Manchester Memoirs, Vol. Ix. (1916), No. 11. 3 



a gas is being liberated. Throughout this paper, cathodic 

 overvoltage with liberation of hydrogen is generally im- 

 plied, unless the contrary is stated. 



If we attempt to reduce a substance electrolytically 

 (in acid solution for example), then on applying a given 

 E.M.F. to the ceil, one of two things must occur at the 

 cathode, (a) either the discharged hydrogen ions will 

 react with the reducible substance present, or (b) they 

 will combine with each other and escape in the form of 

 gas bubbles. 



Overvoltage hinders the formation of gas bubbles 

 and thus encourages the former action to take place to 

 the exclusion of the latter. 



It might appear therefore that in order to reduce any 

 substance electrolytically, we have only to choose a 

 cathode of the right overvoltage, and given other suitable 

 conditions (temperature, circulation of electrolyte, etc.). 

 we should be able to obtain complete reduction before 

 any hydrogen is liberated. 



In practice the problem is by no means so simple. 



Many electrolytic reductions may take place along 

 two or more different paths leading to different end 

 products, and in some cases the substance of which the 

 electrode is composed may exert a specific influence in 

 determining the proportions of the original substance 

 which are reduced along the specified paths. 



In such cases, overvoltage may be only of secondary 

 importance. 



The following example will illustrate this : 



Sodium Nitrate in acid solution is reduced by a 

 metallic cathode with formation of hydroxylamine and 

 ammonia. Tafel studied this reaction with different 

 electrodes and obtained the following results ; current 

 density 0*24 amp. per sq. cm. — 



