Chemical elements
  Sulphur
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    Compounds
      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
      Sulphites
      Sulphur Trioxide
      Pyrosulphuric Acid
      Pyrosulphates
      Sulphuric Acid
      Persulphuric Anhydride
      Persulphuric Acid or Perdisulphuric Acid
      Perdisulphates
      Permonosulphuric Acid
      Amidopermonosulphuric Acid
      Thiosulphuric Acid
      Thiosulphates
      Polythionic Acids
      Dithionic Acid
      Trithionic Acid
      Trithionates
      Tetrathionic Acid
      Tetrathionates
      Pentathionic Acid
      Pentathionates
      Wackenroders Solution
      Hexathionic Acid
      Polythionic Acids
      Sulphur Sesquioxide
      Hydrosulphurous Acid
      Hydrosulphites
      Nitrogen Sulphide
      Nitrogen Persulphide
      Nitrogen Pentasulphide
      Sulphammonium
      Hexasulphamide
      Nitrogen Chlorosulphide
      Trithiazyl Chloride
      Thiotrithiazyl Chloride
      Dithiotetrathiazyl Chloride
      Nitrogen Bromosulphide
      Thiotrithiazyl Bromide
      Thiotrithiazyl Iodide
      Thiotrithiazyl Nitrate
      Thiotrithiazyl Hydrogen Sulphate
      Thiotrithiazyl Thiocyanate
      Thionylamide
      Sulphamide
      Imidodisulphamide
      Sulphimide
      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
      Thioformaldehyde
      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
      Thiocarbamide
      Azidodithiocarbonic Acid
      Thiocyanogen
      Cyanogen Monosulphide
      Cyanogen Trisulphide
      Sulphur Thiocyanate
      Disulphur Dithiocyanate
      Thiocyanic Acid
      Thiocyanates
      Dithiocyanic Acid
      Trithiocyanuric Acid
      Perthiocyanic Acid
      Perthiocyanogen
      Sulphates

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.


Properties

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.
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