THALLIUM ELECTRODE AND THALLOUS IODIDE. 225 



1914, Lewis and Randall *^ state that numerous investigations in their 

 laboratory had fixed the value of this e. m. f. as —0.0530 v. at the same 

 temperature. This value is uncorrected for the liquid junction 

 potential. While the potential between these two liquids cannot be 

 calculated with accuracy, still, inasmuch as it is not very large, the 

 data at hand for its calculation should be good enough to furnish us 

 with a good check upon our measurements. Calculation according 

 to the Henderson equation gives the value 0.0004— volt at 25° and 

 0.0008— volt at 0°, for this potential. The corrected potential be- 

 tween the two calomel electrodes at 25° thus becomes 0.0533— v. It 

 will be seen that the difference between the first two cells in Table IX. 

 is exactly the same: 0.0533 v. 



Slade ** also measured the potential of the normal calomel electrode 

 against the decinormal electrode. Three electrodes of each kind were 

 made up according to the directions given in the handbook by Ost- 

 wald-Luther, and measured against one another from time to time. 

 The observed value at 25° was -0.0534 ± 0.0002 volt. 



At 0°, we measured by independent experiment the potential be- 

 tween the two calomel electrodes, the normal and decinormal, in a 

 bath of cracked ice and water. The mean of four determinations gave 

 as the value for this combination (see Table VIII) at 0°: —0.0481 volt; 

 so that the corrected value becomes —0.0489 volt, when we take into 

 account the liquid junction as above mentioned. The corrected 

 difference between our two cells at 0° is seen to be 0.0488 volt. 



A further check upon the values given in the last column of Table 

 IX is obtained from the following considerations. Richards*^ found 

 the temperature coefficients of the normal and tenth-normal calomel 

 electrodes to be these: 



The difference between the temperature coefficients of the N/10 and 

 N/1 electrodes is seen from these figures to be .00079 — .00061 = 

 .00018 volt. This value should be equal to the temperature coefficient 

 of the potential difference observed between the two electrodes as 

 seen in the last column of Table IX, and in turn equal to the tempera- 

 ture coefficient of the difference between the two pairs of cells measured 

 at 25° and 0°. In each case the observed difference is .0045 v. ; the 



43 G. N. Lewis and M. Randall, Jour. Amer. Chem. Soc, 36, 1974 (1914). 



44 R. E. Slade, Jour. Chem. Soc, 99, 1977 (1911"). 



45 T. W. Richards, Proc. Amer. Acad., 33, 1 (1897). 



