A REVISION OF THE ATOMIC WEIGHT OF IODINE. 



THE SYNTHESIS OF SILVER IODIDE AND THE RATIO OF 

 SILVER IODIDE TO SILVER BROMIDE 

 AND SILVER CHLORIDE. 



INTRODUCTION. 



The atomic weight of iodine was for some time considered one of the best de- 

 termined of chemical constants, owing to the extremely concordant results of Stas 

 and Marignac, who both deduced the value 126.79 (O = 16.000 and Ag = 

 107.880) from syntheses of silver iodide. Recently, however, two series of 

 determinations, one by Ladenburg and one by Scott, have yielded results over 

 0.1 higher than the above. Ladenburg's method consisted in heating silver 

 iodide in a current of chlorine until all the iodine had been displaced, and gave 

 the result 126.90, while Scott, like Stas and Marignac, synthesized silver iodide 

 and obtained in two analyses the values 1 26.90 and 1 26.92.^ The anomaly in the 

 atomic weights of iodine and tellurium has always been of the greatest in- 

 terest, and has led to a large number of investigations upon the atomic weight 

 of tellurium during the last few years, which have shown conclusively that the 

 value of this constant lies in the neighborhood of 127.6. The doubt thrown upon 

 the atomic weight of iodine by the work of Ladenburg and Scott made it 

 imperative to subject iodine to further careful investigation. 



In choosing a compound for an atomic weight determination, two of the most 

 important points to be considered are the stability of the substance under vari- 

 ous conditions, and the certainty with which the atomic weights are known of 

 those elements, besides the one under examination, which compose it. One 

 of the few compounds of iodine which satisfy both the above conditions is silver 

 iodide. Accordingly synthesis of silver iodide from a weighed amount of silver 

 was selected for the first method of investigation. 



* The higher values of Scott and Ladenburg were confirmed, shortly before the publication 

 of the first portion of this paper, by Kothner and Aeuer, in a preliminary notice of experiments 

 involving syntheses of silver iodide as well as a repetition of Ladenburg's work, from which they 

 conclude that the atomic weight of iodine can not be lower than 1 26.90. Details of their work 

 are not given. See Bcr. d. d. chem. Gesell., 37, 2536 (1904). A critical discussion of all the 

 earlier work upon the atomic weight of iodine is to be found at the end of this paper. 



91 



