704 



electrolyte, however, is dependent on the coneeiitnitioii ; 6jy resp. 6^.1/2 

 varies witli tlie total concentration, and together witii it accoi'ding 

 to equation (2) also the potential difference A. If, therefore, also the 

 total metal-ion concentration has been fixed, everj- thing is perfectly 

 dete rminate. 



Let us put that for given T, P and total metal-ion concentration, 

 for which our Fig. 1 holds, the internal equilibrium between the 

 metal-ions is indicated by the point Z, it follows from the A, .r- figure 

 that for unarj^ electromotive equilibrium the solution L will coexist 

 with the metal pha^e S for a potential difference indicated by the 

 situation of the line LS. It has been said that the internal equilibrium 

 in the metal phase is solely determined by T and P, i.e. the point 

 y, but the situation of S in the A, .v-figure depends of course on the 

 total metal-ion concentration in the electrolyte. 



We see, however, that when we prolong the lines ac and nd 

 metastable, the same solution Z can be in unary electromotive 

 equilibrium with another metal phase for a higher potential difference, 

 viz. with the phase S' . This second unary electromotive equilibiium 

 is, however, metastable, whereas the first is stable. 



If we first of all assume that we have always to do with internal 

 equilibrium we may question what will happen when the solution 

 Z is electrolysed, while the total metal-ion concentration is kept 

 constant. It is clear that for the separation of a metal phase, in 

 which another internal equilibrium prevails than in the electrolyte, 

 a molecular transformation will be necessary, which in our case 

 consists of the reaction 2M^^f^. We now see from Fig. 1 that 

 the metastable phase S' lies much closer to the liquid L' than the 

 stable phase S to the liquid phase L. It follows from this that the 

 deposition of the metastable metal S' requires a much smaller internal 

 transformation than that of S, ami the consequence of this luill 

 be that lühen electrolytic metal depositions is carried out at tempera- 

 tures at which the velocity of transformation of the metastable modi- 

 fcation to the stable modification is small, the metastable state is 

 deposited. 



It is, therefore, seen from tiiis that for so far as Ostwald's ''6rV5^fo 

 der Umvmndlungsstufen'" holds also here, the explanation is quite 

 analogous to that given by me for the succession in the appearance 

 of different allotropic states of the same substance in the cases in 

 which the deposition was not effected by the supply of electric 

 energy ^). On that occasion I already pointed out that Ostwald's rule 



1) Zeitschr. f. phys. Ghem. 84, 385 (1U13). 



