Polymerization in Cadmium Iodide Solutions. 455 



trations of both solutions were known, which was true in the 

 case of the cell made up of the two potassium iodide electrodes, 

 but not for a cell with a cadmium iodide electrode. For the 

 potassium iodide concentration cell the value of the diffusion 

 potential was calculated with the aid of the following data 

 taken from the article of Braj and MacKay : 





Solution 1 

 01 molar 



Solution 2 

 0-01 molar 



Migration 

 velocities 



(K+) 



0-0865 

 0-0430 

 0-0435 



000941 

 0-00484 

 0-00457 



: 74-8 

 : 76-5 

 : 41-5 



Using the Planck formula the value of the potential so 

 obtained was 0-0070 volt, while the Henderson formula gave 

 from the same data the value 0'0069 volt. The measured 

 value, as given by the difference between the electromotive 

 force as observed with and without the use of the intermediate 

 nitrate solution, was approximately 0*003 volt. 



In the opinion of the writers this difference between the 

 measured and calculated values of the diffusion potential is too 

 large to be explained by experimental error, or by uncertainty 

 in the data employed in the calculation, and is to be ascribed 

 to some peculiarity in the behavior of liquid junctions which 

 involve the tri-iodide equilibrium. Support for this view 

 is given by the fact that the diffusion of iodine in an 

 iodide solution is abnormal in at least one important respect, 

 for the rate of diffusion of iodine in potassium iodide is 

 known to increase with increasing concentration of the latter, 

 while by the rule of Abegg and Bose^ we should expect just 

 the reverse. That the discrepancy is in all probability not due 

 to the incomplete elimination of the diffusion potential by the 

 nitrate solution is shown by the fact that the electromotive 

 force of this concentration cell as measured with the nitrate 

 solution as intermediate liquid (mean value of many determina- 

 tions, 0'0660 volt), was in close agreement with the value cal- 

 culated from Equation 1 (0-0561 volt). 



Apparatus. — The type of half-cell used is shown in the 

 figure. The tube carrying the electrode (of bright platinum 

 foil) was fitted into the neck of the half-cell by a ground joint, 

 so that the iodine solution came in contact with nothing but 

 platinum and glass. Two such cells were clamped in a frame 

 with their siplion tubes dipping into opposite arms of a U tube 

 containing the intermediate solution, and the whole w^as 

 immersed up to the necks of the half-cells in a thermostat kept 

 at 25°. With stop-cocks closed the cell could be left set up 

 for many days without danger of contamination of the solu- 



*Zeitsclir. phys. Chem., xxx, 551, 1899. 



