THALLIUM ELECTRODE AND THALLOUS IODIDE. 219 



use of a siphon in the plating out process. The thallous chloride 

 solution in B becomes turbid in the vicinity of the platinum anode, 

 but the liquid in C is not contaminated. The siphon-tube, S, is pro- 

 vided so that the cell may be rinsed out with fresh liquid from the 

 saturation bottle without the passage of the liquid through the anode 

 chamber. The rubber stopper of chamber C has four holes, one for 

 the inlet tube, A, one for the outlet tube and trap, F, and two for the 

 thallium electrodes. 



The chamber, C, is connected to the calomel electrode chamber by a 

 side-arm, which contains a three-way cock, M. One limb of this cock 

 consists of a siphon-tube, through which, by the adjustment of the 

 stopcock, liquid may be run out from either half of the cell, thus 

 making a fresh junction. The calomel electrode chamber was pro- 

 vided with a small separatory fmanel, which served as a reservoir of 

 potassium chloride solution saturated with calomel. The cell when 

 in use was connected to the saturation bottle by means of a short 

 piece of rubber tubing, and could be immersed to the line indicated 

 in the figure, either in the thermostat at 25°, or in a tub of equal size 

 filled with cracked ice and distilled water, for the 0° measurements. 



The electrodes themselves consisted of platinum wire, wound into 

 small spirals, and sealed through glass tubes, electrical connection 

 being made as usual with mercury. 



Thallous chloride was prepared as already described ; see page 206. 



Mercury was purified by passing it 20 to 25 times through a five- 

 foot tower of mercurous nitrate solution acidified with dilute nitric 

 acid, followed by redistillation at reduced pressure, allowing a very 

 small current of air to bubble through the liquid during distillation. 

 Part of the mercury thus purified was converted into calomel, by 

 dissolving it in dilute, redistilled nitric acid in the presence of excess 

 mercury, and precipitation of the mercurous chloride by means of 

 dilute, pure hydrochloric acid. The precipitate was washed by 

 shaking up with pure water in a glass-stoppered bottle at least twelve 

 times, decanting the supernatant liquid each time after the precipitate 

 had settled. Then a portion of the solid was placed m a smaller 

 (250-cc.) bottle and the washing continued. The precipitate was now 

 filtered off on a Biichner funnel with the aid of suction, and after a 

 further washing on the funnel, as much of the adhering water as would 

 drain off was allowed to do so, and the moist solid was transferred to a 

 Jena bottle and shaken up several times with approximately 0.1 N 

 potassium chloride solution. Finally it was shaken up with a care- 

 fully prepared 0.1 N solution of potassium chloride for about an hour. 



