Determination of total and free sulphur dioxide in wine with a continuous-flow microdistillation system

Abstract

A continuous-flow microdistillation system for the determination of SO 2 (free and total) in wine samples is described. The assembly permits the successive addition of discrete volumes of sample to the microdistillation module with the aid of a nitrogen carrier stream. The nitrogen carrier also transports the volatilized SO 2, after the distillation step, to the absorption module, where the analyte reacts with a solution of 2,2′-dinitro-5,5′-dithiodibenzoic acid in phosphate buffer (pH 6) to give a yellow derivative, which is monitored spectrophotometrically at 410 nm. Free and total so 2 signals are obtained by operating the microdistillation module at room temperature (ca. 20°C) and 100°C, respectively. Total SO 2 requires a previous hydrolysis step, which is carried out on-line with the aid of 4 M NaOH. Chemical and physical parameters were optimized. The influence of some common compounds usually present in wine samples on the analytical signal was also studied.The proposed procedure was compared with two recommended methods: the EEC reference distillation method and the EEC iodimetric method. The proposed method was found to be equally suitable to the reference distillation method, for the rapid and accurate control of wine samples, and additionally competes advantageously with the iodometric method.