C H A P T E R - IV

DETgJiMINATION OF ZINC BY PRECIPITATION OF ZINC AMMONIUM

PHOSPHATE FROM HOMOGENEOUS SOLUTION

DET,RMINATION OF ZINC BY PRECIPITATION OF ZINC AMMONIUM

PHOSPHATE FROM HOMOGENEOUS SOLUTION

SUMMARY

An invesUgation was carried out for the preoipi tation

of Zino ammonium phosphate from homogeneous solution using

disodium hydrogen phosphate as precipitant and a quantitative

method has been developed. The method is based on the

solubility of zino ammonium phosphate in ammonioal solution ++ due to the forma·tion of zino ammonia complex, Zn(NH.3) 4

•

When the ammonioal solution is heated, ammonia is volatalised

and free zino ions are gradually liberated causing a slow

precipitation. Optimum pH for the precipitation is found

to be 7. 0 - 7.8. Hydroxides of iron, chromium and aluminium,.

e.nd phosphates of barium, strontium, oaloium and manganese

will be preoeipitated, if present, from the ammon1a1 solution

whereas Zinc will remain in solution. This property is used

for the separation of these metals from zino. Anions suoh as

sulphate, acetate and chloride in the form of ammonium salta

do not interfere upto 50 mmoles, but show interference if

present as the alkali salts in concentrations mo.re than

10 mmoles. Zino has been determined in presence of nickel

after oomplexing with tartrate. Copreoipitetion of cadmium

with zino in this determination hae been studied using Cd-115

traoer.The ~ethod has been applied for the determination of zinc

in Brass.

•

DET~RMINATION OF ZINC BY PRECIPITATION OP ZINC AMMONIUM

PHOSPHATE PROM HOMOGENEOUS SOLUTION

95

All the ti trimetrio methods for the determination

of zinc, with the exception of oomplexometric procedures,

are based on precipitation reactions. It ia observed that

these methode suffer trom most of the errors of the

corresponding gravimetric procedures, and in aJdition,

other errors introduced by nature of titrimetrio procedures

themselves1• The electrolytic determination of zinc is not

so accurate and is used chiefly for the separation of zinc

from certain elements and it can not be applied for quant:Hative

determination until preliminary separatiore have been made.

A number of methods are available for the

gravimetric detdrmination of zinc bu.t none of the methods

are useful for the separE.tion and determinatlo.n of the metal.

in presence of aeveral common ions. Considerable research

has been performed o.a the g:cavimetric analysi3 of zinc.

Vance anJ. Borup2 conducted a number of experiments with

radioactive zinc using various known gravimetric methode

and oonoluded that the determimltion as sulphide, phosphate

and o:xelate have comparable ac~racy.

Precipitation of sulphide by hydrogen sulphide

and of phosphate using diammonium hyirogen phosphate are

the most important methods tor the gravimetric estimat1ol\

ot zinc. Zinc ~.ie also determi.aed as zinc meroury thiocyanate3,

96

zinc oxinate4 , zino anthranilite5 and zino quinoldinate6•

The seldom used forme of determination are ae baeio zino 7 8 9 carbonate 1 dipyridil zino thiocyanate , zinc oxalate ,

10 ll zinC 5-bromo an thranile. te 1 ZinC BalicylaldO Xina te and

In the determination of zinc, in the form of zinc

sulphide, the solubility and morphological properties of

the precipitate are greatly affected by hydrogen ion

conce,ltration of the solution. One of the most widely

used methods for the precipitation of zinc sulphide is

that of Fales ani ·ware13 , in which formt.te ·buffer is

used to maintain the pH between 2 and 3. In more ao:kdio

solution precipitation is incomplete, whereas in more

alkaline solution, slimy precipitates SI'e produced. In

this method of determination, presence of an electrolyte,

such as ammonium sulphate or thiocyanate, is desirable to

prevent the fo~ation of colloidal precipitates. Chloride

and sulphate ions contaminate the precipitate from 0.2 to

2.0 percent, if zino is precipitated in their presence.

Zinc sulphide, however, is not sui table as a weighing form

and it must be converted to zinc sulphate (ignition to

300 - 600°0) or zinc oxide (igni'tion above 940°0).

Thermogravimetric analysis shows that no region of stability

exists for either zino sulphide or zino sulphate14. Partial

separatio4 of zinc :!rom oetions o:t groups IV and V and

m&ng&Aese, oobal t and nickel oan be achieved by this method.

97

oomplex Zinc can

salt, zinc mercury

be preoipi tated aa the white 3,15

thiocyanate, In Rg (SCN)4, in neutral

solution containi•lg not mOL'II than 1 ml of concentrated nitric

acid end not mor·e than 0.15 gma of zinc, by the addition o!

the precipi taut (mixture of potassium thiocyanate end

mercuric chloride). Hydrochloric aoid interferes. The

ions - oadmium, nickel, cobalt, manganese, mercury, chromium

and to some extent bismuth also interfere. Preliminary

seeding of the solution is essential to get a precipitate

of theoretical composition ani a slight excess of reagent

is to be added for complete precipitation.

Zinc can be determiue:l by using anthranilic ac!d5

at pH 4. 72 but it requ.!res different empericel gravimetric

factors depending on whethei' the precipitation is carried

out from hot or cold solution. Exoept allcali and alkaline

earth metals all o~;he? QatioJJ.s interfere. The zinc can also

be determined as q_uinoldinate6 in the pH range 2. 3 to 6. 5

but eubj eot to a numbe.r of inte.1.·ferences, particularly from

copper, sil•er, mercurous, merauric, tungsten and molybdenu~.

Zinc determinatio"l as oxinate 4

achieves separation from neuly

all common interferences by :proper control of pH end using

masking agents except large amounts of nickel, oobalt and

cadmium.

Zinc has been precipitated as oxalate from homogeneous

solution by the hydrolysis of diethyl oxalate16 , but the authors

concluded that the method is of limited use because of

• 98

interferences of several ions. 17-19 Many workers studied

the precipitation of zino sulphide from homogeneous solution

by using thioaoetamide as a aouroe for sulphide. The usa

of thiooarbonic acid20 and th1ouraa21 have also been reported.

Zino 8-hydroxyquinolinate has been precipitated by hydrolysis

of 8-aoetoxyquinoline22 and by volatalisation of ammonia

method23• Zinc 8-hydroxyquinaldate has alRo been precipitated

by the hydrolysis of 8-e.oetoxyquinaldine24.

'.!:he present work describes the determiaation of

zinc ns zinc ammonium phosphate by precipitation from homogeneous

solution. The conventional zinc ammonium phosphate method was

developed by Dakin25. In this method the pH of the solution

is quite critical because it affects the composition as well

as solubility of the precipitate. Nitrate ions interfere.

Even ~all amounts of O.OlM ammonium nitrate cause low results.

Accordiag to Dakin, BOdium OI' potaet:~iUl!l phosphates are not

satisfactory precipitentsae they yield precipitates that are

0onteminated by sodiWD or potassiu'll. The one redeeming

feature of the method is the ease with which the precipitate

can be dried.

In the present method disodium hyd.rogen phosphate

is used as the precipitant. The solubility of zinc ammonium

phosphate at higher pH due to formation of zino ammonia ++

complex Zn(NH3)4 in presence of free ammonia and the slow

liberation of the zinc ions into solution by volatalisin&

ammonia causing preoipi tation of zinc ammonium phosphate,

forms the basis of the method.

' '

EXPERIMENT.Al;t

REAGENTS ;-

Zinc Solution a-

A stock solutioa was prepareQ. by weighing about

4. 5 gms of 1 ./malaR' 2inc oxide, d.isso:i.ving 1 t in ni trio

acid and diluted with distilled water to one litre. The

zinc content was checked by titration with standard EDTA 26

solution, using eriochrome black - T indicator •

Reagent Solution •-

1M solution. of disodium hyv.l'Ogan phoapb.ate

(B. D. H. 'AnalaR' ) was prepared.

All other chemicals u.sed 1o1ere of A. R. grade.

EC)UIPMENT •-

Bausch & Lomb spectroniol-20 colorimeter WI·-•

used for colour measurements.

G. M. Counter (Medical Spectrometer, IlARC) was

used for mea~ing radioactivity.

99

pH measurements were made on Becknoan

Thermogravimetric curve was recorded

electro be.l.anoe.

pH 11eter, H2

•

usin& Cahn aa

Carl Zeiea Zena Allphivsl with af. •etio auto1uuo

exposure oontrol wee used for \akin& pboto•ioroarapbe.

1Ufl

MIN 8 microsoope with low power ob~ective 9x &!ld

micrometric eye piece 5x were used for measuring sizes of

the crystal.s.

PROCEDURE a-

lO ml of ooncentrated ammonia (Sp.gr. 0.910)

and 20 ml of 1M ammonium acetate were added to a sample

solution oontaining about 1.12 mmoles of zinc in a beaker.

The volume was adjusted to about 200 ml. To this solution,

2.5 mmoles of diaodium hydrogen phosphate were added. About

1 ml of O.l% phenol red indicator was then added.

The beaker was heated on a steam bath for about

4 -5 hours, maintainin6 the volume by occasional additions

of distilled water. The completeness of the precipitation

was indicated 'by the change of red oolour of the supernate

to yellow. The pH of the supernate was found to be between

7. 2 to 7.8.

The precipitate was tranRfered to G4 sintered

e:;lass crucible and washed twice 01i th small amounts of 0

distilled water. The precipitate wa~ dried at 130 C in

an oven for about 30 minutes and weighed as zinc ammonium

(Conversion factor Zn/Zn NH4Po4 • 0. 36645)

DB'.rEB.MINA'UOII OP' UIIPBECIPUATED ZIIfC IN THE Jl'ILDATE I-

a) rraoer Technique a-

Zino-69 tracer waa prepared by irradiating 1 A.nalaR1 Zillo metal at 1 Apsara1 , BARO, Trombay.

1 (J 1

A stock solutioll was then prepe.:ced by dissolving

the sui table sample of irradiated zinc metal &lld the

unirradiated Zilla metal in concentrated llitrio aoid and

diluting to one litre with distilled water.

20 ml aliquot of this solution was takt'!n and

zinc was precipitated using recommended procedure. The

filtrate was oolleoted and diluted to 100 ml. A standard

solution was prepared by diluting 20 ml of the stock

solution to 100 ml. Activities were measured ill both the

oases using G. M. Oounter. Prom the rssul ts, the amount of

zinc unpreoipitated was calculated.

b) Colorimetric Method •-

As a check on the zinc content in the filtrate,

colorimetry was also pe·rformed by followi~ the zinc 27

dith1zonate method • Absorbance of the pink zinc

di thizonate was measured at 535 nm.

1U2

COPRECIPITATION OP QADMIUM IN THE DETSRMI~ATION 0' ZINC AS

ZINC AMMONIUM PHOSPHATE a-

Aliquota ( 20 ml) of Zinc solution were taken and

different amounts of cadmium tracer Cd - 115 were added.

Zinc was then precipitated ueing proposed method. The

activity due ~ Cd - 115 in the precipitate was measured.

Simultaneously a blG.tlk oontain.i.a;!; pu1·e Od - 115 Tracer was

also precipitated and the a~tivities were oompared.

AP2LICATION OF THE METHOD

Determination of Zino in Brass, B.C.S.No. 5f a-

The sample containing 70 to 80 mg o:f' Zino was

aoouratel:r weighed and transtered to a 400 ml beaker.

The alloy was dissolved in concentrated ni trio acid and

evaporated nearly- to dry-ness. It was cooled and 50 ml of

water and 2.5 ml of concentrated hydrochloric acid were

added, and saturated with hydrogen sulphide gas. The

solution was furthet• diluted to 100 ml and again saturated

with hydrogen sulphide gas. The precipitated sulphideeof

copper, lead eto. were separated by- filtration. The filtrate

containing zino was neutralised with ammonia a:f'ter expelling

the hydrogen sulphide gas. The zino was then determined by

the re oommended pro oedure.

1U:l

RESULTS A.l'lD DISCUSSION

Tablt~ 20 summa:daea the resul 's obtaiuttd for the

determination uf variou.~ !1lllounta of zino by reoommended

prooedure.

TABLE 20

Precipitation of varying amounts of zino as zino ammonium

phosphate •-

Zino taken (mg)

9.160

9.160

18.320

18.320

36.640

36.640

73.280

73.280

73.280

73.280

73.280

183. 200

183.200

366.400

366.400

Zinc !ouud (mg)

9.165

9.164

lB. 320

18.332

36.645

36.626

73.286

73.280

73.293

73.302

73.288

183.310

183.160

366.520

366.240

Diffel'EI!lue (mg)

Error (~)

+0.005 0.0546

+0.004 0.0437

o.ooo o.oooo +0. 012 0.0655

+o. 005 0.0137

-0.014 0.0382

+O, 006 0.0002

o.ooo o.oooo

+0.013 0.01'77

+0.022 0.0300

+0.008 0.0109

+O.llO 0.0600

-0.040 0.0218

+0.120 0.0327

-0.160 0.0437

The results show that from 9.16 mg to '66.~ mg of zinc

can be determined ~ith fair aoauraoy by this method.

11J4

Photomicrographs were aho~n (magni:tioaticn, x240) 28

for the precipitate formed by direct method , for the

precipitate fo~n~d by using diammonium hydrogen phosphate

in the px·oposed method and for the precipitate obtained by

recommended procedure in Figures 12A, 12B and 12 C

respectively. The photographs show that the particle aize

of the precipitate of recommended procedure was considerablt

improved due to elongated prismatic crystals compared to

the precipitate (fine prismatic crystals) formed either by

direct method or by ·the use of diammonillm hydrogen phosphate

in the routine procedure.

The sizes of the crystals are bigger whe~ compared

to those formed by using diammonium hydrogen phosphate in

the routine procedure. The sizes of the crystals formed

by recommended procedure are - the leagth 0. 080 1JI1!I and

tha breadth 0. 032 mm for bigger orya"tals! o. 048 mm an.i

0.016 mm for smaller crystals. The vi~al peroent of smaller

crystals is about 80, The sizes of the crystals formed by

using dia.mmonium hydrogen phosphate in the recommend~d

procedure are 0.016 mm length and n.OJ5 mm width and all

the c:~:-ystals are of tho same size.

The amount of zino in the filtrate at various pH

values (7.0 to 8.7) using zinc- 69 as a traoer are reported

in Table 21.

. . -~. 41'-_,.. ..

Determination of unpreoipitated zino using Zn- 69 1-

pH

8.7

8.5

8.3

8.1 7.8

7.6

7.5

7.4

7.2

7.0

Zino in fil tn te (mg)

14.66 7. :u 2.92

1.09

0.02

o. 00

c.co o.oo o.o1 o.oo

1U~

----------------·--------------------------------·------!rhe results show that amou.nt of zinc in the fil ti'IJte

between pH 7.0 and. 7.8 is almost nil. Hence, the pH

l'ange ·r.o to 7.8 is ideal fer qurmtitative preci,_;Hdion·

of zinc by this method. Tho preoipite.tion usually IJOrnmenoes

at pH 9. 3. Figu:r(l l3 is the graph of thE' pH of tht! <Jolution

against the amuunt of zinc in the Ziltrate.

The amount of zinc in the filtrate obtained

from reoomme.adsd procedure was also determined usinG

di thizonate metbod 27

• The oalibrt•tion au.rve is sho\'ill in

Figure 14. It is found that the loss of zino is 30 p&•

••r-------r-------,-------~--

~12 ~

E . -, -' 0 0

' ~ 0 , 1 •

'

4

0 70

. 0

I I I I --r-,_

74 7.8 B2 " •o pH

Effect of pH on precipitation of Zinc ammoniUM phosphate.

I) 7 -- -

0.5.

// / -i

I '

fjt ~----t- ·------------··

;=~ ~~--~-' --j .:0 40 50

concerd rct1cr c• 7,rc [j..:g)

Fig 14 · Calibration ct.~rve for determining Zinc )'I 535nm.

TABLE 22

Calibration ou1~e data for determining zino s-

Volume of the solution

(ml)

1.0

2.0

3.0

4.0

pg of Zino present Opticl:ll. denai ty

0.1.6

0.30

0.45

0.60

For th~ filtrate,tbe optical density attained is 0.63

which corresponds to 30 p.g of zinc.

.. '

Conoantration of ammonil.l.l'l.l aoe ta·te is orl. tioal

beoausa it cont:r:ols ·the pH of the aolu1iion. If leas than

Cono".mtreti,ms of arnmouium !1o<3ta.te :from l5 - 50 mmoles

1116

were a tlldied wi·th good .. :eaul ts. The rt~::m.llis are presented

in ·rable 23.

1117

TABLE 2'

Effect of conoeatration of 111111110nium aoetate on the determination

of zinc ammonium phosphate ,_

Ammonium acetate taken (mmoles)

2.0

5.0

10.0

15.0

20.0

30.0

40.0

50.0

pH

8.4

8.2

e.o 7.7

7.4

7.3

7.3

7. 2

Zino taken

(mg)

7'3.280

73. 280

73.280

73.280

73.280

73.280

T5. 280

73.280

Zino found

(mg)

76.580

75.504

74.742

73.875

73.280

1'-'· 290

73.296

73.260

Difference Error

{mg) (%)

... ,. 300 4.5030

1'2. 224 '· 0350 +1.462 1.9950

-tO. 595 0.8119

o.ooo 0.0000

+0.010 0.01:57

+o.ol6 0.0218

-0.020 0.0273

The resalts of the effect of the amount of

precipitant is shown in Table 24. It is found that for

complete preoipitation of 1.12 mmolea of zinc, the reagent

required is l. 2 mmoles. Concentration changes of disodium

hydrogen phosphate between the limits 2 P~d 10 mmoles

always yielded reproducible results, but concentrations

of the precipitant above 10 mmoles, produced higher values.

This might be iue to the conta~inaticn of the precipitate

with sodium ions. A value of 2. 5 mmolea was selected for

routine use.

•

TA})L.E 24

Effect of exoese reagent on the precipitation of zino

ammonium phosphate a-

Na2 HPO 4 taken

( mmolE•s)

l.!)

1.5

2. 5

s.o 10.0

15.0

25.0

Zino teken Zino found

(mg) (mg)

73.280 73.262

73.280 73. '-94

73.280 73. 2fl0

73.280 73.272

73.280 73.312

73.280 74.282

73.280 76.950

Difference Error

(mg) ( ")

-o.ol8 0.0246

+o.ol4 0.0191

o. 000 o.oooo -o.oos 0.0109

+0.032 0.0436

+1.002 1. 3680

+3.670 5.0080

11J8

The effeot of foreign anions on the determination

of zinc ammonium phosphate was studied and the resu.l ts az·e

reported in Table 25. The resul t!'l show that anions like

sulphate, nitrate, acetate and obloride when present upto

50 mmoles do not contaminate the precipitate when used as

their ammonium salts. However, the same studies when conducted

using alkali salts clearly show higher values when the salt

concentrations exceed 10 ll'.llloles. It can be concluded tru:.t

anions do not interfere in the pre~ipitation. It is the

alkali metal, that oontami•1atea the prscipi tate resu.l ting

higher values. Sodium silioate interferes if present in

concentration above l IDII!Ole.

1()~

TABLE 25

Datel"mination of zino in presenoe or foreign anions 1-

Subs tanoe Conoen tra tion Za taken Zn found Difference Error (mmo1es) (mg) (mg) (mg) ( ~)

(NH4

) 2 so4

25.0 73. 280 73.262 -o.ol8 0.0246

.. 50.0 73.280 73. 29 2 +0.012 0.0164

NH4 No3 25.0 73.280 73.204 -0.076 0.1037

.. 50.0 73.280 7'3.200 -0.060 0.1090

NH4 01 25.0 73.280 73.294 •o.o14 0.0191

" 50.0 73.280 73.24-8 -0.032 0.0436

x2 so4 5.0 73.280 73.482 +o. 202 0.2757

• 10.0 73.280 74. 290 +1.010 l. 3780

Na N03 5.0 73.280 73.265 -0.015 0.0205

.. 10.0 73.280 74.030 +o. 750 1. 0230

Na Cl 5.0 73.280 73.381 +O.lOl 0.1378

Na 01 1o.o 73.280 74.000 ..-o. 120 0-:3824

Na{ oH3coo) 5.0 73.280 73.232 -0.048 0.0655

" 10.0 73.280 73.481 ..-o. 201 o. 2743

Na2 :uo3 l.O 73.280 74.000 +O. 720 0.9824

Results of precipitation of zino ammoaium phosphate

after preliminary separation of zino from cations {50 mg eaoh)

are reported in Table 26.

110 •

TABLE 26

Precipitation of zino ammonium phosphate in preeenoe of

diverse oatioue 1- (50 mg eaoh)

Cation taken Zinc taken Zinc found Difference E~r (mg) (mg) (mg) (")

Cu(II), Cd(II), Bi(III),

Hg(II), As(V), Sb(V) & 73.280 73.485 +O. 205 o. 2798

Pb( II)

l'ii( II} 7"j. 280 73.248 -0.032 0.04'36

Ba(II), Sr(II), Ca(II), 73. 280 73.424 +0.144 0.1965

Mg( II) and Mn( II)

Fe( III), Cr(III) and 73.280 73.182 -0.098 0.1338

Al( III)

Zinc was separated and determined in presence of copper,

cadmium, bismuth, mercUl7t arsenic, antimony and lead by

precipitating them as sulphides from hydrochloric acid

solu ticn. Zinc was determined in presence of nickel after

complexing with ammonium tartrate and ammonium chloride.

Barium, strontium, c&.loium, magnesium and manganese 1o1ere

separated as their sparingly soluble phosphates and iron,

chromium and aJ.umiuium were separated as their hydroxides,

keeping zinc in its soluble zinc ammonia complex. Zino

was then determined in the filtrate by the proposed method

and fairly accurate results were o btaiDed.

1 1 t

Cadmium - 115 traoer wae used tor the ooprecipitation

studies. About 26 percent of oadmium was ec1trained b7 zino

ammonium phosphate and hence the interference of cadmium in

this zino det-;.rmi.wtion 11!1 because of ooprecipi tation. The

results are presented in Teble 27.

TABLE 27

Co pre cipi te.tion of oadmium with zino ammonium pho ephate 1-

Zino taken Cadmium tracer taken

(mg) ( mg)

73. 280 24-. 740

73.280 4-9.480

73. 280 74-. 220

73.280 98.960

73.280 123.700

CadmiUlll carried by- zino

(mg)

7.126

14-. 290

21.300

28.190

34.880

" ot Cadmium carried

28.80

28.87

28.70

28.4-8

28.20

Ridsdale' s analysed sample of Brass, B. c. s. No. 5:f'

was used for the application of the proposed method. The

values obtained are in fair agreement with the reported

value, 24.2. The results are shown in Table 28.

Figure 15 is the thermogravimetric aurve of zinc

ammonium phosphate. The curve has a hortzo:~.tal line upto 160°0.

On i'urther heating, water and ammonia gradue.ll.y escape and finally

the pyrophosphate is formed quantitatively at 560°0.

Tom>-•rclur~,·c

r-----"'~'r'c_---,-----'T4~'c_---r----~'~'~'----

I I I

4 70 ·--------.-------, ' !

_ _J_ ------·

- -t-------

- ______ L --- -'-

Frg.IS. Thermogra'Yinetnc curvt for Z1nc ammonrum phosphate.

6

\

TABLE 28

Determination of zino in Erae•, B.c.s. No. 5f ,_

Alloy taken Zino taken calculated

(mg) (mg)

75.7 18.32

151.4 36.64

302.8 73. 28

605.6 146.56

Zino found Zino oontent

( mg) ( IC)

18.32 24.20

36.65 24.21

73.28 24.20

146.62 24.26

CONCLUSIONS

112

The simple new method described for the gravimetric

determination of zinc by precipitation from homogeneous solution

uses the volatalisation of ammonia procedure. It is no doubt,

superior to the conventional methols with respect to crystallinity

of the precipitate and accuracy of the results. It was found by

tracer method that the unp~eoipitated zinc in the filtrate is

almo:::t nil ancl by colorimetric method is 30 Ub• The extent of

copreoipi tation of cadmium alongwi th zinc was found to be 28IC •

Nitrate caused no interference unlike in the conventional method.

The gravimetric factor Zn/Zn NH4Po4 • 0.36649 is also good when

compared to either Zn/Zn S04 • 0.40497 or Zn/Zn 0 • 0.80339.

This method found to applicable for the determination of zinc

in Brass.

---

113

RE:PERENCES

1. J.H. Kanzelmayer, in "Treatise on Analytical Obemietry",

Eda., I.M.Koltho!! and P,J,Elving, Intereoiet!Oe Publiehera,

Lowlo:>n, "Part II, Vol. 3, P.l25 (1961).

2. J,E:,Vanoe and R.S,Borup, Anal.Chem., 1.2 1 610 (1953).

3. N.C.Vosburgh, G.Cooper, w.J.Clayton and H.P.Pfann,

Ind.Eng,Chem.Ana1,Ed,, .!Q, 393 (1938).

5.

6.

B.

g.

10.

11.

12.

14.

R,Berg, Z,ana1.Chem,, ,ll 9 171 (1927).

H.Funk and M.Ditt, Ibid ., j!, 332 (1932).

P.Ray and M.K.Bose, Ibid • , ,22, 400 (1933).

K,Kraut, Z,anorg.Chem., _u, l. (1897).

G. Spaau, z. anal. Cham., g, :na (1924).

A.01assen, Ibid ., ]:!!, 189, 3n (1879).

A.S.Whee1er, J.Chem.Soo., 31, 565 (1905). -S.H.Simonsen and P.C.Kristopher, Anal,Chem., 26, 681 (1954). -W.Mark We.rk and H,Gebhaardt, Z.anorg,Chem., 1471 42 (1925).

H. A. Fales and G.M.Wars, J,Am.Chem.Soc., ,!!, 487 (1919).

C.Duval, "Inorganio Thermogravimetric Analysis", Elsevier,

New York, (1953) P. 277.

15. G,E.F.Lundel1 and H.:Bee, Z,angew.Chem., ~~ II.617 (1915).

16. E. R. \.laley, L. Gordon and o. A. Simmonds, Ana1..-Chem., ~'

1060 (1950).

17. H.:Plasohlta, Chemist.Analyst., 2, 44 (1955). -18. D.H.K1ein and E.H.Swift, Talanta, ]1, 349 (1965).

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

114

R. B. Hahn and D. L. P .L'i~le, Anal. Ohim. A ate.. , 31, 38 2 ( 1964). -W.Pilz, Mh.Ohem., §j, 471 (1952).

E.J • .Nake.no, J.Chem.Soo.Japan Ind.Chem.Seot., .§1, 565 (1963).

J.P.Jonea, O.E.Hileman and L.Gordon, Talanta, !Q, 111(1963).

F.il.Firsohing and J.G.Bre1r1er, Anal.Ohem., ]2, 1630 (1963).

S.Hikime and L.Gordon, Talanta, .!!1 851 (1964).

H. v. J)akin, z. anal. Chem. , ]2, 27'3 ( 190::>).

A.I.Vogel, "A Text Book of Quantitative Inorganic Analysis",

Longman, Green and Co.Ltd, London,(1968) P.433.

G.Charlot, "Colorimetric Determination of Elements",

Elsevier, Amsterdam, (1964) P.432.

Ref. No. 26, P~ 533.

---