CONTRIBUTIONS FROM THE CHEMICAL LABORATORY 

 OF HARVARD COLLEGE. 



A REVISION OF THE ATOMIC WEIGHT OF CHROMIUM. 



SECOND PAPER.— THE ANALYSIS OF SILVER DICHROMATE. 

 By Gregory Paul Baxter and Richard Henry Jesse, Jr. 



Presented January 13, 1909. Received December 11, 1908. 



In the preceding paper 1 is described a successful attempt to prepare 

 pure silver cure-mate and to determine its silver content, with the 

 object of throwing light upon the atomic weight of chromium, the 

 value found in this way, 52.01, being about one-tenth of a unit lower 

 than the one in common use. The preparation and analysis of silver 

 dichromate was next investigated. Since the proportion of chromium 

 in the dichromate is fifty per cent larger than in the chromate, the 

 effect of experimental uncertainty upon the final result is correspond- 

 ingly reduced. 



Silver dichromate possesses another great advantage over silver 

 chromate for exact work in that it may be readily crystallized from 

 nitric acid solutions, and thus may be freed from impurities included 

 or occluded during precipitation, with the exception of nitric acid and 

 moisture. For, the silver and chromium being present in equivalent 

 proportions during the crystallization, the inclusion of mother liquor 

 could do no harm. If the concentration of the nitric acid is sufficiently 

 high, there is no possibility of the separation of silver chromate as 

 such during this crystallization, since Sherrill 2 has shown that silver 

 chromate changes rapidly into silver dichromate under nitric acid solu- 

 tions more concentrated than 0.075 normal. This is primarily due to 

 the low value of the dissociation constant of the second hydrogen of 

 chromic acid, which has been found by Sherrill to be 6 X 10 -7 , the 

 solubility product of silver chromate being 9 X 10~ 12 , and that of silver 

 dichromate being 2 X 10 -7 . Sherrill has also investigated the part 



1 Baxter, Mueller, and Hines, These Proceedings, 44, 399-417 (1909). 



2 Jour. Amer. Chem. Soc, 29, 1641 (1907). 



