304 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XVII. — The Electromotive Force of Iodine Concentration Cells 
with One Electrode saturated with Iodine. By A. P. Laurie, 
D.Sc., M.A. Cantab. 
(MS. received January 16, 1909. Read February 15, 1909.) 
This investigation was undertaken with the view of determining the 
number of free iodine ions present in saturated solutions of iodine in 
potassium iodide of varying strengths. 
The Nernst equation for iodine concentration cells can be written as 
follows : — 
E = *02955( og 
V °(I) 
— — log 
2 * (iy) 
(i); 
where I 2 and I' 2 are the concentrations of free iodine, and I and I' are the 
number of iodine ions present at the two electrodes. 
To consider first the neighbourhood of the electrode surrounded by the 
dilute solution of iodine : The distribution between potassium iodide and 
iodine is conditioned by the mass equation 
KIxL 
KL 
-£ = If 
( 2 ); 
or, if we call a the total number of potassium iodide molecules added, 
and b the total number of iodine molecules added, and x the total number 
of free iodine molecules present, then we have 
(a — b + x)x 
b - x 
h 
On expanding the quadratic we get for the first term 
kb 
— x 
(3). 
a - b + k 
which, within the limits of the solutions actually used, can be utilised for 
determining the value of x. The mean value of k, taken from Jakowkin’s 
first four tables, is '00138.* 
Applying equation (4), we can calculate the number of free iodine 
molecules present — that is, the value of P 2 . Further, if the, amount of 
iodine added is very small as compared with the total amount of potassium 
iodide, we may assume that the number of iodide ions present can be 
calculated from the known ionisation constants for potassium iodide of 
the strength used. We thus get the total value of the expression log 
r 2 
(IT 
If we now consider the solution saturated with iodine surrounding the 
other electrode, and if we assume that the solubility of iodine in water is 
* Zeits. Phys. Ghem xx. 36. 
