Gooch and Bosworth — Silver as the Chr ornate. 241 



Art. XYII. — The Gravimetric Determination of Silver as the 

 Chromate ; by F. A. Gooch and Rowland S. Bosworth. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — cxcv.] 



It lias been shown in a recent paper from this laboratory* 

 that the precipitation of silver chromate from the solution of 

 a soluble chromate made faintly acid with acetic acid may be 

 carried to completion by the addition of silver nitrate in con- 

 siderable excess; and that the exact determination of the chrom- 

 ium of a soluble chromate or dichromate may be effected by 

 treating with silver nitrate the solution of either salt, adding 

 ammonia to alkalinity and then acetic acid to faint acidity, 

 transferring the precipitate and washing it in the filtering 

 crucible with a dilute solution of silver nitrate until foreign 

 material other than that reagent has been removed, finishing 

 the washing with a small amount of water applied judiciously 

 in portions, and weighing the dried or gently ignited residue 

 of silver chromate. The success of this process turns upon 

 keeping the chromium at the moment of precipitation essen- 

 tially in the form of chromate rather than dichromate and in 

 taking care that an excess of silver nitrate shall be present 

 nearly to the end of the washing. The present paper deals 

 with the conditions under which, in reversal of the process 

 just described, silver may be precipitated completely as the 

 chromate; 



In the first experiments to be described, a solution of silver 

 dichromate was added gradually, either in slight excess or in 

 considerable excess, to a solution of silver nitrate maintained 

 at the boiling point ; the mixture was cooled and treated with 

 ammonia until the precipitate first formed had been dissolved ; 

 acetic acid was added to faint acidity ; and the precipitate 

 was settled for a half -hour, filtered off on asbestos in a perfor- 

 ated crucible, washed with a little water, dried and weighed. 

 In every case the weight of silver chromate found was defic- 

 ient and the testing of the filtrate with hydrochloric acid 

 showed the presence of a soluble salt of silver. The errors 

 amounted to several milligrams, varying with the conditions. 

 Dilution of the original solution and prolonged washing with 

 water increased the error. Admixture of alcohol was with- 

 out appreciable eifect upon the solubility of the precipitate. 

 Evaporation to dryness and transfer by a dilute solution of 

 potassium dichromate, and a final washing with water used 

 judiciously in small portions, tended to diminish the amount of 

 soluble salt, but this treatment never reduced the error of loss 



* Gooch. and Weed : this Journal, xxvi, 85, 1908. 



