Distribution constant

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 ${\displaystyle [A]_{org}\propto [A]_{aq}}$), so at any given temperature:

${\displaystyle (K_{D})_{A}={(aA)_{org} \over (a_{A})_{aq}}\approx {[A]_{org} \over [A]_{aq}}}$

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:

${\displaystyle [A]_{i}=({V_{aq} \over V_{org}K_{D}+V_{aq}})^{i}[A]_{0}}$

(where [A]_i is the concentration of A remaining after extracting V_aq millilitres of solution with the original concentration of [A]₀ with i portions of the organic solvent, each with a volume of V_org).