## partition constant, $$K_{\text{D}}^{\text{o}}$$

https://doi.org/10.1351/goldbook.P04438
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
Source:
PAC, 1993, 65, 2373. (Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations 1993)) on page 2385 [Terms] [Paper]