Sulfur dioxide (or Sulphur dioxide) has the chemical formula SO₂. The gas is irritating to the lungs and is frequently described as smelling of burning sulfur.

It is produced by volcanoes and in various industrial processes. In particular, poor-quality coal and petroleum contain sulfur compounds, and generate sulfur dioxide when burned: the gas reacts with water and atmospheric oxygen to form sulfurous acid (H₂SO₃) and thus acid rain.

Preparation of sulfur dioxide for chemical production

Sulfur dioxide is often prepared by burning sulfur in air:

S(s) + O₂ (g) → SO₂ (g)

Hydrogen sulfide from crude oil may also be burned.

H₂S (g) + O₂ {g} → H₂ (g) + SO₂ (g)

Sulfide ores such as iron pyrites and sphalerite (zinc blende) may also be used:

4 FeS₂ (s) + 11 O₂ (g) → 2 Fe₂O₃ (s) + 8 SO₂ (g)

2 ZnS (s) + 3 O₂ (g) → 2 ZnO (s) + 2 SO₂ (g)

When anhydrous CaSO₄, is heated with coke and sand in the manufacture of cement, CaSiO₃, sulfur dioxide is a by-product.

2 CaSO₄ (s) + 2 SiO₂ (s) + C (s) → 2 CaSiO₃ + 2 SO₂ (g) + CO₂

Flue gas desulfurisation in power stations releases SO₂ for the process.

Uses

Sulfur dioxide is sometimes used as a preservative in alcoholic drinks, or dried apricots.

H₂SO₃ is also called "hydrogen sulfite" or sulfurous acid.

See also:

• Sulfur
• Sulfur trioxide

External links

• Food Intolerance Network - Sulphite factsheet
• Sulphur Dioxide, Molecule of the Month