A REVISION OF THE ATOMIC W'EIGHT OF ARSENIC. 75 



with a solution of this salt without the addition of ammonia. Here the mother- 

 liquor becomes more and more acid as precipitation proceeds. 



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 arsenic 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 

 possible 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. The precipitation of silver arsenate by adding a solution of this salt to 

 a solution of silver nitrate resembles the preparation of Sample B, 



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 ammonia to form tri- 

 ammonium arsenate was added to a solution of this salt before the precipi- 

 tation 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 recrystallized 

 sodium carbonate was added in amount sufficient to form disodium arsenate. 

 After the solution had been evaporated to dryness, the salt was recrystallized 

 four times in platinum. Enough ammonia to form disodium ammonium arse- 

 nate was added to a solution of this salt before the precipitation of the silver 

 arsenate. This material resembles Sample A. 



Sample F. A portion of the disodium arsenate prepared for Sample B was 

 converted into trisodium arsenate by means of recrystallized sodium carbonate, 

 and the trisodium arsenate was recrystallized six times in platinum vessels. 



Sample G. Arsenic trioxide was twice resublimed in a current of pure dry air 

 and then once crystallized from dilute hydrochloric-acid solution. Next the 

 arsenious acid was oxidized to arsenic acid exactly as described under Sample C. 

 Finally the arsenic acid was converted into trisodium arsenate by means of 

 pure sodium carbonate, and the salt was crystallized four times in platinum. 

 Samples F and G are evidently very similar. 



In all the foregoing crystallizations the crystals were thoroughly drained in a 

 centrifugal machine employing large platinum Gooch crucibles as baskets,^ and 

 each crop of crystals was once rinsed with a small quantity of pure water and 

 subsequently drained in the centrifugal machine. 



1 Baxter: Jour. Amer. Chem. Soc, 30,-286 (1908). 



