ozone - Physical properties, Chemistry, Ozone in Earth's atmosphere, Ozone and health, Artificial production, Applications

A form of oxygen having molecules O₃. It is formed by the action of ultraviolet radiation on ordinary oxygen, and is a gas, boiling point ?112°C. It is unstable and a strong oxidizing agent, with bacteriocidal properties, but is corrosive to humans in any but very low concentration. Its presence in the upper atmosphere is important in protecting the Earth from excessive ultraviolet radiation.

Ozone, the first allotrope of a chemical element to be described by science, was discovered by Christian Friedrich Schönbein in 1840, who named it after the Greek word for smell (ozein), from the peculiar odor in lightning storms.

Physical properties

Undiluted ozone is a pale blue gas at standard temperature and pressure;

Chemistry

Ozone is a powerful oxidizing agent. Ozone will oxidize metals (except gold, platinum, and iridium) to oxides of the metals in their highest oxidation state:

2 Cu + O3 → 2 Cu3+ + H2O + O2

Ozone converts oxides to peroxides:

SO2 + O3 → SO3 + O2

It also increases the oxidation number of oxides:

NO + O3 → NO2 + O2

The above 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:

PbS + 4 O3 → PbSO4 + 4 O2

Sulfuric acid can be produced from ozone, either starting 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 with tin(II) chloride and hydrochloric acid:

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 in ozone provides higher temperatures than combusting in dioxygen (O2). Following is a reaction for the combustion of carbon subnitride:

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:

H + O3 → HO2 + O 2 HO2 → H2O4

Ozonides can be formed, which contain the ozonide anion, O3-. 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:

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 ions:

CsO3 + Na + NaO3

Treatment with ozone of calcium dissolved in ammonia leads to ammonium ozonide and not calcium ozonide:

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 the water, forming a precipitate which can be filtered:

2 Mn

Ozone will also turn cyanides to the one thousand times less toxic cyanates:

CN + O2

Finally, ozone will also completely decompose urea:

(NH2)2CO + O3 → N2 + CO2 + 2 H2O

Ozone in Earth's atmosphere

The standard way to express total ozone amounts (the amount of ozone in a vertical column) in the atmosphere is by using Dobson units.

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. Ozone in the stratosphere is mostly produced from ultraviolet rays reacting with oxygen:

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

It is destroyed by the reaction with atomic oxygen:

O3 + O → 2 O2

(See Ozone-oxygen cycle for more detail.)

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).

Low level ozone

Low level ozone (or tropospheric ozone) is regarded as a pollutant by the World Health Organisation. It is formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that to form ozone directly at the source of the pollution or many kilometers down wind. For more details of the complex chemical reactions that produce low level ozone see tropospheric ozone or Seinfled and Pandis (1998).

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. The atmospheric lifetime of tropospheric ozone is about 22 days and 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) .

As well as having an impact on human health (see below) there is also evidence of significant reduction in agricultural yields due to increased ground-level ozone and pollution which interferes with photosynthesis and stunts overall growth of some plant species.

Ozone as a greenhouse gas

Although ozone was present at ground level before the industrial revolution, peak concentrations are far higher than the pre-industrial levels and even background concentrations well away from sources of pollution are substantially higher. This increase in ozone is of further concern as ozone present in the upper troposphere acts as a greenhouse gas, absorbing some of the infrared energy emitted by the earth.

Ozone and health

Ozone in air pollution

There is a great deal of evidence to show that high concentrations (ppm) 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 .

Physiology of ozone

Ozone, along with reactive forms of oxygen such as superoxide, singlet oxygen (see 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. In this system, ozone is produced by antibody-catalyzed production of trioxidane from water and neutrophil-produced singlet oxygen.

Ozone has also been proven to form specific, cholesterol-derived metabolites that are thought to facilitate the build-up and pathogenesis of atherosclerotic plaques (A form of heart disease). Volume: Number: Page: 23 DOI:

Safety

Artificial production

Ozone may be formed from O2 by electrical discharges and by action of high energy electromagnetic radiation.

Industrial production

Ozone used in industry is measured in ppm or mg/L (OSHA exposure limits for example), and percent by mass or weight. A high voltage alternating current is applied to the plates and the ozone is formed in the air gap when O2 molecules disassociate and recombine into O3.

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 3M sulfuric acid electrolyte. The half cell reactions taking place are:

3 H2O → O3 + 6 H ΔEo = − 1.53 V 6 H → 3 H2 ΔEo = 0 V 2 H2O → O2 + 4 H 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.

Applications

Industrial applications

Ozone can be used for bleaching substances and for killing bacteria. Ozone does not form organochlorine compounds, but it also does not remain in the water after treatment, so some systems introduce a small amount of chlorine to prevent bacterial growth in the pipes, or may use chlorine intermittently, based on results of periodic testing.

Industrially, ozone or ozonated water is used to

disinfect water before it is bottled;

Ozone is a reagent in many organic reactions in the laboratory and in industry.

Many hospitals in the U.S. and around the world use large ozone generators to decontaminate operating rooms between surgeries.

Consumer applications

Ozone machines, with or without ionization, are currently used to sanitize (high ozone output) and deodorize non-inhabited rooms, ductwork, vehicles, boats, woodsheds, and buildings.

Some models of air purifiers that also emit low levels of ozone have been sold in the US.

Ozone is used in spas or hot tubs with reduced levels of Chlorine or Bromine for keeping the water free of bacteria.

Ozone is also widely used in treatment of water in aquaria and fish ponds.

Pharmaceutical applications

Ozone has a number of medical uses.