Determination of dissociation constants of species oxidizable in aqueous solution by air oxygen on an example of quercetin

The comparison of literature data on the dissociation constant of one of the most abundant natural flavonoids, e.g., quercetin, demonstrates their irreproducibility. The reason for this likely corresponds to the easiness of its oxidation by air oxygen during the titration process. To eliminate such problems, a modified version of potentiometric titration was proposed with bubbling a weak flow of an inert gas (nitrogen) through the solution to be titrated in the presence of minimal amounts of a nonionic surfactant. By virtue of the technique proposed, the values of pK (a) for quercetin were measured to be 6.62 +/- 0.04 and 9.7 +/- 0.3. The first one corresponds to the hydroxyl group in the gamma-pyrone fragment of the molecule, while another agrees with the typical values of pK (a) for phenols.