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

The complex-formation equilibria between aluminum(III) ion and L-(+)-ascorbic acid (AA) in 0.1 M KCl ionic medium at 25 degrees C and 0.15 M NaCl ionic medium at 37 degrees 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 (1H, 13C, 27Al) 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.