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

Hydrosulphurous Acid, H2S2O4

Schonbein in 1852-1858, during a re-investigation of the electrolysis of sulphurous acid solution, examined about twenty years before by Faraday, obtained at the cathode a yellowish solution of marked reducing power; he obtained a solution of similar properties on treatment of sulphurous acid solution with zinc, which reaction was already known from the observations of Berthollet, Fourcroy and Vauquelin to give no free hydrogen.

A further examination of the action of zinc on sulphurous acid and on sodium hydrogen sulphite solutions was made by Schutzenberger in 1869, who arrived at a formula NaHSO2 for the sodium salt, and suggested the name hydrosulphurous acid; although the formula has proved to be incorrect, the name possesses an advantage in precluding any confusion with thiosulphuric acid; the possibility of such confusion is introduced by the term hypo sulphurous acid, which, however, until recent years, received fairly general acceptance in English-speaking countries. Under the present system of nomenclature "hyposulphurous acid" should be H2SO2, which, however, is termed sulphoxylic acid.

The free acid is exceedingly unstable and cannot be isolated, only the salts being of importance. These may be obtained as follows:

Zinc slowly dissolves in sulphurous acid solution without effervescence; the solution is at first yellow and then becomes colourless; air should be excluded. The action probably follows the course

Zn + 2SO2 = ZnS2O4.

It is possible to prepare the hydrosulphites of the metals in anhydrous condition by the action of various metals, for instance zinc, magnesium, sodium, on sulphur dioxide in the presence of moisture-free ether or alcohol. This reaction is of importance as having supplied an early proof of the absence of hydrogen from the salts and hence of the incorrectness of the formulae Zn(HSO2)2, NaHSO2, which had been accepted previously:

Zn + 2SO2 = ZnS2O4.

Moissan in 1902 obtained anhydrous hydrosulphites by passing sulphur dioxide diluted with hydrogen over the hydrides of the alkali and alkaline earth metals. He was able to produce the hydrosulphites of sodium, potassium, lithium, calcium and strontium in this way, and by measurement of the quantity of hydrogen liberated was able to prove the correctness of the general formula Mx.S2O4:

2KH + 2SO2 = K2S2O4 + H2.

Sodium hydrogen sulphite solution can be used in place of the aqueous sulphurous acid in the first method described. With zinc, the resulting reaction is

4NaHSO3 + Zn = ZnSO3 + Na2SO3 + Na2S2O4 + 2H2O.

The result is more satisfactory if the acid sulphite is accompanied by a semi-molecular proportion of sulphurous acid:

2NaHSO3 + SO2 + Zn = Na2S2O4 + ZnSO3 + H2O.

On pouring the solution into alcohol a double sulphite of sodium and zinc is rapidly precipitated, whilst needles of sodium hydrosulphite, Na2S2O4.2H2O, separate slowly afterwards. Another procedure for the removal of the zinc from the solution is to add milk of lime cautiously, when a solution of sodium hydrosulphite remains.

Other metals may be used in place of zinc, e.g. iron, copper, manganese or even sodium amalgam or calcium.

In the electrolysis of aqueous sulphurous acid or sodium hydrogen sulphite solution, a little hydrosulphite is formed at the cathode as a result of the reduction process.

Titanous chloride also reduces sulphurous acid or sodium hydrogen sulphite solution with formation of an orange-yellow solution of hydro- sulphurous acid, from which sodium hydrosulphite is obtainable by further treatment with sodium hydroxide solution:

2NaHSO3 + 2TiCl3 + 2H2O = 2NaCl + 2TiO2 + 4HCl + H2S2O4.

Formic acid, when mixed with aqueous sodium hydrogen sulphite, forms a solution of strong reducing power, due to conversion of some of the sulphite by reduction into hydrosulphite:

2NaHSO3 + H.COONa = NaHCO3 + H2O + Na2S2O4.

Sodium formaldehydesulphoxylate may be used conveniently instead of formic acid.

Hydrosulphurous acid is also formed as an intermediate product in the reduction of sulphurous acid by hypophosphorous acid.

The free acid is exceedingly unstable, and its orange-yellow aqueous solution, obtained directly by any of the methods already described, or by decomposition of a hydrosulphite with a suitable acid such as dilute sulphuric acid, is capable only of short existence, soon decomposing with deposition of sulphur and liberation of sulphurous acid. By measurement of the electrical conductivity of solutions of the acid and of the sodium salt it has been possible to show that hydrosulphurous acid is a stronger acid than sulphurous acid, although weaker than thiosulphuric acid. According to Berthelot the heat of formation of the acid in aqueous solution is a small positive quantity.

Details of the strong reducing character of the acid will be found in the following, under the description of the reactions of the hydrosulphites.

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