PHENOMENOLOGICAL MODEL OF MALIC ACID IONIZATION IN THE PRESENCE OF STRONG ELECTROLYTES AND ITS CONNECTION WITH DETERMINATION OF THE AMOUNT OF ORGANIC AND MINERAL ACIDS IN THE GRAPES AND WINE
An experimental study of the behavior of dilute (c < 2×10<sup>-3</sup>, mol/dm<sup>3</sup>) aqueous solutions of malic acid in the presence of strong electrolytes in the process of continuous potentiometric titration with an electrogenerated base was performed. A phenomenological model of malic acid ionization is proposed, based on the assumption that its molecular and ionized forms exist in dimeric forms in solutions, including highly diluted ones. The proof of the correctness of the proposed model was performed by solving two mathematical models and testing them experimentally. The first classical model was that malic acid under experimental conditions is in monomeric form and in the process of titration is successively ionized by two steps. The second model was based on the fact that malic acid, like its anionic forms, is in aqueous solutions in a stable dimeric form and is titrated as a four-base acid. Ionization constants are found for each of the models, and the relationship between them is described. For the second model, it is shown that the process of ionization of malic acid in the third and fourth stages is difficult due to the fact that protons are located in a ring of carboxyl groups and are titrated much more difficult than the terminal ones, since their bond with oxygen atoms is strengthened during titration. Distribution curves of the assumed molecular and ionic forms of malic acid formed during titration are constructed, and the nature of hydrogen and intermolecular bonds is discussed. The influence of malic acid the appearance of wine titration curves was revealed. The results of comparing the ionization constants of dimernic forms of tartaric, malic, citric, formic, lactic, succinic and acetic acids found by this method are presented. The results obtained are used in titrimetry methods for evaluating the quality of grape raw and wines.