Chloric acid (HClO3): properties, preparation and safety

Structure and chemical characteristics

In chloric acid, chlorine is in the +5 oxidation state and is bonded to three oxygens, one of which carries the proton that gives the material its acidity. In water the molecule is largely dissociated to hydrogen ions and chlorate ions, and solutions act as fairly strong acids and oxidizers. Concentrated solutions are chemically unstable and can decompose to give chlorine oxides, chlorine dioxide and other products.

Preparation and reactions

Chloric acid is commonly produced by acidifying solutions of chlorate salts under controlled, dilute conditions; for example, reacting a soluble chlorate with a strong acid yields an aqueous solution of HClO3 while precipitating an insoluble counterion. Because concentrated acid decomposes violently and may generate gases such as chlorine dioxide, preparations are kept dilute and cooled.

Uses, derivatives and examples

The acid itself is rarely used directly; its main role is as the conceptual parent of chlorate salts such as sodium chlorate and potassium chlorate. These salts have historical and industrial uses—sodium chlorate in pulp bleaching and herbicides, potassium chlorate in pyrotechnics—owing to the oxidizing character of the chlorate ion. For general reference on the compound see chloric acid overview, for the anion see chlorate ion, and for the acidic dissociation process see hydrogen ion.

Safety and handling

Chloric acid solutions are strong oxidizers and can ignite or promote combustion of organic materials and reducing agents. Laboratories treat it with caution: avoid concentration, keep solutions cold, store away from organics and reducing compounds, and never distill or evaporate to dryness. Relevant safety and regulatory information is available from chemical safety databases: safety data.

• Key hazard: oxidizing reactivity and possible release of toxic gases on decomposition.
• Common laboratory practice: prepare only dilute solutions and neutralize before disposal where appropriate.

Chloric acid is one member of a series of chlorine oxoacids that includes hypochlorous, chlorous, chloric and perchloric acids; these differ in oxidation state, acidity and stability, with chloric acid occupying an intermediate role both in oxidation potential and tendency to decompose.