Conditional (formal) potential, E0', is related to the potential of a cell reaction, E cell, by an equation analogous to that relating E cell to the standard potential E0, with the activity, ai, replaced by any composition variable, ci (to be indicated by a subscript). For example: \[E_{\mathrm{cell}}=E_{\text{c}}^{0'}- \frac{R\ T}{n\ F}\ \sum _{\begin{array}{c} i \end{array}}\nu _{i}\ \ln c_{i}\]