Sodium Metabisulfite

What is Sodium metabisulfite?

Sodium metabisulfite is an inorganic compound with a chemical formula Na2S2O5. It is also known as disodium metabisulfite or sodium pyrosulfite. It is a sodium salt of sulfurous acid. This inorganic sodium salt is composed of disulfite ion and sodium ion in a ratio 1:2. It is yellowish-white or white in color and smells slightly like sulfur. It is toxic when inhaled and when mixed with water it is corrosive. It is easily soluble in water but slightly soluble in ethanol. It is widely used as a laboratory reagent and food preservative. It has a pH value of 4.0 – 4.5.

How to make sodium metabisulfite?

Process 1:

Sodium bisulfite solution which is saturated with sulfur dioxide is evaporated to give the following:

\(2NaHSO_{3}\rightarrow H_{2}O +Na_{2}S_{2}O_{5}\)

The above reaction is reversible and hence the compound is stored in an airtight container to avoid the action of atmospheric water vapors.

Process 2: Sodium carbonate is made to pass through sulfur dioxide to give the following reaction:

\(2SO_{2}+Na_{2}CO_{3}\rightarrow Na_{2}S_{2}O_{5}+CO_{2}\)

Properties of Sodium metabisulfite – Na2S2O5

Na2S2O5 Sodium metabisulfite
Molecular Weight/ Molar Mass 190.107 g/mol
Density 1.48 g/cm³
Boiling Point Decomposes
Melting Point 170°C

Sodium metabisulfite structure – Na2S2O5

Structure of Sodium metabisulfite

The above image describes the sodium metabisulfite structure. Na2S2O5 is the chemical formula of sodium metabisulfite and consists of two ions involving sulfur showing two oxidation states: 4 and 6. It is an SO2 group joined to an SO3 group with negative charge more localized on the SO3. Both the groups contain one oxygen atom which is joined by a single bond. These single bonded oxygen atoms are connected to each of the sodium atoms.

Uses of Sodium metabisulfite (Na2S2O5 )

  • Sodium metabisulfite is used as a food preservative.
  • It is used as an oxidant and sterilizer in the brewing of beer and fermenting of wine.
  • It is used as a bleaching agent in textile manufacturing.
  • It is used in the water purification process.
  • It is used in pharmaceuticals as a reducing agent.
  • It is used in cosmetics as a preservative.
  • It is used in darkroom photography.
  • It is used in injectable medicines as an antioxidant additive.
  • It inhibits the growth of bacteria and fungi.
  • It is used in the cotton and wool industry.

Sodium metabisulfite health risks – When this compound is in contact for a prolonged time or ingested or inhaled, it can cause severe irritation to sensitive areas like skin, eyes. People who are suffering from digestive disorders or asthma should keep themselves away from this compound as it could lead to complex digestive or respiratory disease.

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Practise This Question

In the following molecule, the C2  C3 bond length is shorter than the usual CC bond length. The reason for this observation is ______