Na2S9H2O

Na2S9H2O, more commonly known as sodium thiosulfate pentahydrate, is an inorganic compound with the chemical formula Na2S2O3·5H2O. It is the hydrated form of sodium thiosulfate, a salt composed of sodium cations (Na+) and thiosulfate anions (S2O3^2-). The pentahydrate form contains five molecules of water of crystallization per formula unit.

Sodium thiosulfate pentahydrate typically appears as colorless, crystalline solids. It is readily soluble in water, forming a neutral solution. It is important to note that while often referred to as "hypo," this common name is also applied to the anhydrous form and can sometimes cause confusion.

The compound is widely used in various applications. In photography, it serves as a fixer, dissolving unexposed silver halide crystals from photographic film and paper. In analytical chemistry, it is used in redox titrations, particularly for the determination of halogens. It also finds use as a dechlorinating agent in water treatment, neutralizing residual chlorine. In medicine, it has been used as an antidote for cyanide poisoning and in the treatment of calciphylaxis. Sodium thiosulfate pentahydrate is generally considered a safe chemical when handled appropriately, though contact with skin and eyes should be avoided.