Distribution constant

Expression

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

:(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:

:[A]i = \left({V{aq} \over V_{org} K_D + V_{aq}}\right)^i [A]_0

(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).

References

References

1. (2009). "IUPAC Compendium of Chemical Terminology".
2. (1993). "Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations 1993)". Pure and Applied Chemistry.
3. (5 November 2014). "Practical Gas Chromatography: A Comprehensive Reference". Springer.