460 Yan Name and Brown — Ionization and 



that two metallic salts of like type and as closely related as are 

 cadmium iodide and cadmium tri-iodide should show any large 

 difference in ionization, the possibility of slight differences is 

 not excluded, which if present would cause small positive or 

 negative errors in the value of the active fraction, calculated as 

 above. 



According to McBain* the observed transport numbers indi- 

 cate that the complex ion present in predominating amount is 

 Cdig', formed by ionization of (Cd^g according to the reaction : 



(Cdlj3 ■^±. 2Cdl3' + Cd+ + . 



Whether or not this inference is correct it is probable that the 

 above equilibrium is typical, and that the inactive fraction con- 

 sists essentially of [a) associated molecules, (&) complex anions, 

 and (c) simple Cd+"*" cations. It follows, therefore, that the 

 degree of ionization of the simple Cdl^ molecules may be cal- 

 culated, at least approximately, by dividing the concentration 

 of the iodine ion, taken from Table I, by the equivalent con- 

 centration of the ^' active " cadmium iodide as derived from 

 the data of the previous investigation. 



Table II shows the results obtained in this way. The first 

 three columns contain the concentrations of total cadmium 

 iodide, of active cadmium iodide, and of the iodine ion, respec- 

 tively. The fourth column gives the degree of ionization of 

 the simple Cdl^ molecules as calculated from columns two and 

 three, and the fifth shows, for comparison, the degrees of 

 ionization of cadmium nitrate, a normally ionized salt of like 

 type, at the same concentrations as those of (Cdljactive i^^ col- 

 umn two. The data for cadmium nitrate were obtained by 

 interpolation from values given by Noyes and Falk.f 



Table II. 



7 

 _ (I') for Cd(N03)2 



(Cdl^) (Cdl2)active (I') ^ ~ 3(Cdl2)active at 18° 



500 30- 30-4 -51 'VS 



250 26 5 31-2 '59 '79 



125 21- 27-5 -65 - '80 



10 5-5 10-5 -95 -87 



According to these results the degree of ionization of the 

 Cdlj molecules is considerably lower, except in the most dilute 

 solution, than that of the cadmium nitrate. A difference in 

 this direction, though smaller in .amount, would be expected 

 on account of the presence in the cadmium iodide solution of 



*Zeitschr. f Elektrochem., xi, 215, 1905. 

 f Jour. Am. Chem. Soc, xxxiv, 475, 1912. 



