BAXTER AND COFFIN. — ANALYSIS OF SILVER ARSENATE. 1S1 



existence of only one solid in equilibrium with the arsenate solution ex- 

 cept at certain fixed concentrations, the possibility of the occlusion of 

 either basic or acid arsenates by the silver arsenate still exists. Ex- 

 periments only are able to throw light on this point. Accordingly 

 arsenate solutions of different conditions of acidity and alkalinity were 

 used in the precipitations, and the compositions of the different precipi- 

 tates were compared. 



Sample A. Commercial C. P. disodium arsenate was recrystal- 

 lized four times, all but the first crystallization being conducted in 

 platinum vessels. The mother liquor from the fourth crystallization, 

 after the removal of the arsenic by hydrogen sulphide, gave no test for 

 phosphate. The calculated amount of redistilled ammonia to make 

 disodium ammonium arsenate was added to a solution of the purified 

 salt before the precipitation of the silver arsenate. 



Sample B. This sample was made from disodium arsenate which 

 had been recrystallized five times in platinum vessels. Silver arsenate 

 was precipitated with a solution of this salt without the addition of 

 ammonia. 



Sample C. Commercial C. P. arsenic trioxide was recrystallized 

 three times from dilute hydrochloric acid solution, and, after being 

 rinsed with water and centrifugally drained, it was converted into ar- 

 senic acid by means of nitric and hydrochloric acids in a porcelain 

 dish. The hydrochloric and nitric acids were expelled by evaporation 

 nearly to dryness, and the residue was twice evaporated to dryness with 

 nitric acid in a platinum dish. After the residue had been dissolved 

 in water, the solution was allowed to stand for some time in order to 

 allow pyro and meta arsenic acids to be converted as completely as pos- 

 sible into ortho arsenic acid. Then sodium carbonate which had been 

 twice crystallized in platinum was added to the solution in amount 

 sufficient to form disodium arsenate, and the product was crystallized 

 four times in platinum vessels. 



Sample D. A portion of the arsenic acid made for the preparation 

 of Sample C was converted into ammonium dihydrogen arsenate by 

 adding the calculated amount of redistilled ammonia, and the salt 

 was recrystallized five times in platinum. A sufficient quantity of am- 

 monia to form triammonium arsenate was added to a solution of this 

 salt before the precipitation of the silver arsenate. One specimen of 

 silver arsenate, made in this way was discarded, since its composition 

 was very irregular. 



Sample E. To a portion of the arsenic acid used for Sample C re- 

 crystallized sodium carbonate was added in amount sufficient to form 

 disodium arsenate. After the solution had been evaporated to dryness, 



