Sodium sulfite

Sodium sulphite
anhydrous	hydrate
Names
	IUPAC name Sodium sulfite
	Other names Hypo clear (photography); E221;
Identifiers
CAS Number	7757-83-7 ;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:86477 ;
ChemSpider	22845 ;
ECHA InfoCard	100.028.929
EC Number	231-821-4;
E number	E221 (preservatives)
PubChem CID	24437;
RTECS number	WE2150000;
UNII	VTK01UQK3G ;
CompTox Dashboard (EPA)	DTXSID2044260 ;
	InChI InChI=1S/2Na.H2O3S/c;;1-4(2)3/h;;(H2,1,2,3)/q2+1;/p-2 Key: GEHJYWRUCIMESM-UHFFFAOYSA-L ; InChI=1/2Na.H2O3S/c;;1-4(2)3/h;;(H2,1,2,3)/q2+1;/p-2 Key: GEHJYWRUCIMESM-NUQVWONBAK;
	SMILES [O-]S(=O)[O-].[Na+].[Na+];
Properties
Chemical formula	Na2SO3
Molar mass	126.043 g/mol
Appearance	White solid
Odor	Odorless
Density	2.633 g/cm3 (anhydrous); 1.561 g/cm3 (heptahydrate)
Melting point	33.4 °C (92.1 °F; 306.5 K) (dehydration of heptahydrate); 500 °C (anhydrous)
Boiling point	Decomposes
Solubility in water	27.0 g/100 mL water (20 °C)
Solubility	Soluble in glycerol ; Insoluble in ammonia, chlorine
log P	−4
Acidity (pKa)	~9 (heptahydrate)
Refractive index (nD)	1.565
Structure
Crystal structure	Hexagonal (anhydrous); Monoclinic (heptahydrate)
Hazards
NFPA 704 (fire diamond)	2 0 0
Flash point	Non-flammable
Safety data sheet (SDS)	ICSC 1200
Related compounds
Other anions	Sodium selenite
Other cations	Potassium sulfite
Related compounds	Sodium bisulfite; Sodium metabisulfite; Sodium sulfate
	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

Sodium sulfite (sodium sulphite) is the inorganic compound with the chemical formula Na₂SO₃. A white, water-soluble solid, it is used commercially as an antioxidant and preservative. It is also suitable for the softening of lignin in the pulping and refining processes of wood and lignocellulosic materials. A heptahydrate is also known but it is less useful because of its greater susceptibility toward oxidation by air.