Acid–base behavior and Al3+ complex formation of synthesized 2,3-dihydroxyterephthalic acid (DHTPA) at pH 3 as a model compound of Inogashira fulvic acid (IFA)

The interaction between the aluminum ion (Al3+) and Inogashira fulvic acid (IFA) at pH 3 was investigated using the calibration curve method for 27Al NMR spectra. The average conditional stability constant (log K) can be calculated to be 2.00–2.04 (M−1) (bidentate–monodentate) from the results of 27Al NMR measurements. In addition, because IFA has various coordination sites, including COOH, phenolic and/or alcoholic OH groups, we attempted to synthesize 2,3-dihydroxyterephthalic acid (DHTPA) as a model compound with the functional groups of IFA for the investigation of the microscopic coordination mechanism between Al3+ and IFA by potentiometric titration and 27Al NMR measurements. The pKai values of DHTPA could be determined successfully (pKa1 = 2.3, pKa2 = 3.4 and pKa3 = 7.2; pKa4 was not determined), and these values indicate that DHTPA can act as a powerful chelating ligand, even at pH 3. DHTPA can interact with Al3+, and it predominantly forms 1:1 and 2:1 Al–DHTPA complexes. The calculated average conditional formation constant (log K1 and K2) of each complex can be determined as 1.09 (1:1) and 4.81 (M−1) (2:1). The results obtained showed that both the 1:1 and 2:1 Al–DHTPA complexes are formed and Al3+ interacts with IFA at pH3.

We have attempted to synthesize 2,3-dihydroxyterephthalic acid (DHTPA) as a model compound with the functional groups of Inogashira fulvic acid (IFA) for the investigation of the microscopic coordination mechanism between Al3+ and IFA. The complexation of aluminum ions was investigated by means of potentiometric titration and 27Al NMR, and the estimated structures of the complexes formed with the aluminum ion are discussed.Figure optionsDownload as PowerPoint slide