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

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
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.
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