Sulfur Trioxide

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sulfur trioxide

[′səl·fər trī′äk‚sīd]
(inorganic chemistry)
SO3 A toxic, irritating liquid in three forms, α, β, γ, with respective melting points of 62°C, 33°C, and 17°C; a strong oxidizing agent and fire hazard; used for sulfonation of organic chemicals.

Sulfur Trioxide


(also sulfuric anhydride), SO3, an oxide of sulfur, which in this case has an oxidation state of +6. Solid sulfur trioxide exists in α, β, γ, and δ forms, with melting points of 16.8°, 32.5°, 62.3°, and 95°C, respectively. The forms differ in crystal shape and degree of SO3 polymerization. The unstable α form is formed during the solidification of liquid sulfur trioxide and gradually undergoes transition in the presence of moisture to the stable β form, which occurs as silky crystals, similar to those of asbestos. The β form may be reconverted to the α form only through the gaseous state of SO3. At normal pressure and a temperature of 44.7°C, gaseous SO3 is converted into a liquid, which, upon solidification at 16.8°C, forms α-SO3. The mutual transitions of the other modifications proceed very slowly.

The critical temperature of sulfur trioxide is 218.3°C, and the critical pressure is 83.8 atmospheres. As a vapor, SO3 is monomolecular. Dissociation of sulfur trioxide into SO2 and O2 from heat begins at approximately 450°C and is practically complete at 1200°C. SO3 dissolves in water with the formation of H2 SO4: it reacts with basic oxides and bases. As a strong oxidizing agent, SO3 oxidizes sulfur, phosphorus, and hydrocarbons, thereby undergoing reduction to SO2. In the laboratory, sulfur trioxide is produced by roasting Fe2(SO4)3 or by the action of an excess of P2 O5 on concentrated H2 SO4. The industrial process used in producing SO3 involves the catalytic oxidation of SO2.

Sulfur trioxide is used as a sulfonating agent in the production of many organic products. It is also used in the production of oleum and anhydrous HNO3.