Pyrosulphates

The pyrosulphates of the alkali metals are obtained most easily by heating the corresponding acid sulphates to 300°-350° C:

2KHSO₄ = K₂S₂O₇ + H₂O.

This dehydration, however, does not proceed to completion even under reduced pressure, and in order to obtain the pure salts it is preferable to allow the finely divided anhydrous normal sulphate to interact with sulphur trioxide. This latter process stands in fairly close relationship with the methods based on the treatment of a normal sulphate with chlorosulphonic acid, with the aid of heat, and on the crystallisation of the solution produced by dissolving a normal sulphate in warm "fuming sulphuric acid":

K₂SO₄ + SO₃ = K₂S₂O₇;
BaSO₄ + Cl.SO₂.OH = BaS₂O₇ + HCl;
K₂SO₄ + H₂S₂O₇ = K₂S₂O₇ + H₂SO₄.

When heated strongly, for instance above 450° C., the pyrosulphates undergo decomposition into sulphur trioxide and the corresponding normal sulphate. The sodium salt, which melts at 400.9° C., exhibits appreciable dissociation at about 460° C. In contact with moist air or water the pyrosulphates become converted into the corresponding acid sulphates:

K₂S₂O₇ + H₂O = 2KHSO₄.