The factor by which the ratio of two substances in the feed must be multiplied to give their ratio after treatment. \[\frac{Q_{\rm{A}}}{Q_{\rm{B}}} = S_{\rm{A,B}}\ \frac{Q_{\rm{A}}^{'}}{Q_{\rm{B}}^{'}}\] where \(Q_{\rm{A}}\) and \(Q_{\rm{A}}^{'}\) are the final and initial amounts of species A and \(Q_{\rm{B}}\) and \(Q_{\rm{B}}^{'}\) are the final and initial amounts of species B. Hence \(S_{\rm{A,B}}=\frac{E_{\rm{A}}}{E_{\rm{B}}}\) where \(E\) is the fraction extracted. In terms of \(D\),\(n\),\(r\) (where \(n\) is the number of stages and \(r\) the phase ratio), \[S_{\rm{A,B}} = \frac{1- (1\,+\,r\ D_{\rm{A}})^{-n}}{1- (1\,+\,r\ D_{\rm{B}})^{-n}}\]