chlorate

chlorate and perchlorate, 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;.

The chlorate anion has the formula ClO₃⁻. In this case, the chlorine atom is in the +5 oxidation state. "Chlorate" can also refer to chemical compounds containing this anion; chlorates are the salts of chloric acid. As predicted by VSEPR, chlorate anions have trigonal pyrimidal structures.

Chlorates are powerful oxidizers and should be kept away from organics or easily oxidized materials. Chlorates were once widely used in pyrotechnics, though their use has fallen due to their instability. Most pyrotechnic applications which used chlorates in the past now use perchlorates instead.

Examples of chlorates include:

• potassium chlorate, KClO₃
• sodium chlorate, NaClO₃
• magnesium chlorate, Mg(ClO₃)₂

Preparation

Metal chlorates can be prepared by adding chlorine to hot metal hydroxides, for example, KClO₃:

3Cl₂ + 6KOH → 5KCl + KClO₃ + 3H₂O

The industrial scale synthesis for sodium chlorate starts from sodium chloride. If the electrolysis is not done with the method described at chlorine, but a mixing of the chlorine evolved and the sodium hydroxide is allowed, the reaction mentioned above takes place. The heating to 50-70°C is done by the electrical power applied for electrolysis.

Stock notation

If a Roman numeral in brackets follow the word "chlorate", this refers to the oxyanion containing chlorine in the respective oxidation state, namely:

Common name	Stock name	Oxidation state	Formula
Hypochlorite	Chlorate(I)	+1	ClO−
Chlorite	Chlorate(III)	+3	ClO2−
Chlorate	Chlorate(V)	+5	ClO3−
Perchlorate	Chlorate(VII)	+7	ClO4−

Using this convention, "chlorate" means any chlorine oxyanion. However, Stock notation for chlorine is quite uncommon, and "chlorate" typically refers only to the +5 oxidation state.