sulfur dioxide

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sulfur dioxide,

chemical compound, SO2, a colorless gas with a pungent, suffocating odor. It is readily soluble in cold water, sparingly soluble in hot water, and soluble in alcohol, acetic acid, and sulfuric acid. It is corrosive to organic materials and dissolves in water to form sulfurous acid, H2SO3. Sulfur dioxide is used in bleaching and in chemical manufacture and as a refrigerant and a food preservative, e.g., for fumigating fruit. It may be produced by reaction of sulfur with oxygen, e.g., by burning sulfur in air, and it is often produced during the roasting of sulfide ores, e.g., in zinc smelting. Sulfur dioxide is a dangerous air pollutant because of its corrosive properties; it irritates the eyes, nose, and lungs. It is produced by combustion of coal, fuel oil, and gasoline, since these fuels contain sulfur. The sulfur content of a fuel can be reduced by refining, so that less sulfur dioxide is emitted when the fuel is burned.

Sulfur Dioxide


(also sulfurous acid anhydride), SO2, an oxide of sulfur where the oxidation state is +4. A colorless gas with a characteristic sharp odor, sulfur dioxide is encountered in nature in volcanic gases. It condenses at - 10.5°C to a colorless liquid and solidifies at - 75°C to a crystalline mass. It has a critical temperature of 157.3°C and a critical pressure of 77.8 atmospheres.

Sulfur dioxide dissolves readily in water with the formation of sulfurous acid (H2 SO3). Oxygen oxidizes SO2 at high temperatures in the presence of catalysts. Sulfur dioxide can be oxidized to SO3 and H2 SO4 or reduced to sulfur. It forms salts of sulfurous acid with aqueous alkaline solutions. SO2 is very stable thermally, dissociating to S and O2 or to SO and O only at around 2800°C.

In the laboratory, sulfur dioxide is prepared by the action of H2 SO4 on hydrosulfites, for example

2NaHSO3 + H2 SO4 = Na2 SO4 + 2SO2 + 2H2 O

It is also prepared by heating copper filings with concentrated sulfuric acid:

Cu + 2H2 SO4 = CuSO4 + SO2 + 2H2 O

SO2 is used chiefly in the production of sulfuric acid; it is also used in the paper and textile industries and in the sulfation of vegetables and fruits. Sulfur dioxide can be used in refrigeration because it condenses easily and has a high heat of vaporization. As a strong reducing agent in aqueous solutions, SO2 decolorizes many organic dyes and is used in the bleaching of fabrics and sugar.

Sulfur dioxide is toxic. It can enter the organism through the respiratory passages during the roasting of sulfur ores (in the production of sulfuric acid) at copper-smelting plants and the burning of fuels containing sulfur in forges and boilers at, for example, superphosphate plants and thermal electric power stations. In mild cases, sulfur dioxide poisoning produces a cough, rhinitis, tearing, a dry sensation in the throat, hoarseness, and chest pains. With moderately severe acute poisoning, there is also headache, dizziness, general fatigue, and pain in the epigastric region. Examination will reveal symptoms of chemical burns on the mucous membrane of the respiratory passages. Prolonged exposure to sulfur dioxide may produce chronic poisoning, which manifests itself through atrophic rhinitis and damage of the teeth and is often accompanied by toxic bronchitis with attacks of asphyxia. Damage to the liver and circulatory system and the development of pneumosclerosis are possible.

Preventive measures include the hermetic sealing of production equipment, effective ventilation, the removal of sulfur dioxide from flue and waste gases, and the personal protection of respiratory organs (gas mask). The maximum permissible concentration of sulfur dioxide in the air at production facilities is 1.0 mg/m3. The daily average concentration in populated areas should not exceed 0.15 mg/m3. Every year tens of millions of tons of sulfur dioxide from the industrial combustion of coals and petroleum containing compounds of sulfur are released into the atmosphere. The removal of sulfur dioxide from waste gases is an important scientific and engineering problem.


sulfur dioxide

[′səl·fər dī′äk‚sīd]
(inorganic chemistry)
SO2 A toxic, irritating, colorless gas soluble in water, alcohol, and ether; boils at -10°C; used as a chemical intermediate, in artificial ice, paper pulping, and ore refining, and as a solvent. Also known as sulfurous acid anhydride.
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