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.

Web links

Wiktionary: sulphurous acid  - explanations of meanings, word origins, synonyms, translations

Commons : Sulphurous Acid  - Collection of images, videos and audio files

Individual evidence

1. ↑ ^{a b c d e f} Entry on sulphurous acid in the GESTIS substance database of the IFA , accessed on January 8, 2018(JavaScript required) .
2. ^ ^{A b c d} A. F. Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 .
3. ^ Arnold Willmes, paperback chemical substances , Harri Deutsch, Frankfurt (M.), 2007, p. 775.
4. ↑ L. Kolditz, Inorganic Chemistry, VEB Deutscher Verlag der Wissenschaften, Berlin 1983, p. 476.