Sodium sulfite (sodium sulphite) is the

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistr ...

with the chemical formula Na₂ SO₃. A white, 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.

Preparation

Sodium sulfite can be prepared by treating a solution of

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alka ...

with

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...

. 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. :SO2 + 2 NaOH -> Na2SO3 + H2O Sodium sulfite is made industrially by treating sulfur dioxide with a solution of sodium carbonate. The overall reaction is: :SO2 + Na2CO3 -> Na2SO3 + CO2

Applications

Sodium sulfite is primarily used in the

pulp and paper industry The pulp and paper industry comprises companies that use wood as raw material and produce pulp, paper, paperboard and other cellulose-based products. Manufacturing process The pulp is fed to a paper machine where it is formed as a paper web an ...

. 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 Sodium thiosulfate (sodium thiosulphate) is an inorganic compound with the formula . Typically it is available as the white or colorless pentahydrate, . The solid is an efflorescent (loses water readily) crystalline substance that dissolves well in ...

) 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 A preservative is a substance or a chemical that is added to products such as food products, beverages, pharmaceutical drugs, paints, biological samples, cosmetics, wood, and many other products to prevent decomposition by microbial growth or by ...

to prevent

dried fruit Dried fruit is fruit from which the majority of the original water content has been removed either naturally, through sun drying, or through the use of specialized dryers or dehydrators. Dried fruit has a long tradition of use dating back to th ...

from discoloring, and for preserving meats. It is used as a reagent in sulfonation and sulfomethylation agent. It is used in the production of

. The

Wellman–Lord process The Wellman–Lord process is a regenerable process to remove sulfur dioxide from flue gas (flue-gas desulfurization) without creating a throwaway sludge product. In this process, sulfur dioxide from flue gas is absorbed in a sodium sulfite solutio ...

utilizes sodium sulfite for

flue gas desulfurization Flue-gas desulfurization (FGD) is a set of technologies used to remove sulfur dioxide () from exhaust flue gases of fossil-fuel power plants, and from the emissions of other sulfur oxide emitting processes such as waste incineration. Methods S ...

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.Merck Index of Chemicals and Drugs, 9th ed. monograph 8451

Sodium bisulfite Sodium bisulfite (or sodium bisulphite, sodium hydrogen sulfite) is a chemical mixture with the approximate chemical formula NaHSO3. Sodium bisulfite in fact is not a real compound, but a mixture of salts that dissolve in water to give solutio ...

, NaHSO₃, is mixture of salts that dissolve in water to give solutions composed of sodium and bisulfite ions.

Structure

According to

X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angle ...

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

References

{{Sodium compounds Photographic chemicals Sodium compounds Sulfites E-number additives