Potassium metabisulfite, K₂S₂O₅, also known as potassium pyrosulfite, is a white

crystalline A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

powder with a pungent odour. It is mainly used as an antioxidant or chemical sterilant. As a disulfite, it is chemically very similar to

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 substance is sometimes referred to as disodium metabisulfite. It is used a ...

, with which it is sometimes used interchangeably. Potassium metabisulfite has a

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...

crystal structure.

Preparation and reactions

Potassium metabisulfite can be prepared by treating a solution of

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which exp ...

with sulfur dioxide. :2 SO₂ + 2 KOH → K₂S₂O₅ + H₂O It decomposes at 190 °C, yielding potassium sulfite and sulfur dioxide: :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 Wine is an alcoholic drink typically made from fermented grapes. Yeast consumes the sugar in the grapes and converts it to ethanol and carbon dioxide, releasing heat in the process. Different varieties of grapes and strains of yeasts are m ...

or must additive, in which it forms sulfur dioxide (SO₂). Sulfur dioxide is a disinfectant. It also acts as a potent antioxidant, 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 industry to inhibit the growth of wild

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

and

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

. This step is called 'stabilizing'. It is also used to neutralize

monochloramine Monochloramine, often called chloramine, is the chemical compound with the formula NH2Cl. Together with dichloramine (NHCl2) and nitrogen trichloride (NCl3), it is one of the three chloramines of ammonia. It is a colorless liquid at its melting p ...

from tap water. It is used both by homebrewers and commercial brewers alike. It is not used as much for brewing

beer Beer is one of the oldest and the most widely consumed type of alcoholic drink in the world, and the third most popular drink overall after water and tea. It is produced by the brewing and fermentation of starches, mainly derived from ce ...

, because the

wort Wort () is the liquid extracted from the mashing process during the brewing of beer or whisky. Wort contains the sugars, the most important being maltose and maltotriose, that will be fermented by the brewing yeast to produce alcohol. Wort als ...

is almost always boiled, which kills most microorganisms.

Other uses

*Potassium metabisulfite is sometimes added to

lemon The lemon (''Citrus limon'') is a species of small evergreen trees in the flowering plant family Rutaceae, native to Asia, primarily Northeast India (Assam), Northern Myanmar or China. The tree's ellipsoidal yellow fruit is used for culin ...

juice as a

preservative A preservative is a substance or a chemical that is added to products such as food products, beverages, pharmaceutical drugs, paints, biological samples, cosmetics, wood, and many other products to prevent decomposition by microbial growth or by ...

. *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 Sodium sulfite (sodium sulphite) is the inorganic compound with the chemical formula Na2 SO3. A white, water-soluble solid, it is used commercially as an antioxidant and preservative. A heptahydrate is also known but it is less useful because of ...

). *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 Aam papad is an Indian fruit leather made out of mango pulp mixed with concentrated sugar solution and sun dried. It is also known as ' (Odia), ' ( Assamese), ' (Hindi), ' (Malayalam), ' (Telugu), ' (Bengali) and ' ( Marathi). Traditional Aam P ...

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