Oxides And Oxy-acids Of Chlorine : Chlorous Acid

Chlorine dioxide dissolves in water, forming a yellow solution without acid reaction which is stable for several weeks in the dark at 0°. With solutions of alkalies, however, it acts as an acidic oxide, forming a mixture of two salts in equivalent amounts: 2KOH + 2ClO₂ = KClO₃ + KClO₂ + H₂O. KClO₂ is the salt of chlorous acid, HClO₂. The salts may be separated by evaporation in vacua over sulphuric acid, when the less soluble KClO₃ is first deposited.

Pure chlorites are obtained by the action of alkali and hydrogen peroxide on a concentrated aqueous solution of chlorine dioxide. The latter is conveniently prepared by warming at 60° a mixture of 40 gm. of KClO₃, 150 gm. of crystalline oxalic acid, and 20 c.c. of water, and passing the gas into water: 2KClO₃ + 2C₂H₂O₄ = K₂C₂O₄ + 2H₂O + 2CO₂ + 2ClO₂. When diluted with carbon dioxide, chlorine dioxide is not liable to explode during preparation. The hydrogen peroxide reduces chlorine dioxide to chlorous acid: 2ClO₂ + H₂O₂ = 2HClO₂ + O₂. Barium chlorite is formed by suspending barium peroxide in hydrogen peroxide and passing in chlorine dioxide. Free chlorous acid is obtained in solution by treating barium chlorite with dilute sulphuric acid.

The alkali chlorites have a caustic taste, and bleach vegetable colours. They may be distinguished from hypochlorites by the bleaching action after addition of sodium arsenite. Silver and lead nitrates precipitate yellow crystalline AgClO₂ and Pb(ClO₂)₂. These explode on heating; lead chlorite detonates violently on percussion when mixed with sugar, and has been used for detonators.

Chlorites liberate iodine from iodides:

NaClO₂ + 4KI + 2H₂O = 2I₂ + 4KOH + NaCl.

They react only slowly with arsenious oxide. Chlorous acid gives a characteristic violet colour with ferrous sulphate.

The gas prepared by heating a mixture of potassium chlorate and sugar, benzene, or arsenious oxide, with nitric acid, believed to be chlorous anhydride, Cl₂O₃, by Millon (1843), was shown by Garzarolli-Thurnlack in 1881 to be a mixture of chlorine dioxide with chlorine. The mixture of chlorine and the dioxide obtained by treating potassium chlorate with concentrated hydrochloric acid, supposed by Davy to be an oxide of chlorine, Cl₂O, called eucmorine, was examined by Pebal in 1875, and the method used was applied by Garzarolli-Thurnlack to the supposed trioxide.

A measured volume of the gas was decomposed by heating, and the increase in volume determined. The chlorine was then absorbed by potash solution, and the residual oxygen measured. A comparison of the expansion on explosion with the volume of oxygen produced showed that the latter was double the former.

The different oxides of chlorine would give the following results:

	Expansion on Explosion	Residual oxygen after absorption of chlorine.
Chlorine monoxide: Cl₂O; 2Cl₂O = 2Cl₂ + O₂	3-2=1 vols.	1 vols.
Chlorine dioxide, ClO₂; 2ClO₂ = Cl₂ + 2O₂	3-2=1 vols.	2 vols.
Chlorous anhydride, Cl₂O₃ (?); 2Cl₂O₃ = 2Cl₂ + 3O₂	5-2=3 vols.	3 vols.

With euchlorine and the supposed trioxide, the volume relations (2) were found, hence both contained only chlorine and chlorine dioxide. By passing the "trioxide," and euchlorine, through tubes immersed in a freezing mixture, pure chlorine dioxide was liquefied and chlorine passed on.

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Chemistry guide
Oxides And Oxy-acids Of Chlorine
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