1908-9.] Electromotive Force of Iodine Concentration Cells. 315 
is evident that in a normal KI solution containing very little iodine the 
dissociation constant for I.,±^I 0 + I is the same as in dilute solutions. 
But as the amount of iodine in the solution is increased this dissocia- 
tion constant alters, the X 3 molecules becoming more stable. In dilute 
solutions the amount of iodine can be varied without affecting this constant, 
but in presence of a very strong iodine solution, such as that present in 
normal KI, the tendency of the Ivl 3 to dissociate diminishes. It is for this 
reason that it is possible to dissolve more iodine in these solutions. For 
the total amount of iodine which can be dissolved is controlled by the 
solubility of iodine in water and by the value of k. If k is smaller, then, 
considering the equation — - - — MyC _ ^ p. - g evident that the ratio of 
total iodine dissolved to total KI present can be pushed further, till the 
limiting value of x is reached, namely -00134. 
a 
While, therefore, the increased solubility of the iodine is accounted for 
up to a certain strength of KI by the alteration of the dissociation constant 
of I 3 ions, the formation of still higher polyiodides in still stronger 
solutions is no doubt due to the fact that it is only under the influence 
of very high concentrations that they are stable, and therefore begin to 
be present in considerable quantities. 
In conclusion, I have to thank Mr King for his assistance, and the 
Carnegie Trust for the grant which enabled me to carry out these 
experiments. 
( Issued separately May 11 , 1909 .) 
