C-13 AND AL-27 NMR-STUDY ON THE INTERACTION IN ACIDIC AQUEOUS-SOLUTION BETWEEN ALUMINUM ION AND TIRON, SALICYLIC-ACID AND PHTHALIC-ACID - AS MODEL COMPOUNDS WITH FUNCTIONAL-GROUPS OF FULVIC-ACID

The interaction between aluminium ion and tiron (Tr), salicylic acid ( Sa), and phthalic acid (Pal as model compounds for functional groups o f fulvic acid in aqueous solution of pH 3 was investigated by the C-13 and Al-27 NMR method. In the Al-27 NMR spectra peaks due to 1:1 alumi nium-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 C-13 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 6x10(2) (Tr), 3x10(2) (Sa) and 3x10 (Pal mol(-1) . dm(3). These findings show that the stability of these chelate complexes depends o n the chelate effect (stability: five > six >> seven membered ring).