A REVISION OF THE ATOMIC WEIGHT OF CADMIUM. 



THE ANALYSIS OF CADMIUM BROMIDE. 



Since the research described in the preceding paper indicates that the atomic 

 weight of cadmium is nearly one tenth of a unit higher than the results of 

 recent prior determinations by other investigators, in order to confirm or dis- 

 prove the higher value the analysis of cadmiiun bromide was undertaken. 



PURIFICATION OF MATERIALS. 

 CADMIUM BROMIDE. 



The cadmium material employed for the work consisted of fractions II and 

 III of sulphide, which were obtained in the earlier investigation (page 4). 

 Here also the sulphide was first converted into sulphate, and after crystalli- 

 zation of the sulphate the cadmium was deposited electrolytically upon a 

 platinum dish which had been coated with a thin film of paraffine. The 

 deposit was separated from the dish mechanically and after washing with water 

 was freed from paraffine with redistilled ether and alcohol. By the method 

 finally adopted for converting the cadmium into bromide, it was covered in a 

 quartz dish with water slightly acidified with hydrobromic acid to prevent the 

 formation of basic cadmium salts, and the purest bromine was added in small 

 quantities until the metal was almost wholly dissolved. The solution was 

 heated with the residual metallic cadmium upon a steam-bath until every 

 trace of bromine had disappeared. Then it was filtered with a platinum 

 funnel into a platinum dish, and was recrystaUized three times, with centrif- 

 ugal drainage in the platinum funnel after each crystallization.^ The original 

 solution contained only traces of sulphate, and, when tested with barium hy- 

 droxide, the mother-liquors of the third crystallization gave absolutely no test 

 for sulphate, hence the crystals themselves must have been pure (Samples II 

 and III). The crystals were dried over potassium hydroxide in a vacuum 

 desiccator. 



BROMINE. 



Commercial bromine was freed from chlorine by two distillations from a con- 

 centrated solution of a bromide, the bromide in the second distillation being 

 almost free from chloride. The bromine was covered with water, and hydrogen 

 sulphide, which had been thoroughly washed with water, was passed into the 



' Richards: Jour. Amer. Chem. Soc, 27, no (1905), 



21 



