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 (S2O52-). 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.
As mentioned earlier, sodium metabisulfite is made up of two sodium ions and one metabisulfite ion. The structure of an 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.
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– ⇌ S2O52- + 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
|Molar Mass||190.107 grams per mole|
|Density||1.48 grams per cubic centimetre|
|Melting Point||170oC but begins to decompose at 150oC|
- 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 20o
- This compound is highly soluble in glycerol but not very soluble in ethanol.
- 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
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.
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