Hybrid sequential injection–flow injection manifold for the spectrophotometric determination of total sulfite in wines using o-phthalaldehyde and gas-diffusion

A new automated spectrophotometric method for the determination of total sulfite in white and red wines is reported. The assay is based on the reaction of o-phthalaldehyde (OPA) and ammonium chloride with the analyte in basic medium under SI conditions. Upon on-line alkalization with NaOH, a blue product is formed having an absorption maximum at 630 nm. The parameters affecting the reaction – temperature, pH, ionic strength, amount concentration and volume of OPA, amount concentration of ammonium chloride, flow rate and reaction coil length – and the gas-diffusion process – sample and HCl volumes, length of mixing coil, donor flow rate – were studied. The proposed method was validated in terms of linearity (1–40 mg L−1, r = 0.9997), limit of detection (cL = 0.3 mg L−1) and quantitation (cQ = 1.0 mg L−1), precision (sr = 2.2% at 20 mg L−1 sulfite, n = 12) and selectivity. The applicability of the analytical procedure was evaluated by analyzing white and red wine samples, while the accuracy as expressed by recovery experiments ranged between 96% and 106%.