Sodium metabisulfite

Sodium metabisulfite or sodium pyrosulfite (IUPAC spelling; Br. E. sodium metabisulphite or sodium pyrosulphite) is an inorganic compound of chemical formula Na2S2O5. The name is sometimes referred to as disodium (metabisulfite, etc). It is used as a sterilizer and antioxidant/preservative.

Food additive
It is used as a food additive, mainly as a preservative and is sometimes identified as E223. As an additive, it may cause allergic reactions, particularly skin irritation e.g. eczema; gastric irritation and asthma. It is not recommended for consumption by children. It is present in many dilutable squashes (i.e. fruit juice concentrates) and in candy bars such as Mounds

Sterilization / Cleaning agent
It is commonly used in homebrewing preparations to sanitize equipment. It is used as a cleaning agent for potable water reverse osmosis membranes in desalination systems. It is also used to remove chloramine from drinking water after treatment.

Stump Removing
In the brand Stump-Out, it is used in almost a pure form (98%) to cause degradation of lignin, creating pores for fuel adsorption, and consequently, ignition.

Packaging
It can be purchased in powdered form. Either sodium or potassium metabisulfite will be the primary ingredient in Campden tablets and some stump removers. In solid form it ranges in color from white to slightly yellow.

Chemical properties
When mixed with water, sodium metabisulfite releases sulfur dioxide (SO2), a pungent, unpleasant smelling gas that can also cause breathing difficulties in some people. For this reason, sodium metabisulfite has fallen from common use in recent times, with agents such as hydrogen peroxide becoming more popular for effective and odorless sterilization of equipment. Released sulfur dioxide however makes the water a strong reducing agent.

Alternatives
Include ascorbates 300-304 in some applications. Calcium sulfate 516 and ascorbic acid 300 are safe alternatives to sodium metabisulfite used as a flour improver.