Comparative sulfite assay by voltammetry using Pt electrodes, photometry and titrimetry: Application to cider, vinegar and sugar analysis

Abstract

Sulfite is a widely applied preservation agent, against oxidative decay in foodstuffs and beverages. Among the analytical methods applied, electrochemical techniques exploit its facility to undergo oxidation. This paper aims at the comparative investigation of the performances exhibited by three analytical methodologies: cyclic voltammetry at different Pt working electrodes, the volumetric method employing sodium thiosulfate as titrating agent and the photocolorimetric method relying on the reduction of Fe3+-orthophenanthroline complex by sulfite. The cyclic voltammetric assay was performed at Pt strip and Pt ring electrodes, by linearly sweeping the potential between -100 and 1,500 mV. The linear range corresponded to 7.5 mg L-1 - 4.0 g L-1 for Pt strip working electrode and to 15.5 mg L-1 - 4.0 g L-1 for Pt ring working electrode. Relative standard deviations smaller than 3% showed repeatability. RSD values smaller than 3% were also obtained in the photometric assay, but the latter was characterised by a narrower linear range. The Pt strip electrode allowed wider linear range and lower sensitivity, whereas the Pt ring electrode with Ag/AgCl reference in the same unit was characterised by better repeatability. Applications involved sulfite assay in vinegar, brown sugar and cider samples with consistency between cyclic voltammetry and titrimetry.