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 chemist ...

of chemical formula Na₂S₂O₅. The substance is sometimes referred to as disodium metabisulfite. It is used as a

disinfectant A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than st ...

antioxidant Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricant ...

, 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 by ...

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 alkali ...

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 activ ...

. 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. :SO₂ + 2 NaOH → Na₂SO₃ + H₂O :SO₂ + Na₂SO₃ → Na₂S₂O₅ which yields a residue of colourless solid Na₂S₂O₅.

Chemical structure

The anion

metabisulfite A disulfite, commonly known as metabisulfite or pyrosulfite, is a chemical compound containing the ion . It is a colorless dianion that is primarily marketed in the form of sodium metabisulfite or potassium metabisulfite. When dissolved in wat ...

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.

Reactivity

Upon dissolution in water, bisulfite is generated: :Na₂S₂O₅ + H₂O → 2 Na⁺ + 2 HSO₃⁻

Uses

Sodium and

potassium metabisulfite Potassium metabisulfite, K2S2O5, also known as potassium pyrosulfite, is a white crystalline powder with a pungent odour. It is mainly used as an antioxidant or chemical sterilant. As a disulfite, it is chemically very similar to sodium metabis ...

have many major and niche uses. It is widely used for preserving food and beverages. *Sodium metabisulfite is added as an

excipient An excipient is a substance formulated alongside the active ingredient of a medication, included for the purpose of long-term stabilization, bulking up solid formulations that contain potent active ingredients in small amounts (thus often referred ...

to medications which contain adrenaline (

epinephrine Adrenaline, also known as epinephrine, is a hormone and medication which is involved in regulating visceral functions (e.g., respiration). It appears as a white microcrystalline granule. Adrenaline is normally produced by the adrenal glands and ...

), in order to prevent the oxidation of adrenaline. For example, it is added to

combination drug A combination drug or a fixed-dose combination (FDC) is a medicine that includes two or more active ingredients combined in a single dosage form. Terms like "combination drug" or "combination drug product" can be common shorthand for a FDC prod ...

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 an ...

and adrenaline, and to the formulation in

epinephrine autoinjector An epinephrine autoinjector (or adrenaline autoinjector, also known by the trademark EpiPen) is a medical device for injecting a measured dose or doses of epinephrine (adrenaline) by means of autoinjector technology. It is most often used for th ...

s, 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 Sodium dithionite (also known as sodium hydrosulfite) is a white crystalline powder with a sulfurous odor. Although it is stable in dry air, it Chemical decomposition, decomposes in hot water and in acid Solution (chemistry), solutions. Structu ...

it is used as a rust-stain remover *It is used in photography. *Concentrated sodium metabisulfite can be used to remove

tree stump After a tree has been cut and felled, the stump or tree stump is usually a small remaining portion of the trunk with the roots still in the ground. Stumps may show the age-defining rings of a tree. The study of these rings is known as dendrochrono ...

s. 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

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 inferior ...

. *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 Sickle cell disease (SCD) is a group of blood disorders typically inherited from a person's parents. The most common type is known as sickle cell anaemia. It results in an abnormality in the oxygen-carrying protein haemoglobin found in red blo ...

(and other similar forms of

haemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...

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, mi ...

). The substance causes defunct cells to sickle (through a complex

polymerisation In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many for ...

) 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 a SO₂ source (mixed with air or oxygen) for the destruction of cyanide in commercial

gold cyanidation Gold cyanidation (also known as the cyanide process or the MacArthur-Forrest process) is a hydrometallurgical technique for extracting gold from low-grade ore by converting the gold to a water-soluble coordination complex. It is the most commonly ...

processes. *It is used as a 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. *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. *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