# What is the reasoning behind deriving electrode potential of a metal-metal sparingly soluble salt electrode?

Considering a sparingly soluble salt $$M_xN_y$$ of the metal $$M$$, in which the corresponding reaction is $$M_xN_y$$ = $$xM^{y+}$$ + $$yN^{x-}$$, the standard electrode potential is given by $$E^{o}_{M_xN_y/M/N^{x-}}$$ = $$E^{o}_{M^{y+}/M}$$ + $$\frac{RT}{xy\times 96485}$$ $$\ln{K_\text{sp}}$$.

This derivation is done by writing $$E_{M_xN_y/M/N^{x-}}$$ = $$E_{M^{y+}/M}$$, i.e. equating the electrode potentials (not the standard) of $$M_xN_y/M$$ and $$M^{y+}/M$$. I do not understand the reason behind doing so. Why are they equal at this instant?