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 (A1) 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 A1. 27A1 NMR spectroscopy provides a non-invasive technique to study the bonding of A1 with potential ligands without disturbing the equilibrium of the system. In single ligand systems containing oxalic acid, three 27A1 resonance peaks were observed at 6.4, 11.4 and 16.0 ppm downfield from the A13+ 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 27A1 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 AI 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 27A1 NMR measurements to calculate stability constants is discussed.

Characterization of root mucilage from Melastoma malabathricum, with emphasis on its roles in aluminum accumulation

“…To identify the form of Al in M. malabathricum mucilage directly, 27 Al NMR was applied. 27 Al NMR is a very efficient tool for the speciation of Al in solution (Kerven et al. , 1995), although Al tightly bound to the solid phase cannot be detected by solution‐state NMR.…”

Purity analysis of hydrogen cyanide, cyanogen chloride and phosgene by quantitative 13C NMR spectroscopy

“…Accurate results from quantitative NMR spectroscopy require a high degree of specificity to ensure that analyte and impurity signal intensities are derived properly. The broad chemical shift ranges inherent to nuclei such as 15 N, 19 F, 31 P, and 13 C used in this method, predispose analytical methods based on their observation for high spectral resolution and method specificity, especially at higher magnetic field strengths. It is possible, however, for an impurity signal to converge or even superimpose with the analyte signal.…”

High Aluminum Resistance in Buckwheat1

“…5), which are assigned to the 1:1, 1:2, and 1:3 Al-oxalate complex, respectively (Kerven et al, 1995). This suggests that the Al in buckwheat roots is present in the form of Al-oxalate complexes at a molar ratio of 1:3 and 1:2.…”