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}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/1d996bf761ee7079e170b8a94fd3f3a6bb63a6a8)
), so at any given temperature:
![{\displaystyle (K_{D})_{A}={(aA)_{org} \over (a_{A})_{aq}}\approx {[A]_{org} \over [A]_{aq}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/5c669fb1edb9ff07b15b57179aa71725a62106bb)
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}=\left({V_{aq} \over V_{org}K_{D}+V_{aq}}\right)^{i}[A]_{0}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/3db8d36364746bb883df42eb890109f9afab1ddf)
(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).
