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 ²⁷Al NMR, ¹³C NMR and ESI–MS techniques. The five peaks including a peak due to Al³⁺ were observed in ²⁷Al 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 (AlGA²⁺) and three bidentate complexes (AlGA⁺, AlGA₂⁻, and AlGA₃³⁻) complexes. From the deconvolution of ²⁷Al NMR spectra and pKa value of GA, the conditional formation constants (log₁₀ K) of each complex (GA/Al molar ratio of 25 in mixed solution) can be determined to be 0.94 (AlGA²⁺), − 0.96 (AlGA⁺), − 0.77 (AlGA₂⁻) and − 2.21 (AlGA₃³⁻), respectively. In addition, the overall formation constant of three bidentate complex at pH 3 was also calculated to be − 1.65.