Sanitizer - use 2 oz per 1 gallon of water. Antioxidant and bactericide - use 1/4 teaspoon to 6 gallons. Dissolve sulphite in warm water before adding. Potassium metabisulfite, K2S2O5, also known as potassium pyrosulfite, is a white crystalline powder with a pungent sulfur odour. The main use for the chemical is as an antioxidant or chemical sterilant.