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

The complexation of boric acid with chromotropic acid in aqueous solution w as examined thoroughly by B-11 NMR measurements. Two peaks with chemical sh ift values of delta -17.7 and -18.0 were observed besides the free boric ac id/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 prevai ls in a higher pH range. The formation constants for these complexes were e valuated based on the signal intensities of B-11 NMR spectra to be log beta (1)=-1.57 and log beta(2)=2.35, which are well consistent with those report ed previously as well as that (beta(2)) obtained kinetically in this work. The chromatographic separation of the 1:2 complex from the other species en abled 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 for mation. The pH dependences of the rate constants of the forward and backwar d reactions of the 1:2 complexation could be interpreted by the catalytic r ole of hydrogen ions in the reactions. Plausible mechanisms for both the re actions of 1:2 complex formation and decomposition were proposed. On the ba sis of the equilibrium and kinetic information on the complexation, the opt imum conditions for practical applications of the ligand so far reported co uld be well understood.