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:
Its value should not vary with composition but depends on the choice of standard states
and on the temperature (and eventually the pressure).
Source:
PAC, 1993, 65, 2373
(Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations
1993))
on page 2385
