A REVISION OF THE ATOMIC WEIGHT OF BROMINE. 



THE SYNTHESIS OF SILVER BROMIDE AND THE RATIO OF SILVER 

 BROMIDE TO SILVER CHLORIDE. 



INTRODUCTION. 



In numerous investigations in this laboratory upon the atomic weights of 

 certain metals, in which metallic bromides were first titrated against the purest 

 silver, and then the precipitated silver bromide was collected and weighed, the 

 relation between the silver used in the titrations and the silver bromide ob- 

 tained has )delded data from which the atomic weight of bromine may be calcu- 

 lated. Furthermore, in all these investigations, as a check upon the purity of 

 the silver and bromine employed, silver bromide was synthesized directly from 

 weighed quantities of silver and an excess of ammonium bromide or hydrobro- 

 mic acid. Many of these results have already been collected and discussed by 

 Richards,^ nevertheless they are cited in the following table together with a 

 few more recent determinations. (See table on page 52.) 



From the first of these ratios the atomic weight of bromine, referred to silver 

 107.880, is found to be 79.919, and from the second 79.918. 



Very recently, in experiments in which silver iodide was heated first in a cur- 

 rent of air and bromine imtil the iodine was completely displaced, and then in 

 a current of chlorine to displace the bromine, the ratio of silver bromide to 

 silver chloride was determined in six cases. From the results of these experi- 

 ments the atomic weight of bromine was calculated to be 79.916,^ if the atomic 

 weight of chlorine is assumed to be 35.457. 



These values for bromine are in close agreement with those of Stas.' In his 

 experiments weighed quantities of pure silver and bromine were first titrated 

 against each other, and then the precipitate of silver bromide was collected and 

 weighed. Of the four results by the first method, one should be rejected ac- 

 cording to his own statements, since the bromine was not thoroughly dried. 

 The remaining three, 79.922, 79.924, and 79.923, give as an average 79.923. 

 From the weight of silver bromide four values were obtained, 79.913, 79-9iS> 

 79.918, and 79.920, with an average of 79.917. 



* Proc. Atner. Phil. Soc, 43, 119 (1904). 



* Baxter: Proc. Amer. Acad., 41, 82 (1905); Jour. Amer. Chem. Soc, 27, 884; Zeit. anor f- 

 Chem., 46, 45. (See page m-) 



^ (Euvres Completes, i, 603. 51 



