Wide pH Range Potentiometric and Spectrophotometric Investigation into the Acidic Constants of Quercetin, Luteolin and l-Ascorbic Acid in Aqueous Media

It is now well established that the dissociation constants of an organic compound are characteristic of the types of groups, or the combinations of groups, contained in it. Furthermore, the acid-base dissociation constants are important parameters to fully understand the properties of a molecule in biological systems. In this framework, the aim of the present study was to determine the acidic constants of three natural molecules with well-known antioxidant properties, namely quercetin, luteolin and l-ascorbic acid. The evaluation was carried out in aqueous media (i.e., 0.16 M NaCl) at 37 degrees C in a wide pH range by using a combined approach based on potentiometric and spectrophotometric measurements. The results underline the necessity to employ both experimental techniques to obtain accurate values for acidic constants, preventing uncertainties related to undesirable oxidation reactions.