A REVISION OF THE ATOMIC WEIGHT OF POTASSIUM. 



THE ANALYSIS OF POTASSIC BROMIDE. 



INTRODUCTION. 



The foregoing quantitative study of potassic chloride by Dr. Arthur 

 Staehler and one of the present authors affords strong evidence that 

 the atomic weight of potassium is about 39.114, slightly lower than the 

 value based upon the work of Stas. The authors fully appreciated, how- 

 ever, that the investigation of a single compound is not enough to 

 establish a chemical constant so important as this, and, accordingly, the 

 present investigation was prosecuted simultaneously. It was expected 

 that the two investigations might either support one another or else, by 

 affording incompatible results, lead to the discovery of a constant error in 

 one or the other, and thus pave the way for further advance in knowledge. 

 As will be seen, the work on the chloride was satisfactorily confirmed by 

 the work on the bromide. 



In the present case the careful study of potassic bromide was particu- 

 larly necessary, because there already exist two concordant series of 

 experiments upon this substance, performed by the old masters Marignac 

 and Stas, pointing to a value in the neighborhood of 39.14, instead of the 

 before-mentioned new value 39.11. In this case the higher value for 

 potassium is not diminished, as in the case of the chloride, by an additive 

 correction in the atomic weight of the halogen, because Baxter has 

 shown Stas's estimate for bromine to have been nearly correct. 1 Hence 

 the discrepancy remains one too serious to be tolerated. 



The careful study of this work of Stas and Marignac affords convincing 

 evidence that the potassic bromide used by them for analysis was not 

 sufficiently pure for the purpose. Stas admitted that some of his prepara- 

 tions were not even wholly soluble in water, and his method of procedure 

 was such that some of them probably contained platinum and hydroxide. 

 Because speculations of this kind concerning work so long past are of 

 but little value, it was clearly necessary to repeat this work with modern 

 care ; and the following pages recount the details of the repetition. 



1 A brief review of Stas's and Marignac's work may be found in Clarke's "Recal- 

 culations" (1897), p. 47. Baxter's work is to be found in Proc. Am. Acad.. 42, 201, 

 1906; Journ. Am. Chem. Soc., 28, 1322 (1906). See also Richards, Trans. Am. 

 Phil. Soc., 43, 116 (1904). 



2? 



