228 JONES AND SCHUMB. 



certain Thomson effect in the solution as well as the temperature 

 effect on the calomel electrode, and is therefore objectionable. Sam- 

 met considers that a correction of —0.004 volts should be applied to 

 his results to make them equivalent to direct measurement against a 

 normal calomel electrode at 25°C. When this correction is applied, 

 his results agree with those of Kiister and Crotogino within one 

 millivolt. Sammet then calculated the normal potential of the iodine 

 electrode. In making this calculation, he assumed on the basis of the 

 investigations of Le Blanc and Noyes, Jakovkin, and Noyes and 

 Seidensticker already referred to, that in a solution saturated with 

 iodine, one-half of the original iodide is converted into triiodide (the 

 later work of Bray and MacKay shows- that these assumptions are 

 only approximately true). He found a fairly constant value for the 

 normal potential in concentrated solutions but a rapid rise in the 

 dilute solutions, and therefore neglected the results obtained with the 

 dilute solutions. The measurements obtained with the stronger solu- 

 tions gave a value of —0.341 for the normal potential of the iodine 

 electrode, but the more dilute solutions gave higher values rising to 

 -0.363 with the 0.001 N solution. 



Maitland ^^ measured the potential of cells containing less iodine 

 than sufficient to cause saturation, the amount of free iodine being 

 determined by distribution experiments. His results with 0.1 N KI 

 gave a value for the normal potential of —0.3409, but show a varia- 

 tion with concentration of potassium iodide in the opposite direction 

 to the measurements of Kiister and Crotogino. Unfortunately, 

 Maitland's measurements did not extend below 0.1 N potassium iodide. 



Lewis and Randall ^^ report that the value —0.3407 was found by 

 P. V. Farragher, but no details are given beyond the bare statement 

 that this value was obtained with dilute solutions. 



So far as we have been able to find, no measurements of this poten- 

 tial at any other temperature have been published. Since the work 

 described above was published, the equilibrium between iodine and 

 dilute (0.001 N to 0.1 N) potassium iodide solutions has been studied 

 more carefully by Bray and MacKay ^* at 25°C., and later a similar 

 investigation was carried out at 0°C. by Jones and Hartmann.^^ 

 These investigations show that the assumptions made by Sammet in 

 his calculation of the normal potential are only approximately true. 



52 W. Maitland, Z. Elektrochem., 12, 263 (1906). 



53 G. N. Lewis and M. Randall, Jour. Amer. Chem. Soc, 36, 2264 (1914). 



54 W. C. Bray and G. M. J. MacKav, Jour. Amer. Chem. Soc, 32, 914 (1910); 

 and W. C. Bray, ibid., 32, 9.32 (1910)." 



5 5 Grinnell Jones and M. L. Hartmann. Jour. Amer. Chem. Soc, 37, 241 

 (1915). 



