Chemistry. — ''On the Periodic Passivity of Iron, 11". By Prof. 

 A. Smits and C. A, Lobry dk Bruyn. (Coinnmiiicated by 

 Prof. P. Zeeman). r ^-. ^ ^ yw^ k aV 



(Communicated in the meeting of June 29, 1918). 



Periodic passivity in experiments with sealea-in iron electrodes. 



In a previous communication ^) on this subjoct we have shown 

 how we have succeeded in calling forth the phenomenon of 

 periodic passivity on anodic poUirisation of i)-on in a solutioji of 

 0,473 gr. mol Fe SO, -|- 0,023 gr. mol Fe Cl, per litre. In these 

 experiments we made use of an iron electrode 0.3 cm. long with 

 an area of db 0.3 cm*, which was sealed into the sliort leg of a 

 U-shaped tube by means of shellac. The considerations that led us 

 to these investigations were the following. During the anodic solution 

 of iron in a solution of Fe SO^ the internal equilibrium in the 

 metal surface above a certain density of current, can be disturbed 

 so greatly that passivity appears. When into the solution CI, Br., or 

 1-ions are introduced in a suflicient concentration, which need, 

 however, be only exceedingly small, activation of the iron suddenly 

 makes its appearance. It follows from this that for a definite density 

 of current, given by the velocity of solution of the iron, it must 

 be possible to find a halogen-ionconcentration, for which at a definite 

 moment the chance that the iron remains passive, is equally great 

 as the chance that it becomes active. 



When at this moment the density of current is slightly diminished, 

 the transition passive-active is sure to take place. 



The iron anode in the passive state will dissolve only exceedingly 

 little, the iron, which has now become active, will, however, go 

 very greatly into solution. 



Tn consequence of this the contact of the halogen-ions with the 

 iron will diminish, and as the iron is now almost entirely with- 

 drawn from the catalytic influence of the halogen-ions, it can again 

 pass into the passive state. 



Since, however, as has been said, the passive iron dissolves very 

 little, and the processes which now take place at the anode consist 



1) These Proc. 



