extraction and micro determination of niobium(v) with n-α-phenylstyrylacrylohydroxamic acids

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Extraction and Micro Determination of Niobium(V) withN-a-Phenylstyrylacrylohydroxamic Acids

KALPESHPATEL, S. K. MENON, AND Y. K. AGRAWAL*

Chemistry Department, School of Sciences, Gujarat University, Ahmedabad 380 009, India

Received September 29, 1994; accepted March 1, 1995

A sensitive and selective method for the extraction and microgram determination of niobium(V)with N-p-chlorophenyl-a-phenylstyrylacrylohydroxamic acid (PCPPSAHA) is described. The nio-bium-PCPPSAHA complex has almaxof 385 nm and a molar absorptivity of 3.7 × 104 l mol−1 cm−1.The system obeys Beer’s law in the range 0.2–4 ppm of niobium(V). The Sandell sensitivity is 0.0025mg cm2. The sensitivity of the method is increased using thiocyanate. The Nb-PCPPSAHA-SCNcomplex has almax of 390 nm and a molar absorptivity 4.7 × 104 l mol−1 cm−1. Niobium is direct-ly determined by graphite furnace atomic absorption spectrophotometry with detection limit20 pg. © 1996 Academic Press, Inc.

INTRODUCTION

Several spectrophotometric methods have been reported for the determination of nio-bium (1–10). These methods are nonspecific and are interfered with by many closelyassociated diverse ions, such as tantalum, molybdenum, titanium, and zirconium. Theatomic absorption method for niobium with a nitrous oxide acetylene flame is less sen-sitive and not free from interference (11). Niobium is also determined by an amplificationprocedure in which molybdoniobophosphoric acid is extracted into butanol and deter-mined by AAS (12), but the method is not free from interferences. Several hydroxamicacids have also been used for the gravimetric and spectrophotometric determination ofniobium, but either they are less sensitive or they have diverse ions interferences. It hasbeen observed that sometimes introduction of a double bond in the side chain of hydrox-amic acids increases the selectivity and sensitivity of the reagent (13–15). With this viewthe newa-phenyl substituted styrylacrylohydroxamic acids have been synthesized andused for the extraction and spectrophotometric determination of niobium(V). Further, theextract is directly aspirated to the furnace, which increases the sensitivity and detectionlimits 25-fold by virtue of efficient nebulization of the solvent and concentration ofniobium in a smaller volume of the solution by extraction into chloroform. The method isselective, as most of the diverse ions are not extracted with niobium(V).

EXPERIMENTAL

Reagent

All the chemicals used were of analytical reagent grade, unless otherwise stated.N-p-chlorophenyl-a-phenylstyrylacrylohydroxamic acid (PCPPSAHA) and its analogues were

* To whom correspondence should be addressed.

MICROCHEMICAL JOURNAL 53, 158–163 (1996)ARTICLE NO. 0022

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0026-265X/96 $18.00Copyright © 1996 by Academic Press, Inc.All rights of reproduction in any form reserved.


synthesized as described elsewhere (16). Their 0.2% solutions in toluene or methanol anda 2% w/v solution of Aliquat 336 in toluene was prepared.A standard niobium(V) solution was prepared by fusing 0.626 g of niobium pentoxide

with potassium bisulfate and the fused mass was dissolved in 1M ammonium oxalatesolution by boiling and made up to 250 ml. Its final concentration, 9.42 × 10−3 M, wasdetermined spectrophotometrically (17). The toluene was purified as described by Weiss-berger (18).

Apparatus

The spectral measurements were made on a Hitachi 3210 spectrophotometer using 10mm quartz cells. A GBC 902 atomic absorption spectrophotometer with a 1000 graphitefurnace system was used for AAS measurements. The analytical wavelength 334.4 nmwas used for niobium.

Procedure

Into a 60 ml separatory funnel was transferred an aliquot of niobium(V) solution (5–100mg/ml). Adjustment of aqueous phase to a total volume of 25 ml with 10M HCl was madewith concentrated HCl and distilled water. After 5 ml of 0.2% reagent solution(PCPPSAHA) in toluene was added the contents were shaken gently for 5 min. Theorganic phase was separated, dried over anhydrous sodium sulfate, and transferred into a25–ml volumetric flask. The anhydrous sodium sulfate was washed with toluene and the

TABLE 1Special Data and Effect of Acidity on the Extraction of Niobium–a-Phenylstyryl–Acrylohydroxamic

Acid Complexes

a-phenyestyryl– Acidity Molar absorptivityacrylohydroxamic acid (M) % Ea liter mol−1 cm−1

N-Phenyl- 10 99.9 2.5 × 104

N-p-Tolyl- 10 99.9 2.2 × 104

N-m-Tolyl- 10 99.9 2.2 × 104

N-m-Chlorophenyl- 10 99.9 2.4 × 104

N-p-Bromophenyl- 10 99.8 2.7 × 104

N-Phenyl-p-chloro- 10 99.9 2.4 × 104

N-p-chlorophenyl-p-chloro- 10 99.8 2.3 × 104

N-p-chlorophenyl- 10 99.9 3.7 × 104

N-p-chlorophenyl- 8 99.9 3.7 × 104

N-p-chlorophenyl- 7.5 94.5 3.5 × 104

N-p-chlorphenyl- +SCN* 10 99.9 4.7 × 104

N-p-chlorophenyl- +SCN* 8 99.9 4.7 × 104

N-p-chlorophenyl- +SCN* 7.5 95.1 4.5 × 104

N-p-chlorophenyl- + 10 99.9 4.7 × 104

Aliquat 336 + SCN*

Note.Niobium, 3 ppm; solvent, toluene; color of complex, yellow;lmax 385 nm;l*max 390 nm.a% E 4 100D + (Vaq/Vorg); whereVaq andVorg are the volumes of the aqueous and organic phase and the

distribution coefficient isD. D 4 Concentration of niobium in organic phase/(total niobium–niobium extractedin organic phase).

NIOBIUM DETERMINATION WITH N-a-PHENYLSTYRYLACRYLOHYDROXAMIC ACIDS 159


washings were collected. Finally the combined extract was diluted to 25 ml with toluene.The maximum absorbance of the yellow colored complex was measured against thereagent blank at 385 nm.Niobium was determined directly using graphite AAS at 334.4 nm by aspirating the

25-fold diluted toluene extract into the furnace.

RESULTS AND DISCUSSION

The spectroscopic data for the extraction of niobium(V) witha-phenylstyrylacrylohy-droxamic acids are given in Table 1. The wavelength of maximum absorbance is 385 nm.It has been observed thatN-p-chlorophenyl-a-phenylstyrylacrylohydroxamic acid(PCPPSAHA) is the most selective and sensitive reagent for the extraction and determi-nation of niobium (e 4 3.7 × 104 l mol−1 cm−1) and hence it is studied in detail. Thesystem obeys Beer’s law in the range 0.2–4 ppm of Nb with Sandell sensitivity 0.0025mgcm2. Additions of thiocyanate have a synergetic effect for the extraction of niobium,which increases the molar absorptivity to 4.7 × 104 l mol−1 cm−1 (Table 1). However, theliquid ion exchange, Aliquat 336 does not have any effect on the extraction equilibria(Table 1).

Acidity

Maximum color intensity for niobium(V)-hydroxamate complexes was obtained from8–10M HCl (Table 1), but as the concentration of acid decreases, the intensity alsodecreases. Hence 10M HCl was used for all extractions. The color intensity is stable forseveral days and is unaffected by temperature.

FIG. 1. Variation of log [DM] vs ligand concentration of (a) −log [PCPPSAHA], (b) −log [Aliquat 336], and(c) −log [SCN−].

PATEL, MENON, AND AGRAWAL160


Reagent Concentration

The absorbance of niobium(V) was constant with the use of excess of the reagent(PCPPSAHA). Extraction with various concentrations of the reagent showed that 3–5 mlof 0.2% PCPPSAHA was adequate for quantitative extraction of the niobium, whereasextraction was incomplete at lower concentration of PCPPSAHA.The use of ammonium thiocyanate favors extraction and increases the intensity of the

color. For maximum extraction and color intensity, 1 ml of 10% solution of ammoniumthiocyanate was found to be sufficient.

Diluents

The niobium-PCPPSAHA complex was extracted with various dilutents, viz., toluene,chloroform, isoamyl alcohol, andN-butylalcohol. It has been observed that toluene is thebest solvent for quantitative extraction of niobium.

TABLE 2Effect of Diverse Ions

ToleranceForeign ion Added as limit (mg)

Ag+ AgNO3 50As3+ As2O3 40Be2+ BeSO4 35Mg2+ MgSO4 40Ca2+ Ca(NO3)2 45Sn2+ Sn(NO3)2 45Cd2+ CdSO4 45CO2+ COCl2 40Cu2 CuSO4 40Cr3+ CrCl3 40Hg2+ HgCl2 35Ni2+ NiCl2 45Fe2+ FeSO4 45Fe3+ FeCl3 35Mn2+ MnCl2 40Zn2+ ZnSO4 40Al3+ AlCl3 45V3+ NH4VO3 50a

MOO24−6 (NH4)6MO7O24 40

Ti+4 TiCl4 40b

Ta Ta2O5 25Zr4+ Zr(NO3)4 35Cl− NaCl 50Br− NaBr 50I− NaI 50CH3COO CH3COONa 50PO4

3− Na3PO4 30SO4

3− Na2SO4 50

a Stripped with 0.15N H2SO4.b Stripped with 0.2M oxalic acid.



Type of Liquid Anion Exchanger

Niobium was extracted with three extractants, Aliquat 336, Amberlite LA-2, and trioc-tylamine, in various dilutents. Aliquat 336 in toluene was found to be the best for ex-traction.

Composition

The composition of niobium(V)–PCPPSAHA was studied by the slope ratio method(19, 20), i.e. by plotting a graph of the logarithm of the distribution coefficient of metal[log D(M)] against the logarithm of the ligand concentration, -[log (ligand)]. The extractionwas carried out by taking a fixed amount of niobium in the presence of (a) varyingconcentration of PCPPSAHA, (b) a constant amount of PCPPSAHA and a varying con-centration of Aliquat 336, or (c) a constant amount of PCPPSAHA and a varying con-centration of thiocyanate. In all the instances the plot of logD(M) against -log(ligand) (Fig.1) gave a straight line. The compositions of the complexes (a) niobium:PCPPSAHA4 1:2 (b) niobium:PCPPSAHA:SCN + Aliquat 3364 1:2:1:1, and (c)niobium:PCPPSAHA:SCN4 1:2:1, respectively:

NbO2 + 2(PCPPSAHA)+ Cl− → [NbO Cl(PCPPSAHA)2] + H+

[NbO Cl(PCCPSAHA)2] + SCN− → [NbO (PCPPSAHA)2 SCN] + Cl−

[NbO (PCPPSAHA)2 SCN] + R4N+ Cl− → [NbO (PCPPSAHA)2 SCN[R4N

+ Cl−]]

Effect of Diverse Ions

Niobium was extracted and separated in the presence of large number of different ions(Table 2). Interference studies were made by measuring the absorbance of the extractedphase. The tolerance limit was set as the amount of foreign ion causing a change of ±0.02absorbance or 2% error in the recovery of niobium. Moderate amounts of various metalions commonly associated with niobium were tolerated, as were most common anions.Large concentrations of vanadium(V) and titanium(IV) (>50 mg) interfere, but can beeliminated by stripping them with 0.15N H2 SO4 and 0.2M oxalic acid, respectively.

Determination of Niobium in Standard Samples

To test the reliability of the method, niobium was determined in standard alloys androck samples. The results given in Table 3 show that niobium can be determined accu-rately.

TABLE 3Determination of Niobium in Alloys and Steel Samples

Niobium found (ppm)a

SampleNiobium certifiedvalue (ppm) Spectrophotometry GF-AAs

NBS 123b (Stainless steel) 0.700 0.706 ± 0.02 0.710NBS 345 (G. Ni steel) 0.210 0.208 ± 0.02 0.20032E low-alloy steel 15.000 15.030 ± 0.05 15.00033d low-alloy steel 15.000 14.900 ± 0.03 15.005

a Average of 10 determinations.

PATEL, MENON, AND AGRAWAL162


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extraction and micro determination of niobium(v) with n-α-phenylstyrylacrylohydroxamic acids

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