Polymerization in Cadmium Iodide Solutions. 467' 



To calculate the molecular conductivity we must obtain a 



value for the conductivity of Cdig'. If we assume with 



McBain that the value 1'25, the limit which the observed 



(anion) transport number of a cadmium iodide solution tends 



to approach with increasing concentration, is that of the 



anion Cdlg^, then the relative velocity of this ion must be 



1*25 42 



— - — = 0*42, that is, ^ that of Cd"*"^. For the equivalent con- 



ductivity of the Cd++ ion we may use the value 47 for 18°,. 

 which corresponds to 56 at 25°. This gives, for CdIg', 34 at 

 18° and 40*5 at 25°. For iodine ion the values are h'i^'Q and 

 76*5 respectively. The molecular conductivity of the O'Ol 

 molar solution at 25° should therefore be 



{ (0-00645)(ll2)-f-(0-0024)(40-5) + (0-0105)('76-5) } ^O'Ol = 162-2,. 



and for the 0*125 molar solution, calculated in the same way,, 

 69-2. 



These calculated conductivities are much too high, the meas- 

 ured values being 120 and 57 respectively, a result whichi 

 seems to be due to some fault in the assumptions made rather 

 than to experimental errors. It is not clear, however, how 

 this discrepancy can be eliminated without introducing some 

 other one. The evidence at hand is in some respects conflict- 

 ing and is obviously insufficient for an exact solution of the 

 problem. In short, though some of the values in Table YI 

 are probably nearly correct, the figures as a whole can repre- 

 sent, at best, no more than a rough approximation to the truth. 



The Iodine- Cadmium. Iodide EquiXihriurn at 0°. — Each of 

 the starred values in Table Y represents the solubility of iodine 

 in the given solution at its freezing point, which, as an approx- 

 imation, may be assumed to be the same as the solubility at 0° 

 in the same medium. By subtracting the solubility of iodine 

 in pure water at 0° (0*000638 mols/liter^) we obtain (SIg), the 

 equivalent concentration of the tri-iodide formed, and can 

 therefore calculate the approximate value of the equilibrium 

 constant K, = (1l){l,)/{1i;). 



The results so obtained are given in Table YII, which com- 

 pares the values of K^ for cadmium iodide at 0° and 25° with 

 those for potassium iodide at the same temperatures and con- 

 centrations. All of these figures refer to solutions saturated. 

 with iodine. The values for cadmium iodide at 25° were 

 taken from our previous article ; those for potassium iodide 

 at 0° were calculated in the way just described from data given 

 by Jones and Hartmann;f those for potassium iodide at 25° 

 were taken from the article of Bray and MacKay. 



* Jones and Hartmann, Jour. Am. Chem. Soc, xxxvii, 256, 1915. 

 f Loc. cit., p. 250. 



