an allotropic form of oxygen. Melting point, — 192.7°C; boiling point, — 112°C. Ozone is an explosive blue gas with a characteristic pungent odor. Unlike the diatomic molecule of ordinary oxygen (O2), the ozone molecule is triatomic (O3). Ozone was first discovered in 1785 by the Dutch physicist M. van Marum, who determined that air acquires characteristic oxidizing properties and a fresh odor after electric sparks are passed through the medium. One liter of ozone weighs 2.1445 g under normal conditions. Gaseous ozone is 1.5 times as dense as oxygen and 1.62 times as dense as air. At temperatures below the boiling point, ozone converts to a dark blue liquid with a density of 1.71 g/cm3 at — 183°C. It is diamagnetic in the gaseous state and slightly paramagnetic in liquid form. Ozone has a water solubility of 0.394 g/l at 0°C, which is 15 times higher than the solubility of oxygen. Ozone forms in the reversible reaction 3O2 + 68 kilocalories (285 kilojoules) ⇆ 2O3. The ozone molecule, O3, is unstable and undergoes spontaneous conversion into O2 with the liberation of heat energy. Small concentrations of pure ozone decompose gradually; the decomposition of large concentrations is marked by an explosion. The heating and subsequent interaction of ozone with small quantities of organic substances, certain metals, or metal oxides significantly accelerate the conversion process. Inversely, the presence of small quantities of HNO3 stabilizes ozone. Storage in containers made of glass, certain plastics, or pure metals at a constant temperature of —78°C also nearly prevents ozone decomposition.
Ozone is one of the strongest oxidizing agents—considerably stronger than O2. It is capable of oxidizing most elements, including all metals except gold and the platinum metals. Ozonides form upon interaction of ozone with certain inorganic and organic compounds. The presence of ozone in a gas mixture can be determined by the reaction
O3 + 2KI + H2O = I2 + O2 + 2KOH
O2 does not react with KI.
Ozone forms in processes that are accompanied by the liberation of atomic oxygen, for example, the decomposition of peroxides and phosphorus oxidation. It is commercially prepared in ozonizers by passing air or oxygen through a silent electrical discharge at low temperatures. Ozone and diatomic oxygen are easily distinguished, since O3 liquefies more readily than O2.
Because of its strong oxidizing properties, ozone is used to synthesize many organic substances and to bleach paper and oils. Its destructive effect on microorganisms makes it a highly suitable disinfectant for water and air; the disinfection process is called ozonization. Since ozone is extremely toxic (more so than carbon monoxide), the maximum permissible concentration of O3 in the air is 10-5 percent.