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Preparation of Perdisulphates
Apparatus for Preparation of Perdisulphates.
A similar process to the electrolytic preparation of perdisulphuric acid is possible for the preparation of the perdisulphates. The corresponding alkali hydrogen sulphate or a mixture of the normal sulphate and sulphuric acid in cold aqueous solution is electrolysed in a cell so arranged that the anode and cathode are separated by a porous diaphragm, or at least arranged so that the cathode gases shall not pass near the anode; a convenient experimental apparatus is represented in the diagram (fig.). The yield of alkali perdisulphate is increased by the presence of nickel sulphate, aluminium sulphate, alkali fluoride, potassium thiocyanate or potassium ferricyanide in the electrolyte, and also by the application of a high current density at the anode, which should therefore be as small as is convenient. A platinum anode is commonly used, and this should be of smooth metal; electrodes of aluminium and tin have also been recommended.

As in the electrolytic formation of perdisulphuric acid, the first anodic product is the discharged HSO4' ion, which by coupling then gives rise to perdisulphuric acid. If the metal perdisulphate is less soluble than the acid sulphate, it crystallises from the solution; this is the case in the electrolysis of a concentrated solution of potassium or ammonium hydrogen sulphate,8 the changes in the case of the latter salt being as follows:

HSO4' + + = HSO4,
2HSO4 = H2S2O8,
H2S2O8 + (NH4)HSO4 = (NH4)2S2O8 + 2H2SO4.

Ammonium perdisulphate has also been prepared by passing fluorine into a cold saturated aqueous solution of ammonium hydrogen sulphate; ozonised oxygen escapes as oxidation proceeds and crystals of the persulphate slowly separate.

The perdisulphates of the alkaline earth metals may be prepared by grinding together, with a little water, molecular proportions of the metal oxide and ammonium perdisulphate, and drying the mixture in a vacuum. The resulting mass is carefully powdered and the metal perdisulphate extracted by solution in absolute alcohol, from which it may be obtained by the addition of absolute ether, or by evaporation in a vacuum.

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