Sodium Metabisulfite, also known as sodium pyrosulfite, is an inorganic compound with the chemical formula Na₂S₂O₅.

It should be noted that, when introduced to water, Sodium Metabisulfite liberates sulfur dioxide gas which has a highly pungent and unpleasant odour. This gas can also lead to respiratory problems in humans.

Uses

Sodium Metabisulfite is widely used as a preservative, antioxidant, bleach, color fixative and leavening agent in food production. As preservative, antioxidant, color fixative: in Canned mushrooms, candied, vermicelli to whiten products, keep fresh and extend shelf life. As a leavening agent: in biscuit, baked food and bread to ferment.

In pharmaceuticals, Sodium Metabisulfite is widely used as intermediate in Pharmaceutical. As intermediate: in manufacturing of chloroform, styrene sulfone and benzaldehyde.

Sodium Metabisulfite is widely used as an antichlor, coagulants and contrast agent in various different industries. 

• As antichlor: in Dyeing Process to get rid of chlorine.

• As coagulants: in the Rubber industry to dye.

• As contrast agent: in the Photographic Industry to develop.

Synthesis

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.

2HSO₃ˉ ?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.

Hazards

Sodium Metabisulfite can affect you when breathed in. If contact, it can irritate the skin and eyes. Breathing Sodium Metabisulfite can irritate the nose, throat and lungs, causing coughing, wheezing and shortness of breath. Also, Sodium Metabisulfite may cause an asthma-like allergy. Future exposure can cause asthma attacks with shortness of breath, wheezing, cough, and chest tightness.