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