Aluminium(III) and Iron(III) 1,2-Diphenolato Complexes: a Potentiometric Study

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Abstract
The equilibrium reactions between aluminium(III) and the phenols catechol (LH2), protocatechuic acid (LH3) and catechin (LH4), and between iron(III) and protocatechuic acid have been studied by potentiometric titration in aqueous solution, I 0.10M ( KCl ), 25°C. Stability constants are reported for the mononuclear diphenolato complexes AlLHn-2, Al(LHn-2)2and Al(LHn-2)3, and the hydroxo complexes Al(OH)LHn-2 and Al(OH)(LHn-2)2, n = 2 ( catechol ) or 4 ( catechin ). For protocatechuic acid, the carboxylate -coordinated species lLH22+ and the species AlLH + (carboxyl- protonated ) are also postulated. Analogous species were characterized for the iron(III)- protocatechuic acid system. Stoichiometric end-points were obtained only if an excess of ligand was used (L/M > 4). Monomeric and polymeric aluminium- hydroxo species and ternary metal- ligand - hydroxo species were found to be unimportant under the experimental conditions employed.