Nitrosulphuric Acid, N₂O₅.4SO₃.3H₂O

Nitrosulphuric Acid, N₂O₅.4SO₃.3H₂O, may be obtained by the prolonged action of the vapour of sulphur trioxide on concentrated nitric acid. It forms brilliant white crystals which on heating yield brown vapours and a sublimate of nitrosulphonic acid. It is soluble in water, yielding a mixture of nitric and sulphuric acids.

According to Somiya, the gradual addition of fuming sulphuric acid to an aqueous mixture of sulphuric and nitric acids results in two successive temperature maxima being reached, the second being the less well-defined. The first corresponds with the formation of sulphuric acid from the free sulphur trioxide and the compound SO₃.HNO₃ or OH.SO₂.0NO₂, and the second with conversion of the latter into N₂O₅.4SO₃.