Sodium metabisulfite, also known as sodium pyrosulfite is an inorganic compound, used as a preservative and reducing agent. Its chemical formula is Na2S2O5.
- Na2S2O5 → Na2SO3 + SO2
The same thing happens when a strong acid is added.
- Na2S2O5 + 2 HCl → 2 NaCl + H2O + 2 SO2
Sodium metabisulfite itself can be used as a reducing agent as well as the sulfur dioxide it produces. Perhaps the most famous example of this is the reduction of chloroauric acid to produce very pure elemental gold. It can also be added to a solution of copper(II) chloride to precipitate copper(I) chloride.
The little-known copper compound Chevreul's Salt can be produced by boiling a mixture of copper(II) sulfate and sodium metabisulfite solutions. Both of these reactions should be done outside or in a fume hood as sulfur dioxide is released, though to a much lesser extent in the case of Chevreul's salt.
Sodium metabisulfite is a white salt. It has a faint sulfur dioxide smell.
Sodium metabisulfite is available as a wine preservative, either alone or mixed with potassium metabisulfite, product known as Campden tablets. The metabisulfite salt is usually stored in an air-tight bag inside the outer paper or plastic bag, to keep it dry and reduce the sulfur dioxide smell. Some stump removers, namely the one maunfactured by Bonide (found at Home Depot), consist of sodium metabisulfite. Other stump removers may be potassium nitrate or other chemicals, so it is important to read the label/MSDS.
Sodium metabisulfite can be prepared by evaporating a solution of sodium bisulfite saturated with sulfur dioxide:
- 2 HSO3- ⇌ H2O + S2O52-
- Precipitate gold from gold compounds
- Generate sulfur dioxide for use as a reducing agent
- Bleach neutralization
It may cause allergic reactions to people who are sensitive to sulfites. Sulfur dioxide is very irritating and unpleasant as well as toxic.
Sodium metabisulfite should be kept out of hot or humid environments and away from water and acids while indoors.
Sodium metabisulfite can be neutralized with bleach.