Oxides And Oxy-acids Of Chlorine : Perchloric Acid

The most stable oxy-acid of chlorine is that containing most oxygen, viz., perchloric acid, HClO₄. Small quantities of the very soluble sodium salt, NaClO₄, occur in crude sodium nitrate, or Chile nitre: it acts prejudicially on vegetation if the impure nitrate is used as a fertiliser.

Perchloric acid is formed by the evaporation of a solution of chloric acid, and if the latter is distilled aqueous perchloric acid comes over, chlorine and oxygen escaping at the same time:

3HClO₃ = HClO₄ + Cl₂ + 2O₂ + H₂O.

Potassium perchlorate, KClO₄, is prepared by heating the pure chlorate at 510° in a new porcelain dish, or better at 480° in a silica flask for 8 hours, separating from the chloride by treatment with cold water and crystallising the residue from hot water: 4KClO₃ = 3KClO₄ + KCl. Any chlorate remaining may be decomposed by hydrochloric acid, which is without action on the perchlorate.

If potassium perchlorate is distilled with four times its weight of very concentrated sulphuric acid in a small retort, perchloric acid comes over as a colourless, or slightly yellow, strongly fuming liquid: KClO₄ + H₂SO₄ = KHSO₄ + HClO₄. The yield is increased by carrying out the distillation under 10-20 mm. pressure, when the acid distils over between 90° and 160°. It is purified by distilling under 60 mm. pressure, when it boils at 40°-6o°. It boils, with partial decomposition, at 90° under 760 mm. pressure, or without decomposition at 19° under 11 mm. pressure.

During the distillation under ordinary pressure, the liquid in the receiver gradually solidifies to white crystals of the monohydrate, HclO₄,H₂O m pt. 50°. Other crystalline hydrates are known: HclO₄,2H₂O (m. pt. -17.8°); 2HClO₄,5H₂O (m. pt. -30°); 2HClO₄,7H₂O (m. pt. -41.4°) and two forms of HclO₄,3H₂O (m. pt. -43.2° and -37°) The anhydrous acid explodes violently at 92° and sometimes spontaneously on keeping, when it undergoes decomposition. It is very hygroscopic, and dissolves in water with a hissing noise and great evolution of heat. The hydrate HClO₄,H₂O was regarded as the acid itself by its discoverer, Stadion (1816); pure HClO₄ was first prepared by Roscoe (1863). On heating, HClO₄,H₂O breaks up into anhydrous acid, which distils over, and an oily solution of maximum boiling point, 203°, containing 71.6 per cent, of HClO₄.

The oily aqueous acid, which is quite stable, is conveniently prepared by adding ammonium perchlorate (a commercial substance), dissolved in concentrated hydrochloric acid, to warm concentrated nitric acid in a porcelain dish. Nitrogen, chlorine, and nitrosyl chloride are evolved, and on evaporation aqueous perchloric acid remains:

NH₄ClO₄ + HCl = NH₄Cl + HClO₄;

HNO₃ + 3HCl = NOCl + Cl₂ + 2H₂O;

2NH₄Cl + 3Cl₂ = N₂ + 8HCl.

Paper and wood catch fire when anhydrous perchloric acid is dropped on them. If a few drops of the acid are poured on recently-ignited wood charcoal, there is a violent explosion.

The aqueous acid dissolves iron and zinc to form perchlorates: 2HClO₄ + Zn = Zn(ClO₄)₂ + H₂, and the acid is not reduced (cf. HClO₃). It is reduced by sodium hyposulphite (Na₂S₂O₄), titanium trichloride, or in alkaline solution by ferrous hydroxide. It is therefore a much less powerful oxidising agent than chloric acid. Silver perchlorate is peculiar in being a metallic salt which is soluble in toluene.

Protein crystallography
Chemistry guide
Oxides And Oxy-acids Of Chlorine
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