Sulphonic Acids

Just as chlorosulphonic acid may be regarded as derived from hydrochloric acid by the substitution of the group - SO₂.OH in place of the hydrogen atom, so also many other compounds are known which likewise may be considered as produced by the introduction of the sulphonic acid group in place of hydrogen.

For example, ammonia and its hydroxy derivatives yield a series of sulphonic acids, their relationships being represented by the following table:

NH3, Ammonia.	HO.NH2, Hydroxylamine.	NH(OH)2, Dihydroxyammonia.	N(OH)3, "Ortho-nitrous " acid.
NH2.SO2.OH, Amidosulphonic acid.	HO.NH.SO2.OH, Hy droxylami ne-sulphonic acid.	OH.SO2.n(OH)2, Dihydroxylarnine-sul- phonic acid.
NH(SO2.OH)2, Imidosulphonic acid.	HO.N(SO2.OH)2, Hydroxylamine-disul- phonic acid.
N(SO2.OH)3, Nitrilosulphonic acid.

The foregoing sulphonie acids are all known, at least in the form of their salts, and are obtainable, directly or indirectly, by the interaction of sulphurous and nitrous acids or their salts. Isomerides of some of the sulphonic acids are also known, for example, hydroxylamine-isosulphonic acid, NH₂.O.SO₂.OH, which is obtainable by the action of chlorosulphonic acid on hydroxylamine hydrochloride. As its constitution indicates, this isomeric acid is in reality a derivative of permonosulphuric acid.

By the action of sulphur dioxide on ammonia three different compounds may be formed, the product depending on the conditions of the reaction. The proportions in which the gases combine depend largely on the extent to which the temperature is allowed to rise, the heat of union being considerable. The product also varies according to which gas is present in excess, unless the temperature is kept very low, in which case ammonium amidosulphinate is formed. When sulphur dioxide is in excess the yellow, crystalline amidosulphinic acid, NH₃.SO₂ or NH₂.SO₂H, is formed. With excess of ammonia the product may be either the white, crystalline salt, ammonium amidosulphinate, 2NH₃.SO₂ or NH₂.SO₂.NH₄, or triammonium imidodisulphinate, 4NH₃.2SO₂ or NH₄.N:(SO₂.NH₄)₂, a red compound, having the same percentage composition as ammonium amidosulphinate but of double molecular weight. That in the molecule of this latter substance three of the nitrogen atoms are placed differently from the fourth is evident from the formation of a silver salt, Ag.N:(SO₂Ag)₂, which is also red in colour. Triammonium imidodisulphinate is also obtainable by the action of thionyl chloride on liquid ammonia, a di amide first being formed:

2SOCl₂ + 7NH₃ = HN:(SO.NH₂)₂ + 4NH₄Cl.

The diamide is hydrolysed by water to NH(SO₂.NH₄)₂, which reacts with more ammonia to form the triammonium salt. On evaporation, and digestion of the residue with absolute alcohol at -5° C., a red solution is obtained, which, when evaporated in a vacuum, yields the triammonium salt in the form of red flakes.

Ammonia combines with sulphur trioxide to form at least six different compounds:

NH₃.2SO₃ or HN(SO₃H)₂, Imidosulphonic acid.
NH₃.SO₃ or H₂N.SO₃H, Amidosulphonic acid.
4NH₃.3SO₃ or N(SO₃.NH₄)₃, Ammonium nitrilosulphonate.
3NH₃.2SO₃ or HN(SO₃NH₄)₂, Diammonium imidosulphonate (parasulphat-ammon).
4NH₃.2SO₃ or (NH₄)N(SO₃.NH₄)₂, Normal ammonium imidosulphonate (sulphat-ammon).
2NH₃.SO₃ or H₂N.SO₃.NH₄, Ammonium amidosulphonate.

These six compounds can all be derived the one from the other, with the exception of the nitrilosulphonate, which cannot be re-formed from the others although it is itself the most convenient source of them. Nitrilosulphonic acid would, if it existed in the free state, be the seventh of these compounds, heading the column as NH₃.3SO₃ or N(SO₃H)₃.