A Headspace Gas Detection Tube Method to Measure SO₂ in Wine without Disrupting SO₂ Equilibria

Abstract

The headspace gas detection tube (HS-GDT) method to measure molecular and free sulfur dioxide (SO₂) in wine is a simple and inexpensive procedure using commercial industrial safety colorimetric tubes. A syringe is used to sample a wine and to create a closed headspace, which is expelled through the GDT after equilibrium is obtained. The vapor-phase concentration of SO₂ (P_SO2) is determined from the manufacturer’s printed markings and then related to the molecular SO₂ concentration in the wine based on Henry’s law coefficients or, more accurately, on calibration curves. Typical wine ethanol concentrations had a significant effect on the pK_a of SO₂, as previously reported, but no effect on Henry’s law coefficients. Calibration curves in model wine and aqueous buffer yielded indistinguishable results when appropriate pK_a values were used. Best results for calibration curves were achieved with 200 mL headspace of model wines (5 to 40 mg/L free SO₂, equivalent to 0.14 to 1.12 mg/L molecular SO₂), which yielded satisfactory linearity (r² = .99), reproducibility (mean CV = 8% for molecular SO₂ >0.4 mg/L), and detection limits (0.21 mg/L molecular SO₂). Molecular SO₂ measured by HS-GDT and by aeration-oxidation (A-O) showed a strong correlation between methods for white and blush wines (r² = 0.97) and a poor correlation for red wines (r² = 0.72). A-O values averaged double the HS-GDT values in red wines. The difference in molecular SO₂ values by A-O and HS-GDT correlated well with the estimated molecular SO₂ decrease due to formation of anthocyanin-bisulfite adduct (r² = 0.936), supporting the hypothesis that dissociation of anthocyanin-bisulfite adducts occurs during A-O analysis. By not perturbing this equilibrium, HS-GDT accurately reports free and molecular SO₂ values.