Sulfur trioxide (alternative spelling sulphur trioxide, also known as nisso sulfan) is the chemical compound with the formula SO3. It has been described as "unquestionably the most [economically] important sulfur oxide".[1] It is prepared on an industrial scale as a precursor to sulfuric acid.

Sulfur trioxide

Preferred IUPAC name
Sulfur trioxide
Systematic IUPAC name
Other names
Sulfuric anhydride, Sulfur(VI) oxide
3D model (JSmol)
ECHA InfoCard 100.028.361 Edit this at Wikidata
EC Number
  • 231-197-3
RTECS number
  • WT4830000
UN number UN 1829
  • InChI=1S/O3S/c1-4(2)3 checkY
  • InChI=1/O3S/c1-4(2)3
  • (monohydrate): InChI=1S/O3S.H2O/c1-4(2)3;/h;1H2
  • monomer: O=S(=O)=O
  • γ-trimer: O=S0(=O)OS(=O)(=O)OS(=O)(=O)O0
  • α/β polymer: OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)O
  • (monohydrate): O.O=S(=O)=O
Molar mass 80.066 g/mol
Appearance Colorless to white crystalline solid which will fume in air.[2] Colorless liquid and gas.[3]
Odor Varies. Vapor is pungent; like sulfur dioxide.[4] Mist is odorless.[3]
Density 1.92 g/cm3, liquid
Melting point 16.9 °C (62.4 °F; 290.0 K)
Boiling point 45 °C (113 °F; 318 K)
Reacts to give sulfuric acid
256.77 JK−1mol−1
−395.7 kJ/mol
Occupational safety and health (OHS/OSH):
Main hazards
Highly corrosive, extremely strong dehydrating agent
GHS labelling:
GHS05: Corrosive GHS07: Exclamation mark
H314, H335
P261, P280, P305+P351+P338, P310[5]
NFPA 704 (fire diamond)
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
rat, 4 hr 375 mg/m3[citation needed]
Safety data sheet (SDS) ICSC 1202
Related compounds
Other cations
Selenium trioxide
Tellurium trioxide
Polonium trioxide
Related sulfur oxides
Sulfur monoxide
Sulfur dioxide
Related compounds
Sulfuric acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Sulfur trioxide exists in several forms: gaseous monomer, crystalline trimer, and solid polymer. Sulfur trioxide is a solid at just below room temperature with a relatively narrow liquid range. Gaseous SO3 is the primary precursor to acid rain.[6]

Molecular structure and bonding




The molecule SO3 is trigonal planar. As predicted by VSEPR theory, its structure belongs to the D3h point group. The sulfur atom has an oxidation state of +6 and may be assigned a formal charge value as low as 0 (if all three sulfur-oxygen bonds are assumed to be double bonds) or as high as +2 (if the Octet Rule is assumed).[7] When the formal charge is non-zero, the S-O bonding is assumed to be delocalized. In any case the three S-O bond lengths are equal to one another, at 1.42 Å.[1] The electrical dipole moment of gaseous sulfur trioxide is zero.

Ball-and-stick model of the cyclic trimer of SO3



Both liquid and gaseous[8] SO3 exists in an equilibrium between the monomer and the cyclic trimer. The nature of solid SO3 is complex and at least 3 polymorphs are known, with conversion between them being dependent on traces of water.[9]

Absolutely pure SO3 freezes at 16.8 °C to give the γ-SO3 form, which adopts the cyclic trimer configuration [S(=O)2(μ-O)]3.[10][1]


An ampoule of sulfur trioxide

If SO3 is condensed above 27 °C, then α-SO3 forms, which has a melting point of 62.3 °C. α-SO3 is fibrous in appearance. Structurally, it is the polymer [S(=O)2(μ-O)]n. Each end of the polymer is terminated with OH groups.[1] β-SO3, like the alpha form, is fibrous but of different molecular weight, consisting of an hydroxyl-capped polymer, but melts at 32.5 °C. Both the gamma and the beta forms are metastable, eventually converting to the stable alpha form if left standing for sufficient time. This conversion is caused by traces of water.[11]

Relative vapor pressures of solid SO3 are alpha < beta < gamma at identical temperatures, indicative of their relative molecular weights. Liquid sulfur trioxide has a vapor pressure consistent with the gamma form. Thus heating a crystal of α-SO3 to its melting point results in a sudden increase in vapor pressure, which can be forceful enough to shatter a glass vessel in which it is heated. This effect is known as the "alpha explosion".[11]

Chemical reactions


Sulfur trioxide undergoes many reactions.[1]

Hydration and hydrofluorination


SO3 is the anhydride of H2SO4. Thus, it is susceptible to hydration:

SO3 + H2O → H2SO4 fH = −200 kJ/mol)[12]

Gaseous sulfur trioxide fumes profusely even in a relatively dry atmosphere owing to formation of a sulfuric acid mist. SO3 is aggressively hygroscopic. The heat of hydration is sufficient that mixtures of SO3 and wood or cotton can ignite. In such cases, SO3 dehydrates these carbohydrates.[11]

Akin to the behavior of H2O, hydrogen fluoride adds to give fluorosulfuric acid:

SO3 + HF → FSO3H



SO3 reacts with dinitrogen pentoxide to give the nitronium salt of pyrosulfate:

2 SO3 + N2O5 → [NO2]2S2O7



Sulfur trioxide is an oxidant. It oxidizes sulfur dichloride to thionyl chloride.

SO3 + SCl2 → SOCl2 + SO2

Lewis acid


SO3 is a strong Lewis acid readily forming adducts with Lewis bases.[13] With pyridine, it gives the sulfur trioxide pyridine complex. Related adducts form from dioxane and trimethylamine.

Sulfonating agent


Sulfur trioxide is a potent sulfonating agent, i.e. it adds SO3 groups to substrates. Often the substrates are organic, as in aromatic sulfonation.[14] For activated substrates, Lewis base adducts of sulfur trioxide are effective sulfonating agents.[15]



The direct oxidation of sulfur dioxide to sulfur trioxide in air proceeds very slowly:

SO2 + 12O2 → SO3 H = −198.4 kJ/mol)



Industrially SO3 is made by the contact process. Sulfur dioxide is produced by the burning of sulfur or iron pyrite (a sulfide ore of iron). After being purified by electrostatic precipitation, the SO2 is then oxidised by atmospheric oxygen at between 400 and 600 °C over a catalyst. A typical catalyst consists of vanadium pentoxide (V2O5) activated with potassium oxide K2O on kieselguhr or silica support. Platinum also works very well but is too expensive and is poisoned (rendered ineffective) much more easily by impurities.[16] The majority of sulfur trioxide made in this way is converted into sulfuric acid.



Sulfur trioxide can be prepared in the laboratory by the two-stage pyrolysis of sodium bisulfate. Sodium pyrosulfate is an intermediate product:[17]

  1. Dehydration at 315 °C:
    2 NaHSO4 → Na2S2O7 + H2O
  2. Cracking at 460 °C:
    Na2S2O7 → Na2SO4 + SO3

The latter occurs at much lower temperatures (45–60 °C) in the presence of catalytic H2SO4.[18] In contrast, KHSO4 undergoes the same reactions at a higher temperature.[17]

Another two step method involving a salt pyrolysis starts with concentrated sulfuric acid and anhydrous tin tetrachloride:

  1. Reaction between tin tetrachloride and sulfuric acid in a 1:2 molar mixture at near reflux (114 °C):
    SnCl4 + 2 H2SO4 → Sn(SO4)2 + 4 HCl
  2. Pyrolysis of anhydrous tin(IV) sulfate at 150 °C - 200 °C:
    Sn(SO4)2 → SnO2 + 2 SO3

The advantage of this method over the sodium bisulfate one is that it requires much lower temperatures and can be done using normal borosilicate laboratory glassware without the risk of shattering. A disadvantage is that it generates significant quantities of hydrogen chloride gas which needs to be captured as well.

SO3 may also be prepared by dehydrating sulfuric acid with phosphorus pentoxide.[19]



Sulfur trioxide is a reagent in sulfonation reactions. Dimethyl sulfate is produced commercially by the reaction of dimethyl ether with sulfur trioxide:[20]

CH3OCH3 + SO3 → (CH3)2SO4

Sulfate esters are used as detergents, dyes, and pharmaceuticals. Sulfur trioxide is generated in situ from sulfuric acid or is used as a solution in the acid.

B2O3 stabilized sulfur trioxide was traded by Baker & Adamson under the tradename "Sulfan" in the 20th century.[21]



Along with being an oxidizing agent, sulfur trioxide is highly corrosive. It reacts violently with water to produce highly corrosive sulfuric acid.

See also



  1. ^ a b c d e f Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 703–704. ISBN 978-0-08-037941-8.
  3. ^ a b Lerner, L. (2011). Small-Scale Synthesis of Laboratory Reagents with Reaction Modeling. CRC Press. p. 10. ISBN 9781439813133. LCCN 2010038460.
  4. ^ "Substance:Sulfur trioxide - Learn Chemistry Wiki".
  5. ^ "Sulfur trioxide 227692" (PDF). SO3. Archived from the original on 2020-09-01. Retrieved 1 September 2020.
  6. ^ Thomas Loerting; Klaus R. Liedl (2000). "Toward elimination of descrepancies between theory and experiment: The rate constant of the atmospheric conversion of SO3 to H2SO4". Proceedings of the National Academy of Sciences of the United States of America. 97 (16): 8874–8878. Bibcode:2000PNAS...97.8874L. doi:10.1073/pnas.97.16.8874. PMC 16788. PMID 10922048.
  7. ^ Housecroft, Catherine E.; Sharpe, Alan G. (2012). Inorganic Chemistry (4 ed.). Essex, England: Pearson. p. 575.
  8. ^ Lovejoy, R. W.; Colwell, J. H.; Eggers, D. F.; Halsey, G. D. (February 1962). "Infrared Spectrum and Thermodynamic Properties of Gaseous Sulfur Trioxide". The Journal of Chemical Physics. 36 (3): 612–617. Bibcode:1962JChPh..36..612L. doi:10.1063/1.1732581.
  9. ^ Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, ISBN 0-12-352651-5
  10. ^ Westrik, R.; Mac Gillavry, C. H. (1941). "The crystal structure of the ice-like form of sulphur trioxide (γ-modification)". Recueil des Travaux Chimiques des Pays-Bas. 60 (11): 794–810. doi:10.1002/recl.19410601102.
  11. ^ a b c Merck Index of Chemicals and Drugs, 9th ed. monograph 8775
  12. ^ "The Manufacture of Sulfuric Acid and Superphosphate" (PDF). Chemical Processes in New Zealand. Archived from the original (PDF) on 2018-01-27. Retrieved 2016-04-22.
  13. ^ Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5
  14. ^ Weil, J. K.; Bistline Jr., R. G.; Stirton, A. J. (1956). "α-Sulfopalmitic Acid". Organic Syntheses. 36: 83. doi:10.15227/orgsyn.036.0083.
  15. ^ Rondestvedt Jr., Christian S.; Bordwell, F. G. (1954). "Sodium β-Styrenesulfonate and β-Styrenesulfonyl Chloride". Organic Syntheses. 34: 85. doi:10.15227/orgsyn.034.0085.
  16. ^ Hermann Müller "Sulfuric Acid and Sulfur Trioxide" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim. 2000 doi:10.1002/14356007.a25_635
  17. ^ a b K.J. de Vries; P.J. Gellings (May 1969). "The thermal decomposition of potassium and sodium-pyrosulfate". Journal of Inorganic and Nuclear Chemistry. 31 (5): 1307–1313. doi:10.1016/0022-1902(69)80241-1.
  18. ^ GarageChemist. "Preparation of Sulfur Trioxide and Oleum" (PDF). pp. 1–2.
  19. ^ "How to make sulfur trioxide - YouTube". Retrieved 1 September 2020.
  20. ^ Weisenberger, Karl; Mayer, Dieter; Sandler, Stanley R. (2000). "Dialkyl Sulfates and Alkylsulfuric Acids". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a08_493. ISBN 978-3-527-30385-4.
  21. ^ Habashi, Fathi; Dugdale, Raymond (June 1973) [1972-11-06]. "The Action of Sulfur Trioxide on Chalcopyrite". Metallurgical and Materials Transactions. B-4 (6): 1553–1556. Bibcode:1973MT......4.1553H. doi:10.1007/BF02668007. S2CID 93744787. p. 1553: Sulfur trioxide used was pure, colorless liquid SO3 marketed under the trade name Sulfan by Baker and Adamson