BAXTER. — ATOMIC WEIGHT OF CHROMIUM. 407 



cantation with water eight times more, each wash water being allowed 

 to stand in contact with the precipitate for many hours, and the pre- 

 cipitate being shaken with the wash water very thoroughly at intervals, 

 in order to leach out any accidentally enclosed or adsorbed soluble 

 salts. The prolonged extra washing evidently was unnecessary, since the 

 results are practically the same as those obtained with Sample I. 



Sample III This sample was prepared from the four times recrys- 

 tallized potassic chromate. A quantity of this material in about tenth 

 normal solution was precipitated with an equivalent amount of silver 

 nitrate, equally dilute. The precipitation took place in Jena glass, the 

 silver solution being slowly poured into the chromate, in order to 

 accentuate the effect of the hydrolysis if possible. It will be recalled 

 that in the case of Samples I and II prepared with the ammonic salt, 

 the chromate was added to the silver solution. The precipitate was 

 then transferred to platinum and washed seven times with the purest 

 water, the chromate being thoroughly agitated with each washing. 

 After the removal of the greater part of the adhering water by centrifu- 

 gal settling, this sample was dried in a preliminary fashion at 150° 

 and was pulverized in an agate mortar, as in the case of Samples I and 



II. The salt was soft and crystalline, and greenish black in color. 

 Sample IV. A fourth sample also was prepared from recrystallized 



potassium chromate, which in turn was made from recrystallized 

 chromic acid. In the first place, potassic hydroxide was prepared by 

 the electrolysis of three times recrystallized potassic oxalate, with the 

 use of a mercury cathode and decomposition of the amalgam with pure 

 water in a platinum dish, as in the preparation of potassium hydroxide 

 in an investigation upon the atomic weight of potassium. 22 The solu- 

 tion of the pure hydroxide was added to a solution of three times 

 recrystallized chromic acid, contained in a platinum dish, until the 

 normal chromate had been formed as indicated by the yellow color. 

 From this solution, by three systematic crystallizations, potassium 

 chromate was separated. 



The silver chromate was prepared from this material and the purest 

 silver nitrate by slowly adding a six hundredths normal solution of the 

 chromate to a silver nitrate solution of equivalent concentration, this 

 procedure being the reverse of that used in the preparation of Sample 



III. The dark brownish-red precipitate was allowed to settle in the 

 flask in which precipitation took place. Then, the supernatant solution 

 having been decanted, the silver chromate was transferred to a platinum 

 dish and washed very thoroughly with water. After being freed from 



22 Richards and Mueller, Jour. Amer. Chem. Soc, 29, 645 (1907). 



