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Ozone, from the Greek word ozein, means to smell. It is a pale blue, highly poisonous gas with a strong odor. Ozone is considered a pollutant at ground level, but the ozone layer of the upper atmosphere protects life on Earth from the Suns harmful ultraviolet radiation.

Ozone is one of three forms, called allotropes, of the element oxygen. Ozone is triatomic, meaning that it has three atoms in each molecule (formula O3). Ordinary, or diatomic, oxygen (O2) is more stable than ozone and accounts for the bulk of oxygen in the atmosphere. Electrical sparks and ultraviolet light can cause ordinary oxygen to form ozone. The presence of ozone sometimes causes a detectable odor near electrical outlets.


Chemical Names:                     Ozone, triatomic oxygen, O3

Other Name:                        Trioxygen

CAS Number:                   100028-15-6

Other Codes:                      NIOSH RTECS #RS8225000


At normal temperatures and pressures, ozone is a bluish gas with a specific gravity of 2.144 (about 1.5 times the density of ordinary oxygen gas). It has a characteristic pungent odor that is detectable at concentrations as low as 0.02 to 0.05 ppm. At greater concentrations it is irritating to eyes and the respiratory tract and at high concentrations ozone may be fatal.

Atmosphere at sea level contains an ozone concentration of about 0.05 ppm. In cities with smog conditions ozone concentration may reach 0.5 ppm or higher at times. The gas condenses to a liquid at -111.9C (-169.52F) and freezes at -192.5C (-314.5F). Liquid ozone is deep blue, and is diamagnetic (repelled by magnetic fields). Solid ozone is dark purple.

Ozone is much more active chemically than ordinary oxygen. It is used in purifying water, sterilizing air, and bleaching certain foods. Ozone decomposes spontaneously in water. The reaction generates hydroxyl free radicals, which are very reactive oxidizing agents but have a half-life of microseconds. In aqueous solution, ozone can react by direct oxidation of compounds or can oxidize compounds by hydroxyl free radicals that are produced during ozone decomposition.


The solubility of ozone depends on the water temperature and the ozone concentration in the gas phase: 

O3 Gas

Water Temperature

5 o C

10 o C

15 o C

20 o C
















If oxidizable chemicals are present in the water, larger amounts of ozone will dissolve to satisfy the demand. One limiting factor is the efficiency of the mass transfer device used. Higher concentrations of Ozone in water cause more vigorous oxidation of even more resistant organic compounds.

Formation of Ozone

Ozone is usually formed by combining an oxygen molecule with an oxygen atom in an endothermic reaction. Naturally occurring ozone is produced in the outer atmosphere by the photoreaction of solar ultraviolet (UV) radiation on oxygen. At ground level, ozone may be produced by reactions caused by changes in entropy, e.g. water falling on rocks in a waterfall. Ozone is also produced by photoreactions with nitrogen oxides (NOx) and volatile organic compounds (VOC) from industrial emissions, vehicles and other sources.

Because ozone is unstable it is generated at the point of use. It can be generated by irradiating oxygen-containing gas with UV light and other technologies but the primary industrial method is by the corona discharge method. The oxygen containing gas is passed through two electrodes separated by a dielectric and a discharge gap. When voltage is applied to the electrodes, electrons flow across the gap and provide energy for the disassociation of oxygen molecules, which leads to the formation of ozone.

There are generally four system components to an ozone generating process: a power source or ozone generator, a gas source, an ozone delivery system and an off-gas destruction system. The gas source may be air, high purity oxygen or a combination of the two. Air feed systems are more complicated than liquid oxygen feed systems because the air must be clean, dry, free of contaminants and with a maximum dew point of -60o C to prevent damage to the generator.


Ozone formation

Ozone has an endothermic standard free energy of formation (+163 kJ mol-1). Ozone formation can be initiated by electrical discharge, for instance, that faint garlicy aroma around the back of televisions, laser printers, or other electronic equipment indicates ozone is being produced. It can also be generated photolytically- by the action of light (hv in the first equation above)- cleaving an oxygen molecule. The oxygen radical formed then quickly reacts with an oxygen molecule to form ozone (see the second reaction above).

The ozone molecule is not particularly stable, and can be easily thermolytically or photolytically cleaved. Light energy splits an oxygen radical from ozone as indicated in the third reaction. This radical can then attack another ozone molecule to form two oxygen molecules (see the fourth reaction).

                        energy of ozone formation

The wavelength associated with the photolytic formation and degradation of ozone lies in the ultraviolet range of the electromagnetic spectrum. According to the first reaction, oxygen molecules can absorb the "UVC" end of the ultraviolet spectrum and the ozone molecules (see the third reaction) are cleaved by absorbing "UVB" radiation.





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