Equilibrium and structural studies of silicon(IV) and aluminium(III) in aqueous solution. Part 9. A potentiometric study of mono- and poly-nuclear aluminium(III) 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 Al^III in natural waters, the solubility of a clay mineral (kaolinite) was calculated within the range 4 ⩽ pH ⩽ 9.