A gas diffusion sequential injection system for the determination of sulphur dioxide in wines

In the present work, a sequential injection system with spectrophotometric detection was developed for the determination of free and total sulphur dioxide in wines. It was based on the formation of a coloured product from the reaction among SO₂, formaldehyde and pararosaniline. A gas diffusion unit (GDU) was incorporated into the manifold to prevent the wine matrix interference in the spectrophotometric measurement. An acid solution was added to the sample prior to its passage through the donor channel of the GDU to promote gaseous SO₂ formation. For the free SO₂ determination, the sample was directly aspirated into the holding coil; for the total SO₂ determination, the sample was processed after previous in-line hydrolysis of bound SO₂ with an alkali solution. Two second-order calibration curves were established, defining two concentration ranges: 2-40 mg l^-1 for the free SO₂ determination and 25-250 mg l^-1 for the total SO₂ determination. Relative standard deviations (n = 10) were lower than 1.2% for the determination of free SO₂ and lower than 2.3% for the determination of total SO₂. The sample frequency was about 16 h^-1. This methodology was applied to the determination of free and total sulphur dioxide in 10 table wines and the results were statistically comparable with those furnished by the recommended procedure. (C) 2001 Elsevier Science B.V.