Dithionous acid

Names
IUPAC name Dithionous acid
Other names Hydrosulfurous acid; Hyposulfurous acid
Identifiers
CAS Number	15959-26-9 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:29253 Y
ChemSpider	22898 Y
PubChem CID	24490
CompTox Dashboard (EPA)	DTXSID70274771
InChI InChI=1S/H2O4S2/c1-5(2)6(3)4/h(H,1,2)(H,3,4) Y Key: GRWZHXKQBITJKP-UHFFFAOYSA-N Y InChI=1/H2O4S2/c1-5(2)6(3)4/h(H,1,2)(H,3,4) Key: GRWZHXKQBITJKP-UHFFFAOYAX
SMILES O=S(O)S(=O)O
Properties
Chemical formula	H2S2O4
Molar mass	130.144 g/mol
Acidity (pKa)	0.35, 2.45 [1]
Conjugate base	Dithionite
Related compounds
Related compounds	Oxalic acid Sodium dithionite Potassium dithionite
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Dithionous acid is a sulfur oxoacid with the chemical formula H₂S₂O₄. It is unstable in pure form,^[2] but its salts, known as dithionites, are stable.

It was initially assumed that the C₂ symmetric structure HOS(=O)-S(=O)OH is the most stable among molecules with the formula H
₂S
₂O
₄ using ab initio calculations.^[2] The reason for this is the existence of intermolecular hydrogen bonds. It is now known that dithionous acid spontaneously decomposes to SO₂ and S(OH)₂.

Sodium dithionite is a white powder used as a reductant and a bleaching agent. It is also used to reduce the nitro group to an amino group in organic reactions.