Diselenotrithionic Acid, H₂Se₂SO₆

If in the interaction of selenious and sulphurous acids an excess (that is, more than a semi- molecular proportion) of the former is taken, the odour of sulphur dioxide soon disappears and apparently a Diselenotrithionic Acid, H₂Se₂SO₆, is formed in the solution:

2H₂SO₃ + 2H₂SeO₃ = H₂Se₂SO₆ + H₂SO₄ + 2H₂O.

The acid is even less stable than selenotrithionic acid; its aqueous solution decomposes in the light or on warming; in fact, it cannot even be preserved in the dark. Acids and alkalis also bring about its decomposition. Treatment with barium carbonate, as in the case of the preceding acid, gives a solution of the barium salt which, however, is less stable than barium selenotrithionate, and the solution decomposes slowly in the dark.