Hydrosulphurous Acid, H₂S₂O₄

Schonbein in 1852-1858, during a re-investigation of the electrolysis of sulphurous acid solution, examined about twenty years before by Faraday, obtained at the cathode a yellowish solution of marked reducing power; he obtained a solution of similar properties on treatment of sulphurous acid solution with zinc, which reaction was already known from the observations of Berthollet, Fourcroy and Vauquelin to give no free hydrogen.

A further examination of the action of zinc on sulphurous acid and on sodium hydrogen sulphite solutions was made by Schutzenberger in 1869, who arrived at a formula NaHSO₂ for the sodium salt, and suggested the name hydrosulphurous acid; although the formula has proved to be incorrect, the name possesses an advantage in precluding any confusion with thiosulphuric acid; the possibility of such confusion is introduced by the term hypo sulphurous acid, which, however, until recent years, received fairly general acceptance in English-speaking countries. Under the present system of nomenclature "hyposulphurous acid" should be H₂SO₂, which, however, is termed sulphoxylic acid.

The free acid is exceedingly unstable and cannot be isolated, only the salts being of importance. These may be obtained as follows:

Zinc slowly dissolves in sulphurous acid solution without effervescence; the solution is at first yellow and then becomes colourless; air should be excluded. The action probably follows the course

Zn + 2SO₂ = ZnS₂O₄.

It is possible to prepare the hydrosulphites of the metals in anhydrous condition by the action of various metals, for instance zinc, magnesium, sodium, on sulphur dioxide in the presence of moisture-free ether or alcohol. This reaction is of importance as having supplied an early proof of the absence of hydrogen from the salts and hence of the incorrectness of the formulae Zn(HSO₂)₂, NaHSO₂, which had been accepted previously:

Zn + 2SO₂ = ZnS₂O₄.

Moissan in 1902 obtained anhydrous hydrosulphites by passing sulphur dioxide diluted with hydrogen over the hydrides of the alkali and alkaline earth metals. He was able to produce the hydrosulphites of sodium, potassium, lithium, calcium and strontium in this way, and by measurement of the quantity of hydrogen liberated was able to prove the correctness of the general formula M_x.S₂O₄:

2KH + 2SO₂ = K₂S₂O₄ + H₂.

Sodium hydrogen sulphite solution can be used in place of the aqueous sulphurous acid in the first method described. With zinc, the resulting reaction is

4NaHSO₃ + Zn = ZnSO₃ + Na₂SO₃ + Na₂S₂O₄ + 2H₂O.

The result is more satisfactory if the acid sulphite is accompanied by a semi-molecular proportion of sulphurous acid:

2NaHSO₃ + SO₂ + Zn = Na₂S₂O₄ + ZnSO₃ + H₂O.

On pouring the solution into alcohol a double sulphite of sodium and zinc is rapidly precipitated, whilst needles of sodium hydrosulphite, Na₂S₂O₄.2H₂O, separate slowly afterwards. Another procedure for the removal of the zinc from the solution is to add milk of lime cautiously, when a solution of sodium hydrosulphite remains.

Other metals may be used in place of zinc, e.g. iron, copper, manganese or even sodium amalgam or calcium.

In the electrolysis of aqueous sulphurous acid or sodium hydrogen sulphite solution, a little hydrosulphite is formed at the cathode as a result of the reduction process.

Titanous chloride also reduces sulphurous acid or sodium hydrogen sulphite solution with formation of an orange-yellow solution of hydro- sulphurous acid, from which sodium hydrosulphite is obtainable by further treatment with sodium hydroxide solution:

2NaHSO₃ + 2TiCl₃ + 2H₂O = 2NaCl + 2TiO₂ + 4HCl + H₂S₂O₄.

Formic acid, when mixed with aqueous sodium hydrogen sulphite, forms a solution of strong reducing power, due to conversion of some of the sulphite by reduction into hydrosulphite:

2NaHSO₃ + H.COONa = NaHCO₃ + H₂O + Na₂S₂O₄.

Sodium formaldehydesulphoxylate may be used conveniently instead of formic acid.

Hydrosulphurous acid is also formed as an intermediate product in the reduction of sulphurous acid by hypophosphorous acid.

The free acid is exceedingly unstable, and its orange-yellow aqueous solution, obtained directly by any of the methods already described, or by decomposition of a hydrosulphite with a suitable acid such as dilute sulphuric acid, is capable only of short existence, soon decomposing with deposition of sulphur and liberation of sulphurous acid. By measurement of the electrical conductivity of solutions of the acid and of the sodium salt it has been possible to show that hydrosulphurous acid is a stronger acid than sulphurous acid, although weaker than thiosulphuric acid. According to Berthelot the heat of formation of the acid in aqueous solution is a small positive quantity.

Details of the strong reducing character of the acid will be found in the following, under the description of the reactions of the hydrosulphites.