thiosulfate

n.
A salt or ester of thiosulfuric acid.

A salt containing the negative ion S₂O₃²⁻. This species is an important reducing agent and may be viewed as a structural analog of the sulfate ion (SO₄²⁻) where one of the oxygen (O) atoms has been replaced by a sulfur (S) atom. The sulfur atoms of the thiosulfate ion are not equivalent. Thiosulfate is tetrahedral, and the central sulfur is in the formal oxidation state 6+ and the terminal sulfur is in the formal oxidation state 2−.

Principal uses of thiosulfates include agricultural, photographic, and analytical applications. Ammonium thiosulfate [(NH₄)₂S₂O₃] is exploited for both the nitrogen and sulfur content, and it is combined with other nitrogen fertilizers such as urea. Thiosulfate ion is an excellent complexing agent for silver ions (bound through sulfur). The sodium salt and the ammonium salt are well known as the fixing agent “hypo” used in photography. The aqueous thiosulfate ion functions as a scavenger for unreacted solid silver bromide on exposed film and therefore prevents further reaction with light. In nature, thiosulfate is converted into hydrogen sulfide (H₂S) via enzymatic reduction. Hydrogen sulfide, in turn, is converted into the thiol group of cysteine by the reaction with O-acetylserine. See also Coordination complexes; Oxidation-reduction; Photographic materials; Sulfur.

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(thī'ō-sŭl'fāt')
n.

A salt or an ester of thiosulfuric acid.

Any salt of thiosulfuric acid. See also sodium thiosulfate.