Sodium sulfite

Template:Chembox image cellTemplate:Chembox AllOtherNamesTemplate:Chembox headerbarTemplate:Chembox IndexlistTemplate:Chembox JmolTemplate:Chembox ChEMBLTemplate:Chembox ECHATemplate:Chembox E numberTemplate:Chembox IUPHAR ligandTemplate:Chembox UNIITemplate:Chembox CompToxTemplate:Chembox headerbarTemplate:Chembox SolubilityInWaterTemplate:Chembox headerbarTemplate:Chembox HazardsTemplate:Chembox headerbarTemplate:Chembox Datapage checkTemplate:Yesno

Sodium sulphite

Template:Chembox image sbs cell
Template:Longitem	Template:Unbulleted list
ChEBI	Template:Unbulleted list
ChemSpider	Template:Unbulleted list
DrugBank	Template:Unbulleted list
EC Number	Template:Unbulleted list
KEGG	Template:Unbulleted list
Template:Longitem	Template:Unbulleted list
RTECS number	Template:Unbulleted list
Template:Longitem	Na2SO3
Molar mass	126.043g/mol
Appearance	White solid
Odor	Odorless
Density	2.633g/cm3 (anhydrous) 1.561g/cm3 (heptahydrate)
Melting point	Template:Chembox CalcTemperatures
Boiling point	Template:Chembox CalcTemperatures
Solubility	Soluble in glycerol Insoluble in ammonia, chlorine
log P	−4
Acidity (pKa)	~9 (heptahydrate)
Template:Longitem	1.565
Template:Longitem	Hexagonal (anhydrous) Monoclinic (heptahydrate)
Template:Longitem	Sodium selenite
Template:Longitem	Potassium sulfite
Template:Longitem	Sodium bisulfite Sodium metabisulfite Sodium sulfate

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.

Preparation

File:Sodium-sulfite-xtal-3x3x3-c-3D-bs-17.png

Structure of anhydrous sodium sulfite

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 white solid. With more SO₂, the solid dissolves to give the disulfite, which crystallizes upon cooling.

Template:Chem2

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

Template:Chem2

Uses

Sodium sulfite is primarily used in the pulp and paper industry. It has been also applied in the thermomechanical conversion of wood to fibres (defibration) for producing medium density fibreboards (MDF).

As an oxygen scavenger agent, it is used to treat water being fed to steam boilers to avoid corrosion problems, 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

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.

Structure

According to X-ray crystallography sodium sulfite heptahydrate features pyramidal SO₃^2- centers. The S-O distances are 1.50 and the O-S-O angles are near 106º.