The concentration of all species involved in the species involved in the electrode reaction is unity.This need not be always true.
Nernst shows that for the electrode reaction:
the electrode potential at any concentration measured with respect to standard hydrogen electrode can be represented by:
but concentration of solid M is taken as unity as we have
R is gas constant (8.314 JK–1 mol–1),
F is Faraday constant (96487 C mol–1), T is temperature in kelvin and [Mn+] is the concentration of the species, Mn
Let us take a electrode reaction
The Nernst equation of this electrode
Instead of activity, we can take molar concentration.
For pure solid and liquid molar concentration is taken as unity.