Thiosulfate
Thiosulfate (S2O3 2-) is a sulfur oxyanion built like sulfate but with one oxygen replaced by a loosely-bound terminal sulfur. Its sodium salt is the classic photographic fixer (dissolving silver halides), a sulfur-donor cyanide antidote (via rhodanese, forming thiocyanate), a water dechlorinator, and a treatment for calciphylaxis and cisplatin toxicity.
Thiosulfate is the sulfur oxyanion with the formula S2O3 carrying a 2- charge (S₂O₃²⁻). Structurally it resembles sulfate (SO4 2-) but with one of the four oxygen atoms replaced by a second sulfur. The ion is tetrahedral around the central sulfur, and the central-to-terminal sulfur-sulfur bond (about 201 pm) behaves as a single bond. The two sulfur atoms are not equivalent: although the classic textbook assignment gives them oxidation states of +6 and -2 (mirroring sulfate and sulfide), XANES spectroscopy indicates the charge distribution is closer to +5 for the central atom and -1 for the terminal atom. That terminal, loosely held sulfur is what makes thiosulfate a useful sulfur donor. Sodium thiosulfate ("hypo") is the workhorse salt. In black-and-white photography it serves as the photographic fixer, dissolving unexposed silver halide (such as silver bromide) into stable water-soluble silver-thiosulfate complexes that wash away, leaving a permanent image. In medicine it is the slow-acting half of the classic cyanide antidote: the enzyme Rhodanese: The Cyanide Detoxification Enzyme That Can't Save You from Poisoning transfers thiosulfate's terminal sulfur onto the cyanide ion, producing thiocyanate, which is roughly 200 times less toxic and readily excreted (it complements faster antidotes like Hydroxocobalamin: The Vitamin B12 Cyanide Antidote and links to the broader story of Cyanide poisoning). Thiosulfate also rapidly dechlorinates water and quenches bleaching in paper-making, and newer clinical uses include treating calciphylaxis (dissolving calcium deposits into soluble calcium-thiosulfate complexes) and limiting the kidney and hearing toxicity of the chemotherapy drug cisplatin. Thiosulfate sits within the wider chemistry of biological organosulfur compounds and sulfur metabolism—the same family of reactive sulfur species that includes the onion-and-garlic toxins described in N-Propyl Disulfide: The Organosulfur Toxin in Onions and Garlic. As a reducing sulfur reagent it is chemically related to sulfite salts such as Sodium Metabisulfite: The Gold Precipitant and Food Preservative.