1908-9.] Electromotive Force of Iodine Concentration Cells. 305 
not affected by the presence of KI, except in so far as they enter into com- 
bination, then we get for the value of I 2 the solubility of iodine in water 
at 25° C. — that is, '00134 molecules in 1000 c.c. 
If we could neglect the contact electromotive force between the solutions 
we could therefore calculate the number of iodine ions present in the 
saturated solution by simply putting the known values into equation (1) 
and measuring the E.M.F. of the cell. 
As far as potassium iodide is concerned, there is so little difference in 
the velocity of the potassium and iodide ions that this must be very small, 
but we have also the presence of I 3 ions to consider. Where the number 
of I 3 ions is very small compared with the number of iodide ions, this 
electromotive force is no doubt negligible, and in former measurements of 
the electromotive force of such iodine cells I showed that when the total 
amount of iodine present is small as compared with the potassium iodide, 
the experimental results showed that the contact E.M.F.’s may be neglected. 
In these experiments, however, we have a very considerable concentra- 
tion of iodine in proportion to the total potassium iodide round one 
electrode, and a very small concentration round the other electrode. We 
can therefore no longer neglect this possible source of error in the calcula- 
tion of the number of iodine ions present. 
With the view of determining the amount of this contact electromotive 
force I used the method devised by Cumming.* Having first measured 
the electromotive force of the cell, with potassium iodide solution between 
the respective solutions of iodine in potassium iodide, I replaced the 
potassium iodide by 10 normal ammonium nitrate solution, and remeasured 
the electromotive force. 
In the subsequent tables the electromotive force in potassium iodide is 
given, and also the correction obtained by replacing the potassium iodide 
by ammonium nitrate as the connecting liquid. 
Before giving the experimental results it is necessary to consider a little 
more closely the mass equation investigated by Jakowkin. It is evident 
that in the form in which it is written above no consideration is taken of 
the relative amount of ionised and un-ionised potassium iodide, and potassium 
tri-iodide present. Dawson f shows that it is necessary in this connection 
to consider separately the three mass equations 
KI ^±K + 1 
KI 3 ^±K + r 3 
I 2 + I 
* A. C. Gumming, Trans. Faraday Society , vol. ii. part 3. 
+ Chem. Soc. Trans., vol. lxxix. p. 239. 
VOL. XXIX. 
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