Distribution of a solute, S, between two immiscible solvents (such as aqueous and organic phases) is an equilibrium condition that is described by the following equation:
Saq 
Sorg
The equilibrium constant for this equilibrium condition is:
K = [Sorg] / [Saq]
where [Sorg] and [Saq] are the solute concentrations in the organic and aqueous phases, respectively, and the equilibrium constant, K, is called the partition or distribution coefficient.
Partitioning of a solute between two phases is the basis for extractions and chromatographic separations.