Hyposulfiitti

Hyposulfiitti, also known as thiosulfate, is an oxyanion of sulfur with the chemical formula [S₂O₃]²⁻. It is the conjugate base of hyposulfurous acid. In the hyposulfiitti ion, one sulfur atom is in a formal oxidation state of +2 and the other is in a formal oxidation state of -2, though its bonding is more complex. The ion consists of a central sulfur atom bonded to three oxygen atoms and another sulfur atom, which is in turn bonded to the central sulfur atom.

Hyposulfiitti salts are generally soluble in water. Sodium hyposulfiitti (Na₂S₂O₃), commonly known as sodium thiosulfate, is perhaps the most well-known compound. It is widely used in photography as a fixing agent, due to its ability to dissolve silver halides. It is also employed in analytical chemistry for iodometric titrations, in medicine as an antidote for cyanide poisoning, and in water treatment to remove chlorine.

Other hyposulfiitti salts, such as potassium hyposulfiitti (K₂S₂O₃) and ammonium hyposulfiitti ((NH₄)₂S₂O₃), also exist. These compounds share many of the chemical properties of sodium hyposulfiitti.

Hyposulfiitti can be formed by the oxidation of sulfides or sulfites. It can also be produced by the reaction of sulfur dioxide with sulfides. In solution, hyposulfiitti can undergo disproportionation reactions, especially under acidic conditions, yielding sulfite and elemental sulfur. Under more vigorous conditions, it can be further oxidized to sulfate.