Precipitation of Silver Phosphate from Homogeneous Solution F. H. FlRSCHlNG Department o f Chemisfry, hiversify o f Georgia, Athens, Ga.
b Silver phosphate is precipitated from homogeneous solution by the volatilization of ammonia. Silver phosphate is soluble in ammoniacal solution because of the formation of the silver-ammonia complex. As ammonia escapes from the heated solution, large, easily filtered crystals of silver phosphate precipitate. The high gravimetric factor resulting from the use of silver ion as the precipitating agent makes a precise determination of phosphate possible.
T
HE two most widely used gravimetric methods for the determination of phosphate have limitations. The ammonium phospho-12-molybdate method (1) is subject to a variety of errors. Despite the low gravimetric factor, the weighing of magnesium pyrophosphate has been considered to be the best method available. A more precise determination is made possible by using silver phosphate, which is 7.40y0 phosphorus or 16.96% phosphorus pentoxide. Precipitation of insoluble silver salts from homogeneous solution using the volatilization of ammonia has been applied to the separation of iodide, bromide, and chloride ( 2 ) . This technique produces large, well-formed crystals of silver phosphate which are easily filtered and washed. This method is based on the solubility of silver phosphate in ammoniacal solution, due to the formation of the silver - ammonia complex. The ammoniacal solution is heated, ammonia escapes, and the free silver ion concentration gradually increases, causing silver phosphate to precipitate slowly, From 2 to 200 mg. of phosphorus pentoxide can be determined by this method. Quantitative precipitation is achieved, for less than 0.1 mg. of phosphorus pentoxide is left unprecipitated.
PROCEDURE
Add a soluble sample containing (1.3 mmoles of about 90 mg. of P z O ~ HaP04) to a 250-ml. beaker. Add 0.5 ml. of concentrated ammonium hydroxide and 25 mmoles of ammonium nitrate and adjust the volume to about 100 ml.
Prepare the precipitant solution by adding 15 mmoles of silver nitrate and 3 ml. of concentrated ammonium hydroxide to a beaker and adjusting the volume to about 75 ml. Add the precipitant solution directly to the phos-, phate solution in one portion. A clear uniform solution should result. (If a precipitate forms, add more ammonium hydroxide.) Place the beaker on a low temperature hot plate for 3 to 4 hours. Maintain the volume by occasional additions of water. The p H of the solution, a t room temperature, must be below 7.5 before filtration. Continue the heating until this pH range is achieved. Then filter through a medium-porosity tared glass crucible. R a s h the precipitate with several small portions of distilled water, dry a t 150' C. for a t least an hour, and weigh as silver phosphate.
was made of these three crystal shapes; the powder pictures were identical, indicating the same crystal lattice. The quantity of ammonium nitrate is not critical, but it affects the pH of precipitation. If 9 mmoles of ammonium nitrate are added, precipitation is quantitative a t about p H 8. If 55 mmoles are added, precipitation is quantitative a t about pH 7. A value of 25 mmoles of ammonium nitrate was selected, precipitation becoming quantitative a t about 7.5.
Table 1. Determination of Unprecipitated Phosphate Using Phosphorus-32
Phosphate
Phosphate in
in
DISCUSSION
The solubility of silver phosphate was determined a t various p H values (Table I) using phosphorus-32 as a tracer. Two methods were used to establish the solubility of silver phosphate by precipitation from homogeneous solution: volatilization of ammonia from basic solution and hydrolysis of urea from acid solution. Precipitation begins a t about pH 8.5 from basic solution when ammonia volatilization is used and a t about pH 2.8 from acid solution when urea hydrolysis is used. Gravimetric results for the acid p H values could not be obtained because silver carbonate begins to precipitate about the point where silver phosphate precipitation becomes quantitative. The precipitation of silver carbonate is caused by the carbon dioxide released from the hydrolysis of urea. As the pH gradually increases the carbonate concentration also increases, resulting in the precipitation of silver carbonate. The form of the silver phosphate crystals produced by various precipitation methods was unusual. From ammoniacal solution (volatilization of ammonia), the crystals were dense, geometric shapes. From acid solution (hydrolysis of urea), the crystals were slender needlelike tetrahedrons. From acid solution (hydrolysis of urea in an acetate buffer), the crystals were flat, platelike forms. An x-ray analysis
4.0 4.7 5.1 6.2 7.1
0.6 0.1 0.02 0.05 0.07
7.4 7.5 7.7 7.9
0.05 0.04 0.4 3 .O
Table II. Precipitation of Varying Amounts of Silver Phosphate P206, Mg. Error, Taken Found Mg. 0.0 1.9 1.9 $0.1 1.9 2.0 0.0 3.7 3.7 -0.1 9.3 9.3 0 .0 18.6 18.5 --0.1 18.6 18.6 --0.1 46.6 46.5 0.0 46.6 46.5 0.0 93.1 93.1 G 93.1 0.0 _ _i . 1 93.1 0.0 93.1 93.1 0.0 93.1 186.3 0.0 186.3 186.2 --0.1 186.3 Table 111. Precipitation of Silver Phosphate in Presence of Diverse Ions (0.65 mmole of PzOs,93.1 mg. taken) PZ06
Substance Found, Taken Mmolea Mg. 50 93.1 KNOs 50 93.4 NaNOa 50 93.6 NaOAc 25 93.1 NHiOAc 93.6 (NK)zSO, 25 93.3 (Na.hS01 5 NaZSiOz
0.5
94.6
VOL. 33, NO. 7, JUNE 1961
Error, hfg. 0.0 +0.3 $0.5 0.0 +0.5 +0.2 +1.5
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The concentration of silver nitrate is not critical. When the quantity of silver nitrate was varied from 7 to 30 mmoles, the determination of 0.65 mmole of phosphorus pentoxide (93.1 mg.) was reproducible within 0.1 mg. A value of 15 mmoles of silver nitrate was selected for routine use. Results (Table 11) show that from about 2 to 200 mg. of phosphorus pentoxide can be determined within 0.1 mg. The possible interference of common substances was studied. Table I11 shows that potassium, ammonium, nitrate, and acetate ions give no significant change in results. Both sodium and sulfate ions cause slightly high results when present in large
amounts. Silicate is the only substance studied that is a serious interference. This method would not be satisfactory if arsenate, chromate, tungstate, vanadate, etc., were present. Multivalent cations would also interfere. A prior removal by some other technique, such as ion exchange, is necessary before this method can be used. Chloride, bromide, iodide, cyanide, thiocyanate, and oxalate, which form insoluble silver compounds, can be removed by a preliminary volatilization with nitric acid. The solution containing nitric acid must be heated almost t o dryness to remove these anions completely. The resulting salts must be hydrolyzed for about 4 hours in warm
nitric acid solution to remove any polyphosphate that may have formed during the volatilization and heating. When nitric acid volatilization was used in the presence of 1 mmole of these interfering anions, the determination of 0.65 mmole of phosphorus pentoxide (93.1 mg.) was reproducible within 0.3 mg. LITERATURE CITED
(1) Cannon, P., TaZanta 3, 219 (1960). (2) Firsching, F. H., ANAL. CHEM.32,
1876 (1960). RECEIVED for review December 9, 1960. Accepted March 23, 1961. Southeastern Regional Meeting, ACS, Birmingham, Ala., November 1960.
Determination of Radioantimony by Extraction into Dii s 0 b uty Ica rbino I R. W. LOWE,'S. H, PRESTWOOD, R. R. RICKARD, and E. 1. WYATT Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Tenn. A simple liquid-liquid extraction procedure has been developed for determining radioantimony in mixed fission products. Antimony(V) is extracted from a 7M HCI-6M t i 3 P 0 4 aqueous solution into diisobutylcarbinol-n-heptane ( 1 to 1 ). For quantitative measurement, the Sb(V) is stripped from the organic phase into 1 M NaOH and counted on a gamma scintillation spectrometer. At least 9570 of the Sb is recovered; no carrier is required. The procedure is rapid (30 minutes) and is also suitable for remotely controlled analyses for radioantimony. The behavior of 25 radioisotopes and of U and Th is reported.
The use of diisobutylcarbinol as an extractant for radioantimony was suggested from the work of Moore and Reynolds (8), who observed the coextraction of Sb(V) and Pa23ainto diisobutylcarbinol from an aqueous phase made strongly acid with HCl or H804. The evaluation of diisobutylcarbinol as an extractant for the isolation of
ECHNIQUES for the extraction of antimony have been reviewed by West (16) and Sandell (1.2). Coppins and Price ( d ) , Edwards and Voigt (4, and Schweitzer and Storms (14) studied isopropyl ether as an extractant for antimony. Others have suggested the use of amyl acetate (6), tri-n-octylphosphine oxide in cyclohexane (IO), and diethylammonium diethyldithiocarbamate in chloroform (17) as extractants. Most analytical methods for radioactive antimony employ distillation and/or precipitation techniques ( I , 3, 6), in all of which an antimony carrier is required. Extraction techniques are used in only a few methods.
50-
40-
RESULTS AND DISCUSSION
1
0 HCI 0 HzS04 HsP04
90- A 80
* "03
-
1 Present address, Vanderbilt University Medical School, Nashville, Tenn.
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ANALYTICAL CHEMISTRY
g 60n W
6
EXPERIMENTAL DETAILS
Reagents. All reagents were analytical reagent grade. Diisobutylcarbinol (2,6-dimethyl-4heptanol), diluted 1 to 1by volume with n-heptane and the resulting solution equilibrated with an aqueous solution of the same acid composition as the aqueous phase. Ceric ammonium sulfate, solid, Ce(SO4)2.2("4) 2S04.2H20. 7 M HCl-6M HsP04. Apparatus. Gamma-ray scintillation counter having a well-type sodium iodide crystal and equipped with a differential pulse-height analyzer, a scaler, and/or a recorder.
70 -
T
radioantimony is reported herein. Throughout the study, the organic phase was diisobutylcarbinol-n-heptane (1 to 1 by volume), solutions of various acids were the aqueous phases, and Sb125 was used as a tracer.
a a k-
In -i
IA 30-
20
-
to-
CONCENTRATION OFACIO IN AQUEOUS PHASE, M
Figure 1. Effect of type and concentration of acid in aqueous phase on extractab!lity of Sb(V)126 into diisobutylcarbinol-n-heptane
It was observed that Sb(II1) is extracted to only a limited extent into diisobutylcarbinol-n-heptane, whereas Sb(V) is extracted almost quantitatively from HCl solutions of high concentrations (10M or greater), from solutions, and to some extent from H3P0h solutions. Cerium(1V) readily oxidizes Sb(II1) to Sb(V) in strong HC1 solution (11, IS). However, Ce(IV) will not oxidize Sb(1V) to Sb(V) (16); the Sb(IV) must first be reduced to Sb(II1). The reduction is done with sodium bisulfite. The results
