In pH testing of bleach substances, sodium thiosulfate neutralizes the color-removing effects of bleach and allows one to test the pH of bleach solutions with liquid indicators. The reduction reaction is analogous to the iodine reduction reaction. It is used to dechlorinate tap water including lowering chlorine levels for use in aquariums, swimming pools, and spas (e.g., following superchlorination) and within water treatment plants to treat settled backwash water prior to release into rivers. Ammonium thiosulfate is typically preferred to sodium thiosulfate for this application. It is used for both film and photographic paper processing the sodium thiosulfate is known as a photographic fixer, and is often referred to as 'hypo', from the original chemical name, hyposulphite of soda. Silver halides, e.g., AgBr, typical components of photographic emulsions, dissolve upon treatment with aqueous thiosulfate.This application as a photographic fixer was discovered by John Herschel. See also: Collodion § Wet-plate collodion photography

Sodium thiosulfate as poison antidote