Sulphonitronic Acid

Sulphonitronic Acid or "Purple Acid"-For a considerable time the existence of an unstable compound which yields a purple solution in sulphuric acid has been known. Of various names which have been suggested for this compound, the designation "sulphonitronic acid" possesses the advantage of indicating the nature of the chief constituent elements without committing itself to any definite conception of the structure or constitution. The acid has been regarded as a derivative of quadrivalent nitrogen, viz. " nitrosisulphonic acid," O:N(OH).SO₂.OH, or peroxylaminic acid, NO(SO₂.OH)₂. Unfortunately the question of the actual constitution is still undecided, and even the composition is uncertain. According to more recent investigations it appears probable that the " acid" is either an oxide of nitrogen intermediate between NO and N₂O₃, or a compound of sulphuric acid with such an oxide. On account of this uncertainty the compound is frequently referred to merely as "purple acid" (also "blue acid " and "violet acid"). It has possibly acquired undue importance on account of its occurrence as an intermediate product in the "lead chamber" process for the manufacture of sulphuric acid.

The coloured solution in sulphuric acid is obtainable by passing nitrogen dioxide with air into a saturated solution of sulphur dioxide in diluted sulphuric acid (1:1 by volume) at 0° C. It can also be formed by the addition of sodium hydrogen sulphite to a solution of nitrosulphonic (nitrosylsulphuric) acid, produced by dissolving sodium nitrite in slightly diluted sulphuric acid. These methods depend on the reduction of the nitrosulphonic acid by sulphurous acid or sulphur dioxide; the reduction can also be effected by metals, e.g. mercury.

The blue solution is unstable and decomposes slowly, with formation of sulphuric acid, sulphur dioxide and nitrogen dioxide. When shaken with air or submitted to oxidation by chlorine, nitric acid or hydrogen peroxide, conversion into nitrosulphonic acid is effected, brown fumes being liberated. Dilution with water also destroys the coloured substance. If strongly cooled, the solution changes to an intense red, so that if a solution is too weak to possess a marked colour at the ordinary temperature, the presence of the "purple acid" can easily be detected by cooling in a mixture of acetone and solid carbon dioxide.

Certain of the salts in solution have a stronger colour than the acid, and in some cases are more stable; thus, a deep blue solution of the copper salt may be obtained by the reduction of nitrosulphonic acid (in sulphuric acid) by mercury in the presence of copper. A suggestion has been made that the colour in the "brown ring" test for a nitrate is due to the formation of the ferrous salt of "purple acid," but this is improbable.