188 HYDROGEN ION CONCENTRATION 



acetate-ion yielding gas to use for our electrodes. And so k2 and 

 ki, are in reality nothing but integration constants for purposes of 

 calculation. In fact, k2/ki is but a single constant to which we 

 attribute the same significance. Hence we can state that 



^&^ 



TT = RT In — + K (1) 



C2 



And yet this constant K does possess important meaning, its 

 value relates specifically to the ion-species, being a definite charac- 

 teristic value of every ion-species. 



If a current-yielding chain be arranged to contain two such phase 

 boundaries with a potential at each, then the total potential E of 

 the system will be 



E = ,r - TTi = RT In — + K - ( RT In ^' + K' ) 



C2 y C 2 J 



Only in that case, where all the phases have only one ion-species 

 in common will the individual values of K be identical and disappear 

 and Ci, C2, c/, 0,% will then signify only the concentrations of the 

 common ion. 



But if the various phases involved contain more than one ion- 

 species in common the equation can just as well be appHed to each 

 of these ions. But since in all combinations of phases in which 

 water is included H-ions are present, it would follow for all such 

 chains 



rTT+] 



TT = RT In —-^ + Constant 



IH'^J2 



where the constant depends upon the nature of the solvent only. 

 This result may be easily deduced from Beutner's general equation 

 (even though Beutner himself did not come to this conclusion). 

 Nevertheless it is still very difficult to determine experimentally the 

 hydrion concentration in non-aqueous phases. 



So much for the thermodynamic explanation of phase boundary 

 potentials. It will be worth while to add another method of repre- 

 sentation, that of Nernst, which is the older one in point of time. 

 It follows from Nernst's conception of the distribution of the salt 

 in the two solvents. The final state attained corresponds, as was 

 shown above, neither to the true distribution equiUbrium of the 



