THE ANALYSIS OF SILVER BICHROMATE. 159 



Next the silver bromide was washed at least eight times by decantation with 

 pure water and collected upon a weighed Gooch crucible. Then it was dried 

 in an electric oven, first at 100° for 2 hours, then at 175° for about 18 hours. 

 After cooUng in a desiccator near the balance for several hours, the weight of 

 the silver bromide was determined. 



The use of an asbestos mat in the Gooch crucible made it necessary to collect 

 and determine the fibers detached during the filtration. This was done by 

 passing the entire filtrate and wash-waters through a small filter paper. The 

 paper was ignited in a weighed porcelain crucible, and the ash was treated with 

 nitric acid and then hydrobromic acid to convert a trace of reduced silver to the 

 state of bromide. In order to avoid any danger from adsorption of chromic 

 salts by the filter paper, at the end of the filtration the paper was rinsed with 

 hot dilute hydrobromic acid. The correction for asbestos could have been 

 avoided if it had been possible to employ a Gooch-Munroe-Neubauer crucible 

 with a mat of platinum sponge. It has already been shown, however, in the 

 preceding paper (page 146), that such crucibles lose markedly in weight when 

 exposed to the action even of the dilute aqua regia of the mother-Uquors of 

 these analyses. 



The moisture retained by the silver bromide was found by fusing the dried 

 salt in a porcelain crucible, the loss in weight on fusion being determined. The 

 fused silver bromide was always light yellow and gave every indication of purity. 



As in the preceding research a small quantity of silver bromide dissolved in 

 the filtrate and wash-waters was found by evaporating the combined filtrate 

 and wash-waters until nearly all the excess of acid had been expelled, and then, 

 after slight dilution, precipitating the silver as sulphide. The sulphide was col- 

 lected on a small paper, the ash of which, after ignition, was treated with nitric 

 acid. The amount of silver thus obtained was found by comparing in a nephel- 

 ometer precipitates of silver bromide produced in this solution and in very 

 dilute standard solutions of silver. 



In analysis 9 the silver was precipitated as silver chloride, the only other dif- 

 ference in the procedure being that the precipitate was washed with dilute 

 hydrochloric acid instead of pure water. 



DETERMINATION OF MOISTURE AND NITRIC ACID IN 



SILVER DICHROMATE. 



Silver dichromate which has been crystallized from nitric acid, after being 

 dried at 200°, contains traces of both nitric acid and water. Both of these sub- 

 stances can be expelled from the salt by fusion, although slight decomposition 

 of the salt takes place simultaneously. Since the only readily volatile sub- 

 stance which can be formed by the decomposition of the salt is oxygen gas, the 

 problem of the determination of the moisture and nitric acid consisted in that 

 of absorbing in a quantitative fashion the water, nitric acid, and nitric peroxide 

 formed by decomposition of the nitric acid. This was effected by passing ths 

 current of air containing the moisture and nitrogen compounds through two 



