﻿LEAD FROM PERCHLORATE BATHS 



85 



The lead for the anodes had been once refined, but even then a 

 small quantity of slime was formed. Betts has sho^Yn^° that anodes 

 of triple refined lead, which dissolve in the lead fluo-silicate bath 

 without the formation of slime, gave corrosion efficiencies greater 

 than 100 per cent. In some of his experiments, the anode corrosion 

 efficiencies, in per cent, were as follows: 101.4, 101.8, 101.3 and 

 101.8. This shows that the free liuo-silicic acid is gradually 

 changed to lead fiuo-silicate. After a time the bath might even 

 become alkaline. This extra loss from the anode is, no doubt, the 

 result of chemical solution by the free acid in the bath. The per- 

 chloric acid in the perchlorate bath dissolves lead on open circuit, as 

 shown by the following table. The bath contained 5 per cent lead 

 and 5 per cent free perchloric acid. 



Contact with platinum wire increased the action, as was to be 

 expected. The finely divided lead in the anode slime would be 

 more rapidly attacked than the massive lead, owing to the greater 

 surface exposed and to contact with more positive metallic impuri- 

 ties. The above experiments were made with refined lead, which 

 would dissolve more slowly tlian bullion. The free perchloric acid 

 which is thus gradually neutralized must be restored. Two methods 

 were tried: Electrolyzing with platinum anodes. The objection 

 is that some lead peroxide is formed on the anode. Of course, 

 the cost of platinum would present difficulties in commercial work. 

 Grraphite cannot be used, because it disintegrates during electrolysis. 

 A simpler method is to add the required amount of sulphuric acid 

 to a portion of the electrolyte. Lead sulphate is precipitated and 

 perchloric acid remains in solution. The filtrate is returned to the 

 bath. This diminution in acidity is not rapid. It is greater, of 

 course, A^-itli baths containing large amounts of acid. This may be 

 seen from the following table : 



Per Cent of . Loss of HOIO^ 



Lead Suspended by 

 Platinum wire 

 Cotton string 



Area of Lea l 

 10.2 

 10.2 



Grams Dissolved per Sq. 

 Dm. per 24 Hours. 



0.495 



0.339-0.414 



Acid in the Bath. 



in Per Cent Per Day. 



1.6 

 4.5 

 4.9 



0.025 



0.07 



0.05 



Current Density. Many things have an effect upon the prac- 

 ticable current density. A high current density is favored by con- 



" Betts, Transactions American Electrochemical Society. VI, 67 (1904). 



