Ozone (O3) is an allotrope of oxygen, the molecule consisting of three oxygen atoms instead of the more stable diatomic O2.
At standard temperature and pressure ozone is a blue gas. Ozone forms a dark blue liquid, below -112 °C, and a dark blue solid, below -193 °C. Ozone is a powerful oxidizing agent, and is unstable, decaying to ordinary oxygen through the reaction:
2O3 → 3O2. This reaction proceeds more rapidly with increasing temperature and increasing pressure.
Ozone has a sharp, pungent odor. It is present in low concentrations throughout the Earth's atmosphere. It is also formed from O2 by electrical discharges, e.g., lightning, and by action of high energy electromagnetic radiation.
Some kinds of electrical equipment generate levels of ozone that a human can easily smell. This is especially true of devices using high voltage, such as television sets and photocopiers. Electric motors using brushes can generate ozone from repeated sparking inside the unit. Large motors, such as those used by elevators or hydraulic pumps, will generate more ozone than smaller motors.
The highest levels of ozone in the atmosphere are in the stratosphere, in a region also known as the ozone layer. Here it filters out much ultraviolet light from the Sun that would be harmful to most forms of life. The standard way to express ozone amounts in the atmosphere is by using Dobson units. Ozone used in industry is measured in ppm (OSHA exposure limits for example), and percent by mass or weight.
Ozone was discovered by Christian Friedrich Schonbein in 1840.
Stratospheric ozone
Ozone is notable for its ability to absorb UV-B radiation. Ozone is created naturally within the ozone layer. Ozone depletion and ozone holes occur because of chlorofluorocarbons and other upper atmospheric contaminants.
Ozone in the earth's atmosphere is generally created by ultraviolet light which breaks apart O2 molecules, creating atomic oxygen. The atomic oxygen then combines with an unbroken molecule, to create O3. Sometimes the individual oxygen atoms will combine with N2 to create a nitrogen oxide; which may be dissociated by visible light to re-create ozone.
When ultraviolet light hits ozone it splits into a molecule of O2 and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle. This cycle can be disrupted by the presence of atomic chlorine, fluorine or bromine in the atmosphere; these elements are found in certain stable compounds, especially chlorofluorocarbons (CFCs) which may find their way to the stratosphere and there be liberated by the action of ultraviolet light on them.
The nitrogen oxide cycle for the formation of ozone can also be broken by the presence of atmospheric water vapor, reducing nitrogen oxides to a more stable form.
Use in industry
Ozone can be used for bleaching substances and for killing bacteria. Many municipal drinking water systems kill bacteria with ozone instead of the more common chlorine. Ozone does not form organochlorine compounds, but it also does not remain in the water after treatment.
Industrially, ozone or ozonated water is used to:
- disinfect water before it is bottled,
- kill bacteria on food-contact surfaces
- scrub yeast and mold spores from the air in food processing plants
- wash fresh fruits and vegetables to kill yeast, mold and bacteria
- chemically attack contaminants in water (iron, arsenic, hydrogen sulfide, nitrites, and complex organics lumped together as "color"),
- provide an aid to flocculation (a process of agglomeration of molecules, which aids in filtration... this is where the iron and arsenic get removed),
- clean and bleach fabrics (the latter use is patented),
- assist in processing plastics to allow adhesion of inks,
- age rubber samples to determine the useful life of a batch of rubber.
Use in medicine
Ozone, along with hypochlorite ions, is naturally produced by white blood cells and the roots of marigolds as a means of destroying foreign bodies. When ozone breaks down it gives rise to oxygen free radicals, which are highly reactive and damage or destroy most organic molecules.
Ozone has a number of medical uses. It can be used to affect the body's antioxidant-prooxidant balance, since the body usually reacts to its presence by producing antioxidant enzymes.
Ozone therapy has blossomed into a thriving field of alternative medicine, and there are a host of successful applications. Ozone treatments are very safe when used correctly.
In the USA ozone therapy is illegal, as the FDA has not approved its use on humans, although it has been used and still is used succesfully in many other countries. Air purifiers which produce "activated oxygen", i.e. ozone, are often sold in the US in spite of FDA regulations.