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)

Abstract The interaction between the aluminum ion (Al 3+ ) and Inogashira fulvic acid (IFA) at pH 3 was investigated using the calibration curve method for 27 Al 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 27 Al 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 Al 3+ and IFA by potentiometric titration and 27 Al NMR measurements. The p K ai values of DHTPA could be determined successfully (p K a1  = 2.3, p K a2  = 3.4 and p K a3  = 7.2; p K a4 was not determined), and these values indicate that DHTPA can act as a powerful chelating ligand, even at pH 3. DHTPA can interact with Al 3+ , and it predominantly forms 1:1 and 2:1 Al–DHTPA complexes. The calculated average conditional formation constant (log  K 1 and K 2 ) 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 Al 3+ interacts with IFA at pH3.