The complexation between Al 3+ ([Al(H 2 O) 6 ] 3+ ) and glycolic acid (GA, C 2 H 4 O 3 ) which has a carboxyl group and a hydroxyl group in a molecule was investigated under acidic condition using 27 Al NMR, 13 C NMR and ESI-MS techniques. The ve peaks including a peak due to Al 3+ were observed in 27 Al NMR spectra for the mixed solution of Al 3+ and GA, suggesting the existence of at least four Al-GA complexes. The results of NMR and ESI-MS measurements revealed that GA and Al 3+ can form one monodentate complex (AlGA 2+ ) and three bidentate complexes (AlGA + , AlGA 2 − , and AlGA 3 3− ) complexes. From the deconvolution of 27 Al NMR spectra and pKa value of GA, the conditional formation constants (log 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.

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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 (log 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.