Ozone : Ozone, Preparation

The most convenient method of preparing ozonised oxygen is by the action of a silent electric discharge on oxygen, preferably dry. Many types of apparatus are used, all very similar in principle. One of the simplest

Fig: Brodies ozoniser

is that of Brodie (1872).

Expt. 3. - The oxygen passes slowly through the annular space between two glass tubes, the inner tube filled with dilute copper sulphate solution, and the whole apparatus placed in a jar of the same liquid. The wires from a coil dip into the liquids, which form electrodes and at the same time serve to cool the apparatus. A bluish-violet glow is seen in the glass surfaces, usually accompanied by a hissing noise; there should be very few sparks, as these destroy ozone. The gas is conducted away through glass tubes with ground-glass joints, or joints made with ordinary corks or paraffin wax. Rubber is quickly destroyed by ozone, dry cork is more resistant. Air may be used instead of oxygen, but less ozone is obtained, and oxides of nitrogen may be formed.

The original ozoniser of Siemens (1858) consists

Fig: Siemens ozoniser

of two concentric glass tubes, the outer covered, and the inner lined, with tinfoil, but Brodie's is superior in many ways. By cooling the whole apparatus at 0°, using a powerful coil, and avoiding sparks, as much as 25 per cent, by weight of the oxygen may be converted into ozone; usually the yield is less.Warburg found that ultraviolet light of wave-length 209 mμ, produces ozone, and since the line 185 mμ, is the only one from the mercury lamp strongly absorbed by oxygen it is probably the chemically active one, corresponding with the energy 165 kcal. This could dissociate the O₂ molecule into normal atoms (118 kcal.). Warburg found that two molecules of ozone are formed per absorbed quantum. It may be assumed that the formation of ozone, both by the action of the electric discharge and of ultraviolet light on oxygen, involves the dissociation of the oxygen molecules into atoms, which then react with oxygen molecules to form ozone: O₂=O+O; O₂+O=O₃.

This agrees with the quantum yield found by Warburg if one molecule is dissociated per quantum:

O₂ +hν = 2O.

Expt. 4. - Ozonised oxygen is formed by the electrolysis of sulphuric acid (sp. gr. 1.1). The apparatus is shown in.

Fig: Ozone from sulphuric acid by electrolysis

A very good yield is obtained with a heavy current and an anode (positive electrode) composed of a narrow platinum tube coated with glass, having a narrow line of metal exposed, and cooled by a stream of calcium chloride solution at -14°.

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Ozone
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