Oxides And Oxy-acids Of Chlorine : Chlorine Dioxide

Thos. Hoyle in 1797 obtained a yellow explosive gas from potassium chlorate and concentrated sulphuric acid, but did not recognise it as an oxide of chlorine. The gas was also obtained by Chenevix, but its composition was first ascertained by Davy in 1815. He showed that it was chlorine dioxide, ClO₂. On explosion, two volumes of this gas gave three volumes of gas, consisting of two volumes of oxygen and one volume of chlorine: 2ClO₂ = Cl₂ + 2O₂. The density of chlorine dioxide gas was found by Pebal and Schacherl (1882) to correspond with the formula ClO₂.

Powdered previously fused, potassium chlorate is added in small quantities at a time to cooled concentrated sulphuric acid in a small retort The orange-yellow paste is very cautiously warmed by placing the retorl in KikewaJm water, and the gas collected by downward displacement, since it is heavier than air. It dissolves in water and attacks mercury slowly, being completely absorbed by it. There is considerable danger of violent explosion in the preparation of chlorine dioxide and it is recommended that no attempt should be made to prepare it by this method.

The reaction 3KClO₃ + 2H₂SO₄ = KClO₄ + 2KHSO₄ + H₂O + 2ClO₂ appears to take place as follows:

KClO₃ + H₂SO₄ = KHSO₄ + HClO₃.(chloric acid);

3HClO₃ = HClO₄ (perchloric acid) + 2ClO₂ + H₂O.

Pure chlorine dioxide is prepared by passing dry chlorine over_silver chlorate at 90° and condensing it from the gas in a freezing mixture (King and Partington, 1926):

2AgClO₃ + Cl₂ = 2 AgCl + 2ClO₃ + O₂.

When passed through a tube cooled in a freezing mixture the gas condenses to a dark red liquid, boiling at 11.0°, and at -59° this freezes to an orange-coloured crystalline solid. The liquid and solid are slowly decomposed by exposure to light. The liquid is violently explosive, although it may be distilled without decomposition in the entire absence of organic matter. The gas also explodes readily on heating with a hot wire or glass rod, according to some experimenters at 6o°-63°, by an electric spark, and in contact with turpentine, alcohol, or ether.

Expt. 5. - Add one c.c. of cold concentrated sulphuric acid to two portions of 0.5 gm. of potassium chlorate in two test-tubes. A yellow gas with a peculiar smell is generated. Insert a hot glass rod into one tube; into the other throw a small piece of phosphorus. The gas in the first tube explodes; the phosphorus in the second tube inflames spontaneously and explodes the gas. Care must be used in these experiments, as some acid is usually projected from the tubes.

Chlorine dioxide (sometimes called chlorine peroxide) is a powerful oxidising agent. This is evident from the following experiments, due to Hoyle and to Fourcroy and Vauquelin.

Expt. 6. - Equal parts of powdered sugar (or starch) and potassium chlorate are mixed with a spatula on a sand bath, and a drop of concentrated sulphuric acid is allowed to fall on the mixture from a glass rod. The whole mass ignites, and burns violently.

Expt. 7. - A little potassium chlorate is placed in a glass of water, and one or two small fragments of phosphorus are thrown in. If a few C.c. of concentrated sulphuric acid are carefully poured down a thistle funnel on to the chlorate, ClO₂ is evolved. When this comes in contact with the phosphorus, there is a series of flashes of light, accompanied by slight and harmless explosions.

Chlorine hexoxide, ClO₃ or Cl₂O₆, was observed by Millon in 1843 as a product of the action of light on chlorine dioxide, but was overlooked until it was again obtained in 1925 by Bodenstein, etc. It is best obtained by the interaction of ozone and chlorine dioxide: ClO₂ + O₃ = ClO₃ + O₂. It is a dark red liquid, sp. gr. 1.65, m.pt. 3.5°, less explosive than chlorine dioxide. It dissolves unchanged in water, but with alkali gives a mixture of chlorate and perchlorate: Cl₂O₆ + 2OH' = ClO₃' + ClO₄' + H₂O. The vapour density corresponds with ClO₃, but the liquid is perhaps Cl₂O₆. The vapour is very unstable.

Protein crystallography
Chemistry guide
Oxides And Oxy-acids Of Chlorine
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