Equilibrium and structural studies of silicon(IV) and aluminium(III) in aqueous solution. Part 13. A potentiometric and27Al nuclear magnetic resonance study of speciation and equilibria in the aluminium(III)-oxalic acid-hydroxide system

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Abstract
Equilibria between Al3+, oxalic acid (H2L), and OH– were studied in 0.6 mol dm–3 NaCl medium at 25 °C. Potentiometric titrations (glass electrode) and 27Al n.m.r. measurements were performed to study speciation and equilibria within the concentration ranges 0.2 ⩽–log [H+]⩽ 7.2, 0.0005 ⩽b⩽ 0.02 mol dm–3, 0.0005 ⩽c⩽ 0.025 mol dm–3, and 0.5 ⩽c/b⩽ 16 [b and c are the total concentrations of aluminium(III) and oxalic acid respectively]. Besides a series of [AILn]3 – 2n complexes with n= 1,2, and 3, the formation of the polynuclear mixed hydroxo-complexes Al3(OH)3L3 and [Al2(OH)2L4]4 – were established. N.m.r. data also indicated the formation of an [AlHL]2+ complex in strongly acidic solutions: pKa([AlHL]2+)≈ 0.0. The significance of the different Al complexes to conditions prevailing in natural waters is discussed, including a model calculation of the solubility of a clay mineral (kaolinite) in the presence of oxalate. Data were analysed using the least-squares computer program LETAGROPVRID.