Pentathionic Acid, H₂S₅O₆

Pentathionic Acid, H₂S₅O₆ is present in "Wackenroder's solution", and can be separated by removing the sulphuric acid by treatment with a little barium carbonate, the remaining acid liquid after filtration being capable of concentration to a specific gravity of 1.3 by evaporation on a water-bath and to 1.6 by evaporation in a vacuum at the ordinary temperature.

The decomposition of sulphur monochloride by water gives rise to a complex mixture of substances including sulphur and pentathionic acid. The equation

5S₂Cl₂ + 6H₂O = 5S + 10HCl + H₂S₅O₆

originally suggested to represent the change does not represent all the facts, because trithionic and tetrathionic acids are also present, and it is probable that the result may depend on a primary formation of thiosulphuric acid by the combination of nascent sulphur with sulphurous acid.

When thiosulphates are decomposed by acids a small quantity of pentathionic acid, together with tetrathionic acid, is produced, in addition to sulphur and sulphur dioxide. Lead thiosulphate appears especially to be well adapted to this reaction and gives some pentathionic acid on treatment with iodine and hydriodic acid or with hydrogen sulphide.

The acid may be prepared free from tri- and tetra-thionic acids by treating a cold aqueous solution of sodium thiosulphate containing sodium arsenite with hydrochloric acid. On concentrating the liquid at 35° C. sodium pentathionate gradually separates. After filtering, a solution is obtained containing about 60 per cent, of pentathionic acid together with a further 12 per cent, of the sodium salt.

By allowing the reaction to proceed at -10° to -15° C. mixed crystals of penta- and hexa-thionates may be obtained.

Pentathionic acid is formed by the action of sulphur dioxide on a suspension of sulphur in water:

5S + 5SO₂ + 2H₂O = 2H₂S₅O₆.

This reaction probably explains the formation of the acid by decomposition of thiosulphates by acids as already described.

Traces of pentathionic acid are also stated to be found in the condensed liquid from the interaction of steam and sulphur vapour at a red heat, and in a mixture of sulphur and water after exposure to atmospheric oxidation. In the former case the pentathionic acid probably results after the high temperature reaction by the interaction of sulphur dioxide and hydrogen sulphide in the condensate.

Like the lower members of the series of thionic acids, free pentathionic acid is known only in aqueous solution; a solution of the pure acid is obtained by treating an aqueous solution of the potassium salt with the requisite quantity of tartaric acid for the removal of the potassium in the form of hydrogen tartrate. The solution is denser than water; it cannot be concentrated beyond a limit of 50 to 60 per cent, acid without decomposition.

The heat of formation of the acid is given by the equation:

H₂ + 5S + 3O₂ + Aq. = H₂S₅O₆,Aq. + 216 Calories.

The solution is a colourless, odourless liquid of strongly acid taste and conducts the electric current. When cold it is comparatively stable, and may be kept practically unchanged for two or three months, but there is a tendency, especially with more concentrated solutions, to gradual decomposition with formation of tetrathionic acid, trithionic acid and sulphur; on boiling, the solution gives hydrogen sulphide and sulphur, together with sulphurous and sulphuric acids, the relative proportions varying with the concentration.

Hydrogen sulphide slowly decomposes aqueous pentathionic acid, the final state being represented by the equation

H₂S₅O₆ + 5H₂S = 6H₂O + 10S,

whilst excess of sulphurous acid causes a partial degradation into tetrathionic acid and trithionic acid.

Oxidising agents such as chlorine water, nitric acid or potassium permanganate cause conversion into sulphuric acid with the intermediate formation of sulphur. The reaction is quantitative with a mixture of potassium chlorate and hydrochloric acid.