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Microdetermination of phosphate with EDTA
MICROCHEMICAL
JOURNAL
18, 137-141 (1973)
Microdetermination with
of Phosphate EDTA
ARTHUR DE SOUSA
A few indirect complexometric methods for the determination of the phosphate ion are known (2, 6-8, 12, 1.3). Beside being time consuming, owing to the hours or days required for complete precipitation of the phosphate, these methods cannot always be applied to microanalysis due to the solubility product of the precipitate. A suitable method for the microdetermination of phosphate is described, based on the instantaneous formation of the yellow orthophosphate, Ag,PO, (KS,, = 1.3 x 10~““) (9, resulting from the reaction of silver nitrate with a slightly alkaline or neutral solution of the phosphate sample. In this reaction the phosphate ion is completely precipitated almost instantaneously in the presence of an excess of silver nitrate. However, it is necessary that Cl-, Br-, I-, vanadate, molybdate, chromate, sulfate and tungstate ions be absent so as to avoid co-precipitation. The freshly formed silver orthophosphate dissolves in an ammoniacal solution of potassium cyanonickelate as used by Flaschka and Huditz (3) and Flaschka (4) for dissolving silver halides: 2Ag,,PO, + 3K,Ni(CN),
--+
3K,Ag,(CN),
+ 3Ni2+ + 2PO,“-.
TWO phosphate ions coexist with three of nickel in the solution. The freed nickel is titrated with EDTA to a visual end-point with murexide as indicator. The required time for a determination of phosphate in solution is about I hr, while the classical phosphomolybdate and magnesium pyrophosphate procedures need several hours and in many cases a microbalance has to be used. EXPERIMENTAL
METHODS
The following procedure requires the phosphate solution to be neutral or very slightly alkaline. 137
I38
DE SOUSA
Reagents
u. AgNOB solution. Prepared by stirring 30 g of AgN03 with 100 ml of cold water and filtering the solution. Store it in a dark brown bottle. b. Murexide indicator. As murexide solution is only stable for 1 or 2 days (I), it is preferable to dilute the indicator with solid sodium chloride and to add the finely powdered solid mixture to the liquid to be titrated (5). c. Potassium cyunonickefate. K,Ni(CN), solution: Titrate a measured portion of 0.2 M nickel sulfate solution, with 1 M potassium cyanide solution until murexide changes from yellow to purple. Repeat the titration and average the results. Mix equivalent amounts of the two solutions, add some ammonia, and dilute with water to acyanonickelate concentration of about 0.1 M. The resulting solution, if sufficiently alkaline, is stable for some months (10, I I). d. 0.01 M EDTA. Dissolve 3.721 g ofdisodium EDTA dihydrate in distilled water in a 1000 ml graduated flask and complete the volume up to the mark. Generally it is not necessary to check the titer, but if desired, this can be done by titrating a known volume of the EDTA solution using a known molar solution of nickel sulfate and murexide as an indicator. The end point is indicated by the change in color of the solution from purple to yellow. In case a 0.001 M EDTA solution is needed, it can be prepared by diluting the 0.01 M with water in the ratio of 1 : 10. e. Wash solution (saturated aqueous solution of Ag,PO,). Stir mechanically during 1 hr about 5 g of freshly precipitated silver orthophosphate in 1 liter of water. Let the solid matter settle down and then decant the clear supernatant liquid. Pipet a known volume of the phosphate solution and make it neutral or slightly alkaline (NH,OH or HNO,) so as to have a phosphate content of l-5 mg. Add water if necessary to bring the volume to about 50 ml. Then in a dark corner of the laboratory, add about 5 ml of the silver nitrate solution, stir well and allow the precipitate to settle. Add a drop or two of the AgNO, solution to the clear supernatant liquid and check that no cloudiness is produced after addition of silver nitrate. In this case all the phosphate has been precipitated. If not, add more reagent for complete precipitation. Filter the precipitate through a fine sintered glass filter with a capacity of 15-25 ml. Wash the precipitate three or four times with the wash solution, then once with water and discard filtrate and washings. Place the filter and precipitate in a beaker containing suf-
DETERMINATION
139
OF PHOSPHATE
ficient potassium cyanonickelate to cover the filter. Warm gently while stirring until all the yellow precipitate has dissolved. Remove the filter carefully from the beaker and rinse it well with water, collecting the washings in the beaker. The nickel displaced by the silver in the complex, is titrated with EDTA solution, using murexide as indicator. The end point is indicated by the sudden change in color from yellow-orange to purple.
Let us consider the reaction after dissolution of the silver orthophosphate in the potassium cyanonickelate according to the equation given above. We conclude that 2PO,:+ correspond to 3Ni’+ as the end products of the reaction. In other words, 189.9 14 g of PO,“- correspond to 176.13 g of nickel. As I ml of 0.01 M EDTA is equivalent to 0.587 I mg of Ni, it thereby results, that I ml of 0.01 M EDTA corresponds indirectly to 0.633 mg PO,“-. or I ml of 0.001 M EDTA is equivalent to 63.3 pg PO,“-. When the end point is reached. the number of ml 0.01 M EDTA used is multiplied by 0.633 to obtain the phosphate content of the sample (mg), or in the case of titration with 0.001 M EDTA by 63.3 to obtain the phosphate content in micrograms. RESULTS
AND
CONC‘LUSIONS
The above described procedure gives very satisfactory results and a microdetermination of phosphate in minerals, soils, fertilizers, biological samples, pharmaceutical products. and organo-phosphorus compounds after the phosphate is in solution, can be carried out in about I hr. The accuracy is about 1% (see Table 1). The described method has the advantage of not requiring previous removal of Fe, Al. Ni, -1ABLE TI-ORATION
PO,:‘-
taken
ok, SODIUM
PHOWH~TE
I WITH
0.01 M EDTA
PO,“- found
(me)
(mg)
"VJ
50.0 40.0 30.0 20.0 10.0 5.0
so.2
40. I 29.9 20. I IO. I 5. I
ro.7 t 0. I -0.1 to. I to. I 1 0. I
140
DE
SOUSA
TABLE TITRATION
OF SODIUM
2
PHOSPHATE
WITH
0.001
M EDTA Difference
PO,:‘- taken (PLg)
PO,:‘+ found (Pl.g)
5000 4000 3000 2000 1000 500 300 200 100
5050 4030 3050 2040 1020 510 310 190 I10
% +50 +30 +50 +40 +20 +10 +10 -10 +10
1 1
2 2 2 2 3 5 10
Co, Mn, Zn, Cd, Pb, Ca, Sr, Ba, and Mg, as is the case for the usual gravimetric methods. The procedure can also be used for microquantities of phosphate, by titrating the freed nickel with a 0.001 M EDTA solution. In this case 1 ml of the titrant will correspond to 63.3 pg POd3- (see Table 2). SUMMARY An indirect microdetermination of phosphate via EDTA titration is described, which can be applied to minerals, soils, fertilizers, biological samples, drugs and organo-phosphorus compounds. The method is based on the precipitation of phosphate as the very insoluble silver orthophosphate (K,, = 1.3 X IO-““), dissolution of this precipitate in a solution of potassium cyanonickelate and titration of the nickel displaced by silver. The phosphate content is obtained indirectly by multiplying the number of ml of the titrant by a factor. The method takes about an hour after the phosphate is brought into solution. The accuracy is about 1% for samples containing 5-50 mg PO,“- and about 3-5% for samples with 100 to 5000 pg PO,:r+. REFERENCES 1. Brunisholz, G., Murexide-stabilized indicator for the determination of fluorine. Hdv. Chim. Actu 37, 1546-1547 (1954). 2. Flaschka, H., and Holasek, A., Determination of small quantities of phosphate. Mikrochem. Ver. Mikrochim. Acta 39, 101-104 (1952). 3. Flaschka, H., and Huditz, F., Indirect titrimetric determination of silver and halogen ions with EDTA. Z. And. Chem. 137, 104- 107 (1952). 4. Flaschka H., Microtitrations with EDTA. V. indirect determination of silver and halogens. Mikrochim. Actor 40, 21-26 (1952). 5. Flaschka, H., Microchemical titrations with EDTA. Mikrorhem. Ver. Mikruchim. Acta 39, 38-50 (1952).
DETERMINATION
OF
PHOSPHATE
141
6. Fleischer, K. D., Southworth, R. C., Hodecker, J. H., and Tuckerman, M. M., Determination of phosphorus in organic compounds. A/ttr/. Ci~em. 30, 152-154 ( 1958). 7. Huditz, F., Flaschka, H., Petzold, I.. Precipitation of the phosphate ion with magnesium ions in the presence of calcium and other cations, as well as its analytical determination with EDTA, Z. A/rtr/. Cl~r,?l. 135, 333-340 (I 952). X. Ishihashi, M., and Tahushi. M.. Quantitative analysis of phosphoric acid. VII. Concentration of a trace amount of phosphate in dilute solution or in sea water by coprecipitation with magnesium hydroxide. RuuscXi KtrgrrXrr 6, 7-l 1 t 1957). 0. Meites,L.,“HandhookofAnalyticalChemistry.” lsted.,p. I-14. McGraw-Hill,New York. 10. de Sousa. A., Indirect determination of potassium. A,~cr/. Chim. Ac~ttr 22, 522-523 (1960). II. de Sousa, A., Complexometric determination of bromide and iodide present together. A,~tr/. C/rim. Ar,trr 22, 520-522 ( 1960). Sporek, K. F., Determination of phosphate in uranium concentrates and liquors via an EDTA titration. C‘12(,llli.~t-A1~(1!\..\/ 47, II- 13 (I 958). 13. Ueda. S., Yamamoto, Y., and Wakizaka. H., Chelometric determination of phosphate after precipitation with bismuth nitrate. Nipporl Ktrgtrhtt Ztrs.\hi 82, X73-875 (1961). /I.
JOURNAL
18, 137-141 (1973)
Microdetermination with
of Phosphate EDTA
ARTHUR DE SOUSA
A few indirect complexometric methods for the determination of the phosphate ion are known (2, 6-8, 12, 1.3). Beside being time consuming, owing to the hours or days required for complete precipitation of the phosphate, these methods cannot always be applied to microanalysis due to the solubility product of the precipitate. A suitable method for the microdetermination of phosphate is described, based on the instantaneous formation of the yellow orthophosphate, Ag,PO, (KS,, = 1.3 x 10~““) (9, resulting from the reaction of silver nitrate with a slightly alkaline or neutral solution of the phosphate sample. In this reaction the phosphate ion is completely precipitated almost instantaneously in the presence of an excess of silver nitrate. However, it is necessary that Cl-, Br-, I-, vanadate, molybdate, chromate, sulfate and tungstate ions be absent so as to avoid co-precipitation. The freshly formed silver orthophosphate dissolves in an ammoniacal solution of potassium cyanonickelate as used by Flaschka and Huditz (3) and Flaschka (4) for dissolving silver halides: 2Ag,,PO, + 3K,Ni(CN),
--+
3K,Ag,(CN),
+ 3Ni2+ + 2PO,“-.
TWO phosphate ions coexist with three of nickel in the solution. The freed nickel is titrated with EDTA to a visual end-point with murexide as indicator. The required time for a determination of phosphate in solution is about I hr, while the classical phosphomolybdate and magnesium pyrophosphate procedures need several hours and in many cases a microbalance has to be used. EXPERIMENTAL
METHODS
The following procedure requires the phosphate solution to be neutral or very slightly alkaline. 137
I38
DE SOUSA
Reagents
u. AgNOB solution. Prepared by stirring 30 g of AgN03 with 100 ml of cold water and filtering the solution. Store it in a dark brown bottle. b. Murexide indicator. As murexide solution is only stable for 1 or 2 days (I), it is preferable to dilute the indicator with solid sodium chloride and to add the finely powdered solid mixture to the liquid to be titrated (5). c. Potassium cyunonickefate. K,Ni(CN), solution: Titrate a measured portion of 0.2 M nickel sulfate solution, with 1 M potassium cyanide solution until murexide changes from yellow to purple. Repeat the titration and average the results. Mix equivalent amounts of the two solutions, add some ammonia, and dilute with water to acyanonickelate concentration of about 0.1 M. The resulting solution, if sufficiently alkaline, is stable for some months (10, I I). d. 0.01 M EDTA. Dissolve 3.721 g ofdisodium EDTA dihydrate in distilled water in a 1000 ml graduated flask and complete the volume up to the mark. Generally it is not necessary to check the titer, but if desired, this can be done by titrating a known volume of the EDTA solution using a known molar solution of nickel sulfate and murexide as an indicator. The end point is indicated by the change in color of the solution from purple to yellow. In case a 0.001 M EDTA solution is needed, it can be prepared by diluting the 0.01 M with water in the ratio of 1 : 10. e. Wash solution (saturated aqueous solution of Ag,PO,). Stir mechanically during 1 hr about 5 g of freshly precipitated silver orthophosphate in 1 liter of water. Let the solid matter settle down and then decant the clear supernatant liquid. Pipet a known volume of the phosphate solution and make it neutral or slightly alkaline (NH,OH or HNO,) so as to have a phosphate content of l-5 mg. Add water if necessary to bring the volume to about 50 ml. Then in a dark corner of the laboratory, add about 5 ml of the silver nitrate solution, stir well and allow the precipitate to settle. Add a drop or two of the AgNO, solution to the clear supernatant liquid and check that no cloudiness is produced after addition of silver nitrate. In this case all the phosphate has been precipitated. If not, add more reagent for complete precipitation. Filter the precipitate through a fine sintered glass filter with a capacity of 15-25 ml. Wash the precipitate three or four times with the wash solution, then once with water and discard filtrate and washings. Place the filter and precipitate in a beaker containing suf-
DETERMINATION
139
OF PHOSPHATE
ficient potassium cyanonickelate to cover the filter. Warm gently while stirring until all the yellow precipitate has dissolved. Remove the filter carefully from the beaker and rinse it well with water, collecting the washings in the beaker. The nickel displaced by the silver in the complex, is titrated with EDTA solution, using murexide as indicator. The end point is indicated by the sudden change in color from yellow-orange to purple.
Let us consider the reaction after dissolution of the silver orthophosphate in the potassium cyanonickelate according to the equation given above. We conclude that 2PO,:+ correspond to 3Ni’+ as the end products of the reaction. In other words, 189.9 14 g of PO,“- correspond to 176.13 g of nickel. As I ml of 0.01 M EDTA is equivalent to 0.587 I mg of Ni, it thereby results, that I ml of 0.01 M EDTA corresponds indirectly to 0.633 mg PO,“-. or I ml of 0.001 M EDTA is equivalent to 63.3 pg PO,“-. When the end point is reached. the number of ml 0.01 M EDTA used is multiplied by 0.633 to obtain the phosphate content of the sample (mg), or in the case of titration with 0.001 M EDTA by 63.3 to obtain the phosphate content in micrograms. RESULTS
AND
CONC‘LUSIONS
The above described procedure gives very satisfactory results and a microdetermination of phosphate in minerals, soils, fertilizers, biological samples, pharmaceutical products. and organo-phosphorus compounds after the phosphate is in solution, can be carried out in about I hr. The accuracy is about 1% (see Table 1). The described method has the advantage of not requiring previous removal of Fe, Al. Ni, -1ABLE TI-ORATION
PO,:‘-
taken
ok, SODIUM
PHOWH~TE
I WITH
0.01 M EDTA
PO,“- found
(me)
(mg)
"VJ
50.0 40.0 30.0 20.0 10.0 5.0
so.2
40. I 29.9 20. I IO. I 5. I
ro.7 t 0. I -0.1 to. I to. I 1 0. I
140
DE
SOUSA
TABLE TITRATION
OF SODIUM
2
PHOSPHATE
WITH
0.001
M EDTA Difference
PO,:‘- taken (PLg)
PO,:‘+ found (Pl.g)
5000 4000 3000 2000 1000 500 300 200 100
5050 4030 3050 2040 1020 510 310 190 I10
% +50 +30 +50 +40 +20 +10 +10 -10 +10
1 1
2 2 2 2 3 5 10
Co, Mn, Zn, Cd, Pb, Ca, Sr, Ba, and Mg, as is the case for the usual gravimetric methods. The procedure can also be used for microquantities of phosphate, by titrating the freed nickel with a 0.001 M EDTA solution. In this case 1 ml of the titrant will correspond to 63.3 pg POd3- (see Table 2). SUMMARY An indirect microdetermination of phosphate via EDTA titration is described, which can be applied to minerals, soils, fertilizers, biological samples, drugs and organo-phosphorus compounds. The method is based on the precipitation of phosphate as the very insoluble silver orthophosphate (K,, = 1.3 X IO-““), dissolution of this precipitate in a solution of potassium cyanonickelate and titration of the nickel displaced by silver. The phosphate content is obtained indirectly by multiplying the number of ml of the titrant by a factor. The method takes about an hour after the phosphate is brought into solution. The accuracy is about 1% for samples containing 5-50 mg PO,“- and about 3-5% for samples with 100 to 5000 pg PO,:r+. REFERENCES 1. Brunisholz, G., Murexide-stabilized indicator for the determination of fluorine. Hdv. Chim. Actu 37, 1546-1547 (1954). 2. Flaschka, H., and Holasek, A., Determination of small quantities of phosphate. Mikrochem. Ver. Mikrochim. Acta 39, 101-104 (1952). 3. Flaschka, H., and Huditz, F., Indirect titrimetric determination of silver and halogen ions with EDTA. Z. And. Chem. 137, 104- 107 (1952). 4. Flaschka H., Microtitrations with EDTA. V. indirect determination of silver and halogens. Mikrochim. Actor 40, 21-26 (1952). 5. Flaschka, H., Microchemical titrations with EDTA. Mikrorhem. Ver. Mikruchim. Acta 39, 38-50 (1952).
DETERMINATION
OF
PHOSPHATE
141
6. Fleischer, K. D., Southworth, R. C., Hodecker, J. H., and Tuckerman, M. M., Determination of phosphorus in organic compounds. A/ttr/. Ci~em. 30, 152-154 ( 1958). 7. Huditz, F., Flaschka, H., Petzold, I.. Precipitation of the phosphate ion with magnesium ions in the presence of calcium and other cations, as well as its analytical determination with EDTA, Z. A/rtr/. Cl~r,?l. 135, 333-340 (I 952). X. Ishihashi, M., and Tahushi. M.. Quantitative analysis of phosphoric acid. VII. Concentration of a trace amount of phosphate in dilute solution or in sea water by coprecipitation with magnesium hydroxide. RuuscXi KtrgrrXrr 6, 7-l 1 t 1957). 0. Meites,L.,“HandhookofAnalyticalChemistry.” lsted.,p. I-14. McGraw-Hill,New York. 10. de Sousa. A., Indirect determination of potassium. A,~cr/. Chim. Ac~ttr 22, 522-523 (1960). II. de Sousa, A., Complexometric determination of bromide and iodide present together. A,~tr/. C/rim. Ar,trr 22, 520-522 ( 1960). Sporek, K. F., Determination of phosphate in uranium concentrates and liquors via an EDTA titration. C‘12(,llli.~t-A1~(1!\..\/ 47, II- 13 (I 958). 13. Ueda. S., Yamamoto, Y., and Wakizaka. H., Chelometric determination of phosphate after precipitation with bismuth nitrate. Nipporl Ktrgtrhtt Ztrs.\hi 82, X73-875 (1961). /I.