Trithionic Acid, H₂S₃O₆

From Thiosulphates

Potassium thiosulphate in concentrated aqueous solution reacts with sulphur dioxide forming potassium trithionate; the reaction is sometimes represented as

2K₂S₂O₃ + 3SO₂ = 2K₂S₃O₆ + S,

but this does not quantitatively represent the change, some tetrathionate and pentathionate also being produced, whilst the quantity of sulphur liberated is correspondingly less. The trithionate crystallises from the solution, and the free acid is obtained in the solution by treatment with hydrofluosilicic acid.

A similar formation of trithionate can be effected by recrystallising a mixture of potassium thiosulphate and potassium hydrogen sulphite in aqueous solution. It is probable that the method by which potassium trithionate was first prepared depended on the same reaction. Potassium hydrogen sulphite solution was warmed with sulphur for several days, with the result that sulphate, thiosulphate and trithionate were obtained, the formation of the last-named in all probability occurring by way of the thiosulphate.

By the gradual addition of hydrogen peroxide to an ice-cold aqueous solution of sodium thiosulphate, it is possible to convert the latter into trithionate, the changes

3Na₂S₂O₃ + 4H₂O₂ = 2Na₂S₃O₆ + 3H₂O + 2NaOH
and
Na₂S₂O₃ + 2NaOH + 4H₂O₂ = 2Na₂SO₄ + 5H₂O

occurring simultaneously.

Certain double salts of thiosulphuric acid when heated with water undergo decomposition with production of trithionate; thus the sodium-mercurous salt decomposes according to the equation:

2NaHgS₂O₃ = Hg₂S + Na₂S₃O₆.

At one time it was believed that trithionate could be synthetically produced by the action of iodine on an aqueous mixture of sodium sulphite and thiosulphate, the reaction being assumed to accord with the equation:

Na₂SO₃ + Na₂S₂O₃ + I₂ = Na₂S₃O₆ + 2NaI.

Subsequent investigation indicated that this view of the reaction was erroneous, and that any trithionate obtained was actually due to a secondary reaction between tetrathionate and sulphite.

From Sulphites

Potassium hydrogen sulphite, when kept in aqueous solution for a long time with exclusion of air, undergoes spontaneous change with formation of sulphate and trithionate. The change, which is commonly represented as

10KHSO₃ = 5K₂SO₄ + H₂S₃O₆ + 2S + 4H₂O,

is possibly connected with the decomposition of sulphites into sulphate and sulphur.

Trithionate is also produced when sulphur dioxide is passed into a mixture of solutions of potassium sulphide and potassium hydrogen sulphite:

K₂S + 4KHSO₃ + 4SO₂ = 3K₂S₃O₆ + 2H₂O.

Sulphur chloride or dichloride can convert potassium sulphite into trithionate, the equations being

2K₂SO₄ + S₂Cl₂ = K₂S₃O₆ + 2KCl + S,
2K₂SO₄ + SCl₂ = K₂S₃O₆ + 2KCl.

Trithionic acid is found together with sulphuric, sulphurous and thiosulphuric acids in the reaction product from the decomposition of nitrogen sulphide, N₄S₄, with water.

In the oxidation of alkali sulphides or polysulphides by potassium permanganate solution at the ordinary temperature, trithionic acid has been found amongst the reaction products, in addition to sulphuric acid and sulphur.

The spontaneous degradation of the higher polythionates gives rise to trithionate.

Trithionic acid is the least stable of the polythionic acids. The aqueous solution of the free acid, which is generally obtained from a cold concentrated solution of the potassium salt by the addition of a suitable acid, such as hydrofluosilicic or perchloric acid, which will remove the metal as a sparingly soluble salt, slowly decomposes, even at the ordinary temperature, with formation of sulphur, sulphur dioxide and sulphuric acid:

H₂S₃O₆ = H₂SO₄ + S + SO₂.

The decomposition is really due to hydrolysis, the primary products being sulphuric and thiosulphuric acids, the formation of the former causing the decomposition of the latter, the presence of which is consequently difficult to detect.

The pure acid has not been isolated, but the moderately concentrated solution is an odourless, clear, slightly viscous liquid, which is not very strongly acidic, although it possesses an acid taste. The heat of formation of the acid from its elements is given by the equation:

H₂ + 3S + 3O₂ + Aq. = H₂S₃O₆, Aq. + 270.1 Calories.

Oxidising acids such as nitric, chloric and iodic acids induce rapid decomposition of trithionic acid with formation of sulphur and sulphuric acid; the presence of other acids, for example hydrochloric, perchloric or dilute sulphuric acid, also hydrogen sulphide, is without any harmful effect. Addition of sulphurous acid causes the gradual formation of a mixture of all the polythionic acids.

Copper nitrate and mercuric nitrate when heated with the aqueous solution give a black precipitate of the corresponding sulphide; mercuric chloride in excess causes the precipitation of the white substance 2HgS.HgCl₂; silver nitrate produces a white precipitate which gradually becomes black due to the formation of sulphide.