422 PROCEEDINGS OF THE AMERICAN ACADEMY. 



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

 ruates and dichromates in solution and has found the following equa- 

 tion to hold: 



(Cr 2 7 =) 



(HCr0 4 f 



=75. 



Although obviously the concentration of the hydrochromate ion in 

 dichromate solutions (in a 0.1 molal solution of potassic dichromate 

 fifteen per cent of the salt existing as hydrochromate) is always con- 

 siderable, the precipitation of the solid phase AgHCr0 4 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, since the water 

 content of our material was carefully investigated, the presence of 

 hydrochromate in traces could do no harm ; for the latter substance 

 upon sufficient heating would yield dichromate and water according to 

 the following equation : 



2 AgHCr0 4 = Ag a Cr 2 7 + H 2 0. 



Although the presence of polychromates other than the dichromate 

 seemed improbable, their absence from our material was shown by 

 crystallizing silver dichromate from nitric acid of different concen- 

 trations. Since this variation was without effect, it may be reason- 

 ably supposed that more highly acid salts than the dichromates were 

 neither precipitated as solid phases nor occluded. 



Purification of Materials. 



Only slight changes were made in the methods of purifying the ma- 

 terials used in the various preparations of silver dichromate and in the 

 analyses from those described in the preceding paper. 



Nitric Acid. — Nitric acid was freed from chlorine by several dis- 

 tillations through a platinum condenser. 



Hydrochloric Acid. — This acid also after dilution was purified by 

 distillation with a quartz condenser. 



Hydrobromic Acid. — Hydrobromic acid was prepared from bromine 

 which had been twice distilled from solution in potassium bromide, the 

 bromide in the second distillation being essentially free from chlorine. 

 The hydrobromic acid was synthesized by passing carefully cleansed 

 hydrogen ("made from the lead-sodium alloy "hydrone" and water) 

 through the bromine at about 40° and then over hot platinized asbestos, 

 the acid being collected in pure water. Iodine was eliminated from the 

 acid by boiling with free bromine several times. Finally it was redis- 



