Chloric acid, HClO3, is an oxoacid of chlorine, and the formal precursor of chlorate salts. It is a strong acid (pKa ≈ −1) and oxidizing agent.
It is prepared by the reaction of sulfuric acid with barium chlorate, the insoluble barium sulfate being removed by precipitation:
- Ba(ClO3)2 + H2SO4 → 2HClO3 + BaSO4
Another method is the heating of hypochlorous acid, of which productions include chloric acid and hydrogen chloride:
- 3HClO → HClO3 + 2 HCl
It is also produced by the reaction of sulfuric acid with potassium chlorate in the combustion of sugar using potassium chlorate, sulfuric acid, and sugar.
It is stable in cold aqueous solution up to a concentration of approximately 30%, and solution of up to 40% can be prepared by careful evaporation under reduced pressure. Above these concentrations, and on warming, chloric acid solutions decompose to give a variety of products, for example:
- 8HClO3 → 4HClO4 + 2H2O + 2Cl2 + 3 O2
- 3HClO3 → HClO4 + H2O + 2 ClO2
The decomposition is controlled by kinetic factors: indeed, chloric acid is never thermodynamically stable with respect to disproportionation.
Chloric acid is a dangerously powerful oxidizing agent and will cause most organics and flammables to deflagrate on contact. For example a mixture of potassium chlorate and sugar will burn when concentrated sulfuric acid is added due to chloric acid production. Because sulfur tends to contain acidic impurities, it will form highly unstable mixtures with potassium chlorate due to chloric acid being produced.