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Selective microdetermination of phosphate based on Volhard's titration
MICROCHEMICAL
JOURNAL
Selective
24, 212-216 (1979)
Microdetermination Based on Volhard’s
SALAH
SHAHINE
Facult>l of Engineering,
AND
of Phosphate Titration
SAMIR
EL-MEDANY
Ain Shams University, Received
Abbasia,
Cairo, Egypt
June 5, 1978
INTRODUCTION Silver nitrate reacts with the phosphate ion in neutral or slightly alkaline solutions to form a sparingly soluble precipitate of silver phosphate, Ag,P04 (K,, = 1.3 x IO-““). De Sousa (2) made use of this reaction for the complexometric determination of phosphate. The phosphate is precipitated as the silver salt, which after being filtered and washed is dissolved in potassium nickelocyanide solution, and the demasked nickel is titrated with EDTA solution. However, this method is of limited applicability since it lacks specificity. Chloride, bromide, iodide, and sulfate (the latter in high concentrations) react similarly and must be absent. In the present method, however, microamounts of phosphate can selectively be determined in the presence of large excesses of the above anions. The phosphate is precipitated as silver phosphate from homogeneous solutions to avoid coprecipitation with other anions. The precipitate is filtered off, washed, and treated with dilute nitric acid to dissolve silver phosphate; any silver halide or sulfate that may have formed remains unaffected on the filter. The silver ions set free in the acid are then estimated by Volhard’s titration. The whole procedure takes about 30 min. EXPERIMENTAL Apparatus and Reagents Grade A microburettes (5-ml capacity) and grade A 2-ml pipets were used throughout. All chemicals used were of analytical grade. Potassium dihydrogen phosphate (Analar BDH; minimum assay, 99.5- 100.5%) was recrystallized twice from conductivity water for use as a standard. Cadmium ammonium phosphate monohydrate, CdNH,P04.H,0, magnesium ammonium phosphate hexahydrate, MgNH,P0,*6Hz0, manganese ammonium phosphate monohydrate, MnNH,PO,.H,O, and zinc ammonium phosphate, ZnNH4P04, were prepared according to the standard gravimetric methods (3). Calcium hydrogen phosphate dihydrate was prepared according to Bailar (I). These salts were used as samples for analysis. Ammoniacal silver nitrate solution was freshly prepared (every day) by 212 0026-265X/79/020212-05$01.00/0 Copyright @ 1979 by Academic Press, Inc. All rights of reproduction in any form reserved.
DETERMINATION
OF PHOSPHATE
213
mixing 6 ml of 30% (w/v) silver nitrate solution with 8 ml of 7 N ammonia solution, adding more ammonia if necessary to dissolve any persisting precipitate, and filtering. Procedure
Transfer an aliquot of the slightly acidic test solution (l-2 ml) containing 2 -6 mg of phosphate into a lo-ml filter beaker, then add 2 drops of 2 N nitric acid, 3 drops of 3 M ammonium nitrate solution, and 2 ml of the ammoniacal silver nitrate solution. Heat gently on a mild hot plate until the pH reaches 6.5-7 (pH paper). Allow to cool to room temperature, filter with suction, and wash with 5 ml of saturated silver phosphate solution, then with two 2-ml portions of cold distilled water. Rinse the beaker with three 2-ml portions of 1 N nitric acid, allowing each portion to percolate through the filter disk to dissolve the precipitate, and collecting the solution in a 50-ml Btichner flask. Add 5 drops of ferric alum indicator and titrate with 0.05 N potassium thiocyanate.
Notes
(1) The test solution must be made slightly acidic before analysis and boiled gently for 2-3 min to decompose any carbonate, sulfite, or sulfide that may coexist. (2) If the sample solution contains cations such as Ca2+, Ba2+, Mg2+, A13+,etc., that form insoluble phosphates in basic medium, they must be removed before analysis by passing the solution through a cationexchange column (0.8 x 6 cm) in the hydrogen form (Amberlite IR-120). RESULTS AND DISCUSSION General
Before establishing the procedure outlined above, the phosphate was precipitated by the addition of silver nitrate in the conventional manner. Good results were obtained in the absence of halides, but very low results were obtained in their presence, as shown in Table 1. This is clearly due to the occlusion of some silver phosphate precipitate in the silver halide precipitate which prevents the dissolution of the former in dilute nitric acid. The extent of occlusion depends on the relative solubilities of the precipitates, the relative concentrations of ions, and the rate of addition of the precipitant. The effect of relative solubilities can be studied qualitatively from Table 1. Thus, among the halides, the iodide caused the smallest extent of occlusion. This is because silver iodide is much less soluble than silver phosphate. When silver nitrate is added gradually, most of the
214
SHAHINE
AND
EL-MEDANY
TABLE EFFECT
1
OF HALIDES ON PHOSPHATE DETERMINATION BY CONVENTIONAL PRECIPITATION (SAMPLE TAKEN, 2 ml 0.0100 M KH,PO,)
Halide ion
Concentration (M)
Added as
P0,3m found P-9
-
-
-
0.0998 0.0998 0.0079 0.0076 0.0081 0.0083 0.0091 0.0093
0.01 0.02 0.01 0.02 0.01 0.02
ClBrI-
NaCl KBr KI
% Error -0.2 -0.2 -21 -24 -19 -17 -9 -7
silver iodide will be precipitated before silver phosphate starts to form. However, when the difference between the solubilities of the silver halide and silver phosphate is not too large, the two precipitates will form, more or less, simultaneously. This increases occlusion, as in the case of chloride and bromide. Trials to minimize occlusion by slow and dropwise addition of silver nitrate solution and by digestion of the precipitate did not improve the results to a measurable extent. A prior precipitation of silver halides in acidic medium followed by neutralization to precipitate silver phosphate gave much better results, but the precision was still unsatisfactory. Using the technique of precipitation from homogeneous solutions, however, no interferences were observed from the halides. In this technique, the precipitant, (Ag+), is slowly and homogeneously generated in the solution by gradual decomposition of silver ammine complex. [ AgW-Mz] + -
heat
Ag+ + 2 NH3.
Under this condition, the different precipitates are formed sequentially in the order of increasing solubility. The final product is thus a mixture of precipitates and not a mixed precipitate. Validity
of the Method
To study the validity of the procedure, synthetic mixtures of known phosphate concentrations and containing different amounts of foreign anions were analyzed. The results obtained were satisfactory (Table 2), the maximum error being 1% and the standard deviation 0.61%. Analysis
of Inorganic
Phosphates
As a test of the applicability of the method, representative inorganic
DETERMINATION
215
OF PHOSPHATE
TABLE 2 DETERMINATION OF PHOSPHATE BY THE NEW METHOD; EFFECT OF FOREIGN ANIONS (SAMPLE, 2 ml xM KH,PO, + ADDED SALT) Phosphate concentration Foreign anion
Concentration CM)
Added as
Cl-
0.1 0.2 0.2 0.1 0.2 0.2 0.1 0.2 0.2 0.1 0.2 0.2
Br
I-
SO,‘-
Standard deviation
NaCl
KBr
KI
KXh
(M)
Taken
Found
0.0100 0.0200 0.0250 0.0100 0.0200 0.0250 0.0100 0.0200 0.0250 0.0100 0.0200 0.0250 0.0100 0.0200 0.0250
0.0101 0.0200 0.0250 0.0999 0.0201 0.0250 0.0100 0.0200 0.0249 0.0999 0.0200 0.0252 0.0100 0.0198 0.0251
from true value = d\/C(%E)‘/(n-1)
‘3%Error +I 0.0 0.0 -1 +0.5 0.0 0.0 0.0 -0.4 -1 0.0 +0.8 0.0 -1 +0.4
= 0.61%.
TABLE 3 ANALYSIS OF INORGANIC PHOSPHATES
Sample CaHPO,.2H,O
CdNH,PO,.H,O
MgNH,PO,.6H,O
MnNH,PO,
ZnNH,PO,
H,O
mg of POd3-
Taken (ms)
Calc.
Found
Recovery (o/o)
6.78 7.54 7.65 10.32 10.88 9.98 10.52 11.24 10.06 7.92 8.18 8.24 7.82 7.4s 7.18
3.74 4.16 4.22 4.03 4.24 3.89 4.07 4.35 3.89 4.04 4.18 4.21 4.16 3.97 3.82
3.70 4.20 4.20 3.98 4.22 3.86 4.04 4.36 3.92 4.00 4.16 4.16 4.16 4.00 3.84
98.93 100.96 99.53 98.76 99.53 99.23 99.26 100.23 100.77 99.01 99.52 98.81 100.00 100.76 100.52
Mean recovery (%o) 99.81
99.17
100.09
99.11
100.43
216
SHAHINE
AND
EL-MEDANY
phosphates were analyzed. The test samples were dissolved in dilute nitric acid and freed from cations by ion exchange before determination. The method gave satisfactory results for all samples; the mean recoveries ranged between 99.1 and 100.4% (Table 3). SUMMARY A selective method for the microdetermination of phosphate has been developed. The phosphate is precipitated from homogeneous solutions as silver phosphate, which after being filtered and washed is dissolved in dilute nitric acid, and the silver ions set free are determined by Volhard’s titration. Whereas the halide ions interfere if the conventional precipitation is adopted, they do not interfere if precipitation is conducted from homogeneous solutions. The method is simple and accurate to a maximum error of 21%.
REFERENCES I. Bailar, J. C., “Inorganic Syntheses,” Vol. IV, p. 20. McGraw-Hill, New York, 1953. 2. DeSousa, A., Microdetermination of phosphate with EDTA. Microchem. J. 18, 137-141 (1973). 3. Vogel, A. I., “A Text Book of Quantitative Inorganic Analysis,” 3rd printing, pp. 477, 493, 532, and 536. Longmans, London, 1962.
JOURNAL
Selective
24, 212-216 (1979)
Microdetermination Based on Volhard’s
SALAH
SHAHINE
Facult>l of Engineering,
AND
of Phosphate Titration
SAMIR
EL-MEDANY
Ain Shams University, Received
Abbasia,
Cairo, Egypt
June 5, 1978
INTRODUCTION Silver nitrate reacts with the phosphate ion in neutral or slightly alkaline solutions to form a sparingly soluble precipitate of silver phosphate, Ag,P04 (K,, = 1.3 x IO-““). De Sousa (2) made use of this reaction for the complexometric determination of phosphate. The phosphate is precipitated as the silver salt, which after being filtered and washed is dissolved in potassium nickelocyanide solution, and the demasked nickel is titrated with EDTA solution. However, this method is of limited applicability since it lacks specificity. Chloride, bromide, iodide, and sulfate (the latter in high concentrations) react similarly and must be absent. In the present method, however, microamounts of phosphate can selectively be determined in the presence of large excesses of the above anions. The phosphate is precipitated as silver phosphate from homogeneous solutions to avoid coprecipitation with other anions. The precipitate is filtered off, washed, and treated with dilute nitric acid to dissolve silver phosphate; any silver halide or sulfate that may have formed remains unaffected on the filter. The silver ions set free in the acid are then estimated by Volhard’s titration. The whole procedure takes about 30 min. EXPERIMENTAL Apparatus and Reagents Grade A microburettes (5-ml capacity) and grade A 2-ml pipets were used throughout. All chemicals used were of analytical grade. Potassium dihydrogen phosphate (Analar BDH; minimum assay, 99.5- 100.5%) was recrystallized twice from conductivity water for use as a standard. Cadmium ammonium phosphate monohydrate, CdNH,P04.H,0, magnesium ammonium phosphate hexahydrate, MgNH,P0,*6Hz0, manganese ammonium phosphate monohydrate, MnNH,PO,.H,O, and zinc ammonium phosphate, ZnNH4P04, were prepared according to the standard gravimetric methods (3). Calcium hydrogen phosphate dihydrate was prepared according to Bailar (I). These salts were used as samples for analysis. Ammoniacal silver nitrate solution was freshly prepared (every day) by 212 0026-265X/79/020212-05$01.00/0 Copyright @ 1979 by Academic Press, Inc. All rights of reproduction in any form reserved.
DETERMINATION
OF PHOSPHATE
213
mixing 6 ml of 30% (w/v) silver nitrate solution with 8 ml of 7 N ammonia solution, adding more ammonia if necessary to dissolve any persisting precipitate, and filtering. Procedure
Transfer an aliquot of the slightly acidic test solution (l-2 ml) containing 2 -6 mg of phosphate into a lo-ml filter beaker, then add 2 drops of 2 N nitric acid, 3 drops of 3 M ammonium nitrate solution, and 2 ml of the ammoniacal silver nitrate solution. Heat gently on a mild hot plate until the pH reaches 6.5-7 (pH paper). Allow to cool to room temperature, filter with suction, and wash with 5 ml of saturated silver phosphate solution, then with two 2-ml portions of cold distilled water. Rinse the beaker with three 2-ml portions of 1 N nitric acid, allowing each portion to percolate through the filter disk to dissolve the precipitate, and collecting the solution in a 50-ml Btichner flask. Add 5 drops of ferric alum indicator and titrate with 0.05 N potassium thiocyanate.
Notes
(1) The test solution must be made slightly acidic before analysis and boiled gently for 2-3 min to decompose any carbonate, sulfite, or sulfide that may coexist. (2) If the sample solution contains cations such as Ca2+, Ba2+, Mg2+, A13+,etc., that form insoluble phosphates in basic medium, they must be removed before analysis by passing the solution through a cationexchange column (0.8 x 6 cm) in the hydrogen form (Amberlite IR-120). RESULTS AND DISCUSSION General
Before establishing the procedure outlined above, the phosphate was precipitated by the addition of silver nitrate in the conventional manner. Good results were obtained in the absence of halides, but very low results were obtained in their presence, as shown in Table 1. This is clearly due to the occlusion of some silver phosphate precipitate in the silver halide precipitate which prevents the dissolution of the former in dilute nitric acid. The extent of occlusion depends on the relative solubilities of the precipitates, the relative concentrations of ions, and the rate of addition of the precipitant. The effect of relative solubilities can be studied qualitatively from Table 1. Thus, among the halides, the iodide caused the smallest extent of occlusion. This is because silver iodide is much less soluble than silver phosphate. When silver nitrate is added gradually, most of the
214
SHAHINE
AND
EL-MEDANY
TABLE EFFECT
1
OF HALIDES ON PHOSPHATE DETERMINATION BY CONVENTIONAL PRECIPITATION (SAMPLE TAKEN, 2 ml 0.0100 M KH,PO,)
Halide ion
Concentration (M)
Added as
P0,3m found P-9
-
-
-
0.0998 0.0998 0.0079 0.0076 0.0081 0.0083 0.0091 0.0093
0.01 0.02 0.01 0.02 0.01 0.02
ClBrI-
NaCl KBr KI
% Error -0.2 -0.2 -21 -24 -19 -17 -9 -7
silver iodide will be precipitated before silver phosphate starts to form. However, when the difference between the solubilities of the silver halide and silver phosphate is not too large, the two precipitates will form, more or less, simultaneously. This increases occlusion, as in the case of chloride and bromide. Trials to minimize occlusion by slow and dropwise addition of silver nitrate solution and by digestion of the precipitate did not improve the results to a measurable extent. A prior precipitation of silver halides in acidic medium followed by neutralization to precipitate silver phosphate gave much better results, but the precision was still unsatisfactory. Using the technique of precipitation from homogeneous solutions, however, no interferences were observed from the halides. In this technique, the precipitant, (Ag+), is slowly and homogeneously generated in the solution by gradual decomposition of silver ammine complex. [ AgW-Mz] + -
heat
Ag+ + 2 NH3.
Under this condition, the different precipitates are formed sequentially in the order of increasing solubility. The final product is thus a mixture of precipitates and not a mixed precipitate. Validity
of the Method
To study the validity of the procedure, synthetic mixtures of known phosphate concentrations and containing different amounts of foreign anions were analyzed. The results obtained were satisfactory (Table 2), the maximum error being 1% and the standard deviation 0.61%. Analysis
of Inorganic
Phosphates
As a test of the applicability of the method, representative inorganic
DETERMINATION
215
OF PHOSPHATE
TABLE 2 DETERMINATION OF PHOSPHATE BY THE NEW METHOD; EFFECT OF FOREIGN ANIONS (SAMPLE, 2 ml xM KH,PO, + ADDED SALT) Phosphate concentration Foreign anion
Concentration CM)
Added as
Cl-
0.1 0.2 0.2 0.1 0.2 0.2 0.1 0.2 0.2 0.1 0.2 0.2
Br
I-
SO,‘-
Standard deviation
NaCl
KBr
KI
KXh
(M)
Taken
Found
0.0100 0.0200 0.0250 0.0100 0.0200 0.0250 0.0100 0.0200 0.0250 0.0100 0.0200 0.0250 0.0100 0.0200 0.0250
0.0101 0.0200 0.0250 0.0999 0.0201 0.0250 0.0100 0.0200 0.0249 0.0999 0.0200 0.0252 0.0100 0.0198 0.0251
from true value = d\/C(%E)‘/(n-1)
‘3%Error +I 0.0 0.0 -1 +0.5 0.0 0.0 0.0 -0.4 -1 0.0 +0.8 0.0 -1 +0.4
= 0.61%.
TABLE 3 ANALYSIS OF INORGANIC PHOSPHATES
Sample CaHPO,.2H,O
CdNH,PO,.H,O
MgNH,PO,.6H,O
MnNH,PO,
ZnNH,PO,
H,O
mg of POd3-
Taken (ms)
Calc.
Found
Recovery (o/o)
6.78 7.54 7.65 10.32 10.88 9.98 10.52 11.24 10.06 7.92 8.18 8.24 7.82 7.4s 7.18
3.74 4.16 4.22 4.03 4.24 3.89 4.07 4.35 3.89 4.04 4.18 4.21 4.16 3.97 3.82
3.70 4.20 4.20 3.98 4.22 3.86 4.04 4.36 3.92 4.00 4.16 4.16 4.16 4.00 3.84
98.93 100.96 99.53 98.76 99.53 99.23 99.26 100.23 100.77 99.01 99.52 98.81 100.00 100.76 100.52
Mean recovery (%o) 99.81
99.17
100.09
99.11
100.43
216
SHAHINE
AND
EL-MEDANY
phosphates were analyzed. The test samples were dissolved in dilute nitric acid and freed from cations by ion exchange before determination. The method gave satisfactory results for all samples; the mean recoveries ranged between 99.1 and 100.4% (Table 3). SUMMARY A selective method for the microdetermination of phosphate has been developed. The phosphate is precipitated from homogeneous solutions as silver phosphate, which after being filtered and washed is dissolved in dilute nitric acid, and the silver ions set free are determined by Volhard’s titration. Whereas the halide ions interfere if the conventional precipitation is adopted, they do not interfere if precipitation is conducted from homogeneous solutions. The method is simple and accurate to a maximum error of 21%.
REFERENCES I. Bailar, J. C., “Inorganic Syntheses,” Vol. IV, p. 20. McGraw-Hill, New York, 1953. 2. DeSousa, A., Microdetermination of phosphate with EDTA. Microchem. J. 18, 137-141 (1973). 3. Vogel, A. I., “A Text Book of Quantitative Inorganic Analysis,” 3rd printing, pp. 477, 493, 532, and 536. Longmans, London, 1962.