376 Proceedings of the Royal Society of Edinburgh. [Sess. 
It is also obvious in general that if both solvents are saturated with 
the salt, they will be in equilibrium with each other, though this is not 
necessarily the case if the solvents themselves act on each other, as in the 
case of alcohol and water, where admixture results in contraction and 
evolution of heat and precipitation of dissolved salt. 
If the solvents do not mix, the saturated solutions will be in equilibrium. 
In the case of the cell already described, ignoring for the present 
the disturbing factor due to the interdiffusion and chemical action on each 
other of alcohol and water, it is evident that we have here at least two 
partition coefficients to deal with, namely, the partition coefficient between 
iodine in water and iodine in alcohol, and between potassium iodide in water 
and potassium iodide in alcohol. A consideration of the solubilities of these 
substances in the respective solvents will at once show that we should 
expect to find, when the solutions on both sides are of equal strength, an 
E.M.F. tending to transfer iodine from water to alcohol and potassium 
iodide from alcohol to water, until a ratio of strength of solution for each 
was established approximately equal to the ratio of their solubilities. 
Unfortunately, however, the necessary data for exact calculations in 
the case of this cell are wanting, as, while we know the ratio between KI, 
I 2 , and KI 3 in the water solution, we have not similar data for alcohol and 
so cannot determine the amount of free iodine present. 
The next paper of importance for our present purpose is by Abel, 
dealing with the mathematical theory of such cells.* 
The following is Abel’s treatment of the problem : — 
To take the case of a simple salt like potassium iodide, distributed 
between two solvents and partially ionised in each, it is evident that there 
is a partition coefficient between the unionised solvent and a partition 
coefficient between the ions in the two solvents respectively. 
The ionic partition coefficient will here be the same for both ions. 
Professor Abel investigated this case as follows : — 
In the first place, let us suppose the solutions to be in partition 
equilibrium, then 
Phase I. 
Phase II. 
M 
Solvent L x 
Solvent L n 
M-ion concentration c x 
M-ion concentration c u 
M, M are the two metal electrodes, and ci, Cu the ionic concentrations of 
the metallic ions in the two solvents respectively. 
* Zeit. phys. Chem ., lvi. p. 612 (1906). For the researches of other investigators on 
cells with organic solvents, along with fresh results, see the Dissertation by Joseph 
Neustadt, Breslau University, 4th Aug. 1909. 
