CHAPTER VIII 



Potentials at Phase Boundaries 



summary of contents 



At the boundary of two phases there generally arises a potential 

 difference which depends upon the specific distribution of a dissolved 

 electrolyte between the phases. If we assume that every ionic species 

 has a characteristic distribution coefficient, then the coefficients of the 

 cations and of the anions of an electrolyte will not be equal. Hence 

 it follows, in the first place, that the distribution law does not hold for 

 the whole electrolyte when it does hold for its individual ion components, 

 and secondly, that a potential difference arises at the phase boundary 

 and that this potential bears a thermodynamically calculable relation 

 to the electrolytic solution pressures of the various ionic species in each 

 phase. The potential difference can be calculated from the concentra- 

 tions of any ionic species common to the two phases, and the E.M.F. 

 of a chain involving two such boundaries may vary, according to the 

 conditions, from zero to a certain maximal value. The latter is com- 

 parable to that of an ordinary concentration chain with metal electrodes. 

 With respect to their electromotive behavior the different ions as well 

 as the solvents are arranged in a serial sequence. These phase boundary 

 potentials appear to be of great physiological importance, for it is prob- 

 able that many of the potential differences found in living organisms 

 are of this nature. 



53. The origin and calculation of phase-boundary potentials 



Properly speaking, electrode potentials are phase boundary poten- 

 tials as well. But here we shall consider as such only those potentials 

 which arise without the participation of metals at phase boundaries. 

 Both phases in the case to be considered shall be solutions — not, 

 as in diffusion potentials, miscible liquids which can become homo- 

 geneous through diffusion, but rather immiscible fluids separated by 

 a sharply defined boundary at the place of contact. These phases 

 can exist side by side in complete equilibrium, whereas the diffusion 

 potential is characterized by a lack of equihbrium. 



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