Sulfur dioxide

Names IUPAC nameOther names Identifiers 3535237 ChEBIChEMBLChemSpiderECHA InfoCard100.028.359EC NumberE numberE220 (preservatives)1443 KEGGMeSHSulfur+dioxideRTECS numberUNIIUN number1079, 2037 Properties SO
₂Molar mass64.066 g/mol Appearance Colorless gas OdorPungent; similar to a just-struck match Density2.619 kg m Melting point−72 °C; −98 °F; 201 K Boiling point−10 °C (14 °F; 263 K) 94 g/L
forms sulfurous acidVapor pressure230 kPa at 10 °C; 330 kPa at 20 °C; 462 kPa at 30 °C; 630 kPa at 40 °C Acidity (pK_a) ~1.81 Basicity (pK_b) ~12.19 −18.2·10 cm/mol Viscosity12.82 μPa·s Structure C_2vDigonal Dihedral 1.62 D Thermochemistry 248.223 J K mol −296.81 kJ mol Hazards GHS labelling: DangerH314, H331, H370NFPA 704 (fire diamond) Lethal dose or concentration (LD, LC): 3000 ppm (mouse, 30 min)
2520 ppm (rat, 1 hr) 993 ppm (rat, 20 min)
611 ppm (rat, 5 hr)
764 ppm (mouse, 20 min)
1000 ppm (human, 10 min)
3000 ppm (human, 5 min) NIOSH (US health exposure limits): TWA 5 ppm (13 mg/m) TWA 2 ppm (5 mg/m) ST 5 ppm (13 mg/m) 100 ppm Related compounds Sulfur monoxide
Sulfur trioxide
Disulfur monoxideOzone
Selenium dioxide
Tellurium dioxide
Polonium dioxide

Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula SO
₂. It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is released naturally by volcanic activity and is produced anthropogenically as a by-product of metals refining and the burning of fossil fuels for energy.

Sulfur dioxide is somewhat toxic to humans, although only when inhaled in relatively large quantities for a period of several minutes or more. It was known to medieval alchemists as "volatile spirit of sulfur".