1908-9.] Electromotive Force of Iodine Concentration Cells. 313 
beyond *508 of iodine ; but if the changing values of k , subtracted from 
'001365, be plotted against the iodine concentrations, it will be found that 
the graph is very nearly a straight line. If this straight line be produced 
until it passes through the value for saturation with iodine ('71), and if the 
value for k so obtained be used to calculate the amount of free iodine present 
in a saturated solution, then the value obtained is very nearly '00134. It 
is therefore justifiable to use this graph to determine the values of k 
between '508 and '71 of iodine. The following table contains the number 
of iodine ions calculated, as above described, from the E.M.F.’s as measured. 
Table IV. — E.M.F.’s of Jodine Solutions in Normal KI with -00139 I., 
in the Dilute Electrode. 
No. of Iodine 
Molecules. 
E.M.F. 
nh 4 no 3 
Correction. 
No. of Free 
Iodine Ions. 
•0322 
•0110 
•0003 
•758 
•056 
•0187 
0005 
•738 
T09 
•0277 
•0010 
•708 
•209 
•0382 
•0020 
•636 
'279 
•0441 
•0026 
•566 
•362 
•0498 
•0033 
•522 
*508 
•0597 
•0051 
•402 
•57 
•0644 
•006 
•353 
•61 
•0672 
•0065 
•325 
•65 
•0701 
•0071 
•299 
•70 
•0734 
•0078 
•274 
•712 
•0745 
•008 
•262 
It will be noted that the number of iodide ions present in the saturated 
solution with '0139 molecules of iodine in the weak electrode agrees very 
closely with the number as determined from the solution containing '0005 
of iodine. (If k — '00138 is used instead of k — '00136 in making the 
calculation, this number is slightly altered.) When it is remembered that 
in the one case the concentration of the iodine was '0005 and in the other 
case it was '0138, the agreement between these numbers is very satisfactory. 
At the other end of the table is to be found the number of iodide ions 
for a solution containing '03 of iodine. If the number of I 3 ions correspond- 
ing to this amount of iodine be added to the number of iodide ions as 
measured from the E.M.F., then the total number of iodide ions agrees with 
that derived from the conductivity of a normal solution of KI. 
A very interesting graph (5) can be constructed in the following manner 
from these results. Take the number of iodine ions as ordinates, and the 
respective strengths of iodine as abscissae. Mark on the base line the 
number of iodide ions present in a normal KI solution, and opposite '71 of 
