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For other
uses, see Ozone (disambiguation).
Ozone
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Identifiers
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Properties
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O3
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47.998 g·mol−1
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Appearance
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bluish colored gas
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2.144 g·L−1 (0 °C), gas
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80.7 K,
−192.5 °C
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161.3 K,
−111.9 °C
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Solubility in water
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0.105 g·100mL−1 (0 °C)
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Thermochemistry
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Std enthalpy of |
+142.3 kJ·mol−1
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Standard molar |
237.7 J·K−1.mol−1
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Hazards
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Oxidant
(O)
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Except where noted otherwise, data are given for |
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Ozone or trioxygen (O3) is a triatomic molecule,
consisting of three oxygen atoms. It is an allotrope of
oxygen that is much less stable than the diatomic O2.
Ground-level ozone is an air pollutant with harmful effects on the respiratory
systems of animals. The ozone layer in the upper atmosphere filters potentially damaging ultraviolet
light from reaching the Earth's surface. It is present in low concentrations throughout
the Earth's atmosphere. It has many industrial and
consumer applications.
Ozone,
the first allotrope of a chemical element to be recognized by science, 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]
Most
people can detect about 0.01 ppm in air. Exposure
of 0.1 to 1 ppm produces headaches, burning eyes, and
irritation to the respiratory passages.[5]
At
-112 °C, it forms a dark blue liquid. At temperatures below -193 °C, it forms a
violet-black solid.[6]
Ozone is diamagnetic,
meaning that it will resist formation of a magnetic
field and will decrease the energy stored in the field once the field is
established.
The
structure of ozone, according to experimental evidence from microwave spectroscopy, is bent, with C2v symmetry (similar to the water molecule), O –
O distance of 127.2 pm and O – O – O angle of 116.78°.[7] The central atom forms an sp² hybridization
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.
Ozone is
a powerful oxidizing agent, far better than dioxygen. It is also unstable
at high concentrations, decaying to ordinary diatomic oxygen (in about half an
hour in atmospheric conditions[9]):
2 O3 → 3 O2
This
reaction proceeds more rapidly with increasing temperature and decreasing
pressure. Deflagration of ozone can be triggered by a spark, and
can occur in ozone concentrations of 10 wt% or higher [10].
Ozone
will oxidize metals (except gold, platinum, and iridium) to oxides of the metals
in their highest oxidation state:
2 Cu+ (aq) + 2 H3O+ (aq)
+ O3 (g) → 2 Cu2+ (aq) +
3 H2O (l) + O2 (g)
Ozone
also increases the oxidation number of oxides, such as the oxidation of nitric
oxide to nitrogen dioxide:
NO + O3 → NO2 + O2
This
reaction is accompanied by chemiluminescence.
The NO2 can be further oxidized:
NO2 + O3 → NO3 + O2
The NO3 formed can react with NO2 to form N2O5:
NO2 + NO3 → N2O5
Ozone
reacts with carbon to form carbon dioxide, even at room temperature:
C + 2 O3 → CO2 + 2 O2
Ozone
does not react with ammonium salts but it reacts with ammonia to form ammonium
nitrate:
2 NH3 + 4 O3 → NH4NO3 + 4 O2 + H2O
Ozone
reacts with sulfides to make 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, starting
either from elemental sulfur or from sulfur dioxide:
S + H2O + O3 → H2SO4
3 SO2 + 3 H2O
+ O3 → 3 H2SO4
All three atoms of ozone may
also react, as in the reaction of tin(II) chloride with hydrochloric
acid and NaCl:
3 SnCl2 + 6 HCl + O3 → 3 SnCl4 + 3 H2O
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
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
Solid nitryl perchlorate can be made from NO2, ClO2, and O3 gases:
2 NO2 + 2 ClO2 + 2 O3 → 2 NO2ClO4 + O2
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
Ozonides
can be formed, which contain the ozonide anion, O3-. These compounds 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
Treatment
with ozone of calcium dissolved in ammonia leads to ammonium ozonide and not calcium ozonide:[14]
3 Ca + 10 NH3 + 6 O3 → Ca•6NH3 + Ca(OH)2 +
Ca(NO3)2 + 2 NH4O3 + 2 O2 + H2
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 turn cyanides to the one thousand times less toxic cyanates:
CN- + O3 → CNO- + O2
Finally,
ozone will also completely decompose urea:[15]
(NH2)2CO +
O3 → N2 + CO2 + 2 H2O
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. Concentrations at a point are measured in parts per billion (ppb) or in μg/m³.
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, a vitamin also produced by the human body. 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
Main
articles: Tropospheric ozone and Photochemical smog
Low level
ozone (or tropospheric ozone) is regarded as a
pollutant by the World Health Organization[16] and the United States
Environmental Protection Agency (EPA). 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).[17]
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.[18][19]
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.[19]
Ozone
cracking in Natural rubber tubing
Ozone gas
attacks any polymer possessing olefinic or double bonds within its chain structure, such materials
including 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.
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.[20][21] 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 scientific review on the climate
change (the IPCC Third Assessment Report[22])
suggests that the radiative forcing of tropospheric ozone is about 25% that of carbon dioxide.
Red Alder leaf, showing the typical
discolouration caused by ozone pollution.[23]
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.[16][24] A connection has also been known to exist between increased ozone caused by
thunderstorms and hospital admissions of asthma sufferers.[25] 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 cardiovascular problems. According to scientists with the United States
Environmental Protection Agency (EPA), susceptible people can be adversely effected by ozone levels as low as 40 ppb.[26]
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 standard 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.[26] 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".[27]
Ozone can
also be present in indoor air pollution.
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.[28] 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.[29]
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.[30][31]
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.[32]
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[33] as well as a secondary condensation product via aldolization.[34]
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."[35]
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." [36]
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. [37] 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.
Production
Ozone
often forms in nature under conditions where O2 will not react.[5] Ozone used in industry is measured in g/Nm³ 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.
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.[38] 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..
UV 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.[39] While standard UV ozone generators tend to be less expensive, 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.
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 anions are even more reactive than ordinary O3.
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.
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 troposhere. It is the world's largest single natural
generator of ozone, lending calls for it to be designated a UNESCO World Heritage Site.[40]
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.[41] 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;
so that 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:[5]
3 O2 — electricity → 2 O3
Some air
filters and purifiers create ozone.
The
largest use of ozone is in the preparation of pharmaceuticals, synthetic lubricants, as well as many other
commercially useful organic compounds, where it is used to sever carbon-carbon
bonds.[5] It can also be used for bleaching substances and for killing microorganisms in air and water sources.[42] Many municipal drinking water systems kill bacteria with ozone instead of the
more common chlorine.[43] Ozone has a very high oxidation potential.[44] Ozone does not form organochlorine compounds,
nor does it remain in the water after treatment. The Safe Drinking Water Act
mandate 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:
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.[49]
Ozone is
used as an alternative to chlorine or chlorine
dioxide in the bleaching of wood pulp[50] . 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[51]
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.[52]
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 lower levels of ozone have been sold. This kind of air
purifier is sometimes claimed to imitate nature's way of purifying the air[53] 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."[54] 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 exceeds 20,000
times the WHO recommended limits stated above.[55][46] Ozone can be used to remove pesticide residues from fruits and vegetables.[56][57]
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 these halogens.
Gaseous ozone created by ultraviolet light or by corona discharge is injected
into the water.[58]
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.
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