•151 Va7i Name and Broimi — Ionization and 



ing free iodine, have been proved by the work of a number of 

 investigators.^ In its relation to the present problem the work 

 of Laurief is especially important. Laurie used the electro- 

 motive force of concentration cells composed of two such 

 iodine electrodes as a means of calculating the iodine ion con- 

 centration in a potassium iodide solution saturated with iodine. 

 The l^ernst equation for the electromotive force of such a cell 

 at 25° may be written 



The electromotive force is thus stated in terms of four concen- 

 trations. Under the conditions of Laurie's experiments three 

 of these four concentrations were calculable from known data, 

 and the fourth, the desired iodine-ion concentration, was calcu- 

 lated from the observed electromotive force. Concentrated 

 ammonium nitrate solution was used to eliminate diffusion 

 potentials. 



The method pursued in our own experiments was similar to 

 that of Laurie. The cells measured were of the type 



Pt : KI + I, (saturated) : NH^X03 : CdL + I, : Pt. 



Each cadmium iodide electrode was measured against two 

 different potassium iodide electrodes of different concentra- 

 tions, designated hereafter as electrodes A, and B, respectively, 

 the former containing 0*1 molar KI, the latter O'Ol molar KI, 

 both saturated with iodine. These two electrodes were then 

 measured against each other, thus furnishing a check upon the 

 results. All necessary data concerning these iodine-potassium 

 iodide solutions have been given by Bray and MacKay.J 



As intermediate solution to eliminate diffusion potentials, a 

 concentrated solution of ammonium nitrate was employed, to 

 which, following a suggestion due to Luther,§ enough sodium 

 nitrate was added to make the mean cation velocity the same 

 as that of the anion. This solution contained 8'3 mols NH^NOg 

 and 1 mol Xa XOg per liter. For comparison a few measure- 

 njents were made in which saturated potassium chloride solu- 

 tion was substituted for the mixed nitrates, but the results 

 failed to show any difference large enough to be of importance 

 in the present work. The nitrate solution was used in all the 

 experiments recorded below. 



Calculation of the diffusion potential by the Planck or the 

 Henderson equation was practicable only when the ion concen- 



* See. for example, Maitland, Zeitsehr. f. Elektrocliem., xii, 263,1906; 

 also, Jones and Hartmann, Jour. Am. Chem, Soc, xxxvii, 757, 1915. 

 t Zeitsehr. phvs. Chem., lxvii,'627, 1909. 

 i Jour. Am. Chem. Soc, xxxii, 914, 1910. 

 g See Bjei-rum, Zeitsehr. phys. Chem., liii, 438, 1905. 



