36 THE ANALYSIS OF POTASSIC BROMIDE. 



THE DRYING AND WEIGHING OF POTASSIC BROMIDE. 



For reasons already mentioned in the paper on potassic chloride, the 

 salt under investigation must be fused before weighing. Unfortunately, 

 potassic bromide when fused in air in a platinum vessel attacks the plati- 

 num to a considerable extent. Although it is shown later that this action 

 does not seriously affect the weight of silver needed to precipitate the 

 bromine in the salt, it was nevertheless desirable to avoid all such irregu- 

 larities. No such action is observed when the fusion takes place in an 

 atmosphere of pure dry nitrogen. Accordingly, in all work to be re- 

 counted, the potassic bromide was protected in this way during its fusion 

 in a well-seasoned platinum boat, and was finally, by means of the Harvard 

 "bottling apparatus," shut up in a tight weighing bottle in an atmosphere 

 of pure dry air. The details have already been adequately given in several 

 places, especially in the account of the recent work on the atomic weight 

 of caesium. 1 The only difference in the present procedure was demanded 

 by the high fusing-point of potassic bromide (750) ; on account of this, 

 a porcelain ignition-tube was substituted for one of glass, as in the case 

 of calcic chloride. 2 



Even with all possible care, however, the platinum boat was distinctly 

 attacked, almost always losing in weight during the fusion. In two of the 

 cases analyses 1 and 14 where the potassic bromide was fused for 

 some time in the boat, the corrosive action was very considerable, causing 

 losses of weight of 0.0007 and 0.0014 gram, respectively. One would be dis- 

 posed to reject these determinations entirely, except for the fact that their 



KBr 

 average result, : - = 1.10317, is almost exactly identical with that of 



the average of results 5 and 6, in which the boat was scarcely attacked at 



KBr 

 all, namely, r- - = 1.10318. Instead of being rejected, these results 



6 



are therefore retained and used as evidence that even a considerable 

 amount of dissolved platinum has no perceptible effect on the weight of 

 silver precipitated by the salt. In a majority of the other cases, the change 

 in weight of the boat during the fusion was less than 0.0001 gram. The 

 average loss in analyses 10, 11, 12, and 16, in which the argentic bromide 

 was weighed, was less than 0.00009 gram ; hence even if platinum had 

 been present in the potassic bromide in a state of fine division it could not 

 have exerted any essential effect upon the result of this series of analyses. 



'Richards and Archibald, Proc. Amer. Acad., 38, 451 (1903) ; Zcit. anorg. Chem., 

 34, 362 (1903). 



2 Richards, Journ. Amer. Chem. Soc., 24, 374 (1902). 



