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
Safety data sheet	ICSC 1200
NFPA 704 (fire diamond)	0 2 0
Flash point	Non-flammable
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).
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	Infobox references

Sodium sulfite (sodium sulphite) is the inorganic compound with the chemical formula Na₂ SO₃. A pale yellow, water-soluble solid, it is used commercially as an antioxidant and preservative. A heptahydrate is also known but it is less useful because of its greater susceptibility toward oxidation by air.[1]

Preparation

Sodium sulfite can be prepared by treating a solution of sodium hydroxide with sulfur dioxide. When conducted in warm water, Na₂SO₃ initially precipitates as a yellow solid. With more SO₂, the solid dissolves to give the disulfite, which crystallizes upon cooling.[1]

SO₂ + 2 NaOH → Na₂SO₃ + H₂O

Sodium sulfite is made industrially by treating sulfur dioxide with a solution of sodium carbonate.[2] The overall reaction is:

SO₂ + Na₂CO₃ → Na₂SO₃ + CO₂

Applications

Sodium sulfite is primarily used in the pulp and paper industry.[3]

As an oxygen scavenger agent, it is used to treat water being fed to steam boilers to avoid corrosion problems,[4] in the photographic industry, it protects developer solutions from oxidation and (as hypo clear solution) to wash fixer (sodium thiosulfate) from film and photo-paper emulsions

As a reducing agent it is used in the textile industry as a bleaching, desulfurizing, and dechlorinating agent (e.g. in swimming pools). Its reducing properties are exploited in its use as a preservative to prevent dried fruit from discoloring, and for preserving meats.

It is used as a reagent in sulfonation and sulfomethylation agent. It is used in the production of sodium thiosulfate.

Reactions

Heptahydrate

If sodium sulfite is allowed to crystallize from aqueous solution at room temperature or below, it does so as a heptahydrate.[1] The heptahydrate crystals effloresce in warm dry air. Heptahydrate crystals also oxidize in air to form the sulfate. The anhydrous form is much more stable against oxidation by air.[5]

References

Johnstone, H. F. (1946). "Sulfites and Pyrosulfites of the Alkali Metals". Inorganic Syntheses. 2: 162–167. doi:10.1002/9780470132333.ch49. ISBN 9780470132333.
Weil, Edward D.; Sandler, Stanley R. (1999). "Sulfur Compounds". In Kroschwitz, Jacqueline I. (ed.). Kirk-Othmer Concise Encylclopedia of Chemical Technology (4th ed.). New York: John Wiley & Sons, Inc. p. 1937. ISBN 978-0471419617.
Barberá, José Jiménez; Metzger, Adolf; Wolf, Manfred (2000). "Sulfites, Thiosulfates, and Dithionitesl Chemistry". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a25_477.
"Pre-boiler and Boiler Corrosion Control | GE Water".
Merck Index of Chemicals and Drugs, 9th ed. monograph 8451

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[IS-1] Johnstone, H. F. (1946). "Sulfites and Pyrosulfites of the Alkali Metals". Inorganic Syntheses. 2: 162–167. doi:10.1002/9780470132333.ch49. ISBN 9780470132333.

[Kirk-Othmer-2] Weil, Edward D.; Sandler, Stanley R. (1999). "Sulfur Compounds". In Kroschwitz, Jacqueline I. (ed.). Kirk-Othmer Concise Encylclopedia of Chemical Technology (4th ed.). New York: John Wiley & Sons, Inc. p. 1937. ISBN 978-0471419617.

[3] Barberá, José Jiménez; Metzger, Adolf; Wolf, Manfred (2000). "Sulfites, Thiosulfates, and Dithionitesl Chemistry". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a25_477.

[4] "Pre-boiler and Boiler Corrosion Control | GE Water".

[merck-5] Merck Index of Chemicals and Drugs, 9th ed. monograph 8451