A REVISION OF THE ATOMIC WEIGHTS OF IODINE 



AND SILVER. 



THE ANALYSIS OF IODINE PENTOXIDE. 



INTRODUCTION. 



For some time it has been apparent that Stas's researches upon the atomic 

 weight of silver, upon which the value now in use depends, are somewhat at 

 fault. The need for a redetermination of the ratio of this constant to the atomic 

 weight of oxygen is especially pressing, since silver has been very frequently 

 used as the basis for exact work upon atomic weights, both directly, and also 

 indirectly through its relation to the atomic weights of the halogens in the analy- 

 sis of metallic haUdes. 



The problem is made diflficult, however, by the fact that the only known definite 

 compounds of silver with oxygen are difficult if not impossible to prepare in a 

 pure state, besides containing so small a percentage of oxygen that the ex- 

 perimental error would be greatly magnified in the calculation. All other meth- 

 ods involve the knowledge of the exact ratio of at least one other atomic weight 

 to that of silver or oxygen, the accuracy of the process diminishing with the 

 number of atoms involved. Among the various classes of compounds the 

 analysis of which may afford the desired information, oxides are of especial 

 interest because the knowledge of the ratio of only one atomic weight to that of 

 silver is involved. The difficulty and uncertainty in analyzing metallic oxides ^ 

 make them unsuited for the purpose. The ratios of the atomic weights of silver 

 and the halogens are known with greater exactness than in the case of most 

 elements, hence to determine the ratio of any one of the halogens to that of 

 oxygen will serve the purpose equally well. The only compound of a halogen 

 and oxygen of considerable stability is iodine pentoxide. This substance is, 

 however, quite stable through a fairly wide range of conditions, and its analysis 

 offered enticing possibilities. To be sure, the compound contains only 25 per 

 cent of oxygen, but this disadvantage is in part counterbalanced by the fact 

 that the ratio of the atomic weight of iodine to that of silver is greater than in 

 the case of most elements. 



' Richards and Baxter: Proc. Amer. Acad., 35, 61 (1899); 35, 253 (1900); Zeit. anorg. 

 Chem., 22, 221; 23, 245. 



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