Sulfur trioxide

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Structural formula
Structural formula of sulfur trioxide
Surname Sulfur trioxide
other names
  • Sulfuric anhydride
  • Sulfur (VI) oxide
  • Acidum sulfuricum anhydricum
Molecular formula SO 3
Brief description

colorless mass that smokes in the air (γ-form)

External identifiers / databases
CAS number 7446-11-9
EC number 231-197-3
ECHA InfoCard 100.028.361
PubChem 24682
Wikidata Q242715
Molar mass 80.06 g mol −1
Physical state

solid (α-, β-form), liquid (γ-form)


2.00 g cm −3 (γ form)

Melting point

62.3 ° C (α-), 32.5 ° C (β-) 16.8 ° C (γ-form)

boiling point

44.45 ° C (β-, γ-form), α-form decomposes at 50 ° C

Vapor pressure

260 hPa (20 ° C)


explosive hydrolysis in water

safety instructions
GHS labeling of hazardous substances
05 - Corrosive 07 - Warning


H and P phrases H: 314-335
EUH: 014
P: 261-280-305 + 351 + 338-310
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Sulfur trioxide , SO 3 , is the anhydride of sulfuric acid . Under normal conditions it forms colorless, needle-shaped crystals, which are extremely hygroscopic and react very violently (explosively) with water. There are three different modifications for the solid sulfur trioxide . Inhalation causes irritation, and in the lungs it becomes sulfuric acid, which can cause life-threatening pulmonary edema .

Approx. 60 g sulfur trioxide in an ampoule


Sulfur trioxide is produced industrially in the contact process by vanadium pentoxide- catalyzed oxidation of sulfur dioxide with atmospheric oxygen at 420 ° C. Pure sulfur trioxide is obtained by distilling off of oleum , which is an intermediate in the production of sulfuric acid is. The initially gaseous sulfur trioxide distilled off is then condensed by cooling to form liquid sulfur trioxide. Narrow temperature limits must be observed during condensation and storage, since the fixed point and boiling point are very close to one another.

In order to obtain smaller amounts of sulfur trioxide, it can also be distilled off from a mixture of sulfuric acid and phosphorus pentoxide or metaphosphoric acid (H 4 P 4 O 12 , cyclo-phosphate). However, sulfur trioxide can also be produced by the oxidation of sulfur dioxide with nitrogen dioxide.


Gaseous sulfur trioxide is mainly used to produce sulfuric acid:

The gaseous SO 3 dissolves very slowly in water and is therefore introduced into concentrated sulfuric acid, with disulfuric acid (H 2 S 2 O 7 ) being formed. With water this is then converted to sulfuric acid:

Further to the production of fluorosulfonic acid and chlorosulfonic acid

If sulfur trioxide is mixed with alcohols , sulfuric acid esters are formed:

This reaction is used in the manufacture of surfactants :

The sulfuric acid esters are neutralized with sodium hydroxide solution and thus provide fatty alcohol sulfates . Sulfur trioxide is also suitable as an oxidizing agent . It was also used to make smoke grenades , as just a drop of liquid sulfur trioxide can completely fog a large room.


Gaseous sulfur trioxide is present as a monomer. This is built in a trigonal-planar manner and contains three S – O double bonds of equal length :

Structural formula of sulfur trioxide

The structural formula given is composed of the following mesomeric boundary structures:

Examples of mesomeric boundary structures of sulfur trioxide.
Ball and stick model of sulfur trioxide

It is in equilibrium with S 3 O 9 molecules. If (gaseous) SO 3 is cooled below −80 ° C, so-called γ-SO 3 is formed . This consists of S 3 O 9 molecules. It is not constructed in a planar manner, but rather forms a corrugated ring. The sulfur atoms are surrounded by oxygen in a distorted tetrahedral manner .

γ-sulfur trioxide

Sulfur trioxide instantly carbonizes rubber and most plastics such as PVC , only special plastics such as Teflon and perfluoroalkoxy are relatively resistant.

Metrological recording of sulfur trioxide emissions

Sulfur trioxide emissions can be determined with a condensation process in which a partial flow of the exhaust gas to be sampled is taken from the exhaust gas by means of a heated sampling probe and fed into a condensation vessel. The boundary conditions in the condensation vessel mean that sulfur trioxide is separated out as sulfuric acid. The sulphate is quantitatively determined by ion chromatography or titration .

Another method for measuring sulfur trioxide is the 2-propanol method. In this process, the laden exhaust gas is passed through a solution containing 2-propanol . In contrast to sulfur dioxide, sulfur trioxide is very well absorbed by this solution. After the sampling is complete, the absorbed sulfur trioxide is titrated against thorin with a barium perchlorate solution. This method has not proven itself in practice.

Individual evidence

  1. Entry on sulfur trioxide. In: Römpp Online . Georg Thieme Verlag, accessed on October 1, 2014.
  2. a b c d e f g Entry on sulfur trioxide in the GESTIS substance database of the IFA , accessed on January 10, 2017(JavaScript required) .
  3. Page no longer available , search in web archives: THOMAFLUID-PFA Chemical Tubing - Standard.  ( Page no longer available , search in web archives ) Retrieved July 16, 2012.@1@ 2Template: Toter Link / @1@ 2Template: Toter Link /
  4. VDI 2462 sheet 2: 2011-11 measurement of gaseous emissions; Determination of sulfur trioxide in waste gases containing water vapor; Condensation method (Measurement of gaseous emissions; Determination of sulfur trioxide in water vapor containing exhaust gas; Condensation method). Beuth Verlag, Berlin. P. 3.
  5. Wolfgang Guse: Sulfur trioxide in combustion exhaust gases. In: Dust - cleanliness. Air . 41, No. 6, 1981, ISSN  0949-8036 , pp. 204-210.
  6. VDI 2462 sheet 7: 1985-03 measurement of gaseous emissions; Measuring sulfur trioxide concentration; 2-propanol method (Measurement of Gaseous Emissions; Measurement of the Sulfur-Trioxide Concentration; 2-propanol method). VDI-Verlag, Düsseldorf, p. 2.
  7. VDI 2462 sheet 2: 2011-11 measurement of gaseous emissions; Determination of sulfur trioxide in waste gases containing water vapor; Condensation method (Measurement of gaseous emissions; Determination of sulfur trioxide in water vapor containing exhaust gas; Condensation method). Beuth Verlag, Berlin, p. 2.