THALLIUM ELECTRODE AND THALLOUS IODIDE. 



229 



Although the existing data on the potential of iodine electrode at 

 25° appeared to be unusually reliable, it was necessary for our purpose 

 to measure this potential at 0°C. Since measurements could also 

 be made at 25°C. with little extra effort and since we were primarily 

 interested in the temperature coefficient of this potential, we deter- 

 mined to make the measurements at both temperatures. The 

 temperature coefficient thus obtained may be expected to be more 



Figure 6. 



reliable than one deduced by comparison of measurements made by 

 different experimenters, since as far as possible all the conditions 

 except the temperature remain unchanged. 



E:cpcrvmental Details: — The iodine and potassium iodide used 

 were prepared by the method used by Jones and Hartmann.^^ The 

 solutions were saturated with iodine b}' long continued shaking with 

 excess of solid iodine at the proper temperature before being added to 

 the cells for the potential measurements. The cells used are shown 

 in Figure 6. The tlaree way cock between the two halves of the cell 

 was closed during the filling of the cell, thus preventing mixing, and 

 also facilitating the formation of a fresh junction by removing solution 

 from both halves of the cell through the siphon tube. The platinum 

 electrodes were about 1. X 1.5 cm. in dimension, and were ignited in a 

 blast lamp immediately before being placed in the cells. A con- 

 siderable excess of solid iodine was used to ensure saturation. Two 



56Grinne]l Jones and M. L. Hartmann, loc. cit. 



