386 
Proceedings of the Royal Society of Edinburgh. [Sess. 
In calculating the E.M.F. of this cell, we have to take into account the 
following conditions : — 
In the first place, an E.M.F. is produced owing to the different osmotic 
pressures of the iodine at the two poles of the cell, which can be calculated 
p 
from the formula E= 0291 log ^7 at 20'4° C., where P, P' are the pressures 
due to the iodine solutions. This equation is true when the concentration 
of iodine ions is the same at both ends of the cell, and consequently the 
osmotic pressure of the iodine ions need not be considered. But, owing to 
the fact that iodine in presence of potassium iodide forms complexes, such 
as KI S , both the amount of free iodine present and the amount of potassium 
iodide actually present in the solution are affected. If we consider the mass 
equation ‘ - 2 = C, it is evident that the formation of KI S not only diminishes 
the amount of free iodine present, but also the amount of potassium iodide 
present. If, then, we compare the E.M.F. of two solutions to which the 
same amount of potassium iodide has been added, but which contain 
different quantities of added iodine, then in the solution containing the 
larger proportion of iodine the amount of free potassium iodide will be less 
than on the side containing the weak solution of iodine ; consequently the 
osmotic pressure of the iodine ions must be taken into account at both sides 
of the cell, the concentration of the ions being equal to the concentration 
of the free potassium iodide molecules, with a correction for the amount of 
dissociation. Consequently, if p, p' represent the iodine ion concentrations, 
the complete formula for the E.M.F. will be E = -0291 
When the potassium iodide is in great excess as compared with the iodine 
on both sides of the cell, the E.M.F. of the cell will be very nearly 
proportional to the iodine added. But when the amount of iodine added on 
one side sensibly equals the amount of potassium iodide, then the E.M.F. will 
be very sensibly increased, owing not only to the free iodine being no longer- 
proportional to the iodine added, but also owing the larger diminution of 
free KI on the strong iodine side. This increase, as we shall find, exists in 
practice, but is not so great as is to be expected from the mass equation and 
equation for calculating the E.M.F. given above. 
Taking, then, the mass equation jrj ~ 2 = C, and accepting the results 
■ _ 3 
obtained by Jakowkin, we can take the value of C as ='0014 with 
sufficient nearness for these calculations. 
If, then, a be the total number of potassium iodide molecules added to 
