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