THALLIUM ELECTRODE AND THALLOUS IODIDE. 227 



of thallous iodide. An examination of the existing literature shows 

 that apparently trustworthy measurements of this quantity are 

 available at 25°C., but we were unable to find any measurements at 

 0°C. 



The early potential measurements of Beetz, Doat, Regnauld, 

 Peirce, Laurie, Bancroft, and Smale*^ on various cells containing 

 an iodine electrode need not be discussed in detail, since they 

 either do not permit the computation of the normal potential or 

 when judged by modern standards are very crude and inaccurate. 

 In this early work the influence of ion concentration on elec- 

 trode potential was not sufficiently appreciated. Moreover, these 

 earlier measurements were, carried out before it had been shown 

 by Le Blanc and Noyes *^ by means of freezing point measurements 

 that iodine dissolves in iodide solutions with the formation of some 

 complex ion, and before Jakovkin *^ had demonstrated by distribution 

 measurements that the complex ion is a triiodide ion (I3"). These 

 conclusions of Jakovkin were confirmed and extended by Noyes and 

 Seidensticker,*^ who found that in solutions saturated with iodine at 

 25°C. the amount of triiodide formed is almost exactly half of the 

 original iodide present or equal to the remaining iodide. 



Klister and Crotogino ^° were the first to carry out a systematic 

 study of the potential of the iodine electrode with some degree of 

 accuracy. They measured the potential of a bright platinum elec- 

 trode immersed in solutions of potassium iodide varying in concen- 

 tration from 1 N to about 0.001 N containing varying amounts of free 

 iodine against a normal calomel electrode at 25°C. 



These measurements were repeated in part by Sammet,^^ who 

 measured the potential of a series of cells containing a platinum 

 electrode immersed in solutions of potassium iodide varying from 

 1 -normal to iV-normal and saturated with iodine at 25° against a 

 normal calomel electrode at 18°C. This procedure involves an un- 



46 W. Beetz, Pogg Ann., 90, 42 (1853). 



V. Doat, Compt. rend., 42, 855 (1856). 



J. Regnauld, Compt. rend., 43, 47 (1856). 



B. O. Peirce, Wied. Ann., 8, 98 (1879). 



A. P. Laurie, Phil. Mag., (5) 21, 409; Jour. Chem. Soc, 49, 700 (1886). 



W. D. Bancroft, Z. physik. Chem., 10, 387 (1892). 



F. J. Smale, Z. physik. Chem., 14, 577 (1894). 



47 M. Le Blanc and A. A. Noyes, Z. physik Chem., 6, 401 (1890). 



48 A. A. Jakovkin, Z. phvsik. Chem., 13, 539 (1894); 20, 19 (1896). 



49 A. A. Noyes and J. Seidensticker, Z. physik. Chem.-, 27, 357 (1898). 



50 F. W. Kuster and F. Crotogino, Z. anorg. Chem., 23, 87 (1900) and F. 

 Crotogino, Z. anorg. Chem., 24, 247 (1900). 



51 V. Sammet, Z. physik. Chem., 53, 674 (1905). 



