104 



Electeolytic I'KoDrcTioN (F Selexic Actd from 

 Lead Selenate, 



F. C. Mathers. 



Meutzer* has shown that the electrolysis of a solutiou of copper sel- 

 enate results in tlie deposition of metallic copper upon the cathode and 

 the formation of selenic acid in the solution. To obtain pure selenic acid 

 the copper selenate must hrst be carefully jjurified by recrystallizing be- 

 fore electrolysis. In the experiments that are described in this paper, 

 lead selenate was used as the salt to be electrolysed on account of the 

 ease with which it could be prepared and pnrirted. 



Selenic acid was first prepared by oxidizing selenium dioxide in a 

 nitric acid solution with potassium permanganate. After the precipitate 

 of manganese dioxide had been removed by filtration, the selenic acid in 

 the filtrate was precipitated as lead selenate by the addition of lead nit- 

 rate. Lead selenate is verj'^ insoluble in water and so can be filtered and 

 easily washed free from the other salts in the solution. 



For electrolj'sis, the lead selenate was placed in a platinum dish that 

 was filled with water. The platinum dish wa.>< used as the cathode and 

 a platinum wire coil was used as the anode. The resistance of the solu- 

 tiou was very high at first, but it rapidly dropped as the electrolysis pro- 

 ceeded and the free selenic acid was formed. 



To determine the amount of selenic acid that was formed during au 

 exi>eriment, the electrolyte was filtered and the acid in the filtrate was 

 titrated with standard sodium hydroxide sohitiDU. 



The current yield was best with low current density at the cathode, 

 hot solution, and a large quantity of lead selenate upon the cathode, and 

 decreased by the addition of powdered lead to the lead selenate. An in- 

 crease in the volume of the solution or the use of a mercury cathode were 

 without effect. The current yields were quite low — tlie maximum being 

 about 12%. 



♦Mentzer, Compt. Rend., 127. 54 (1898). 



