468 Va?i Name and Brown — Cadmium Iodide Solutions. 



Table VII. 



Potassium Iodide 

 Concentra- Cadmium Iodide K^ at 0° Ki at 35° 



tion. Ki at 0° Ki at 25° Jones and Bray and 



equiv./liter (approximate) Hartmann. MacKay 



1-0 0-0154 0-0234 0-00046 



0-5 0-0090 0-0137 0-00088 



0-25 0-0053 0-0082 



0-10 0-000696 0-00131 



0-02 0-0011 0-00247 0-000716 0-00137 



These figures show that the value of K^ for cadmium iodide 

 at.O°is abnormal in the same way as at 25°, being larger 

 throughout than for a normal iodide like potassium iodide, and 

 increasing rapidlj with the concentration. The effect upon K^ 

 of a change in temperature, however, is practically the same 

 for the cadmium as for the potassium salt. 



Summary. 



1. Measurements have been made : {a) by the electromotive 

 force method, of the iodine ion concentration in cadmium 

 iodide solutions of 0-5, 0-25, 0*125, and 0-01 molar strength, 

 containing various amounts of di^^solved iodine; also yh) of the 

 freezing point lowering of each of these cadmium iodide solu- 

 tions, and of the further lowering produced bj the addition of 

 known amounts of iodine. 



2. Yalues of (I^) calculated in the ordinary way from the 

 cryoscopic measurements should be slightly lower, if complexes 

 are present, than those electrically measured. This was found 

 to be the case in the 0*01 and 0-125 molar solutions, but not in 

 the two stronger solutions. 



3. The freezing point of a cadmium iodide solution was 

 depressed by the addition of iodine in a nearly constant ratio, 

 which in the stronger solutions was about 1*4° per mol, and 

 only slightly smaller in the weakest solution. This indicates 

 the presence of complexes in considerable quantity even in the 

 0*01 molar solution. Neither this result nor the abnormally 

 low power of cadmium iodide to unite with iodine can be 

 accounted for, in the absence of complexes, by the assumption 

 that the degree of ionization of the cadmium iodide is very 

 small, unless this low ionization is accompanied by high ioni- 

 zation of the cadmium tri-iodide, a state of affairs which is 

 decidedly improbable. 



4. A tentative calculation of the composition of the two 

 more dilute cadmium iodide solutions, based upon the assump- 

 tion that the ion Cdlg' and its parent molecule (Cdl2)3 are the 

 only complexes present, failed to give results in quantitative 

 agreement with other experimental data. 



