If dilute sulphuric acid be added to a solution of potassium chlorate, chloric acid is liberated, but it cannot be separated by distillation, as it is decomposed in the process. To obtain the free acid, sulphuric acid must be added to a solution of barium chlorate.41 The sulphuric acid gives a precipitate of barium sulphate, and free chloric acid remains in solution. The solution may be evaporated under the receiver of an air-pump. This solution is colourless, has no smell, and acts as a powerful acid (it neutralises sodium hydroxide, decomposes sodium carbonate, gives hydrogen with zinc, &c.); when heated above 40°, however, it decomposes, forming chlorine, oxygen, and perchloric acid: 4HClO3 = 2HClO4 + H2O + Cl2 + O3. In a concentrated condition the acid acts as an exceedingly energetic oxidiser, so that organic substances brought into contact with it burst into flame. Iodine, sulphurous acid, and similar oxidisable substances form higher oxidation products and reduce the chloric acid to hydrochloric acid. Hydrochloric acid gas gives chlorine with chloric acid (and consequently with KClO3 also) acting in the same manner as it acts on the lower acids: HClO3 + 5HCl = 3H2O + 3Cl2.
By cautiously acting on potassium chlorate with sulphuric acid, the dioxide (chloric peroxide), ClO2,42 is obtained (Davy, Millon). This gas is easily liquefied in a freezing mixture, and boils at +10°. The vapour density (about 35 if H = 1) shows that the molecule of this substance is ClO2.43 In a gaseous or liquid state it very easily explodes (for instance, at 60°, or by contact with organic compounds or finely divided substances, &c.), forming Cl and O2, and in many instances44 therefore it acts as an oxidising agent, although (like nitric peroxide) it may itself be further oxidised.45 In dissolving in water or alkalis chloric peroxide gives chlorous and hypochlorous acids—2ClO2 + 2KHO = KClO3 + KClO2 + H2O—and therefore, like nitric peroxide, the dioxide may be regarded as an intermediate oxide between the (unknown) anhydrides of chlorous and chloric acids: 4ClO2 = Cl2O3 + Cl2O3.46
As the salts of chloric acid, HClO3, are produced by the splitting up of the salts of hypochlorous acid, so in the same way the salts of perchloric acid, HClO4, are produced from the salts of chloric acid, HClO3. But this is the highest form of the oxidation of HCl. Perchloric acid, HClO4, is the most stable of all the acids of chlorine. When fused potassium chlorate begins to swell up and solidify, after having parted with one-third of its oxygen, potassium chloride and potassium perchlorate have been formed according to the equation 2KClO3 = KClO4 + KCl + O2.
The formation of this salt is easily observed in the preparation of oxygen from potassium chlorate, owing to the fact that the potassium perchlorate fuses with greater difficulty than the chlorate, and therefore appears in the molten salt as solid grains ( see Chapter III. Note 12). Under the action of certain acids—for instance, sulphuric and nitric—potassium chlorate also gives potassium perchlorate. This latter may be easily purified, because it is but sparingly soluble in water, although all the other salts of perchloric acid are very soluble and even deliquesce in the air. The perchlorates, although they contain more oxygen than the chlorates, are decomposed with greater difficulty, and even when thrown on ignited charcoal give a much feebler deflagration than the chlorates. Sulphuric acid (at a temperature not below 100°) evolves volatile and to a certain extent stable perchloric acid from potassium perchlorate. Neither sulphuric nor any other acid will further decompose perchloric acid as it decomposes chloric acid. Of all the acids of chlorine, perchloric acid alone can be distilled. The pure hydrate HClO 4 is a colourless and exceedingly caustic substance which