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EC number: 231-197-3 | CAS number: 7446-11-9
Endpoint summary
Administrative data
Description of key information
Additional information
Sulphur trioxide has several different forms due to structural changes caused by traces of water. Absolutely pure gaseous sulphur trioxide condenses to form a cyclic trimeric structure also known as γ-SO₃, a colourless solid with a melting point of 16.8 °C. Condensation of sulphur trioxide at temperatures >27 °C results in the formation of α-SO₃, a fibrous polymeric substance with a melting point of 62.3 °C. An additional polymeric form of sulphur trioxide (β-SO₃) is similarly fibrous in appearance but has a melting point of 32.5 °C. Both γ-SO₃ and β-SO₃ are metastable and will convert to α-SO₃ in the presence of trace amounts of moisture. The vapour pressure of solid sulphur trioxide also varies with polymeric state: α-SO₃ < β-SO₃ < γ-SO₃). The vapour pressure of α-SO₃ is similar to that of the liquid form.
Appearance
Sulphur trioxide is reported to be a colourless fuming liquid at 20 °C (Kirk-Othmer). Its various forms exist as 'asbestos-like' needles (alpha- and beta-forms), or as an ice-like mass or liquid (gamma-form) (Merck Index).
Melting point
The melting point of sulphur trioxide is reported to be 16.8 °C. However, it should be noted that the different polymerised forms of the substance have different melting points: α form (62.4 °C), β form (32.5 °C), γ form (16.8 °C). Melted sulphur trioxide usually exists in the γ form.
Boiling point
The boiling point of sulphur trioxide is reported to be 44.8 °C at 1013 hPa (CRC Handbook)
Density
The density of sulphur trioxide (γ phase) is reported to be 1.9224 g/mL at 20 °C (Kirk-Othmer).
Granulometry
Waiver. The substance is generally present in a liquid form at room temperature but can exist in several solid polymeric forms. However, measurement is not technically possible due to the rapid reaction of the substance with atmospheric moisture.
Vapour pressure
The vapour pressure of sulphur trioxide varies with polymeric state: 97.3 hPa (α form), 458.6 hPa (β form) and 577.2 hPa (γ form) at 25 °C (Merck Index, HSDB). The vapour pressure of the α form is similar to that of the liquid form.
Partition coefficient
Waiver. It is not possible to measure this property of sulphur trioxide as it rapidly hydrolyses to form sulphuric acid. Additionally, this property is not applicable to inorganic substances.
Water solubility
Waiver. It is not possible to measure this property of sulphur trioxide as it rapidly hydrolyses to form sulphuric acid.
Surface tension
Waiver. It is not possible to measure this property of sulphur trioxide as it rapidly hydrolyses to form sulphuric acid. Additionally, based on structure, surface activity is not expected nor can it be predicted and surface activity is not a desired property of the material.
Flash point
Waiver. Measurement of flash-point is not required for an inorganic substance; measurement is not technically possible due to the rapid reaction of the substance with atmospheric moisture.
Auto-flammability
Waiver. Sulphur trioxide itself is non-flammable. Measurement is not technically possible due to the rapid reaction of the substance with atmospheric moisture.
Flammability
Waiver. Sulphur trioxide itself is non-flammable; however, from experience of use and handling, it is known to react with substances such as wood or fabrics with enough heat of hydration to cause ignition.
Explosivity
Waiver. Sulphur trioxide is not explosive; however, it is known that heating α-SO₃ to its melting point can result in a sudden increase in vapour pressure which may be enough to shatter a glass vessel.
Oxidising properties
Sulphur trioxide does not meet the criteria for classification as an oxidiser.
Stability in organic solvents
Waiver. Not applicable to an inorganic substance.
Dissociation constant
Waiver. It is not possible to measure this property of sulphur trioxide as it rapidly hydrolyses to form sulphuric acid.
Viscosity
The dynamic viscosity of sulphur trioxide is reported to be 1.820 mPa s at 25 °C (Chemistry of the Elements, 2nd ed.).
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