Manchester lUemoirs, Vol. (xi. (1917) j^o. 9 15 



the lead accumulator, lead peroxide ionises 'directly, giving a 

 PbOg" ion and forming plumbites in which the lead is definitely 

 bivalent. 



Overvoltage iji different electrolytes. 



Generally speaking, anodic overvoltages measured in alkali 

 are more ireliable than those measured in acid, since we only 

 have two ainions, O" and OH" present, while in acid the four 

 anions O", OH', SO4", and HSO/ may all take part. 



Similarly, cathodic overvoltages in acid, where only one 

 cation is present, are more reliable^ than those in alkali where 

 two 'exist. How far we are justified in assuming those in alkali 

 , to 'be hydrogen overvoltages is somewhat doubtful, although in 

 viiew taf the fact that in the greater number of cases the values 

 are almost the same in both electrolytes, the assumption is 

 probably correct in most cases. Mercury is certainly exceptional, 

 owing to its affinity for metallic sodium. Hence mercury and 

 amalgamated electrodes may show abnormally high values in 

 alkali, due to the visible formation of sodium amalgam. 



Since the cathodic overvoltage of a given metal depends to 

 some extent upon the electrolyte, the table of values g'iven must 

 :n|ot be looted lipon as complete. There seems to be no reaso'n 

 why jany metal should not acquire the typical overvoltage of any 

 group if it is capable of showing the valency characteristic of 

 that group. P'urther determinations under certain conditions, 

 specially in difTercnt electrolytes, will undoubtedly supply new 

 values for many of these metals. Nickel, for example, in nickel 

 suJiphate solution, shows a hydrogen overvoltage of 0.7 volt, 

 which is typical of a divalent metal, and it may be noted that 

 the metal actually ha^s this valency in the given compound. 

 Iron, again, in ferrous sulphate solution, shows the high value 

 typical of a divalent metal, while in ferric sulphate solution the 

 value 0.5 volt typical of a trivalent metal is shown. 



Metal overvoltages. 



Metal overvoltages arc remarkable as being generally so 

 low that they might ble nearly all put down as approximately 

 zero, compared with gas overvoltages. Iron, nickel, and cobalt 

 are striking exceptions to the above rule, but in all three cases 

 hydrogen is evolved in quantity at the cathodes, specially at hig^h 

 current densities. 



■With some of the other metals (copper, in copper sulphate 

 solution, for example), if a very high current density is em- 

 ployed, hydrogen is also liberated, owing to the inability of the 

 copper ions to 'dififuse fast enough to carry all the current. At 

 the same time the overvoltage rises to the high values shown (by 

 divalent metals. This effect, is increased by further hindering 

 diffusion of copper ions by the addition of glue, gum, etc. 



