Quantitative ²⁷Al NMR spectroscopic studies of Al (III) complexes with organic acid ligands and their comparison with GEOCHEM predicted values

Abstract

The toxic inorganic monomeric forms of aluminium (Al) that limit plant growth have been shown to be effectively detoxified by complexation with organic acid ligands released by breakdown of added organic materials. The binding capacity of these acids is dependent on the degree of dissociation of their carboxyl groups and their ability to form bonds with Al. ²⁷Al NMR spectroscopy provides a non-invasive technique to study the bonding of Al with potential ligands without disturbing the equilibrium of the system. In single ligand systems containing oxalic acid, three ²⁷Al resonance peaks were observed at 6.4, 11.4 and 16.0 ppm downfield from the Al³⁺ reference peak at 0 ppm. These were assigned to Alox, Alox₂ and Alox₃ complexes respectively and were observable at pH values down to 3.5. In the presence of the citrate ligand, two ²⁷Al resonance peaks at 6.1 and 11.3 ppm, assigned respectively to the Alcit and Alcit₂ complexes, were observed at pH 3.4. At pH 4.3 and an Al:citrate molar ratio of 1:2, the 6.1 ppm peak was not visible, and the second peak further downfield was split into two unresolved peaks at 10.8 and 12.4 ppm indicating the presence of two forms of the Alcit₂ complex. Distribution of Al between the various species, based on integration of the resonance peaks and equilibrium calculations carried out using GEOCHEM, is discussed in light of the stability constants present in the database of GEOCHEM version (v.) 1.23 and GEOCHEM-PC v. 2.0. Large discrepancies between the computed values and the NMR measured values indicate the need to incorporate more recent literature values in the database for realistic equilibrium calculations in systems containing organic acid ligands. The potential of using quantitative ²⁷Al NMR measurements to calculate stability constants is discussed.