Potassium Metabisulfite (E224)

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.