Potassium metabisulfite, K₂S₂O₅, also known as potassium pyrosulfite, is a white crystalline powder with a pungent odour. It is mainly used as an

antioxidant Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricant ...

or chemical sterilant. As a

disulfite A disulfite, commonly known as metabisulfite or pyrosulfite, is a chemical compound containing the ion . It is a colorless dianion that is primarily marketed in the form of sodium metabisulfite or potassium metabisulfite. When dissolved in wat ...

, it is chemically very similar to sodium metabisulfite, with which it is sometimes used interchangeably. Potassium metabisulfite has a monoclinic crystal structure.

Preparation and reactions

Potassium metabisulfite can be prepared by treating a solution of potassium hydroxide with

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...

. :2 SO₂ + 2 KOH → K₂S₂O₅ + H₂O It decomposes at 190 °C, yielding potassium sulfite and

: :K₂S₂O₅ → K₂SO₃ + SO₂

Uses

It is used as a food additive, also known as E224.List of E-number food additives
/ref> It is restricted in use and may cause allergic reactions in some sensitive persons.

Wine

Potassium metabisulfite is a common wine or

must Must (from the Latin ''vinum mustum'', "young wine") is freshly crushed fruit juice (usually grape juice) that contains the skins, seeds, and stems of the fruit. The solid portion of the must is called pomace and typically makes up 7–23% of t ...

additive, in which it forms sulfur dioxide (SO₂). Sulfur dioxide is a disinfectant. It also acts as a potent

, protecting both the color and delicate flavors of wine. A high dose would be 3 grams of potassium metabisulfite per six-gallon bucket of must (yielding roughly 75 ppm of SO₂) prior to fermentation; then 6 grams per six-gallon bucket (150 ppm of SO₂) at bottling. Some countries regulate the SO₂ content of wines. Winemaking equipment is sanitized by spraying with a 1% SO₂ (2 tsp potassium metabisulfite per L) solution.

Beer

Potassium metabisulfite is sometimes used in the

brewing Brewing is the production of beer by steeping a starch source (commonly cereal grains, the most popular of which is barley) in water and #Fermenting, fermenting the resulting sweet liquid with Yeast#Beer, yeast. It may be done in a brewery ...

industry to inhibit the growth of wild bacteria and fungi. This step is called 'stabilizing'. It is also used to neutralize monochloramine from tap water. It is used both by

homebrewers Homebrewing is the brewing of beer or other alcoholic beverages on a small scale for personal, non-commercial purposes. Supplies, such as kits and fermentation tanks, can be purchased locally at specialty stores or online. Beer was brewed dom ...

and commercial brewers alike. It is not used as much for brewing beer, because the wort is almost always boiled, which kills most microorganisms.

Other uses

*Potassium metabisulfite is sometimes added to lemon juice as a preservative. *Potassium metabisulfite is used in the textile industry for dyeing and cotton printing. *Potassium metabisulfite is sometimes used to precipitate gold from solution in aqua regia (as an alternative to sodium sulfite). *It is a component of certain photographic developers and solutions used in photographic fixing. *It is used as a bleaching agent in the production of coconut cream. *It is used in some pickles as a preservative. *It is used in tint etching iron-based metal samples for microstructural analysis. *It is used in Aam papad as a preservative.

Safety

Potassium metabisulfite can irritate skin, eyes, and respiratory tract.

References

{{DEFAULTSORT:Potassium Metabisulfite Metabisulfites Potassium compounds Photographic chemicals E-number additives