Ozone: Encyclopedia - Ozone

Ozone

Ozone (O₃) is an allotrope of oxygen, the molecule consisting of three oxygen atoms instead of the more stable diatomic O₂.

Ozone is a pale blue gas at standard temperature and pressure. It forms a dark blue liquid below -112 °C and a dark blue solid below -193 °C. Ozone is a powerful oxidizing agent. It is also unstable, decaying to ordinary oxygen through the reaction:

2O₃ → 3O₂. This reaction proceeds more rapidly with increasing temperature and decreasing pressure.

Ozone is a highly corrosive, poisonous substance and a common pollutant. It 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 such as 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 voltages, such as television sets, laser printers, 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.

Ozone - Ozone layer

See main article: Ozone layer.

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 the shorter wavelengths (less than 320 nm) of ultraviolet light (270 to 400 nm) from the Sun that would be harmful to most forms of life in large doses. These same wavelengths are also responsible for the production of vitamin D, which is essential for human health. The standard way to express total 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 - Discovery of ozone

Ozone was discovered by Christian Friedrich Schönbein in 1840, who named it after the Greek word for smell (ozein), from the peculiar odor in lightning storms. [1]. Even so, the odor from a lightning strike is usually from electrons freed during the rapid chemical changes, not the ozone itself. [2].

Ozone - Industrial production

Industrially, ozone is produced with short wavelength ultraviolet radiation from a mercury vapor lamp or the application of a high voltage electrical field in a process called cold discharge. The cold discharge apparatus consists of two metal plates separated by an air gap and a high dielectric strength electrical insulator such as borosilicate glass or mica. A high voltage alternating current is applied to the plates and the ozone is formed in the air gap when O₂ molecules disassociate and recombine into O₃. A faint corona may be present in the air gap, but the voltage is maintained below that which would cause punch-through of the insulator with subsequent arcing and plasma formation. In the laboratory ozone can be produced by electrolysis using a 9 volt battery, a pencil graphite rod cathode, a platinum wire anode and a 3M sulfuric acid electrolyte ^[3]. The half cell reactions taking place are:

so that in the net reaction three equivalents of water are converted into one equivalent of ozone and one equivalent of hydrogen. Oxygen formation is a competing reaction.

Ozone - 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, so some systems introduce a small amount of chlorine to prevent bacterial growth in the pipes, or may use chlorine intermittently, based on results of periodic testing. Where electrical power is abundant, ozone is a cost-effective method of treating water, as it is produced on demand and does not require transportation and storage of hazardous chemicals. Once it has decayed, it leaves no taste or odor in drinking water.

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 are 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.

Ozone is a reagent in many organic reactions in the laboratory and in industry. Ozonolysis is the cleavage of an alkene to carbonyl compounds.

Ozone - 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. Many hospitals in the U.S. and around the world use large ozone generators to decontaminate operating rooms between surgeries. The rooms are cleaned and then sealed airtight before being filled with ozone which effectively kills or neutralizes all remaining bacterium.

Ozone therapy has blossomed into a thriving field of alternative medicine, and there are a host of claimed applications above and beyond what has actually been verified by studies. Ozone treatments are dangerous, however, since ozone is highly corrosive.

In the United States ozone therapy is illegal, as the Food and Drug Administration (FDA) has not approved its use on humans. At least one death has been attributed to application of ozone through insufflation in the U.S. "Air cleaners" which produce "activated oxygen", i.e., ozone, are often sold in the U.S. nonetheless. See Air ioniser.

Ozone - Air pollution

See main articles: Tropospheric ozone and Air pollution.

Ozone is not directly emitted by car engines or by industrial operations themselves. These sources emit hydrocarbons and nitrogen oxides that react with sunlight to form ozone directly at the source of the pollution being emitted and in the atmosphere's boundary layer (1 to 3 km altitude). The mix of hydrocarbons, nitrogen oxides, and ozone are the major components of smog that frequently occurs in urban and suburban areas. Recent satellite maps of nitrogen dioxide (NO₂) clearly show the worldwide distribution of polluted regions associated with industrial activity (automobiles, factories, and fossil fuel power generation).

There is a great deal of evidence to show that ozone at the earth's surface can harm lung function and irritate the respiratory system (WHO Europe reports, cited below). Ozone has been found to convert cholesterol in the blood stream to plaque (which causes hardening and narrowing of arteries). This cholesterol product has also been implicated in Alzheimer's disease, suggesting a link between the inflammatory response associated with head injury and Alzheimer's. Air quality guidelines such as those from the World Health Organization are based on detailed studies of what levels can cause measurable health effects.

Although ozone was present at ground level before the industrial revolution, peak concentrations are far higher than the pre-industrial levels [4] and even background concentrations well away from sources of pollution are substantially higher [5].

Ozone reacts directly with some hydrocarbons such as aldehydes and thus begins their removal from the air, but the products of ozonolysis are themselves key components of smog. Ozone photolysis by UV light leads to production of the hydroxyl radical and this plays a part in the removal of hydrocarbons from the air, but is again a step in the creation of components of smog such as peroxyacyl nitrates which are powerful eye irritants. Ultimately, ozone is one component of smog which is harmful in itself and contributes both to the production and ultimate removal of other air pollutants.

Ozone - Other uses

During the 1992 U.S. Presidential election, George H.W. Bush referred to his opponents Bill Clinton and Al Gore as "Bozo and Ozone", respectively, the latter in connection with Gore's well known stance on environmental issues.

See also

• Ozone depletion, including the phenomenon known as the Ozone Hole.
• Ozone layer
• Tropospheric ozone

Adapted from the Wikipedia article "Ozone", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki