Nitrosodisulphonic Acid, NO(SO₃H)₂

Nitrosodisulphonic Acid, Peroxylaminic Acid, or Sulphazilic Acid, NO(SO₃H)₂ is only known in solution; it may be obtained by the passage of equal volumes of nitric oxide and air into a cooled saturated solution of sulphur dioxide in sulphuric acid. It forms an intense bluish-violet solution:

2NO + O + 2SO₂ + H₂O = 2[NO(SO₃H)],
2[NO(SO₃H)] = NO + NO(SO₃H)₂.

It is also formed by passing sulphur dioxide into a solution of sodium nitrite in sulphuric acid:

2NO₂(SO₃H) + 3SO₂ + 2H₂O = 2NO(SO₃H)₂ + H₂SO₄.

Nitrosodisulphonic acid decomposes spontaneously into sulphuric acid, sulphur dioxide and nitric oxide. Its solution is rapidly decolorised on shaking with air or on mixing with hydrogen peroxide, per- mono- or perdi-sulphuric acid, nitric acid, chlorine or potassium chlorate. Most metallic oxides and carbonates decompose the solution, being converted into sulphates.