Ozone : Ozone, Formation

Van Marum in 1785 noticed that the air in the vicinity of an electrical machine in active operation acquired a peculiar smell, and tarnished mercury. Cruickshank in 1801 observed the same smell in electrolytic oxygen, but the fact that the odour was due to a peculiar gas was only recognised in 1840 by Schonbein, who gave the substance the name ozone (Greek ozo, I smell). He found that it is also produced by the slow oxidation of phosphorus in moist air, and is capable of liberating iodine from potassium iodide.

Expt. 1. - Place a few sticks of freshly scraped phosphorus in a stoppered bottle with a little water. When the fumes have subsided, introduce a piece of paper dipped into a solution of potassium iodide and starch ("starch-iodide paper"). This is at once turned blue. The peculiar smell of the gas is also noticeable. The ozonisation is most pronounced at 24°; below 6° no action occurs, except under reduced pressure. A greenish, phosphorescent, light, which can be seen in the dark, accompanies the formation of ozone.

Ozone is said to occur in traces in country, especially sea, air, but many of the effects attributed to ozone are doubtless caused by hydrogen peroxide, or oxides of nitrogen. There is some spectroscopic evidence for the existence of ozone in the upper atmosphere, where it may be formed by the action of ultra-violet light on oxygen. It has been stated that the maximum amount of ozone in the air never exceeds 1 in 10⁷. The evaporation of salt water in the form of spray is said to produce the ozone of sea air. If present in larger amounts than 1 in 20,000, ozone in air has an irritant action on the mucous membrane, and is poisonous.

Ozone is contained in electrolytic oxygen and in the oxygen evolved by the action of fluorine on water, or by the action of concentrated sulphuric acid on barium peroxide. It is produced by heating crystalline periodic acid, by passing oxygen over heated manganese dioxide, by the action of radium salts on oxygen, and by heating ammonium persulphate with nitric acid.

Expt. 2. - Stir a little barium peroxide with concentrated sulphuric acid. The smell of ozone is perceptible.

Ozone is formed in traces in flames of burning hydrogen or coal-gas, but not by the combustion of carbon or carbon monoxide, it was supposed to be formed by the slow combustion of ether vapour on glowing platinum wire, but the substance produced is probably hydrogen peroxide. Ozone is formed in fairly large quantities when oxygen (or air) is exposed to ultra-violet light. If a quartz mercury lamp is operated under a glass bell-jar for a few minutes, the air in the jar smells strongly of ozone. This gas does not contain oxides of nitrogen. Liquid oxygen exposed to ultra-violet light becomes dark blue, owing to the production of liquid ozone.

In all cases, ozone is obtained mixed with oxygen in varying amounts: the product is ozonised oxygen (or ozonised air).

Protein crystallography
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Ozone
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