Acid–base chemistry of red wine: analytical multi-technique characterisation and equilibrium-based chemical modelling

Abstract

In this paper an attempt is made to model the alkalimetric titration trend (pH<7) of a series of red wines on the basis of an equilibrium-based calculation involving as input the chemical equilibria of the acid–base active substances, whose analytical concentration was determined in each wine under consideration. The chemical model (14 acid–base active substances and 22 protonation equilibria were considered) has been built in reliable conditions with respect to the chemical environment of the fluids under study, in order to describe the acid–base properties at a thermodynamic level using specific professional software. In this connection, protonation constant values of the substances involved were necessary, paying attention to: solvent composition, background salt, ionic strength and temperature. A series of synthetic solutions (based on mixed ethanol/water solvents at known ionic strength) was then pH-metrically titrated (25 °C) in order to refine suitable protonation constant values. Analytical measurements of carboxylic acids, amino acids, inorganic anions and metals were executed to know the concentration of a series of acid–base active reactants (pH<7) and the ionic strength state of each wine. Finally, a comparison between the experimental trend of the alkalimetric titration of each wine and the calculated one via the chemical model tested by means of a computer-assisted simulation has been performed. As an overall check of our job, the charge balance principle was adopted. During the alkalimetric titrations of each wine conductometric detection was performed as well, with the aim of confirming the values of ionic strength calculated using both analytical and equilibrium concentrations. The agreement we found when comparing the calculated values of ionic strength with varying pH with the trend of the conductometric signal can be considered as a significant validation of the chemical model proposed. The jump from the analytical to the equilibrium composition may allow the prediction of the effect on red wines consequent to chemical actions (addition of substances) or natural phenomena.