200 JONES AND SCHUMB. 



iodide. An electrolytic cell was prepared, containing as one electrode, 

 metallic thallium coated with thallous iodide immersed in a solution 

 of potassium iodide, and as the other electrode, an iodine electrode. 

 This type of cell, however, proved to be lacking in constancy and 

 reproducibility. After many attempts to overcome its defects, it was 

 reluctantly abandoned. The direct method having failed to give 

 satisfactory results, an indirect method was developed. An electrode 

 of metallic thallium in a saturated solution of thallous chloride was 

 found to give a constant and reproducible potential when atmospheric 

 oxygen was rigorously excluded. From determinations of the con- 

 ducti\ity of saturated solutions of thallous chloride and of thallous 

 iodide, conclusions can be drawn in regard to the relati^■e concentra- 

 tion of the thallous ion in these solutions, and hence the potential of 

 thallium in a saturated solution of thallous iodide can be computed. 

 These computations require a knowledge of the mobility of the thal- 

 lous ion, and therefore the conductivity of a series of solutions of 

 thallous nitrate of progressively increasing dilution was determined. 

 Finally measurements of the potential of an iodine electrode in solu- 

 tions of potassium iodide of known strength were needed. All the 

 measurements were carried out at both 25°C. and 0°C., these tempera- 

 tures being selected partly because they are convenient working 

 temperatures and partly because the other data needed are available 

 in the existing literature for these temperatures with sufficient ac- 

 curacy. 



Section 1. The Conductivity of Solutions of Thallous Nitrate 

 from 0.001 N to 0.1 N and the Equivalent Conductance of the Thal- 

 lous Ion at 25°C. and 0°C. 



In 1895 E. Franke ^ published measurements of the equivalent 

 conductance at 25°C. of solutions of thallous nitrate and of several 

 other thallous salts. Franke calculated the equivalent conductance 

 of these salts at infinite dilution by extrapolation by Ostwald's " valence 

 and dilution rule." He concluded that the equivalent conductance 

 of the thallous ion at 25° is 71.2 Siemens units. His data, when con- 

 verted into modern units and extrapolated by Noyes' method, give 

 75.9 mhos as the equivalent conductance of the thallous ion at 25°C. 



Hunt * has measured the equivalent conductance of solutions of 

 thallous sulphate at 18° and 25°C. At 25°C. his data, when extra- 

 polated by Noyes' method, give 156.0 as the equivalent conductance 

 of thallous sulphate, and if the equivalent conductance of the sulphate 



3 E. Franke, Z. phys. Chem., 16, 463 (1895). 



4 Franlvlin L. Hunt, Jour. Amer. Chem. Soc, 33, 795 (1911). 



