Sodium sulfite: Wikis


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Sodium sulfite
Sodium sulfite
IUPAC name
Other names Hypo clear (photography)
CAS number 7757-83-7 Yes check.svgY
PubChem 24437
RTECS number WE2150000
Molecular formula Na2SO3
Molar mass 126.043 g/mol
Appearance white solid
Density 2.633 g/cm3 (anhydrous)
1.561 g/cm3 (heptahydrate)
Melting point

33.4 °C (dehydration of heptahydrate)
500°C (anhydrous)

Boiling point


Solubility in water 67.8 g/100 ml (18 °C, heptahydrate)
Crystal structure hexagonal (anhydrous)
monoclinic (heptahydrate)
EU Index Not listed
NFPA 704
NFPA 704.svg
Flash point Non-flammable
Related compounds
Other anions Sodium selenite
Other cations Potassium sulfite
Related compounds Sodium bisulfite
Sodium metabisulfite
Sodium sulfate
 Yes check.svgY (what is this?)  (verify)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Sodium sulfite (sodium sulphite) is a soluble compound of sodium. 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 the purification of TNT for military use. 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 froth flotation of ores, oil recovery, food preservatives, making dyes.


Sodium sulfite 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

Simple English

Sodium sulfite structure

Sodium sulfite is a chemical compound. It is made of sodium and sulfite ions. It is one of the sulfites used to preserve dried food. It is used to remove chlorine from pools. It reacts with acids to produce sulfur dioxide. It is a weak reducing agent. When it is oxidized, it is converted to sodium sulfate. Its chemical formula is Na2SO3.

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