1 6 NEWBERY, The Theory of Overvoltage. 



When a given ion is to be liberated at a given elec- 

 trode, the choice of electrolyte has comparatively little 

 effect on the overvoltage so long as no chemical action 

 takes place between the liberated ion and the electrolyte. 

 Thus the cathodic overvoltage of most metals is nearly 

 the same in N/i H 2 SO„ N/io H 2 S0 4 , N/i NaOH, etc. 

 As a general rule the overvoltages in the last electrolyte 

 are slightly higher than those in the other two, but there 

 are exceptions. Change of concentration of the electro- 

 lyte produces comparatively little change of overvoltage 

 in most cases, but as a rule cathodic overvoltages are 

 lower in more concentrated electrolytes. This is partly 

 due to small concentration changes near the electrodes, 

 and the difference therefore is less if the electrolyte is kept 

 in motion. It is most marked in very dilute solutions, 

 and since this concentration polarisation is not a part of 

 true overvoltage, it should be avoided where possible by 

 using fairly strong solutions. 



Certain impurities in, or additions to the electrolyte 

 produce marked effects upon the overvoltage, and in the 

 case of metallic deposition, still more marked effects upon 

 the nature of the deposit. 



The presence of an oxidising agent such as chromic 

 acid, permanganate, etc., reduces the cathodic overvoltage 

 by assisting in the removal of the liberated hydrogen, and 

 also by increasing its ionisation. 



The action of colloids and poisons is remarkable. 

 Generally speaking, small quantities of colloids (ooi to 

 o*i %) such as gelatine, dextrin, gum arabic, etc., increase 

 the overvoltage of most metals by anything up to o - i 

 volt. Larger quantities sometimes produce still greater 

 effects, but often produce no effect at all. 



Thus C05 % dextrin raised the metal overvoltage of 

 zinc by C04 volt, but had no effect on the hydrogen over- 



