Editing Ozone (section)

==Production==
[[File:THC 2003.902.021 E. H. Johnston Ozone Production.jpg|thumb|Ozone production demonstration, Fixed Nitrogen Research Laboratory, 1926]]

'''Ozone generators''', or '''ozonators''',<ref>{{cite web |title=Visual Encyclopedia of Chemical Engineering |website=encyclopedia.che.engin.umich.edu |url=http://encyclopedia.che.engin.umich.edu/Pages/TransportStorage/Ozonators/Ozonators.html}}</ref> are used to produce ozone for cleaning air or removing smoke odours in unoccupied rooms. These ozone generators can produce over 3&nbsp;g of ozone per hour. Ozone often forms in nature under conditions where O<sub>2</sub> will not react.<ref name=brown/> Ozone used in industry is measured in μmol/mol (ppm, parts per million), nmol/mol (ppb, parts per billion), μg/m<sup>3</sup>, mg/h (milligrams per hour) or weight percent. The regime of applied concentrations ranges from 1% to 5% (in air) and from 6% to 14% (in oxygen) for older generation methods. New electrolytic methods can achieve up 20% to 30% dissolved ozone concentrations in output water.

Temperature and humidity play a large role in how much ozone is being produced using traditional generation methods (such as corona discharge and ultraviolet light). Old generation methods will produce less than 50% of nominal capacity if operated with humid ambient air, as opposed to very dry air. New generators, using electrolytic methods, can achieve higher purity and dissolution through using water molecules as the source of ozone production.

===Coronal discharge method===
[[File:Shkarkese elektrike.jpg|thumb|A homemade ozone generator. Ozone is produced in the corona discharge.]]
This is the most common 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|corona discharge tube]] or ozone plate.<ref>{{cite web |title=Ozone Cell vs Ozone Plate – A2Z Ozone |work=A2Z Ozone |url=https://www.a2zozone.com/blogs/news/ozone-tube-vs-ozone-plate |access-date=2020-01-10 |archive-url=https://web.archive.org/web/20200110211607/https://www.a2zozone.com/blogs/news/ozone-tube-vs-ozone-plate |archive-date=2020-01-10}}</ref><ref>{{OrgSynth|last1=Smith|first1=L. I.|last2=Greenwood|first2=F. L.|last3=Hudrlik|first3=O. |collvol=3|collvolpages=673 |volume=26|pages=63|year=1946|title=A laboratory ozonizer|prep=cv3p0673}}</ref> They are typically cost-effective and do not require an oxygen source other than the ambient air to produce ozone concentrations of 3–6%. Fluctuations in ambient air, due to weather or other environmental conditions, cause variability in ozone production. However, they also produce [[nitrogen oxide]]s as a by-product. Use of an [[air dryer]] can reduce or eliminate nitric acid formation by removing water vapor and increase ozone production. At room temperature, nitric acid will form into a vapour that is hazardous if inhaled. Symptoms can include chest pain, shortness of breath, headaches and a dry nose and throat causing a burning sensation. 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===
{{See also|Photochemistry}}
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.<ref>{{cite journal |last=Dohan |first=J. M. |author2=W. J. Masschelein |year=1987 |journal=Ozone Sci. Eng. |title=Photochemical Generation of Ozone: Present State-of-the-Art |volume=9 |pages=315–334 |issue=4 |bibcode=1987OzSE....9..315D |doi=10.1080/01919518708552147}}</ref>

UV ozone generators use ambient air for ozone production, no air prep systems are used (air dryer or oxygen concentrator), therefore these generators tend to be less expensive. However, UV ozone generators usually produce ozone with a concentration of about 0.5% or lower which limits the potential ozone production rate. Another disadvantage of this method is that it requires the ambient 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 (microbiology)|sterilization]], for example). Production of ozone is one of the [[ultraviolet germicidal irradiation#Potential dangers|potential dangers]] of [[ultraviolet germicidal irradiation]]. VUV ozone generators are used in swimming pools 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 (physics)|plasma]] created by [[dielectric barrier discharge|DBD]]. The diatomic oxygen is split into single atoms, which then recombine in triplets to form ozone.

It is common in the industry to mislabel some DBD ozone generators as CD Corona Discharge generators. Typically all solid flat metal electrode ozone generators produce ozone using the dielectric barrier discharge method. Cold plasma machines use pure oxygen as the input source and produce a maximum concentration of about 24% ozone. They produce far greater quantities of ozone in a given time compared to ultraviolet production that has about 2% efficiency. 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 [[electrical insulation|insulator]] must be used to separate the metallic electrodes and to prevent arcing.

===Electrolytic===
Electrolytic ozone generation (EOG) splits water molecules into H<sub>2</sub>, O<sub>2</sub>, and O<sub>3</sub>.

In most EOG methods, the hydrogen gas will be removed to leave oxygen and ozone as the only reaction products. Therefore, EOG can achieve higher [[dissolution (chemistry)|dissolution]] in water without other competing gases found in corona discharge method, such as nitrogen gases present in ambient air. This method of generation can achieve concentrations of 20–30% and is independent of air quality because water is used as the source material. Production of ozone electrolytically is typically unfavorable because of the high [[overpotential]] required to produce ozone as compared to oxygen. This is why ozone is not produced during typical water electrolysis. However, it is possible to increase the overpotential of oxygen by careful catalyst selection such that ozone is preferentially produced under electrolysis. Catalysts typically chosen for this approach are [[lead dioxide]]<ref>{{cite journal |author1=Foller, Peter C. |author2=Tobias, Charles W. |title=The Anodic Evolution of Ozone |journal=Journal of the Electrochemical Society |volume=129 |issue=3 |page=506 |year=1982 |bibcode=1982JElS..129..506F |doi=10.1149/1.2123890}}</ref> or boron-doped diamond.<ref>{{cite journal |author1=Arihara, Kazuki |author2=Terashima, Chiaki |author3=Fujishimam Akira |title=Electrochemical Production of High-Concentration Ozone-Water Using Freestanding Perforated Diamond Electrodes |journal=Journal of the Electrochemical Society |volume=154 |issue=4 |pages=E71 |year=2007 |bibcode=2007JElS..154E..71A |doi=10.1149/1.2509385}}</ref>

The ozone-to-oxygen ratio is improved by increasing current density at the anode, cooling the electrolyte around the anode close to 0&nbsp;°C, using an acidic electrolyte (such as dilute sulfuric acid) instead of a basic solution, and by applying pulsed current instead of DC.<ref name="Hale1919">{{cite book |last=Hale |first=Arthur J. |title=The Manufacture of Chemicals by Electrolysis |year=1919 |publisher=D. Van Nostrand Co. |pages=15, 16 |url=https://books.google.com/books?id=eDNDAAAAIAAJ |access-date=12 September 2019}}</ref>

===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&nbsp;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 {{convert|2|bar|lk=on}} absolute in oxygen and {{convert|3|bar}} absolute in air. Several megawatts of [[electric power|electrical power]] may be installed in large facilities, applied as single phase AC [[electric current|current]] at 50 to 8000&nbsp;Hz and peak [[voltage]]s 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 a 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 comes 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 rubber]]s are usually adequate for use as [[gaskets]] in ozone concentrations below 1 wt%, such as in equipment for accelerated aging of rubber samples.

===Incidental production===
Ozone may be formed from {{chem|O|2}} by electrical discharges and by action of high energy [[electromagnetic radiation]]. [[Arc suppression|Unsuppressed arcing]] in electrical contacts, motor brushes, or mechanical switches breaks down the chemical bonds of the atmospheric oxygen surrounding the contacts [{{chem|O|2}} -> 2O]. Free radicals of oxygen in and around the arc recombine to create ozone [{{chem|O|3}}].<ref>{{cite web |title=Lab Note #106 ''Environmental Impact of Arc Suppression'' |publisher=Arc Suppression Technologies |date=April 2011 |url=http://www.arcsuppressiontechnologies.com/arc-suppression-facts/lab-app-notes/ |access-date=October 10, 2011}}</ref> Certain [[electrical equipment]] generate significant levels of ozone. This is especially true of devices using [[high voltage]]s, such as [[air ioniser|ionic air purifiers]], [[laser printer]]s, [[photocopier]]s, [[electroshock weapon|tasers]], and [[arc welding|arc welders]]. [[Electric motor]]s using [[brush (electric)|brushes]] can generate ozone from repeated [[spark gap|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|Catatumbo lightning storms]] phenomenon on the [[Catatumbo River]] in [[Venezuela]], though ozone's instability makes it dubious that it has any effect on the ozonosphere.<ref name="Zulia">[http://www.agenciadenoticias.luz.edu.ve/index.php?option=com_content&task=view&id=4965&Itemid=151 ¿Relámpagos del Catatumbo regeneran la capa de ozono?] {{Webarchive|url=https://web.archive.org/web/20160305205225/http://agenciadenoticias.luz.edu.ve/index.php?id=4965&itemid=151&option=com_content&task=view |date=2016-03-05}}. Agencia de noticias de la [[Universidad del Zulia]].</ref>
It is the world's largest single natural generator of ozone, lending calls for it to be designated a [[UNESCO World Heritage Site]].<ref name="meteo">{{cite web |title=Fire in the Sky |url=http://www.meteogroup.co.uk/uk/home/weather/weather_news/news_archive/archive/2007/november/ch/f540146dcc/article/fire_in_the_sky.html |access-date=2008-08-16 |archive-url=https://web.archive.org/web/20110721215231/http://www.meteogroup.co.uk/uk/home/weather/weather_news/news_archive/archive/2007/november/ch/f540146dcc/article/fire_in_the_sky.html |archive-date=2011-07-21}}</ref>

===Laboratory production===
[[File:Siemen's Ozoniser.jpg|thumb|A laboratory method for the preparation of ozone by using Siemen's Ozoniser]]
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 (concentration)|molar]] [[sulfuric acid]] [[electrolyte]].<ref>{{cite journal |last=Ibanez |first=Jorge G. |author2=Rodrigo Mayen-Mondragon |author3=M. T. Moran-Moran |year=2005 |title=Laboratory Experiments on the Electrochemical Remediation of the Environment. Part 7: Microscale Production of Ozone |journal=Journal of Chemical Education |volume=82 |issue=10 |page=1546 |bibcode=2005JChEd..82.1546A |doi=10.1021/ed082p1546}}</ref> The [[half cell]] reactions taking place are:
:<math chem>\begin{align}
& \ce{3 H2O -> O3 + 6 H+ + 6 e-} && (\Delta E^\circ=-\text{1.53 V}) \\
& \ce{6 H+ + 6 e- -> 3 H2} && (\Delta E^\circ=\text{0 V}) \\
& \ce{2 H2O -> O2 + 4 H+ + 4 e-} && (\Delta E^\circ=\text{1.23 V})
\end{align}</math>

where {{mvar|E°}} represents the [[standard electrode potential (data page)|standard electrode potential]].

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 generated by a [[high voltage]] [[electric arc|arc]]. In its simplest form, high voltage AC, such as the output of a [[neon-sign transformer]] is connected to two metal rods with the ends placed sufficiently close to each other to allow an arc. The resulting arc will convert atmospheric oxygen to ozone.

It is often desirable to contain the ozone. This can be done with an apparatus consisting of two concentric glass tubes sealed together at the top with gas ports 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. A source of dry {{chem|O|2}} is applied to the bottom port. When high voltage is applied to the foil leads, [[electricity]] will discharge between the dry dioxygen in the middle and form {{chem|O|3}} and {{chem|O|2}} which will flow out the top port. This is called a Siemen's ozoniser. The reaction can be summarized as follows:<ref name=brown/>
: <chem>3O2 ->[\text{electricity}] 2O3</chem>