Potentiometric determination of aqueous dissociation constants of flavonols sparingly soluble in water

Two different approaches were evaluated and used to estimate the aqueous pKa values of some flavonols sparingly soluble in water (morin, fisetin and quercetin) from their pKa values in methanol/water mixtures obtained by potentiometry. Both approaches lead to similar results, although one of them requires only one pKa value at one unique methanol/water mixture, whereas the other (the classical Yasuda–Shedlovsky plot) requires several pKa data at different methanol/water mixtures. Thus, the first approach is recommended because it is faster and simpler. The automated potentiometric method is strongly recommended for pKa determination of these types of compounds because of its simplicity and speed of operation.