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Sodium sulfite

Sodium sulfite
IUPAC name	Sodium sulfite
Other names	Sodium sulphite hypo clear (photography)
Identifiers
CAS number	7757-83-7
Properties
Molecular formula	Na₂SO₃
Molar mass	126.0418 g/mol
Appearance	white, granular
Density	2.633 g/cm³ (anhydrous) 1.561 g/cm³ (heptahydrate)
Melting point	251.8°C (525 K) (anhydrous) heptahydrate loses 7H₂O at 33.4°C
Boiling point	decomposes
Solubility in water	67.8 g/100 ml of heptahydrate (18°C)
Solubility in other solvents	3 g/100 ml (?°C) (heptahydrate)
Structure
Crystal structure	hexagonal (anhydrous) monoclinic (heptahydrate)
Hazards
Flash point	?°C
Related Compounds
Related compounds	SO₂, H₂SO₃, Na₂SO₄
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Sodium sulfite (sodium sulphite) is a soluble compound of sodium. Its chemical formula is Na₂S O₃. It has a molecular weight of 126.04. It is a product of S O₂ scrubbing, a part of the flue gas desulfurization process. It is also used as a preservative to prevent dried fruit from discoloring, and for preserving meats, and is used in the same way as sodium thiosulfate to convert elemental halides to their respective acids, in photography and for reducing chlorine levels in pools.

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Applications

Sodium sulfite is primarily used in the pulp and paper industry. It is used in water treatment as an oxygen scavenger agent, in the photographic industry to protect developer solutions from oxidation and (as hypo clear solution) to wash fixer (sodium thiosulfate) from film and photo-paper emulsions, in the textile industry as a bleaching, desulfurizing and dechlorinating agent and in the leather trade for the sulfitization of tanning extracts. It is used in chemical manufacturing as a sulfonation and sulfomethylation agent. It is used in the production of sodium thiosulfate. It is used in other applications, including ore flotation, oil recovery, food preservatives, making dyes, and detergent. It forms a bisulfite adduct with aldehydes, and with ketones forms a sulfonic acid. It is used to purify or isolate aldehydes and ketones.

Descriptive chemistry

Sodium sulfite is decomposed by even weak acids, giving up sulfur dioxide gas.

Na₂SO₃ + 2 H⁺ → 2 Na⁺ + H₂O + SO₂

A saturated aqueous solution has pH of ~9. Solutions exposed to air are eventually oxidized to sodium sulfate. If sodium sulfite is allowed to crystallize from aqueous solution at room temperature or below, it does so as a heptahydrate. 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.^[1]

References

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

Categories: Sulfites | Sodium compounds | Photographic chemicals

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Sodium_sulfite". A list of authors is available in Wikipedia.