THALLIUM ELECTRODE AND THALLOUS IODIDE. 213 



of Rocs shows that the two phases present in the eutectic mixture are 

 TliHgs, containing sufficient excess thalHum in solid solution to bring 

 its composition up to 31.3% Tl, and solid thallium, containing 18% 

 of mercury in solid solution. 



Richards and Daniels,^'' in the course of an extensive investigation 

 of the thermodynamic properties of imsaturated thallium amalgams, 

 have also measured the freezing points and electromotive force of a 

 series of thallium amalgams, and their results are entirely in accord 

 with the later independent work of Roos, but not in accord with those 

 of Kurnakow and Puschin. Although Richards and Daniels did not 

 measure the cooling curves of these amalgams and therefore do not 

 confirm this part of Roos' work directly, they have shown by direct 

 comparison that the electromotive force of metallic thallium is not 

 the same as the electromotive force of a two-phase thallium amalgam. 

 At 20°, Richards and Daniels find the potential of metallic thallium 

 to be 2.49 millivolts more negative than the potential of the two-phase 

 amalgam. 



Sucheni's results, on which Lewis and von Ende relied in part, do 

 not justify the conclusions which Sucheni drew from them. He 

 measured the potential of a series of thallium amalgams in contact 

 with a 0.1 N potassium chloride solution saturated with thallium 

 chloride against the decinormal calomel electrode. "The potential 

 values, which are given later, represent the mean of several series in 

 which measurements were made through the entire series of amalgams 

 from 0% Tl to 100% Tl. The individual values for the same thallium 

 content differ from each other in the different series by from 0.001 to 

 0.004 volt. Up to 100% Tl the value remains constant at 0.830," 

 (i.e., from a Tl content of 49.17%, which is shown in his table of results 

 — for 37°C — to have a potential of 0.830 volt, whereas an amalgam 

 of 43.59% and all weaker amalgams are shown to have a lower po- . 

 tential). Therefore his statement that the potential of thallium 

 metal and that of a two-phase amalgam at 37°C. agree, means only 

 that they are within four millivolts of each other. "The electro- 

 motive force, as one perceives from the measurements, remains con- 

 stant from the point where the potential curve becomes horizontal up 

 to pure thallium. From this it follows that in all these amalgams 

 which contain over 43 atomic per cent, of thallium, thallium is present 

 as an independent phase and that thallium does not appreciably 

 dissolve the compound TlHgo." ^^ 



37 T. W. Richards and F. Daniels, Jour. Amer. Chem. Soc, 41, 1765 (1919). 

 This work was completed in 1914, but the pubUcation was delayed by the war. 



38 A. Sucheni, Z. Elektrochemie, 12, 729 (1906). 



