398 



The maxiiniiiri negative potentials are however no eqnilibrium 

 potentials, that follows already from this, that the potential of jMg 

 activated by aiiialgation in a solution of 1 gr. mol of MgSO^ per 

 litre, is more negative, i.e. — 1.856 Volt, instead of — 1.790 Volt, 

 which value will also lie still below the real normal potential of 

 equilibrium of Mg, as will be shown below. 



Maffnesium. 



After this introduction we shall examine the metals Mg more 

 closely. 



The difiiculties which are usually encountered in the study of 

 the electromotive behaviour of magnesium and aluminium are owing 

 (o the fact that extraordinary phenomena appear when the usual 

 methods of determining the equilibrium potential are ap|)lied to 

 these strongly Imsio nietaLs. 



For example, suppose that the Mg potential is — J. 86 Volt. Since 

 the Mg electrode develops hydrogen, this means that the above 

 potential corresponds lo the potential of the three phase eqnilibrium, 

 magnesium (inner equilibrium) — hydrogen (by inner equilibrium) 

 and the surrounded liquid layer. 



The liberation of gaseous hydrogen takes place because hydrogen 

 ions from outside diffuse into the surrounding liquid layer and com- 

 bine with the electrons. 



The above assumption holds for Fe and Zn because it can be 

 shown by calculation that the surrounding liquid layer can coexist 

 with metal and hydrogen, the two latter in inner equilibrium. 



If however we take now strongly basic metals, we can see that 



(M") 

 the quotient would be so large, that the electrolyte would 



(2H ) 



become inconsistent. 



The question now arises: "Can the above negative potential ( — 1.86 

 Volt) be the potential of magnesium and unary hydrogen (that is 

 to say hydrogen in inner equilibrium) with respect to the surround- 

 ing liquid layer containing say 1 gr. ion Mg per litre." 



0.058 

 Applying the formula E = — log. L^^^^ — 2.8') and sub- 

 stituting for E the value — 1.86 we can calculate that 



L^,^ = (Mg^) ((9^)' ==10^^-16. 



If we consider that for hydrogen in inner equilibrium 



1) The Theory of Allotropy p. 123. 



