THE POTENTIAL OF THE THALLH'IM ELECTRODE AND 

 THE FREE ENERGY OF FORMATION OF THALLOUS 



IODIDE. 



The Conductivity and Ionization of Thallous Nitrate Solu- 

 tion; THE Solubility of Thallous Chloride and of Thl^l- 

 Lous Iodide; the Normal Potentials of the Thallium and 

 OF the Iodine Electrodes. 



By Grinnell Jones and Walter Cecil Schumb. 



Received Oct. 19, 1920. Presented Dec. 8, 1920. 



In a paper by Grinnell Jones and M. L. Hartmann ^ it was demon- 

 strated that the free energy of formation of silver iodide is greater at 

 25°C. than at 0°C., thus justifying the inference that the mutual 

 atomic attraction of silver and iodine increases with a rise of tempera- 

 ture, and furnishing, with the aid of Richards' ^ theory of compressible 

 atoms, a plausible explanation of the hitherto inexplicable fact that 

 silver iodide, when heated, contracts — an almost unique property. 

 But the results, when interpreted with the existing data on the specific 

 heats and heat of formation of the substances involved, are not 

 cjuantitatively in accord with the reciuirements of the so-called 

 "Nernst Heat Theorem." 



The present paper records the results of an attempt to test the 

 generality of the relations observed by the investigation of another 

 case. 



Thallous iodide was selected as the first case to be studied on ac- 

 count of the marked similarity between the iodides of thallium and of 

 silver. 



It is true that neither the essential specific heat data nor meas- 

 urements of the coefficient of expansion of thallous iodide are as yet 

 available, but it is hoped to measure these properties of thallous iodide 

 at the earliest opportunity. The present paper records measurements 

 of the free energy of formation of thallous iodide at 25°C. and atO°C., 

 together with the results of some subsidiary measurements which were 

 required to interpret these observations. At first an attempt was 

 made to measure directly the free energy of formation of thallous 

 iodide by a method analogous to that used successfully^ with silver 



r~ 1 Grinnell Jones and Miner L. Hartmann, Jour. Amer. Chem. Soc, 37, 752 

 :m (1915). 



^ 2 T. W. Richards, Jour. Amer. Chem. Soc, 36, 2417 (1914). This paper 

 gives a summary of the theory and a bibUography of earlier papers. 



