Sequential spectrophotometric determination of methanol and iron in vinegar by a flow injection-pervaporation method

An easily automatable sequential flow-injection–pervaporation method is proposed for the photometric determination of methanol and iron in vinegar. The method is based on separation of the methanol from the sample matrix by pervaporation followed by its oxidation to formaldehyde with permanganate, decolouration of the latter with S2O52–, and subsequent reaction of formaldehyde with p-rosaniline to yield a violet reaction product with maximum absorption at 567 nm. Iron is determined by an existing method based on reaction with thiocyanate in acidic medium and monitoring at 508 nm. After optimisation by either the univariate or multivariate approaches, as required, the linear range was established for methanol (4–1000 mg L–1) and iron (0.18–20 mg L–1). The proposed method was then compared with reference methods for methanol and iron in terms of repeatability (2.452 mg L–1 and 0.245 mg L–1, respectively), reproducibility (4.435 mg L–1 and 0.356 mg L–1, respectively), detection limit (LOD=82 and 0.234 mg L–1, respectively), and traceability. The throughput was nine samples per hour.