25 No. 4
July - August 2003
Evaluation of Stability Constants for a-Hydroxycarboxylic
Acid Complexes with Protons and Metal Ions and the Accompanying
Enthalpy Changes. Part II: Aliphatic a-Hydroxycarboxylic
Acids (IUPAC Technical Report)
R. Portanova, L. Lajunen, M. Tolazzi, and J. Piispanen
and Applied Chemistry,
Vol. 75, No. 4, pp. 495–540 (2003)
hydroxycarboxylic acids and their derivatives are important
chemicals for the pharmaceutical, biological, food, and other
industries. Various hydroxycarboxylic acids occur naturally.
The technological, practical, and industrial applications
of these compounds have led to numerous studies of their properties
hydroxycarboxylic acids contain two donor groups, the hydroxyl
and the carboxylate groups, and therefore are all potentially
bidentate ligands. The proton and metal ion complexation constants
of these ligands depend strongly on the relative positions
of the two donor groups in the molecule. The a-hydroxycarboxylic
acids form considerably stronger complexes with most metal
ions, through bidentate chelation involving both functional
groups, than do the corresponding simple carboxylic acids.
Hydroxyl groups more distant from the carboxylic groups do
not generally participate in the formation of chelate complexes,
and the ligand coordination to metal ions occurs via the carboxyl
hydroxycarboxylic acids form stable complexes with most metal
ions. Owing to the considerable amount of reported data, in
the form of protonation and metal ion binding constants, the
critical evaluation of the equilibrium data in this paper
is restricted to proton and metal ion complexation with aliphatic
acids, in aqueous solution, published between 1960 and 1994.
Little relevant data that was published earlier are included
in this paper. Enthalpy changes, when available, are also
reported. Equilibrium data obtained in non-aqueous or in solvent
mixtures, as well as those dealing with the formation of mixed
complexes, are not surveyed in this paper.
determinations of acid dissociation constants and metal ion
complexation constants with hydroxycarboxylic acids have been
reported. Various experimental methods have been used for
the measurements, mainly potentiometry, spectroscopy, and
distribution between two phases. Several studies were carried
out in media of constant ionic strength (commonly in the range
0.1 - 2.0 mol dm-3), the supporting electrolytes being sodium
perchlorate, but also sodium chloride, potassium chloride,
and nitrate. The majority of the measurements were performed
at 25 °C.
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