Amidosulphonic Acid, NH₂.SO₃H

Amidosulphonic Acid, NH₂.SO₃H, may be obtained by the action of sulphur trioxide on dry ammonia. When an aqueous solution of hydroxylamine hydrochloride is saturated with sulphur dioxide and afterwards evaporated, amidosulphonic acid is obtained. Potassium imidosulphonate, NH(SO₃K)₂, and potassium nitrilosulphonate, N(SO₃K)₃, are both decomposed by hot water with formation of potassium amidosulphonate:

NH(SO₃K)₂ + H₂O = NH₂.SO₃K + HO.SO₃K,
N(SO₃K)₃ + 2H₂O = NH₂.SO₃K + 2HO.SO₃K.

Amidosulphonic acid may also be prepared by passing sulphur dioxide into an aqueous solution of sodium nitrite containing sodium carbonate, until the solution is acid to litmus; the nitrilosulphonate which is formed is hydrolysed by the addition of a little concentrated sulphuric acid, amidosulphonate and acid sulphate being formed. The solution is neutralised by the addition of sodium carbonate and the sodium sulphate formed separated by crystallisation; the addition of a large excess of concentrated sulphuric acid to the solution then results in the precipitation of amidosulphonic acid.

Amidosulphonic acid is a colourless, odourless, crystalline solid, specific gravity 2.03 at 12° C. It decomposes to a large extent during melting but has an apparent melting-point of 205° C. It is slowly soluble in water, requiring five parts of water at 0° C. and half this amount at 70° C. to dissolve it. It is stable in air and not deliquescent. Although it is not precipitated by solutions of barium chloride it retards the precipitation of small quantities of sulphuric acid by barium chloride. It forms a compound with oxide or nitrate of mercury, which is insoluble in dilute nitric acid and probably has the composition Hg₃N₂(SO₃H)₂(OH)₂.2H₂O. When amidosulphonic acid, or any of its salts, is heated, partial conversion into imidosulphonate and ammonia occurs

2NH₂.SO₃H = NH(SO₃H)₂ + NH₃,

and partial conversion into sulphate and gaseous products. Thus, in the case of the barium salt, the decomposition may be represented by the equation:

3Ba(SO₃.NH₂)₂ = 3BaSO₄ + NH(SO₃.NH₄)₂ + NH₃ + NS + N.

The potassium salt reacts in the cold with an equivalent amount of hypochlorous acid to form potassium chloramidosulphonate, NHCl.SO₃K, which may be isolated by evaporation to small bulk in vacuo and precipitation with alcohol. It is a comparatively stable, hygroscopic, crystalline salt, hydrolysed by mineral acids to form ammonium chloride and sulphuric acid. The corresponding bromine compound is similar in properties. Barium forms a less stable chloro-compound.

Amidosulphonic acid reacts quantitatively with the alkalis, and with carbonates and borates, and its use as a primary standard in volumetric analysis has been suggested.