Complexation Between Aluminum Ion and Glycolic Acid Under Acidic Condition

The complexation between Al3+ ([Al(H2O)(6)](3+)) and glycolic acid (GA, C2H4O3) which has a carboxyl group and a hydroxyl group in a molecule was investigated under acidic condition using Al-27 NMR, C-13 NMR and ESI-MS techniques. The five peaks including a peak due to Al3+ were observed in Al-27 NMR spectra for the mixed solution of Al3+ and GA, suggesting the existence of at least four Al-GA complexes. The results of NMR and ESI-MS measurements revealed that GA and Al3+ can form one monodentate complex (AlGA(2+)) and three bidentate complexes (AlGA(+), AlGA(2)(-), and AlGA(3)(3-)) complexes. From the deconvolution of Al-27 NMR spectra and pKa value of GA, the conditional formation constants (log(10)K) of each complex (GA/Al molar ratio of 25 in mixed solution) can be determined to be 0.94 (AlGA(2+)), - 0.96 (AlGA(+)), - 0.77 (AlGA(2)(-)) and - 2.21 (AlGA(3)(3-)), respectively. In addition, the overall formation constant of three bidentate complex at pH 3 was also calculated to be - 1.65.