The Chemical Structure, Properties, and Uses of Sodium Metabisulfite

Fact about Sodium metabisulfite
Sodium metabisulfite is used in numerous applications due to its unique properties. Here's more about the structure, chemical formula, and uses of sodium metabisulfite.
Did You Know?
Sodium metabisulfite and potassium metabisulfite are chemically very similar, and are often used interchangeably in their industrial applications. Potassium metabisulfite is often preferred in the food industry, due to the risks of excess dietary sodium.
Sodium metabisulfite, also known as sodium pyrosulfite or disodium metabisulfite, has the formula Na2S2O5.

It can be readily obtained by evaporating a solution of sodium bisulfite saturated with sulfur dioxide. The reaction is as follows:

2NaHSO3 ➙ H2O + Na2S2O5

This reaction is easily reversible. Hence, sodium metabisulfite is stored in airtight containers to prevent the action of atmospheric water vapors.

It can also be obtained by passing sulfur dioxide through sodium carbonate. The reaction for that is as follows:

2SO2 + Na2CO3 ➙ Na2S2O5 + CO2
The structure of sodium metabisulfite consists of two ions involving sulfur showing two oxidation states of sulfur: 4 and 6. It is an SO2 group joined to an SO3 group. The net negative charge leans towards the SO3 group. Both contain one oxygen atom joined by a single bond; these oxygen atoms are connected to one sodium atom each. Here is a simple representation.
sodium metabisulfite
This is a crude representation of the 3D structure of the compound. The actual chemistry of the structure of sodium metabisulfite is complicated. If this is a bit difficult to understand, here's a 2D version.
sodium metabisulfite 2d
As with the 3D version, this too is a crude representation of the structure. Explaining the actual structure would require lengthy digressions, and is beyond the scope of this article.
► At room temperature and pressure, sodium metabisulfite is a white powder (may be yellowish) with a slight smell of sulfur dioxide.

► It starts melting at about 302°F (150°C), but doesn't melt until at least 338°F (170°C).

► Its density is 1.48 g/mL.

► It releases sulfur dioxide in various situations, including upon reaction with water and strong acids, and heating. This has made it an unpopular cleaning agent, due to the unpleasant smell of sulfur dioxide.
It is used in the food industry as an antioxidant and preservative. It has antimicrobial properties that keep the food item safe. This may cause allergic reactions in those allergic to sulfites. In healthy humans with no allergies, sodium metabisulfite is metabolized in the liver, and excreted in the form of sulfates through urine.

It is often a component of Campden tablets, which are used in beer breweries and winemaking; either sodium or potassium metabisulfite is used. It provides sulfur dioxide, which helps preserve the wine. It is also used in these enterprises as a sanitizing agent.

It is used as a bleaching agent in the production and/or processing of paper, wool, cotton, etc.

Sodium metabisulfite should be handled with extreme care, since it can cause severe irritation upon inhalation, ingestion, or prolonged contact, particularly with sensitive areas such as the eyes. Those who suffer from asthma or any digestive disorder should stay well away from this compound, since its presence can cause potentially fatal complications in respiratory or digestive diseases.