Chemistry. — ''The Passivity of Chromium". (Second Cornmii- 

 iiicatioii). By Dr. A. H. W. Aten. (Communicated bij Prof. 

 A. F. Holleman). 



(Communicated in the meeting of Jan. 26, 1918) 



In the first communication on this subject ') the potentials were 

 discussed, which different kinds of chromium present, when they are 

 in contact with an electrolyte in a curreulless condition. This second 

 paper gives the results of determinations of the potential presented 

 by chromium in anodic and cathodic polarisation. 



'1. Anodic polarisation of electrolytic chromium, in solutions of 

 chromous-sulpliate. 



The phenomena that make their appearance on anodic polai-isation 

 of electrolytic chromium, are dependent on the nature of the metal 

 on which the chromium has been deposited. The results given below 

 refer to chromium that has been deposited from a solution of chromic 

 chloride on copper. The measurements took place according to the 

 meihod of Le Blanc 'j. The commutator used in this was arranged 

 so, that the electrode was polarized during a certain time, was then 

 currentless for a time of equal duration, was then connected with 

 the compensation apparatus, and was finally currentless again. 



The potential as function of the strength of the current had in 

 this the following course. In currentless condition the potential 

 was — 0.55. With a very feeble current the potential rose to -f- 0.1, 

 and increased only little when the current was strengthened. 



Above a certain limiting value of the strength of the current the 

 potential rapidly rose again to -{- 0.7 V, and increased but little on 

 further strengthening of the current. This course is represented by 

 line I in figure i. It appears from the shape of the line that from 

 ^ to Ö a slowly progressing reaction takes place at the anode, 

 from B io C a, rapidly pi-oceeding reaction. Between C and D the 

 limiting current for this reaction is reached. At D a new reaction 



1) These Proc. Oct. 1917. 



«) Zeitschr. fur physik. Chemie. 5, 469 ^1890). 



