406 PROCEEDINGS OF THE AMERICAN ACADEMY. 



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

 uble eliminates can safely be employed for the precipitation of silver 

 chroniate without the least danger of the precipitation of silver dichro- 

 ruate, and even that the presence of a slight amount of free acid could 

 do no harm. 



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

 the first hydrogen dissociating to the same extent as that of hydro- 

 chloric acid, 20 but the second hydrogen having the constant 6.0 x 1(T 7 

 at 1S°, 21 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 ammonium chromate in 

 0.05 molal solution is 2.7 per cent hydrolyzed. The basicity of the solu- 

 tions, on the other hand, will be greater the stronger the base. In order 

 to determine whether this hydrolysis is sufficient to produce precip- 

 itation 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 presence 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 dis- 

 tilled 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, centrifn gaily drained to re- 

 move 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 pre- 

 cipitate 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 de- 



20 Walden, Zeit. physikul. Chem., 2, 49 (1888). 



21 Sherrill, loc. cit. 



