Determination of S o h ble Ortho-, Pyro-, and Triphosphate in Presence of Each Other L. E. NETHERTON, A. R. WREATH,
and D. N. BERNHART
Victor Chemical Works, Research Laboratories, Chicago Heights, 111.
phosphate content before and after hydrolysis is calculated to . . triphosphate. Pyrophosphate. The m i x h e is hydrolyzed completely in hot acid solution. cooled. and total orthoDhosohate determined colorimetricaliy. The'total phosphorus conteit, minus that due to triphosphate and orthophosphate present originally, is equivalent to the phosphorus content of the pyrophosphate in the original sample.
A rapid, simple, and accurate procedure for the analysis of mixtures containing water-soluble ortho-, pyro-, and triphosphates has been developed based upon the selective alkaline hydrolysis of triphosphate combined with a modified colorimetric method for determining orthophosphate.
I
Since most samples of con~mercialsodium triphosphate contain very little, if any, orthophosphate or metaphosphate, and since pyrophosphate is unaffected under the prescribed conditions. the estimation of the sodium triphosphate content can be made by a colorimetric orthophosphate determination, following alkaline hydrolysis of the sample. ;ill other chain and cyclic phosphates will interfere.
T HAS been shown that sodium triphosphate hydrolyzes slov-ly in 1% sodium hydroxide solution at 100' C. to form
equimolar quantities of orthophosphate and pyrophosphate ( 1). Cnder these conditions, approximately 60 hours were required for complete hydrolysis. This same reaction takes place in 40 to 60 minutes 1% hen the triphosphate is heated a t the boiling temperature with a 10 to 20% sodium hydroxide solution. By contrast, sodium pyrophosphate is stable toward hydrolysis under the prescribed conditions. These observations have served as the basis for a new and superior method for the determination of sodium triphosphate bv measuring colorimetrically the amount of orthophosphate developed following alkaline hydrolysis. The phosphorus content determined as orthophosphate following hydrolysis, multipled by three, represents the phosphorus content due to the triphosphate originally present. The hydrolysis of sodium triphosphate in hot concentrated sodium hydroxide solution may be represented by Equation 1.
+
+ XasPOa + H20
K a ~ P ~ O ,2NaOH ~ +Na4PzOr
EXPERIMENTAL
Apparatus. A Klett-Summerson photoelectric colorimeter was used a t 430 mfi with a 10-mm. test tube for the colorimetric determination of orthophosphate. Reagents. PHOSPHATES. The tetrasodium pyrophosphate ( 2 ) was prepared from the commercial grade product by three recrystallizations and by heatin the resulting hydrated tetrasodium pyrophosphate a t 400' t o convert it t o the anhydrous form. The sodium triphosphate was prepared by recrystallixation of high purity (approximately 98%) commercial sodium triphosphate, using ethyl alcohol as the precipitant. The sodium triphosphate hexahydrate ( 3 ) was dehydrated by heating a t 380" to400" C. for 20 hours. Reagent grade potassium dihydrogen phosphate was used without further purification. Results for these reagents are summarized in Table I. B M M O N ~ MOLYBDATE UM SOLUTION.This solution was prepared by dissolving 18.75 grams of ammonium molybdate, reagent grade, in 300 ml. of water, adding carefully 150 ml. of concentrated sulfuric acid, cooling, and diluting to 500 ml. ACETONE,C.P. SODIUM HYDROXIDE (50% solution). One thousand grams of C.P. sodium hydroxide were dissolved in 1000 ml. of water. The solution was filtered through an asbestos pad and stored in a plastic bottle. STANDARD PHOSPHATE SOLUTION.Potassium dihydrogen phosphate, reagent grade (0.9578 gram) which had been dried for 1 hour at 110' C. was dissolved in a small amount of water and diluted to 500 ml. One milliliter of this solution contains 1 mg. of phosphorus pentoxide. Procedure. A 1-gram sample was dissolved in water and diluted t o 250 ml. Using an appropriate aliquot, the original orthophosphate (5') content of the sample was determined. On another aliquot of the same size the total phosphorus content (5') of the sample was determined following acid hydrolysis. To a third aliquot (25 ml.) in a 250-ml. beaker, 25 ml. of 50% sodium hydroxide solution and 100 ml. of water were added. The solution was covered with a watch glass and placed on a hot plate. .4fter the sample had been boiled for 45 minutes and had been concentrated to about 50 to 80 ml., it was removed from the hot plate, and cooled to room temperature, then transferred to a 100-ml. volumetric flask and diluted to volume. B 10-ml. aliquot was then transferred to a 50-ml. flask in an ice bath. In the meantime, a mixture containing 100 ml. of the ammonium molybdate solution, 150 ml. of acetone, and 150 ml. of water was made up and chilled in the ice bath. The sample was diluted to 50 ml. with this acetone-ammonium molybdate-water solution, mixed well, and the absorbance determined after 1 minute. The phosphorus content of the orthophosphate present a t this stage, minus the phosphorus content of the orthophosphate originally present in the sample, multiplied by three represents the phosphorus content of the sodium triphosphate in the original sample. Tetrasodium pyrophosphate is determined by difference. The total phosphorus content of the sample minus that due to triphosphate and orthophosphate represents the phosphorus content of the pyrophosphate in the original sample.
8.
(1)
Most commercial samples of sodium triphosphate contain approvimately 10% tetrasodium pyrophosphate and zero to a few tenths of 1% of sodium orthophosphate. I n a review of methods for determining sodium triphosphate and tetrasodium pyrophosphate in the presence of each other, Dewald and coworkers ( 4 ) concluded that no accurate method exists for triphosphate samples containing between 4 and 10% of pyrophosphate. A simple, rapid, and accurate method for analyzing such products is described by the authors and has been applied to the analysis of commercial sodium triphosphate and to synthetic mixtures containing varying percentages of ortho-, pyro-, and triphosphates.
Table I.
KaaPzO7 NarPaOio KHzPOI
Analysis of Phosphates
pzos, % Calcd. Found 53.4 53.4 57.9 57.8 52.2 52.1
Bell Method" Tri,
Pyre,
% 99 8
%
Kll
100.0
..
...
Results (2) are for sodium triphosphate hexahydrate and not anhydrous sodium triphosphate. a
The following procedure is recommended. Orthophosphate. The orthophosphate content of the mixture is determined colorimetrically prior t o alkaline hydrolysis. Triphosphate. A sample of the mixture is hydrolyzed in 10 to 20% sodium hydroxide solution a t the boiling temperature for 40 t o 60 minutes, diluted to a known volume, and orthophosphate colorimetrically determined. The difference in ortho-
860
V O L U M E 27, NO. 5, M A Y 1 9 5 5
861
Hydrolysis of Sodium Triphosphate to Pyrophosphate and Orthophosphate. Aliquots containing 0.1000 gram of sodium triphosphate or tetrasodium pyrophosphate were boiled in 150 ml. of 5, 10, 20, and 30% sodium hydroxide solution. Solut'ion samples were removed a t time intervals of 15, 30, 45, 60, and 120 minutes, cooled, and diluted to 100 ml. The solutions had concentrated approximately to one third to one half of their original volume during the boiling. No attempt was made to keep them a t constant volume. Then 10-ml. aliquots (equivalent to 10 mg. of the original sample) were analyzed for orthophosphate using a modified colorimetric method ( 3 ) . The data in Table I1 show that after 45 minutes in 10% sodium hydroxide solution or 30 minutes in 20% and 30% sodium hydroxide solution, one mole of orthophosphate was formed per mole of sodium triphosphate originally present, as required by Equation 1. The tetrasodium pyrophosphate did not hydrolyze to orthophosphate under any of the conditions as shown in Table 11. I t was therefore established that the hydrolysis of sodium triphosphate in hot alkaline solutions proceeds according to Equation 1.
Table 11. Hydrolysis of Sodium Triphosphate and Pyrophosphate i n Boiling Sodium Hydroxide Solutions Original Concn. of XaOH,
% 5 10 20 30
Moles of Ortho Formed KasPaOlo 15 min. 30 min. 45 min. 60 min. 120 min.
0.25 0.75 0.85 0.95
0.40 0.95 1.00 1.00
0.65 1.00 1.00 1.00
0.78 1.00 1.00 1.00
0.90 1.00
1.00 1.00
SatPlOl
120 min.
0.0 0.0 0.0 0.0
Modified Colorimetric Method. For determining orthophosphate in presence of pyrophosphate and sodium hydroxide, the modified colorimetric method of Bernhart and Wreath (3)employing a sulfuric acid solution of ammonium molybdate in an acetone-water medium was used. A standard curve for phosphorus pentoxide content versus absorbence, ranging from 0 to 3 mg. of phosphorus pentoxide, was prepared using a Klett-Summerson photoelectric colorimeter a t 430 mp with a 10-mm. test tube. No changes in the values of this standard curve n-ere found when the standard phosphate solution (0, 1, 2, and 3 mg. of phosphorus pentoxide) contained 10 mg. of tetrasodium pyrophosphate and 2.5 ml. of 50% sodium hydroxide solution and the absorbence was read within 30 minutes.
During the investigation of the effect of tetrasodium pyrophosphate and sodium hydroxide on the standard phosphate curve, certain changes had to be made in the original colorimetric method. When the ammonium molybdate reagent was added first, the heat evolved due to neutralization of the base was sufficient to cause rapid hydrolysis of the pyrophosphate in the acid solution, yielding high results. Cooling in an ice bath did not entirely eliminate hydrolysis of pyrophosphate to orthophosphate. When the ammonium molybdate reagent was added last an offcolor developed. Increasing the acetone concentration or decreasing the sulfuric acid concentration caused a separation into two liquid layers. Excellent results were obtained when the acetone, water, and ammonium molybdate reagent were mixed, cooled in an ice bath, and added to the chilled aliquot containing the phosphates and sodium hydroxide. The volume ratio of the acetone, water, and ammonium molybdate solutions was 1.5 to 1.5 to 1.0. No hydrolysis of the pyrophosphate occurred when this procedure was followed. RESULTS
The results obtained on analyzing triphosphate and pyrophosphate mixtures are given in Table I11 and demonstrate the accuracy and reproducibility of this method of analysis. Although data are presented only for mixtures containing 40% or more of sodium triphosphate and 60% or less of tetrasodium pyrophosphate, the method is also applicable for mixtures richer in tetrasodium pyrophosphate. The average time required for an individual analysis amounts to approximately 70 minutes. The time may be drastically reduced in the analysis of a series of samples by hydrolyzing all of the samples simultaneously. Synthetic mixtures of orthophosphate, pyrophosphate, and triphosphate were prepared from the purified reagents and analyzed by this procedure. The results as presented in Table IV show that the new method gives satisfactory results for these and similar types of mixtures.
Table IV.
Analysis of Ortho-, Pyro-, and Triphosphate Mixtures
Composition
% Ortho 5 . 0 Pyro 5.0 Tri 90.0 Ortho
5.0 10.0 85.0 Ortho 5.0 Pyro 15.0 Tri 80.0 Ortho 5.0
Pyro
Tri
Table 111. Determination of NajP3010in &fixturesof Na4P20, and NasP3010 Triphosphate Addeda,
% 100
95
90
85
80 60 40
NasP101o Found, %
100.0 99.8 99.7 99.3 94.8 94.4 94.5 94.5 90.6 90.3 90.3 90.2 85.5 85.0 84.1 85.0 80.5 79.3 80.4 59.6 59.7 59.6 39.8 40.0 40.2
Remainder is NatPrOl.
Difference
Deviation, Parts per Thousand
0.0 -0.2 -0.3 -0.7
0 2 3 7
-0.2 -0.6 -0.5
2 6
-0.5
5 5
+O. 6 +0.3 +0.3
7 3 3 2
+0.2 +0.5
0.0
-0.9 0.0 +0.5
-0.7 $0.4 -0.4 -0.3
-0.4 -0.2 0.0 $0.2
Pyro Tri
20.0
75.0
% Found 5.0, 5.0 4.4, 4.3 90.3, 90.5 5 . 0 , 5.0 9.1, 9.3 85.5, 85.2 5.0. 5.0 14.2, 14.0 80.4, 80.8 5.0, 5.0 19.0, 19.4 75.6, 75.2
Difference 0.0,
0.0
-0.6, -0.7 $0.3, $0.5 0.0, 0.0 -0.9, -0.7 +0.5, + 0 . 2 0.0, 0.0 -0.8, -1.0 $0.4, +0.8 0.0, 0.0 -1.0, -0.6 $0.6, + 0 . 2
ACKh-OW LEDGJIENT
The authors wish to acknowledge the assistance of Esther Paciorek, who performed most of the analyses reported, and the helpful criticism of L. F. Audrieth and R. K.Bell during the preparation of the manuscript.
6
0 11 0
LITERATURE CITED
(1) Bell, R.N., Ind. Eng. Chem., 39, 136 (1947). ( 2 ) Bell, R. N., Wreath, A. R., and Curless, W. T., ANAL.CHEM., 24, 1997 (1952). (3) Bernhart, D. N.,and Wreath, A. R., Ibid., 27, 440 (1955). (4) Dewald, W., Schmidt, hl., and Neeb, K. H., Fette u . Seifeen, 56, 105 (1954). RECEIVED for review J u n e 28, 1954. Accepted November 11, 1954. Presented before t h e Division of Analytical Chemistry a t the 126th Meeting of the AMERICAN CHEMICAL SOCIETY,New York.
