Potassium Metabisulfite

Brief Overview 

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.

Manufacturing Process

Potassium metabisulfite (K₂S₂O₅) also known as potassium pyrosulfite has already been prepared from solutions containing potassium hydroxide and,or potassium carbonate by treatment with gases containing sulfur dioxide. For this purpose, gases containing sulfur dioxide are passed into an aqueous solution of potassium hydroxide and,or potassium carbonate. Potassium sulfite K 80 is first formed and this remains dissolved in the excess potassium hydroxide and,or potassium carbonate solution provided saturation has not been reached. When further sulfur dioxide is passed in, the potassium sulfite is converted to potassium bisulfite (KHSO) and then an aqueous solution of pure potassium metabisulfite is formed. To obtain the potassium metabisulfite therefrom in solid form, the solutions obtained may be evaporated, but appreciable decomposition of the salt takes place by disengagement of sulfur dioxide and by oxidation.

Food Industry

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.

Other Application 

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.