# Sodium metabisulfite

Sodium metabisulfite or sodium pyrosulfite (IUPAC spelling; Br. E. sodium metabisulphite or sodium pyrosulphite) is an inorganic compound of chemical formula Na$_2$S$_2$O$_5$. The substance is sometimes referred to as disodium metabisulfite. It is used as a disinfectant, antioxidant, and preservative agent. When dissolved in water it forms sodium bisulfite.

## Preparation

Sodium metabisulfite can be prepared by treating a solution of sodium hydroxide with sulfur dioxide. When conducted in warm water, Na$_2$SO$_3$ 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$_2$SO$_3$ + H$_2$O
    SO$_2$ + Na$_2$SO$_3$ → Na$_2$S$_2$O$_5$
which yields a residue of colourless solid Na$_2$S$_2$O$_5$.

## 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$_2$S$_2$O$_5$ + H$_2$O → 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 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 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 and adrenaline, and to the formulation in epinephrine autoinjectors, 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.
* 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.
* 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 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 an SO$_2$ source (mixed with air or oxygen) for the destruction of cyanide in commercial gold cyanidation processes.
* It is used as an 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™) 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.
* 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.