Sulfur dioxide

Sulfur dioxide

Names
IUPAC name Sulfur dioxide
Other names Sulfur(IV) oxide Sulfurous anhydride
Properties
Chemical formula	SO2
Molar mass	64.066 g/mol
Appearance	Colorless gas
Odor	Sulfurous
Density	2.6288 kg/m3
Melting point	−72 °C (−98 °F; 201 K)
Boiling point	−10 °C (14 °F; 263 K)
Solubility in water	9.4 g/100 ml
Solubility	Soluble in acetone, chloroform, diethyl ether, ethanol, formic acid
Vapor pressure	237.2 kPa
Acidity (pKa)	1.81
Thermochemistry
Std molar entropy (So298)	248.223 J·K−1·mol−1
Std enthalpy of formation (ΔfHo298)	−296.81 kJ/mol
Hazards
Safety data sheet	Sigma-Aldrich
Flash point	Non-flammable
Lethal dose or concentration (LD, LC):
LC50 (Median concentration)	3,000 ppm (mouse, 30 min) 2,520 ppm (rat, 1 hr)
Related compounds
Related compounds	Sulfur trioxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Sulfur dioxide is a gas with an irritating smell, industrially used mainly to make sulfuric acid. It has the chemical formula SO₂.

Properties

Chemical

Sulfur dioxide will react with alkali to form sulfites:

SO₂ + 2 NaOH → Na₂SO₃ + H₂O

Sulfur dioxide will reduce hydrogen sulfide to elemental sulfur:

SO₂ + 2 H₂S → 3 S + 2 H₂O

Sulfur dioxide can be further oxidized by halogens, to sulfuryl halides, such as sulfuryl chloride:

SO₂ + Cl₂ → SO₂Cl₂

Sulfur dioxide can be further oxidized by oxygen to sulfur trioxide:

2 SO₂ + O₂ → 2 SO₃

Sulfur dioxide dissolves in water, forming sulfurous acid. This acid is very similar to carbonic acid in that it only exists in very small quantities as an ionic species in such solutions, and can only be observed as an anhydrous substance in very extreme conditions.

Physical

Sulfur dioxide is a toxic white gas, with a poignant smell of burned matches. It is soluble in water at low temperature. SO₂ boils at −10 °C, and melts at −72 °C.

Availability

Liquified sulfur dioxide is sold as compressed gas in chemical industry.

More accessible, you can buy aq. solutions of sulfur dioxide from many winemaking suppliers. Usually the solutions have a concentration of 5-10% and are very cheap.

Preparation

Sulfur is usually cheap, so burning sulfur in air produces plenty of sulfur dioxide. However, set up limitations sometimes prevent the use of this method, usually due to having a closed system where the sulfur cannot be open to the air. Mixing an oxidizer such as potassium chlorate or potassium nitrate can solve this issue but a wet method may be preferred. In that case, the action of a strong acid on a sulfite or bisulfite will create sulfur dioxide gas.

When burning elemental sulfur, a small amount will aerosolize and may contaminate the installation and the purity of the product. There are a few ways to remove the impurities: one requires to dissolve the gas in cold water, filter the resulting sulfurous acid solution and then boil the solution to desorb the sulfur dioxide from solution. Another way is to wash the resulting gas in a wash column, usually with warm water to reduce the adsorption of SO₂ or aprotic solvents that do not react with it. Electrostatic precipitators are even better, as they do not require adsorption-desorption processes and are more efficient.

Sulfur dioxide can also be produced by roasting metal sulfides, a common process applied in the sulfuric acid industry. It can also be produced by mixing calcium sulfate with sand (silicon dioxide) and carbon (coke), though this process requires very high temperatures.

"A lovely generator can be made by mixing H₂SO₄ with an equal volume of water, and then adding sulfite or (meta)bisulfite to this. Slight heating gives a smooth and not too fast generation of lots of SO2. You don't need strong heating, and that is a pleasant thing" - Thread on SO₂ generation

Projects

• Make sulfuric acid
• Make sulfur trioxide
• Sulfuryl chloride synthesis
• Reduce gold compounds to gold metal
• Make manganese sulfate

Handling

Safety

Sulfur dioxide is corrosive and toxic. Inhalation of a high quantity can irritate and damage human tissues.

Storage

Storing liquified sulfur dioxide should only be done in corrosion-resistant cylinders.

Disposal

Sulfur dioxide can be neutralized by bubbling it in alkaline solutions or mixing it with hydrogen sulfide or ammonia. Bubbling it through hydrogen peroxide yields sulfuric acid.

References

Relevant Sciencemadness threads

• SO2 Generator
• Clean Sulfur Dioxide Gas