The interaction between aluminium ion and tiron (Tr), salicylic acid (Sa), and phthalic acid (Pa) as model compounds for functional groups of fulvic acid in aqueous solution of pH 3 was investigated by the 13C and 27Al NMR method. In the 27Al NMR spectra peaks due to 1:1 aluminium-ligand complexes were observed at 15.7, 2.6 and -3.5 ppm. No peak due to a 1:2 complex was observed. By comparing the 13C NMR chemical shifts between free ligands and coordinated ligands, it was evidenced that the three ligands act as bidentate ligands for aluminium ions at pH 3. The binding constants of these complexes at pH 3 were estimated to be 6× 102 (Tr), 3×102 (Sa) and 3×10 (Pa) mol-1 · dm3. These findings show that the stability of these chelate complexes depends on the chelate effect (stability: five>six≫seven membered ring).
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