Page 193 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

QUANTITATIVE TREATMENT OF SOLVENT EXTRACTOR EQUlLlBRlA 6.3

4. Resonance and steric efects. The stability of chelate structures is enhanced
by contributions of resonance structures of the chelate ring: thus copper
acetylacetonate (see formula above) has greater stability than the copper
chelate of salicylaldoxime. A good example of steric hindrance is given by
2,9-dimethylphenanthroline (neocuproin), which does not form a complex
with iron(I1) as does the unsubstituted phenanthroline; this hindrance is at
a minimum in the tetrahedral grouping of the reagent molecules about a
univalent tetracoordinated ion such as that of copper(1). A nearly specific
reagent for copper is thus available.
The choice of a satisfactory chelating agent for a particular separation should,
of course, take al1 the above factors into account. The critical influence of pH
on the solvent extraction of metal chelates is discussed in the following section.

6.3 QUANTITATIVE TREATMENT OF SOLVENT EXTRACTION EQUlLlBRlA
The solvent extraction of a neutral metal chelate complex formed from a
chelating agent HR according to the equation

may be treated quantitatively on the basis of the following assumptions: (a) the
reagent and the metal complex exist as simple unassociated molecules in both
phases; (b) solvation plays no significant part in the extraction process; and
(c) the solutes are uncharged molecules and their concentrations are generally
so low that the behaviour of their solutions departs little from ideality. The
dissociation of the chelating agent HR in the aqueous phase is represented by
the equation

The various equilibria involved in the solvent-extraction process are expressed
in terms of the following thermodynamic constants:
Dissociation constant of complex, K, = [Mn+],[R -];/[MR,],
Dissociation constant of reagent, Kr = [H +] ,[R -1 ,/[HR],
Partition coefficient of complex, p, = [MRn],/[MRn],
Partition coefficient of reagent, pr = [HR] ,/[HR],
where the subscripts c and r refer to complex and reagent, and w and o to
aqueous and organic phase respectively.
The distribution ratio, i.e. the ratio of the amount of metal extracted as
complex into the organic phase to that remaining in al1 forms in the aqueous
phase, is given by

which can be shown' to reduce to
D = K[HR]O/[H+];
where K = (Krpr)"/KCp,
If the reagent concentration remains virtually constant
D = K*/[H+]", where K* = K[HR]O