Preferred Label : extraction (equilibrium) constant;
IUPAC 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: \[{M}_{{aq}} {n } n{HL}_{{org}} \rightleftharpoons {ML}_{n,{org}}
n{H}_{{aq}} { }\] in which the reagent HL initially dissolved in an organic phase
reacts with a metal ion M sup i n /i /sup in aqueous solution to form a product
ML sub i n /i /sub which is more soluble in the organic phase than in water, \[K_{{ex}}
{{0}} \frac{a_{{ML } n,{org}}\hspace{2pt}a_{{H} { },{aq}} {n}}{a_{{M} {n },{aq}}\hspace{2pt}a_{{HL},{org}}
{n}}\];
Scope note : 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 kex which should be explicitly defined in this context.; the qualification equilibrium is often omitted.; use of ringbom's conditional extraction constant, kexeff ah n n
mln' org m' aq hl'
org n in conjunction with alpha coefficients is useful.; the terms partition constant and distribution constant must not be used in this sense.; the extraction constant is related to other terms relevant to such systems by: kexdmlnßnkan
dhl n where ßn is the overall formation constant of m ln and ka is the dissociation
constant of hl. where the reagent hl is more soluble in water than the other immiscible
phase it may be more convenient to define a special equilibrium constant in terms
ofhlaq:kex dmlnßnkan; 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, kex 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:
m aq n l org n a aq -¿ ml n . a n n - org kex
mln ann- org mn aq
l org a- aq n; 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.; when concentrations are used instead of activities or mixed terms are employed as
when h1 and/or mn are measured with an electrode, the appropriate name is extraction
constant, symbol kex , accompanied by a careful definition. kex0 may be termed
the thermodynamic extraction constant.;
Origin ID : E02304;
See also
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: \[{M}_{{aq}} {n } n{HL}_{{org}} \rightleftharpoons {ML}_{n,{org}}
n{H}_{{aq}} { }\] in which the reagent HL initially dissolved in an organic phase
reacts with a metal ion M sup i n /i /sup in aqueous solution to form a product
ML sub i n /i /sub which is more soluble in the organic phase than in water, \[K_{{ex}}
{{0}} \frac{a_{{ML } n,{org}}\hspace{2pt}a_{{H} { },{aq}} {n}}{a_{{M} {n },{aq}}\hspace{2pt}a_{{HL},{org}}
{n}}\]
