Equilibrium of a Species Between Two Phases in Contact

Consider Eq. (2.4) for the case of a species at equilibrium in two different phases - for instance, an electron in a copper wire and a nickel wire welded together. Since equilibrium is assumed, we can write 

In these and all following equations the subscript represents the species and the superscript shows the phase in which it is. 

The physical significance of Eq. (2.8) is that, at the contact between two dissimilar metals, a certain potential difference will develop, generated by the difference in chemical potential of the electrons in the two metals. It might at first seem odd to have a potential drop inside a metal wire, (even if it is made of two metals welded together), unless a current is flowing. In the present case, however, one might consider ( ) as 

Consider a monovalent metal ion in solution, at equilibrium with the same ion in the crystal lattice. In this case 

Remembering that the activity of a species in a pure phase, Jj+, is always defined as unity, we can rewrite Eq. (2.12) in a simplified form as follows  

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