THE ANALYSIS OF SILVER CHROMATE. 143 



Owing to the weak nature of the second hydrogen of chromic acid, the first 

 hydrogen dissociating to the same extent as that of hydrochloric acid,^ but the 

 second hydrogen having the constant 6.0 X 10"^ at 18°,^ appreciable hydrolysis 

 of solutions of its salts takes place, to a greater extent the weaker the base with 

 which the chromic acid is combined. Sherrill has found, for instance, that am- 

 monium chromate in 0.05 molal solution is 2.7 per cent hydrolyzed. The basic- 

 ity of the solutions, on the other hand, will be greater the stronger the base. In 

 order to determine whether this hydrolysis is sufficient to produce precipita- 

 tion or occlusion of basic chromates, precipitates of silver chromate were 

 formed by means of solutions of both ammonium and potassium chromates. 

 The comparison of precipitates formed in this way will show whether the pres- 

 ence of basic salts is to be feared. 



Sample I. — Ammonic chromate was prepared by adding to a solution of the 

 pure chromic acid a slight deficiency of the purest freshly distilled ammonia. 

 The solution was diluted until about tenth normal, and was slowly poured with 

 constant shaking into a solution of an equivalent quantity of silver nitrate of 

 about the same concentration. The dark red precipitate of silver chromate was 

 washed six times by decantation with large portions of water, centrifugally 

 drained to remove as much water as possible and dried at gradually increasing 

 temperatures in an electric oven, finally at 160° for a long time. The dried 

 lumps were then gently ground to a fine powder in an agate mortar in order to 

 facilitate further drying as well as to insure homogeneity. During the addition 

 of the chromate to the silver solution, since the chromate solution was slightly 

 deficient in ammonia, acid accumulated in the silver nitrate solution. Hence 

 each succeeding portion of precipitate was formed under conditions of greater 

 acidity, although the concentration of acid in the solution could never have 

 approached that found by Sherrill to be necessary for the existence of the silver 

 dichromate. 



Sample II. — This preparation was practically identical with Sample I, since 

 part of the precipitate obtained as above was washed by decantation with 

 water eight times more, each wash-water being allowed to stand in contact 

 with the precipitate for many hours, and the precipitate being shaken with the 

 wash-water very thoroughly at intervals, in order to leach out any acciden- 

 tally inclosed 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 recrystallized 

 potassium chromate. A quantity of this material in about tenth normal solu- 

 tion 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 hy- 

 drolysis if possible. It will be recalled that in the case of Samples I and II pre- 



^ Walden: Zeit. physikal. Chem., 2, 49 (1888). ^ Sherrill, Loc. cit. 



