200 GUGGENHEIM art. e 



remark of Willard Gibbs (Collected Works, I, 429): "Again, the 

 consideration of the difference of potential in the electrolyte, 

 and especially the consideration of the difference of potential 

 in electrolyte and electrode, involves the consideration of quan- 

 tities of which we have no apparent means of physical measure- 

 ment, while the difference of potential in 'pieces of metal of the 

 same kind attached to the electrodes' is exactly one of the things 

 which we can and do measure." Unfortunately not all chemists 

 have been as careful as Willard Gibbs in avoiding the expres- 

 sion "difference of electric potential" when referring to two 

 phases of different composition. 



16. Combinations of Ions with Zero Net Electric Charge. The 

 potential [/xj of a given ionic species in a certain phase is the 

 increase in the characteristic function when one mol of the 

 given species is added to the phase, keeping all the other inde- 

 pendent variables unaltered. In particular it is the increase in f 

 when one mol is added at constant temperature and pressure. 

 If we consider, not the addition of a single ionic species but the 

 simultaneous addition or removal of several species, say the addi- 

 tion of Xi mols of the species S„ where Xi may be positive or 

 negative, then the corresponding increase in f will be ^ Xi [m]. 



i 



Making the substitution in (71) we have formally 



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Suppose now that the net electric charge of the ions added is 

 zero. The condition for this is 



2 ^i ^i = 0- (73) 



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If this condition is satisfied then (72) becomes 



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Thus, although the chemical potential of an individual ionic 

 species is indeterminate, certain linear combinations of the 



