The aluminium(III)-4-nitrocatechol system: potentiometry, voltammetry and application to the determination of reactive Al(III)

Potentiometric and voltammetric investigations of the Al(III)-4-nitrocatechol system indicate that 4-nitrocatechol (4ncat) is a suitable ligand for amperometric determination of Al(III) at environmental concentrations. The potentiometric study of solution equilibria was undertaken over the pH range 3–11 and [4ncat]: [Al(III)] ratios from 1 to 4. Complexes [Al(4ncat)]+, [Al(4ncat)2]−, [Al(4ncat)3]3− and [Al(4ncat)2OH]2− were characterised; evidence for [Al(4ncat)OH]0 was equivocal. The voltammetric responses of 4ncat and its Al(III) complexes were examined by cyclic voltammetry and hydrodynamic voltammetry at the rotating disk electrode. The potential for oxidation of free 4ncat decreases as the solution pH increases from ca. 4 to 9, and the potential for oxidation of coordinated 4ncat decreases as the number of coordinated ligands increases. At pH ca. 9, with a suitable excess of ligand, formation of [Al(4ncat)3]3− is almost quantitative, and close to 300 mV separates the oxidation of free and coordinated ligand. These conditions were chosen for amperometric determination of Al(III) in a flow injection analysis (FIA) system. Targeting of the reactive Al(III) fraction and removal of interferents were achieved online using an oxine-derivatised microcolumn. The detection limit for reactive Al(III) was 0.08 μM (2σ for a 1 μM Al3+ standard). The method was applied to the determination of reactive Al(III) in soil solutions and measurement of the Al(III) complexation capacity of a soil-derived fulvic acid.