180 PROCEEDINGS OF THE AMERICAN ACADEMY. 



A glance at this rather discordant series of results shows the neces- 

 sity for a redetermination of the atomic weight of arsenic. Even in 

 the more recent investigations of Hibbs and* Ebaugh there exists an 

 extreme variation of nearly two tenths of a unit in the averages of the 

 five series. 



In this research silver arsenate was chosen for analysis, first, be- 

 cause the compound is unchanged by moderate heating, and hence 

 1 may be dried at a temperature high enough to expel all but a very 

 small amount of moisture. In the second place, silver compounds may 

 be analyzed with great ease as well as accuracy by precipitation of the 

 silver as silver halogen compounds. Furthermore, preliminary ex- 

 periments confirmed the statement by Ebaugh that it is possible com- 

 pletely 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. 



Purification of Materials. 



Silver arsenate. — All the samples of silver arsenate were prepared 

 by adding to a fifteenth normal solution of silver nitrate a solution of 

 similar concentration of an equivalent amount of an arsenate of sodium 

 or ammonium, the differences between the different samples consisting 

 chiefly in the nature of the soluble, arsenate employed. 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 settling in 

 platinum crucibles, and then by being heated in an electric oven at a 

 temperature of 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 diphenylamine that the 

 arsenate could be washed free from nitrates. 



Although one of the hydrogens of arsenic acid resembles the hy- 

 drogen of strong acids in its dissociating tendency, the other two hy- 

 drogens are those of weak acids. 3 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 



3 Washburn calculates from Walden's conductivity measurements the 

 constant for the first hydrogen of arsenic acid to be 4.8 • 10 -3 . Jour. Amor. 

 Chem. Soc, 30, 35 (1908). The constants for the second and third hydro- 

 gens are probably lower than those of phosphoric acid, 2.1 • 10 -7 and 

 5.6 • 10- 13 . Ibid., 38. 



