104 



Kt^ecteota'tic I'koduction (f Selenic Acid i-'rom 

 Lead Selenate. 



F. C. Mathers. 



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

 euate results in the 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 first be carefully purified by recrystallizing be- 

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

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

 ease with which it could be prepared and purified. 



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 very insoluble in water and so can be filtered and 

 easily washed free from the other salts in tlie solution. 



For electrolysis, the lead selenate was placed in a platinum dish that 

 was filled with water. The i)latinum dish was used as the cathode and 

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

 tion 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 an 

 exiierimeut, the electrolyte was filtered and the acid in the filtrate was 

 titrated with standard sodium hydroxide solution. 



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 — tin' niaxinnim being 

 about 12%. 



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



