142 RESEARCHES UPON ATOMIC WEIGHTS. 



eighths being distilled from the solution of these bromides. 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 hydrobromic 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 was prepared from Merck's "Highest Purity Chromic Acid." 

 The material was dissolved in pure water, and the solution was filtered through 

 a Gooch-Munroe-Neubauer crucible with a mat of platinum sponge, a quan- 

 tity of sandy material being thus separated. The solution was then evap- 

 orated to saturation and three times systematically recrystallized in platinum 

 dishes with centrifugal draining, each mother-liquor being used for the crystalli- 

 zation of three crops of crystals on account of the small temperature coefficient 

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

 on testing in the nephelometer, indicated only traces of sulphates and halogens. 



Some of the purest commercial potassium chromate, after solution 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. 



PREPARATION OF SILVER CHROMATE. 



The point in the investigation requiring the most attention was the prepa- 

 ration of normal silver chromate free from both basic and acid salts. Since 

 the salt can not be crystallized, owing to its slight solubility in water, it is neces- 

 sary so to regulate the conditions during precipitation that neither acid nor 

 basic salts can separate as distinct solid phases. 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 com- 

 position. 



Fortunately data are available which indicate the conditions under which 

 silver dichromate or hydrochromate can exist. Sherrill ^ 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 di- 

 chromate changes into silver chromate under a saturated solution in nitric acid 

 less concentrated than 0.06 normal. Some time before, Kriiss ^ had shown that 

 silver dichromate is converted into silver chromate by contact with water. 



In the light of these facts it is obvious that the solutions of the soluble chro- 

 mates can safely be employed for the precipitation of silver chromate without 

 the least danger of the precipitation of silver dichromate, and even that the 

 presence of a slight amount of free acid could do no harm. 



^ Jotir. Amer. Chem. Soc, 29, 1673 (1907). 

 * Ber. d. d. Chem. Gescll., 22, 2050 (18S9). 



