Conditional (formal) potential, \(E^{0'}\), 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: \[E_{\mathrm{cell}}=E_{\rm{c}}^{0'}- \frac{R\ T}{n\ F}\ \sum _{\begin{array}{c} i \end{array}}\nu _{i}\ \ln c_{i}\]