Sulfur trioxide (SO3) structure, properties, risks, uses

Anthony Golden
Sulfur trioxide (SO3) structure, properties, risks, uses

The sulfur trioxide It is an inorganic compound formed by the union of a sulfur atom (S) and 3 oxygen atoms (O). Its molecular formula is SO3. At room temperature, SO3 it is a liquid that releases gases into the air.

The structure of the OS3 gaseous is flat and symmetrical. All three oxygens are equally located around sulfur. The SO3 Reacts violently with water. The reaction is exothermic, which means that heat is produced, in other words, it gets very hot.

Sulfur trioxide molecule SO3. Author: Benjah-bmm27. Source: Wikimedia Commons.

When the SO3 liquid cools, turns into a solid that can have three types of structure: alpha, beta and gamma. The most stable is alpha, in the form of layers joined together constituting a network.

Gaseous sulfur trioxide is used to prepare fuming sulfuric acid, also called oleum, because of its resemblance to oil or oily substances. Another of its important applications is in the sulfonation of organic compounds, that is, the addition of -SO groups3- to these. Thus, useful chemicals such as detergents, colorants, pesticides, among many others can be prepared..

The SO3 It is very dangerous, it can cause severe burns, eye and skin damage. Nor should it be inhaled or ingested as it can cause death from internal burns, in the mouth, esophagus, stomach, etc..

For these reasons, it must be handled with great caution. It should never come into contact with water or combustible materials such as wood, paper, fabrics, etc., as fires can occur. Neither should it be disposed of nor should it enter sewers due to the danger of explosion..

The SO3 Gaseous generated in industrial processes should not be released into the environment, as it is one of those responsible for the acid rain that has already damaged large areas of forests in the world.

Article index

  • 1 Structure
  • 2 Nomenclature
  • 3 Physical properties
    • 3.1 Physical state
    • 3.2 Molecular weight
    • 3.3 Melting point
    • 3.4 Triple point
    • 3.5 Boiling point
    • 3.6 Density
    • 3.7 Vapor pressure
    • 3.8 Stability
  • 4 Chemical properties
  • 5 Obtaining
  • 6 Uses
    • 6.1 In the preparation of oleum
    • 6.2 In sulfonation chemical reactions
    • 6.3 In the extraction of metals
    • 6.4 In various uses
  • 7 Risks
    • 7.1 For health
    • 7.2 From fire or explosion
  • 8 Environmental impact
  • 9 References


The sulfur trioxide molecule SO3 in the gaseous state it has a flat triangular structure.

This means that both sulfur and the three oxygens are in the same plane. Furthermore, the distribution of oxygens and all electrons is symmetric.

Lewis resonance structures. Electrons are evenly distributed in SO3. Author: Marilú Stea.

In solid state, three types of SO structure are known3: alpha (α-SO3), beta (β-SO3) and gamma (γ-SO3).

The gamma γ-SO form3 contains cyclic trimers, that is, three units of SO3 together forming a cyclic or ring-shaped molecule.

Gamma-type solid sulfur trioxide ring-shaped molecule. Author: Marilú Stea.

The beta phase β-SO3 possesses infinite helical chains of tetrahedra of composition SO4 linked together.

Structure of a chain of beta-type solid sulfur trioxide. Author: Marilú Stea.

The most stable form is alpha α-SO3, similar to beta but with a layered structure, with the chains joined to form a network.


-Sulfur trioxide

-Sulfuric anhydride

-Sulfuric oxide

-SW3 gamma, γ-SO3

-SW3 beta, β-SO3

-SW3 alpha, α-SO3

Physical properties

Physical state

At room temperature (around 25 ºC) and atmospheric pressure, SO3 it is a colorless liquid that emits fumes into the air.

When the SO3 liquid is pure at 25 ºC it is a mixture of SO3 monomeric (single molecule) and trimeric (3 molecules attached) of formula S3OR9, also called SOgamma γ-SO3.

As the temperature drops, if the SO3 it is pure when it reaches 16.86 ºC, it solidifies or freezes to γ-SO3, also called "SO ice3".

If it contains small amounts of moisture (even traces or extremely small amounts) the SO3 polymerizes to the beta β-SO form3 which forms crystals with a silky shine.

Then more bonds are formed generating the alpha α-SO structure3, which is a needle-shaped crystalline solid that resembles asbestos or asbestos.

When alpha and beta merge they generate gamma.

Molecular weight

80.07 g / mol

Melting point

SW3 gamma = 16.86 ºC

Triple point

It is the temperature at which the three physical states are present: solid, liquid and gas. In the alpha form the triple point is at 62.2 ºC and in the beta it is at 32.5 ºC.

Heating the alpha form has a greater tendency to sublimate than to melt. Sublimate means to go from the solid state to the gaseous state directly, without going through the liquid state.

Boiling point

All forms of OS3 boil at 44.8 ºC.


The SO3 liquid (gamma) has a density of 1.9225 g / cm3 at 20 ºC.

The SO3 gaseous has a density of 2.76 relative to air (air = 1), which indicates that it is heavier than air.

Vapor pressure

SW3 alpha = 73 mm Hg at 25 ºC

SW3 beta = 344 mm Hg at 25 ºC

SW3 gamma = 433 mm Hg at 25 ºC

This means that the gamma form tends to evaporate more easily than beta and this than alpha..


The alpha form is the most stable structure, the others are metastable, that is, they are less stable.

Chemical properties

The SO3 reacts vigorously with water to give sulfuric acid HtwoSW4. When reacting, a lot of heat is produced, which is why water vapor is quickly released from the mixture.

Being exposed to the air the SO3 absorbs moisture quickly, emitting dense vapors.

It is a very strong dehydrating agent, this means that it removes water easily from other materials.

Sulfur in SO3 it has an affinity for free electrons (that is, electrons that are not in a bond between two atoms) so it tends to form complexes with compounds that possess them such as pyridine, trimethylamine or dioxane.

Complex between sulfur trioxide and pyridine. Benjah-bmm27 [Public domain]. Source: Wikimedia Commons.

By forming complexes, sulfur “borrows” the electrons from the other compound to fill its lack of them. Sulfur trioxide is still available in these complexes, which are used in chemical reactions to provide SO3.

It is a powerful sulfonating reagent for organic compounds, which means that it is used to easily add an -SO group3- to molecules.

Reacts easily with the oxides of many metals to give sulfates of these metals.

It is corrosive to metals, animal and plant tissues.

The SO3 It is a difficult material to handle for several reasons: (1) its boiling point is relatively low, (2) it has a tendency to form solid polymers at temperatures below 30 ºC and (3) it has a high reactivity towards almost all organic substances And the water.

It can polymerize explosively if it does not contain a stabilizer and there is presence of moisture. Dimethyl sulfate or boron oxide are used as stabilizers..


It is obtained by the reaction at 400 ºC between sulfur dioxide SOtwo and molecular oxygen Otwo. However, the reaction is very slow and catalysts are required to increase the rate of the reaction..

2 SOtwo + ORtwo ⇔ 2 SO3

Among the compounds that accelerate this reaction are the platinum metal Pt, vanadium pentoxide VtwoOR5, ferric oxide FetwoOR3 and nitric oxide NO.


In the preparation of oleum

One of its main applications is the preparation of oleum or fuming sulfuric acid, so called because it emits vapors visible to the naked eye. To obtain it, the SO is absorbed3 in concentrated sulfuric acid HtwoSW4.

Oleum or fuming sulfuric acid. You can see the white smoke coming out of the bottle. W. Oelen [CC BY-SA 3.0 (]. Source: Wikimedia Commons.

This is done in special stainless steel towers where the concentrated sulfuric acid (which is liquid) goes down and the SO3 soda is rising.

The liquid and gas come into contact and join, forming oleum, which is an oily-looking liquid. This possesses a mixture of HtwoSW4 And so3, but it also has disulfuric acid molecules HtwoStwoOR7 and trisulfuric HtwoS3OR10.

In sulfonation chemical reactions

Sulfonation is a key process in large-scale industrial applications for the manufacture of detergents, surfactants, colorants, pesticides, and pharmaceuticals..

The SO3 it serves as a sulfonating agent to prepare sulfonated oils and alkyl-aryl-sulfonated detergents, among many other compounds. The following shows the sulfonation reaction of an aromatic compound:

ArH + SO3 → ArSO3H

Sulfonation of benzene with SO3. Pedro8410 [CC BY-SA 3.0 (]. Source: Wikimedia Commons.

Oleum or SO can be used for sulfonation reactions.3 in the form of its complexes with pyridine or with trimethylamine, among others.

In the extraction of metals

SO gas3 It has been used in the treatment of minerals. Simple oxides of metals can become much more soluble sulfates by treating them with SO3 at relatively low temperatures.

Sulfide minerals such as pyrite (iron sulfide), chalcosine (copper sulfide) and millerite (nickel sulfide) are the most economical sources of non-ferrous metals, so treatment with SO3 allows these metals to be obtained easily and at low cost.

Iron, nickel and copper sulphides react with SO gas3 even at room temperature, forming the respective sulfates, which are very soluble and can be subjected to other processes to obtain the pure metal.

In various uses

The SO3 used to prepare chlorosulfuric acid also called chlorosulfonic acid HSO3Cl.

Sulfur trioxide is a very powerful oxidant and is used in the manufacture of explosives..


To health

The SO3 It is a highly toxic compound by all routes, that is, inhalation, ingestion and contact with the skin.

Irritating and corroding mucous membranes. Causes skin and eye burns. Its vapors are very toxic when inhaled. Internal burns, shortness of breath, chest pain, and pulmonary edema occur.

Sulfur trioxide SO3 is very corrosive and dangerous. Author: OpenIcons. Source: Pixabay.

It is poisonous. Its ingestion generates severe burns of the mouth, esophagus and stomach. In addition, it is suspected of being a carcinogen..

From fire or explosion

It represents a fire hazard when in contact with materials of organic origin such as wood, fibers, paper, oil, cotton, among others, especially if they are wet..

There is also a risk if you come into contact with bases or reducing agents. Combines with water explosively, forming sulfuric acid.

Contact with metals can produce hydrogen gas Htwo which is very flammable.

Heating in glass jars should be avoided to prevent possible violent rupture of the container..

Environmental impact

The SO3 It is considered one of the major pollutants present in the earth's atmosphere. This is due to its role in the formation of aerosols and its contribution to acid rain (due to the formation of sulfuric acid HtwoSW4).

Forest damaged by acid rain in the Czech Republic. Lovecz [Public domain]. Source: Wikimedia Commons.

The SO3 is formed in the atmosphere by the oxidation of sulfur dioxide SOtwo. When forming the SO3 This reacts rapidly with water to form sulfuric acid HtwoSW4. According to recent studies, there are other mechanisms of transformation of SO3 in the atmosphere, but due to the large amount of water present in it, it is still considered much more likely that SO3 becomes primarily HtwoSW4.

The SO3 Gas or gaseous industrial waste containing it should not be discharged into the atmosphere because it is a dangerous pollutant. It is a highly reactive gas and, as mentioned above, in the presence of humidity in the air, SO3 becomes sulfuric acid HtwoSW4. Therefore, in the air the SO3 persists in the form of sulfuric acid forming small droplets or aerosol.

If the sulfuric acid droplets enter the respiratory tract of humans or animals, they grow rapidly in size due to the moisture present there, so they have the possibility of penetrating the lungs. One of the mechanisms by which the acid mist of HtwoSW4 (i.e. SO3) can produce strong toxicity is because it changes the extracellular and intracellular pH of living organisms (plants, animals and humans).

According to some researchers, fog from SO3 It is the cause of the increase in asthmatics in an area of ​​Japan. SO fog3 It has a very corrosive effect towards metals, so that metal structures built by humans, such as some bridges and buildings, can be severely affected..

The SO3 Liquid should not be disposed of in sewage drains or sewers. If spilled into sewers, it may create a fire or explosion hazard. If spilled by accident, do not direct a stream of water at the product. It should never be absorbed in sawdust or other combustible absorbent, as it can cause fires.

It should be absorbed in dry sand, dry earth or other totally dry inert absorbent. The SO3 It should not be released into the environment and it should never be allowed to come into contact with it. It should be kept away from water sources because with this it produces sulfuric acid that is harmful to aquatic and terrestrial organisms.


  1. Sarkar, S. et al. (2019). Influence of Ammonia and Water on the Fate of Sulfur Trioxide in the Troposphere: Theoretical Investigation of Sulfamic Acid and Sulfuric Acid Formation Pathways. J Phys Chem A. 2019; 123 (14): 3131-3141. Recovered from
  2. Muller, T.L. (2006). Sulfuric acid and sulfur trioxide. Kirk-Othmer Encyclopedia of Chemical Technology. Volume 23. Recovered from
  3. U.S. National Library of Medicine. (2019). Sulfur trioxide. Recovered from
  4. Kikuchi, R. (2001). Environmental Management of Sulfur Trioxide Emission: Impact of SO3 on Human Health. Environmental Management (2001) 27: 837. Recovered from
  5. Cotton, F. Albert and Wilkinson, Geoffrey. (1980). Advanced Inorganic Chemistry. Fourth Edition. John Wiley & Sons.
  6. Ismail, M.I. (1979). Extraction of Metals from Sulfides Using Sulfur Trioxide in Fluidised Bed. J. Chem. Tech. Biotechnol. 1979, 29, 361-366. Recovered from

Yet No Comments