A REVISION OF THE ATOMIC WEIGHT OF IODINE. 

 By Gregoky Paul Baxteu. 



Presented by T. W. Richards, Oct. 12, 1904. Received August 6, 1904. 



The atomic weight of iodine has been for some time considered one 

 of the best determined of chemical constants, owing to the extremely 

 concordant results of Stas and Marignac, who both deduced the value 

 126.85 (O = 16.000) from syntheses of silver iodide. Within the last 

 two years, however, two series of determinations, one by Ladenburg and 

 one by Scott, have yielded results over one tenth of a unit higher than 

 the above. Ladeuburg's method consisted in heating silver iodide in a 

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

 result 126.96, while Scott, like Stas and Marignac, synthesized silver 

 iodide and obtained in two analyses the values 126.96 and 126.98.* 

 The anomaly in the atomic weights of iodine and tellurium has always 

 been of the greatest interest, and has led to a large number of investiga- 

 tions upon the atomic weight of tellurium during the last few years, 

 which have shown conclusively that this value lies in the neighborhood 

 of 127.6. The doubt thrown upon the atomic weight of iodine by the 

 work of Ladenburg and Scott makes it imperative to subject iodine to 

 further careful investigation. 



PART I. 



The Ratio of Silver to Silver Iodide. 



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

 the most important points to be considered are the stability of the sub- 

 stance under various conditions, and the certainty with which the atomic 

 weights are known of those elements, besides the one under examination, 

 which compose it. In the case of iodine, few of its comjiounds are 



* Verj' recently, since the work described in this paper was completed, K()thner 

 and Aeuer have published a preliminary notice of experiments involving syntheses 

 of silver iodide as well as a repetition of Ladenburg's work, from which tliey con- 

 clude that the atomic weight of iodine cannot be lower thati 12G.963. Details of 

 their work are not given. See lier. d. d. chem. Gesell., 37, 2530. 



