14 Newbery, Recent Work on Overvoltage. 



Reviewing the table as a whole, a rise of overvohage of 

 two equal steps is observed from Group o to Group II., followed 

 by a gradual fall through the rest of the table. 



Anodic (oxygen) overvoltage. 



The overvoltages so far discussed have all been cathodic 

 hydrogen overvoltages. 



Anodic overvoltages do not show so great regularity for 

 two .reasons : — 



(i) It is impossible to obtain an aqueous electrolyte having 

 only one .anion, whilst any dilute acid gives only one cation. 



(2) The true potential of the oxygen electrode is stilj 

 doubtful, land further, it is not clear as to whether this poten- 

 tial should be taken as the standard from which to measure 

 anodic overvoltages, since in .any aqueous solution hydroxyl 

 ions are far more plentiful than oxygen ions. Hence, anodic 

 overvoltages calculated from an oxygen electrode as standard 

 are abnormally high, compared) with cathodic overvoltages. 



Caspari, using an indefinitely low current density, obtained 

 a value for the anodic overvoltage of platinum about 0.3 volt 

 lower 'than the average of those given in these tables. Under 

 the experimental conditions of Caspari's work, the oxygen ions 

 piresent were probably sufficient to carry the small current used, 

 whilst in the present work other ions certainly took part. 



If the anodic overvoltages are recalculated on the assump- 

 tion that the potential of the normal standard is 0.3 volt above 

 that of an oxygen electrode, we obtain a series of values which 

 in many cases sh!ow a considerable resemblance to cathodic 

 lovervoltages of the same metal. The parallelism is, however, 

 very rough, and in many cases an anodic overvoltage measured 

 in alkali only corresponds; with a cathodic overvoltage found 

 in acid. 



A few of the best examples are given below, where column 

 I. shows the average anodic overvoltage, generally in alkali, 

 and column II. the corresponding cathodic overvoltage. 



I. Anodic. II. Cathodic. 



Metal. Volt. Volt. 



Copper 032 0-34 



Silver 0.41 0.43 



Zinc 0.63 0.60 



Lead 0.64 0.67 



Nickel 0.27 0^24 



Cobalt 0.27 0.25 



Palladium 0.35 0.34 



Iridium 0.18 0.18 



Platinum 0.60 0.65 



Manganese 0.62 0.60 



It may be noted here that the anodic overvoltage of lead 

 Ciovered with peroxide corresponds with the cathodic over- 

 voltage of a bivalent metal. According to Liebenou's theory of 



