Sodium metabisulfite or sodium pyrosulfite (IUPAC spelling; Br. E. sodium metabisulphite or sodium pyrosulphite) is an

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

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 ...

antioxidant Antioxidants are Chemical compound, compounds that inhibit Redox, oxidation, a chemical reaction that can produce Radical (chemistry), free radicals. Autoxidation leads to degradation of organic compounds, including living matter. Antioxidants ...

, 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. When dissolved in water it forms

sodium bisulfite Sodium bisulfite (or sodium bisulphite, sodium hydrogen sulfite) is a chemical mixture with the approximate chemical formula NaHSO3. Sodium bisulfite is not a real compound, but a mixture of salts that dissolve in water to give solutions compose ...

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 . It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly corrosive base (chemistry), ...

with

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...

. 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 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 metabi ...

have many major and niche uses. It is widely used for preserving food and beverages. *Sodium metabisulphite is one of the main ingredients in "Drywhite", a composition used to prevent chipped potatoes oxidising during storage prior to use in many fish and chip shops. *Sodium metabisulfite is added as an

excipient An excipient is a substance formulated alongside the active ingredient of a medication. They may be used to enhance the active ingredient’s therapeutic properties; to facilitate drug absorption; to reduce viscosity; to enhance solubility; to i ...

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 a ...

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

combination drug A combination drug is a combination of two or more pharmaceutical drugs as active ingredients combined into a single dosage form, typically as a ''fixed-dose combination'', with each constituent standardized to specifications of a fixed dose. Fix ...

formulations which contain a

local anaesthetic A local anesthetic (LA) is a medication that causes absence of all sense, sensation (including pain) in a specific body part without loss of consciousness, providing local anesthesia, as opposed to a general anesthetic, which eliminates all sen ...

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 t ...

s, such as the EpiPen. This lengthens the shelf life of the formulation, although the sodium metabisulfite reacts with adrenaline, causing it to degrade and form epinephrine sulfonate. *In combination with sodium hydrosulfite it is used as a rust-stain remover *It is used in photography as an antioxidant in photographic film development. *Concentrated sodium metabisulfite can be used to remove

tree stump After a tree has been cut and has fallen, 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 dendr ...

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 rigidit ...

in the stumps, facilitating removal. *It is also used as an

in some tablets, such as

paracetamol Paracetamol, or acetaminophen, is a non-opioid analgesic and antipyretic agent used to treat fever and mild to moderate pain. It is a widely available over-the-counter drug sold under various brand names, including Tylenol and Panadol. Parac ...

. *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), also simply called sickle cell, is a group of inherited haemoglobin-related blood disorders. The most common type is known as sickle cell anemia. Sickle cell anemia results in an abnormality in the oxygen-carrying ...

(and other similar forms of

haemoglobin Hemoglobin (haemoglobin, Hb or Hgb) is a protein containing iron that facilitates the transportation of oxygen in red blood cells. Almost all vertebrates contain hemoglobin, with the sole exception of the fish family Channichthyidae. Hemoglobi ...

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, ...

). 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 form ...

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

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. *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 an additive in the extraction of starch from tubers, fruit, and cereal crops. *It is used as a

pickling Pickling is the process of food preservation, preserving or extending the shelf life of food by either Anaerobic organism, anaerobic fermentation (food), fermentation in brine or immersion in vinegar. The pickling procedure typically affects t ...

agent to treat high pressure

reverse osmosis Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane, semi-permeable membrane to separate water molecules from other substances. RO applies pressure to overcome osmotic pressure that favors even distribu ...

and

nanofiltration Nanofiltration is a Membrane technology, membrane filtration process that uses nanometer sized pores through which particles smaller than about 1–10 nanometers pass through the membrane. Nanofiltration membranes have pore sizes of about 1–10 n ...

water desalination membranes for extended storage periods between uses. *It is used to create a bisulfite adduct from ketones to aid in separation of the ketone product. The usage of metabisulfite versus the sulfite is also more entropically favourable.

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. Some people who are sulfite sensitive may experience an allergic reaction to sodium meta bisulfite, sometimes severe, resulting in labeling requirements for food safety. In 2024, it was named ‘allergen of the year 2024’ by the American Contact Dermatitis Society.

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