Sulfur Dioxide
Properties
| State | Gas at room temperature (liquefies under modest pressure) |
| Color | Colorless |
| Solubility | Very soluble in water (94 g/L at 25°C, forming aqueous SO2/H2SO3 equilibrium); soluble in ethanol and acetic acid |
| Melting Point | -72°C |
| Boiling Point | -10°C |
About Sulfur Dioxide
Sulfur dioxide is the bent, polar molecule SO2 (64.058 g/mol, O–S–O angle 119°), instantly recognizable to anyone who has lit a kitchen match or stood downwind of a copper smelter. The S–O bonds are intermediate between single and double in length (143 pm) because SO2 is a resonance hybrid with the lone pair on sulfur giving it a dipole moment of 1.63 D and pronounced Lewis basicity. SO2 forms naturally from volcanic eruptions (Mount Pinatubo injected about 17 million tonnes into the stratosphere in 1991, dropping global temperatures 0.5°C for two years) and biologically from bacterial sulfur reduction in anoxic sediments, but the dominant anthropogenic source is combustion of sulfur-containing fossil fuels — primarily coal-fired power generation and bunker-grade marine fuel oil. In the atmosphere, SO2 oxidizes within hours to days to SO3 via OH-radical chemistry, then hydrates to sulfuric acid aerosol that nucleates cloud droplets and falls as acid rain. Global emissions peaked around 150 Mt/year in 1980 and have since dropped below 80 Mt/year thanks to flue-gas desulfurization (wet limestone scrubbers) on coal plants, IMO 2020 marine fuel sulfur cap (3.5% → 0.5% S), and the collapse of high-sulfur coal use in OECD countries. Industrially, SO2 is a feedstock — the Contact process burns sulfur or roasts pyrite to make SO2, which is catalytically oxidized over V2O5 to SO3, then absorbed into 98% H2SO4 to make oleum and back to acid.
Where you'll encounter it
If you've ever pulled the cork on a bottle of dry white wine and caught a brief sulfur whiff before the fruit comes through, that's free SO2 added by the winemaker (typically 25–45 mg/L as molecular SO2) doing its antimicrobial and antioxidant job — the EU mandates the warning 'contains sulfites' on any wine above 10 mg/L. In a sun-dried apricot or a pale golden raisin, the bright color is preserved by SO2 fumigation that inhibits polyphenol oxidase browning; the unsulfured versions are the dark brown ones. In a paper mill, the kraft pulping recovery boiler emits SO2 that is captured in a wet scrubber (the characteristic rotten-cabbage-plus-matchhead odor downwind of an old paper mill is mostly mercaptans plus SO2). And if you have asthma and have ever wheezed within minutes of finishing a glass of red wine or a bowl of dried fruit, the trigger is almost certainly the free SO2 — sulfite-induced bronchoconstriction affects roughly 5–10% of asthmatics.
Common Uses
- Antimicrobial and antioxidant in wine, dried fruit, fruit juice concentrates (E220, 25–250 mg/kg)
- Feedstock for the Contact process producing sulfuric acid (250 Mt/year globally)
- Bleaching agent for chemical pulp, wool, silk, and straw products
- Reducing agent in dechlorination of municipal water and wastewater effluent
- Refrigerant in pre-1930 industrial cooling systems (replaced by ammonia and CFCs)
- Solvent for free-radical polymerization studies and inert gas reactions in coordination chemistry
- Ozone-layer-friendly foam blowing agent for some specialty polystyrene foams
Safety Information
OSHA PEL is 5 ppm 8-hr TWA; ACGIH TLV is 0.25 ppm STEL — much lower because asthmatics show bronchoconstriction at 0.5 ppm within minutes. NIOSH IDLH is 100 ppm. Detection threshold by smell is around 0.5 ppm so the gas warns before reaching dangerous levels for healthy adults, but asthmatics can react below the odor threshold. GHS: H331 toxic if inhaled, H314 causes severe skin burns and eye damage, H280 contains gas under pressure. Acute high-dose exposure causes chemical bronchitis, pulmonary edema, and reactive airway dysfunction syndrome (RADS). Use only with engineering controls (fume hood, local exhaust); cylinders require a chlorine-style regulator and a sulfite scrubber on the vent line. Liquid SO2 causes severe frostbite on skin contact.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.