Thiosulfate
Thiosulfate () (IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur. The prefix thio- indicates that the thiosulfate ion is a sulfate ion with one oxygen replaced by sulfur. Thiosulfate is a tetrahedral ion with C symmetry. It occurs naturally and is produced by certain biochemical processes. It rapidly dechlorinates water and is notable for its use to halt bleaching in the paper-making industry. Thiosulfate is mainly used in dying in textiles and the bleaching of natural substances. Sodium thiosulfate, commonly called "hypo" (from "hyposulfite"), was widely used in photography to fix black and white  negatives and prints after the developing stage; modern 'rapid' fixers use ammonium thiosulfate as a fixing salt because it acts three to four times faster. Some bacteria can metabolise thiosulfates. Formation. Thiosulfate is produced by the reaction of sulfite ion with elemental sulfur, and by incomplete oxidation of sulfides (pyrite oxidation), sodium thiosulfate can be formed by disproportionation of sulfur dissolving in sodium hydroxide (similar to phosphorus). Reactions. Thiosulfates are stable only in neutral or alkaline solutions, but not in acidic solutions, due to disproportionation to sulfite and sulfur, the sulfite being dehydrated to sulfur dioxide: This reaction may be  used to generate an aqueous suspension of sulfur and demonstrate the Rayleigh scattering of light in physics. If white light is shone from below, blue light is seen from sideways and orange from above, due to the same mechanisms that color the sky at mid-day and dusk. Thiosulfate reacts with iodine to give tetrathionate: This reaction is key for iodometry. With bromine (X = Br) and chlorine (X = Cl), thiosulfate is oxidized to sulfate: Reactions with metals and metal ions. Thiosulfate extensively forms diverse complexes with transition metals. In the era of silver-based photography, thiosulfate was consumed  on a large scale as a "fixer" reagent. This application exploits thiosulfate's ability to dissolve silver halides. Thiosulfate is also used to extract or leach gold (sodium thiosulfate) and silver from their ores as a less toxic alternative to cyanide. Also reflecting its affinity for metals, thiosulfate rapid corrodes metals in acidic conditions. Steel and stainless steel are particularly sensitive to pitting corrosion induced by thiosulfate. Molybdenum improves the resistance of stainless steel toward pitting (AISI 316L hMo). In alkaline aqueous conditions and medium temperature (60 °C), carbon steel and stainless steel (AISI 304L, 316L) are not attacked , even at high concentration of base (30%w KOH), thiosulfate (10%w) and in presence of fluoride ion (5%w KF). Occurrence. The very rare mineral sidpietersite, Pb(SO)O(OH), as the presence of this anion in the mineral bazhenovite was recently disputed. Nomenclature. Thiosulfate is an acceptable common name (but used almost always); functional replacement IUPAC name is sulfurothioate; the systematic additive IUPAC name is trioxidosulfidosulfate(2−) or trioxido-1"κ""O"-disulfate("S"—"S")(2−). The external sulfur has an oxidation state of –1 (−I) while the central sulfur atom has an oxidation number of +5 (+V). Biochemistry. The enzyme rhodanase catalyzes the detoxification of cyanide by thiosulfate: CN +  → SCN + . Sodium thiosulfate has been considered as an empirical treatment for cyanide poisoning, along with hydroxocobalamin. It is most effective in a pre-hospital setting, since immediate administration by emergency personnel is necessary to reverse rapid intracellular hypoxia caused by the inhibition of cellular respiration, at complex IV. It activates thiosulfate sulfur transferase (TST) in mitochondria. TST is associated with protection against obesity and type II (insulin resistant) diabetes.