Voltammetric determination of free sulfur dioxide in wines using platinum and gold disc microelectrodes

Abstract

A voltammetric method is described for the determination of free SO₂ in untreated wine samples by using platinum and gold disc microelectrodes, based on the oxidation of SO₂ at the solid electrodes. The effects on this process caused by some natural components of wine, such as ethanol, tartaric acid, potassium chloride, anthocyanins, acetaldehyde and pH, were examined. Calibration graphs were obtained under the weakly acidic conditions characteristic of wine (pH = 3–3.2) and were linear for concentrations of SO₂ in the range 1–100 and 10–100 mg 1^–1 at gold and platinum electrodes, respectively. Correlation coefficients of 0.9998 and 0.9995 and detection limits of 0.58 and 2.35 mg l^–1 were found for gold and platinum electrodes, respectively. The free SO₂ contents of several Italian red and white wines at their natural pH values were evaluated by interpolating the current recorded directly in the wine on the calibration graphs obtained with synthetic standards. Free SO₂ was determined at a level as low as 2.8 mg l^–1, with a relative standard deviation of about 5%, by using gold electrodes; platinum electrodes were suitable for determining concentrations of free SO₂ above 10 mg l^–1, with a relative standard deviation of <5%. The results obtained with the proposed method were compared with those given by classical iodimetric titration, and the differences between the data from the two methods were <10 and <25% for white and red wines, respectively.