Sulfur Trioxide
Title: Sulfur Trioxide
CAS Registry Number: 7446-11-9
Additional Names: Sulfuric anhydride
Molecular Formula: O3S
Molecular Weight: 80.06
Percent Composition: O 59.95%, S 40.05%
Line Formula: SO3
Literature References: Prepd by the contact process, i.e., by the action of oxygen on sulfur dioxide in the presence of catalysts such as platinized asbestos, platinized magnesium sulfate, ferric oxide, or vanadium compds. Extensive review and bibliography: Fairlie, Sulfuric Acid Manufacture, A.C.S. Monograph Series no. 69 (New York, 1936). Exists in 3 modifications. May be prepd in the laboratory by heating fuming sulfuric acid and collecting the sublimate in a cooled receiver. If the vapor is condensed above 27°, the g-form is obtained as a liquid. If the vapor is condensed below 27° and in the presence of a trace of moisture, a mixture of all 3 forms is obtained. The 3 forms can be separated by fractional distillation. The a-form is the stable modification, the b- and g-forms are metastable. Review of toxicology and human exposure: Toxicological Profile for Sulfur Trioxide and Sulfuric Acid (PB99-122038, 1998) 224 pp.
Properties: Melted SO3 exists in the g-form and on solidifying tends to the a-form. The difference in vapor pressures explains the so-called "alpha explosion" of sulfur trioxide. Heating of high-melting SO3 in glass vessels should be avoided to prevent possible shattering of the container. Absolutely dry SO3 is not corrosive to metals and shows no acid reaction. On exposure to air, it absorbs moisture rapidly, emitting dense white fumes. Combines with water with explosive violence (heat of dilution 504 cals/g) forming sulfuric acid. Due to this avidity for water, SO3 chars many organic substances. On contact with wood shavings the heat produced by dehydration is sufficient to cause fire.
Derivative Type: a-Form
Properties: Asbestos-like needles, mp 62.3°. Vapor pressure at 25° = 73 mm.
Melting point: mp 62.3°
Derivative Type: b-Form
Properties: Asbestos-like neeedles, mp 32.5°. Vapor pressure at 25° = 344 mm.
Melting point: mp 32.5°
Derivative Type: g-Form
Properties: Ice-like mass, mp 16.8° or liquid, d 1.9224. bp760 44.8°. Vapor pressure at 25° = 433 mm.
Melting point: mp 16.8°
Boiling point: bp760 44.8°
Density: d 1.9224
CAUTION: Potential symptoms of overexposure by inhallation are tooth erosion and respiratory tract irritation; by ingestion are mouth and throat burns, gastric erosion; direct contact may cause irritation or chemical burns of skin, eyes, respiratory and gastrointestinal tracts. See PB99-122038.
Use: Intermediate in sulfuric acid manuf; in sulfonations for formation of addition compds with amines; in the manuf of explosives.

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