- Email: [email protected]
Determination of phosphorus and tungsten in heteropoly acids by EDTA-titration
0039-9140/82/l 10943-03$03.00/O Pergamon Press Ltd
Talanta, Vol. 29, pp. 943 to 945, 1982 Printed in Great Britain
DETERMINATION OF PHOSPHORUS AND TUNGSTEN IN HETEROPOLY ACIDS BY EDTA-TITRATION HIROMU HAYASHI*
Guelph-Waterloo
and J. B.
MOFFAT~
Centre for Graduate Work in Chemistry and Department University of Waterloo, Waterloo, Ontario, Canada
of Chemistry,
(Received 2 February 1982. Accepted 14 June 1982) Summary-Previous methods for the titrimetric are found to be unsatisfactory for heteropoly described.
The application
of heteropoly
acids
and
their
determination of phosphorus and tungsten with EDTA acids, Successful modifications of the procedures are
salts
to tungstophosphates
have produced less than satisfactory results. We report here some modifications of earlier procedures that make them satisfactory for analysis of 12-tungstophosphoric acid and dimeric 9-tungstophosphoric acid by EDTA titration.
in
in derecent years. 1,2 Tungsto- and molybdophosphates compose in alkaline media to form simple tungstate or molybdate ions and hydrogen phosphate,1*3 which finds use in a popular method4 for the titrimetric determination of phosphorus after precipitation as ammonium 12-molybdophosphate.3
catalytic
processes
has received
[PMo,~O,,]~-
increased
attention
EXPERIMENTAL
Samples
+ 230H-
+ 12MoO:-
+ HPO:-
+ llH20
(1)
[P2W18062]6+ 340H+ lSWO:+ ZHPO:-
+ 16H20
(2)
Analytical-reagent grade 12-tungstophosphoric acid, authentic simple tungstates and phosphates were obtained from BDH Chemicals. The 12-acid was shown to be the 2Chydrate by thermogravimetry.’ The dimeric 9-tungstophosphoric acid was prepared from the A-form of the ammonium salt.”
However, calcination of the hydrated 12-tungstophosphoric acid at high temperatures results in decomposition of the heteropoly structure with some loss of the matrix oxide by sublimation2 The stoichiometry given above no longer applies to the catalyst thus obtained and it is necessary to analyse separately for both phosphorus and tungsten. EDTA methods have been reported for the determination of phosphorus in uranium ores, concentrates and liquors, by precipitation as ammonium molybdophosphate and titration with EDTA.’ Phosphates, metaphosphates, pyrophosphates, and tripolyphosphates have also been determined by precipitation as magnesium ammonium phosphate followed by EDTA titration.6 Further discussion of EDTA techniques for the determination of phosphorus can be found in the literature.4.7 The determination of tungsten by precipitation as lead tungstate and subsequent EDTA titration was suggested some years ago’ and has been applied to tungsten-iron and tungsten-molybdenum alloysg and to tungsten oxides.” Though these methods have been satisfactorily used for analysing
a variety
of phosphorus
and
of relatively
tungsten,
simple
attempts
Reagents Stock solutions of 0.01M magnesium chloride and 1M ammonia/l M ammonium chloride buffer. Eriochrome Black T, 1% solution in 10% ammonia solution,” prepared fresh every month. Stock solutions of 0.02M lead nitrate and of acetate buffer lo (50 g of sodium acetate and 12 ml of glacial acetic acid diluted to 1000 ml). Aqueous 1% Xylenol Orange solution. Procedures Phosphorus. Add 5 ml of 1M sodium hydroxide to an aliquot of heteropoly acid solution (containing -50 pmole of phosphorus) and dissolve the resultant precipitate by gentle boiling. Neutralize with 5 ml of 1M hydrochloric acid, then add 20 ml of ammonia/ammonium chloride buffer and 10 ml of 0.01M magnesium chloride. Titrate the surplus magnesium with O.OlM EDTA (Eriochrome Black T indicator) after leaving sealed overnight. Tungsten. To an aliquot of heteropoly acid solution (cu. 100 pmole of tungsten), add 1.6 ml of IM sodium hydroxide and 20 ml of water. Boil gently to dissolve the sample and dilute with water to about 450 ml. Quickly add 20 ml of acetate buffer to make the pH 5.0, and then 10 ml of 0.02M lead nitrate. Boil for 15 min, cool, and filter off the white precipitate. Titrate the surplus lead in the filtrate with O.OlM EDTA (Xylenol Orange indicator). Correct the tungsten value for phosphate as discussed in the text.
compounds
to apply
them RESULTS
*On leave from Department of Chemical Engineering, Tokushima University, Minamijosanjima, Tokushima, 770, Japan. tTo whom correspondence should be addressed.
of 943
AND DISCUSSION
Table 1 shows the results of alkalimetric tungstophosphoric acids and their
titrations calcined
944
SHORT COMMUNICATIONS
Table
1. Alkalimetric
analysis
of tungstophosphoric products*
Sample
L
products,
in which
a known
mentioned
(ca.
100 pmole
amount
of phosphorus)
of heteropoly was
their
calcined
mm&
Taken
Found
Recovery, %
0.112, O.llO* 0.125*
0.111, 0.109, 0.121,
99.3 99.4 96.8
0.100s 0.1136
0.101, o.1104
100.7 97.2
in the text.
in 20 ml of 0.25M sodium hydroxide and the excess of alkali was titrated with 0.25M hydrochloric acid (phenolphthalein as indicator).3 The uncalcined 12-tungstophosphoric acid was the 24-hydrate, calcination of which at 400” gave the anhydrous acid and at 525” resulted in decomposition. This thermal behaviour was confirmed by thermogravimetric analysis and powder X-ray diffraction.’ The calcined products and the dimeric g-acid required complete hydrolysis by gentle boiling in alkali before the titration. This simple technique seemed suitable for routine analysis of the stoichiometric heteropoly acid. The EDTA determination of phosphorus6 is based on the complete precipitation of phosphate as ammonium magnesium phosphate (NH4MgP0,.6H,0) with a known (and excessive) amount of magnesium chloride at pH 10.5 in ammonia/ammonium chloride medium. Fine crystals are deposited on the walls of the container within 5-10 min and titration after 30 min gives fairly good results for simple phosphates (Table 2), but not for the heteropoly acids. The modified procedure described above gives results in good acid
and
Phosphorus,
H,PW,,O,, Uncalcined 24-hydrate Calcinedt at 400” (anhydrous) Calcinedt at 525” (decomposed) H~PzWI@GZ Uncalcined 35-hydrate Calcinedt at 400” (anhydrous) *Literature orocedure3 tin helium, for 2 hr.
acids
dissolved
agreement with the expected values for the uncalcined tungstophosphoric acids (Table 2). Evidence for some loss of phosphorus on calcination appears in these results though it is not shown by those obtained by alkalimetry (Table 1). The modifications are simple, but essential both for complete decomposition of the heteropoly acid and quantitative precipitation of NH,MgP0,.6H,O. Tungsten(V1) forms a white precipitate of lead tungstate with lead ions. I3 Titration” of the remaining lead with EDTA at pH 5 gives excellent results for a simple tungstate and an isopolytungstate (Table 3). The complete decomposition of the isopoly salt into simple tungstate by gentle boiling in alkaline medium is again important. However, the values obtained for tungsten in heteropoly acids were about 15% too high. The discrepancy was suspected to arise from the precipitation of lead phosphate, which is only slightly soluble in water,14 along with the lead tungstate. Precipitates were obtained from both Na*HPO, and Na,PO, with lead in acetate buffer. EDTA titration of the excess of lead in the filtrate confirmed the precipitation of PbJPO,), Since 1 mmole of phosphate will react with 1.5 mmole of lead (which in turn will
Table 2. EDTA determination
of phosphorus Recovery,
Phosphorus, Sample Authentic phosphates* Na2HP0, (NH&HP% Na,PO,. 12Hz0 Heteropoly acids5 H3PW,2040.24HZ0 Calcined at 400” Calcined at 525” H,P,W,,062~35H20
%
mmole
Independent analysis
Taken
Found
This work
0.049 1 0.0560 0.0527
0.0470 0.0555 0.0560
0.95, 0.99, 1.062
0.99jt 1.00,t 0.97,t
0.0453 0.0571 /I 0.061011 0.042 1
0.0460 0.0555 0.0540 0.0420
l.015 0.97, 0.88, 0.99,
l.OOa$
*Literature procedure4 in the text. TLaboratory A. fLaboratory B. @Modified procedure (this work). /iBased on anhydrous H,PW1204,,.
1.024:
SHORT
945
COMMUNICATIONS
Table 3. EDTA determination
of tungsten Recovery, %
Tungsten, mm& Sample Simple and isopoly tungstateg Na,WO,.2H,O (NH&oW~#U .5HzO Heteropoly acids 11 H,PW,2040.24H20 HsP2W,,0,2.35H,0
Taken
Found*
0.1052 0.1218
0.1070 0.1221
0.1025 0.1051
0.1170 0.1236
Correctedt
0.1041 0.1060
This work
Independent analysis
101.7 100.2
lOO.S$ 107.9$
101.6 100.8
101.27 99.lT
*Calculated as W, but including P for heteropoly species. tcorrected for P by deduction of 1.5 mmole of W per mmole of P. SLaboratory A. $Literature procedure.” /IModified procedure. TLaboratory B. be equivalent to 1.5 mmole of tungsten), for each mmole of phosphorus present 1.5 mmole of tungsten should be deducted from the apparent total amount found. The lead phosphate was observed to react slowly with EDTA in the titration if it was not removed by filtration, but the modified procedure gives satisfactory results for the total amount of tungsten and phosphorus (Table 3).
Acknowledgement-The financial support provided by a Strategic Grant from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
REFERENCES
1. G. A. Tsigdinos, Topics Curr. Chem., 1978, 76, 20. 2. H. Hayashi and J. B. Moffat, J. Catal., in the press. 3. D. Stockdale, Analyst, 1958, 83, 24. 4. W. Rieman and J. Beukenkamp, in I.M. Kolthoff and
5. 6.
7. 8.
P. J. Elving (eds.), Treatise on Analytical Chemistry, Part II, Vol. 5, p. 346. Interscience, New York, 1961. K. F. Sporek, Chemist-Analyst, 1958, 47, 12. G. Schwarzenbach and H. Flaschka, Complexometric Titrations, 2nd Ed. Methuen, London, 1969, and references therein. R. Piibil, Applied Complexometry. Pergamon Press, Oxford, in the press. Yu. I. Bykovskaya, Trudy Inst. Metallury. Akad. Nauk
SSSR, 1962, 244. 9. 0. I. Popova and 0. G. Seraya, Zh. An&.
Khim.,
1968, 23, 791. 10. P. Bourret, J.-M. Lecuire and C. Weis, Chim. Anal. (Paris), 1970, SO, 1114. 11. H. Wu, 1. Biol. Chem., 1920, 43, 189. 12. G. V. Zavarov, G. A. Zhitarev and N. T. Karabanov, Zavodsk. Lab., 1956, 22. 650; Chem. Abstr., 1956, SO, 13661. 13. G. A. Parker, in I. M. Kolthoff and P. J. Elving (eds.), Treatise on Analytical Chemistry, Part II, Vol. 10, p. 478. Interscience, New York, 1978. 14. R. C. Weast (ed.), Handbook of Chemistry and Physics, 52nd Ed., B-102. Chem. Rubber Co., Cleveland, 1971.
Talanta, Vol. 29, pp. 943 to 945, 1982 Printed in Great Britain
DETERMINATION OF PHOSPHORUS AND TUNGSTEN IN HETEROPOLY ACIDS BY EDTA-TITRATION HIROMU HAYASHI*
Guelph-Waterloo
and J. B.
MOFFAT~
Centre for Graduate Work in Chemistry and Department University of Waterloo, Waterloo, Ontario, Canada
of Chemistry,
(Received 2 February 1982. Accepted 14 June 1982) Summary-Previous methods for the titrimetric are found to be unsatisfactory for heteropoly described.
The application
of heteropoly
acids
and
their
determination of phosphorus and tungsten with EDTA acids, Successful modifications of the procedures are
salts
to tungstophosphates
have produced less than satisfactory results. We report here some modifications of earlier procedures that make them satisfactory for analysis of 12-tungstophosphoric acid and dimeric 9-tungstophosphoric acid by EDTA titration.
in
in derecent years. 1,2 Tungsto- and molybdophosphates compose in alkaline media to form simple tungstate or molybdate ions and hydrogen phosphate,1*3 which finds use in a popular method4 for the titrimetric determination of phosphorus after precipitation as ammonium 12-molybdophosphate.3
catalytic
processes
has received
[PMo,~O,,]~-
increased
attention
EXPERIMENTAL
Samples
+ 230H-
+ 12MoO:-
+ HPO:-
+ llH20
(1)
[P2W18062]6+ 340H+ lSWO:+ ZHPO:-
+ 16H20
(2)
Analytical-reagent grade 12-tungstophosphoric acid, authentic simple tungstates and phosphates were obtained from BDH Chemicals. The 12-acid was shown to be the 2Chydrate by thermogravimetry.’ The dimeric 9-tungstophosphoric acid was prepared from the A-form of the ammonium salt.”
However, calcination of the hydrated 12-tungstophosphoric acid at high temperatures results in decomposition of the heteropoly structure with some loss of the matrix oxide by sublimation2 The stoichiometry given above no longer applies to the catalyst thus obtained and it is necessary to analyse separately for both phosphorus and tungsten. EDTA methods have been reported for the determination of phosphorus in uranium ores, concentrates and liquors, by precipitation as ammonium molybdophosphate and titration with EDTA.’ Phosphates, metaphosphates, pyrophosphates, and tripolyphosphates have also been determined by precipitation as magnesium ammonium phosphate followed by EDTA titration.6 Further discussion of EDTA techniques for the determination of phosphorus can be found in the literature.4.7 The determination of tungsten by precipitation as lead tungstate and subsequent EDTA titration was suggested some years ago’ and has been applied to tungsten-iron and tungsten-molybdenum alloysg and to tungsten oxides.” Though these methods have been satisfactorily used for analysing
a variety
of phosphorus
and
of relatively
tungsten,
simple
attempts
Reagents Stock solutions of 0.01M magnesium chloride and 1M ammonia/l M ammonium chloride buffer. Eriochrome Black T, 1% solution in 10% ammonia solution,” prepared fresh every month. Stock solutions of 0.02M lead nitrate and of acetate buffer lo (50 g of sodium acetate and 12 ml of glacial acetic acid diluted to 1000 ml). Aqueous 1% Xylenol Orange solution. Procedures Phosphorus. Add 5 ml of 1M sodium hydroxide to an aliquot of heteropoly acid solution (containing -50 pmole of phosphorus) and dissolve the resultant precipitate by gentle boiling. Neutralize with 5 ml of 1M hydrochloric acid, then add 20 ml of ammonia/ammonium chloride buffer and 10 ml of 0.01M magnesium chloride. Titrate the surplus magnesium with O.OlM EDTA (Eriochrome Black T indicator) after leaving sealed overnight. Tungsten. To an aliquot of heteropoly acid solution (cu. 100 pmole of tungsten), add 1.6 ml of IM sodium hydroxide and 20 ml of water. Boil gently to dissolve the sample and dilute with water to about 450 ml. Quickly add 20 ml of acetate buffer to make the pH 5.0, and then 10 ml of 0.02M lead nitrate. Boil for 15 min, cool, and filter off the white precipitate. Titrate the surplus lead in the filtrate with O.OlM EDTA (Xylenol Orange indicator). Correct the tungsten value for phosphate as discussed in the text.
compounds
to apply
them RESULTS
*On leave from Department of Chemical Engineering, Tokushima University, Minamijosanjima, Tokushima, 770, Japan. tTo whom correspondence should be addressed.
of 943
AND DISCUSSION
Table 1 shows the results of alkalimetric tungstophosphoric acids and their
titrations calcined
944
SHORT COMMUNICATIONS
Table
1. Alkalimetric
analysis
of tungstophosphoric products*
Sample
L
products,
in which
a known
mentioned
(ca.
100 pmole
amount
of phosphorus)
of heteropoly was
their
calcined
mm&
Taken
Found
Recovery, %
0.112, O.llO* 0.125*
0.111, 0.109, 0.121,
99.3 99.4 96.8
0.100s 0.1136
0.101, o.1104
100.7 97.2
in the text.
in 20 ml of 0.25M sodium hydroxide and the excess of alkali was titrated with 0.25M hydrochloric acid (phenolphthalein as indicator).3 The uncalcined 12-tungstophosphoric acid was the 24-hydrate, calcination of which at 400” gave the anhydrous acid and at 525” resulted in decomposition. This thermal behaviour was confirmed by thermogravimetric analysis and powder X-ray diffraction.’ The calcined products and the dimeric g-acid required complete hydrolysis by gentle boiling in alkali before the titration. This simple technique seemed suitable for routine analysis of the stoichiometric heteropoly acid. The EDTA determination of phosphorus6 is based on the complete precipitation of phosphate as ammonium magnesium phosphate (NH4MgP0,.6H,0) with a known (and excessive) amount of magnesium chloride at pH 10.5 in ammonia/ammonium chloride medium. Fine crystals are deposited on the walls of the container within 5-10 min and titration after 30 min gives fairly good results for simple phosphates (Table 2), but not for the heteropoly acids. The modified procedure described above gives results in good acid
and
Phosphorus,
H,PW,,O,, Uncalcined 24-hydrate Calcinedt at 400” (anhydrous) Calcinedt at 525” (decomposed) H~PzWI@GZ Uncalcined 35-hydrate Calcinedt at 400” (anhydrous) *Literature orocedure3 tin helium, for 2 hr.
acids
dissolved
agreement with the expected values for the uncalcined tungstophosphoric acids (Table 2). Evidence for some loss of phosphorus on calcination appears in these results though it is not shown by those obtained by alkalimetry (Table 1). The modifications are simple, but essential both for complete decomposition of the heteropoly acid and quantitative precipitation of NH,MgP0,.6H,O. Tungsten(V1) forms a white precipitate of lead tungstate with lead ions. I3 Titration” of the remaining lead with EDTA at pH 5 gives excellent results for a simple tungstate and an isopolytungstate (Table 3). The complete decomposition of the isopoly salt into simple tungstate by gentle boiling in alkaline medium is again important. However, the values obtained for tungsten in heteropoly acids were about 15% too high. The discrepancy was suspected to arise from the precipitation of lead phosphate, which is only slightly soluble in water,14 along with the lead tungstate. Precipitates were obtained from both Na*HPO, and Na,PO, with lead in acetate buffer. EDTA titration of the excess of lead in the filtrate confirmed the precipitation of PbJPO,), Since 1 mmole of phosphate will react with 1.5 mmole of lead (which in turn will
Table 2. EDTA determination
of phosphorus Recovery,
Phosphorus, Sample Authentic phosphates* Na2HP0, (NH&HP% Na,PO,. 12Hz0 Heteropoly acids5 H3PW,2040.24HZ0 Calcined at 400” Calcined at 525” H,P,W,,062~35H20
%
mmole
Independent analysis
Taken
Found
This work
0.049 1 0.0560 0.0527
0.0470 0.0555 0.0560
0.95, 0.99, 1.062
0.99jt 1.00,t 0.97,t
0.0453 0.0571 /I 0.061011 0.042 1
0.0460 0.0555 0.0540 0.0420
l.015 0.97, 0.88, 0.99,
l.OOa$
*Literature procedure4 in the text. TLaboratory A. fLaboratory B. @Modified procedure (this work). /iBased on anhydrous H,PW1204,,.
1.024:
SHORT
945
COMMUNICATIONS
Table 3. EDTA determination
of tungsten Recovery, %
Tungsten, mm& Sample Simple and isopoly tungstateg Na,WO,.2H,O (NH&oW~#U .5HzO Heteropoly acids 11 H,PW,2040.24H20 HsP2W,,0,2.35H,0
Taken
Found*
0.1052 0.1218
0.1070 0.1221
0.1025 0.1051
0.1170 0.1236
Correctedt
0.1041 0.1060
This work
Independent analysis
101.7 100.2
lOO.S$ 107.9$
101.6 100.8
101.27 99.lT
*Calculated as W, but including P for heteropoly species. tcorrected for P by deduction of 1.5 mmole of W per mmole of P. SLaboratory A. $Literature procedure.” /IModified procedure. TLaboratory B. be equivalent to 1.5 mmole of tungsten), for each mmole of phosphorus present 1.5 mmole of tungsten should be deducted from the apparent total amount found. The lead phosphate was observed to react slowly with EDTA in the titration if it was not removed by filtration, but the modified procedure gives satisfactory results for the total amount of tungsten and phosphorus (Table 3).
Acknowledgement-The financial support provided by a Strategic Grant from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
REFERENCES
1. G. A. Tsigdinos, Topics Curr. Chem., 1978, 76, 20. 2. H. Hayashi and J. B. Moffat, J. Catal., in the press. 3. D. Stockdale, Analyst, 1958, 83, 24. 4. W. Rieman and J. Beukenkamp, in I.M. Kolthoff and
5. 6.
7. 8.
P. J. Elving (eds.), Treatise on Analytical Chemistry, Part II, Vol. 5, p. 346. Interscience, New York, 1961. K. F. Sporek, Chemist-Analyst, 1958, 47, 12. G. Schwarzenbach and H. Flaschka, Complexometric Titrations, 2nd Ed. Methuen, London, 1969, and references therein. R. Piibil, Applied Complexometry. Pergamon Press, Oxford, in the press. Yu. I. Bykovskaya, Trudy Inst. Metallury. Akad. Nauk
SSSR, 1962, 244. 9. 0. I. Popova and 0. G. Seraya, Zh. An&.
Khim.,
1968, 23, 791. 10. P. Bourret, J.-M. Lecuire and C. Weis, Chim. Anal. (Paris), 1970, SO, 1114. 11. H. Wu, 1. Biol. Chem., 1920, 43, 189. 12. G. V. Zavarov, G. A. Zhitarev and N. T. Karabanov, Zavodsk. Lab., 1956, 22. 650; Chem. Abstr., 1956, SO, 13661. 13. G. A. Parker, in I. M. Kolthoff and P. J. Elving (eds.), Treatise on Analytical Chemistry, Part II, Vol. 10, p. 478. Interscience, New York, 1978. 14. R. C. Weast (ed.), Handbook of Chemistry and Physics, 52nd Ed., B-102. Chem. Rubber Co., Cleveland, 1971.