Sodium metabisulfite

Names
Other names Sodium pyrosulfite Sodium disulfite
Identifiers
CAS Number	7681-57-4 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:114786 N
ChEMBL	ChEMBL2016976 N
ECHA InfoCard	100.028.794
EC Number	231-673-0
E number	E223 (preservatives)
PubChem CID	656671
RTECS number	UX8225000
UNII	4VON5FNS3C Y
CompTox Dashboard (EPA)	DTXSID0029684
InChI InChI=1S/2Na.H2O5S2/c;;1-6(2)7(3,4)5/h;;(H,1,2)(H,3,4,5)/q2*+1;/p-2
SMILES [O-]S(=O)S(=O)(=O)[O-].[Na+].[Na+]
Properties
Chemical formula	Na2S2O5
Molar mass	190.107 g/mol
Appearance	White to yellow powder
Odor	Faint SO2
Density	1.48 g/cm3
Melting point	170 °C (338 °F; 443 K) decomposition begins at 150 °C
Solubility in water	45.1 g/100 mL (0 °C) 65.3 g/100 mL (20 °C) 81.7 g 100 mL (100 °C)
Solubility	Very soluble in glycerol Slightly soluble in ethanol
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H302, H318
Precautionary statements	P264, P270, P280, P301+P312, P305+P351+P338, P310, P330, P501
NFPA 704 (fire diamond)	2 0 1
NIOSH (US health exposure limits):
PEL (Permissible)	None[1]
REL (Recommended)	TWA 5 mg/m3[1]
IDLH (Immediate danger)	N.D.[1]
Safety data sheet (SDS)	Mallinckrodt MSDS
Related compounds
Other anions	Sodium sulfite Sodium bisulfite
Other cations	Potassium metabisulfite
Related compounds	Sodium dithionite Sodium thiosulfate Sodium sulfate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Sodium metabisulfite or sodium pyrosulfite (IUPAC spelling; Br. E. sodium metabisulphite or sodium pyrosulphite) is an inorganic compound of chemical formula Na₂S₂O₅. The substance is sometimes referred to as disodium metabisulfite. It is used as a disinfectant, antioxidant, and preservative agent.^[2] When dissolved in water it forms sodium bisulfite.

Preparation

Sodium metabisulfite can be prepared by treating a solution of sodium hydroxide with sulfur dioxide.^[3] When conducted in warm water, Na₂SO₃ initially precipitates as a yellow solid. With more SO₂, the solid dissolves to give the disulfite, which crystallises upon cooling.^[4]

SO₂ + 2 NaOH → Na₂SO₃ + H₂O

SO₂ + Na₂SO₃ → Na₂S₂O₅

which yields a residue of colourless solid Na₂S₂O₅.

Chemical structure

The anion metabisulfite consists of an SO₂ group linked to an SO₃ group, with the negative charge more localised on the SO₃ end. The S–S bond length is 2.22 Å, and the "thionate" and "thionite" S–O distances are 1.46 and 1.50 Å, respectively.^[5]

Reactivity

Upon dissolution in water, bisulfite is generated:

Na₂S₂O₅ + H₂O → 2 Na⁺ + 2 HSO₃⁻

Uses

Sodium and potassium metabisulfite have many major and niche uses. It is widely used for preserving food and beverages.

• Sodium metabisulfite is added as an excipient to medications which contain adrenaline (epinephrine), in order to prevent the oxidation of adrenaline.^[6] For example, it is added to combination drug formulations which contain a local anaesthetic and adrenaline,^[6] and to the formulation in epinephrine autoinjectors, such as the EpiPen.^[7] This lengthens the shelf life of the formulation,^[6] although the sodium metabisulphite reacts with adrenaline, causing it to degrade and form epinephrine sulphonate.^[8]
• In combination with sodium hydrosulfite it is used as a rust-stain remover^[9]
• It is used in photography.^[10]
• Concentrated sodium metabisulfite can be used to remove tree stumps. Some brands contain 98% sodium metabisulfite, and cause degradation of lignin in the stumps, facilitating removal.^[11]
• It is also used as an excipient in some tablets, such as paracetamol.
• A very important health related aspect of this substance is that it can be added to a blood smear in a test for sickle cell anaemia (and other similar forms of haemoglobin mutation). The substance causes defunct cells to sickle (through a complex polymerisation) hence confirming disease.
• It is used as a bleaching agent in the production of coconut cream
• It (or liquid SO₂) is commonly used as an antimicrobial and antioxidant in winemaking; bottled wine indicates its use with the label "Contains Sulfites" in the US.
• It is used as a reducing agent to break sulfide bonds in shrunken items of clothing made of natural fibres, thus allowing the garment to go back to its original shape after washing
• It is used as an SO₂ source (mixed with air or oxygen) for the destruction of cyanide in commercial gold cyanidation processes.
• It is used as an SO₂ source (mixed with air or oxygen) for the precipitation of elemental gold in chloroauric (aqua regia) solutions.
• It is used in the water treatment industry to quench residual chlorine.
• It is used in tint etching iron-based metal samples for microstructural analysis.^[12][13]
• It is used as a fungicide for anti-microbe and mould prevention during shipping of consumer goods such as shoes and clothing. Plastic stickers and packaging (such as Micro-Pak™) containing the anhydrous, sodium metabisulfite solid active ingredient are added prior to shipping. The devices absorb moisture from the atmosphere during shipping and release low levels of sulfur dioxide.^[14]
• It is used for preserving fruit during shipping.^[15]
• It is used as a solvent in the extraction of starch from tubers,^[16] fruit,^[17] and cereal crops.^[18][19]
• It is used as a pickling agent to treat high pressure reverse osmosis and nanofiltration water desalination membranes for extended storage periods between uses.

Safety

Sodium metabisulfite, despite not being flammable, decomposes in 150°C of heat releasing toxic gasses when decomposed. It is corrosive when dissolved in water.

It is harmful if swallowed, is corrosive to metals and should be stored safely.^[20]