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: \[\left(K_{\rm{D}}^{\rm{o}}\right)_{\rm{A}} = \frac{a_{\rm{A,org}}}{a_{\rm{A,aq}}}\] Its value should not vary with composition but depends on the choice of standard states and on the temperature (and eventually the pressure).