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

摘要

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 (logK) 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 pKai 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 Al~(3+), and it predominantly forms 1:1 and 2:1 Al-DHTPA complexes. The calculated average conditional formation constant (logK_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.