Polymerization in Cadmium Iodide Solutions. 457 



measured from time to time until the constant value 0*056 volt 

 was reached. A half-cell was then filled with the iodine-cad- 

 mium iodide solution to be investigated, and measured in turn 

 against each of the reference electrodes. Finally, as a check, 

 the reference electrodes were again combined and measured. 

 Obviously the difference between the electromotive forces of 

 the two cells in which the iodine-cadmium iodide electrode was 

 used should agree with the electromotive force of the cell made 

 up of the two reference electrodes (0'056 volt). All measure- 

 ments in which the difference above mentioned was within 

 dr O'OOl volt of 0*056 vohs were considered trustworthy. 



The cadmium iodide solutions used were of four different 

 concentrations : 0*5, 0*25, 0*125 and 0*01 molar, from each of 

 which a number of electrodes containing varying amounts of 

 dissolved iodine were prepared and measured against electrodes 

 A and B. Since these concentrations of cadmium iodide were 

 the same as employed in the determinations of distribution 

 coefficients described in our previous article* the results of 

 that work were used in calculating (Ij, the concentration of 

 uncombined iodine, from the concentration of dissolved iodine 

 as found by direct titration. For electrodes A and B (I^) was 

 obviously equal to 0*00132, the solubility of iodine in pure 

 water, and the values of (1^) for these two solutions were those 

 given on p. 455 of this article. It was, therefore, possible to 

 calculate {!') for each cadmium iodide solution from the 

 observed electromotive force by means of Equation 1. 



A summary of the results is given in Table I. Except 

 where otherwise stated, concentrations in this and the follow- 

 ing tables are expressed in millimols per liter. In the third 

 column are the potential differences given by each iodine-cad- 

 mium iodide electrode against reference electrode A, while the 

 potential difference for the same electrode against reference 

 electrode B is found in the fourth column on the horizontal 

 line next below. A negative sign prefixed to the recorded 

 potential indicates that the reference electrode formed the neg- 

 ative pole. Usually the reverse was true. The iodine-ion con- 

 centrations calculated from these potential differences are 

 recorded in the fifth and sixth columns, the former giving the 

 two independent values for each solution, derived from the 

 two potentials measured, and the latter the mean of the two. 

 Finally, by graphical extrapolation of the values in column six 

 to zero concentration of iodine, a value has been obtained for 

 each cadmium iodide solution which should represent the con- 

 centration of iodine ion in the given cadmium iodide solution 

 if it contained no dissolved iodine. These four extrapolated 

 values are the important ones for our present purpose. Since 

 *This Journal (4), xliv, 105, 1917. 

 Am. Jour. Sci.— Fourth Series, Vol. XLIV, No. 264. 



