Polymerization in Cadmium Iodide Solutions. 459 



Table I. — Continued. 









E. 



M. F. 









Dissolved 





volt 





d') 





iodine 



(I.) 



against 

 A 



against 

 B 



(D 



mean 





f *8-16 



1-32 



0-0080 



-0-0468 



6-62 

 6-95 



6-79 





2-540 



0-2967 



0-0342 



-0-0193 



8-69 

 9-62 



9-16 





1-397 



0-1569 



0-0431 





8-95 



9-23 



0-01 









-0-0114 



9-52 





molar \ 



0-646 



0-0713 



0-0535 





8-94 



8-94 



Cdl, 

















0-3104 



0343 



0-0654 



0-0116 



9-93 

 10-90 



10-42 





0-1411 



0-0168 



0-0755 



0-0216 



10-36 

 11-27 



10-77 





0-0 



0-0 









10-50 



* Solution saturated with iodine. 



all these iodine-ion concentrations are very low we can certainly 

 afford to disregard here the slight error involved in the assump- 

 tion that the results of electromotive force measurements repre- 

 sent concentrations rather than " activities " as defined by 

 Lewis.* 



Discussion. — In the previous investigation, as has already 

 been stated, values were obtained for the proportion of simple 

 molecules, ionized and non-ionized, in a pure solution of cad- 

 mium iodide at various concentrations. This quantity we shall 

 designate, as in our former article, by the term '' active frac- 

 tion." The rest of the cadmium iodide must consist of asso- 

 ciated cadmium iodide molecules, and of the ions, simple or 

 complex, formed therefrom. 



These values of the active fraction are calculated upon the 

 assumption that the abnormally low power of cadmium iodide 

 to unite with iodine is due entirely to the presence of com- 

 plexes, a condition which may be only approximately fulfilled. 

 The amount of iodine taken up by an iodide should be practi- 

 cally independent of its degree of ionization, provided that the 

 tri-iodide is ionized to about the same extent, as is no doubt 

 generally the case.f But although it is contrary to experience 



* For a discussion of the relation between activity and concentration for 

 the iodine ion in potassium iodide solutions, see Bray and MacKay (Jour. Am. 

 Chem. Soc, xxxii, 925, 1910). 



f This is confirmed by the fact that for all metallic iodides so far investi- 

 gated, with the exception of those of cadmium and mercury, the value of the 

 equilibrium constant Ki — (21) (I2) / (2I3) is the same. The list includes the 

 iodides of di- and trivalent metals, Ba, Sr, Zn, Ni, and La, whose degrees of 

 ionization are certainly somewhat different from those of the univalent 

 iodides. 



