Tetrathionic Acid, H₂S₄O₆

1. From Thiosulphates: As is well known, sodium tetrathionate is produced by the interaction of sodium thiosulphate and iodine:
   
   2Na₂S₂O₃ + I₂ = Na₂S₄O₆ + 2NaI.
   
   This reaction, discovered by Fordos and Gelis in 1842, is a general one and applicable to all thiosulphates; it is ionic in character and therefore proceeds rapidly:
   
   2S₂O₃'' + I₂ = S₄O₆'' + 2I'.
   
   By using lead or barium thiosulphate a solution of the corresponding tetrathionate can be obtained from which an aqueous solution of the free acid may be prepared by treatment with the calculated quantity of dilute sulphuric acid. Lead thiosulphate gives better results than the barium salt, but it is not advisable to use hydrogen sulphide for the removal of the lead on account of the possible interaction of the hydrogen sulphide or the lead sulphide with the tetrathionic acid. On adding the requisite quantity of tartaric acid to a solution of potassium tetrathionate, the potassium is gradually deposited as the hydrogen tartrate and a pure solution of tetrathionic acid obtained after one or two days.
   
   Chlorine, bromine, hypochlorous acid and hypobromous acid also convert thiosulphate into tetrathionate, but their activity is so great that the reaction tends to proceed still further, with formation of sulphate.
   
   Many other oxidising agents can effect the same change in a thiosulphate, for example permanganates, chlorates, selenium dioxide, barium peroxide, hydrogen peroxide and lead dioxide, in the presence of sufficient sulphuric acid to neutralise the alkali as it is produced:
   
   2Na₂S₂O₃ + O + H₂O = Na₂S₄O₆ + 2NaOH.
   
   Perdisulphates likewise oxidise thiosulphates to tetrathionates, but do not need the presence of additional acid:
   
   2M₂S₂O₃ + M₂S₂O₈ = M₂S₄O₆ + 2M₂SO₄.
   
   Ferric salts and cupric salts are also able to convert thiosulphate into tetrathionate. The reaction with copper sulphate is as follows:
   
   3Na₂S₂O₃ + 2CuSO₄ = Cu₂S₂O₃ + 2Cu₂SO₄ + Na₂S₄O₆.
   
   A convenient method of preparing the potassium salt is to use this reaction, filter off the copper salt and to the concentrated filtrate add potassium acetate, when the tetrathionate separates; this, after removal, should be washed with alcohol.
   
   The observation that in the electrolysis of ammonium thiosulphate solution tetrathionic acid is formed at the anode also belongs to this class, because the process is not one of the coupling of discharged anions, but an anodic oxidation of the thiosulphate.
   
   Sulphur dioxide when passed through a solution of a thiosulphate gives rise not only to tetrathionate but also to trithionate and pentathionate. Under suitable conditions, however, with a high concentration of sulphur dioxide, the tetrathionate separates. The reaction is accelerated by traces of potassium arsenite.
   
   In the interaction of sulphur monochloride with potassium thiosulphate, tetrathionate is obtained as the highest polythionate product.
2. Tetrathionic acid is present in the mixture obtained by the action of sulphur dioxide on hydrogen sulphide in aqueous solution (see "Wackenroder's solution,").
3. Pentathionic acid solution when treated with lead dioxide or when merely allowed to undergo spontaneous decomposition, yields tetrathionic acid.

The free acid is known only in the form of its aqueous solution, which is without colour or odour.

From the strongly acid taste, the heat of neutralisation with dilute sodium hydroxide and the electrical conductivity of the solution, it appears that tetrathionic acid is a fairly strong acid, comparable with dithionic acid in this respect. The heat of formation is given by the equation:

H₂ + 4S + 3O₂ + Aq. = H₂S₄O₆,Aq. + 260.8 Calories.

It is the most stable of the polythionic acids, and a dilute aqueous solution can be heated to boiling without decomposition, although the concentrated solution undergoes decomposition giving sulphurous and sulphuric acids together with sulphur. Mineral acids, excluding those of decided oxidising or reducing character, do not induce decomposition of the cold solutions. Alkalis, however, cause the formation of a mixture of thiosulphate and trithionate:

4K₂S₄O₆ + 6KOH = 5K₂S₂O₃ + 2K₂S₃O₆ + 3H₂O,

whilst in hot solution some sulphide may also be produced. With sodium carbonate the reaction is

4Na₂S₄O₆ + 4Na₂CO₃ = 6Na₂S₂O₃ + Na₂S₃O₆ + Na₂SO₄ + 4CO₂.

Sulphur dioxide abstracts sulphur from aqueous tetrathionic acid giving trithionic acid, the sulphur remaining in the solution and converting part of the tetrathionic acid into pentathionic acid, so that the final solution contains all three acids.

Hydrogen sulphide in excess causes gradual decomposition, sulphur and pentathionic acid being the chief products. In neutral solution the reaction is more rapid and appears to take the following course:

1. S₄O₆'' + H₂S = 2S₂O₃'' + 2H^• + S,
2. S₂O₃'' + 2H₂S + 2H^• = 3H₂O + 4S,
3. 5S₂O₃'' + 6H^• = 2S₅O₆'' + 3H₂O.

With the free acid only reactions (i) and (iii) occur. The decomposition takes place more readily than in the case of trithionic acid. Alkali sulphides in boiling solution convert tetrathionate into thiosulphate, with liberation of sulphur:

K₂S₄O₆ + K₂S = 2K₂S₂O₈ + S.

Freshly precipitated lead sulphide also induces the decomposition of tetrathionic acid.

Vigorous oxidising agents, such as chlorine and bromine, convert tetrathionic acid into sulphuric acid, whilst reducing agents, for example hydrogen, especially in contact with platinum, and sodium or potassium amalgam, act on the tetrathionates with formation of thiosulphates,

K₂S₄O₆ + 2Na = K₂S₂O₃ + Na₂S₂O₃,

the reaction being capable of proceeding further, with the production of some sulphide.