Equilibrium and structural studies of silicon(<scp>IV</scp>) and aluminium(<scp>III</scp>) in aqueous solution. Part 9. A potentiometric study of mono- and poly-nuclear aluminium(<scp>III</scp>) citrates

Abstract

Complexation in the Al3+–H3L–OH– system (H3L = citric acid) has been studied within the limits 2 ⩽–log [H+]⩽ 8, 0.000 25 ⩽B⩽ 0.016 mol dm–3, and 0.000 25 ⩽C⩽ 0.016 mol dm–3 with C: B ratios equal to 1, 2, 4, 8, 16, and 32 (B and C represent the total concentrations of Al3+ and citric acid respectively). The measurements were performed as potentiometric (glass electrode) titrations in 0.6 mol dm–3 NaCl at 25 °C. Data are explained with the mononuclear species [Al(HL)]+, [AlL], and [AlL2]3– together with a very stable trinuclear complex [Al3(OH)4L3]4–. Equilibrium constants including acidity constants of H3L have been obtained. Data were analyzed using the least-squares computer program LETAGROPVRID. To demonstrate the complexing ability of citrate ions to AlIII in natural waters, the solubility of a clay mineral (kaolinite) was calculated within the range 4 ⩽ pH ⩽ 9.