464 Van Name and Brown — Ionization and 



limn two. This quantity proves to be roughly constant irre- 

 spective of the concentration and to have a value between 1'3 

 and 1'4, or about three-fourths of 1*86, the normal molecular 

 lowering for a non-electrolyte in pure water. 



The relatively large amount of this increase proves that the 

 cadmium tri-iodide formed is derived ultimately from some 

 source which previously contributed a much smaller number 

 of molecules and ions to the solution. Unless, therefore, we 

 are willing to admit that the cadmium tri-iodide may have in 

 the solution a degree of ionization many times greater than that 

 of the simple cadmium iodide molecules,* these results must 

 be regarded as clear proof of the existence of complexes in 

 these solutions. On the other hand, if the effect is largely or 

 wholly due to complexes, as is probably the case, the relative 

 constancy of the values in the last column of Table lY is an 

 indication that the complexes are present in considerable quan- 

 tity even in the more dilute solutions. On account of the low 

 solubility of iodine in water at 0° the depression due to the 

 iodine which remains uncombined is so smallf as not to affect 

 the validity of this reasoning. 



Owing to the presence of complexes it is of course impos- 

 sible to get any accurate measure of the concentration of the 

 iodine ion from the value of ^, but in the present case the error 

 so involved would not necessarily be very large. Of the 

 various kinds of complex ions to be expected here the two 

 simplest and most probable are Cdlg^ and Cdl/^, formed as 

 products of the equilibria 



2Cdi,:;=t(Cdij,:^± Cd++ + cdi;' 



and 3CdI^:^Z^ (Cdlj3 :^z± Cd++ + 2Cdl3'. 



In both of these cases the number of ions produced is the same 

 as the number of Cdl^ molecules disappearing, so that the net 

 result of the complex formation will be to diminish the freez- 

 ing point depression by a small amount due to that part which 

 remains in the form of non-ionized polymerized molecules, 

 (Cdl2)2 or (Cdl2)3 as the case may be. Hence, if values for the 

 concentrations of the iodine ion are calculated in the usual way 

 from the freezing point lowerings the results will in general be 

 low, but in sufficiently dilute solutions should not be very far 

 from the truth. 



In Table Y the iodine ion concentrations, so calculated, are 

 tabulated for comparison with those derived from the measure- 

 ments of electromotive force. For the two lowest concentra- 



* A rough calculation shows that to account for the results in the absence 

 of complexes the ratio of these two degrees of ionization would have to be 

 over 2 in the 0*01 molar solution, about 8 in the next, and about 25 in the 

 strongest. 



t Its maximum value is 0'0013°, which is reached only when the solution 

 is saturated with iodine. 



