Sodium thiosulfate is an inorganic compound with the formula Na2S2O3·xH2O. Typically it is available as the white or colorless pentahydrate, which is a white solid that dissolves well in water. The compound is a reducing agent and a ligand, and these properties underpin its applications. Historically known as "hyposulphite of soda", sodium thiosulfate is used in medicine as a treatment for cyanide poisoning. It is also a reliable photographic fixer and played a key role in the development of photography in the 19th century.