(3 ) 



The zinc emitting' positive Zii-ion^, the surrounding solution becomes 

 electro-positive, and the zinc itself electro-negative. As known, this 

 gives rise to the formation of a so-called electric double-layer in 

 the bounding-layer between the metal and the electrolyte, consisting 

 of positive Zn-ions on the side of the electrolyte and an equivalent 

 amount of negative electricity or electrons in the metal. 



By the formation of this electric double-layer an electric potential 

 difference between metal and electrolyte is brought about, which at 

 first increases, but very soon becomes constant. This takes place 

 when the potential difference has become great enough to prevent 

 the further solution of the Zn-ions. 



In order to compute the potential difference between the metal 

 and the solution, we shall apply the principle of the virtual dis- 

 placement, as has been done before by Mr. van Laar. ^) 



If we have to do with a purely chemical equilibrium then with 

 virtual displacement of this equilibrium the sum of the changes ot 

 molecular potential will be = 0, which is expressed by the equa- 

 tion of equilibrium : 



If the equilibrium is a purely electrical e(|uilil»riiiin then with 

 a virtual displacement of this equilibrium the sum of the changes 

 of electric energy will be = 0. 



If however we have an equilibrium that is neither purely che- 

 mical, nor purely electrical, but a combination of the two, as is 

 the case with electromotiv^e equilibrium, then with virtual displace- 

 ment of this equilibrium, the sum of the changes of the molecular- 

 potential -j- the sum of the changes of the electric energy will 



have to be =z 0. 



+ 

 If we represent the mol. potential of the Zn-ions by ;/;„ in case 



+ 

 of electromotive equilibrium, we know that (i~,t is much smaller 



than Hsn or the mol. potential of the zinc in the bar of zinc. 



If we now suppose that a Zn-ion emitted by the zinc virtually 

 carries a quantity of electricity de from the metal towards the solu- 

 tion, then this quantity of electricity being carried by a ponderable 



de 

 quantity" — when v = valency of the metal and e = the charge 

 ve 



of a univalent ion, the increase of the thermodynamic potential 

 during this process will be equal to 



1) Chem. Weekbl. N'. 41, 1905. 



1* 



