What is Sodium Metabisulfite?

Sodium metabisulfite is a chemical compound with the formula Na₂S₂O₅. It is also known as sodium pyrosulfite and sodium disulfite. Na₂S₂O₅ is an ionic compound containing the sodium cation (Na⁺) and the metabisulfite anion (S₂O₅^2-). In its standard state, sodium metabisulfite exists as a white or yellowish-white powder, as shown below.

This compound has a faintly pungent odour similar to that of sulfur dioxide. It is important to note that sodium metabisulfite may induce allergic reactions in humans who are sensitive to sulfites.

Structure of Sodium Metabisulfite

As mentioned earlier, sodium metabisulfite is made up of two sodium ions and one metabisulfite ion. The structure of a Na₂S₂O₅ molecule is illustrated below.

The disulfite (or metabisulfite) anion features a direct connection between the two sulfur atoms. The first sulfur atom is bonded to three oxygen atoms and exhibits an oxidation state of +5 whereas the other sulfur atom exhibits an oxidation state of +3 and is bonded to two oxygen atoms.

Preparation of Sodium Metabisulfite

When a solution of NaHSO₃ (sodium bisulfite) is saturated with SO₂ (sulfur dioxide) and then allowed to evaporate, a residue of sodium metabisulfite is obtained. The equilibrium reaction can be written as follows.

2HSO₃^– ⇌ S₂O₅^2- + H₂O

The free sodium ions in the solution form an ionic bond with the metabisulfite ion, resulting in the formation of solid Na₂S₂O₅ residue.

Properties of Sodium Metabisulfite

Chemical Data

Sodium Metabisulfite	Na2S2O5
Molar Mass	190.107 grams per mole
Density	1.48 grams per cubic centimetre
Melting Point	170oC but begins to decompose at 150oC
Boiling Point	Decomposes

Physical Properties

• Sodium sulfite has a white or whitish-yellow appearance in its solid-state.
• It has a faintly pungent smell similar to SO₂.
• Na₂S₂O₅ is fairly soluble in water – its solubility corresponds to 65.3g/100mL at a temperature of 20^oC.
• This compound is highly soluble in glycerol but not very soluble in ethanol.

Chemical Properties

• When introduced to water, sodium metabisulfite liberates sulfur dioxide gas which has a very pungent and unpleasant odour. This gas can also lead to respiratory problems in humans.
• SO₂ gas is liberated by this compound on exposure to strong acids such as hydrochloric acid. The chemical equation for this reaction is given by: 2HCl + Na₂S₂O₅ → 2SO₂ + 2NaCl + H₂O
• When heated, sodium metabisulfite undergoes decomposition to form sodium sulfite and sulfur dioxide. The chemical reaction is: Na₂S₂O₅ → SO₂ + Na₂SO₃

Uses of Sodium Metabisulfite

Sodium sulfite has several applications as an antioxidant, a preservative and as a disinfectant. Some important uses of this compound are listed below.

• Na₂S₂O₅ is one of the primary ingredients used in Campden tablets. It is also used to sanitize the equipment used for winemaking.
• This compound is also used to increase the shelf life of several local anaesthetics.
• Concentrated Na₂S₂O₅ can be used for the removal of tree stumps since it degrades the lignin in it.
• Sodium sulfite is also used to test for sickle cell diseases.
• When gold is dissolved in aqua regia, it can be precipitated out with the help of this compound.
• Since it can act as an oxygen scavenger, Na₂S₂O₅ is used as a corrosion inhibitor in the oil industry.
• This compound is also used in the treatment of wastewater since it can reduce hexavalent chromium into its trivalent form.

Frequently Asked Questions

What are the uses of sodium metabisulfite?

Sodium metabisulfite, a chemical compound with the formula Na2S2O5, is widely used as an antioxidant in pharmaceutical formulations. It is also used as a disinfectant and as a food preservative in many industries.

List some examples of foods that contain sodium metabisulfite

Examples of food items that contain sodium metabisulfite are listed below:

• Jams
• Baked products
• Potato chips
• Pickled food items
• Dried fruits

Is sodium metabisulfite organic or inorganic?

Sodium metabisulfite, also known as sodium pyrosulfite, is an inorganic compound with the chemical formula Na2S2O5. The IUPAC spelling of the compound is sodium metabisulphite or sodium pyrosulphite.

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