Sodium dithionate

For the sterilizing agent, see sodium metabisulfite. For the reducing agent, see sodium dithionite.

Sodium dithionate Na₂S₂O₆ is an important compound for inorganic chemistry. It is also known under names disodium dithionate, sodium hyposulfate, and sodium metabisulfate.

Can be produced by following reactions:

2 NaHSO₃ + MnO₂ → Na₂S₂O₆ + Mn(OH)₂

3 Cl₂ + Na₂S₂O₃·5H₂O + 6 NaOH → Na₂S₂O₆ + 6 NaCl + 8 H₂O

The dithionate ion represents sulfur that is oxidized relative to elemental sulfur, but not totally oxidized. Sulfur can be reduced to sulfide or totally oxidized to sulfate, with numerous intermediate oxidation states in inorganic moieties, as well as organosulfur compounds. Example inorganic ions include sulfite and thiosulfate.

Sodium dithionate is a very stable compound which is not oxidized by permanganate, dichromate or bromine. It can be oxidized to sulfate under strongly oxidizing conditions: these include boiling for one hour with 5 M sulfuric acid with an excess of potassium dichromate, or treating with an excess of hydrogen peroxide then boiling with concentrated hydrochloric acid. The Gibbs free energy change for the oxidation is about −300 kJ/mol.

It should not be confused with sodium dithionITE, Na₂S₂O₄, which is a very different compound, and is a powerful reducing agent with many uses in chemistry and biochemistry. Confusion between dithionate and dithionite is commonly encountered, even in manufacturers' catalogues!

Sodium metabisulfate is also encountered occasionally as a food preservative, especially in combination with sulfur dioxide in sweet, moist foods such as those containing coconut-flavored interiors.