-hydroksy-2-quinolylazo)- 1-naphthol 142 2-(8-quinolylazo)-4,5-diphenylimidazole 268 2-(o-hydroxyphenyl)benzothiazoline 244 2-amino-4-chlorobenzenethio1393 2-aminodiphenylamine 383 2-aminoperimidine409 2-benzoyl-4-(4-nitrophenyl)acetohydrazine 291 2-carboxy-2"-hydroxy-5'-sulphoformazylbenzene

475 2-furaldehyde-2-pyridylhydrazone 173 2-hydroxyacetophenone 291 2-mercapto-4-methyl-5-phenylazopyrimidine 416 2-mercaptobenzimidazole 240, 360 2-mercaptobenzothiazole 240 2-nitroso-5-diethylaminophenol 225 2-nitroso-5-dimethylaminophenol 463 2-nitroso-l-hydroxynaphthol-6-sulphonic acid 171 2-nitroso-l-naphthtol 174, 291, 371 2-oximinodimedonedithiosemicarbazone 130 2-phenoxyquinalizarin-3,4'-disulphonic acid 109 2-pyridinecarbaldehyde-2-(5-nitro)pyridylhydra-

zone 291 2-pyridinecarbaldehyde-3,5-dinitro-2-pyridylhydra-

zone 291 2-pyridinediaminobenzene 136 2-pyridyl-2-thienyl-13-ketoxime 173 2-pyridylthiourea 357

2-pyrilydene-2-aminopheno1380 2-quinolyl-2-pyridylketone hydrazone 69 2'-quinolylfluorone 484

2-thiophenealdehyde-2-benzothiazolylhydrazone 184

3,3'-diaminobenzidine 384, 386, 467 3,3'-dimetylnaphthydine 46, 317 3,4-diaminobenzoic acid 386 3,4-dihydroxyazobenzene 483 3,5-dinitropyrocatecho1208,277,280, 299, 447,

465,466, 484 3,5-dinitrosalicylic acid 466 3-(2-pyridil)-5,6-diphenyl- 1,2,4-triazine 291,504 3-(2'-thiazolylazo)-)-2,6-diaminotoluene 173 3-(4-metoksyphenyl)-2-mercaptopropenoic acid

291 3-(4-phenyl-2-pyridyl)-5,6-diphenyl- 1,2,4-triazine

233 3-hydroxy-2-methyl-l,4-naphthoquinone monoxime

291 3-hydroxyflavone 299, 484 3-hydroxyflavone 480 3-hydroxypicolinaldehyde azine 290 3-mercapto-l,5-diphenylformazane 63 3-nitroalizarin 438 3-nitrophenylfluorone 117 3-nitroso-2,6-pyridinediol 173, 371,375 3-pyridylfluorone 438, 439, 466 3-thianaphthenoyltrifluoroacetone 346 4,4'-bis(dimethylamino)thiobenzophenone 399 4,5,6-triaminopyrimidine 386 4,5-diamino-2,6-dimercaptopyrimidine 386 4,5-dibromopyrocatecho1296 4,7-diphenyl- 1,I0-phenanthroline 231,429, 488,

503 4,5-dibromo-o-nitrophenylfluorone 300 4-(2-pyridylazo)resorcinol 45, 53, 54, 218, 219,

242, 458, 460, 461,495 4-(2-quinolylazo)pheno1400 4-(2-thiazolylazo)-6-chlororesorcinol 452 4-(2-thiazolylazo)resorcinol 4, 55 4-(2-thiazolylazo)rezacetophenone 429 4-(2'-thiazolylazorezacetophenon) oxime 452 4-(3,5-dibromo-2-pyridylazo)-N,N-diethylaniline

397 4-(3,5-dibromo-2-pyridylazo)-N-ethyl-N-(3-sul-

phopropyl)aniline 184 4-(4-methyl-2-thiazolylazo)resorcino1233,363 4-(p-nitrophenylazo)-2-amino-3-pyridinol 397 4-amino-4'-chlorobiphenyl 412 4-aminoantipyrine 318, 437 4-butyl-2-(a-methylbenzyl)pheno177 4-butylnioxime 290 4-chloro-2-nitroso-l-naphthol 318 4-Me-cyclam- 14 135

516 Index

4-methyl-2-(2'-hydroxy- 1 '-naphthylazo)-thiazole 184

4-methylpiperazinedithiocarbamate 413 4-nitro-l,2-diaminobenzene 386 4-nitropyrocatechol 103,208 4-picrylaminobenzo- 15-crown-5 ether 69

5,5-dimethyl-l,3-cycloheksanedione 386 5,5'-dithiobis(2-dinitrobenzoic) acid 409, 410 5,7-dichloro-8-hydroksyquinoline 68,299 5,7-dichlorooxine 346 5-(2- thiazolylazo)-ethylamino-p-cresol 363 5-(2-carboxyphenylazo)-8-hydroxyquinoline 268 5-(2-pyridylazo)-2-monoethylamino-p-cresol 439 5-(2-pyridylazo)-p-cresol 363 5-(8-quinolylazo)-2-monoethylamino p-cresol 466 5-bromosalicylfluorone 87,438 5-bromosalicylhydroxamic acid 466 5-chloro-7-iodo-8-hydroxyquinoline 298 5-methylfurfural- 1 -phthalazinehydrazone 291

5-nitrobarbituric acid 78 5-phenyl-3-(4-phenyl-2-pyridyl)- 1,2,4-triazine 233 5-phenylazo-8-aminoquinoline 324 5-sulphobutyloamide-2'-methoxyphenylanthranilic

acid 149 6-amino-l-hydroxynaphthalenesulphonic acid 386 6-nitrodimethylenecarbocyanine 173 7-(4,5-dimethyltiazolylazo)-8-hydroxyquinolino-5-

sulphonic acid 423 7-iodo-8-hydroxyquinoline-5-sulphonic 234 8-hydroxyquinaldine 82, 109 8-hydroxyquinoline 4, 7, 10, 11, 13, 14, 45, 57 8-hydroxyquinoline-5-sulphonic acid 299 8-mercaptoquinoline 68, 116 9-(2'-hydroksyphenyl)-2,3,7-trihydroxy-6-fluorone

300 absorbance 29 absorbance additivity law 29 absorption spectra 48 acetylacetone 7, 12, 69, 82, 106, 167, 168,227, 378

ajatin 148 Alizarin Black 431 Alizarin Complexone 191, 192, 196, 208, 209, 277 Alizarin Red see: Alizarin S Alizarin S 87, 117, 142, 195, 196, 221, 234, 240,

253,467,480, 481,484, 513, Aluminon 86, 109, 492

amiloride 225, 351,353 aminobenzenethiol 276 anion-exchangers 19 anthranilic acid 132,147,149,456 antipyrine 9, 130, 208, 214, 218, 244, 318, 393,

439,446, 480, 483,484, 497,

antraquinone-2-sulphonate 316 Arsenazo DAL 431 Arsenazo 1 21, 55, 56, 195,240, 301,347,349, 380 Arsenazo II121, 45, 53, 55, 56, 71, 116, 135, 142,

143, 163, 195,233,244,252, 312, 323,330, 343,347, 378,401,426, 427,448, 449, 452, 471,476, 478,493,501,510, 512, 516

arsonophenylazochromotropic acid 295 auxochrome 28 aza-12-crown-4 ether 77 Azo-azoxy BN 139, 403 Azo-azoxy BT 140 Azomethine H 124, 125 barbituric acid 146, 147, 148, 149 bathochromic effect 28 bathocuproine 182, 183, 185, 186, 500 bathocuproinedisulphonic acid 183 bathophenanthroline 148, 227, 228, 231,232, 233,

234, 244, 386 Beer's law 28, 29, 30, 33, 35, 37, 40, 48, 84, 195 Benzhydrazide 225,357 benzidine 147, 155,412 benzil-2-pyridylketo-2-quinolylhydrazone 173 benzohydroxamic acid 68, 106, 446, 466 benzo-14-crown-4 ether 77 benzo-18-crown-6 ether 78, 112, 272 benzothiazole-2-aldehyde-2-quinolylhydrazone 184 benzoylacetone 447 benzoyltrifluoroacetone401,452 Berbelin Blue 317 Berthelot reaction 305 Beryllon II 109, 110, 251 biacetylo-bis(4-phenyl-3-thiosemicarbazone) 290 Bindschedler's Green 63,268,497 bis(2-ethylheksyl)phosphoric acid 206 bis(nonylphenyl)polyoxyethylene ether 470 bis-cycloheksanone oxalyldihydrazone 183 Bismuthiol II 116, 244, 357,375,384, 386, 417,

418,420 blank test 41, 42, 43, 49, 103,473 Bouguer-Lambert-Beer's law 28, 29 Brilliant Green 60, 87, 94, 103, 117, 124, 130, 154,

202, 208, 214, 220, 224, 244, 267, 279, 298, 323, 330, 354, 372, 390, 397, 420, 421,435, 443,452, 458, 461,489

Brilliant Violet B 109,380 Bromobenzothiazo 135 Bromocresol Green 78 Bromo-oxine 68, 142, 253,280, 431 Bromophenol Blue 63, 128, 135,234, 244,475,500

Bromophenol Red 234, 475

Index 517

Bromopyrogallol Red 11,45, 59, 87, 95, 117, 148, 203,209, 215, 220, 234, 244, 245, 274, 276, 277,280, 296, 297, 298,301,348, 389, 400, 401,426, 431,439, 446, 456, 466, 484, 493, 517

Bromothymol Blue 78 Butylrhodamine B 103, 116, 244, 268, 301, 419,

438,456 Cacotheline Blue 316 Cacotheline149 Cadion 135, 136 Cadion 2B 268,400, 412, 475 Cadion A 268 Calcein 59, 142 Calcichrome 109, 142, 143 calibration curve see: standard curve 47, 48, 153 calixarene-hydroxamic acid 429, 520 Calmagite57, 78, 252, 348 Capri Blue 62, 124, 301,340, 369, 370, 371,375 Carboxyarsenazo 347,348,405,485,498 Carboxybenzene S 260 Carboxynioxime 290 Carboxynitrazo 142,348,404 Carmine Red see: carminic acid carminic acid 109, 122, 203,277,432, 456, 467 carriers 12, 13, 14, 240, 345 cation-exchangers 20 Cationic Rose 2S 426 cetyldimethylbenzylammonium chloride 135, 218,

474 cetylpyridinium chloride 201,243, 516, 519 cetylpyridinium ion 4, 45,207, 232, 506 cetyltrimethylammonium bromide 85, 108,202,

207, 219, 232, 277 cetyltrimethylammonium chloride 85, 108, 202,

219, 232, 277 cetyltrimethylammonium ion 4, 45,232 chelating resins 20, 21,239, 337,480. 488 chloramine T 148 chloranilic acid 195,393 chlorex 227

chloro-oxine 68, 132, 173,259, 290, 435,436, 462 chlorophosphonazo DAL 348 chlorophosphonazo I 86,252 chlorophosphonazo III 53, 56, 140, 141, 142, 252,

348,380, 404, 405,412, 431,456, 457 chlorophosphonazo p-C1 378 chlorophosphonazo R 109 chlorpromazine 280

Chromal Blue 86, 109, 202, 233,324, 380, 456 Chromazol KS 86

Chrome Azurol S 4, 45, 58, 83, 84, 85, 87, 95, 107, 108, 142, 163, 195, 202, 220, 232, 317, 323, 346, 361, 370, 378, 428, 443, 452, 462, 480, 488,490

chromophore 27, 28, 244, 510 Chromotrope 2B 57 Chromotrope 2R 252 chromotropic acid 55,404, 405,443,444, 445,447,

498 Chromoxane Blue 86, 202 Chrompyrazole I 214, 244, 363 Claytona yellow see: Titan Yellow Cobaltone see: 1-nitrozo-2-naphthol Congo Red 252 Coprecipitation 5, 12, 13, 14, 16, 91, 93, 139, 153,

159, 206, 213,223,322, 416, 436 crown ethers 11, 45, 69, 76, 77, 78, 79, 178, 199,

227,244, 264, 403,422, 480, 488 cryptand 10, 69, 76, 78, 136, 140, 244, 403,451,

480 cryptates 10 Crystal Violet 16, 60, 61, 94, 103, 104, 124, 130,

135, 136, 154, 155, 202, 208, 220, 224, 225, 268, 277, 280, 291, 301, 312, 331, 340, 341, 356, 365, 371, 372, 374, 392, 400, 412, 426, 438,456, 503

cupellation 211,320, 321,337,369 cupferron 7, 8, 10, 12, 13, 82, 83,91,199, 261,295,

442, 461,479 cupral see: sodium diethyldithiocarbamate 180 cuprizone 69, 179, 183, 184, 186 cuproine 43, 78, 178, 179, 182, 183, 185,500 curcumin 121, 122, 125, 291 cuvette30, 31, 2, 40, 41, 48, 75, 193, 194, 242, 489 cyanolyzis 409 cycloheksylfluorone 185 cycloheptane-l,2-dione dioxime see: heptoxime cyclohexane-l,3-dione bisthiosemicarbazone 225 derivative spectrophotometry 3, 33, 35, 36, 38, 45,

142, 163, 173, 174, Devarda's alloy 306 di-(2-ethyl)hexylsulphoxide 447 di-(2-naphthyl)thiocarbazone 53 di-2-pyridyl-ketothiosemicarbazone 173 diacetylglyoxime 4, 10,14,69, 133, 173,266, 287,

288, 289,290, 291,292, 313, 321,322, 324, 357,508

diantipyrylmethane 4, 7, 9, 69, 276, 442, 443,484, 513

dibenzo-18-crown-6 ether 273

dibenzoylmethane 69,132, 452,454, 457, 517 dibromofluorescein 135,475

518 Index

dibromophenylfluorone 95,280, 438 dicarboxidine 154 Dicarboxyarsenazo 348 dicyclohexyl-18-crown-6 199, 239, 264, 396 diethazine 278 diethylammonium diethylditiocarbamate 67, 98,

181 diethyldithiophosphoric acid 91, 98 differential spectrophotometry 34, 39 dilituric acid 78 dimedro1445 dimethyloglyoxime 4, 10, 14, 69, 133, 173,266,

286, 287 dimethylosulphonazo III 41"2 dinaphthtizone 66, 401 dinaphthylthiourea 419 di-n-butyldithiophosphoric acid 8 diphenylcarbazone 66, 130, 136, 148, 154, 159,

184, 225,245,267, 276, 467, 471,501,515 diphenylglyoxime 290 diphenylguanidine 9, 54, 87, 192, 202, 209, 220,

221,280, 288,324, 348,363,437,446 diphenylthiocarbazide 419 diphenylthiocarbazone 66, 130, 136, 148, 154, 159,

225,245,268,471,505,519 dipicrylamine 79 Dispergator BO 85 distribution coefficient disulphophenylfluorone 208,438, 446 dithiobenzoylmethane 116 dithiol 103,274, 275,276, 277,278, 279, 280, 281,

331,353, 357,438, 439 dithiopyrilmethane 416 dithizone 7, 10, 21, 40, 45, 63, 64, 65, 66, 78, 92,

112, 113, 117, 132, 133, 138, 148, 167, 174, 178, 179, 184, 214, 217, 225, 239, 240, 241, 419, 420, 421, 423, 435, 567, 468, 469, 471, 495

ditiooxamide 277,357,371,375,386 dual wavelength spectrophotometry 34 emodine 142, 253 eosine 46, 508 Eriochrome Azurol B 324, 456 Eriochrome Azurol G 324, 446 Eriochrome Black B 252 Eriochrome Black T 45, 57, 92, 109, 142, 179, 202, 220, 249,251,252, 253 Eriochrome Brilliant Violet 380 Eriochrome Cyanine R 4, 58,83, 86, 87, 107, 109,

163, 185, 191, 193, 194, 195,200, 201,202, 218,219, 220, 232, 233,253,324, 380, 446, 456, 466, 488, 492, 518

Erythrosin 63, 135, 136, 140, 260, 348,404, 475 ethyl eosin 340 Ethyl Rhodamine B 61,484 Ethyl Violet 103, 124, 185, 186, 244, 331 ethylenediaminotetraacetic acid 4, 162 extraction, efficiency 5, 471 extraction, percent 5 extraction, separation of matrix extraction, separation of traces 10, 11, 12, 13, 14,

15, 17, 63, 159, 320 extraction 7 extraction systems 6, 7, 8, 10, 508 ferroin 124, 148, 155,224, 230, 234 ferron 68, 87,185,234, 348, 439 ferrozine 103, 163, 173, 185, 233, 234, 313, 372,

412, 439, 467,504 fire assay 211,320, 337,369, 397 flow injection analysis 3, 26, 36, 37, 117, 125 fluorescein 59, 63, 129, 130, 135,209, 412, 475 fluoresceincomplexone see: Calcein fluorexone 59 formaldoxime 69, 233,255,257, 258, 261,290,

348,467,506 formazan21, 63, 77,269, 492 Fuchsin(e) 60, 130, 244, 356, 495 Gallein 59, 220, 280, 348,439 glycinymine diacetylmonoxime 29 glyoxal bis(2-hydroxyanil) 140, 141 Griess method 309, 310 guanylhydrazone-3,4-dihydroxybenzaldehyde Gutzeit method 17, 98, 99, 100, 101,102, 103 haematein 203 haematoxylin 87,203,221,234, 372, 439, 447 heptanone oxime 324 heptoxime 290 Hydroxynaphthol Blue 142 hyperchromic effect 232, 276 hypsochromic effect 28 indigocarmine 317 indigodisulphonate 318 indigotrisulphonate 318 indoferron 380 iodo-oxine 252 ion flotation 15 ion-exchange 5, 18, 19, 20, 248, 342, 414 isobutyldithiopyrilmethane 416 isonitrosoacetophenone 37 I isonitrosobenzoylacetone 234 Kjeldahl method 306, 508, 513 1,r-diantraquinonylamine (see: 1,1'-dianthrimide) lead tetramethylenedithiocarbamate 114 leuco-quinizarin 253 Lumogallion 86, 202, 220, 299

Index 519

macrocyclic compounds 9, 76, 185,396, 400, 488 Magneson I1252 Magon 252 Magon sulphonate 252 Malachite Green 60, 95, 103, 104, 124, 135, 136,

154, 163 mandelic acid 95, 124, 203,277, 479 mannito173, 128 masking 5, 15 masking reagents 44

m-carboxychlorophosphonazo 431,432 measuring error 34, 42 Meldola's Blue 422

mercaptopropionic acid 357 Metalphthalein 142, 143 Metanilic Yellow 244, 277 Methyl Blue 208 Methyl Green 95,301,340, 356, 426, Methyl Orange 46, 63, 78, 130, 154, 155,234, 363 Methyl Red 151 Methyl Violet 60, 94, 103,268,299, 300, 302 methyl-2-pyridylketoxime 357 Methylene Blue 16, 46, 62, 95, 104, 121-125, 148,

154, 163, 185, 202, 208, 214, 215, 224, 268, 277, 301, 312, 317, 325, 331, 348, 375, 386, 393,400, 407,408, 412, 413,426, 447,498

Methylene Green 62, 393 methylfluorone 60, 95,277 methylglyoxalobis(4-phenyl-3-thiosemicarbazone)

471 Methylthymol Blue 58, 86, 130, 148, 154, 196, 202,

203,220, 348,380, 412, 431,484, 496, 505 molar absorption coefficient 39, 40, 75 molar absorptivity 39, 40 morin 163,234, 235,277, 431,439, 484 morpholin-N-dithiocarbamate 181 murexide 142, 496

N,N-diphenylbenzamidine 115,278,435,493 N-( 1-naphthyl)ethylenediamine 310 N-(ditiocarboxy)sarcosine 178 N-(m-tolyl)-N-phenylhydroxylamine 462 naphthoquinone dioxime 289 naphthylbismuthiol 418

N-benzoyl-N-(o-tolyl)hydroxylamine 466 N-benzoyl-N-phenylhydroxylamine 4, 7, 68, 462, 465

N-benzyl-2-naphthohydroxamic acid 466 N-benzylbenzohydroxamic acid 466 N-cinnamoyl-N-phenylhydroxylamine 462 neocuprizone 184

neocuproine 155, 182, 183, 184, 185,224, 318, 500 Neonickelone 290

Neothoron 431 Nessler method 26, 306, 308,309 N-ethyl-N-(sulphopropyl)aniline 318 N-furoylphenylhydroxylamine 68 N-hydroksy-N,N-diphenylbenzamidine 456, 505 nickelone 290 Nile Blue 62,124, 208, 214, 215, 277, 301, 312,

340, 357, 380, 412, 431,484, 496, 505 Nile Red 277 Nitchromazo 404, 405, 412 Nitroanthranylazo 77 nitrobenzoazopyrocatechol 259 nitrochromazo 401 nitro-orthanilic acid S 401 nitrophosphonazo 1 252 nitrosodibenzylaniline 342 nitroso-R salt 167, 170, 171, 174, 363,488 Nitrosulphonazo III 277 Nitrotetrazolium Blue 357,499 N-methylanabazine-ct'-azo p-cresol 466 N-methyl-o-phenylenediamine 386 N-p-chlorophenyl-2-furohydroxamic acid 205,206 N-phenylbenzohydroxamic acid 68 N-phenylbenzylhydroxamic acid 480 N-phenylcinnamohydroxamic acid 518 N-phenyl-laurohydroxamic acid 446 N-p-tolylbenzohydroxamic acid 348 N-p-tolyl-p-methoxybenzohydroxamic acid 482 N-salicylohydroxamic acid 466 N-a-phenylstyrylacrylhydroxamic acid 289 o-arsanilic acid 130 o-chlorophenylofluorone 208 o-cresolphthalein 142, 143,253 octadecyldithiocarbamate 45,363,372 o-dianisidine 225 o-hydroxyhydroquinonephthalein 361 o-hydroxythiobenzhydrazide 355 o-nitroaniline 310 o-nitrophenylfluorone 163,277,280, 298, 299, 301,

438,484

o-phenylenediamine 383,386, 467,504 organic solvents, properties 6 Orthanilic K acid 412 Orthanilic S acid 404 o-tolidine 46, 130, 154, 155,318, 348 oxine 4, 21, 67, 68, 82.83, 84, 107, 116, 132, 139,

142, 173,217,220, 221,227, 244, 249, 252,347, 348, 422, 431,435,438,439, 461, 463,464, 466, 471

p- phenylenediamine 147, 148,509 p-Acetylarsenazo 345

520 Index

p-acetylchlorophosphonazo 345,347, 378 Palladiazo 324, 325 pararosaniline 46, 407, 410, 411, 413 partition coefficient 6, 41,206 p-bromomandelic acid 479 p-chloromandelic acid 277 p-chlorophosphonazo 345 p-dimethylaminobenzylidenerhodanine 214, 400 p-dimethylarsenazo II143 phenanthrenequinone monoxime 373 phenazo 252 Phenol Red 128, 129, 149, 224 phenolphthalein 148, 149, 154, 193, 194, 289, 318,

347 phenolsulphophthalein see Phenol Red phenoxyquinalizarin 109 phenyl-2-pyridylketoxime 233,357 phenylfluorone 60, 87, 95, 163, 173,202, 206, 207,

208,209, 220, 234, 276, 279, 297, 300, 330, 378, 390, 428, 433,435,443,463,471,480, 498,516

phenylthiourea 357 phthalimidebisthiosemicarbazone 225 piazselenol 381 Picramine M 202, 220 Picramine-epsilon 57, 301,484 piperazinebis(dithiocarbamate) 181 p-nitrobenzenediazoaminoazobenzene see Cadion p-nitrochlorophosphonazo 348,380 p-nitrophenoxide 76 p-nitrosodiphenylamine 324 p-phenylazo-3-aminorhodanine 267 precipitation flotation precision 26, 42 promazine 280, 297,375 propericiazine 277 propylfluorone 277,438 Purpurin 209, 234 pyridine-2-aldehyde-2-quinolylhydrazone 69, 291 pyrocatecho157, 87, 95, 103, 117, 124, 196, 202, 234, 276, 295,443 Pyrocatechol Violet 58, 86, 95, 117, 196, 202, 209, 220, 233,240, 243,280, 348,436, 437,439, 446, 466, 483,485,496, 514 pyrogallo1277,299,300, 301,302, 466, 467 Pyrogallol Red 59, 117,130, 163, 202, 220, 221,

233,244, 275,436, 443,452, 463 pyrogallolbenzoin 220 pyrogallolsulphonic acid 301 Pyronine G 267,409, 412 pyrrolidinedithiocarbamate 181 quercetin 277,280, 431,439, 484

quinalizarin 203,277 quinalizarin complexone 192 quinizarin 77,281 quinoline 148 quinoline-2-aldehyde-2-quinolylhydrazone 184 quinonylphenylphosphoric acid 451 quinoxaline-2,3-ditiol 291 Rezarson 439 Rezazurin 244 Rhodamine 3B 61 Rhodamine 6G 16, 61, 62, 95, 116, 135, 163, 164,

173,208 rhodanine 21,214, 215,269, 324, 341,397,400 Rodamine B 61, 62, 92, 93, 95, 116, 142, 148, 200,

201,208 rosaniline 129 Rose Bengal 135, 185,244, 291,400 Rosocyanin 121, 125 rubeanic acid 277,357, 371,375,386 rubrocurcumin 121, 125 rutin 195,277, 280 Safranine T 62, 95, 317,357,393,426 salicylaldehyde guanylhydrazone 225 salicylfluorone 60, 208, 261, 277, 278, 280, 281,

299, 301,438,446, 466, 506 salicylic acid 309, 348, 447, 451 salicyloylhydrazone 87 Seignette salt 241,308 selectivity sensitivity 39 silver diethyldithiocarbamate 100, 101,494 silver p-sulphamoylbenzoate 149 Sintanol DS-10 85

Sintanol DS-7 85 sodium diethyldithiocarbamate 66, 67, 114, 181 specific absorptiviy 40 spectrophotometry 26 spektrophotometric titration 34 standard curve 33, 48, 75 standard curve technique 33 standard solutions 33, 34, 47, 75, 141, 153, 213,

228,250, 297,380, 418 Stilbazo 86, 516 sulphanilic acid 149, 225,234, 309 Sulpharsazen 179, 220, 244, 475 sulphoalthiox Sulphochlorophenol S 57, 109, 196, 298, 302 Sulphochlorophenolazorhodanine 363,372 Sulphochrome 86, 233 Sulphonazo III 21,348,404, 405, 412 Sulphonitrazo E 298 Sulphonitrophenol 325

Index 521

Sulphonitrophenol K 466 Sulphonitrophenol M 57, 298,302, 324, 341

Sulphonitrophenol S 86, 109, 277, 331 sulphosalicylic acid 195,234, 261,424, 456 surfactants 45 tetrabromofluorescein 63, 130, 209 tetrabromophenolsulphophthalein 63, 128 tetrabromopyrocatecho1299, 447 tetrabromosalicylfluorone 208 tetradecyldimethylbenzylammonium ion 45,484,

504 tetrahydroxyphenazine 447 tetraiodofluorescein 63 tetrakis(1-methylpyridinium-3-yl)porphyrin 471 tetrakis(4-sulphophenyl)porphyrin 475 tetrakis(3-chloro-4-sulphophen yl)porphyrin 475 tetramethyltetra-azacyclotetradecane 135 tetramethylthiourea 419 tetraphenylarsonium ion 9, 120, 158, 161,199, 255, 273,277,294, 324, 351,353, 358,362, 366, 442 Tetrazolium Violet 476 Tetron 348 thenoyltrifluoroacetone 4, 7,345,348, 429, 479,

488,520 Thiazole Yellow 250 thiazole-2-carbaldehyde-2-quinolylhydrazone 291 Thiazolyl Blue 286, 499, 507 thiazolylazopyrocatecho1446 thiobarbituric acid 116, 371 thiobenzhydrazide 357 thiobenzoylacetone 173,268 thiocarbamide 438 thiocyanates 11, 20, 146, 445 thiodibenzoylmethane 136, 173,268,280, 324, 426 thioglycolic acid 84, 86, 277,278, 280, 299, 461 thio-Michler's ketone 69, 155, 184, 212, 214, 265,

267, 268, 321, 323, 324, 325, 387, 399, 401, 501

thiourea 20, 91, 94, 112, 116, 168,221,231,239, 274, 289, 324, 329, 337,355,357,360, 369, 371,375,385,419, 472, 488,491,512

Thoron I 55, 77, 109, 220, 412, 429, 516 Thymol Blue 58, 78,86, 130, 135, 148, 149, 154,

196, 202, 204, 220, 348, 372, 380, 412, 431, 484, 493,, 496, 502, 505,506, 508

thymolphthalein 142, 149 tiocyanates 11, 20, 146, 445

tiooxine see: 8-mercaptoquinoline Tiron 163,234, 277, 281,446, 447,504 Titan Yellow 249, 250, 253 toluene-3,4-ditiol see: dithiol Toluidine Blue 95,277 trace analysis 49 traces, separation by distillation 92, traces, separation by flotation 15 traces, separation by precipitation 12, 13 traces,extractive separation 10, 11 tribenzylamine 9, 91,158, 161,264, 363,439, 471 tribromocarboxyarsenazo 485 trifluoroethyl xanthate 416 tri-n-butyl phosphate 4, 8 tri-n-butylacetohydroxamic acid 482 tri-n-octylamine 4, 9, 474 tri-n-octylphosphine 4, 8 triphenylphosphine oxide 296, 339, 513 tris(2-ethylhexyl)phosphate 439 tropolone 451 turbidimetry 37 Turquoise Blue 173 tyrodine 244, 341 Unithiol 371 Variamine Blue 154 Viktoria Blue 4R 61,202, 244, 301,340, 357, 419,

426, 475, Violet Red 416 Volatility 17, 66, 422 Xylenol Orange 21, 58,86, 109, 113, 115, 117, 154,

163, 195, 202, 220, 240, 244, 268, 280, 291, 299, 318, 348, 379, 380, 386, 431, 438, 466, 480, 481,482, 483,484, 504, 508,520

Xylidyl Blue 252 Zephiramine 45, 59, 85, 86, 108, 141, 163, 201,

202, 207, 209, 219, 220, 232, 233, 252, 276, 280, 291, 324, 331, 380, 431, 453, 464, 466, 483

zinc dibenzyldithiocarbamate 114, 181 Zincon 11, 291,475,476, 500, 519 a,~3,7,~-tetrakis(4-sulphonyl)porphyrin 173 a,13,7,~-tetraphenylporphyrintrisulphonic acid 185 ct-benzoinoxime 272, 275,278, 295,357, 488,507 a-benzoyldioxime 355 a-benzyldioxime 357 a-furyldioxime 69,288, 357, 372 a-nitroso-13-naphthol 169

separation preconcentration and spectrophotometry in inorganic analysis

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