Physical Properties of Ozone

The characteristic odour of ozone can always be observed in the atmosphere, near electrical apparatus working with a high voltage; indeed, so penetrating is the odour that it will betray the presence of one part in 2½ millions of air. As already mentioned, it was this odour that led Schonbein to suggest the name ozone for the gas.

The blue colour of ozone is easily perceptible in the gas obtained by evaporation of the liquid and can also be detected in oxygen containing only 10 per cent, of ozone if layers of at least 1 metre in depth are examined. Ozone exerts so marked an absorption for ultra-violet light, especially in the neighbourhood of 258 μμ, as to allow the amount of ozone in ozonised oxygen to be determined photo-electrically.

Ozonised oxygen is appreciably denser than pure oxygen. A litre of ozone at N.T.P. would weigh 2.1445 grams, whereas oxygen weighs only 1.4289. Its density, therefore, is 1.50, where that of oxygen is taken as unity. The contraction accompanying the formation of ozone from oxygen was first observed by Andrews and Tait in 1860. The higher density of ozone is indicated by the relative velocities of diffusion of pure oxygen and ozonised oxygen, the results of Soret and of Ladenburg obtained with a mixture of known ozone content indicating for ozone a vapour density 1.5 times that of oxygen.

Ozone is distinctly more soluble in water than oxygen, but it has not been found possible to obtain concordant figures for the solubility, because in aqueous solution the gas decomposes so rapidly that equilibrium is difficult to attain. Moufang states that 1 litre of water dissolves about 10 mg. of ozone at 2° C., but only 1.5 mg. at 28° C. If this is correct, the solubility curve is remarkably steep.

The solution possesses the characteristic odour and oxidising properties of the gas. In dilute acid solution ozone is much more stable and the absorption coefficient in decinormal sulphuric acid has been experimentally determined as 0.487 at 0.

The " ozone water " of commerce usually contains no ozone. Its activity is due to such substances as hypochlorites, etc.

Carbon tetrachloride dissolves approximately seven times as much ozone as does water, and when oxygen containing 6 per cent, of ozone is passed through the former solvent a distinctly blue solution is easily obtained.

The specific heat ratio has been determined by extrapolation from observed values for mixtures of oxygen and ozone and leads to the value

γ = 1.29.

The magnetic properties of ozone are more marked than those of oxygen.

The gas can be liquefied by cooling, or by combined cooling and pressure, but great care is necessary in working with liquid ozone because in the liquid or compressed state the substance is exceedingly unstable, and tends to explode if the pressure is suddenly reduced or the temperature suddenly raised; contact with grease or organic matter generally also may cause liquid ozone to explode.

Liquid ozone has a deep blue, almost black, colour, and is opaque in layers exceeding 2 mm. in thickness. It boils under ordinary pressure at -112.4° C.; the considerable difference between the boiling-points or liquefying-points of ozone and oxygen supplies a convenient method for separating the former from a mixture of the two gases, oxygen remaining uncondensed at temperatures far below the temperature of liquefaction of ozone.

On cooling liquid ozone in liquid hydrogen, it solidifies in violet-black crystals, melting at -249.7° C. Its critical temperature is -5° C.; critical density 0.537; critical pressure 64.8 atmospheres. Its density at the boiling-point is 1.46.