Jump to content

Ozone: Difference between revisions

From Wikipedia, the free encyclopedia
Browse history interactively
← Previous editNext edit →
Content deleted Content added
VisualWikitext
Line 216: Line 216:
====Ozone cracking====
====Ozone cracking====
[[File:Ozone cracks in tube1.jpg|thumb|right|Ozone cracking in [[natural rubber]] tubing]]
[[File:Ozone cracks in tube1.jpg|thumb|right|Ozone cracking in [[natural rubber]] tubing]]
Ozone gas attacks any [[polymer]] possessing olefinic or [[double bond]]s within its chain structure, such as [[natural rubber]], [[nitrile rubber]], and [[styrene-butadiene]] rubber. Products made using these polymers are especially susceptible to attack, which causes cracks to grow longer and deeper with time, the rate of crack growth depending on the load carried by the product and the concentration of ozone in the atmosphere. Such materials can be protected by adding [[antiozonant]]s, such as waxes, which bond to the surface to create a protective film or blend with the material and provide long term protection. [[Ozone cracking]] used to be a serious problem in car tires for example, but the problem is now seen only in very old tires. On the other hand, many critical products like [[gasket]]s and [[O-ring]]s may be attacked by ozone produced within compressed air systems. [[Fuel line]]s are often made from reinforced rubber tubing and may also be susceptible to attack, especially within engine compartments where low levels of ozone are produced from electrical equipment. Storing rubber products in close proximity to [[Direct Current|DC]] [[electric motors]] can accelerate the rate at which ozone cracking occurs. The [[Commutator (electric)|commutator]] of the motor creates sparks which in turn produce ozone. Ozone can be utilized to degrade rubber products.
Ozone gas attacks any [[polymer]] possessing olefinic or [[double bond]]s within its chain structure, such as [[natural rubber]], [[nitrile rubber]], and [[styrene-butadiene]] rubber. Products made using these polymers are especially susceptible to attack, which causes cracks to grow longer and deeper with time, the rate of crack growth depending on the load carried by the product and the concentration of ozone in the atmosphere. Such materials can be protected by adding [[antiozonant]]s, such as waxes, which bond to the surface to create a protective film or blend with the material and provide long term protection. [[Ozone cracking]] used to be a serious problem in car tires for example, but the problem is now seen only in very old tires. On the other hand, many critical products like [[gasket]]s and [[O-ring]]s may be attacked by ozone produced within compressed air systems. [[Fuel line]]s are often made from reinforced rubber tubing and may also be susceptible to attack, especially within engine compartments where low levels of ozone are produced from electrical equipment. Storing rubber products in close proximity to [[Direct Current|DC]] [[electric motors]] can accelerate the rate at which ozone cracking occurs. The [[Commutator (electric)|commutator]] of the motor creates sparks which in turn produce ozone.


====Ozone as a greenhouse gas====
====Ozone as a greenhouse gas====

Revision as of 17:49, 25 June 2010

For other uses, see Ozone (disambiguation).
Ozone
Names
IUPAC name
Trioxygen
Identifiers
ChemSpider
ECHA InfoCard 100.030.051 Edit this at Wikidata
RTECS number
  • RS8225000
  • InChI=1S/O3/c1-3-2
    Key: CBENFWSGALASAD-UHFFFAOYSA-N
  • InChI=1/O3/c1-3-2
    Key: CBENFWSGALASAD-UHFFFAOYAY
Properties
O3
Molar mass 47.998 g·mol−1
Appearance bluish colored gas
Density 2.144 g/L (0 °C), gas
Melting point 80.7 K, −192.5 °C
Boiling point 161.3 K, −111.9 °C
0.105 g/100mL (0 °C)
1.2226 (liquid)
Thermochemistry
237.7 J·K−1.mol−1
+142.3 kJ·mol−1
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Ozone (O3), or Trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope (O2). Ozone in the lower atmosphere is an air pollutant with harmful effects on the respiratory systems of animals and will burn sensitive plants; however, the ozone layer in the upper atmosphere is beneficial, preventing potentially damaging ultraviolet light from reaching the Earth's surface. Ozone is present in low concentrations throughout the Earth's atmosphere. It has many industrial and consumer applications.

History

Ozone, the first allotrope of a chemical element to be recognized, was proposed as a distinct chemical compound by Christian Friedrich Schönbein in 1840, who named it after the Greek verb ozein (ὄζειν, "to smell"), from the peculiar odor in lightning storms.[1][2] The formula for ozone, O3, was not determined until 1865 by Jacques-Louis Soret[3] and confirmed by Schönbein in 1867.[1][4]

Physical properties

Ozone is a pale blue gas, slightly soluble in water and much more soluble in inert non-polar solvents such as carbon tetrachloride or fluorocarbons, where it forms a blue solution. At –112 °C, it condenses to form a dark blue liquid. It is dangerous to allow this liquid to warm to its boiling point, because both concentrated gaseous ozone and liquid ozone can detonate. At temperatures below –193 °C, it forms a violet-black solid.[5]

Most people can detect about 0.01 ppm of ozone in air where it has a very specific sharp odor somewhat resembling chlorine bleach. Exposure of 0.1 to 1 ppm produces headaches, burning eyes, and irritation to the respiratory passages.[6] Even low concentrations of ozone in air are very destructive to organic materials such as latex, plastics, and animal lung tissue.

Ozone is diamagnetic, which means that its electrons are all paired. In contrast, O2 is paramagnetic, containing two unpaired electrons.

Structure

According to experimental evidence from microwave spectroscopy, ozone is a bent molecule, with C2v symmetry (similar to the water molecule). The O – O distances are 127.2 pm. The O – O – O angle is 116.78°.[7] The central atom is sp² hybridized with one lone pair. Ozone is a polar molecule with a dipole moment of 0.5337 D.[8] The bonding can be expressed as a resonance hybrid with a single bond on one side and double bond on the other producing an overall bond order of 1.5 for each side.

Resonance Lewis structures of the ozone molecule

Reactions

Ozone is a powerful oxidizing agent, far stronger than O2. It is also unstable at high concentrations, decaying to ordinary diatomic oxygen (with a half-life of about half an hour in atmospheric conditions):[9]

2 O3 → 3 O2

This reaction proceeds more rapidly with increasing temperature and increased pressure. Deflagration of ozone can be triggered by a spark, and can occur in ozone concentrations of 10 wt% or higher.[10]

With metals

Ozone will oxidize most metals (except gold, platinum, and iridium) to oxides of the metals in their highest oxidation state. For example:

2 Cu+ + 2 H3O+ + O3 → 2 Cu2+ + 3 H2O + O2

With nitrogen and carbon compounds

Ozone also oxidizes nitric oxide to nitrogen dioxide:

NO + O3 → NO2 + O2

This reaction is accompanied by chemiluminescence. The NO2 can be further oxidized:

NO2 + O3 → N2O3 + O2

The N2O3 formed can react with NO2 to form N2O5:

Solid nitryl perchlorate can be made from NO2, ClO2, and O3 gases:

2 NO2 + 2 ClO2 + 2 O3 → 2 NO2ClO4 + O2

Ozone does not react with ammonium salts but it oxidizes with ammonia to ammonium nitrate:

2 NH3 + 4 O3 → NH4NO3 + 4 O2 + H2O

Ozone reacts with carbon to form carbon dioxide, even at room temperature:

C + 2 O3 → CO2 + 2 O2

With sulfur compounds

Ozone oxidizes sulfides to sulfates. For example, lead(II) sulfide is oxidised to lead(II) sulfate:

PbS + 4 O3 → PbSO4 + 4 O2

Sulfuric acid can be produced from ozone and either elemental sulfur or sulfur dioxide:

S + H2O + O3 → H2SO4
3 SO2 + 3 H2O + O3 → 3 H2SO4

In the gas phase, ozone reacts with hydrogen sulfide to form sulfur dioxide:

H2S + O3 → SO2 + H2O

In an aqueous solution, however, two competing simultaneous reactions occur, one to produce elemental sulfur, and one to produce sulfuric acid:

H2S + O3 → S + O2 + H2O
3 H2S + 4 O3 → 3 H2SO4

Other substrates

All three atoms of ozone may also react, as in the reaction of tin(II) chloride with hydrochloric acid and ozone:

3 SnCl2 + 6 HCl + O3 → 3 SnCl4 + 3 H2O

Iodine perchlorate can be made by treating iodine dissolved in cold anhydrous perchloric acid with ozone:

I2 + 6 HClO4 + O3 → 2 I(ClO4)3 + 3 H2O

Combustion

Ozone can be used for combustion reactions and combusting gases; ozone provides higher temperatures than combusting in dioxygen (O2). The following is a reaction for the combustion of carbon subnitride which can also cause lower temperatures:

3 C4N2 + 4 O3 → 12 CO + 3 N2

Ozone can react at cryogenic temperatures. At 77 K (−196 °C), atomic hydrogen reacts with liquid ozone to form a hydrogen superoxide radical, which dimerizes:[11]

H + O3 → HO2 + O
2 HO2 → H2O4

Reduction to ozonides

Reduction of ozone gives the ozonide anion, O3– . Derivatives of this anion are explosive and must be stored at cryogenic temperatures. Ozonides for all the alkali metals are known. KO3, RbO3, and CsO3 can be prepared from their respective superoxides:

KO2 + O3 → KO3 + O2

Although KO3 can be formed as above, it can also be formed from potassium hydroxide and ozone:[12]

2 KOH + 5 O3 → 2 KO3 + 5 O2 + H2O

NaO3 and LiO3 must be prepared by action of CsO3 in liquid NH3 on an ion exchange resin containing Na+ or Li+ ions:[13]

CsO3 + Na+ → Cs+ + NaO3

A solution of calcium in ammonia reacts with ozone to give to ammonium ozonide and not calcium ozonide:[11]

3 Ca + 10 NH3 + 6 O3 → Ca·6NH3 + Ca(OH)2 + Ca(NO3)2 + 2 NH4O3 + 2 O2 + H2

Applications

Ozone can be used to remove manganese from water, forming a precipitate which can be filtered:

2 Mn2+ + 2 O3 + 4 H2O → 2 MnO(OH)2 (s) + 2 O2 + 4 H+

Ozone will also detoxify cyanides by converting it to cyanate, which is a thousand times less toxic.

CN- + O3 → CNO−
 + O2

Ozone will also completely decompose urea:[14]

(NH2)2CO + O3 → N2 + CO2 + 2 H2O

Ozone will cleave alkenes to form carbonyl compounds in the ozonolysis process.

A generalized scheme of ozonolysis
A generalized scheme of ozonolysis

Ozone in Earth's atmosphere

The distribution of atmospheric ozone in partial pressure as a function of altitude.
Concentration of ozone as measured by the Nimbus-7 satellite.
Total ozone concentration in June 2000 as measured by EP-TOMS satellite instrument.

The standard way to express total ozone levels (the amount of ozone in a vertical column) in the atmosphere is by using Dobson units. Average concentration at a point is measured in parts per billion (ppb) or in μg/m3.

Ozone layer

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 between about 10 km and 50 km above the surface (or between about 6 and 31 miles). Here it filters out photons with shorter wavelengths (less than 320 nm) of ultraviolet light, also called UV rays, (270 to 400 nm) from the Sun that would be harmful to most forms of life in large doses. These same wavelengths are also among those responsible for the production of vitamin D in humans. Ozone in the stratosphere is mostly produced from ultraviolet rays reacting with oxygen:

O2 + photon (radiation < 240 nm) → 2 O
O + O2 → O3

It is destroyed by the reaction with atomic oxygen:

O3 + O → 2 O2

The latter reaction is catalysed by the presence of certain free radicals, of which the most important are hydroxyl (OH), nitric oxide (NO) and atomic chlorine (Cl) and bromine (Br). In recent decades the amount of ozone in the stratosphere has been declining mostly because of emissions of CFCs and similar chlorinated and brominated organic molecules, which have increased the concentration of ozone-depleting catalysts above the natural background. Ozone only makes up 0.00006% of the atmosphere.

See also: Ozone-oxygen cycle and Ozone depletion

Low level ozone

Main articles: Tropospheric ozone and Photochemical smog

Low level ozone (or tropospheric ozone) is an atmospheric pollutant [15]. It is not emitted directly by car engines or by industrial operations, but formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that react to form ozone directly at the source of the pollution or many kilometers down wind.

Ozone reacts directly with some hydrocarbons such as aldehydes and thus begins their removal from the air, but the products are themselves key components of smog. Ozone photolysis by UV light leads to production of the hydroxyl radical OH and this plays a part in the removal of hydrocarbons from the air, but is also the first step in the creation of components of smog such as peroxyacyl nitrates which can be powerful eye irritants. The atmospheric lifetime of tropospheric ozone is about 22 days; its main removal mechanisms are being deposited to the ground, the above mentioned reaction giving OH, and by reactions with OH and the peroxy radical HO2· (Stevenson et al., 2006).[16]

There is evidence of significant reduction in agricultural yields because of increased ground-level ozone and pollution which interferes with photosynthesis and stunts overall growth of some plant species.[17][18]

Certain examples of cities with elevated ozone readings are Houston, Texas, and Mexico City, Mexico. Houston has a reading of around 41 ppb, while Mexico City is far more hazardous, with a reading of about 125 ppb.[18]

Ozone cracking

Ozone cracking in natural rubber tubing

Ozone gas attacks any polymer possessing olefinic or double bonds within its chain structure, such as natural rubber, nitrile rubber, and styrene-butadiene rubber. Products made using these polymers are especially susceptible to attack, which causes cracks to grow longer and deeper with time, the rate of crack growth depending on the load carried by the product and the concentration of ozone in the atmosphere. Such materials can be protected by adding antiozonants, such as waxes, which bond to the surface to create a protective film or blend with the material and provide long term protection. Ozone cracking used to be a serious problem in car tires for example, but the problem is now seen only in very old tires. On the other hand, many critical products like gaskets and O-rings may be attacked by ozone produced within compressed air systems. Fuel lines are often made from reinforced rubber tubing and may also be susceptible to attack, especially within engine compartments where low levels of ozone are produced from electrical equipment. Storing rubber products in close proximity to DC electric motors can accelerate the rate at which ozone cracking occurs. The commutator of the motor creates sparks which in turn produce ozone.

Ozone as a greenhouse gas

Although ozone was present at ground level before the Industrial Revolution, peak concentrations are now far higher than the pre-industrial levels, and even background concentrations well away from sources of pollution are substantially higher.[19][20] This increase in ozone is of further concern because ozone present in the upper troposphere acts as a greenhouse gas, absorbing some of the infrared energy emitted by the earth. Quantifying the greenhouse gas potency of ozone is difficult because it is not present in uniform concentrations across the globe. However, the most widely accepted scientific assessments relating to climate change (e.g. the IPCC Third Assessment Report[21]) suggest that the radiative forcing of tropospheric ozone is about 25% that of carbon dioxide.

Health effects

Air pollution

Red Alder leaf, showing the typical discolouration caused by ozone pollution.[22]

There is a great deal of evidence to show that high concentrations of ozone, created by high concentrations of pollution and daylight UV rays at the Earth's surface, can harm lung function and irritate the respiratory system.[15][23] A connection has also been known to exist between increased ozone caused by thunderstorms and hospital admissions of asthma sufferers.[24] Air quality guidelines such as those from the World Health Organization are based on detailed studies of what levels can cause measurable health effects. Exposure to ozone and the pollutants that produce it has been linked to premature death, asthma, bronchitis, heart attack, and other cardiopulmonary problems. According to scientists with the United States Environmental Protection Agency (EPA), susceptible people can be adversely affected by ozone levels as low as 40 ppb.[25]

The Clean Air Act directs the EPA to set National Ambient Air Quality Standards for several pollutants, including ground-level ozone, and counties out of compliance with these standards are required to take steps to reduce their levels. In May 2008, the EPA lowered its ozone standard from 80 ppb to 75 ppb. This proved controversial, since the Agency's own scientists and advisory board had recommended lowering the standard to 60 ppb, and the World Health Organization recommends 51 ppb. Many public health and environmental groups also supported the 60 ppb standard. On the other hand, the EPA had already designated over 300 mostly urban counties as out of compliance, and lowering the standard to 75 ppb put hundreds more in non-compliance. Lowering it further to 60 ppb would likely have left most of the US in non-compliance. Manufacturers, employers, and others argued that the cost of compliance with the lower standard would be prohibitive.[25] The EPA has also developed an Air Quality Index to help explain air pollution levels to the general public. Eight-hour average ozone concentrations of 85 to 104 ppb are described as "unhealthy for sensitive groups", 105 ppb to 124 ppb as "unhealthy" and 125 ppb to 404 ppb as "very unhealthy".[26]

Ozone can also be present in indoor air pollution, partly as a result of electronic equipment such as photocopiers.

A common British folk myth dating back to the Victorian era holds that the smell of the sea is caused by ozone, and that this smell has "bracing" health benefits.[27] Neither of these is true. The characteristic "smell of the sea" is not caused by ozone but by the presence of dimethyl sulfide generated by phytoplankton, and dimethyl sulfide, like ozone, is toxic in high concentrations.[28]

Long-term exposure to ozone has been shown to increase risk of death from respiratory illness. A study of 450,000 people living in United States cities showed a significant correlation between ozone levels and respiratory illness over the 18-year follow-up period. The study revealed that people living in cities with high ozone levels such as Houston or Los Angeles had an over 30% increased risk of dying from lung disease.[29][30]

Physiology

See also: trioxidane

Ozone, along with reactive forms of oxygen such as superoxide, singlet oxygen, hydrogen peroxide, and hypochlorite ions, is naturally produced by white blood cells and other biological systems (such as the roots of marigolds) as a means of destroying foreign bodies. Ozone reacts directly with organic double bonds. Also, when ozone breaks down to dioxygen it gives rise to oxygen free radicals, which are highly reactive and capable of damaging many organic molecules. Ozone has been found to convert cholesterol in the blood stream to plaque (which causes hardening and narrowing of arteries). Moreover, it is believed that the powerful oxidizing properties of ozone may be a contributing factor of inflammation. The cause-and-effect relationship of how the ozone is created in the body and what it does is still under consideration and still subject to various interpretations, since other body chemical processes can trigger some of the same reactions. A team headed by Dr. Paul Wentworth Jr. of the Department of Chemistry at the Scripps Research Institute has shown evidence linking the antibody-catalyzed water-oxidation pathway of the human immune response to the production of ozone. In this system, ozone is produced by antibody-catalyzed production of trioxidane from water and neutrophil-produced singlet oxygen.[31]

When inhaled, ozone reacts with compounds lining the lungs to form specific, cholesterol-derived metabolites that are thought to facilitate the build-up and pathogenesis of atherosclerotic plaques (a form of heart disease). These metabolites have been confirmed as naturally occurring in human atherosclerotic arteries and are categorized into a class of secosterols termed atheronals, generated by ozonolysis of cholesterol's double bond to form a 5,6 secosterol[32] as well as a secondary condensation product via aldolization.[33]

Ozone has been implicated to have an adverse effect on plant growth: "... ozone reduced total chlorophylls, carotenoid and carbohydrate concentration, and increased 1-aminocyclopropane-1-carboxylic acid (ACC) content and ethylene production. In treated plants, the ascorbate leaf pool was decreased, while lipid peroxidation and solute leakage were significantly higher than in ozone-free controls. The data indicated that ozone triggered protective mechanisms against oxidative stress in citrus."[34]

Safety regulations

Due to the strongly oxidizing properties of ozone, ozone is a primary irritant, affecting especially the eyes and respiratory systems and can be hazardous at even low concentrations. The Canadian Center for Occupation Safety and Health reports that:

"Even very low concentrations of ozone can be harmful to the upper respiratory tract and the lungs. The severity of injury depends on both by the concentration of ozone and the duration of exposure. Severe and permanent lung injury or death could result from even a very short-term exposure to relatively low concentrations." [35]

To protect workers potentially exposed to ozone, OSHA has established a permissible exposure limit (PEL) of 0.1 ppm (29 CFR 1910.1000 table Z-1), calculated as an 8 hour time weighted average. Higher concentrations are especially hazardous and NIOSH has established an Immediately Dangerous to Life and Health Limit (IDLH) of 5 ppm.[36] Work environments where ozone is used or where it is likely to be produced should have adequate ventilation and it is prudent to have a monitor for ozone that will alarm if the concentration exceeds the OSHA PEL. Continuous monitors for ozone are available from several suppliers.

Elevated ozone exposure can occur on passenger airplanes, with levels depending on altitude and atmospheric turbulence.[37] U.S. FAA regulations set a limit of 250 ppb with a maximum four-hour average of 100 ppb.[38] Some planes are equipped with ozone converters in the ventilation system to reduce passenger exposure.[37]

Production

Ozone often forms in nature under conditions where O2 will not react.[6] Ozone used in industry is measured in ppm(parts per million), ppb, μg/m3, mg/hr(milligrams per hour) or weight percent. The regime of applied concentrations ranges from 1 to 5 weight percent in air and from 6 to 14 weight percent in oxygen.

Temperature and humidity plays a large role in how much ozone is being produced. Any ozone machine, when operated in very humid ambient air, will produce up to 50% less ozone than when operated in very dry ambient air.

Corona discharge method

This is the most popular type of ozone generator for most industrial and personal uses. While variations of the "hot spark" coronal discharge method of ozone production exist, including medical grade and industrial grade ozone generators, these units usually work by means of a corona discharge tube.[39] They are typically very cost-effective and do not require an oxygen source other than the ambient air. However, they also produce nitrogen oxides as a by-product. Use of an air dryer can reduce or eliminate nitric acid formation by removing water vapor and increase ozone production. Use of an oxygen concentrator can further increase the ozone production and further reduce the risk of nitric acid formation by removing not only the water vapor, but also the bulk of the nitrogen.

Ultraviolet light

UV ozone generators, or vacuum-ultraviolet (VUV) ozone generators, employ a light source that generates a narrow-band ultraviolet light, a subset of that produced by the Sun. The Sun's UV sustains the ozone layer in the stratosphere of Earth.[40]

While standard UV ozone generators tend to be less expensive,[clarification needed] they usually produce ozone with a concentration of about 0.5% or lower. Another disadvantage of this method is that it requires the air (oxygen) to be exposed to the UV source for a longer amount of time, and any gas that is not exposed to the UV source will not be treated. This makes UV generators impractical for use in situations that deal with rapidly moving air or water streams (in-duct air sterilization, for example). Production of ozone is one of the potential dangers of ultraviolet germicidal irradiation. VUV ozone generators are used in swimming pool and spa applications ranging to millions of gallons of water. VUV ozone generators, unlike corona discharge generators, do not produce harmful nitrogen by-products and also unlike corona discharge systems, VUV ozone generators work extremely well in humid air environments. There is also not normally a need for expensive off-gas mechanisms, and no need for air driers or oxygen concentrators which require extra costs and maintenance.

Cold plasma

In the cold plasma method, pure oxygen gas is exposed to a plasma created by dielectric barrier discharge. The diatomic oxygen is split into single atoms, which then recombine in triplets to form ozone.

Cold plasma machines utilize pure oxygen as the input source and produce a maximum concentration of about 5% ozone. They produce far greater quantities of ozone in a given space of time compared to ultraviolet production. However, because cold plasma ozone generators are very expensive, they are found less frequently than the previous two types.

The discharges manifest as filamentary transfer of electrons (micro discharges) in a gap between two electrodes. In order to evenly distribute the micro discharges, a dielectric insulator must be used to separate the metallic electrodes and to prevent arcing.

Some cold plasma units also have the capability of producing short-lived allotropes of oxygen which include O4, O5, O6, O7, etc. These species are even more reactive than ordinary O3.[citation needed]

Special considerations

Ozone cannot be stored and transported like other industrial gases (because it quickly decays into diatomic oxygen) and must therefore be produced on site. Available ozone generators vary in the arrangement and design of the high-voltage electrodes. At production capacities higher than 20 kg per hour, a gas/water tube heat-exchanger may be utilized as ground electrode and assembled with tubular high-voltage electrodes on the gas-side. The regime of typical gas pressures is around 2 bar absolute in oxygen and 3 bar absolute in air. Several megawatts of electrical power may be installed in large facilities, applied as one phase AC current at 50 to 8000 Hz and peak voltages between 3,000 and 20,000 volts. Applied voltage is usually inversely related to the applied frequency.

The dominating parameter influencing ozone generation efficiency is the gas temperature, which is controlled by cooling water temperature and/or gas velocity. The cooler the water, the better the ozone synthesis. The lower the gas velocity, the higher the concentration (but the lower the net ozone produced). At typical industrial conditions, almost 90% of the effective power is dissipated as heat and needs to be removed by a sufficient cooling water flow.

Because of the high reactivity of ozone, only few materials may be used like stainless steel (quality 316L), titanium, aluminium (as long as no moisture is present), glass, polytetrafluorethylene, or polyvinylidene fluoride. Viton may be used with the restriction of constant mechanical forces and absence of humidity (humidity limitations apply depending on the formulation). Hypalon may be used with the restriction that no water come in contact with it, except for normal atmospheric levels. Embrittlement or shrinkage is the common mode of failure of elastomers with exposure to ozone. Ozone cracking is the common mode of failure of elastomer seals like O-rings.

Silicone rubbers are usually adequate for use as gaskets in ozone concentrations below 1 wt%, such as in equipment for accelerated ageing of rubber samples.

Incidental production

Ozone may be formed from O2 by electrical discharges and by action of high energy electromagnetic radiation. Certain electrical equipment generate significant levels of ozone. This is especially true of devices using high voltages, such as ionic air purifiers, laser printers, photocopiers, tasers and arc welders. Electric motors using brushes can generate ozone from repeated sparking inside the unit. Large motors that use brushes, such as those used by elevators or hydraulic pumps, will generate more ozone than smaller motors. Ozone is similarly formed in the Catatumbo lightning storms phenomenon on the Catatumbo River in Venezuela, which helps to replenish ozone in the upper troposphere. It is the world's largest single natural generator of ozone, lending calls for it to be designated a UNESCO World Heritage Site.[41]

Laboratory production

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 3 molar sulfuric acid electrolyte.[42] The half cell reactions taking place are:

3 H2O → O3 + 6 H+ + 6 e− (ΔEo = −1.53 V)
6 H+ + 6 e− → 3 H2 (ΔEo = 0 V)
2 H2O → O2 + 4 H+ + 4 e− (ΔEo = −1.23 V)

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

It can also be prepared by passing 10,000-20,000 volts DC through dry O2. This can be done with an apparatus consisting of two concentric glass tubes sealed together at the top, with in and out spigots at the top and bottom of the outer tube. The inner core should have a length of metal foil inserted into it connected to one side of the power source. The other side of the power source should be connected to another piece of foil wrapped around the outer tube. Dry O2 should be run through the tube in one spigot. As the O2 is run through one spigot into the apparatus and 10,000-20,000 volts DC are applied to the foil leads, electricity will discharge between the dry dioxygen in the middle and form O3 and O2 out the other spigot. The reaction can be summarized as follows:[6]

3 O2 — electricity → 2 O3

Ionic air purifiers

Some air purifiers create ozone.[43]

Applications

Industry

The largest use of ozone is in the preparation of pharmaceuticals, synthetic lubricants, and many other commercially useful organic compounds, where it is used to sever carbon-carbon bonds.[6] It can also be used for bleaching substances and for killing microorganisms in air and water sources.[44] Many municipal drinking water systems kill bacteria with ozone instead of the more common chlorine.[45] Ozone has a very high oxidation potential.[46] Ozone does not form organochlorine compounds, nor does it remain in the water after treatment. The Safe Drinking Water Act mandates that these systems introduce an amount of chlorine to maintain a minimum of 0.2 ppm residual free chlorine in the pipes, based on results of regular testing. Where electrical power is abundant, ozone is a cost-effective method of treating water, since 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.

Although low levels of ozone have been advertised to be of some disinfectant use in residential homes, the concentration of ozone in dry air required to have a rapid, substantial effect on airborne pathogens exceeds safe levels recommended by the U.S. Occupational Safety and Health Administration and Environmental Protection Agency. Humidity control can vastly improve both the killing power of the ozone and the rate at which it decays back to oxygen (more humidity allows more effectiveness). Spore forms of most pathogens are very tolerant of atmospheric ozone in concentrations where asthma patients start to have issues.

Industrially, ozone is used to:

  • Disinfect laundry in hospitals, food factories, care homes etc;[47]
  • Disinfect water in place of chlorine[6]
  • Deodorize air and objects, such as after a fire. This process is extensively used in fabric restoration
  • Kill bacteria on food or on contact surfaces;[48]
  • Sanitize swimming pools and spas
  • Kill insects in stored grain[49]
  • Scrub yeast and mold spores from the air in food processing plants;
  • Wash fresh fruits and vegetables to kill yeast, mold and bacteria;[48]
  • Chemically attack contaminants in water (iron, arsenic, hydrogen sulfide, nitrites, and complex organics lumped together as "colour");
  • Provide an aid to flocculation (agglomeration of molecules, which aids in filtration, where the iron and arsenic are removed);
  • Manufacture chemical compounds via chemical synthesis[50]
  • Clean and bleach fabrics (the former use is utilized in fabric restoration; 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;
  • Eradicate water borne parasites such as Giardia lamblia and Cryptosporidium in surface water treatment plants.

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

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 bacteria.[51]

Ozone is used as an alternative to chlorine or chlorine dioxide in the bleaching of wood pulp.[52] It is often used in conjunction with oxygen and hydrogen peroxide to eliminate the need for chlorine-containing compounds in the manufacture of high-quality, white paper.[53]

Ozone can be used to detoxify cyanide wastes (for example from gold and silver mining) by oxidizing cyanide to cyanate and eventually to carbon dioxide.[54]

Consumers

Devices generating high levels of ozone, some of which use ionization, are used to sanitize and deodorize uninhabited buildings, rooms, ductwork, woodsheds, and boats and other vehicles.

In the U.S., air purifiers emitting low levels of ozone have been sold. This kind of air purifier is sometimes claimed to imitate nature's way of purifying the air[55] without filters and to sanitize both it and household surfaces. The United States Environmental Protection Agency (EPA) has declared that there is "evidence to show that at concentrations that do not exceed public health standards, ozone is not effective at removing many odor-causing chemicals" or "viruses, bacteria, mold, or other biological pollutants." Furthermore, its report states that "results of some controlled studies show that concentrations of ozone considerably higher than these [human safety] standards are possible even when a user follows the manufacturer’s operating instructions."[56] The government successfully sued one company in 1995, ordering it to stop repeating health claims without supporting scientific studies.

Ozonated water is used to launder clothes and to sanitize food, drinking water, and surfaces in the home. According to the U.S. Food and Drug Administration (FDA), it is "amending the food additive regulations to provide for the safe use of ozone in gaseous and aqueous phases as an antimicrobial agent on food, including meat and poultry." Studies at California Polytechnic University demonstrated that 0.3 ppm levels of ozone dissolved in filtered tapwater can produce a reduction of more than 99.99% in such food-borne microorganisms as salmonella, E. coli 0157:H7, and Campylobacter. This quantity is 20,000 times the WHO recommended limits stated above.[48][57] Ozone can be used to remove pesticide residues from fruits and vegetables.[58][59]

Ozone is used in homes and hot tubs to kill bacteria in the water and to reduce the amount of chlorine or bromine required by reactivating them to their free state. Since ozone does not remain in the water long enough, ozone by itself is ineffective at preventing cross-contamination among bathers and must be used in conjunction with halogens. Gaseous ozone created by ultraviolet light or by corona discharge is injected into the water.[60]

Ozone is also widely used in treatment of water in aquariums and fish ponds. Its use can minimize bacterial growth, control parasites, eliminate transmission of some diseases, and reduce or eliminate "yellowing" of the water. Ozone must not come in contact with fish's gill structures. Natural salt water (with life forms) provides enough "instantaneous demand" that controlled amounts of ozone activate bromide ion to hypobromous acid, and the ozone entirely decays in a few seconds to minutes. If oxygen fed ozone is used, the water will be higher in dissolved oxygen, fish's gill structures will atrophy and they will become dependent on higher dissolved oxygen levels.

See also

References

  1. ^ a b Rubin, Mordecai B. (2001). "The History of Ozone. The Schönbein Period, 1839–1868" (PDF). Bull. Hist. Chem. 26 (1). Retrieved 2008-02-28.
  2. ^ "Today in Science History". Retrieved 2006-05-10.
  3. ^ Jacques-Louis Soret (1865). "Recherches sur la densité de l'ozone". Comptes rendus de l'Académie des sciences. 61: 941.
  4. ^ "Ozone FAQ". Global Change Master Directory. Retrieved 2006-05-10.
  5. ^ "Oxygen". WebElements. Retrieved 2006-09-23.
  6. ^ a b c d e Nicole Folchetti, ed. (2003). "22". Chemistry: The Central Science (9th ed.). Pearson Education. pp. 882–883. ISBN 0-13-066997-0. {{cite book}}: |first= missing |last= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |origdate= ignored (|orig-date= suggested) (help) Cite error: The named reference "brown" was defined multiple times with different content (see the help page).
  7. ^ Takehiko Tanaka; Yonezo Morino. Coriolis interaction and anharmonic potential function of ozone from the microwave spectra in the excited vibrational states Journal of Molecular Spectroscopy 1970, 33, 538–551.
  8. ^ Kenneth M. Mack; J. S. Muenter. Stark and Zeeman properties of ozone from molecular beam spectroscopy. Journal of Chemical Physics 1977, 66, 5278–5283. doi:10.1063/1.433909
  9. ^ Earth Science FAQ: Where can I find information about the ozone hole and ozone depletion? Goddard Space Flight Center, National Aeronautics and Space Administration, March 2008.
  10. ^ Koike, K; Nifuku, M; Izumi, K; Nakamura, S; Fujiwara, S; Horiguchi, S (2005). "Explosion properties of highly concentrated ozone gas" (PDF). Journal of Loss Prevention in the Process Industries. 18: 465. doi:10.1016/j.jlp.2005.07.020.
  11. ^ a b Horvath M., Bilitzky L., Huttner J. (1985). Ozone. pp. 44–49.{{cite book}}: CS1 maint: multiple names: authors list (link)
  12. ^ Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. p. 439. ISBN 978-0-13-039913-7.
  13. ^ Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. p. 265. ISBN 978-0-13-039913-7.
  14. ^ Horvath M., Bilitzky L., Huttner J. (1985). Ozone. pp. 259, 269–270.{{cite book}}: CS1 maint: multiple names: authors list (link)
  15. ^ a b WHO-Europe reports: Health Aspects of Air Pollution (2003) (PDF)
  16. ^ Stevenson; et al. (2006). "Multimodel ensemble simulations of present-day and near-future tropospheric ozone". American Geophysical Union. Retrieved 2006-09-16. {{cite web}}: Explicit use of et al. in: |author= (help)
  17. ^ "Rising Ozone Levels Pose Challenge to U.S. Soybean Production, Scientists Say". NASA Earth Observatory. 2003-07-31. Retrieved 2006-05-10.
  18. ^ a b Mutters, Randall (1999). "Statewide Potential Crop Yield Losses From Ozone Exposure". California Air Resources Board. Retrieved 2006-05-10. {{cite web}}: Unknown parameter |month= ignored (help)
  19. ^ "Tropospheric Ozone in EU - The consolidated report". European Environmental Agency. 1998. Retrieved 2006-05-10.
  20. ^ "Atmospheric Chemistry and Greenhouse Gases". Intergovernmental Panel on Climate Change. Retrieved 2006-05-10.
  21. ^ "Climate Change 2001". Intergovernmental Panel on Climate Change. 2001. Retrieved 2006-09-12.
  22. ^ Jeannie Allen (2003-08-22). "Watching Our Ozone Weather". NASA Earth Observatory. Retrieved 2008-10-11.
  23. ^ Answer to follow-up questions from CAFE (2004) (PDF)
  24. ^ Anderson, W. (2001). "Asthma admissions and thunderstorms: a study of pollen, fungal spores, rainfall, and ozone". QJM: an International Journal of Medicine. 94 (8). Oxford Journals: 429–433. doi:10.1093/qjmed/94.8.429. PMID 11493720. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  25. ^ a b Weinhold B (2008). "Ozone nation: EPA standard panned by the people". Environ. Health Perspect. 116 (7): A302–A305. doi:10.1289/ehp.116-a302. PMC 2453178. PMID 18629332.
  26. ^ "Smog - Who does it hurt? What You Need to Know About Ozone and Your Health". AIRNow.gov. Retrieved 2007-07-10.
  27. ^ Ashfield District Council: Monitored Air Pollutants, downloaded February 2, 2007
  28. ^ University of East Anglia press release, Cloning the smell of the seaside, February 2, 2007
  29. ^ Jerrett, Michael (March 12, 2009). "Long-Term Ozone Exposure and Mortality". N. Engl. J. Med. 360 (11): 1085–1095. doi:10.1056/NEJMoa0803894. PMID 19279340. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  30. ^ Wilson, Elizabeth K. (March 16, 2009). "Ozone's Health Impact". Chemical & Engineering News. 87 (11). American Chemical Society Publications: 9.
  31. ^ Hoffmann, Roald (2004). "The Story of O". American Scientist. 92 (1): 23. doi:10.1511/2004.1.23. Retrieved 2006-10-11. {{cite journal}}: Unknown parameter |month= ignored (help)
  32. ^ Smith, LL (2004). "Oxygen, oxysterols, ouabain, and ozone: a cautionary tale". Free radical biology & medicine. 37 (3): 318–24. doi:10.1016/j.freeradbiomed.2004.04.024.
  33. ^ Paul Wentworth; Nieva, J; Takeuchi, C; Galve, R; Wentworth, AD; Dilley, RB; Delaria, GA; Saven, A; Babior, BM (2003). "Evidence for Ozone Formation in Human Atherosclerotic Arteries". Science. 302 (5647): 1053. doi:10.1126/science.1089525. PMID 14605372.
  34. ^ Iglesias, Domingo J. (2006). "Responses of citrus plants to ozone: leaf biochemistry, antioxidant mechanisms and lipid peroxidation". Plant Physiology and Biochemistry. 44 (2–3): 125–131. doi:10.1016/j.plaphy.2006.03.007. PMID 16644230. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  35. ^ 2-Health Effects of Ozone, Canadian Centre for Occupational Health and Safety
  36. ^ Documentation for Immediately Dangerous to Life or Health Concentrations (IDLH): NIOSH Chemical Listing and Documentation of Revised IDLH Values (as of 3/1/95)
  37. ^ a b http://www.portfolio.com/views/blogs/daily-brief/2008/05/08/airplane-air-heavy-on-the-ozone
  38. ^ http://www.sciencedaily.com/releases/2007/09/070905140105.htm
  39. ^ Organic Syntheses, Coll. Vol. 3, p.673 (1955); Vol. 26, p.63 (1946). (Article)
  40. ^ Dohan, J. M. (1987). "Photochemical Generation of Ozone: Present State-of-the-Art". Ozone Sci. Eng. 9: 315–334. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  41. ^ "Fire in the Sky". Retrieved 2008-08-16.
  42. ^ Ibanez, Jorge G. (2005). "Laboratory Experiments on the Electrochemical Remediation of the Environment. Part 7: Microscale Production of Ozone". Journal of Chemical Education. 82 (10): 1546. doi:10.1021/ed082p1546. Retrieved 2006-05-10. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  43. ^ Phillips, TJ (1999 Nov-Dec). "Ozone emissions from a "personal air purifier"". J Expo Anal Environ Epidemiol. 6: 9: 594–601. Retrieved 29 May 2009. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: location (link)
  44. ^ "Ozone and Color Removal". Ozone Information. Retrieved 2009-01-09.
  45. ^ Hoigné, J. (1998). Handbook of Environmental Chemistry, Vol. 5 part C. Berlin: Springer-Verlag. pp. 83–141.
  46. ^ "Oxidation Potential of Ozone". Ozone-Information.com. Retrieved 2008-05-17.
  47. ^ "Decontamination: Ozone scores on spores". Hospital Development. Wilmington Media Ltd. 2007-04-01. Retrieved 2007-05-30.
  48. ^ a b c Montecalvo, Joseph. "Application of Ozonation in Sanitizing Vegetable Process Washwaters" (PDF). California Polytechnic State University. Retrieved 2008-03-24. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  49. ^ Steeves, Susan A. (January 30, 2003). "Ozone may provide environmentally safe protection for grains". Purdue News.
  50. ^ "Chemical Synthesis with Ozone". Ozone-Information.com. Retrieved 2008-05-17.
  51. ^ de Boer, Hero E. L. (2006). "Use of Gaseous Ozone for Eradication of Methicillin-Resistant Staphylococcus aureus From the Home Environment of a Colonized Hospital Employee". Infection Control and Hospital Epidemiology. 27 (10): 1120–1122. doi:10.1086/507966. PMID 17006820. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  52. ^ Sjöström, Eero (1993). Wood Chemistry: Fundamentals and Applications. San Diego, CA: Academic Press, Inc. ISBN 0126474818.
  53. ^ Su, Yu-Chang; Chen, Horng-Tsai (2001). "Enzone Bleaching Sequence and Color Reversion of Ozone-Bleached Pulps". Taiwan Journal of Forest Science. 16 (2): 93–102.
  54. ^ Bollyky, L. J. (1977). Ozone Treatment of Cyanide-Bearing Wastes, EPA Report 600/2-77-104. Research Triangle Park, N.C.: U.S. Environmental Protection Agency.
  55. ^ "The Unknown Truth Regarding Ozone!". Retrieved 2006-09-16.
  56. ^ EPA report on consumer ozone air purifiers
  57. ^ Long, Ron (2008). "POU Ozone Food Sanitation: A Viable Option for Consumers & the Food Service Industry" (pdf). (report also shows tapwater removes 99.95% of pathogens from lettuce; samples were first inoculated with pathogens before treatment)
  58. ^ Tersano Inc (2007). "lotus Sanitises Food without Chemicals". Retrieved 2007-02-11.
  59. ^ Jongen, W (2005). Improving the Safety of Fresh Fruit and Vegetables. Boca Raton: Woodhead Publishing Ltd. ISBN 1855739569.
  60. ^ "Alternative Disinfectants and Oxidant Guidance Manual" (PDF). United States Environmental Protection Agency. Retrieved 2008-01-14.

Further reading

  • Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  • Series in Plasma Physics: Non-Equilibrium Air Plasmas at Atmospheric Pressure. Edited by K.H. Becker, U. Kogelschatz, K.H. Schoenbach, R.J. Barker; Bristol and Philadelphia: Institute of Physics Publishing Ltd; ISBN 0-7503-0962-8; 2005
Retrieved from "https://en.wikipedia.org/w/index.php?title=Ozone&oldid=370124945"