160 



concentration acidified with lijdrochloric acid, the, potential dijference 

 retains the same value, notiinthsfamJinc/ a strong hydrogen-generation 

 takes place in the latter case. 



The metal zinc is, tlierefoie, not distnrbed throngh solution in 

 hydrochloric acid, and this resnlt is in pei-fect agreement with what 

 is fonnd when zinc is anodically brought to solution in a zinc- 

 chloride solution. In this process, which likewise rests on the with- 

 drawal of electrons from the metal, the potential difference, zinc- 

 electroljte, does not change appreciabl}-, even for compaiatively 

 great densities of current, so that our investigations about the |)ütential 

 difTerence during the solution of zinc in a hydiochloric acid solution, 

 as well as the measurements of the potential difference of the same 

 metal on anodic solntion in a solution of zinc-chloride lead to the 

 residt that the equilibiium in the metal zinc in contact with the 

 above-mentioned electrol^'te sets in with a velocity which is very 

 great compared with tlie velocity with which electrons and ions are 

 withdrawn from the metal. 



3. General consideration. 



When we. now consider the phenomenon in general, we can 

 distinguish the following cases. • 



On immersion of a metal in an acid we have in the simplest 

 case among others the two following equilibria in the electrolyte: 



and 



Ml ^ .I/'a + ^L 



The electron-concentrations of these two equilibria are in general 

 different, and a consequence of this is that either the electrons of 

 the metal equilibrium, in the liquid, combine with the hydrogen 

 ions of the hydrogen equilibrium, which causes electrons -j- ious 

 from the metal to go into solution, or the electrons of the hydrogen 

 equilibrium with the metal ions of the metal equilibrium pass fi'om 

 the electrolyte into the metal. 



Let us first imagine the limiting case, viz. this that the internal 

 equilibrium of the metal surface is established with great velocity, 

 so that this velocity is very great with respect to the velocity with 

 which electrons -j- ions are withdrawn from the metal or are added 

 to the metal, then the metal surface will not change independent 

 of whether one process takes place or the other, and the potential 



