Sodium metabisulfite, also known as sodium disulfite or sodium pyrosulfite, is an inorganic compound and has a formula of Na2S2O5. Its anion consists of a SO2 group linked and joined to a SO3 group, making it a hybrid of dithionite and dithionate. The negative charge of the anion is more localized on the SO3 group due to the presence of more electronegaitve oxygen atoms. When sodium metabisulfite is mixed with water or strong acids or upon heating at high temperature, the pungent gaseous sulfur dioxide is released. This unpleasant smelling gas can cause breathing difficulties in individuals; hence alternatives such as odorless hydrogen peroxide are more widely used.
Using Sodium Bisulfite
Sodium metabisulfite can be manufactured using sodium bisulfite and sulfur dioxide. A solution of sodium bisulfite saturated with sulfur dioxide is allowed to evaporate, yielding sodium metabisulfite as shown.
Using Sodium Carbonate/ Sodium Hydroxide
Sodium metabisulfite can be manufactured by treating sodium hydroxide or a suspension of sodium carbonate with sulfur dioxide containing gases in saturated sodium hydrogen sulfite solution. Sodium metabisulfite yielded is removed by centrifugation and dried.
Sodium metabisulfite is used as a preservative and antioxidant in food and is also known as E223. It may cause allergic reactions in those who are sensitive to sulfites, including respiratory reactions in asthmatics, anaphylaxis and other allergic reactions in sensitive individuals. Sodium metabisulfite and potassium metabisulfite are the primary ingredients in Campden tablets, used for wine and beer making.
Sodium metabisulfite is used as a bleaching and dechlorination agent for bleached cotton. It is also used in the deliming process during leather processing and prevents the formation of toxic sulfide gas. It bleaches the leather and makes leather soft, well-developed, tough, waterproof, anti-break, wear-ability and etc.
It is commonly used in home brewing and winemaking 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.
In photography industry, sodium metabisulfite is used for production of developer solution and acidising fixation baths. Concentrated sodium metabisulfite can be used to remove tree stumps. It is also used as an excipient in some tablets, such as paracetamol.