Sulfurous acid

Sulfurous acid
Names
	IUPAC name Sulfurous acid
	Other names Sulfuric(IV) acid; Thionic acid; Sulfinic acid
Identifiers
CAS Number	7782-99-2 ;
3D model (JSmol)	Interactive image; Tautomer: Interactive image;
ChEBI	CHEBI:48854 ;
ChEMBL	ChEMBL1161699 ;
ChemSpider	1069 ;
ECHA InfoCard	100.029.066
EC Number	231-973-1;
Gmelin Reference	1458
KEGG	C00094 ;
PubChem CID	1100;
UNII	J1P7893F4J ;
CompTox Dashboard (EPA)	DTXSID7042435 ;
	InChI InChI=1S/H2O3S/c1-4(2)3/h(H2,1,2,3) Key: LSNNMFCWUKXFEE-UHFFFAOYSA-N ; InChI=1/H2O3S/c1-4(2)3/h(H2,1,2,3) Key: LSNNMFCWUKXFEE-UHFFFAOYAJ;
	SMILES O[S](=O)O; Tautomer: O[S+](O)[O-];
Properties
Chemical formula	H2SO3
Molar mass	82.07 g/mol
Acidity (pKa)	1.857, 7.172
Conjugate base	Bisulfite
Hazards
	GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H314, H332
Precautionary statements	P260, P261, P264, P271, P280, P301+P330+P331, P303+P361+P353, P304+P312, P304+P340, P305+P351+P338, P310, P312, P321, P363, P405, P501
Flash point	Non-flammable
Safety data sheet (SDS)	ICSC 0074
Related compounds
Related compounds	Sulfur dioxide; Sulfuric acid; Selenous acid
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

Sulfuric(IV) acid (United Kingdom spelling: sulphuric(IV) acid), also known as sulfurous (UK: sulphurous) acid and thionic acid, is the chemical compound with the formula H₂SO₃.

Raman spectra of solutions of sulfur dioxide in water show only signals due to the SO₂ molecule and the bisulfite ion, HSO−3. The intensities of the signals are consistent with the following equilibrium:

SO₂ + H₂O ⇌ HSO−3 + H⁺ K_a = 1.54×10⁻²; pK_a = 1.81.

¹⁷O NMR spectroscopy provided evidence that solutions of sulfurous acid and protonated sulfites contain a mixture of isomers, which is in equilibrium:

[H−OSO₂]⁻ ⇌ [H−SO₃]⁻

Attempts to concentrate the solutions of sulfurous acid simply reverses the equilibrium, producing sulfur dioxide and water vapor. A clathrate with the formula 4SO₂·23H₂O has been crystallised. It decomposes above 7 °C.

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