144 RESEARCHES UPON ATOMIC WEIGHTS. 



pared with the ammonium 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 centrifugal 

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

 pulverized in an agate mortar, as in the case of Sample I and II. The 

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



Sample IV. — A fourth sample also was prepared from recrystallized potas- 

 sium chromate, which in turn was made from recrystalHzed chromic acid. In 

 the first place, potassium hydroxide was prepared by the electrolysis of three 

 times recrystalUzed potassium 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.^ The solution 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 re- 

 verse of that used in the preparation of Sample III. By this method of precipi- 

 tation the solution is maintained as nearly neutral as it is possible to keep it. 

 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 thor- 

 oughly with water. After being freed from water by centrifugal settling, the sil- 

 ver chromate was dried at about 160° in an electric oven, and powdered in an 

 agate mortar. 



Since in the case of Sample III the silver nitrate was added to the chromate, 

 while in preparing Sample IV precipitation took place in the reverse fashion, a 

 comparison of the two samples would not only throw light upon the effect of 

 hydrolysis, but also show whether the occlusion of potassium chromate or 

 silver nitrate was to be feared. 



DRYING OF SILVER CHROMATE. 



The fact that salts dried by prolonged heating at 100°, or at even higher 

 temperatures, usually contain appreciable amounts of moisture, owing to in- 

 cluded mother-Hquor, is a point which has been overlooked by most earlier in- 

 vestigators,2 and the oversight throws doubt on much otherwise very careful 

 work. In exact work the residual water must either be corrected for or entirely 



1 Richards and Mueller: Pub. Car. Inst., 69, 33 (1907); Jour. Amer. Chem. Soc, 29, 645; 

 Zeit. anorg. Chem., 53, 431. 



* Richards: Proc. Amer. Phil. Soc, 42, 28 (1903). 



