A REVISION OF THE ATOMIC WEIGHT OF 



CHROMIUM. 



THE ANALYSIS OF SILVER DICHROMATE. 



INTRODUCTION. 



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

 silver chromate 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 

 (Ag = 107.88), being about o.i unit lower than the one in common use. The 

 preparation and analysis of silver dichromate were next investigated. Since the 

 proportion of chromium in the dichromate is 50 per cent larger than in the 

 chromate, the effect of experimental uncertainty upon the final result is cor- 

 respondingly 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-Hquor could do no harm. If the concentration of the nitric acid is suffi- 

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

 during this crystaUization, since Sherrill^ has shown that silver chromate changes 

 rapidly into silver dichromate under nitric acid solutions 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 Sher- 

 rill to be 6 X lo"'', the solubiUty product of silver chromate being 9 X lo"^, 

 and that of silver dichromate being 2 X lo'^ Sherrill has also investigated the 

 part which the hydrochromate ion plays in the equilibrium relations of chro- 

 mates and dichromates in solution and has found the following equation to 

 hold: 



(Cr,Or) _ 

 (HCrOJ^^ "" ^^ 



Although obviously the concentration of the hydrochromate ion in dichro- 

 mate solutions (in a o.i molal solution of potassic dichromate fifteen per cent 

 of the salt existing as hydrochromate) is always considerable, the precipitation 

 of the solid phase AgHCr04 seems not to be possible. Sherrill was not able to 

 find any indication of the presence of this salt in the precipitate formed by 

 adding silver nitrate to chromic acid in nitric acid solution. Furthermore, 



* Jour. Atner. Chem. Soc, 29, 1641 (1907). 



iSS 



