Ozone : Ozone, Density

Soret, in a second research (1868), made use of Graham's law of diffusion. If ozone has the formula O₃ (density 24) it should diffuse rather more slowly than carbon dioxide, but more rapidly than chlorine. The diffusion rates are inversely proportional to the square roots of the densities:

;

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In order to get over the difficulty of the dilution of ozone with oxygen, Soret allowed the gases to diffuse into pure oxygen and measured the relative diffusion, v/V, of each gas mixed with oxygen, where v is the volume of gas diffusing and V the total volume present in the original mixture. The rate of diffusion of the oxygen in both directions was the same in all cases; the rates of diffusion of the other gases were proportional to the numbers of molecules present in a given volume (measured by F), and inversely proportional to the square roots of the densities. The ratios v/V were therefore inversely proportional to the square roots of the densities of the diffusing gases.

The apparatus

Fig: Density of ozone by diffusion (Soret)

consisted of three glass tubes, B, B', and C, placed over sulphuric acid in E, and separated by sliding glass plates with holes, so that the tubes could be put in communication or separated. B' was in every case filled with pure oxygen. B was first full of acid, and the mixture of one of the gases with oxygen, prepared in C, was transferred to B by sliding the glass partition o. The glass plates between B and B' had perforations, which could be brought between the two cylinders by sliding the plate o'. Diffusion from B to B' was allowed to go on for forty-five minutes, when the plate o' was slid back and the cylinders again isolated. The gas in B' could then be driven out into a solution of baryta, when carbon dioxide was diffused, or potassium iodide, for chlorine or ozone. The ratio of the ozone in the original gas and in the gas in B' was determined from the ratio of the amounts of iodine liberated by the gases. If u, u' are the amounts of iodine liberated by the gas in B', and that remaining in B, respectively, then v/V = u/(u+u'). The relative rates of diffusion were found to be: chlorine, 0.227; ozone, 0.271; carbon dioxide, 0.290.

The ratio of these values for ozone and chlorine is 227/271 = 0.838. The inverse ratio of the square roots of the densities, assuming that ozone is O₃, is √(24/35.5) = 0.822. The diffusion ratio for carbon dioxide and ozone is 271/290 = 0.93, whilst the inverse ratio of the square roots of the densities, again assuming O₃ as the formula of ozone, is √(22/24) = 0.95. The agreement is to 3 per cent., which is satisfactory, as the ozonised oxygen contained only 5 per cent, of ozone by volume.

In 1898 Ladenburg obtained nearly pure ozone by the evaporation of the liquid and compared the times of effusion of equal volumes of this gas and of oxygen in a Bunsen's effusion apparatus; he found 430 secs, and 367.4 sees., respectively. The squares of the times of equal effusion are proportional to the densities, hence 430²:367.4² = x:16, or x = 22. Since the gas contained a little oxygen, which would make the density lower, this result is sufficiently near the value 24 to confirm the formula O₃.

The formula O₃ for ozone was completely established by a determination of the vapour density of pure ozone by Dumas' method. The value 24 was found (Riesenfeld, 1922). All other formulae were shown to be excluded.

Ozone is an allotropic modification of oxygen. The cause of allotropy lies in the different molecular complexities: the molecule of ordinary oxygen, O₂, contains two atoms, whilst ozone contains three atoms of oxygen in the molecule, O₃. Ozone is a polymer of oxygen; the property of a substance existing in two or more forms of different molecular weights is called polymerism.

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