THALLIUM ELECTRODE AND THALLOUS IODIDE. 235 



Thomsen ^° found 30,180 cals. for the heat of formation of thallous 

 iodide. 



Summary. 



1. The equivalent conductance of a series of solutions of thallous 

 nitrate from 0.001 N to 0.1 N was measured at 25°C. and at 0°C. By 

 extrapolation to infinite dilution by Noyes' method the equivalent 

 conductance of the thallous ion at 25°C. was found to be 78.36 mhos, 

 and at 0°C., 41.8 mhos. 



2. The conductivity of saturated solutions of thallous chloride 

 and of thallous iodide was measured at 25°C. and at 0°C. The equi- 

 valent concentrations of the ionized fraction of these salts, calculated 

 from these results, are as follows: 



3. The potential of the metallic thallium electrode in a saturated 

 solution of thallous chloride was measured against the normal and 

 decinormal calomel electrodes at both 25°C. and 0°C. with the fol- 

 lowing observed results (uncorrected for liquid junctions) : 



Tl, TlCl sat., 0.1 N KCl, Hg.Clo, Hg; 

 Tl, TlCl sat., 1.0 N KCl, Ug.CVi, Hg; 



From these results the normal potential of the thallium electrode is 

 computed to be +0.6188 volt at 25°, and 0.5885 volt at 0°. It was 

 found by direct comparison that a metallic thallium electrode is more 

 negative than a saturated two-phase amalgam electrode by 2.8 milli- 

 volts at 25°C. and 1.8 millivolts at 0°C. This is contrary to the 

 assumption commonly made by earlier investigators of this electrode. 



4. The decinormal calomel electrode was measured against the 

 normal calomel electrode at 0°C. with the following results: 



Hg, HgsCls, 0.1 N KCl, 1. N KCl, Hg.Cls, Hg; E = -0.0481 volt; 

 or, corrected for the liquid junction potential, the difference is —0.0489 

 volt. 



5. The potential of an iodine electrode with solutions of 0.1 N, 



60 J. Thomsen, Jour. Prak. Chem., 12, 116 (1875). 



