159 



2. The Potential Difference Metal- lilectro/i/te jvhm 

 the Metal is Attacked. 



Wlien a metal in contact with an electrolyte superficially assumes 

 internal equilibrium with very great velocity, the internal state in 

 the metal surface remains unchanged, in whatever way the metal may 

 be attacked. 



Let us suppose that we immerse zinc in an aqueous solution of 

 hydrochloric acid; then hydrogen generation takes |)lace, because 

 the electron concentration of the metal e(|uilibrium 



Zv,^ :^Zv,^ + Wl (1) 



in the solution is greater than the eleclron-concenti'ation of the 

 hydrogen equilibrium : 



//,,^^2//i4-2^L- (2) 



Hence the electrons of the equilibrium (1) are removed, and 

 through this the equilibi'inm is disturbed. It is now the question 

 how the equilibrium can be restored. 



The concentration of the zinc-atoms in the liquid is so small that 

 even if the reaction constant of the conversion 



Zuf^ _> Zni, + 26»/, 

 was very large, yet only exceedingly few zinc ions and electrons 

 would be split off per second in this way. 



The only way in which the state of equilibrium can be restored 

 is this that the metal sends electrons into solution, which is of course 

 accompanied by zinc ions going into solution, because zinc-ions and 

 electrons, with a difference of only a vei-y small amouni, are always 

 present in the same concentration. 



Through this process the internal equilibrium in the metal surface 

 is disturbed, which can be restored again by the reaction : 



Zvs -► Z}ï, + 2^, . 



As the heterogeneous equilibi'iuni in the l)0uii(lary layei- sets in 

 with very great velocity, the (piestion whether the metal ziiu* dni-ing 

 solution in an acid will be disturbed, comes to this, whether the 

 internal e<juilibi'ium in the surface of the metal sets in with so 

 gi-eat velocity that the concentration remains practically unchanged. 



This is actually the case for zinc under certain circumstances. 

 Mr. HüFFER S. J., who examined some metals at my request, found 

 among others, that when the potential difference between zinc and 

 a solution of zinc-chloride is measured during vigorous stirring, and 

 then that between zinc and a zinc-chloride solution of the same 



