THALLIUM ELECTRODE AND THALLOUS IODIDE. 



205 



respectively, are O.OIGI and 0.00696 mol per liter. At 25°, Noyes ^^ 

 and Geffcken ^^ likewise found 0.0161 mol per liter and Hill ^* found 

 0.01629. Lewis ^^ gives the concentration of the ionized portion of 

 this solution at 25° as 0.0143 mol per liter. 



Abegg ^^ gives the solubility product of thallous iodide at 25° as 

 5.8 X 10"^, corresponding to a solubility of 2.4 x 10~* mol per liter. 

 At 0° we have found no data for the solubility of thallous iodide. 



The teclmique of the determination of the solubility of a slightly 

 soluble salt has been thoroughly worked out, in particular by Kohl- 

 rausch, and later by Bottger.^'^ The only innovation adopted in the 

 present work was in the design of the cell, which was a modification of 

 Bottger's ^'^ and is showTi in Figure 1. Two platinum-iridium elec- 

 trodes were sealed in through one end of the receptacle, 

 in a manner designed to render their position rigidly fixed. 

 The stout sealed-in platinum wires were welded to short 

 copper leads, which extended several cm. into the tube 

 at one end of the cell. These wires were insulated from 

 one another, and at the same time connected with the 

 bridge leads, by surrounding them with two narrow glass 

 tubes and filling the space about these tubes with melted 

 paraffine, which when solidified kept the tvibes in position 

 and sealed the openings at their lower ends. Mercury 

 could now be poured into the tubes and the lead wires 

 from the bridge inserted in the usual way. Another method 

 of making connection between the cell and the outer cir- 

 cuit is shown in Figure 1 at A. Here the copper leads 

 passed entirely outside the cell and were bent into hooks 

 as in Kohlrausch's method, the terminating tube of the 

 cell being completely filled with paraffine as before. This 

 modification was not suited for work at 0°, as during the 

 rotation of the cell in the ice-bath the wires became 

 much bent. The other form could be used at both tem- 

 peratures. 



The stopper and neck of the cell were covered with a thin rubber 

 cap during its immersion in the constant temperature bath, and a 

 similar protection was provided for the other end during rotation. 



Figure 1. 



12 A. A. Noyes, Z. Phys. Chem., B, 249 (1890). 



13 G. Geffcken, Z. Phys. Chem., 49, 296 (1904). 



14 A. E. Hill, Jour. Amer. Chem. Soc, 32, 1189 (1910). 



15 G. N. Lewis, andC. L. vonEnde, Jour. Amer. Chem. Soc, 32, 732 (1910). 



16 R. Abegg, Handb. Anorg. Chem., III. (1), 424. 



17 W. Bottger, Z. Phys. Chem., 46, 531 (1903). 



