# sulphurous acid

Structural formula
General
Surname sulphurous acid
other names
• Sulfur (IV) acid
• Sulphurous acid
• Dihydrogen sulfite
Molecular formula H 2 SO 3
Brief description

colorless liquid with a pungent odor of sulfur dioxide

External identifiers / databases
 CAS number 7782-99-2 EC number 231-973-1 ECHA InfoCard 100,029,066 PubChem 1100 ChemSpider 1069 Wikidata Q206778
properties
Molar mass 82.02 g mol −1
Physical state

liquid

density

1.03 g cm −3

pK s value
• pK s 1 : 1.81
• pK s 2 : 6.99
solubility

completely miscible with water, only stable in solution

safety instructions
GHS labeling of hazardous substances

danger

H and P phrases H: 332-314
P: 280-301 + 330 + 331-302 + 352-304 + 340-305 + 351 + 338-310
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 .

## properties

### Chemical properties

In aqueous solution there is an equilibrium between sulfur dioxide and sulphurous acid:

${\ displaystyle \ mathrm {SO_ {2} (aq) + H_ {2} O (l) \ \ rightleftharpoons \ H_ {2} SO_ {3} (aq)}}$

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:

${\ displaystyle \ mathrm {H_ {2} SO_ {3} + H_ {2} O \ \ rightleftharpoons \ HSO_ {3} ^ {-} + H_ {3} O ^ {+}}}$

The acid constant K S1 (analogous to carbon dioxide ) is formally determined from the sum of the concentrations of dissolved sulfur dioxide and sulfurous acid:

${\ displaystyle \ mathrm {{\ frac {{\ mathit {c}} _ {H_ {3} O ^ {+}} \ cdot {\ mathit {c}} _ {HSO_ {3} ^ {-}}} {{\ mathit {c}} _ {[H_ {2} SO_ {3}]}}} = K_ {S1} = 1 {,} 54 \ cdot 10 ^ {- 2}}}$
With ${\ displaystyle \ mathrm {{\ mathit {c}} _ {[H_ {2} SO_ {3}]} = {\ mathit {c}} _ {H_ {2} SO_ {3}} + {\ mathit { c}} _ {SO_ {2} (aq)}}}$

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 ):

${\ displaystyle \ mathrm {HSO_ {3} ^ {-} + H_ {2} O \ \ rightleftharpoons \ SO_ {3} ^ {2 -} + H_ {3} O ^ {+}}}$
${\ displaystyle \ mathrm {{\ frac {{\ mathit {c}} _ {H_ {3} O ^ {+}} \ cdot {\ mathit {c}} _ {SO_ {3} ^ {2-}} } {{\ mathit {c}} _ {HSO_ {3} ^ {-}}}} = K_ {S2} = 1 {,} 02 \ cdot 10 ^ {- 7}}}$
Sulfite anion

Added to sulfur dioxide with bases , metal oxides or carbonates , so crystallize with the evaporation of the solutions , the salts of sulfurous acid , sulfites , from.

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.

### Tautomerism

Tautomerism of sulphurous acid with sulphonic acid

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 .

## Biological importance

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 .

## use

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.