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agent and may be viewed as a structural analog of the sulfate ion (SO₄²⁻) where one of the oxygen (O) atoms has been replaced by a sulfur (S) atom.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−.