In order to assess the aluminium binding ability of humic and fulvic acids, important organic soil constituents, a pH-potentiometric study was made of the proton and aluminium(III) complexes of various bi-, tri- and tetradentate catechol and salicylic acid derivatives at 25-degrees-C and at an ionic strength of 0.20 mol dm-3 (KCl). The stability data revealed that at low pH the salicylate function, and at high pH the catecholate function, is preferentially bound to the aluminium ion. In the intermediate pH range, mixed hydroxo complexes and other di/oligomeric species are also formed. With an increase of the number of available coordinating sites in the molecule, the tendency to oligomeric complex formation increases, while the tendency to metal ion hydrolysis decreases.
