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Sodium sulfate
Sodium sulfate
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Sodium sulfite
Sodium sulfite

Sodium metabisulfite

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.

Preparation

Sodium disulfite can be prepared by treating a solution of sodium hydroxide with sulfur dioxide. 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.

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Description

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

Main article: Bisulfite § Reactions

Upon dissolution in water, bisulfite is generated:

Na2S2O5 + H2O → 2 Na+ + 2 HSO3−

Uses

Sodium and potassium metabisulfite have many major and niche uses. It is widely used for preserving food and beverages.

  • Sodium metabisulphite 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 metabisulphite reacts with adrenaline, causing it to degrade and form epinephrine sulphonate.
  • 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 substances 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 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 in the water treatment industry to quench chlorine residual
  • 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.
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