Oxides And Oxy-acids Of Chlorine : Chloric Acid

Chloric acid is much more stable than hypochlorous acid; it is formed when the latter, or chlorine water, is exposed to light. If a solution of potassium chlorate is precipitated with hydrofluosilicic acid, sparingly soluble potassium silicofluoride is formed, and the aqueous chloric acid can be filtered off: 2KClO₃ + H₂SiF6 = K₂SiF6 + 2HClO₃. It is more convenient to start with barium chlorate, a solution of which is precipitated with sulphuric acid: Ba(ClO₃)₂ + H₂SO₄ = BaSO₄ (ppd.) + 2HClO₃. The excess of sulphuric acid is precipitated with baryta water, the solution is decanted and is evaporated in a vacuum desiccator over concentrated sulphuric acid until it contains 40 per cent, of HClO₃. On further concentration the acid decomposes into chlorine, oxygen, and perchloric acid.

Barium chlorate is made by evaporating a solution of sodium chlorate and barium chloride: 2NaClO₃ + BaCl₂ <=> 2NaCl + Ba(ClO₃)₂. The sodium chloride is deposited, and the hot filtered solution is evaporated, when monoclinic crystals of Ba(ClO₃)₂,H₂O separate. Chloric acid was first prepared, from barium chlorate, by Gay-Lussac in 1814.

The concentrated acid is colourless, and fairly stable in the dark. When exposed to light it undergoes decomposition and becomes yellow. Organic substances, such as cotton-wool or paper, are ignited by the concentrated acid. It has a pungent smell, rather like that of nitric acid, and strong acidic and bleaching properties. The anhydride corresponding with chloric acid, 2HClO₃ - H₂O = Cl₂O₅, is not known.

Expt. 8. - Pour a concentrated solution of sodium hydrogen sulphite (NaHSO₃) over crystals of potassium chlorate. A trace of free chloric acid is liberated by the weakly acid NaHSO₃. The latter is then oxidised by the chloric acid to the strongly acid NaHSO₄. More chloric acid is liberated, and the velocity of reaction is increased by the action of the products (i.e., by autocatalysis) until in one or two minutes the whole mixture foams over, acid sodium sulphate (NaHSO₄) and hydrochloric acid being formed.

Chloric acid evolves hydrogen with zinc, but in acid solutions chlorates are readily reduced by iron or aluminium powder to hydrochloric acid. In this way they may be estimated:

HClO₃ + 6H = 3H₂O + HCl.

Perchloric acid is not reduced in dilute solution.

Protein crystallography
Chemistry guide
Oxides And Oxy-acids Of Chlorine
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