Potentiometric and multinuclear NMR studies on the interaction of aluminum with ascorbic acid in acidic aqueous solution.

Abstract The complex-formation equilibria between aluminum(III) ion and l -(+)-ascorbic acid (AA) in 0.1 M KCl ionic medium at 25 °C and 0.15 M NaCl ionic medium at 37 °C were studied by glass electrode pH-metric measurements. The obtained experimental results were explained by the formation of the following complexation species: a weak mononuclear 1:1 species AlL 2+ together with two trinuclear mixed-hydroxo species Al 3 H −5 L 4 and Al 3 H −5 L 3+ in acidic aqueous solutions. Meanwhile, the formation of the complexes and structures of Al with AA were proved by multinuclear ( 1 H, 13 C, 27 Al) NMR spectra in the pH range 2.0–5.0. It is supposed that Al directly coordinates with AA at O-3 moiety; also, Al can coordinate with the O-1 and O-2 moieties of ascorbate ion through the weakly binding and the intramolecular hydrogen bonding in acidic aqueous solutions.