Nitrosulphonic Acid, NO₂.SO₂.OH

In 1806, Clement and Desormes, during the manufacture of sulphuric acid by the lead chamber process, observed the formation of a crystalline solid, to which the names nitrosulphonic acid and nitrosylsulphuric acid were later given; the term "chamber crystals," however, is still commonly applied to this acid. The composition and nature of the acid were first investigated by Weber, and by Michaelis and Schumann.

When sulphur dioxide is passed into ice-cold fuming nitric acid, crystals of nitrosulphonic acid separate. The acid is also obtained when sulphuric acid is treated with excess of nitrogen peroxide. It is also formed by the interaction of sulphur dioxide and nitrogen peroxide both in the absence or presence of water, and by the addition in equimolecular proportion of liquid nitrogen tetroxide to chlorosulphonic acid in the absence of moisture.4 Sulphuric acid acts on either nitrous anhydride or nitrosyl chloride with formation of nitrosulphonic acid, according to the equations:

H₂SO₄ + N₂O₃ = NO₂.SO₂.OH + NO.OH,
H₂SO₄ + NOCl = NO₂.SO₂.OH + HCl.

When nitric oxide is passed into a solution of cupric sulphate in concentrated sulphuric acid the following reaction occurs:

3NO + CuSO₄ + 3H₂SO₄ = 2NO₂.SO₂.OH + NO(SO₃)₂Cu + 2H₂O.

Although nitrosulphonic acid may be obtained in the form of quadratic or rhombic prisms, it is more generally obtained as a leafy or feathery crystalline mass which has no definite melting-point but begins to decompose at 73° C., forming the anhydride (see later), nitrogen trioxide and sulphuric acid. No salts with the metals have been obtained. The acid is soluble in water with evolution of heat and decomposition into nitric oxide and sulphuric acid. - The amount of nitric oxide formed increases on heating. If the solution is kept cold the products are sulphuric and nitrous acids, the latter undergoing further decomposition in its characteristic manner, the result being dependent on conditions. Cold concentrated sulphuric acid dissolves the crystals without decomposition. The reduction of a solufion of nitrosulphonic acid in sulphuric acid by sulphur dioxide yields nitric oxide and sulphuric acid,

2NO₂.SO₂.OH + SO₂ + H₂O = 3H₂SO₄ + 2NO,

but in the presence of oxygen this reaction may be hindered or even checked, because the nitrosulphonic acid is re-formed by the interaction of sulphuric acid, nitric oxide and oxygen.

With phosphorus pentachloride nitrosulphonic acid yields chlorosulphonic acid and nitrosyl chloride:

NO₂.SO₂.OH + PCl₅ = Cl.SO₂.OH + NOCl + POCl₃.

Sodium chloride and sodium bromide react with nitrosulphonic acid forming respectively nitrosyl chloride and bromide together with sodium hydrogen sulphate.

It is fairiy generally believed that nitrosulphonic acid plays an important part in the " lead chamber " process for the manufacture of sulphuric acid. The vapour pressures of mixtures of sulphuric acid with nitrous or nitric acid or with both these acids, within the range occurring in the chambers, increase with the nitrogen acid content and with rise in temperature, and the total pressure is always higher than the sum of the individual pressures, especially when the sulphuric acid is concentrated. For nitric acid - sulphuric acid mixtures this may be explained by the occurrence of the following reactions:

NO₂.SO₂.OH + HNO₃ ⇔ H₂SO₄ + N₂O₄,
and
2NO₂.SO₂.OH + H₂O ⇔ 2H₂SO₄ + N₂O₃.

The formation of nitrosyl chloride or bromide by the action of the acid on sodium chloride or bromide is evidence in favour of the nitrosylsulphuric acid structure, ON.O.SO₂.OH. The nitrosulphonic acid structure receives weighty support from the fact that, in addition to the methods already given, the acid can also be obtained by the oxidising action of permonosulphuric acid on hydroxylamine-sulphonic acid, OH.NH.SO₂.OH, in which acid it is certain that the nitrogen is directly attached to sulphur. The conflicting evidence is possibly due to the substance exhibiting tautomerism, in a similar manner to nitrous acid itself, which appears to be able to assume the structures HO.N:O and under varying conditions; this view of the structure of nitrosulphonic acid receives marked confirmation in the action of the acid on certain organic substances, such as dimethylaniline, which give rise concurrently to a nitro- and a nitroso-derivative, the two forms of the acid appearing to act simultaneously and more or less independently. This result indicates the probability of an equilibrium between the two types of molecules NO₂.SO₂.OH and ON.O.SO₂.OH.

On the other hand, Elliott and his co-workers maintain that the evidence for the nitro-structure is unsatisfactory, and that the p-nitro-dimethylaniline produced with dimethylaniline is a secondary product obtained either by oxidation of the nitroso-compound or by direct nitration of the amine. The addition of ethyl hydrogen sulphate to a solution of nitrosulphonic acid in sulphuric acid does not yield nitro-ethane. Elliott suggests that the crystalline acid is essentially the nitroso-form, ON.O.SO₂.OH, but that in the molten condition and in sulphuric acid solution this form is in equilibrium with another of structure . This is in accordance with the behaviour of the acid on reduction and on heating, and also with the synthesis of the anhydride from sulphur dioxide and nitrogen pentoxide, thus: