Sodium sulfite

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Sodium sulfite
IUPAC name Sodium sulfite
Other names Sodium sulphite
hypo clear (photography)
CAS number 7757-83-7
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


Solubility in water 67.8 g/100 ml of heptahydrate (18°C)
Solubility in other solvents 3 g/100 ml (?°C) (heptahydrate)
Crystal structure hexagonal (anhydrous)
monoclinic (heptahydrate)
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 Na2SO3. It has a molecular weight of 126.04. It is a product of SO2 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.


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]


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