Note 45 — Chapter XI · THE HALOGENS: CHLORINE, BROMINE, IODINE, AND FLUORINE

Perchloric acid, which is obtained in a free state by the action of sulphuric acid on its salts, may be separated from a solution very easily by distillation, being volatile, although it is partially decomposed by distillation. The solution obtained after distillation may be concentrated by evaporation in open vessels. In the distillation the solution reaches a temperature of 200°, and then a very constant liquid hydrate of the composition HClO4,2H2O is obtained in the distillate. If this hydrate be mixed with sulphuric acid, it begins to decompose at 100°, but nevertheless a portion of the acid passes over into the receiver without decomposing, forming a crystalline hydrate HClO4,H2O which melts at 50°. On carefully heating this hydrate it breaks up into perchloric acid, which distills over below 100°, and into the liquid hydrate HClO4,2H2O. The acid HClO4 may also be obtained by adding one-fourth part of strong sulphuric acid to potassium chlorate, carefully distilling and subjecting the crystals of the hydrate HClO4,H2O obtained in the distillate to a fresh distillation. Perchloric acid, HClO4, itself does not distil, and is decomposed on distillation until the more stable hydrate HClO4, H2O is formed; this decomposes into HClO4 and HClO4,2H2O, which latter hydrate distils without decomposition. This forms an excellent example of the influence of water on stability, and of the property of chlorine of giving compounds of the type ClX7, of which all the above hydrates, ClO3(OH), ClO2(OH)3, and ClO(OH)5, are members. Probably further research will lead to the discovery of a hydrate Cl(OH)7.