Gooch and Weed — Estimation of Chromium. 85 



Art. X. — The Estimation of Chromium as Silver Chro- 

 mate ; by F. A. Gooch and L. H. Weed. 



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



It has been shown by Autenrieth* that when chromic acid is 

 added to a boiling solution of silver nitrate, or when a soluble 

 chromate or dichromate is added to a solution of silver nitrate 

 previously acidified with nitric acid, or when silver chromate 

 is treated with nitric acid, silver dichromate is formed ; and that, 

 on the other hand, it is silver chromate which is precipitated 

 when silver nitrate in excess is added to a solution of a soluble 

 dichromate, cold or hot, the reaction proceeding according to 

 the equation 



4AgN0 3 + K 2 Cr 2 7 + H 2 = 2Ag 2 Cr0 4 + 2KN0 3 + 2HN0 3 . 



The characteristics of both silver dichromate and silver 

 chromate have recently been summarized and further studied 

 by Margosches,f but so far as we know there is in the literature 

 no account of procedure for the exact quantitative determination 

 of either chromium or silver based upon the characteristics of 

 either of these salts. The solubility of silver dichromate in 

 water and in ordinary solutions is such as to preclude the use 

 of this substance as the final product of a quantitative process 

 depending upon precipitation. The solubility of silver chro- 

 mate in a moderately large volume of water is not inconsider- 

 able, and the solvent action of free acid, even acetic acid in 

 quantity, is marked. We have found, however, that the pre- 

 cipitation of silver chromate is practically complete in a solution 

 only faintly acid with acetic acid and in presence of a large 

 excess of silver nitrate. If such a precipitate is collected in 

 the filtering crucible and washed with a dilute solution of silver 

 nitrate, until no other impurities remain, silver chromate 

 does not dissolve, and the excess of silver nitrate may be re- 

 moved by the cautious use of water without appreciable effect 

 upon the precipitate. The present paper has to do with the 

 determination of chromium as silver chromate. 



In the experiments of which the details are given in the 

 table, the general treatment just described was put to the 

 practical test. Given amounts of potassium dichromate were 

 weighed out and dissolved in hot water, as in experiments (11) 

 and (12), or given amounts of a solution of potassium dichro- 

 mate of known strength were run from a burette into a beaker 

 and heated to boiling, as in experiments (1) to (10). To the 

 hot solution of the dichromate was added, drop by drop, a 



*Ber. Dtsch. chem. Ges., xxxv, 2057. 

 f Zeitschr. anorg. Chem., xli, 68; 1, 231. 



