Sodium metabisulfite or sodium pyrosulfite (IUPAC spelling; Br. E. sodium metabisulphite or sodium pyrosulphite) is an
inorganic compound
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemi ...
of chemical formula Na
2S
2O
5. The substance is sometimes referred to as disodium metabisulfite. It is used as a
disinfectant,
antioxidant, and
preservative
A preservative is a substance or a chemical that is added to products such as food products, beverages, pharmaceutical drugs, paints, biological samples, cosmetics, wood, and many other products to prevent decomposition by microbial growth or ...
agent.
Preparation
Sodium metabisulfite can be prepared by treating a solution of
sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and alkal ...
with
sulfur dioxide
Sulfur dioxide ( IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic ...
.
When conducted in warm water, Na
2SO
3 initially precipitates as a yellow solid. With more SO
2, the solid dissolves to give the disulfite, which crystallises upon cooling.
:SO
2 + 2 NaOH → Na
2SO
3 + H
2O
:SO
2 + Na
2SO
3 → Na
2S
2O
5
which yields a residue of colourless solid Na
2S
2O
5.
Chemical structure
The anion
metabisulfite consists of an SO
2 group linked to an SO
3 group, with the negative charge more localised on the SO
3 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.
Reactivity
Upon dissolution in water, bisulfite is generated:
:Na
2S
2O
5 + H
2O → 2 Na
+ + 2 HSO
3−
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.
For example, it is added to
combination drug formulations which contain a
local anaesthetic
A local anesthetic (LA) is a medication that causes absence of pain sensation. In the context of surgery, a local anesthetic creates an absence of pain in a specific location of the body without a loss of consciousness, as opposed to a general a ...
and adrenaline,
and to the formulation in
epinephrine autoinjectors, such as the EpiPen. This lengthens the shelf life of the formulation,
although the sodium metabisulphite reacts with adrenaline, causing it to degrade and form epinephrine sulphonate.
*In combination with
sodium hydrosulfite it is used as a rust-stain remover
*It is used in photography.
*Concentrated sodium metabisulfite can be used to remove
tree stumps. Some brands contain 98% sodium metabisulfite, and cause degradation of
lignin
Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity ...
in the stumps, facilitating removal.
*It is also used as an
excipient in some
tablets, such as
paracetamol
Paracetamol, also known as acetaminophen, is a medication used to treat fever and mild to moderate pain. Common brand names include Tylenol and Panadol.
At a standard dose, paracetamol only slightly decreases body temperature; it is inferi ...
.
*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
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, m ...
). 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 SO
2) 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 a SO
2 source (mixed with air or oxygen) for the destruction of cyanide in commercial
gold cyanidation processes.
*It is used as a SO
2 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.
*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
™
The trademark symbol is a symbol to indicate that the preceding mark is a trademark, specifically an unregistered trademark. It complements the registered trademark symbol which is reserved for trademarks registered with an appropriate gove ...
) 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.
*It is used for preserving fruit during shipping.
*It is used as a solvent in the extraction of starch from tubers, fruit, and cereal crops.
*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.
Harmful if swallowed, it is corrosive to metals and should be stored safely.
References
External links
International Chemical Safety Card 1461The surprisingly elusive crystal structure of sodium metabisulfite
{{DEFAULTSORT:Sodium Metabisulfite
Metabisulfites
Sodium compounds
Food additives
Disinfectants
Homebrewing
E-number additives