Ozone : Ozone, Properties

Ozonised oxygen usually does not contain more than 15 per cent, by volume of ozone. If cooled by passing through a tube immersed in liquid oxygen, a deep-blue liquid separates. On reducing the pressure, evaporation takes place, and the liquid separates into an upper, deep blue layer, which is a solution of ozone in liquid oxygen, and a lower, violet-black layer, which is a solution of oxygen in liquid ozone. The oxygen may be pumped off from the strongly cooled lower layer, and pure liquid ozone, b. pt. -112.4°, obtained. On careful evaporation this gives a deep-blue gas, containing 100 per cent, of ozone. The liquid is fairly stable below its boiling point and may be distilled in the entire absence of dust or organic matter, the least trace of which, however, brings about its explosive decomposition. The gas is fairly stable in the absence of catalysts, but explodes if heated or brought in contact with organic matter. On cooling the liquid in liquid hydrogen, violet-black crystals of solid ozone, m. pt. -249.7°, are formed. The critical temperature of ozone is -5°.

The decomposition of ozone in admixture with oxygen is slow at low temperatures: it is almost instantaneous at 300°, and takes place according to the equation 2O₃ = 3O₂. It is accompanied by phosphorescence.

Moisture slightly accelerates the decomposition: reduced pressure, chlorine, oxides of nitrogen, and phosphorus pentoxide, rapidly accelerate it.

Ozone is more soluble than oxygen in water: 1 vol. of water at 0° dissolves 0.49 vols. of ozone. It is more soluble in glacial acetic acid, or carbon tetrachloride, than in water, forming blue solutions. It produces a remarkable effect on mercury: the meniscus of the latter is destroyed, and the metal adheres to glass. On shaking with water, the mercury recovers its original form. This reaction is due to oxidation to Hg₂O, which dissolves in mercury (H. B. Baker).

Ozone is catalytically decomposed in contact with metallic silver, platinum, and palladium, and with oxides of manganese, cobalt, iron, lead, and silver, and by shaking it with powdered glass. In the case of silver the metal, if warm, is blackened and an oxide is probably alternately formed and reduced:

2Ag + O₃ = Ag₂O + O₂; Ag₂O + O₃ = 2Ag + 2O₂.

Barium and hydrogen peroxides react with ozone: BaO₂ + O₃ = BaO + 2O₂, but the gas has no action on chromic acid or potassium permanganate (cf. H₂O₂). Sulphur dioxide is oxidised to the trioxide, the ozone being completely absorbed (Brodie): 3SO₂ + O₃ = 3SO₃. This is one of the few reactions in which the ozone molecule oxidises as a whole; another is 3SnCl₂ + 6HCl + O₃ = 3SnCl4 + 3H₂O.

Ozone, containing more energy than oxygen, is a powerful oxidising agent; it bleaches indigo solution and vegetable colours, and converts moist sulphur, phosphorus, and arsenic into their highest oxy-acids. It liberates halogens from their hydracids: (1) 2HCl + O₃ = Cl₂ + H₂O + O₂; (2) 10HI + 4O₃ = 5I₂ + 4H₂O + H₂O₂ + 3O₂ (acidified potassium iodide solution). Ammonia is oxidised to ammonium nitrite and nitrate; a solution of potassium ferrocyanide is oxidised to ferricyanide:

2K₄FeC₆N₆ + H₂O + O₃ = 2K₃FeC₆N₆ + 2KOH + O₂.

The liberation of iodine from potassium iodide constitutes a test for ozone, although iodine is liberated by other oxidising agents (e.g., H₂O₂, chlorine and bromine, and higher oxides of nitrogen). The reaction: O₃ + 2KI + H₂O = O₂ + I₂ + 2KOH, occurs in a neutral solution, which then becomes alkaline. In presence of alkali, the iodine is partly converted into iodide and iodate, but is wholly liberated again on acidifying:

3I₂ + 6KOH = 5KI + KIO₃ + 3H₂O;

5KI + KIO₃ + 3H₂SO₄ = 3K₂SO₄ + 3I₂ + 3H₂O.

Moist iodine is oxidised to iodic acid, HIO₃: I₂ + 5O₃ + H₂O = 2HIO₃ + 5O₂. Dry iodine is converted into a yellow powder, I4O9, without change of volume of the gas: 2I₂ + 9O₃ = I4O9 + 9O₂. An alkaline solution of potassium iodide is oxidised to iodate (KIO₃), and periodate (KIO₄).

Carbon compounds containing double linkages add on ozone to form unstable ozonides, which_ are decomposed by water with the formation of hydrogen peroxide:

H₂C:CH₂ + O₃ = ethylene ozonide

-> 2

+ H₂O₂

This reaction, in which compounds probably containing a chain of three oxygen atoms, -O-O-O-, are produced, points to the structural formula

ozone

for ozone. The readiness with which one atom of oxygen is split off, leaving a residue of oxygen gas, O₂, led to the assumption

that one atom in the ozone molecule was quadrivalent: O = O^IV = O, but the simpler formula is more probable; it also agrees with the formula of hydrogen peroxide.

An aqueous solution of ozone reddens litmus paper before bleaching it, and has been supposed to contain ozonic acid,

. By the action of ozone on solid caustic potash a yellow peroxide, K₂O₄, is obtained. This was regarded by Baeyer and Villiger as potassium ozonate, but on acidification it does not give ozone but oxygen and hydrogen peroxide, H₂O₂.

Protein crystallography
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Ozone
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