sodium sulfite

Sodium sulfite
IUPAC name	Sodium sulfite
Other names	Hypo clear (photography) E221
Identifiers
CAS number	7757-83-7 Y
PubChem	24437
RTECS number	WE2150000
Properties
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	Decomposes
Solubility in water	67.8 g/100 ml (18 °C, heptahydrate)
Structure
Crystal structure	hexagonal (anhydrous) monoclinic (heptahydrate)
Hazards
MSDS	ICSC 1200
EU Index	Not listed
NFPA 704	0 2 0
Flash point	Non-flammable
Related compounds
Other anions	Sodium selenite
Other cations	Potassium sulfite
Related compounds	Sodium bisulfite Sodium metabisulfite Sodium sulfate
Y (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.

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 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.

Reactions

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]

References

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