SELECTIVE CONTINUOUS FLOW-STOPPED FLOW-CONTINUOUS FLOW DETERMINATION OF SULFITE IN WHITE WINES USING IMMOBILIZED SULFITE OXIDASE ON A ROTATING REACTOR

The sulfite ion in white wines has been selectively determined using the sulfite oxidase [EC 1.8.3.1] catalysis of sulfite oxidation in the presence of hexacyanoferrate(III) as the electron acceptor. The hexacyanoferrate(II) produced was detected amperometrically at a stationary Pt ring electrode (pH = 7.50; applied potential, +0.380 V versus Ag-AgCl: 3.0 mol l(-1) NaCl). Ascorbic acid, the most serious interferent in the determination was eliminated before detection by passing the sample through a column with immobilized ascorbate oxidase [EC 1.10.3.3]. Application of the method of standard additions is also possible without the elimination of ascorbate. An unsegmented continuous flow stream passes the sample/reagent(s) through the column and directs it/them to the cell where contact is made with immobilized sulfite oxidase on a rotating disc. This disc is located at the bottom of the cell, while the Pt electrode is located at the top of the cell. The results of determinations using the proposed method correlate well with those of determinations conducted using the official Association of Analytical Chemists (AOAC) modified Monier-Williams method.