SILVER PHOSPHATE in QUANTITATIVE ANALYSIS C. R. JOHNSON AND L. G . NUNN, JR. University of Te3:as, Austin, Texas : 9
M
OST textbooks of elementary quantitative analysis contain at least one laboratory exercise designed to familiarize students with the properties of silver chloride and its use in gravimetric analysis, and also, by analogy, with other determinations in which the same general technic is applied. The specific analysis usually chosen for study is $he determination of chloride, but for instructional purposes a determination of silver is equally useful and much more economical. In the analysis of chloride unknowns by students there is a considerable loss of silver nitrate due to liberal use of excess precipitating reagent, but this is almost literally only a drop in the bucket compared to losses occasioned by the fact that many students do not properly distinguish between silver nitrate solution and water, which after all look very much alike. Two years ago a gravimetric silver determination was substituted for the chloride determination in the quantitative analytical courses here. With a silver unknown to be analyzed, the loss of even a few tenths of a milligram of silver-bearing material became a matter of direct concern to every student. The amount of silver nitrate used in these courses dropped off so markedly that the new laboratory exercise was retained. The unknowns given out for analysis a t first were mixtures of silver nitrate and sodium or potassium ni-
trates containing from twe'nty-five to fifty per cent of silver. The analytical results obtained were excellent, but the unknowns were found to have several undesirable properties. While these mixtures do not absorb moisture in air of moderate humidity, their texture is such that they always seem msst. They are easily light-struck. Every portion spilled on a balance pan develops a permanent stain. Moreover, the grinding and mixing of the samples is a very unpleasant operation, which leaves its mark on everything and everyone concerned for some time. After a preliminary examination of various silver compounds, the phosphate was selected as the most prdmisingsubstitute for the nitrate. Ten large samples for student analysis were made upwith weighed amounts of pure, dry silver phosphate and well-dried, reagentgrade potassium dihydrogen phosphate to contain between 37.0 per cent and 77.3 per cent of silver. The average percentage of silver found in over one hundred student gravimetric analyses of each sample checked the calculated value to better than 1 part in 500 in all ten cases. The same unknowns were used by students in the volumetric determination of silver with 0.0300 iV thiocyanate solution by the Volhard method with uniformly good, but less precise, results. These mixtures containing silver phosphate have none of the objectionable properties of the silver nitrate
unknowns. I t is also of advantage that all of the various mixtures have the same bright yellow color as the pure compound.
thiocyanate solution, which was then used in analyzing samples of pure silver nitrate. Samples 1 and 2 of the silver phosphate were made as described above, from ordinary reagent-grade silver PREPARATION OF PURE SILVER PHOSPHATE nitrate and C.P. phosphoric acid. Sample 3 was preSilver phosphate that will meet most analytical re- pared similarly, but without precautions designed to quirements for purity may be prepared as follows: remove chloride present in the reagents: only the Dissolve 454 grams of reagent-grade silver nitrate in a original silver nitrate solution was filtered, and the liter of distilled water in a 4-liter beaker. Add 70 ml. ammonia was not distilled. Sample 4 was made from of 85 per cent phosphoric acid containing 1.44 grams of silver nitrate purified as described below, and from H3POaper ml., and filter the solution to remove the phosphoric acid prepared by oxidizing vacuum-disslight precipitate which forms, together with any other tilled phosphorus with pure nitric acid. Distilled aminsoluble impurities. Arrange a stirring motor with a monia was used to make aqua ammonia for the preglass propeller to stir the solution, and add 3 N chlo- cipitation. The sample was collected in a Mumoe ride-free aqua ammonia slowly and with constant stir- crucible and dried in quartz. Silver nitrate for the Volhard analyses was purified ring until noticeable precipitation of silver phosphate by precipitation with redistilled 15 N nitric acid, using ceases, or until the pH of the supernatant liquid is about 5.9. About a liter of 3 N aqua ammonia should 0.38 ml. of acid per gram of silver nitrate. The silver nitrate solution, saturated at about 60°C., was filtered be prepared for this neutralization. a t about 80' C. The nitric acid was then added and Collect the silver phosphate on a filter in a 10-inch the mixture was allowed to cool to room temperature. Biichner funnel, and wash it with ten to twenty 500- to 1000-ml. portions of distilled water. Drain the pre- The precipitated silver nitrate was centrifuged and cipitate, applying suction if convenient, and finally dry dried in platinum, dissolved in distilled water, reit to constant weight by 12 to 24 hours' heating on a crystallized, centrifuged, dried, and fused in platinum. In all standardizations and analyses the silver salts hot plate, in a 12-inch porcelain evaporating dish under an inverted funnel, raising the temperature gradually were dissolved in a little water containing 10 ml. of G N from SOT. to 350°C. or higher. Work the material chloride-free nitric acid, diluted to about 50 ml., and occasionally with a spatula or mortar and pestle, to titrated with the thiocyanate solution after addition of 5 ml. of saturated ferric alum solution containing SO break up any lumps. Using this method of preparation we have obtained ml. of 15 N nitric acid per liter. The results of the yields of 370 to 372 grams of silver phosphate containing various standardizations and analyses are summarized from 77.22 to 77.29 per cent of silver. The theoreti- below. .. : cal yield is 373 grams with a silver content of 77.31 S~ANDA~D~A~IONS ANALYSES per cent. 7
Amp01 G r o w
SJLVER PHOSPHATE AS A PRIMARY STANDARD
1
Silver phosphate has been recommended as a standard for the determination of phosphate.' In the course of the present study we have made arplyses to determine its suitability for standardizing potassium thiocyanate solutions for use in Volhard determinations. Four independently prepared samples of silver phosphate were used in standardizing a 0.1 N potassium
MI.
sarn*lc AgaPO< KCNS
2 3 4
0.4868 0.5040 0.5029 0.5034 0.4715 0.5079 0.5023 0.5233
34.53 35.66 35.62 35.70 33.32 35.93 35.57 37.06
Normnlily Gm. AgNOs MI. KCITS G n . A ~ Y O I token wed found
ofKCY.5
0.1010 0.1013 0.1012 0.1011 0.1014 0.1013 0.1012 0.1012
0.6014 0.6539 0.0020, (Normalit;of
34.99 38.06 35.01
0.6016 0.6644 06019
KCNS used = 0.1012)
These experiments show that silver phosphate may easily be prepared in pure form, and that it is a good primary standard for Volhard 'determinations.
