Selenopentathionic Acid, H₂SeS₄O₆

Selenopentathionic Acid, H₂SeS₄O₆, is stated to be formed as the sodium salt when sodium thiosulphate and selenious acid interact in the presence of dilute hydrochloric acid. The reaction is described as taking place according to the equation

H₂SeO₃ + 4Na₂S₂O₃ + 4HCl = Na₂SeS₄O₆ + Na₂S₄O₆ + 4NaCl + 3H₂O,

but the result cannot be considered as absolutely proved, for the salt cannot be isolated, being decomposed when the solution is concentrated either by warming or in a vacuum.