Development of a speciation model for the interpretation of the acid-base properties of grape red wines

Abstract

Starting with known analytical concentrations related to the considered Italian red grape table wines, we now expound the methodology which allows us to establish a relationship among (a) the molar concentration of a series of substances of routine measurement, (b) the total acidity and (c) the pH value of each fluid at a thermodynamic level. The experimental pH value of each wine is compared with the calculated one, obtained as a result of a thermodynamic simulation based on the contemporary treatment of a series of chemical equilibria (17 reactants, 77 equilibria). Accurate equilibrium constant values are previously refined on test substances components of red wines (namely: acetic, tartaric, succinic and citric acid) starting by pH-metric readings (at 25°C) recorded in appropriate experimental conditions. In fact, the chemical conditions take care of both the solvent composition, as regards the amount of ethanol, and the ionic strength value, to take into account the effect due to the background electrolytes. In order to achieve information on ion pair formation in the wine, we have now carried out a series of titration using tetraethylammonium iodide as background salt. Using protonation constant values in both KCl or tetraethylammonium iodide, and a series of measurements with calcium(II) ion, the stability of Ca2+- and K+- complexes (or ion pairs) with some reference ligands has been evaluated. A new hemical model has then been built to interpret in terms of chemical equilibria the pH of each wine, which allows as well to obtain a reliable estimation of the free concentration of both Ca2+ and K+, useful to solve technological problems related to many steps of the wine production and evolution. Operative advantages, with respect to typical oenological problems, are shown passing from the analytical to the equilibrium composition of each wine.