Chemical elements
    Amorphous Sulphur
    Colloidal Sulphur
    Physical Properties
    Chemical Properties
      Hydrogen Sulphide
      Metal Polysulphides
      Hydrogen Polysulphides
      Hydrogen Pentasulphide
      Hydrogen Trisulphide
      Hydrogen Disulphide
      Sulphur Monofluoride
      Sulphur Tetrafluoride
      Sulphur Hexafluoride
      Sulphur Monochloride
      Sulphur Dichloride
      Sulphur Tetrachloride
      Sulphur Monobromide
      Thionyl Fluoride
      Sulphuryl Fluoride
      Fluorosulphonic Acid
      Thionyl Chloride
      Sulphuryl Chloride
      Sulphur Oxytetrachloride
      Pyrosulphuryl Chloride
      Chlorosulphonic Acid
      Thionyl Bromide
      Sodium Sulphoxylate
      Sulphur Dioxide
      Sulphurous Acid
      Sulphur Trioxide
      Pyrosulphuric Acid
      Sulphuric Acid
      Persulphuric Anhydride
      Persulphuric Acid or Perdisulphuric Acid
      Permonosulphuric Acid
      Amidopermonosulphuric Acid
      Thiosulphuric Acid
      Polythionic Acids
      Dithionic Acid
      Trithionic Acid
      Tetrathionic Acid
      Pentathionic Acid
      Wackenroders Solution
      Hexathionic Acid
      Polythionic Acids
      Sulphur Sesquioxide
      Hydrosulphurous Acid
      Nitrogen Sulphide
      Nitrogen Persulphide
      Nitrogen Pentasulphide
      Nitrogen Chlorosulphide
      Trithiazyl Chloride
      Thiotrithiazyl Chloride
      Dithiotetrathiazyl Chloride
      Nitrogen Bromosulphide
      Thiotrithiazyl Bromide
      Thiotrithiazyl Iodide
      Thiotrithiazyl Nitrate
      Thiotrithiazyl Hydrogen Sulphate
      Thiotrithiazyl Thiocyanate
      Sulphonic Acids
      Amidosulphonic Acid
      Imidosulphonic Acid
      Nitrilosulphonic Acid
      Hydroxylamine-monosulphonic Acid
      Nitrososulphonic Acid
      Hydroxylamine-disulphonic Acid
      Hydroxylamine-isodisulphonic Acid
      Hydroxylamine-trisulphonic Acid
      Dihydroxylamidosulphonic Acid
      Sulphazinic Acid
      Sulphazotinic Acid
      Dehydrosulphazotinic Acid
      Nitrosulphonic Acid
      Nitrosulphonyl Chloride
      Nitrosulphonic Anhydride
      Nitrosulphuric Acid
      Nitrosodisulphonic Acid
      Sulphonitronic Acid
      Sulphates of Hydroxylamine
      Hydroxylamine Dithionate
      Hydrazine Dithionate
      Hydrazine Amidosulphonate
      Carbon Subsulphide
      Carbon Monosulphide
      Carbon Disulphide
      Thiocarbonic Acid
      Ammonium thiocarbonate
      Thiolcarbonic Acid
      Xanthic Acid
      Perthiocarbonic Acid
      Sodium perthiocarbonate
      Carbonyl Sulphide
      Thiocarbonyl Chloride
      Thiocarbonyl Tetrachloride or
      Carbon Hexachlorosulphide
      Trichloromethyl Disulphide
      Thiocarbonyl Sulphochloride
      Carbon Bromosulphide
      Amino-derivatives of Thiocarbonic Acid
      Dithiocarbamic Acid
      Azidodithiocarbonic Acid
      Cyanogen Monosulphide
      Cyanogen Trisulphide
      Sulphur Thiocyanate
      Disulphur Dithiocyanate
      Thiocyanic Acid
      Dithiocyanic Acid
      Trithiocyanuric Acid
      Perthiocyanic Acid

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.
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