26 NEWBERY, The Theory of Overvoltage. 



Like Nernst's theory, this probably contains a part of 

 the truth. 



Foerster suggests the formation of a solid solution 01 

 oxide or hydride on the surface of the electrode which 

 would generate a high back E.M.F. 



This theory, better than any other, explains the high 

 anodic and low cathodic overvoltage of platinum but it is 

 unsatisfactory in many important respects. This also may 

 contain part of the truth. 



Le Blanc suggests two factors to account for overvoltage. 



i. The discharged ion does not separate at once but 

 remains dissolved forming a supersaturated solution, in- 

 creasing supersatu ration producing increasing overvoltage. 



2. The ions in the electrolyte are hydrated (com- 

 bined with the solvent) and time is necessary for the 

 reaction — ion hydrate ^ ion + water — in both directions. 

 Hence there will be excess or deficiency of non-hydrated 

 ions near the electrodes which will also produce an excess 

 back E.M.F. 



Le Blanc thus locates the source of overvoltage in the 

 electrolyte while Nernst, Reichenstein, etc., locate it in the 

 electrode. Le Blanc's theory furnishes the most satis- 

 factory explanation of metal overvoltage, but fails to 

 explain the great influence of the nature of the electrode 

 surface during liberation of gas. This again appears to 

 contain part of the truth. 



4. Suggestions for a Fuller Theory of Over- 



VOLTAGE. 



The following are the most important known facts 

 upon which the theory must be built. 



