When wishing to use simple concentrations in place of activities, but that the activity coefficients are far from unity and can no longer be neglected and are unknown or too difficult to determine, it can be convenient to introduce the notion of the "so-called" standard formal reduction potential ( E red ⊖ ⊖ ′ ) which is related to the standard reduction potential as follows: E red ⊖ ⊖ ′ = E red ⊖ ⊖ − − R T z F ln ⁡ ⁡ γ γ Red γ γ Ox So that the Nernst equation for the half-cell reaction can be correctly formally written in terms of concentrations as: E red = E red ⊖ ⊖ ′ − − R T z F ln ⁡ ⁡ C Red C Ox and likewise for the full cell expression.