THALLIUM ELECTRODE AND THALLOUS IODIDE 211 



which the normal potential of thallium is calculated to be 0.617 volt. 

 The average value for the amalgam electrode was 0.690 volt, corre- 

 sponding to a normal potential of 0.618 v. The variations in potential 

 in Kent's measurements were, however, of the order of millivolts; and, 

 furthermore, no especial precautions appear to have been taken to 

 avoid oxidation. 



Brislee,^^ working at room temperature, approximately 17°, but 

 without a thermostat, measured the potential of thallium metal bars 

 in solutions of thallous chloride, thallous nitrate, and thallous hy- 

 droxide, separated from the tenth-normal calomel electrode by satu- 

 rated solutions of ammonium nitrate. He bubbled hydrogen through 

 the solution about the thallium electrodes during measurements. 

 For the cell; Tl, 0.01 N TlCl, NH4NO3 sat., 0.1 N KCl, Hg2Cl2,Hg; 

 he obtained 0.773 v. This corresponds to a normal potential of 

 0.602 volt. Using the temperature coefficient of the normal potential 

 of the thallium electrode, as determined by our experiments, the 

 interpolated value for the normal potential at 25°, according to Brislee, 

 would be 0.612 v. 



Much more important than any of the work referred to above is the 

 investigation of G. N. Lewis and C. L. von Ende.^° First, they in- 

 vestigated the influence of changing concentration of thallous ion on 

 the potential of an unsaturated thallium amalgam. They found that, 

 when adequate precautions were taken to prevent solution of thallium 

 from the amalgam by atmospheric oxidation, the observed potentials, 

 in the case of dilute solutions (0.002 N to 0.0333 N), are in almost 

 perfect accord with the Nernst equation. However, with 0.1 N 

 thallous nitrate solution they find a deviation of about one and one- 

 half millivolts, as is to be expected by analogy with many other similar 

 cases. In other words, in dilute solutions the electromotive " activ- 

 ity" of the ions is the same as its concentration as computed from the 

 conductivity. "Instead then of finding any anomaly in the electro- 

 motive behavior of thallous ions, we may assert that the Nernst equa- 

 tion holds for the thallium electrode over a large range of concentra- 

 tion with greater accuracy than it has as yet been shown to do in the 

 case of any other electrode." The conflicting observations of Neu- 

 mann and of Abegg and Spencer are ascribed to insufficient experi- 

 mental precautions to prevent oxidation. Lewis and von Ende 

 then measured the normal potential of the thallium electrode. They 

 say, "In order to determine the absolute electrode potential of 



29 F. J. Brislee, Trans. Faraday Soc, 4, 159 (1909). 



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



