POTENTIALS AT PHASE BOUNDARIES 187 



that in an aniline chloride solution. Hence it is not possible that the 

 distribution of Cl-ions should always be the same regardless of the 

 kind or of the amount of chloride present, and consequently the 



ratio — cannot be constant under all conditions. 



C2 



Practically, this point was demonstrated by R. Beutner as follows : 

 when water and an oil phase, such as guaiacol, were shaken on the 

 one hand with NaCl, and on the other hand with aniline chloride, 

 it was found that the aqueous phase took up more CI when NaCl 

 was used than when aniline chloride was used. Evidently that 

 which apphes to CI as such need not by all means apply to Cl~-ions, 

 and the same was found when the conductivity of the guaiacol solu- 

 tions in the two cases was determined. In the case of the aniline 

 chloride the Cl-ion content of the guaiacol was much higher than 

 in the case of NaCl. Hence the distribution of Cl-ions, or, in general 

 of any ion species, between water and guaiacol is not in all cases 

 the same. 



Since the ratio Ci/c2 in equation (I) represents a constant value, 

 and ki/k2 being a constant, then tti — 7r2 must always be other than 

 zero. But since the E.M.F. of the whole chain must be zero, it 

 follows that a third potential difference tts must be present, and it 

 must amount to tti — W2. The algebraic sum of these three values 

 must be equal to zero. This potential xs resides in the boundary 

 surface of the water and oil phases. 



The potential difference at the boundary of two phases which 

 contain an ion in common in the concentrations Ci and C2 is 



TT 



RT 



Here k2 is that concentration of this ion in phase II against which 

 a metal electrode of the same kind would produce zero potential, 

 and ki is the concentration of the same ion in phase I against which 

 a metal electrode of the same kind would likewise 3deld zero poten- 

 tial. Or, stated otherwise, ki and k2 are the electrolytic solution 

 tensions of the ionic species concerned in phases I and II respectively. 



Thus these two values, k2 and ki, are quite problematical quanti- 

 ties which frequently have no physical meaning. For, if we should 

 wish to use, for example ammonium acetate as the electrolyte, we 

 find that we have available neither an "ammonium-metal" nor an 



