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


Various methods for the formation of the salts of sulphuric acid will be found under the heading of the properties of the acid, of its anhydride and of sulphur dioxide; further details may be found under the description of the sulphates themselves in the other volumes of this series.

The normal sulphates usually form well-defined crystals containing water of crystallisation, and frequently exhibit isomorphism not only with one another but in some cases also with the corresponding selenates, tellurates and chromates. They are generally fairly soluble in water, the chief exceptions being the sulphates of barium, strontium and lead, which are commonly classed as "insoluble," and of calcium and silver, which are sparingly soluble.

Acid salts of the type XHSO4, where X represents the equivalent weight of a metal, are also known, the hydrogen sulphates or "bi- sulphates" of the alkali metals being the commonest examples. These give acidic aqueous solutions, due to partial conversion into the corresponding normal sulphate and sulphuric acid, or in other words, to the further electrolytic dissociation of the HSO4' ions. Under suitable conditions acid sulphates containing additional molecules of sulphuric acid can be obtained; for example, the following exist: Na2SO4.2H2SO4, 2Na2SO4.9H2SO4, K2SO4.3H2SO4, BaSO4.2H2SO4. Viscosity measurements of aqueous solutions of normal ammonium sulphate, sulphuric acid, and ammonium hydrogen sulphate, indicate that the formation of the acid salt is accompanied by an increase of internal friction. The appreciable solubility of the sulphates of lead, barium and strontium in concentrated sulphuric acid probably is also due to the formation of acid salts, which on dilution of the acid undergo decomposition into sulphuric acid and the original insoluble sulphate. When heated, the alkali hydrogen sulphates undergo dehydration into the corresponding pyrosulphates, further heating then causing decomposition into normal sulphate and sulphur trioxide; the hydrogen sulphates of the other metals yield normal sulphates directly.

Basic sulphates are obtained when the normal sulphates of antimony, bismuth and mercury are treated with water, sulphuric acid being produced simultaneously. These salts are insoluble in water. Many other metals, for example copper, aluminium and tin, yield precipitates of basic sulphates on the addition of alkali to aqueous solutions of their normal sulphates.

The normal sulphates show a marked tendency to the formation of double salts, the best known case being that of the alums, which are isomorphous compounds of the general formula M2(SO4)3.X2SO4.24H2O, where M and X represent a tervalent and univalent metal, respectively; in aqueous solution these double salts are almost entirely resolved into the ions of their constituent salts, recombination taking place as the solution crystallises. Double salts are also formed by the crystallisation of fused mixtures of anhydrous sulphates, the freezing- point curves supplying evidence of the occurrence of combination between the constituents.

Certain halogen salts appear to be isomorphous with potassium sulphate and able to form "alums" with aluminium sulphate; thus, the salts K2BeF4.Al2(SO4)3.24H2O and K2ZnCl2.Al2(SO4)3.24H2O are "alums," crystallising in the cubic system, normally as octahedra.

All sulphates undergo reduction when heated with carbon, the product being the metal, metallic sulphide or metallic carbide, according to the salt in question and the conditions of the treatment. Magnesium sulphate, however, when heated with carbon at 750° C., yields the oxide and free sulphur, the primary reaction being

MgSO4 + C = MgO + SO2 + CO,

sulphur being liberated according to the reversible secondary reaction:

2CO + SO2S + 2CO2,
CO2 + C = 2CO.

Magnesium reduces anhydrous sulphates with vigour at high temperatures. Reduction to sulphide may be brought about by certain micro-organisms in the presence of animal fats, the latter being anaerobic-ally decomposed during the process.

Although sulphuric acid expels many other acids from their salts, it can in a similar manner be displaced from its own salts by heating with still less volatile acids such as phosphoric or boric acid or even with silica or alumina; on account of the high temperature necessary, the liberated sulphuric acid or anhydride is partly decomposed into sulphur dioxide:

At very high temperatures the sulphates of metals such as copper, zinc, iron, aluminium and chromium tend to lose sulphur trioxide (largely in the form of sulphur dioxide and oxygen) and to give residues of the corresponding oxides. Calcium sulphate is stable up to 1300° C., above which temperature it melts and immediately undergoes almost complete decomposition with abundant evolution of fumes. Very slight decomposition has been observed with barium sulphate at 1300° C.

Additive compounds of the type MSO4.2HCl are formed by the sulphates of those metals the chlorides of which do not readily yield hydrogen chloride when treated with sulphuric acid. Thus such compounds of cadmium, copper, lead, mercury, silver, thallium and tin have been prepared; the hydrogen chloride may be expelled by heat.

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