Flow Injection Spectrophotometric Determination of Free and Total Sulfite In Wines Based on the Induced Oxidation of Manganese(II)

Abstract

The sulfur(IV) content in wines was determined with a flow injection spectrophotometric system by treating each sample in the donor channel with sulfuric acid in order to release free SO2, which diffuses through a PTFE membrane to the acceptor solution containing Mn(II) in acetate buffer at pH 5.5. The induced oxidation of Mn(II) takes place proportionally to the free S(IV) content and the oxidized species formed reacts with iodide to form iodine which can be spectrophotometrically measured at 352 nm. A linear plot of absorbance vs S(IV) concentration is observed up to 26 mg SO2 L−1 with a detection limit of 1.0 mg L−1. Matrix effects required the standard additions of sulfite to the samples for obtaining accurate results. A previous treatment of the sample with tris-(hydroxymethylaminomethane) and EDTA decomposed the hydrogen-sulfite/aldehyde adducts and allowed the determination of the total S(IV) content. As expected, total S(IV) concentration in wines has been found to be much higher than free sulfite. A recovery-addition experiment with red and white wines led to recoveries varying from 91.5 to 109.7% by applying the free and total sulfite developed procedures. This is a good indication of the accuracy of both proposed procedures.