CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF 

 HARVARD COLLEGE. 



A REVISION OF THE ATOMIC WEIGHT OF 

 COBALT. 



SECOND PAPER: — THE DETERMINATION OF THE COBALT IN 

 COBALTOUS BROMIDE. 



By Theodore William Richards and Gregory Paul Baxter. 



Presented October 12, 1898. Received December 29, 1898 



In a recent paper upon the atomic weight of cobalt,* we began the 

 analysis of cobaltous bromide by the determination of its bromine, with 

 results which seemed to show that the atomic weight of cobalt is very 

 nearly 58.99 (0 = 16). Although very great care was taken in purifying 

 the cobaltous bromide used in this work, it was never certain that during 

 the sublimation of the bromide in the porcelain tubes small amounts of 

 impurity had not crept in. The situation here was exactly the same as 

 in the research upon nickel, carried on at the same time in this Labora- 

 tory,! and here also the simplest and most convincing way of settling the 

 question was to determine directly the amount of cobalt present in the 

 salt, thus obtaining its total percentage composition. 



Of the three methods of procedure which presented themselves, — 

 precipitation, electrolysis, and reduction of the bromide, — reduction by 

 hydrogen was chosen as being the least complicated and most certain. 

 The slight hope offered in the nickel research that electrolysis might be 

 used for the determination of the metal was here lacking on account 

 of the greater difficulty in obtaining a satisfactory electrolytic deposit. 



A conceivable objection to the use of the reduction of an oxide as an 

 accurate quantitative method is the possibility that this reaction may not 

 be capable of absolutely complete fulfilment. It is well known, for 

 example, that one is rarely able to reoxidize wholly a metal once reduced, 



* These Proceedings, XXXIIL 115. 



t The preceding paper describes this work. This Volume, p. 327. 



