NEWBERY, The Theory of Overvoltage. 



Cathode. 



O.V. 



% Hydroxylamine. 



% Ammonia. 



Lead (rough) 



o"6o 



26-8 



57-6 



,, amalgamated 



o-3 5 



697 



] 9 - 9 



Copper (smooth)... 



°'35 



11 "5 



76-8 



spongy .. 



O'lO 



1 "5 



93'3 



Tin (smooth) 



0-63 



45' 8 



383 



The overvoltages (approximate) at the given current 

 density are given in the second column. From these it 

 appears that the metal with the highest overvoltage gives 

 the best yield of the less reduced product hydroxylamine. 

 This is evidently due to the fact that hydroxylamine 

 and ammonia are not primary products of reduction of 

 nitrates, but are obtained by independent paths since 

 hydroxylamine is not reduced to ammonia by a copper 

 electrode under the conditions of the experiment. 



Nevertheless, in the great majority of cases, over- 

 voltage is the main controlling factor in electrolytic 

 oxidations and reductions, and hence a knowledge of the 

 overvoltage of the electrode in use under the given 

 conditions, is of the greatest service in carrying out such 

 processes. 



Specially is this the case in the electrolytic treatment 

 of organic substances for the preparation of dye stuffs, 

 medicines, etc., where a slight change in the conditions of 

 the experiment, by changing the overvoltage, may com- 

 pletely change the nature of the product obtained. 



Further than this, in order to understand the problem 

 thoroughly, it is necessary to obtain some idea of the 

 nature of overvoltage, its causes and controlling factors. 



