constant at zero ionic strength
is the equilibrium constant
of the distribution reaction expressed in terms of the reacting species. Thus, for the gross reaction:
in which the reagent
initially dissolved in an organic phase reacts with a metal ion
in aqueous solution to form a product MLn
which is more soluble in the organic phase than in water,
- When concentrations are used instead of activities or mixed terms are employed as when
are measured with an electrode, the appropriate name is extraction constant, symbol
, accompanied by a careful definition.
may be termed the thermodynamic extraction constant.
The extraction constant is related to other terms relevant to such systems by:
is the overall formation constant of
is the dissociation constant of
Where the reagent
is more soluble in water than the other immiscible phase it may be more convenient to define a special equilibrium constant in terms of
In distribution equilibria involving non-aqueous systems, e.g. liquid
SO2, molten salts and metals, the mass action equilibrium constant for the relevant extraction process can be identified with
which should be explicitly defined in this context.
In actual practice, it may be necessary to include other terms to take into account other complexes formed by auxiliary reagents and the solvation and/or polymerization of the various species. In such cases,
must be defined with reference to the relevant explicit chemical equation. An example is complex formation between the metal ion and an uncharged crown ether or cryptand molecule followed by ion-pair extraction:
Use of Ringbom's 'conditional extraction constant',
in conjunction with alpha coefficients is useful.
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.
The qualification 'equilibrium' is often omitted.
The terms partition constant and distribution constant must not be used in this sense.
PAC, 1993, 65, 2373
(Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations 1993))
on page 2383
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook