Chemistry. ''The Phenomenon Electrical Stiperfension". Hy Prof. 

 A. Smits. (Communicated l\v Prof. P. Zkkman). 



(Gommunicaled in the meeting of June 29, 191 S). 



It has alread}' been pointed out iu a previous coinuuiiiicatiou ') 

 that the jnetals which furnish the so-called unattackable electrodes, 

 differ from the other metals iu this that they are ideally inert, so that the 

 potential difference of such a metal electrode with respect to an 

 electrolyte is governed by the pievailing electron-concentration in 

 this electrolyte. Let us now suppose that a smooth platinum-electrode 

 immersed in an aqueous solution of hydrochloric acid, is made 

 cathode, it is then easy to see what will happen. 



The two equilibria that ai'e to be considered here, are: 



Pt^Pt' * ■ ■ +4<^ 

 and H,:^2H + 2<9 



the former of which is entirely determined by the electron concen- 

 tration of the hydrogen equilibrium. 



When we immei-se a platinum electrode in a solution of hydrochloric 

 acid, the platinum ion-concentration in the electrolyte will be 

 imperceptibly small. Yet we can speak of a platinum equilibrium in 

 the electrolyte, which, as was already remarked, is entij-ely 

 determined by the electron-concentraticn of the hydrogen equilibrium. 



In virtue of this it may be said that platinum is a hydrogen- 

 electrode from the very first, but so long as the hydrogen has not 

 yet appeared as second phase, the platinum will be a hydrogen- 

 electrode, corresponding with a hydrogen pressure smaller than the 

 pressure under which the electrolyte is. 



When we make the platinum cathode, there aie electrons added 

 to it, and a consequence of this will be that hydrogen-ions from 

 the electrolyte are deposited on the metal surface, and are dissolved 

 in it, from which it appears that the hydrogen is of course not 

 immediately present as a new phase. When the internal equilibrium : 



sets in very i-apidly on the metal surface, this internal e(piilibrium 

 would already have been established in the metal surface in spite 



1) These Proc. 



