THE ANALYSIS OF SILVER BICHROMATE. 157 



PREPARATION OF SILVER BICHROMATE. 



Silver dichromate was prepared by combining either potassium dichromate or 

 chromic acid with silver nitrate in nitric acid solution in platinum vessels. Pre- 

 cipitation was carried out in fairly concentrated solution, since in the subsequent 

 crystallization of the silver salt from nitric acid solution any included substance 

 was sure to be eliminated. Although the inclusion of nitric acid during the 

 crystallization was to be feared, and was actually found to have taken place, a 

 method was devised for the determination of this nitric acid, together with the 

 moisture retained by the solid. 



Sample I. — Silver nitrate and potassium dichromate were dissolved in equiv- 

 alent proportions in 3 normal nitric acid, the concentration of each salt being 

 about 0.7 normal. The cold silver nitrate solution was added very slowly, with 

 constant vigorous stirring, to the dichromate solution. After the precipitate 

 had been allowed to settle, the mother-liquor was decanted, and the precipitate 

 was centrif ugally drained and rinsed with 3 normal nitric acid in the centrifugal 

 machine. 



The salt was then five times recrystallized from solution in 3 normal nitric 

 acid with centrifugal drainage after each crystallization. Owing to the slight 

 solubiHty of silver dichromate in nitric-acid solutions the following scheme of 

 crystallization was adopted. The dichromate was heated with the nitric acid 

 solution upon the electric stove until the acid was saturated \vith silver di- 

 chromate. Then the hot solution was decanted into a dish through a platinum 

 Gooch crucible without a mat of any sort but with small holes, in order to re- 

 move particles of silver dichromate either suspended in the solution or floating 

 on the surface. These particles were always of considerable size, so that the 

 resulting solution was clear. After the saturated solution had cooled and had 

 deposited the greater part of its charge of salt, the mother-liquor was contin- 

 uously used to dissolve fresh portions of salt. About i liter of acid was used 

 for the crystallization of about 50 gm. of dichromate. Although by this method 

 the impurities in the original salt accumulate in the mother-liquor, on account 

 of the relatively large volume of the mother-liquor, there was little danger of 

 these impurities being carried into the second crop of crystals. It was shown, 

 for instance, that the mother-liquor from the third crystallization was free 

 from potassium. This mother-liquor was evaporated to small bulk, neutralized 

 with ammonia, and reduced and precipitated with hydrogen sulphide. The 

 filtrate after evaporation and expulsion of the ammonium salts gave no spec- 

 troscopic flame test for potassium. 



The silver dichromate was not allowed to come in contact with water or any 

 solution except the 3 normal nitric acid solution. 



All of the above operations were carried out in platinum vessels. 



Sample II. — This sample was made exactly as in the case of Sample I, ex- 

 cept that chromic acid was employed instead of potassium dichromate, and 

 that both precipitation and crystallization took place from 0.8 normal nitric 

 acid. The silver dichromate was crystallized five times. 



