BAXTER. — ATOMIC WEIGHT OF CHROMIUM. 405 



mides. The greater part of the chlorine was undoubtedly eliminated 

 by this operation, since the original bromine was fairly pure. In order 

 to be on the safe side, however, the bromine was again reduced to hy- 

 drobromic acid, and this in turn was changed to bromine as above. From 

 the product the final hydrobromic acid was prepared with hydrogen 

 sulphide. After filtration and distillation, it was preserved in Jena 

 glass. 



Chromic Acid. — This was prepared from Merck's "Highest Purity 

 Chromic Acid." The material was dissolved in pure water, and the so- 

 lution was filtered through a Gooch crucible with a mat of platinum 

 sponge, a quantity of sandy material being thus separated. The solu- 

 tion was then evaporated to saturation and three times systemat- 

 ically recrystallized in platinum dishes with centrifugal draining, 

 each mother liquor being used for the crystallization of three crops of 

 crystals on account of the small temperature coefficient of solubility 

 of chromic acid. The mother liquors from the first crystallization, on 

 testing in the nephelometer, indicated only traces of sulphates and 

 halogens. 



Potassic Chromate. — Some of the purest commercial salt, after solu- 

 tion in water, was filtered through a Gooch-Munroe-Neubauer crucible. 

 It was then four times crystallized in platinum, each crop of crystals 

 being centrifugally drained. 



Silver Chromate. — The point in the investigation requiring the most 

 attention was the preparation of normal silver chromate free from both 

 basic and acid salts. Since the salt cannot be crystallized, owing to its 

 slight solubility in water, it is necessary so to regulate the conditions 

 during precipitation that neither acid nor basic salts can separate as a 

 distinct solid phase. Even then the occlusion of traces of either basic 

 or acid salts is still possible, and it is necessary to form the salt 

 under a fairly wide range of conditions in order to show constancy of 

 composition. 



Fortunately data are available which indicate the conditions under 

 which silver dichromate or hydrochromate can exist. Sherrill 18 has 

 recently shown that silver chromate changes into silver dichromate 

 rapidly under a saturated solution in nitric acid more concentrated 

 than 0.075 normal, while silver dichromate changes into silver chro- 

 mate under a saturated solution in nitric acid less concentrated than 

 0.06 normal. Some time before, Kriiss 19 had shown that silver dichro- 

 mate is converted into silver chromate by contact with water. 



18 Jour. Amer. Chem. Soc, 29, 1673 (1907). 



19 Ber. d. d. Chem. Gesell., 22, 2050 (1889). 



