Electrochemical studies of complexation of Pb in red wines

Abstract

Ion-selective electrode potentiometry and cathodic voltammetric techniques were used to study the Pb complexometric properties of four monovarietal wines, ‘Touriga Nacional’ (TN), ‘Touriga Francesa’ (TF), ‘Tinta Roriz’ (TR) and ‘Tinta Barroca’ (TB). The data from titrations of the samples with a Pb solution were treated by two methods suitable for systems involving polymeric heterogeneous ligands: (1) the Scatchard plot, which provided the total complexation capacity of the sample (CC_total) and average conditional stability constants (K_av), and (2) the differential equilibrium function (DEF), which provided values for differential conditional stability constants (K_DEF) as a function of the titration point. The CC_total of the wines ranged between 4.5 × 10⁻³ M (TF) and 1.0 × 10⁻² M (TN). Potentiometric titrations provided values of log(|Pb|_bound/CC_total) = log θ between 0.015 and 0.45, and the respective log K_av between 5.6 (TF) (log K_DEF between 4.7 and 5.5) and 3.0 (TR) (log K_DEF between 2.8 and 3.7). Cathodic voltammetry, carried out only for TR and TN, provided the concentrations of only the strongest ligands of the wines (i.e. those that are operationally inert), which were 27 μM (TR) and 43 μM (TN), and two different log K_av for TN, 7.6 and 6.6, and only one, 7.2, for TR, for log θ 0.0025–0.015 (TN) and 0.0014–0.0075 (TR). Since the values of log θ in wines are lower than those embraced in the experiments, this study indicates that Pb is strongly complexed in wines. Therefore, a low bioavailability of Pb can be expected.