THALLIUM ELECTRODE AND THALLOUS IODIDE. 209 



Section 3. The Potential of the Thallium Electrode at25°C. and 

 at 0°C. 



The earliest work on the thallium electrode is probably that of 

 J. Regnauld/8 who studied the cells: Tl, TI2SO4, ZnS04, Zn; and 

 Tl, TI2SO4, CdS04, Cd; and Tl, TI2SO4, TlHgx (unsaturated thallium 

 amalgam made up from 2.04 grams of Tl and 10 grams of Hg). He 

 found that the metallic thallium is electro-positive to cadmium by an 

 amount equal to 8 times the thermo-electric force between copper 

 and bismuth with the junctions at 0° and 100°C. (or about 57 milli- 

 volts). Goodwin ^^ studied several cells containing thallous chloride 

 or bromide as a depolarizer with electrodes of saturated thallium 

 amalgam. These results, although showing the behavior of these 

 sparingly soluble salts as depolarizers, do not permit the calculation of 

 the normal potential of thallium against any standard electrodes, and 

 need not be discussed in detail here. 



The first measurements of the thallium electrode against the normal 

 calomel electrode are those of Neumann ^° in 1894, who measured both 

 metallic and amalgamated electrodes in saturated solutions of thallous 

 sulphate, thallous chloride, and thallous nitrate against the calomel 

 electrode. For the combination in which we are specially interested, 

 namely, the cell: Tl, TlCl sat., l.X KCl, HgoCl., Hg"; Neumann 

 obtained the value +0.711 volt at 17°.^^ No special precautions to 

 prevent oxidation were taken. Lewis and von Ende ^^ have shown 

 that when his results are plotted against the logarithm of the ion 

 concentration they show no regularity and are, therefore, apparently 

 subject to some fortuitous error. 



Abegg and Spencer ^^ studied the effect of the thallous ion concen- 

 tration on the potential of the thallium electrode, but their values like 

 those of Neumann show marked deviations from the requirements of 

 the Nernst equation. They used amalgamated thallium electrodes 

 on platinum points. They found for TlHgx, TlCl sat., 0.1 N KCl, 

 Hg2Cl2, Hg; E = +0.7752 at 25°. Later work has shown this value 

 to be too low. 



Spencer ^* has measured the potential of a series of thallium amal- 

 gams, varying in composition from 0.001831% thallium up to 55.68% 

 thallium, against metallic thallium at 18°C. For all of the saturated 



18 J. Regnauld, Compt. rend., 64, 611 (1867). 



19 H. M. Goodwin, Z. physik, Chem., 13, 577 (1894). 



20 B. Neumann, Z. physik. Chem., 14, 219 (1894). 



21 Throughout this paper a positive sign of the potential indicates that the 

 positive current flows through the cell as written from left to right. 



22 G. N. Lewis and C. L. von Ende, Jour. Amer. Cliem. Soc, 32, 732 (1910). 



23 R. Abegg and J. F. Spencer, Z. anorg. Chem., 46, 408 (1905). 



24 J. F. Spencer, Z. Elektrochemie, 11, 681 (1905). 



