S2O3

S2O3 refers to the thiosulfate ion. It is a polyatomic anion with the chemical formula [S2O3]2−. The thiosulfate ion consists of a central sulfur atom bonded to three oxygen atoms and another sulfur atom. One of the oxygen atoms is double-bonded to the central sulfur, while the other two oxygen atoms and the second sulfur atom are singly bonded and carry a formal negative charge. The ion has a tetrahedral geometry.

Thiosulfate is the conjugate base of thiosulfuric acid, which is unstable. It is commonly encountered as a salt, such as sodium thiosulfate (Na2S2O3), often known as "hypo" in photography. Sodium thiosulfate is a white, crystalline solid that is highly soluble in water.

Thiosulfate ions are important in various chemical processes. In photography, they are used as a fixer to remove unexposed silver halide crystals from photographic film and paper. They are also used in analytical chemistry, for example, in iodometric titrations to determine the concentration of oxidizing agents. In medicine, sodium thiosulfate is used as an antidote for cyanide poisoning and as a treatment for calciphylaxis. Industrially, it finds applications in textile dyeing and as a bleaching agent. Thiosulfate is generally considered a relatively stable compound under normal conditions, though it can decompose in acidic solutions.