Potassium persulphate, K₂S₂O₈

The Potassium persulphate, K₂S₂O₈, is formed in the electrolysis of the primary sulphate or normal sulphate in concentrated solution, with a current density of 5 amperes per sq. cm. The use of a diaphragm is unnecessary, and the yield is much higher when the electrolyte contains hydrofluoric acid. The compound is also manufactured by double decomposition from ammonium persulphate. Any excess of this salt can be eliminated by crystallizing from hot water in presence of sufficient barium hydroxide to decompose the ammonium derivative. The heat of formation of potassium persulphate is 454.5 Cal., and the solubility at 0° C. is 1.76 grams per 100 grams of water. With concentrated sulphuric acid it forms complex derivatives containing a large proportion of oxygen. It unites with hydrogen peroxide to form an unstable compound, probably having the formula K₂S₂O₈,H₂O₂. This substance decomposes spontaneously into potassium hydrogen sulphate and oxygen. Although it has no action on a cold solution of potassium permanganate, its presence prevents the accurate estimation of hydrogen peroxide by titration with permanganate in the usual manner. This phenomenon is due to a catalytic, mutual reduction of the persulphate and peroxide, the proportion of permanganate required being less than that indicated by theory. If protected from sunlight, dry potassium persulphate can be preserved almost unchanged for years.