1907-8.] Electromotive Force of Iodine Concentration Cells. 387 
the solutions, and b the total number of iodine molecules added to the 
solutions, and x the number of free iodine molecules present, then 
x(a - b + x) _ £ 
b-x 
Expanding this quadratic, we get 
Kb _ K 2 6 2 
X a-b + K (a-6+K) 3+etc '. 
Within the range of the quantities here used, all but the first term can 
be neglected, and we get 
Kb 
a - b + K 
Calculating, then, the amount of free iodine and of free KI molecules 
present from this equation, and making a correction for the dissociation of 
the KI molecules, and putting the values into the equation for calculating 
the E.M.F., we get the following calculated values, which are compared in 
this table with the mean of the two experimental determinations : — 
Table I. 
Calculated 
E.M.F. 
Measured 
E.M.F. 
*344 molecules 'potassium iodide in 1000 c.c. 
*005 molecules of iodine in 1000 c.c. against *0005 molecules . 
•0295 
•0295 
■23 molecules potassium iodide in 1000 c.c . 
*005 molecules of iodine in 1000 c.c. against W0 5 molecules . 
•0298 
•0297 
T15 molecules potassium iodide in 1000 c.c. 
•005 molecules of iodine in 1000 c.c. against '0005 molecules . 
•0305 
•0304 
1 
The E.M.F. for iodine against iodine, if the mass law had not been acting, 
would have been ’0291. It is evident that for those cases where the 
correction for the mass law is small the calculated and measured values 
agree very closely. 
If we now compare the calculated and observed values for cells where 
the proportion of iodine on one side is sensibly of the same order as the 
proportion of potassium iodide, we find the E.M.F. increasing, but sensibly 
lower in value than that calculated from the mass equation. 
