What is Sodium Metabisulfite?

The chemical compound sodium metabisulfite has the formula Na2S2O5. It is also known by the names sodium pyrosulfite or sodium disulfite. Na2S2O5 consists of the sodium cation, (Na+), and the metabisulfite anon (S2O52+). As shown below, sodium metabisulfite is a white powder or yellowish-white in its normal state.

The odor of this compound is faintly pungent, similar to sulfur dioxide. Attention! People who are sensitive to sulfites may experience allergic reactions to sodium metabisulfite.

Structure

As we have already mentioned, sodium metabisulfite contains two sodium ions as well as one metabisulfite. Below is an illustration of the structure of a Na2S2O5 molecular.

Sodium Metabisulfite Structure

Structure of a Sodium Metabisulfite Molecule

The direct connection between two sulfur atoms is the disulfite (or Metabisulfite), anion. The first sulfur atom has three oxygen atoms bonded to it and has an oxidation status of +5. The other sulfur atom has two oxygen atoms bonded to it and has an oxidation status of +3.

Preparation

A solution of NaHSO3 or sodium bisulfite (sodium saltsulfite) can be saturated with SO2 (sulfur dioxide), and allowed to evaporate. Then, a residual of sodium metabisulfite will be formed. This is the equilibrium reaction.

2HSO3- = S2O52- + H2O

The solution's sodium ions form an ionic bond to the metabisulfite, which results in solid Na2S2O5 substance.

Physical Properties

In its solid-state, sodium sulfite appears white or whitish yellow.

It has a mildly pungent scent similar to SO2.

Na2S2O5 can be fairly dissolved in water. Its solubility is 65.3g/100mL at 20oC.

This compound is very well soluble in Glycerol, but not in ethanol.

Chemical Properties

The sulfur dioxide gas is released when sodium metabisulfite is mixed with water. It has a pungent, unpleasant odour. This gas can cause respiratory problems in people.

This compound releases SO2 gas when it is exposed to strong acids like hydrochloric acid. This chemical reaction can be described as: 2HCl + Na2S2O5 + 2SO2 + NaCl + H2O

When heated sodium metabisulfite undergoes a decomposition process to become sodium sulfite or sulfur dioxide. The chemical reaction is Na2S2O5 + SO2 + Na2SO3.

Uses

There are many uses for sodium sulfite, including as an antioxidant and a preservative. Below are some important uses for this compound.

Campden tablets are made from Na2S2O5. It can also be used to clean equipment used in winemaking.

This compound can also be used to prolong the shelf life for several local anaesthetics.

Because it reduces the lignin, concentrated Na2S2O5 is able to be used to remove tree stumps.

Also, sodium sulfite can be used to test for sickle cells diseases.

This compound can be used to precipitate gold when it is dissolved in aqua regia.

It can also act as an oxygen scavenger so Na2S2O5 has been used in the oil industry as a corrosion inhibitor.

This compound can also be used to treat wastewater as it can reduce hexavalent chrome into its trivalent form.