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| The '''distribution constant''' (or '''partition ratio'''), is the [[equilibrium constant]] for the distribution of an [[analyte]] in two [[immiscible]] [[solvent]]s.<ref name=Rice1993>{{citation | last = Rice | first = N. M. | year = 1993 | title = Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations 1993) | journal = Pure and Applied Chemistry | volume = 65 | pages = 2373 | doi = 10.1351/pac199365112373 | last2 = Irving | first2 = H. M. N. H. | last3 = Leonard | first3 = M. A. | issue = 11 }}</ref> For a particular solvent, it is equal to the ratio of its [[molar concentration]] in the [[Stationary phase (chemistry)|stationary phase]] to its molar concentration in the [[mobile phase]], also approximating the ratio of the [[solubility]] of the solvent in each phase. The [[technical terminology|term]] is often confused with partition [[coefficient]] or distribution coefficient, and is often represented by K<sub>D</sub>.
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| ==Expression==
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| The ratio of [[activity (chemistry)|activities]] of a [[solution|solute]], A in an [[aqueous]]/[[organic compound|organic]] system will remain constant and independent of the total [[quantity]] of A (hence <math>[A]_{org} \propto [A]_{aq}</math>), so at any given temperature:
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| <math>(K_D)_A = {(a_A)_{org} \over(a_A)_{aq}} \approx {[A]_{org} \over[A]_{aq}}</math>
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| Distribution constants are useful as they allow the calculation of the concentration of remaining analyte in the solution, even after a number of [[solvent extraction]]s have occurred. They also provide guidance in choosing the most efficient way to conduct an [[Liquid-liquid extraction|extractive separation]].
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| Thus, the concentration of A remaining in an aqueous solution after ''i'' extractions with an organic solvent can be found using:
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| <math>[A]_i = ({V_{aq} \over V_{org} K_D + V_{aq}})^i [A]_0</math> | |
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| (where [A]<sub>i</sub> is the concentration of A remaining after extracting V<sub>aq</sub> [[millilitre]]s of solution with the original concentration of [A]<sub>0</sub> with ''i'' portions of the organic solvent, each with a volume of V<sub>org</sub>). | |
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| ==References==
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| {{Reflist}}
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| ==External links==
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| *[http://goldbook.iupac.org/P04440.html IUPAC Definition of ''partition ratio'']
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| {{chromatography}}
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| [[Category:Chromatography]]
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Revision as of 19:28, 4 March 2014
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