Potassium metabisulfite, K2S2O5, also known as potassium pyrosulfite, is a white crystalline powder with a pungent sulfur odor. The main use for the chemical is as an antioxidant or chemical sterilant. It is a disulfite and is chemically very similar to sodium metabisulfite, with which it is sometimes used interchangeably. Potassium metabisulfite is generally preferred out of the two as it does not contribute sodium to the diet. Potassium metabisulfite has a monoclinic crystal structure which decomposes at 190°C, yielding potassium sulfite and sulfur dioxide:
K2S2O5(s) → K2SO3(s) + SO2(g)
Potassium metabisulfite can be prepared by evaporating a solution of potassium bisulfite, saturated with sulfur dioxide:
2 KHSO3 ⇌ H2O + K2S2O5
Potassium metabisulfite is used as a food preservative, bleaching agent and antioxidant. It is mainly used to retain freshness of wine, fruit juice, beer clarifying agent, fruits and vegetables, meat and grapes. In winemaking industry, it is used as an additive during the bottling process, to prevent molds and bacteria from growing inside the wine bottles. Moreover, potassium metabisulfite is a strong antioxidant that protects the color and flavors of the wine. In beer, it is used to inhibit the growth of wild yeasts, bacteria and fungi.
It is used in textile industry in dyeing and cotton printing. It is a component of certain photographic developer solutions used in fixing. It is sometimes used to precipitate gold from aqua regia, as a replacement for sodium sulfite.