partition constant, \(K_{\text{D}}^{\text{o}}\)
The ratio of activity of a given species A in the extract to its activity in the other phase with which it is in equilibrium, thus: \[\left(K_{\text{D}}^{\text{o}}\right)_{\text{A}} = \frac{a_{\text{A,org}}}{a_{\text{A,aq}}}\] Its value should not vary with composition but depends on the choice of standard states and on the temperature (and eventually the pressure).
See: distribution constant, transfer activity coefficient
PAC, 1993, 65, 2373. 'Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations 1993)' on page 2385 (