Sulphates

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 + SO₂ ⇔ S + 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.