74 RESEARCHES UPON ATOMIC WEIGHTS. 



it is possible completely to convert the arsenate into chloride by heating in a 

 current of hydrochloric-acid gas. Such a process has the advantage that no 

 transfer of material is involved. 



THE PREPARATION OF TRISILVER ARSENATE. 



All the samples of silver arsenate were prepared by adding to a fifteenth nor- 

 mal solution of silver nitrate a solution of similar concentration of an equiva- 

 lent amount of an arsenate of sodium or ammonium, the differences between the 

 different samples consisting chiefly in the nature of the soluble arsenate em- 

 ployed. Precipitation was carried out in a room lighted only with ruby light. 

 After the silver arsenate had been washed by decantation many times with pure 

 water, it was dried in a preliminary way by centrifugal settHng in platinum 

 crucibles, and then by being heated in an electric oven at about 130° C. The 

 salt was powdered in an agate mortar before the final heating in a quartz tube 

 or platinum boat, as explained later. It was shown by tests with diphenyl- 

 amine that the arsenate could be washed free from nitrates. 



Although one of the hydrogens of arsenic acid resembles the hydrogen of 

 strong acids in its dissociating tendency, the other two hydrogens are those 

 of weak acids.^ Hence perceptible hydrolysis takes place in solutions of salts 

 of this acid, even when the base is strong, that of the tertiary salts being of 

 course greatest in extent. It is not an easy matter to predict the effect of this 

 hydrolysis upon the composition of a precipitate of silver arsenate; for while 

 the Phase Rule allows the existence of only one solid in equilibrium with the 

 arsenate solution except at certain fixed concentrations, the possibility of the 

 occlusion of either basic or acid arsenates by the silver arsenate still exists. 

 Experiments only are able to throw light on tliis point. Accordingly arsenate 

 solutions of different conditions of acidity and alkalinity were used in the 

 precipitations, and the compositions of the different precipitates were 

 compared. 



Sample A. Commercial C. P. disodium arsenate was recrystallized four 

 times, all but the first crystallization being conducted in platinum vessels. The 

 mother-Uquor 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. Dur- 

 ing this precipitation the mother-Hquor remains essentially neutral. 



Sample B. This sample was made from disodium arsenate which had been 

 recrystallized five times in platinum vessels. Silver arsenate was precipitated 



^ Washburn calculates from Walden's conductivity measurements the constant for the 

 first hydrogen of arsenic acid to be 4.8X 10-'. Jour. Amer. Chem. Soc, 30, 35 (1908). The con- 

 stants for the second and third hydrogens are probably lower than those of phosphoric acid, 

 2.0 X 10-' and 3.6 X 10-". Abbott and Bray: Ibid., 31, 755 (1909)- 



