sulfurous acid

{{short description|Chemical compound}}

{{distinguish|Sulfuric acid}}

{{chembox

| verifiedrevid = 470482579

| Name = Sulfurous acid

| ImageFile_Ref = {{chemboximage|correct|??}}

| ImageFile = Sulfurous-acid-2D-pyramidal.png

| ImageSize = 150px

| ImageName = Sulfuric(IV) acid

| ImageFile1 = Sulfurous-acid-3D-balls.png

| ImageName1 = Ball-and-stick model of sulfurous acid

| OtherNames = Sulfuric(IV) acid
Thionic acid
Sulfinic acid

| IUPACName = Sulfurous acid

| SystematicName =

| Section1 = {{Chembox Identifiers

| CASNo = 7782-99-2

| CASNo_Ref = {{cascite|correct|CAS}}

| ChEBI_Ref = {{ebicite|correct|EBI}}

| ChEBI = 48854

| ChEMBL_Ref = {{ebicite|correct|EBI}}

| ChEMBL = 1161699

| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}

| ChemSpiderID = 1069

| EC_number = 231-973-1

| Gmelin = 1458

| PubChem = 1100

| UNII_Ref = {{fdacite|correct|FDA}}

| UNII = J1P7893F4J

| KEGG_Ref = {{keggcite|correct|kegg}}

| KEGG = C00094

| InChI = 1/H2O3S/c1-4(2)3/h(H2,1,2,3)

| InChIKey = LSNNMFCWUKXFEE-UHFFFAOYAJ

| SMILES = O[S](=O)O

| SMILES1 = O[S+](O)[O-]

| SMILES1_Comment = Tautomer

| StdInChI_Ref = {{stdinchicite|correct|chemspider}}

| StdInChI = 1S/H2O3S/c1-4(2)3/h(H2,1,2,3)

| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}

| StdInChIKey = LSNNMFCWUKXFEE-UHFFFAOYSA-N

}}

| Section2 = {{Chembox Properties

| Formula = {{chem2|H2SO3}}

| MolarMass = 82.07 g/mol

| ConjugateBase = Bisulfite

| pKa = 1.857, 7.172{{cite book|title=Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution|editor-first=D. D.|editor-last=Perrin|edition=2nd|series=IUPAC Chemical Data|issue=29|publisher=Pergamon|location=Oxford|year=1982|publication-date=1984|orig-date=1969|lccn=82-16524|isbn=0-08-029214-3|at=Entry 217}}

}}

| Section3 =

| Section4 =

| Section5 =

| Section6 =

| Section7 = {{Chembox Hazards

| ExternalSDS = [http://www.inchem.org/documents/icsc/icsc/eics0074.htm ICSC 0074]

| GHSPictograms = {{GHS05}}{{GHS07}}

| GHSSignalWord = Danger

| HPhrases = {{H-phrases|314|332}}

| PPhrases = {{P-phrases|260|261|264|271|280|301+330+331|303+361+353|304+312|304+340|305+351+338|310|312|321|363|405|501}}

| FlashPt = Non-flammable

}}

| Section8 = {{Chembox Related

| OtherCompounds = Sulfur dioxide
Sulfuric acid
Selenous acid

}}

}}

Sulfuric(IV) acid (United Kingdom spelling: sulphuric(IV) acid), also known as sulfurous (UK: sulphurous) acid and thionic acid,{{citation needed|date=July 2023}} is the chemical compound with the formula {{chem2|H2SO3}}.

Raman spectra of solutions of sulfur dioxide in water show only signals due to the {{chem2|SO2}} molecule and the bisulfite ion, {{chem2|HSO3−}}.{{Jolly2nd}} The intensities of the signals are consistent with the following equilibrium:

{{block indent|1={{chem2|SO2 + H2O ⇌ HSO3− + H+}}{{spaces|10}}K_a = 1.54{{e|−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:{{cite book

| title = Inorganic Chemistry, 3rd Edition

| chapter = Chapter 16: The group 16 elements

| author1 = Catherine E. Housecroft

| author2 = Alan G. Sharpe

| publisher = Pearson

| year = 2008

| isbn = 978-0-13-175553-6

| page = 520

}}

{{block indent|{{chem2|[H\sOSO2]− ⇌ [H\sSO3]−}}}}

Attempts to concentrate the solutions of sulfurous acid simply reverse the equilibrium, producing sulfur dioxide and water vapor. A clathrate with the formula {{chem2|4SO2*23H2O}} has been crystallised. It decomposes above 7 °C.