Modern inorganic chemistry

GROUP V! 263

absorption of this light in the process prevents it from reaching the

earth where it would destroy all living matter very rapidly.

Small quantities of ozone are produced when oxygen and air are

subjected to an electrical discharge and it is, therefore, found in the

neighbourhood of working electrical machines. Probably a small

quantity of atomic oxygen is initially produced; most of this reeom-

bines quickly to give oxygen, O 2 , but a few atoms react to form

ozone:

O 2 + O-»O 3

The ozone molecules also decompose by reaction with atomic

oxygen, so that the actual concentration of ozone is small.

Dry —

oxygen

Platinum electrodes

-Ozonised

oxygen

-Dilute sulphuric

acid

Figure 10.2. Preparation of ozone: Brodie's apparatus

Ozone is formed in certain chemical reactions, including the
action of fluorine on water (p. 323) and the thermal decomposition
of iodic(VII) (periodic) acid. It is also formed when dilute (about 1 M)
sulphuric acid is electrolysed at high current density; at low tem-
peratures the oxygen evolved at the anode can contain as much as
30% ozone.
Ozone is normally produced by the use of a silent electrical dis-
charge and a number of ozonisers have been produced. Brodie's
apparatus is shown in outline in Figure 10.2.
Using a potential of approximately 20000 V the ozonised oxygen
produced can contain up to 10% ozone and pure ozone can be
obtained by liquifaction of the mixture followed by fractional distil-
lation (O 2 , b.p. 90 K; O 3 , b.p. 161 K).