|Molecular formula||H 2 SO 3|
colorless liquid with a pungent odor of sulfur dioxide
|External identifiers / databases|
|Molar mass||82.02 g mol −1|
1.03 g cm −3
|pK s value||
completely miscible with water, only stable in solution
|As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .|
The sulfur dioxide is an unstable, existing only in aqueous solution, dibasic acid , which upon release of sulfur dioxide in water is formed; their salts and esters are called sulfites and hydrogen sulfites .
In aqueous solution there is an equilibrium between sulfur dioxide and sulphurous acid:
The equilibrium of the reaction is far to the left.
If you try to evaporate the solution in order to obtain anhydrous sulphurous acid, it decomposes in the reverse of the formation reaction. On cooling, a clathrate SO 2 · 5.75 H 2 O crystallizes out and decomposes again at 7 ° C. Free sulfurous acid H 2 SO 3 cannot therefore be isolated.
The protolysis of sulphurous acid takes place in two stages. In the first stage, hydrogen sulfite is formed:
The actual acid strength is much higher, however, since there are virtually no H 2 SO 3 molecules in aqueous solutions . The following applies to the second stage of protolysis (K S2 ):
The hydrogen sulfites can only be crystallized out with large cations , otherwise disulfites are formed . In the solid, the hydrogen sulfite anion is in the constitution of a sulfonate , the remaining hydrogen is bound to the sulfur.
Sulphurous acid forms two monovalent tautomeric anions in water , hydrogen sulphite and sulphonate ions with the structures HSO 3 - and SHO 3 - , which can be derived as deprotonation products from sulphurous acid or sulphonic acid .
Sulphurous acid is also one of the causes of acid rain , as the sulfur dioxide formed during the combustion of fossil fuels (e.g. coal , oil , petroleum products) or biomass is "washed out" by rainwater from the earth's atmosphere ( see also: smog ). To prevent sulfur dioxide from escaping into the environment, there are various methods of flue gas desulphurisation .
Sulfurous acid and their salts are reducing agents , as they by receiving a further oxygen atom to sulfuric acid or sulfate oxidized are. In aqueous solution, sulfuric acid is oxidized to sulfuric acid by atmospheric oxygen.
- A. F. Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 .
- Arnold Willmes, paperback chemical substances , Harri Deutsch, Frankfurt (M.), 2007, p. 775.
- L. Kolditz, Inorganic Chemistry, VEB Deutscher Verlag der Wissenschaften, Berlin 1983, p. 476.