Distribution Ratio:

One particular aspect that should be amply clear to you that the distribution of a species in the two competing phases is not governed by a simple equilibrium. The situation is complicated by the chemical interactions in the two phases and they can eventually markedly affect the concentration of the distribution species. In this context, you have already been told the case of distribution of benzoic acid between water and benzene.

It is rarely observed that a single species may be present in the two phases. But as an analytical chemist, for the purposes of separations, you are concerned with the overall or stoichiometric distribution of the component of interest in the two phases. In whatever chemical forms the solute remains in the two phases, it is immaterial. Also, it is easier to determine the overall distribution of the component in the two phases.

Therefore, to describe the extraction there is a more practical quantity known as "Distribution Ratio" (D).

D =   Total concentration of S in organic phase/Total concentration of S in aqueous phase

In an ideal condition, if there is no interaction in either of the phases, D would be the same as K_D. If all the interactions of the distributing species are known, they can be evaluated and an expression showing the effect of various parameters on D can be obtained. In order to illustrate the point, we discuss two equilibria one from organic and the other from inorganic chemistry.