106 Yan Name and Brown — Tri- Iodide and 



by the work of Bray and MacKay,* and of Fedotieff.f By the 

 work with cupric iodide it was proved that this iodide behaves 

 " normally," that is, it gives at 25°, and in dilute solution, the 

 same value of K^ as the other five mentioned above. Herz's 

 results with calcium and strontium iodides, however, would 

 seem to show that the former is to some extent abnormal, and 

 the latter decidedly so. This appeared so improbable that the 

 writers have reinvestigated the case of strontium, using 

 materials purified with especial care, and have found, as will be 

 shown later, that strontium iodide is wholly normal in its 

 behavior toward iodine. Since the same is known to be true 

 of barium iodide, there is hardly a doubt that calcium iodide is 

 in reality also normal, and that the discrepancies observed by 

 Herz and Bulla were due to some accidental disturbance. 



Excluding the results of Herz and Bulla, the data in the 

 literature prove that six iodides give at 25° practically the 

 same value of K^^ There is, of course, no reason to doubt that 

 this agreement extends to other temperatores. To the ques- 

 tion whether or not all simple metallic iodides behave alike in 

 this respect, the data hitherto published furnish no definite 

 answer. As a matter of fact, exceptions do occur, as the pres- 

 ent investigation will show. 



Behavior of Divalent Halides. 



Before considering our experimental results we wish to call 

 attention to a point upon which some confusion seems to exist 

 in the literature. Jakow^kin in studying barium iodide 

 assumed that both iodine atoms (or ions) were equally active 

 in the reaction with iodine, and accordingly based his calcula- 

 tions on the equation : 



4BaI.:^=!:iBaI, + 1,. 



As already stated, the value of the equilibrium constant so 

 obtained agreed with that of the univalent iodides. 



Quite recently, Herz and Kurzer;}: have made similar measure- 

 ments with barium iodide, but although they cite Jakowkin's 

 article they ignore his results, and base their calculations on 

 the assumption that only one of the two iodine atoms com- 

 bines with iodine, giving the equation : 



BaI,:^=±BaI, -f- I,. 



The authors seem to consider that this procedure is sufficiently 

 justified by the fact that the equilibrium constant calculated 

 according to this equation shows a fair degree of constancy, 



*Jour. Am. Chem. Soc, xxxii, 1207, 1910. 

 f Zeitschr. anorg. Chem., Ixix, 23, 1910. 

 iZeitschr. Elektrochem., xvi, 869, 1910. 



