Sodium sulfite

Sodium sulfite
anhydrous	hydrate
IUPAC name Sodium sulfite
Other names Hypo clear (photography) E221
Identifiers
CAS number	7757-83-7 Y
PubChem	24437
ChemSpider	22845 Y
UNII	VTK01UQK3G Y
RTECS number	WE2150000
Jmol-3D images	Image 1
SMILES [O-]S(=O)[O-].[Na+].[Na+]
InChI InChI=1S/2Na.H2O3S/c;;1-4(2)3/h;;(H2,1,2,3)/q2*+1;/p-2 Y Key: GEHJYWRUCIMESM-UHFFFAOYSA-L Y InChI=1/2Na.H2O3S/c;;1-4(2)3/h;;(H2,1,2,3)/q2*+1;/p-2 Key: GEHJYWRUCIMESM-NUQVWONBAK
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	678 g/L (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 sulfite (verify) (what is: Y/N?) 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 sodium salt of sulfurous acid. It is a product of sulfur dioxide 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 halogens to their respective hydrohalic acids, in photography and for reducing chlorine levels in pools.

Preparation

Sodium sulfite can be prepared in lab by reacting sodium carbonate with sulfurous acid:

Na₂CO₃ + H₂SO₃ → Na₂SO₃ + CO₂ + H₂O

The second way of prepare it in lab is reacting sodium hydroxide solution with sulfur dioxide gas:

2 NaOH + SO₂ → Na₂SO₃ + H₂O

By adding few drops of concentrated hydrochloric acid it can be tested that sodium sulfite is formed or not. if the solution of Na₂SO₃ gives bubbles of sulfur dioxide gas with HCl, that means the solution of sodium hydroxide is turned to sodium sulfite:

Na₂SO₃ + 2 HCl → 2 NaCl + SO₂ + H₂O

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