20 Newbery, Recent Work on Overvoltage. 



Summary. 



I. — The overvoltages, anodic and cathodic, of a number of 

 electrodes have been measured in acid, in alkali, and in certain 

 solutions of metallic salts undej varying conditions of time and 

 current density. 



2. — Elements in the same group of the periodic system 

 show the same cathodic (hydrogen) overvoltages. 



3. — Overvoltage lis due to the high solution potentials of 

 compounds of the electrode material with the discharged ion, 

 or with a product of the discharged ion. These compounds 

 (hydrides, higher oxides, etc.), form solid solutions in the elec- 

 trode substance, and are usually stable only under the influence 

 of high pressures or high temperatures. 



4. — -Metal overvoltages are due to the presence of the same 

 compounds which produce gas overvoltages, and are in most 

 cases very low, compared with gas overvoltages. Iron, nickel, and 

 cobalt are exceptions to this rule. 



5. — Changes of overvoltage are produced {a) by change? 

 of constitution of the above compounds, and [b) by changes of 

 concentration of the Solid solutions formed. 



6. — Passivity ds due to the insolubility and good electrical 

 conductivity of certain of the above compounds, which form a 

 protective coating over the attackable metal surface. 



7. — The chemical stability of the hydrides or higher oxides 

 formed 'on the electrode surfaces is of great importance when 

 dealing with substances which are only reduced or oxidised 

 with a low velocit)^ 



8.— The ions H', OH', Fe", Ni", Co" are hydrated (com- 

 bined with the solvent) in aqueous solution. 



9— The ions Na", NH'^, K', Cu", Ag", Zn", Cd", Hg', 

 TV, Sn", Pb-, CI', NO3', and SO/ are either non-hydrated, or 

 have !an affinity for the solvent of a quite different order from 

 that of H-, OH', etc. 



In conclusion, the author wishes to express his thanks to 

 Prof. iLapworth for his continued interest and encouragement 

 throughout this work. 



Electro-Chemical Laboratories, 



Manchester University. 



6". Clarke Limited, Printers, Manchester. 



