Van Name and Br oimi— Cadmium Iodide Solutions. 455 



Art. XXXYI. — Ionization and Polymerization in Cadmium 

 Iodide Solutions ; by R. G. Yan Name and W. G. Beowx. 



[Contributions from the Kent Cliemical Laboratory of Yale Univ.— ccxcv.] 



It is a well known fact that water solutions of cadmium 

 iodide show an abnormally low electrical conductivity and 

 freezing point lowerins^ as compared with other salts of like 

 type. This is generally ascribed to the presence in the solu- 

 tion of complex molecules and ions. In a previous article* wq 

 have described a method, based on the measurement of distri- 

 bution coefficients of iodine between such a solution and a non- 

 aqueous phase, by which it is possible, by extrapolation, to cal- 

 culate the percentage of simple molecules and ions in a pure 

 solution of cadmium iodide. The application of this method 

 led to the conclusion that in OS-molar cadmium iodide at 25° 

 about 6 per cent was present in the form of simple Cdl^. 

 molecules, ionized and non-ionized, in 0'25-molar solution 106 

 per cent, in 0'125-molar solution 16-8 per cent, and in 0*01- 

 molar solution 55 per cent. 



These results may be compared with those of Walton, f who 

 showed in 1904 that the rate of decomposition of hydrogen 

 peroxide in a neutral solution containing an iodide is propor- 

 tional to the concentration of the iodine ion, and used this 

 kinetic method to determine the proportion of iodine ion in a 

 series of rather dilute cadmium iodide solutions. With due 

 allowance for ionization and for differences in concentration, 

 our results, as will be shown later, are consistent with those of 

 Walton, at least as to order of magnitude. 



McBain;]: on the other hand, by a mathematical analysis of 

 the data in the literature concerning conductivity, freezing 

 points, and transport numbers of cadmium iodide solutions, has 

 arrived at quite a different result. McBain concludes that in a 

 O'l molar cadmium iodide solution, the only concentration 

 quantitatively dealt with, most of the salt is in the form of 

 simple non-ionized Cdl^ molecules, and that the complexes 

 make up only about 8 per cent of the whole. 



In the present investigation we have attempted to throw 

 further light upon this question by a study of solutions of cad- 

 mium iodide containing dissolved iodine, by means of measure- 

 ments of electromotive force and of freezing point lowering. 



Electro mMive Force Measurements. 



The reversibility and reproducibility of iodine electrodes 

 composed of platinum immersed in an iodide solution contain^ 



* This Journal (4), xliv, 105, 1917. 

 tZeitscbr. phys. Chem., xlvii, 185, 1904. 

 iZeitschr. f. Elektrocbem., xi, 215, 1905. 



