The distribution constant (or partition ratio) (KD) is the equilibrium constant for the distribution of an analyte in two immiscible solvents.
The ratio of activities of a solute, A in an aqueous/organic system will remain constant and independent of the total quantity of A (hence
), so at any given temperature: Distribution constants are useful as they allow the calculation of the concentration of remaining analyte in the solution, even after a number of solvent extractions have occurred.
They also provide guidance in choosing the most efficient way to conduct an extractive separation.
Thus, the concentration of A remaining in an aqueous solution after i extractions with an organic solvent can be found using: (where [A]i is the concentration of A remaining after extracting Vaq millilitres of solution with the original concentration of [A]0 with i portions of the organic solvent, each with a volume of Vorg).