chlorate (klōr´āt, klôr´–) and perchlorate (pərklōr´āt, –klôr´–), salts of chloric acid, HClO₃, and perchloric acid, HClO₄, respectively.

Chloric Acid and Its Salts

Chloric acid, HClO₃·7H₂O, is a colorless substance that occurs only in solution. It is a strong acid and a strong oxidizing agent that decomposes if heated above 40°C. Under certain conditions it forms oxygen, water, and the explosive gas chlorine dioxide, ClO₂; under other conditions it forms perchloric acid and hydrochloric acid.

Formation of Chlorates

A chlorate may be formed (together with the corresponding chloride) by heating the hypochlorite; e.g., 3Ca(ClO)₂→Ca(ClO₃)₂+2CaCl₂. This reaction takes place when chlorine gas is passed into a hot aqueous solution of a metal hydroxide; the hypochlorite is formed and decomposes almost immediately. Commercially, a chlorate is derived when a hot aqueous metal chloride solution is decomposed by electrolysis, forming chlorine gas at the anode and metal hydroxide at the cathode (with evolution of hydrogen); the chlorine reacts with the hydroxide to form the hypochlorite, which decomposes to form the chlorate.

Commercial Uses of Chlorates

The most industrially important chlorate is potassium chlorate, or chlorate of potash, KClO₃; sodium chlorate, or chlorate of soda, NaClO₃, is also used. Potassium chlorate is a colorless crystalline substance that melts at 356°C and decomposes violently at about 400°C. It is a powerful oxidizing agent and is used in making explosives and matches; a mixture of potassium chlorate with phosphorus, sulfur, or any of numerous organic compounds (e.g., charcoal or sugar) explodes upon friction or percussion. When a chlorate is heated, oxygen is evolved, often explosively, and the chloride is formed; e.g., 2KClO₃→2KCl+3O₂. The reaction proceeds controllably at lower temperatures if a catalyst, e.g., manganese dioxide, is used; this provides a convenient source of oxygen. If the chlorate is heated carefully at a lower temperature so that no oxygen is given off, the perchlorate and chloride are formed; e.g., 4KClO₃→3KClO₄+KCl.

Perchloric Acid and Its Salts

Perchloric acid, HClO₄, is a volatile, unstable, colorless liquid that is a strong, corrosive acid and a powerful oxidizing agent, especially when hot. It explodes if heated to about 90°C or on contact with combustible materials. The monohydrate, HClO₄·H₂O, is fairly stable and forms needlelike crystals that melt at 50°C. It explodes if heated to 110°C. The dihydrate, HClO₄·2H₂O, is a stable liquid that boils at 200°C.

Formation of Perchlorates

Perchloric anhydride, or chlorine heptoxide, Cl₂O₇, is a colorless, oily liquid that boils at 82°C without exploding but that may be detonated by shock; it can be prepared by adding phosphorus pentoxide to cold perchloric acid. The perchlorate free radical (chlorine tetroxide, ClO₄) can be prepared by adding bromine to silver perchlorate; it is extremely reactive and unstable.

Commercial Uses of Perchlorates

Perchlorates are safer to handle than chlorates; they are more stable when exposed to heat or shock. Potassium perchlorate, KClO₄, is perhaps most widely used, e.g., in matches, fireworks, and explosives. It is a colorless crystalline substance that melts at about 610°C.