Disulfuric acid

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| ImageFile1 = Dischwefelsäure - Disulfuric acid.svg

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| ImageSize1 = 150

| ImageAltL1 = Structural formula of disulfuric acid

| ImageFile2 = Pyrosulfuric acid molecule ball.png

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| ImageSize2 = 150

| ImageAlt2 = Ball and stick model of the disulfuric acid molecule

| IUPACName = Disulfuric acid {{RedBookRef|page=130}}

| OtherNames = Pyrosulfuric acid, Oleum

| Section1 = {{Chembox Identifiers

| CASNo = 7783-05-3

| CASNo_Ref = {{cascite|correct|??}}

| UNII_Ref = {{fdacite|changed|FDA}}

| UNII = NTC1O8E83E

| PubChem = 62682

| ChemSpiderID = 56433

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

| EINECS = 231-976-8

| MeSHName = Pyrosulfuric+acid

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

| ChEBI = 29211

| SMILES = OS(=O)(=O)OS(O)(=O)=O

| SMILES1 = OS(=O)(=O)OS(=O)(=O)O

| SMILES2 = O=S(=O)(O)OS(=O)(=O)O

| StdInChI = 1S/H2O7S2/c1-8(2,3)7-9(4,5)6/h(H,1,2,3)(H,4,5,6)

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

| InChI = 1/H2O7S2/c1-8(2,3)7-9(4,5)6/h(H,1,2,3)(H,4,5,6)

| StdInChIKey = VFNGKCDDZUSWLR-UHFFFAOYSA-N

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

| InChIKey = VFNGKCDDZUSWLR-UHFFFAOYAZ

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|Section2={{Chembox Properties

| H=2 | O=7 | S=2

| MeltingPtC = 36

| ConjugateBase = Disulfate

| Appearance = colorless

| pKa = 2.5 ({{degC|20}}; in conc. H₂SO₄){{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 62}}

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Disulfuric acid (alternative spelling disulphuric acid) or pyrosulfuric acid (alternative spelling pyrosulphuric acid), also named oleum, is a sulfur oxoacid.{{Greenwood&Earnshaw}} It is a major constituent of fuming sulfuric acid, oleum, and this is how most chemists encounter it. As confirmed by X-ray crystallography, the molecule consists of a pair of SO₂(OH) groups joined by an oxide.{{cite journal |doi=10.1524/zkri.1991.196.14.279|title=Crystal structure of H2S2O7 at 298 K |year=1991 |last1=Hönle |first1=Wolfgang |journal=Zeitschrift für Kristallographie - Crystalline Materials |volume=196 |issue=1–4 |s2cid=101311329 }}