IL\TE OF DIFFUSION OF IODINE IN KI 485 



the effect of which would certainly be to drive the dissociation still further 

 back (21). 



In the case of acetic acid Oholm found that the constant was 0.93 

 as against 1.37 theoretical, using 0.01 n solution. (Acetic acid is, of course, 

 a verj- weakly dissociated acid.) This shows the effect of incomplete 

 dissociation to be very considerable. Of course too, we have no a -priori 

 right to assume that the rate of migration of the ions is the same in KI as 

 in water, or that it is the same, in a weak as in a concentrated solution 

 of KI. 



Molecular condition of the dissolved iodine. Le Blanc and Noyes con- 

 cluded from freezing point experiments, that when iodine is added to a 

 solution of KI it enters into combination to form a complex ion, probably 

 I3. The fact that iodine thus added does not lower the freezing point of 

 the KI solution is excellent evidence that the total number of molecules 

 is not increased, but does not show whether the resulting compound is 

 KI3, KI5, KI7 or some higher iodide (22). 



In 1877 Johnson actually obtained crystals having the definite com- 

 position KI3, by volatilization over concentrated sulphuric acid. This, 

 while very strong evidence in favor of the KI3 solution theory, yet left 

 room for question, for the existence of a solid compound does not neces- 

 sarily prove that it exists in solution (23). 



The fii'st really conclusive work was done by Max Roloff, who made use 

 of the principle of partition coefficients between dissimilar solvents (24). 

 Roloff did not work with iodine in KI solution, but with bromine in KBr 

 solution. The other solvent used was CS2. Roloff showed very conclusively 

 that practically all of the bromine was present in KBr solution as KBrs, 

 and not as any higher bromide. His method was to assume that the 

 bromine formed a given bromide, KBrs, KBr^, or what ever seemed reason- 

 able; and then from the known laws of partition between non-miscible 

 solvents to deduce a function of concentration in each solvent that theoreti- 

 cally should remain constant with varj-ing concentrations. Roloff then 

 tested this hypothesis by actually determining the concentration over as 

 large range -as seemed practical. Thus he showed that the bromine in a 

 KBr solution is present as KBr3 almost exclusivelyj and by analogy iodine 

 in KI should be present as KI3. For a full discussion of the theory involved 

 and for the data the reader should consult the original article (24). 



Later (1896) Jakowkin, using the same methods as those used by 

 Roloff, showed that the reaction was strictly analogous for other bases 

 and for other halogens; in particular for iodine in KI solution. Jakowkin 

 goes into much more detail than does Roloff (25). 



