Sodium tetrathionate, Na₂S₄O₆

The Sodium tetrathionate is prepared from the thiosulphate either by the action of iodine or by oxidation with cupric chloride. It can be precipitated from solution by addition of alcohol. With sodium carbonate it yields sodium thiosulphate:

4Na₂S₄O₆+5Na₂CO₃ = 7Na₂S₂O₃+2Na₂SO₄+5CO₂.

The heat of formation in aqueous solution is given by Berthelot as 387.2 Cal. A dihydrate exists, its heat of solution being –9.5 Cal.

The tetrathionate is converted by the action of sodium sulphite into trithionate and thiosulphate:

Na₂S₄O₆+Na₂SO₃=Na₂S₃O₆+Na₂S₂O₃.

With alkali-metal cyanides the tetrathionate reacts in accordance with the equation

Na₂S₄O₆+3NaCN+H₂O = NaCNS+Na₂SO₄+Na₂S₂O₃+2HCN.

At the boiling-point, in presence of excess of cyanide, the thiosulphate formed reacts further to produce thiocyanate and sulphite.