Equilibrium and kinetic studies on the complexation of boric acid with chromotropic acid

Abstract

The complexation of boric acid with chromotropic acid in aqueous solution was examined thoroughly by ¹¹B NMR measurements. Two peaks with chemical shift values of δ −17.7 and −18.0 were observed besides the free boric acid/borate peak and ascribed to the 1∶1 and the 1∶2 complexes, respectively. The 1∶2 complex is formed in acidic solution, while the 1∶1 complex prevails in a higher pH range. The formation constants for these complexes were evaluated based on the signal intensities of ¹¹B NMR spectra to be log β₁ = −1.57 and log β₂ = 2.35, which are well consistent with those reported previously as well as that (β₂) obtained kinetically in this work. The chromatographic separation of the 1∶2 complex from the other species enabled a kinetic study of the reaction which revealed that the reaction for 1∶1 complex formation takes place much faster than that for 1∶2 complex formation. The pH dependences of the rate constants of the forward and backward reactions of the 1∶2 complexation could be interpreted by the catalytic role of hydrogen ions in the reactions. Plausible mechanisms for both the reactions of 1∶2 complex formation and decomposition were proposed. On the basis of the equilibrium and kinetic information on the complexation, the optimum conditions for practical applications of the ligand so far reported could be well understood.