A chelating resin as a probe for the copper(II) distribution in grape wines

Presented hereafter is the pilot-study of the applicability of resin titration (RT) to mixed solvents in regard to the studies of metal distribution in wines - a beverage that has attracted great attention and that has been extensively studied in the recent years.The RT method of studying the distribution of species is based on the determination of the concentration of metal ions sorbed on the iminodiacetic resin Chelex 100 at different V/w (solution volume/mass of resin). The total metal ion present in the considered solution and the free metal ion are estimated from the titration curves. The method, already applied for water solutions, was tested here in mixed solvent media, at the pH around 3, with known amounts of copper(II) and ligands with different complexing properties. Satisfactory results have been obtained and commented critically.In the application of the RT method, the protonation and the exchange reactions of copper(II) with the considered sorbent were investigated in 12% ethanol, in KCl 0.1 and 1.0 M solutions, according to a well established procedure.Finally, a white grape wine (Riesling) from a winery in Oltrepò Pavese was analyzed with RTs. In the sample obtained before processing the wine, 50% of the total Cu(II) (∼18 μM) was combined with ligands with log αM equal to 3.3; in the final product, the total Cu(II) was 4 μM, almost totally complexed. According to these findings, the ligands accountable for the copper complexation were not the major components in the wine, as they formed considerably weaker complexes.