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 pK _ai values of DHTPA could be determined successfully (pK_a1 = 2.3, pK_a2 = 3.4 and pK_a3 = 7.2; pK_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 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.