Conditional (formal) potential, $E^{0^{\prime }}$, is related to the potential of a cell reaction, $E_{\mathrm{cell}}$, by an equation analogous to that relating $E_{\mathrm{cell}}$ to the standard potential $E_0$, with the activity, $a_i$, replaced by any composition variable, $c_i$ (to be indicated by a subscript). For example: $$E_{\mathrm{cell}}=E_{\text{c}}^{0^{\prime }}-\frac{RT}{nF}\sum _{\begin{array}{c}i\end{array}}{\nu }_i\ln c_i$$