{{chembox | Verifiedfields = changed | Watchedfields = changed | verifiedrevid = 476993033 | Name = Sodium sulphite | ImageFile = Na2SO3.svg | ImageName = Sodium sulfite | ImageFileL1 = Sodium sulfite.jpg | ImageCaptionL1 = anhydrous | ImageFileR1 = Sodium sulfite hydrate.jpg | ImageCaptionR1 = hydrate | IUPACName = Sodium sulfite | OtherNames = {{Unbulleted list|Hypo clear (photography)|E221}} |Section1={{Chembox Identifiers | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | ChemSpiderID = 22845 | ChEBI_Ref = {{ebicite|changed|EBI}} | ChEBI = 86477 | UNII_Ref = {{fdacite|correct|FDA}} | UNII = VTK01UQK3G | InChI = 1/2Na.H2O3S/c;;1-4(2)3/h;;(H2,1,2,3)/q2*+1;/p-2 | InChIKey = GEHJYWRUCIMESM-NUQVWONBAK | SMILES = [O-]S(=O)[O-].[Na+].[Na+] | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChI = 1S/2Na.H2O3S/c;;1-4(2)3/h;;(H2,1,2,3)/q2*+1;/p-2 | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey = GEHJYWRUCIMESM-UHFFFAOYSA-L | CASNo = 7757-83-7 | CASNo_Ref = {{cascite|correct|CAS}} | PubChem = 24437 | RTECS = WE2150000 | EINECS = 231-821-4 }} |Section2={{Chembox Properties | Formula = Na<sub>2</sub>SO<sub>3</sub> | MolarMass = 126.043{{nbsp}}g/mol | Appearance = White solid | Odor = Odorless | Density = 2.633{{nbsp}}g/cm<sup>3</sup> (anhydrous)<br/>1.561{{nbsp}}g/cm<sup>3</sup> (heptahydrate) | Solubility = 27.0{{nbsp}}g/100{{thinsp}}mL water (20{{nbsp}}°C) | SolubleOther = Soluble in glycerol <br>Insoluble in ammonia, chlorine | MeltingPtC = 33.4 | MeltingPt_notes = (dehydration of heptahydrate)<br/> 500 °C (anhydrous) | BoilingPt = Decomposes | RefractIndex = 1.565 | pKa = ~9 (heptahydrate) | LogP = −4 }} |Section3={{Chembox Structure | MolShape = | Coordination = | CrystalStruct = Hexagonal (anhydrous)<br/>Monoclinic (heptahydrate) | Dipole = }} |Section7={{Chembox Hazards | ExternalSDS = [http://www.inchem.org/documents/icsc/icsc/eics1200.htm ICSC 1200] | MainHazards = | FlashPt = Non-flammable | HPhrases = | PPhrases = | GHS_ref = | NFPA-H = 2 | NFPA-F = 0 | NFPA-R = 0 }} |Section8={{Chembox Related | OtherAnions = Sodium selenite | OtherCations = Potassium sulfite | OtherCompounds = Sodium bisulfite<br/>Sodium metabisulfite<br/>Sodium sulfate }} }} '''Sodium sulfite''' ('''sodium sulphite''') is the inorganic compound with the chemical formula Na<sub>2</sub>SO<sub>3</sub>. 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.<ref>{{cite web| title= High Yield Pulp Production by Modified Sulfite Process| url=https://ippta.co/wp-content/uploads/2021/01/IPPTA-VII1-39-42-High-Yield-Pulp-Production.pdf | access-date=2023-10-19}}</ref> A heptahydrate is also known but it is less useful because of its greater susceptibility toward oxidation by air.<ref name=IS/>

==Preparation== Structure of anhydrous sodium sulfite|left|thumb Sodium sulfite can be prepared by treating a solution of sodium hydroxide with sulfur dioxide. When conducted in warm water, Na<sub>2</sub>SO<sub>3</sub> initially precipitates as a white solid. With more SO<sub>2</sub>, the solid dissolves to give the disulfite, which crystallizes upon cooling.<ref name=IS>{{cite book|doi=10.1002/9780470132333.ch49|isbn=9780470132333|chapter=Sulfites and Pyrosulfites of the Alkali Metals|year=1946|last1=Johnstone|first1=H. F.|title=Inorganic Syntheses|pages=162–167|volume=2}}</ref> :{{chem2 | SO2 + 2 NaOH -> Na2SO3 + H2O }}

Sodium sulfite is made industrially by treating sulfur dioxide with a solution of sodium carbonate.<ref Name="Kirk-Othmer">{{cite book |last1=Weil |first1=Edward D. |last2=Sandler |first2=Stanley R. |editor1-first= Jacqueline I. |editor1-last= Kroschwitz |title=Kirk-Othmer Concise Encyclopedia of Chemical Technology |edition=4th |year=1999 |publisher= John Wiley & Sons, Inc.|location=New York|isbn=978-0471419617|page=1937 |chapter=Sulfur Compounds}}</ref> The overall reaction is: :{{chem2 | SO2 + Na2CO3 -> Na2SO3 + CO2 }}

==Uses== Sodium sulfite is primarily used in the pulp and paper industry.<ref>{{Ullmann|doi=10.1002/14356007.a25_477|isbn=3527306730|title=Sulfites, Thiosulfates, and Dithionitesl Chemistry|year=2000|last1=Barberá|first1=José Jiménez|last2=Metzger|first2=Adolf|last3=Wolf|first3=Manfred}}</ref> It has been also applied in the thermomechanical conversion of wood to fibres (''defibration'') for producing medium density fibreboards (MDF).<ref>{{cite web | title=DE19958756A1 - Production of light-colored medium-density fibreboard (MDF) from old fibreboard comprises treatment with sodium sulfite, conversion into pulp and feeding into the blow-line of an MDF plant | website=Google Patents | date=1999-12-07 | url=https://patents.google.com/patent/DE19958756A1/en | access-date=2022-03-01}}</ref>

As an oxygen scavenger agent, it is used to treat water being fed to steam boilers to avoid corrosion problems,<ref>{{cite web | url=http://www.gewater.com/handbook/boiler_water_systems/ch_11_preboiler.jsp | title=Pre-boiler and Boiler Corrosion Control &#124; GE Water | access-date=2016-03-14 | archive-date=2017-10-06 | archive-url=https://web.archive.org/web/20171006062229/https://www.gewater.com/handbook/boiler_water_systems/ch_11_preboiler.jsp | url-status=dead }}</ref> 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.

The Wellman–Lord process utilizes sodium sulfite for flue gas desulfurization.

==Reactions== {{main|sulfite}} Sodium sulfite is primarily used as a mild reducing agent. The heptahydrate crystals effloresce in warm dry air. Heptahydrate crystals also oxidize in air to form sodium sulfate. The anhydrous form is more resistant to oxidation by air.<ref name="merck">Merck Index of Chemicals and Drugs, 9th ed. monograph 8451</ref>

==Structure== According to X-ray crystallography sodium sulfite heptahydrate features pyramidal SO<sub>3</sub><sup>2-</sup> centers. The S-O distances are 1.50 and the O-S-O angles are near 106º.<ref>{{cite journal |doi=10.3891/acta.chem.scand.23-2253|title=The Crystal Structure of Sodium Sulphite. A Refinement Allowing for the Effect of Crystal Twinning|year=1969|last1=Larsson|first1=Lars Olof|last2=Kierkegaard|first2=Peder|last3=Lindberg|first3=Bengt|last4=Holme|first4=Tord|last5=Lindberg|first5=Alf A.|last6=Craig|first6=J. Cymerman|journal=Acta Chemica Scandinavica|volume=23|pages=2253–2260|doi-broken-date=9 September 2025 |doi-access=free}}</ref>

==References== {{reflist}}

{{Sodium compounds}} {{Sulfites}}

Category:Photographic chemicals Category:Sodium compounds Category:Sulfites Category:E-number additives