ELECTROLYTIC REDUCTION 595 



It is this property of rendering difficult the escape of the 

 hydrogen, which lead and mercury possess in such high degree, 

 that allows the reduction of these difficultly reducible organic 

 substances to be accomplished — while at all other cathodes they 

 remain unaffected. 



Quantitative experiments such as those already described 

 show that the time required for the reduction is least by using 

 a small current density {i.e. a large cathode) and as high a current 

 strength as is compatible with holding the temperature within 

 the necessary limits. When working on a larger scale, cooling 

 (by means of hollow electrodes) is usually necessary. 



One of the most interesting phenomena connected with this 

 subject, and one which is also intimately correlated with 

 " super-tension," is the poisonous influence of traces of foreign 

 metals in the cathode chamber. Tafel ^ has shown that "004 

 milligram of platinum per ten square centimetres of cathode 

 surface is sufficient to bring the reduction to a standstill imme- 

 diately. The other enemies of electro-reduction arranged in 

 order are, after platinum, silver, tin, copper, mercury, zinc, 

 and iron ; though none approach platinum in its efficacy in 

 causing the " spontaneous depression " of the reduction. 



The most obvious explanation of this action is that the 

 platinum is electrolytically precipitated on the surface of the 

 cathode, and that then one is dealing virtually with a platinum 

 cathode, which, as has been already shown, by reason of its low 

 super-tension, i.e. the ease with which it allows the hydrogen to 

 escape as gas, does not permit of the reduction being accom- 

 plished. Tafel's^ extensive researches on this problem show 

 that, not only is the amount of platinum present far too minute 

 to form even the thinnest possible skin on the surface of the 

 cathode, but also the action is different on different cathodes, 

 and further, gold, which has almost as low a super-tension as 

 platinum, should have quantitatively almost as great a disturbing 

 effect, whereas gold is not nearly so poisonous to a lead cathode 

 as platinum. 



The conclusion is drawn that the platinum must exercise 

 a catalytic influence on the surface of the cathode material. 

 The disturbing action of minute traces of platinum and other 

 metals has been one of the great difficulties in the application of 



^ Zeitschr. f. phys. Chem. 34(1900), 187-228. 

 * Tafel and Emmert, Ibid. 52 (1905), 349. 



