DISSOCIATION CONSTANTS OF QUERCETIN. TITRATION CURVES

10.25712/ASTU.1811-1416.2024.01.005

Abstract

The flavonoid quercetin is a natural polyphenolic antioxidant. A promising way to increase the solubility and biological activity of quercetin is to conduct its solid-phase mechanochemical reaction with solid alkalis. Some properties of quercetin are more pronounced in a liquid medium, in aqueous solutions. The structural formula of quercetin shows the presence of five hydroxyl groups capable of dissociating. But only one stage of dissociation is often considered. The magnitude of the given values of the dissociation constant is in the range pK1=5-9. An additional complication of the study of dissociation of quercetin is its noticeable oxidation with an increase in the pH of the solution. To prevent this phenomenon, it is proposed to conduct research in an inert gas. The paper presents the results of pH- titration of quercetin with alkali in a water-alcohol solution. Based on the dissociation models of one-, two- and three-base weak acids, the analysis of quercetin titration curves was carried out. A computer program for curve analysis was developed, deviations of experimental titration results from model calculations were processed by LSM. The titration curve is best described by the model of a weak dibasic acid. Dissociation constants for this model are determined: pK1=8.64, pK2=11.09 The value of pK1 is in satisfactory agreement with the literature data. When titrated in an aqueous medium, acids with pK > 12 are not determined. Therefore, the question of the other dissociation constants remains open. The analysis of the initial section of the titration curve by its second derivative was carried out. Depending on the sign of the second derivative, the shape of the titration curve changes at the beginning, this allows to critically evaluate the literature data.