Sulfur dioxide

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Sulfur dioxide
Chemical formula SO2
OTP appearance colorless noxious gas 
Molar Mass(g/mol) 64.066 
Density(g/cc) 0.0026288 
Melting Point(°C) -72 
Boiling Point(°C) -10 
Solubility in water(g/L) 94 
Speed of sound
20°C, 1atm (m/s)
Immediate Danger to Life and Health 100ppm
NFPA 704


The primary use of sulfur dioxide is as a feedstock for sulfuric acid via sulfur trioxide. Heavier than air.

Natural occurence

  • Sulfur dioxide occurs naturally in volcanic vents and sulfur springs


  • Inhalation of large amounts of sulfur dioxide can result in pulmonary edema, which (according to ) is treated with supportive care for breathing and heart issues.[1]



There are many ways to produce sulfur dioxide depending on the raw materials available. Each of the below produces approximately 1 mol of sulfur dioxide:

from Sulfur

Molten sulfur combines with air almost quantitatively producing sulfur dioxide, most efficiently by spraying fine droplets of molten sulfur into preheated (120°C) air:

  • S8 + 8 O2 8 SO2

Thermal decomposition of mineral sulfides and sulfates

  • Roast 1 mol (240g) of galena in air at 1000°C
    2 PbS + 3 O2 2 PbO + 2 SO2
  • Roast 1/2 mol (60g) of iron pyrite in air
    4 FeS2 + 11 O2 2 Fe2O3 + 8 SO2
  • Roast 5/6 mol (113.3g) of gypsum and 1/6 mol (20g) of iron pyrite with 1/2 mol (30g) of silicon dioxide
    10 CaSO4 + 2 FeS2 + 6 SiO2 Fe2SiO4 + 5 Ca2SiO4 + 14 SO2
    This has the interesting characteristic of requiring no external oxygen source
    The fayelite (Fe2SiO4) can be oxidized further to magnetite and silicon dioxide
    The calcium silicate Ca2SiO4 can be used as a fire retardant


sodium sulfite
Very roughly, 1ubm of sulfite to 2 ubm of commercial or 1:3 of azeotropic HCl produces SO2
  1. Gather 126ubm (1M) sodium sulfite
  2. Gather 500ubm of water
  3. Gather 255ubm (222ml, 2M) of commercial (30%-ish, 9M) hydrochloric acid
    or 365ubm (333ml, 2M) of azeotropic (20.2%, 6M) hydrochloric acid
  4. Completely dissolve the sodium sulfite in the water.
  5. Drip hydrochloric acid into sodium sulfite solution producing 2 mols of salt, 1 mol of water, and 1 mol of sulfur dioxide
    Na2SO3 + 2 HCl 2 NaCl + SO2 + H2O
sodium metabisulfite


  • Sulfur dioxide is only slightly soluble in water, whereas sulfur trioxide reacts, and carbon dioxide is very soluble. Bubbling through warm water, therefore, will remove the bulk of these compounds.



  • A practical way to contain and store sulfur dioxide effluent is to combine it with sodium hydroxide, producing sodium bisulfite. Sodium bisulfite is readily convertible back to sulfur dioxide by adding almost any acid.
    SO2(g) + NaOH(aq) NaHSO3 // put
    NaHSO3 + HCl NaCl + H2O(l) + SO2(g) // get


  • Burn with hydrogen sulfide producing elemental sulfur and water
    SO2 + 2 H2S 2 H2O + 3 S
  • Bubble through a solution of potassium permanganate
    5 SO2 + 2 KMnO4 + 2 H2O 2 H2SO4 + 2 MnSO4 + K2SO4

See Also