162 



namel}', that on application of the electron equation for the potential 

 difference, metal -electrolyse, for the derivation of the relation for 

 the electromotive force of a cii'cuit consisting of two metals immersed 

 in the corresponding salt-solutions, we arrive at the following equation : 



A — A := tw • 



' ' F F {8Q 



so that, taking into considei-ation, that the first term of the second 

 member denotes the Volta-efFect of tiie two metals, which is a very 

 small quantity, the electromotive force Aj — A, will be zero in first 

 approximation, when {^^u) = (^u)- 



In the case discussed here the metals 1 and 2 are nickel and 

 hydrogen, and experiment has taught that Ay;— A/^.^ was really 

 practically zero, from which therefore followed i8,\i)L = {^hiv^L- 



Through the inertia of the metal nickel, which inertia was still 

 increased by the hydrogen dissolved in tlje metal, which is here a 

 negative catalyst, as was already stated before, the metal could, 

 therefore, be disturbed so far, that the electron concentration of the 

 nickel equilibrium in the electrolyte had become equal to the elec- 

 tron-concentration of the hydrogen. 



We may, therefore, also express ourselves in a different way, 

 and say, that the nickel had been passivated by the acid. Finally 

 the nickel phase and the hydrogen phase present the same potential 

 difference, accordingly these phases, which are in contact with the 

 same electrolyte, can coexist. As in the case discussed here the 

 nickel will of course be covered by a layer of hydrogen, the found 

 potential difference refers to the three-phase equilibrium Ni -)- H, -|- 

 -|- electrolyte. 



5. Unattachahle Electrodes. 



As follows from the communication cited here, this disturbance 

 is comparatively slowly reached for nickel. There are, however, 

 metals for which this goes much quicker, and these are the metals 

 of which the so-called unattackable electrodes consist, as the plati- 

 num metals. 



These metals belong to the group of the most inert metals that 

 we know. Even in contact with an electrolyte these metals do not 

 get in internal equilibrium, but they are almost always in passive 

 state, so that the potential difference of the unary metal is not even 

 known to us. 



When such a metal is immersed in a solution of HCl or H.^S04, 



