POTENTIALS AT PHASE BOUNDARIES 



185 



logical import of their ideas and pointed them out.^ R. Beutner/ a 

 former student of Haber's and under the encouragement of Jacques 

 Loeb, has recently given us a substantial development of this prob- 

 lem, which we shall discuss next. But we must also give full credit 

 to the theories of Nernst and Haber. While there are no fundamental 

 differences between them, they are both indispensable. 



We shall now show thermodynamically that, in general, between 

 two solutions consisting of immiscible solvents a potential difference 

 must arise, even though they be in a state of chemical eciuilibrium 

 with respect to each other. Let us, for example, shake up portions 

 of water and of guaiacol or of amyl alcohol with some HCl, so that 

 the electrolyte is distributed in the two solvents and is consequently 

 in equiUbrium. The concentration of HCl in the solutions is 

 different. 



Now we shall transfer the mixture of these two 

 solutions into a U-tube, in which they will sepa- 

 rate with a sharply defined boundary between 

 them. If we now dip some metal electrodes into 

 the upper ends of the solutions and connect them 

 by a metal conductor, we shall obtain an arrange- 

 ment of a galvanic chain (fig. 24). We shall 

 choose for our electrodes hydrogen-platinum 

 electrodes which, in contact with a solution con- 

 taining H-ion, have a well defined potential cal- 

 culable from Nernst 's equation. No electric current can originate 

 in this chain, since it is a system in a state of complete equilibrium. 

 Any current should bring about a change in the concentration of the 

 electrolyte, but since we have here a state of equihbrium, no spon- 

 taneously occurring process can be produced which would result in a 

 disturbance of the equihbrium. This is an important part of the 

 second law of thermodynamics. Consequently the E.M.F. of this 

 chain must be equal to zero. 



On the other hand, we can also estimate the potentials at the 

 electrodes. At electrode I the potential is 



Fig. 24 



TTi = RT In 



Si 

 ki 



^ Cf. L. Michaelis, Dynamik der Oberflachen. Dresden 1909. 

 * R. Beutner, Die Entstehung elektrischer Strome in lebenden Geweben. 

 Stuttjrart 1920. 



