Potassium perchlorate, KClO₄

Careful heating of potassium chlorate causes partial autoxidation to Potassium perchlorate, KClO₄:

4KClO₃ = 3KClO₄ + KCl.

The slight solubility of the perchlorate in dilute alcohol affords a means of separating it from the chloride. According to Blau and Weingland, the decomposition of the chlorate is best carried out in quartz flasks at 480° C. without a catalyst, but when between 96 and 97 per cent, of the chlorate has been transformed the perchlorate begins to decompose, so that the change is never complete. The decomposition of the perchlorate is much accelerated by the presence of traces of iron oxide, copper, nickel, boron trioxide, or potassium hydroxide, so that vessels made of iron are unsuitable for its preparation.

When sulphuric acid reacts with potassium chlorate without application of heat, a yield of 11 per cent, of the perchlorate is obtained. Boiling with phosphoric acid of 85 per cent, strength gives a 15 per cent, yield; and evaporation to dryness with nitric acid produces a 30 per cent, yield. Hydrochloric acid does not form any perchlorate.

A good yield of the perchlorate is obtained by the electrolysis of potassium chlorate with a platinum anode and a nickel cathode, the current density being 0.15 ampere per square decimetre. Even when revolving rapidly, the electrodes become incrusted with perchlorate; but this difficulty may be avoided by electrolyzing sodium chlorate, and forming potassium perchlorate by double decomposition. Knibbs and Palfreeman regard the electrolytic formation of perchlorate as being analogous to that of persulphate, and give the scheme

-O₂ClO- + -OClO₂ = [O₂ClO-]₂;
[O₂ClO-]₂ + H₂O = O₂ClO-OH + O₂ClOH.

The salt forms colourless, rhombic crystals, melting at 610° C. Its density is given as 2.524 at 10.8° C., and 2.520 at ordinary temperature. The specific heat is 0.190 between 14° and 45° C. The heat of formation from the elements is 112.5 Cal. The solubility at 25° C. is 1.96 grams per 100 grams of water. Above 400° C. the salt decomposes with evolution of oxygen and formation of KCl. Its insolubility in alcohol renders it of service in the estimation of potassium.