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.

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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.

I. K. MALINA

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References in periodicals archive

The polymerization of sulfur trioxide gives rise to three solid polymeric modifications (Scheme 7) (24).

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In addition, the system is resistant to sulfur trioxides, which can cause the stress cracking that commonly derives from true 98% sulfuric acid.

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