Which element is ozone an allotrope of

Structural formula
Surname ozone
other names

"Active oxygen", tri-oxygen

Molecular formula O3
CAS number 10028-15-6
Brief description colorless gas with an unpleasant pungent odor, blue as a liquid[1]
Molar mass 48.00 g mol−1 [1]
Physical state gaseous[1]
density 2.144 kg m−3 (0 ° C)[1]
Melting point −192.7 ° C[1]
boiling point −110.5 ° C[1]
Vapor pressure

5.5 MPa (−12 ° C)


in water: 494 ml l−1 (0 ° C)[2]

safety instructions

not specified because it is carcinogenic [1]

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions.

ozone (O3) (from Greek ozein “Smell”) is an unstable molecule made up of three oxygen atoms, which breaks down into dimeric oxygen within a short period of time. It is a strong oxidizer.

Ozone is gaseous at room temperature and normal air pressure. Because of its oxidizing effect, it is toxic to humans. Severe headache at the temples is common with ozone intake. In high concentrations, the gas smells characteristic pungent-sharp to chlorine-like in high concentrations due to the oxidizing effect on the nasal mucous membrane, while in low concentrations it is odorless. The odor threshold is 40 µg / m3, however, you quickly get used to the smell and then no longer notice it.

Pure O3, the allotropic form of dioxygen O2, is an unstable, blue, diamagnetic gas at room temperature that solidifies below −192.5 ° C (80 K) to a black-violet solid, which decomposes to O2 tends. The angled molecule is retained in the solid, the O-O distance is 128 picometers, the angle between the three oxygen atoms is 117 °. Ozone maintains combustion much more effectively than dioxygen, and many materials flare up on contact at room temperature.


There are three main ways in which ozone is formed in the atmosphere:

  • High-energy solar radiation splits oxygen molecules in the stratosphere into two individual atoms, which each combine with another oxygen molecule to form ozone. This process of splitting oxygen molecules by high-energy UV radiation with a wavelength of
  • Near the earth, ozone is formed from a reaction between nitrogen dioxide NO2 and oxygen O2 under the influence of UV radiation.
  • Through a thunderstorm: The electrical current flow between the cloud and the ground during the lightning discharge creates ozone (but also nitric acid and other substances).


The amount of ozone in the atmosphere is expressed in Dobson units. The highest concentration of a few ppm is found in ozone in the stratosphere. The ozone-oxygen cycle is responsible for its formation there. Ozone is harmless in the stratosphere and partially absorbs the sun's ultraviolet radiation. In the air we breathe, however, it is already harmful to health in far lower concentrations; in particular, the very different local ozone levels cause irritation of the airways.

In clean air areas, ozone concentrations are often higher in summer than in cities. This is due to the fact that the concentration of nitrogen oxide NO can be very high due to the many car exhaust gases in cities. However, NO counteracts the formation of ozone. The following reactions take place in detail:

Ozone is created as follows:

NO2 + UV radiation → NO + O (reaction 1)

O + O2 → O3 (Reaction 2)

At the same time, ozone is broken down again by NO:

O3 + NO → NO2 + O2 (Reaction 3)

If there were no other substances, so-called volatile hydrocarbons or CO, in the lower air layer, no further ozone would be formed, but instead an equilibrium between O would be established depending on the solar radiation3, NO and NO2 a. The stronger the sun, the more ozone and less NO2 is present, since the latter is split by the UV radiation (reaction 1).

In the (polluted) planetary boundary layer of the atmosphere, however, as already indicated, there are also hydrocarbons that are emitted both by humans (anthropogenic) and by vegetation (biogenic). They are oxidized by OH radicals, the "detergent of the atmosphere", with peroxide radicals being R-O-O arise. These in turn ensure that NO to NO2 is oxidized without an O3 "Consumed" is as in reaction 3, so:

R-O-O + NO → R-O + NO2 (Reaction 4)

When reactions 1 and 2 take place again, net new ozone is formed.

Since NO is emitted by cars and industry, ozone is broken down more quickly in cities (according to Reaction 3) than in rural areas. In addition, in rural areas there are often hydrocarbons that are more easily attacked by OH radicals, which means that reaction 4 takes place more quickly. A prominent example of such an easily degradable biogenic hydrocarbon is isoprene. The exact chain of reactions is described in the article summer smog.

The CFCs (chlorofluorocarbons), which are often mentioned in connection with the ozone layer, are split by UV radiation, creating free chlorine radicals, which in turn contain many ozone molecules "to destroy" can (see ozone layer).


Ozone was discovered by Christian Friedrich Schönbein in 1839.

The degradation reactions of ozone by nitrogen oxides were first described in 1970 by Paul Josef Crutzen (Nobel Prize in Chemistry 1995).

Presentation in the laboratory

Ozone can be obtained from the reaction of potassium permanganate with concentrated sulfuric acid. The unstable dimanganese heptoxide Mn formed as an intermediate2O7 breaks down at room temperature to form manganese dioxide and oxygen, which is rich in ozone.

During the electrolysis of dilute sulfuric acid (approx. 20%), ozone develops on a gold or platinum anode, especially at high current densities. With good cooling, 4–5% ozone content can be achieved in the resulting oxygen, a concentration that is sufficient to be able to carry out all reactions of the ozone on a preparative scale. Using sophisticated equipment (e.g. fine platinum wire coils) and cooling to −14 ° C, significantly higher ozone concentrations can be achieved.

Ozone can also be produced from atmospheric oxygen under the action of ultraviolet radiation or silent electrical discharges. Corresponding devices known as ozonizers are commercially available.

Industrial manufacture

Because of its instability, ozone cannot be stored for long periods of time or bought in pressurized cylinders like other industrial gases. Before it can be used (chemical synthesis, water treatment, as a bleaching agent, etc.) it must be produced on the spot.

In most cases, dried air or oxygen (dew point at least −65 ° C) is used as a carrier gas for production. In rare cases, oxygen is mixed with argon, carbon dioxide, and the like. In the ozone generator, the oxygen molecules are dissociated into oxygen atoms by silent electrical discharge, after which ozone synthesis and ozone enrichment take place in the plasma of the discharge filaments. Typical final concentrations in air are between one and five percent by weight, in oxygen between six and thirteen percent by weight.

The technical devices used in practice can be based on the following electrode configurations:

  • tubes pushed into one another (e.g. glass tube with a metallic inner coating in a steel tube)
  • parallel plates
  • wire-wound electrodes for surface discharges
  • Tip to plate

In systems with more than 20 kg of ozone per hour, only tube ozonizers are usually used.

As a first approximation, ozone enrichment is a function of the electrical energy input per gas volume. The following parameters can be varied to optimize the efficiency:

  • Electrode gap
  • Electrode alignment
  • Dielectric material
  • Peak voltage and frequency

In industrial applications, ozone generator boilers are water-cooled, especially since almost 90 percent of the electrical energy introduced has to be dissipated again. The gas temperature is the dominant factor for the efficiency of ozone synthesis.

Because of the high reactivity of ozone, only a few materials are resistant to ozone. These include stainless steel (grade 316L), glass, polytetrafluoroethylene, polyvinylidene fluoride and perfluorubber. Viton, which must not be exposed to alternating mechanical loads under ozone, is conditionally resistant.


Liquid ozone can be found in the form of a 30 to 75% solution in liquid oxygen at -183 ° C in the presence of stabilizers such as CClF3, F2O, SF6 or store others without the risk of explosion. Gaseous ozone can be stored in its pure state (no contamination by organic compounds, sulfur or certain metals) at -50 to -112 ° C with a slight overpressure.

Ozone in water treatment

In water treatment, ozone is used, among other things, for the environmentally friendly oxidation of iron, manganese, organic matter and for disinfection. Ozonation is one of the central treatment stages in many drinking water works (see web links).

See also: Water treatment in swimming pools

Ozone is also used in the treatment of municipal and industrial wastewater (sewage treatment plant). The goals of a more extensive ozone treatment of conventionally treated wastewater are: (a) Killing pathogenic germs (disinfection) to protect the receiving water (e.g. with regard to the bathing water directive) (b) Oxidative elimination / transformation of organic trace substances that are not or only poorly degradable (especially drug residues).

Furthermore, ozone can be used very well in process combinations with downstream biological systems (biofilters), for example in the oxidation of the chemical oxygen demand (COD) to the biological oxygen demand (BOD), which is then processed further in the biofilter. Ozone is also used in fish cycles in aquaculture or aquarium systems.

Most of the products or processes named “chlorine-free” use ozone, for example when bleaching paper. In this context, the term “active oxygen” is often used.

Ozone treatment of vehicles

A so-called ozone treatment is carried out in professional vehicle preparation. This can be eliminated in particular in the case of used cars with unpleasant odors in the interior. The vehicle interior is completely sealed and flooded with ozone for approx. 24 hours. The oxidizing effect of the ozone converts odorous substances into odorless substances. Germs and odor-causing bacteria are also reliably killed in otherwise inaccessible places. As a result, the vehicle is disinfected and usually odorless after this treatment.


  • Titration method
  • UV analysis
  • Entropy method


With older photocopiers as well as laser printers one can perceive a typical "ozone smell". This smell is only indirectly due to the ozone formed by the ionization of the air in the device; rather, it comes from traces of nitrous gases (NOx), which are formed by the reaction of the ozone with the nitrogen in the air. The functional principle of the devices requires the air to be ionized at voltages of 5–15 kV. Most of the devices have ozone filters that convert the ozone produced into carbon dioxide. However, if possible, these devices should not be used in unventilated rooms. Modern printers and photocopiers work with transfer roller technology, which prevents ozone formation and has largely replaced the older corona wire technology.

Health hazard

The EU has been setting guidelines for ozone concentrations for a long time. According to the EU directive, there is no health risk from ozone below a level of 110 µg / m3. From a one-hour mean value of 180 µg / m3 the population is informed, as this concentration can already impair the performance of sensitive people. From around 200 µg / m3 Ozone can cause symptoms such as tearing, irritation of the mucous membranes in the throat, throat and bronchial tubes, headache, increased coughing, and deterioration in lung function. From a one-hour mean value of 360 µg / m3 Warnings are issued, as from this concentration there can be a risk to human health.


  1. abcdefGHij BGIA GESTIS substance database: http://www.hvbg.de/d/bia/gestis/stoffdb/index.html. 23 Mar 2007
  2. Holeman, Wiberg; Inorganic Chemistry Textbook, de Gruyter, 91st - 100th edition, 1985, p. 460


  • Katrin Palitzsch, Sabine Göllner, Kristina Lupa, Jörg Matschullat, Corinna Messal, Kirsten Pleßow, Mandy Schipek, Ivonne Schnabel, Christian Weller, Frank Zimmermann: Ozone in forest ecosystems from an atmospheric chemical and plant physiological point of view. Environmental sciences and pollutant research 17 (4), pp. 231 - 241 (2005), ISSN 0934-3504
  • Georg Erlwein: About drinking water purification through ozone and ozone waterworks. Leineweber, Leipzig 1904 (digitized as PDF)

See also

Categories: Oxidising substances | Toxic substance | Corrosive substance | Oxygen connection