Na2S4O6

Sodium tetrathionate is an inorganic compound with the chemical formula Na2S4O6. It is the sodium salt of tetrathionic acid. This compound typically exists as a white crystalline solid. It is soluble in water, forming a neutral solution. Sodium tetrathionate is often encountered as a hexahydrate, Na2S4O6·6H2O.

The tetrathionate ion, [S4O6]2-, consists of a chain of four sulfur atoms, with two oxygen atoms bonded to each terminal sulfur atom and one oxygen atom bonded to each of the inner sulfur atoms. The oxidation states of the sulfur atoms in the tetrathionate ion are not uniform; the terminal sulfur atoms have an oxidation state of +1, while the inner sulfur atoms have an oxidation state of +2.5.

Sodium tetrathionate can be synthesized by the reaction of sodium thiosulfate with iodine. This reaction is a common method for its preparation in the laboratory. It finds some limited applications, including in analytical chemistry and as a component in certain photographic processes. It is generally considered a stable compound under normal conditions.