232 ^ JONES AND SCHUMB. 



would be observed in a solution in which the concentration of the 

 iodide ion and free iodine is unity, on the assumption that the Nernst 

 equation is valid from the concentration of the measurement up to 

 unity, 



(3) a correction for the difference between the tenth-normal and 

 the normal calomel electrode. (See pages 224-225.) 



The calculation of the first and of the second corrections requires 

 a knowledge of the concentration and mobilities of the ions present. 

 For the potassium iodide solutions saturated with iodine at 25° this 

 information is supplied by the work of Bray and MacKay ^'^ and at-0° 

 by the work of Jones and Hartmann.^^ Tenth-normal potassium 

 chloride ^^ was assumed to be 85.2% dissociated at 25° and 86.0% at 

 0°. The mobilities of the ions used in these calculations are as fol- 

 lows: 



25° 0° 



K+ 74.8 40.1 



CI- 75.8 41.3 



I- 76.5 43.4 



I3- 41.5 22.8 



Summarizing the results of four different experimenters for the 

 potential of solid iodine in a solution normal with respect to iodide ion, 

 calculated for 25°, and referred to the normal calomel electrode, we 

 have: 



Kiister and Crotogino: —0.256 v. 



Sammet: -0.256 



Maitland: (1) avg. of all solns. -0.2566 



(2) avg. of results with N/10 KI only - . 2560 

 Present paper : — . 2555 



In addition, the corresponding value at 0° is determined in the 

 present work as —0.2525 volt. 



Section 5. The Free Energy of Formation of Thallous Iodide. 



By the aid of the data which we have accumulated in the work 

 described in the foregoing pages, we are now able to carry out the 

 calculation of the free energy of formation of thallous iodide at 25° 

 and at 0°. 



This quantity, expressed in terms of electromotive force, will be 

 seen to equal the algebraic sum of the e. m. f.'s of the following com- 

 binations : 



58 Grinnell Jones and M. L. Hartmann, Jour. Amer. Chem. Soc, 37, 250, 251, 

 253 255 (1915). 



59 A. A. Noyes and K. G. Falk, ibid., 34, 468 (1912). 



