379 



The potential ditFerence, which we therefore measure at the pla- 

 tinum electrode in the case supposed here, is (lie potential difference 

 for the three-phase equilit)rium dc'e', in which a repi-esents the 

 hydrogen phase, c the electrolyte, and e llie hydrogen-containing 

 platiiium phase. It is clear that in this binary figure it is in fact 

 impossible to indicate the composition of the j)Iatiniim electrode as 

 the electrode contains atoms and ions of platinum and hydrogen 

 as well as electrons. The composition of the electrode is in consequence 

 of this indicated in platinum and hydrogen in total. 



As was already said c lies practically on the hydrogen axis, and 

 as in the case that an attackable electrode is used as hydrogen 

 electrode, the iinattackable electrode is immersed in an electrolyte 

 which is practically free from the ions of the electrode material, 

 the concentration a\ lies likewise entirely on the hydrogen side, so 

 that like c also the point c' will practically coincide with a, i.e. 

 the different unattackable electrodes, applied as hydrogen electrode, 

 will practically present the same potential difference under the same 

 circumstances. 



The Sitpertenaion elucidated by Means of the L,x-Fi<i. 



When we immerse a smooth platinum electrode in a large quantity 

 of an electrolyte of the concentration .r,, and when we then make 

 it cathode, fig. 2 gives the successive states. Before the platinum 

 electrode is made cathode, we have electromotive equilibrium between 

 the electrolyte m and the disturbed hydrogen-containing platinum 

 phase n. As soon as the platinum becomes cathode, platinum- and 

 hydrogen ions are deposited on the metal surface, and as the 

 establishment of the equilibrium in the metal surface cannot keep 

 pace with the ion-separation, we get a platinum sui-face that is still 

 more greatly disturbed, in which there are more platinum and more 

 hydrogen ions and also more electrons present than corresponds with 

 the state of equilibrium. Hence a moment after the passage of the 

 current the point ?i' indicates the potential diffei-ence and the com- 

 position of the disturbed, hydrdgen-containing platinum electrode, so 

 that now la and n' repiesent the coexisting phases. 



With increasing density of the cui-rent the electromotive two- 

 phase equilibrium moves continually upwards in our A,.r-figure, and 

 it might be thought that the hydrogen can be separated for the first 

 time as phase at the very moment that the line indicating the elec- 

 trolytes that can coexist with a platinum electrode of definite 



