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.
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]
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]
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]
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
haemoglobinmutation). 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.
^Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 16: The group 16 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 520.
ISBN978-0-13-175553-6.
^K. L. Carter, T. A. Siddiquee, K. L. Murphy, D. W. Bennett "The surprisingly elusive crystal structure of sodium metabisulfite" Acta Crystallogr. (2004). B60, 155–162.
doi:
10.1107/S0108768104003325
^
abcMcGee, Douglas L. (2013).
"Local and topical anesthesia". In Roberts, James R.; Jerris, R. Hedges (eds.). Roberts and Hedges' Clinical Procedures in Emergency Medicine (6th ed.). Elsevier/Saunders. p. 519.
ISBN9781455748594.