Partition equilibria are established for a solute that is exposed to two dissimilar phases.

A classic example of this is the liquid-liquid extraction separation process. If water has some amount of a relatively non-polar substance such as caffeine dissolved in it, the caffeine can be removed by adding a non-polar solvent such as ether and shaking the mixture. The caffeine is more soluble in the ether; most of it will dissolve in the ether, and thus be removed from the water. When water and ether are mixed with caffeine present, an equilibrium is established in terms of the caffeine.

The equilibrium constant of this kind of equilibrium is simply the ratio of the solubility of the solute in one solvent divided by the solubility in the second solvent.

In this equation, S_a is the solubility of the solute in the more soluble solvent and S_b is the solubility of the solvent in the second solvent. This equilibrium constant is also termed a distribution coefficient. The relationship becomes complicated when the solute reacts in one of the liquids.

This concept is of significant interest in the separation field with various chromatographic techniques. Many chromatography techniques fit into the area of partition chromatography either because two liquids are used or because one liquid is held on a solid surface while another moves over that surface.

Akron has an excellent Web resource on solvent extraction.