Sodium sulfite

IUPAC Name: Disodium sulfite
Synonyms of Sodium sulphite (CAS NO.7757-83-7): Anhydrous sodium sulfite ; CCRIS 1429 ; Disodium sulfite ; Disodium sulfite (Na2SO3) ; EINECS 231-821-4 ; Exsiccated sodium sulfite ; HSDB 5043 ; Natrii sulphis ; Natrium sulfurosum ; Natriumsulfid ; Natriumsulfid [German] ; Natriumsulfit ; Natriumsulfit [German] ; S-WAT ; Sodium sulfite ; Sodium sulfite (Na2SO3) ; Sodium sulfite anhydrous ; Sodium sulfite, exsiccated  ; Sulftech ; Sulfurous acid, disodium salt ; Sulfurous acid, sodium salt (1:2) ; UNII-VTK01UQK3G
CAS NO: 7757-83-7
Classification Code: Mutation data ; Tumor data
Molecular Formula: Na₂O₃S
Molecular Weight: 126.0427
Molecular Structure:

Melting Point: 500 °C 
H bond acceptors: 3
H bond donors: 2 
Polar Surface Area: 76.74 Å²
SMILES: [Na+].[Na+].[O-]S([O-])=O
InChI: InChI=1/2Na.H2O3S/c;;1-4(2)3/h;;(H2,1,2,3)/q2*+1;/p-2
InChIKey: GEHJYWRUCIMESM-NUQVWONBAK
Std. InChI: InChI=1S/2Na.H2O3S/c;;1-4(2)3/h;;(H2,1,2,3)/q2*+1;/p-2
Std. InChIKey: GEHJYWRUCIMESM-UHFFFAOYSA-L

The main uses are as follows:
1. Sodium sulphite (CAS NO.7757-83-7) 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.
2. Sodium sulphite 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.
3. Sodium sulphite is used in other applications, including ore flotation, oil recovery, food preservatives, making dyes, and detergent.
4. Sodium sulphite 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.
5. Sodium sulphite forms a bisulfite adduct with aldehydes, and with ketones forms a sulfonic acid. It is used to purify or isolate aldehydes and ketones.

Hazard Codes of Sodium sulphite (CAS NO.7757-83-7): Xn
Risk Statements: 31-36/37/38-22-40 
R31: Contact with acids liberates toxic gas. 
R36/37/38: Irritating to eyes, respiratory system and skin. 
R22: Harmful if swallowed. 
R40: Limited evidence of a carcinogenic effect.
Safety Statements: 24/25-36-26-22 
S24/25: Avoid contact with skin and eyes.
S36: Wear suitable protective clothing. 
S26: In case of contact with eyes, rinse immediately with plenty of water and seek medical advice. 
S22: Do not breathe dust.
WGK Germany: 1
RTECS: WE2150000
F: 3-10
HS Code: 28321000

Sodium sulfite (CAS NO.7757-83-7) 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.