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