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Sodium metabisulfite

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Appearance
  
white to yellow powder

Molar mass
  
190.107 g/mol

Melting point
  
150 °C

Formula
  
Na2S2O5

Density
  
1.48 g/cm³

Soluble in
  
Water


Related compounds
  
Sodium dithioniteSodium thiosulfateSodium sulfate

IUPAC ID
  
Sodium pyrosulphite, Br. E. sodium metabisulphite

Sodium metabisulfite chemical structure properties and uses


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.

Contents

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Sodium metabisulfite


Preparation

Sodium metabisulfite can be prepared by evaporating a solution of sodium bisulfite saturated with sulfur dioxide:

2 HSO3− ⇌ H2O + S2O52−

which leaves solid Na2S2O5 behind.

Chemical structure

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The anion metabisulfite is a hybrid of dithionite (S2O42−) and dithionate (S2O62−). The anion consists of an SO2 group linked to an SO3 group, with the negative charge more localized 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.

Food additive

It is used as a preservative and antioxidant in food and is also known as E223.

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It may cause allergic reactions in those who are sensitive to sulfites, including respiratory reactions in asthmatics, anaphylaxis and other allergic reactions in sensitive individuals.

Sodium metabisulfite and potassium metabisulfite are the primary ingredients in Campden tablets, used for wine and beer making.

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The acceptable daily intake is up to 0.7 milligrams per kilogram of body weight. Sodium metabisulfite oxidizes in the liver to sulfate which is excreted in the urine.

Sanitization and cleaning agent

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It is commonly used in homebrewing and winemaking to sanitize equipment. It is used as a cleaning agent for potable water reverse osmosis membranes in desalination systems. It is also used to remove chloramine from drinking water after treatment.

Other uses

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  • Added to local anaesthetic (lidocaine etc.) solutions to prevent oxidation of vasoconstrictor adrenaline and thus improve the shelf life of the solution
  • 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. Approximately 0.5 mg is used in epinephrine autoinjectors such as the EpiPen.
  • 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 the source of SO2 in wine. An important anti-oxidant and bactericide
  • It is also used to precipitate gold from auric acid (gold dissolved in aqua regia).
  • It is used in waste treatment to chemically reduce hexavalent chromium to trivalent chromium which can then be precipitated and removed from an aqueous waste stream.
  • 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 fibers, 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 oil and gas industry as a corrosion inhibitor/oxygen scavenger.
  • It is used in the water treatment industry to quench chlorine residual

    • Chemical properties

    Sodium metabisulfite Sodium MetabisulfiteSodium MetabisulphiteSodium Metabisulfite

    When mixed with water, sodium metabisulfite releases sulfur dioxide (SO2), a pungent, unpleasant smelling gas that can also cause breathing difficulties in some people. For this reason, sodium metabisulfite has fallen from common use in recent times, with agents such as hydrogen peroxide becoming more popular for effective and odorless sterilization of equipment. Released sulfur dioxide however makes the water a strong reducing agent.

    Sodium metabisulfite releases sulfur dioxide in contact with strong acids:

    Na2S2O5 + 2 HCl → 2 NaCl + H2O + 2 SO2

    On heating to high temperature, it releases sulfur dioxide, leaving sodium sulfite behind:

    Na2S2O5 → Na2SO3 + SO2

    References

    Sodium metabisulfite Wikipedia
    (Text) CC BY-SA


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