815 



metals ions occur of different valency. In active state the metal 

 contains chiefly ions of lower valency, in passive state ions of 

 higher valency. 



According to Smits ') passivity is caused by a deficit of metal 

 ions in the metal in consequence of the splitting up m — ► m + -|- ^ 

 proceeding more slowly than the removal of metal ions from the 

 metal in the anodic solution. Le Bi,anc's and S.mits's views have the 

 advantage that they cannot only account for the anodic passivity, 

 but also for the abnormal polarisation voltages occurring in the 

 cathodic deposition of some metals, the overvoltage in case of 

 gas generation, and more. Besides, gas charges of the metal or 

 solved substances in the electrolyte can also exert a positive or 

 negative catalytic influence on the velocity of the establishment of 

 the homogeneous equilibrium. 



3. The potential of equilibrium of chromium. 



The most characteristic property of passifiable metals is, that 

 during anodic and cathodic polarisation in solutions of their salts, 

 they present abnormal polarisation voltages, i.e. voltages which are 

 greater than can be accounted foi' by concentration polarisation in 

 the liquid. 



For the right understanding of the electromotive behaviour of 

 chromium, in the first place the knowledge of the equilibrium potential 

 is therefore required. 



The records in the literature are very divergent. 



HiTTORF gives 1.184 V for the E. M. F. of the combination 

 Cr I HCl I Na NO, \ Ag NO, | Ag. The normal potential of silver 

 is -|- 0,80 V. From this would follow for chromium in hydrochloric 

 acid Eh = — 0,38"). According to Neum.\nn ') Eh in chromicsulphate is 

 — 0,50, in chromicchloride — 0,48. Chromium amalgam gives the 

 highest negative value. Rathekt ^) found for chromium according to 

 GoLDscHMiDT in 0.1 n H^SO^ Eh ■= — 0,39, for chromium, which 

 was deposited electrolytically according to Carveth and Curhy') 



•) Zeitschr. f. physik. Chemie, 88, 743, (1914); 90, 723, (1915); 92, 1. (1916). 



2) When calculated with respect to tlie hydrogen electrode = 0, the potentials 

 are denoted by Eh. The measurements have all been made with the normal calomel 

 electrode The potentials indicated in the following chapters by £" give the potential 

 difference of the combination Cr | electrolyte | satur. sol. KCl | n. Calomel electrode. 



3) Zeitschr. f. Elektrochemie 7, 656, (1901). 



*) Zeitschr. f physik. Chemie 86, 567, (1914). 

 ') Journ. f. phys. Chemistry 9, 358, (1905). 



58* 



