565 



3, The experimental electrical potential. 



When we measure flie potential difference metal-electrolyte, we 

 do so by the aid of an auxiliary electrode, e.g. a calomel or hydrogen 

 electrode, in other words, we then make an electrical circnit, which 

 is closed during the measurement. The electro-motive force of this 

 circuit, in which we suppose the diffusion potential between L^ 

 and L^ annulled, then becomes: 



in which ^Mi-M^i is a Volta-effect. 



When in this we put ^m^-u ^= ^^ero (N. hydrogen-electrode), we 

 get : 



in wJiich we shall call E the e.rperimental electrical potential. 



This expression, therefore, always contains the Volta-effect, and 

 until this quantity is knowji, and until we introduce further sup- 

 positions about its value, we can of course not determine the differ- 

 ence ^Mi-Li alone in this way, and consequently not the saturation 

 concentration of the metal-ions either (which quantity is equivalent 

 with Nernst's "Lösungstension"), which was calculated froni : 



E=z In ~^~ (1) 



^'.^' {Mr:') ■ 



in which Ljt^^_[^^ has been neglected. 



Nor can we draw conclusions about the order of the satui-ation 

 concentrations of the metal-ions from the so-called tension series, 

 until the Volta-effect shall be liuown. From the fact that the metal 

 3/j immersed in a normal solution of one of its salts appears on 

 measurement to be more negative than the metal J/.^, immersed in 

 a 1 norm. -solution of a corresponding salt, we conclude namely that: 



Aj/i "Ai is more negative than A^/, /^,, 

 but strictly speaking this conclusion is not permissible, because the 

 measurement only says that ^Mx-l^ — ^m^-m-, '■'* more strongly nega- 

 tive than ^M^-L, ' 



We demonstrated that the equations for the exp. elec. potential 

 in the form in which they contain the saturation-concentration of 

 the ions or the "Lösungstension", have the drawback that the un- 

 known Volta-effect occurs in them. It is entirely different with the 

 new already before given equation, in which the .^ioluhilitii product 

 of the metal or the solubility quotient of the metalloid occurs, and 

 the same thing may be said of the electron equation. ') 



1) Zeilschr. f. physik. Chemie 92, 1 (1916). 



37* 



