Term: extraction (equilibrium) constant
https://doi.org/10.1351/goldbook.E02304

Definition:
The extraction constant at zero ionic strength, Kex0, is the equilibrium constant of the distribution reaction expressed in terms of the reacting species. Thus, for the gross reaction: \[\text{M}_{\text{aq}}^{n+} + n\text{HL}_{\text{org}} \rightleftharpoons  \text{ML}_{n,\text{org}} + n\text{H}_{\text{aq}}^{+}\] in which the reagent HL initially dissolved in an organic phase reacts with a metal ion Mn in aqueous solution to form a product MLn which is more soluble in the organic phase than in water, \[K_{\text{ex}}^{\text{0}} = \frac{a_{\text{ML } n,\text{org}}\hspace{2pt}a_{\text{H}^{+},\text{aq}}^{n}}{a_{\text{M}^{n+},\text{aq}}\hspace{2pt}a_{\text{HL},\text{org}}^{n}}\]

Notes:
0) When concentrations are used instead of activities or mixed terms are employed as when H+ and/or Mn are measured with an electrode, the appropriate name is @E02301@ constant, symbol Kex, accompanied by a careful definition. Kex0 may be termed the thermodynamic @E02301@ constant.
1) The @E02301@ constant is related to other terms relevant to such systems by: \[K_{\text{ex}} = \frac{D_{\text{ML},n}\,\beta_{n}\,K_{a}^{n}}{D_{\text{HL}}}\] where βn is the overall @F02485@ of MLn and Ka is the @D01801@ constant of HL. Where the @R05190@ HL is more soluble in water than the other immiscible phase it may be more convenient to define a special @E02177@ in terms of HL(aq): \[K_{\text{ex}}^{0} = D_{\text{ML},n}\,\beta_{n}\,K_{a}^{n}\]
2) In distribution equilibria involving non-aqueous systems, e.g. liquid SO2, molten salts and metals, the mass action @E02177@ for the relevant @E02301@ process can be identified with Kex which should be explicitly defined in this context.
3) In actual practice, it may be necessary to include other terms to take into account other complexes formed by auxiliary reagents and the @S05747@ and/or @P04740@ of the various species. In such cases, Kex must be defined with reference to the relevant explicit chemical equation. An example is complex formation between the metal ion and an uncharged @C01421@ ether or @C01426@ molecule followed by ion-pair @E02301@: \[\text{M}_{\text{aq}}^{n+} + \text{L}_{\text{org}} + n\text{A}_{\text{aq}}^{-} \rightleftharpoons  \left ( \text{ML}^{n+}.\text{A}_{n}^{n-} \right )_{\text{org}}\] \[K_{ex} = \frac{[ \text{ML}^{n+}.\text{A}_{n}^{n-} ]_{\text{org}}}{[ \text{M}^{n+} ]_{\text{aq}} [ L  ]_{\text{org}} [ A^{-} ]_{\text{aq}}}^{n}
\]
4) Use of Ringbom's 'conditional @E02301@ constant', \[K_{\text{ex}}^{\text{eff}} = \frac{{a_{\text{H}^{+}}^{n}}^{n} [ \text{ML}_{n}' ]_{\text{org}}}{ [ \text{M}' ]_{\text{aq}} [ {\text{HL}'  ]_{\text{org}}}^{n}}\] in conjunction with alpha coefficients is useful.
5) The phases can also be specified by the formula of the solvent or by other symbols (preferably Roman numerals) or by overlining formulae referring to one phase, usually the less polar one. The subscript aq (or w) is often omitted; aq is preferable to w as the latter is appropriate only in English and German.
6) The qualification 'equilibrium' is often omitted.
7) The terms @P04438@ and @D01813@ must not be used in this sense.

Related Terms:
1) extraction (http://doi.org/10.1351/goldbook.E02301).
2) formation constant (http://doi.org/10.1351/goldbook.F02485).
3) dissociation (http://doi.org/10.1351/goldbook.D01801).
4) reagent (http://doi.org/10.1351/goldbook.R05190).
5) equilibrium constant (http://doi.org/10.1351/goldbook.E02177).
6) solvation (http://doi.org/10.1351/goldbook.S05747).
7) polymerization (http://doi.org/10.1351/goldbook.P04740).
8) crown (http://doi.org/10.1351/goldbook.C01421).
9) cryptand (http://doi.org/10.1351/goldbook.C01426).
10) partition constant (http://doi.org/10.1351/goldbook.P04438).
11) distribution constant (http://doi.org/10.1351/goldbook.D01813).
12) ionic strength (http://doi.org/10.1351/goldbook.I03180).

Source: Orange Book, 2nd ed., p. 89 (https://media.iupac.org/publications/analytical_compendium/)

Citation: 'extraction (equilibrium) constant' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.E02304

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