146 RESEARCHES UPON ATOMIC WEIGHTS. 



The chromate was next reduced to the chromic state by the addition of a very 

 sHght excess of sulphur dioxide which had been freshly distilled into pure water. 

 The slight excess of sulphurous acid was soon oxidized under the combined 

 influence of heat and nitric acid. In analyses i, 2, 3, 12, 13, and 14 the reduc- 

 tion was effected by means of recrystaUized hydrazine sulphate, in order to 

 avoid to a large extent the presence of sulphuric acid, for Richards and Jones ^ 

 found that silver chloride occludes silver sulphate very tenaciously. The two 

 methods of reduction, however, gave essentially identical results. 



Since in the reduction of the chromate by hydrazine, nitrogen gas is evolved, 

 the flask in which the reduction took place was protected from loss by spatter- 

 ing by means of a long column of bulbs fitting loosely into the neck of the flask. 

 The solution of hydrazine sulphate was added through a funnel with a long fine 

 stem which extended through the column of bulbs nearly to the bottom of the 

 flask. After the addition of the hydrazine, the reaction was allowed to continue 

 slowly, with occasional shaking, and was completed by heating the solution 

 upon a steam bath for a short time. In the presence of acid a dilute solution of 

 hydrazine is without effect upon silver salts. 



After the solution had been allowed to cool, it was diluted to a volume of 1.5 

 liters, and the silver was precipitated as chloride or bromide by the addition 

 of a very dilute solution of an excess of either hydrochloric or hydrobromic acid. 

 The flask with its contents was shaken thoroughly for a few moments and was 

 then allowed to stand several days, until, the silver bromide having settled, 

 the supernatant solution was perfectly clear. 



Since the mother-Uquor of the silver hahde contained both nitric and hydro- 

 bromic acids in excess, the use of a Gooch-Munroe-Neubauer crucible seemed 

 to be attended with danger on account of solution of platinum. Such a possi- 

 bility has already been pointed out,^ and an actual loss was found to take place 

 in blank experiments carried out at the beginning of this research. Accordingly 

 the ordinary platinum Gooch crucible vath an asbestos mat was used. The 

 asbestos had been carefully cleansed by ignition, and washing first with nitric 

 acid and then with water. The crucible was prepared for weighing before and 

 after filtration of the silver halide in exactly the same way. 



The silver halides were washed many times by decantation with dilute hy- 

 drochloric acid in the case of silver chloride, and with very dilute hydrobromic 

 acid in the case of silver bromide. The precipitate was then transferred to the 

 weighed crucible and was dried in an electric oven at 170° for at least 16 hours. 



In order to correct for the small quantity of moisture retained by the silver 

 halides, each precipitate was transferred as completely as possible to a porcelain 

 crucible and fused. From the loss of weight of the portion of silver salt trans- 

 ferred to the crucible, the amount of water in the entire precipitate was cal- 

 culated. 



* Pub. Car. Inst., 69, 73 (1907); Jojir. Amer. Chem. Soc, 29, 831; Zeit. anorg. Chem., 55, 84. 



• Morse, " Exercises in Quantitative Chemistry," p. 203 (1905). 



