CO2

Oxygen

In science, oxygen is a colorless, odorless, tasteless, gaseous chemical element with the chemical symbol O and atomic number 8. It is a group 16, nonmetallic, divalent element that is known to form binary compounds (known as oxides) with almost all the other elements. Oxygen has a valence of -2. On Earth it is usually bonded to other elements covalently or ionically. Oxygen is the 3rd most abundant element in the universe by mass after hydrogen and helium, the most abundant element by mass in the Earth's crust, as well as the most abundant element by mass in the human body.[1]

The word oxygen derives from two roots in Greek, οξύς (oxys) (acid, lit. "sharp," from the taste of acids) and -γενής (-genēs) (producer, lit. begetter). It was recognized in 1777 by Antoine Lavoisier, who coined the name oxygen from the Greek roots mentioned above because he erroneously thought that it was a constituent of all acids.[2]

Diatomic oxygen or dioxygen (O2) is one of the two major components of air (20.95%); the other major component is nitrogen. Oxygen is produced from water by plants during photosynthesis, and is necessary for aerobic respiration in animals. Without oxygen, most organisms will die within minutes.[3] It is toxic to obligate anaerobic organisms and was a poisonous waste product for early life on Earth. Triatomic oxygen (ozone, O3) forms through radiation in the upper layers of the atmosphere and acts as a shield against UV radiation.[2]

The most familiar oxygen compound is water. Other well-known examples include silica (found in sand, glass, rock, etc.), and the compounds of carbon and oxygen, such as carbon dioxide (CO2), alcohols (R-OH where "R" is an organic group), carbonyls (R-CO-H or R-CO-R) such as acetone; and carboxylic acids (R-COOH) such as fatty acids. Oxygenated radicals such as perchlorates (ClO4−) and nitrates (NO3−) are strong oxidizing agents in and of themselves. Phosphorus is biologically important in its oxygenated form as the phosphate (PO43−) ion and as the backbone of RNA and DNA. Many metals bond with oxygen atoms, such as iron in iron(III) oxide (Fe2O3), commonly called ru






The common allotrope of elemental oxygen on Earth, O2, is known as dioxygen. Elemental oxygen is most commonly encountered in this form, as about 21% (by volume) of the Earth's atmosphere. O2 has a bond length of 121 pm and a bond energy of 498 kJ/mol.[18]

Ozone (O3), the less common triatomic allotrope of oxygen, is a poisonous gas with a distinct, sharp odor. It was named ozone by Christian Friedrich Schönbein, in 1840, from the Greek word ÖĮώ (ozo) for smell. [2] It is thermodynamically unstable toward the more common dioxygen form. It is formed continuously in the upper atmosphere of the Earth by short-wave ultraviolet (UV) radiation, and also functions as a shield against UV radiation reaching the ground (see ozone layer).[2] Ozone is also formed by electrostatic discharge in the presence of molecular oxygen. The immune system produces ozone as anicrobial (see below). Liquid and solid O3 have a deeper blue color than ordinary oxygen and they are unstable and explosive. Traces of it can be detected as a sharp, chlorine-like smell coming from electric motors, laser printers and photocopiers.

A newly discovered allotrope of oxygen, tetraoxygen (O4), is a deep red solid that is created by pressurizing O2 to the order of 20 GPa. Its properties are being studied for use in rocket fuels and similar applications, as it is a much more powerful oxidizer than either O2 or O3.[19][20] When tetraoxygen is subjected to a pressure of 96 GPa, it becomes metallic, similarly to hydrogen, and becomes more similar to the heavier chalcogens, such as tellurium and polonium, both of which show significant metallic character.

Oxygen has 17 known isotopes with atomic masses ranging from 12.03 u to 28.06 u. Three are stable, 16O, 17O, and 18O, of which 16O is the most abundant (over 99.7%). The radioisotopes all have half-lives of less than three minutes. Nonetheless, 15O was experimentally used in positron emission tomography.

An atomic mass of 16 was assigned to oxygen prior to the definition of the unified atomic mass unit based upon 12C. Since physicists referred to 16O only, while chemists meant the naturally abundant mixture of isotopes, this led to slightly different atomic mass scales.

The isotopic composition of oxygen atoms in the earth's atmosphere is 99.759% 16O, 0.037% 17O and 0.204% 18O.[8] Water on earth is composed of slightly less 18O than air, with seawater containing 0.1995% of this heavier isotope and fresh water containing 0.1981%.[8] Fresh water contains less 18O because water molecules containing the lighter isotopes are slightly more likely to evaporate and fall as precipitation.


Oxygen constitutes 49.2% of the Earth's crust by mass[23] and is the most common component of the world's oceans (88.81% by mass).[8] It is also the second most common component of the Earth's atmosphere, taking up 20.947% of its volume and 23.14% of its mass (some million billion tonnes).[24][8][25] The Earth is unusual in having such a high concentration of free oxygen in its atmosphere. With only 0.15% oxygen by volume, the atmosphere of Mars has the second most abundant concentration by volume of oxygen of any planet in the solar system and Venus comes in third place.[21] However, their oxygen is only produced by ultraviolet radiation impacting oxygen-containing molecules such as carbon dioxide.

Elemental oxygen also occurs in solution in the world's water bodies. At 25°C under 1 atm of air, a litre of water will dissolve about 6.04 cc (8.63 mg, 0.270 mmol) of oxygen, whereas sea water will dissolve about 4.9 cc (7.0 mg, 0.22 mmol). At 0°C the solubilities increase to 10.29 cc (14.7 mg, 0.460 mmol) for water and 8.0 cc (11.4 mg, 0.36 mmol) for sea water. This difference has important implications for ocean life, as polar oceans support a much higher density of life due to their oxygen content.[28] Polluted water often reduces the amount of oxygen in it by killing off oxygen producing plants, bacteria and algae. Scientists assess this aspect of water quality by measuring the water's biochemical oxygen demand (BOD), or the amount of oxygen needed to restore a normal oxygen concentration.[29]


In nature, free oxygen is produced by the light-driven splitting of water during oxygenic photosynthesis in cyanobacteria, green algae and plants.[30] Algae and cyanobacteria in marine environments provide about 70% of the free oxygen produced on earth.[31] The remainder is produced by terrestrial plants, although almost all oxygen produced in tropical forests is consumed by organisms in those forests.[32]

The formula for photosynthesis is:


Two major methods are employed to produce the 100 million tonnes of oxygen extracted from air for industrial uses annually.[10] The most common method is to fractionally distill liquefied air into its various components, with nitrogen distilling as a vapor while oxygen is left as a liquid.[10]


Oxygen can also be produced through electrolysis of water into oxygen and hydrogen. A similar method is the electrocatalytic oxygen evolution from oxides and oxoacids. Chemical catalysts can be used as well, such as in chemical oxygen generators or oxygen candles that are used as part of the life support equipment on submarines, and which are still part of standard equipment on commercial airliners in case of depressurization emergencies.

Another air separation technology involves forcing air to dissolve through ceramic membranes based on zirconium oxide by either high pressure or an electric current, to produce nearly pure oxygen.[29]

In large quantities, the price of liquid oxygen (2001) is approximately $0.21/kg. [34] Since the primary cost of production is the energy cost of liquefying the air, the production cost will change as energy cost varies.


Oxygen is often transported in bulk as a liquid in specially insulated tankers because one liter of liquefied oxygen is equivalent to 840 liters of gaseous oxygen, at atmospheric pressure and 20 °C.[10] Such tankers are used to refill bulk liquid oxygen storage containers, which stand outside hospitals and other institutions with a need for large volumes of pure oxygen. Liquid oxygen is passed through heat-exchangers, which convert the cryogenic liquid into gas before it enters the building. Oxygen is also stored and shipped in smaller pressure cylinders containing the compressed gas; a form that is useful in certain portable medical applications and Oxy-fuel welding and cutting.[10]

Uptake of oxygen from the air is the essential purpose of respiration, so oxygen supplementation is used in medicine. Oxygen therapy is used to treat emphysema, pneumonia, some heart disorders and any disease that impairs the body's ability to take up and use oxygen.[35] Treatments are flexible enough to be used in hospitals, the patient's home, or increasingly by portable devices. Oxygen tents were once commonly used in oxygen supplementation, but have since been mostly replaced by the use of oxygen masks or nasal cannulas. Hyperbaric medicine uses hyperbaric oxygen chambers to increase the partial pressure of oxygen around the patient and, when needed, the medical staff. Carbon monoxide poisoning, gas gangrene and decompression sickness (the "bends") are sometimes treated using these devices.[35][36]

People who climb mountains or fly in non-pressurized fixed-wing aircraft sometimes have supplemental oxygen supplies.[37] Passengers traveling in commercial airplanes have an emergency supply of oxygen automatically supplied to them in case of cabin depressurization. Sudden cabin pressure loss activates chemical oxygen generators above each seat, causing oxygen masks to drop and forcing iron fillings into the sodium chlorate inside the canister.[29] A steady stream of oxygen gas is produced by the exothermic reaction. A ValuJet airplane crashed after life-expired oxygen canisters, which were being shipped in the cargo hold, activated and burned a hole in the airplane. They were miss-labelled as empty; and were being carried in the hold in contravention of the Dangerous Goods regulations.[38]

A notable application of oxygen as a low-pressure breathing gas is in modern spacesuits, which surround their occupant's body with pressurized air. These devices use nearly pure oxygen at about one third normal pressure, resulting in a normal blood partial pressures of oxygen. This trade-off of higher oxygen concentration for lower pressure is needed to maintain flexible spacesuits.

Oxygen, as a supposed mild euphoric, has a history of recreational use (see oxygen bar). However, the reality of a pharmacological effect is doubtful, a placebo or psychological boost being the most plausible explanation.

In the 19th century, oxygen was often mixed with nitrous oxide to temper its analgesic effect. A stable 50% gaseous mixture (Entonox) is commonly used in medicine today as an analgesic. However, the common basic anesthetic mixture is 30% oxygen with 70% nitrous oxide; the pain-suppressing effects, obviously, are due to the nitrous oxide and not to oxygen.


Smelting of iron ore into steel consumes 55% of commercially produced oxygen.[29] In this process, oxygen is injected through a high-pressure lance into molten iron, which removes sulfur and carbon impurities. The reaction is exothermic, so the temperature increases to 1700 ° C.[29]

Another 25% of commercially produced oxygen is used by the chemical industry.[29] Ethylene is reacted with oxygen to create ethylene oxide, which in turn is converted into ethylene glycol; the primary feeder material used to manufacture a host of products, including antifreeze and polyester polymers (the precursors of many plastics and fabrics).[29]

Most of the remaining 20% of commercially produced oxygen is used in medical applications, metal cutting and welding, as an oxidizer in rocket fuel, and in water treatment.[29] Oxygen is used in oxyacetylene welding burning acetylene with oxygen to produce a very hot flame. In this process, metal up to 60 cm thick is first heated with a small oxy-acetylene flame and then quickly cut by a large stream of oxygen.[39][40] Rocket propulsion requires a fuel and an oxidizer. Larger rockets use liquid oxygen as their oxidizer, which is mixed and ignited with the fuel for propulsion.

Paleoclimatologists measure the ratio of oxygen-18 and oxygen-16 in the shells and skeletons of marine organisms to determine what the climate was like millions of years ago. During periods of lower global temperatures, sea water molecules that contain the lighter isotope, oxygen-16, evaporate at a slightly faster rate than water molecules containing the 12% heavier oxygen-18.[41] Snow and rain from that evaporated water tends to be enriched in oxygen-16 and the seawater left behind tends to be enriched in oxygen-18. Marine organisms then incorporate more oxygen-18 into their skeletons and shells than they would in a warmer climate.[41] Paleoclimatologists also directly measure this ratio in air trapped in ice core samples that are up to several hundreds of thousand years old.

Oxygen presents two spectrophotometric absorption bands peaking at the wavelengths 687 and 760 nanometers. Some scientists have proposed to use the measurement of the radiance coming from vegetation canopies in those oxygen bands to characterize plant health status from a satellite platform.[42] This is because in those bands, it is possible to discriminate the vegetation's reflectance from the vegetation's fluorescence, which is much weaker. The measurement presents several technical difficulties due to the low signal to noise ratio and due to the vegetation's architecture, but it has been proposed as a possibility to monitor the carbon cycle from satellites on a global scale.

Since there are no known ways but the presence of life for planetary atmospheres to maintain high concentrations of oxygen, these same absorption bands have been proposed as a way (when telescopes have improved significantly) to search planets of nearby stars spectrophotometrically for signs of life-as-we-know-it.



In almost all known compounds of oxygen, the oxidation state of oxygen is -2. In a few compounds such as peroxides, the oxidation state is -1. Other oxidation states are quite rare such as:-1/3 (ozonides), +1 (oxygen(I) fluoride) and +2 (oxygen fluoride). The most familiar oxygen-containing compound is H2O. Other well-known examples include silica (found in sand, glass, rock, etc.), and the compounds of carbon and oxygen, such as carbon dioxide (CO2), alcohols (R-OH), carbonyls, (R-CO-H or R-CO-R), and carboxylic acids (R-COOH). Oxygenated radicals such as chlorates (ClO3−), perchlorates (ClO4−), chromates (CrO42−), dichromates (Cr2O72−), permanganates (MnO4−), and nitrates (NO3−) are strong oxidizing agents in and of themselves. Phosphorus is biologically important in its oxygenated form as the phosphate (PO43−) ion. Many metals bond with oxygen atoms, such as iron in iron(III) oxide (Fe2O3), commonly called rust.

One unexpected oxygen compound is dioxygen hexafluoroplatinate O2+PtF6−. It was discovered when Neil Bartlett was studying the properties of platinum hexafluoride (PtF6).[43] He noticed a change in color when this compound was exposed to atmospheric air. Bartlett reasoned that xenon should be oxidized by PtF6. This led him to the discovery of xenon hexafluoroplatinate Xe+PtF6−.Epoxides are ethers in which the oxygen atom is part of a ring of three atoms. O22+ is another cation as in O2F2, it is only formed in the presence of stronger oxidants than oxygen, which limits this cation to oxygen fluorines, e.g. oxygen fluoride.[44]

When dissolved in water, many metallic oxides form alkaline solutions while many oxides of nonmetals form acidic solutions. For example, sodium oxide in solution forms the strong base sodium hydroxide while phosphorus pentoxide in solution forms phosphoric acid.[45]


Although oxygen molecules are not generally reactive at room temperature they do react with certain strong inorganic reducing substances, such as ferrous sulfate in aqueous solution.[45] Oxygen also reacts spontaneously with many organic compounds at or below room temperature in a process called autoxidation.[45] Other substances need to be heated before they will react with oxygen in bulk but some, such as iron, readily forms iron oxide, or rust.

Due to its electronegativity, oxygen forms chemical bonds with almost all other free elements at elevated temperatures to give corresponding oxides. The only elements known to escape the possibility of combination with oxygen are a few of the noble gases and fluorine. So-called noble metals (common examples: gold, platinum) resist direct chemical combination with oxygen, and substances like gold(III) oxide must be formed by an indirect route.

Peroxides retain some of oxygen's original molecular structure. White or light yellow sodium peroxide (Na2O2) is formed when metallic sodium (Na) is burned in oxygen. Each oxygen atom in it's peroxide ion may have a full octet of 4 pairs of electrons.[46] Superoxides are a class of compounds that are very similar to peroxides, but with just one unpaired electron for each pair of oxygen atoms.[46]. These compounds form from oxidation of alkali metals with larger ionic radii (K, Rb, Cs). For example, Potassium superoxide ( KO2) is an orange-yellow solid formed when potassium (K) reacts with oxygen.

Hydrogen peroxide (H2O2) can be produced by passing a volume of 96 to 98% hydrogen and 2 to 4% oxygen through an electric discharge.[45] A more commercially viable method is allow autoxidation of an organic intermediate; 2-ethylanthrahydroquinone dissolved in an organic solvent is oxidized to H2O2 and 2-ethylanthraquinone.[45] The 2-ethylanthraquinone is then reduced and recycled back into the process.


Most chemically combined oxygen is locked in a class of minerals called silicates (which in turn are the major component of rocks and clays). The basic structure of silicates consists of two parts; units of silicon surrounded by four oxygen anions in a tetrahedral arrangement and units of metal-oxygen polyhedra that contain metal cations (examples: aluminium, calcium, iron and sodium).[46] Both units are linked together by sharing oxygen anions, forming complex polymers in the process.

Water- soluble silicates in the form of Na4SiO4, Na2SiO3, and Na2Si2O5 are used as detergents and adhesives.[46] NaxSixOx with a higher ratio of SiO2 to Na2O has a greater molecular weight and a lower solubility. Silica is a crystalline polymer with the chemical formula (SiO2)n. Quartz is the mineral form of silica in nature and the most common deposits of quartz are in sand.


Most of the thousands of organic compounds that contain oxygen are not made by direct action of oxygen. Many of the compounds that are directly created by a reaction with oxygen are commercially important. Examples and the reactions that form them include:[47]

Acetone ((CH3)2CO) and phenol (C6H5OH) are used as feeder materials in the synthesis of many different substances. The cumene (C6H5-CH(CH3)2) in the below reaction is commonly derived from petroleum.

C6H5-CH(CH3)2 + O2 -> C6H5-COOH(CH3)2 -> (CH3)2CO + C6H5OH

Acetylene (C2H2) along with a mix of carbon monoxide, hydrogen and water are formed directly by the oxidation of natural gas in the below simplified equation.[48]

CH4 + C2H6 + O2 -> C2H2 + CO + 3H2 + H2O

Ethylene oxide (C2H4O) is used to make the antifreeze ethylene glycol.

C2H4 + 1/2 O2 -catalyst-> C2H4O

Peracetic acid (CH3(COOH)O) is the feeder material used to make various epoxy compounds.

CH3CHO + O2 -catalyst-> CH3(COOH)O
from the lungs of most animals

Parts of DNA are made of oxygen and the element is found in almost all molecules that are important to life. Molecular oxygen, O2, is essential for cellular respiration in all aerobic organisms. Almost all animals use hemoglobin in their blood to transport oxygen from their lungs to their tissues, but some, such as spiders and lobsters, use hemerythrin.[24] A liter of blood can dissolve 200 cc of oxygen gas, which is much more than water can dissolve.[24] The below description of oxygen uptake and use is typical for vertebrates.

Oxygen diffuses through membranes and into red blood cells after inhalation into the lungs. The Heme group of hemoglobin by now already has carbon dioxide in its active site, but releases it for exhalation when oxygen is present. After being carried in blood to a body tissue in need of oxygen, it is handed-off from the Heme group to monooxygenase, an enzyme that also has an active site with an atom of iron.[24] Monooxygenase uses oxygen to catalyze many oxidation reactions in the body. Oxygen is also used as an electron acceptor in mitochondria to generate chemical energy in the form of adenosine triphosphate (ATP) during oxidative phosphorylation. Carbon dioxide, one of the waste products produced, is released from the cell and into the blood, where it combines with empty Heme groups. Blood circulates back to the lungs and the process repeats.[49] On average, a oxygen atom is used in respiration once every 3,000 years.[12]

Reactive oxygen species are dangerous by-products that sometimes result from the use of oxygen in organisms. Important examples include; oxygen free radicals such as the highly dangerous superoxide O2-, and the less harmful hydrogen peroxide ( H2O2).[24] The body uses superoxide dismutase to reduce superoxide radicals to hydrogen peroxide. Glutathione peroxidase and similar enzymes, then convent the H2O2 to water and dioxygen.[24]

Parts of the immune system of higher organisms, however, create peroxide, superoxide and singlet oxygen to destroy invading microbes. Recently, singlet oxygen has been found to be a source of biologically-produced ozone: this reaction proceeds through an unusual compound dihydrogen trioxide, also known as trioxidane, (HOOOH) which is an antibody-catalyzed product of singlet oxygen and water. This compound in turn disproportionates to ozone and peroxide, providing two powerful antibacterials. The body's range of defense against all of these active oxidizing agents is hardly surprising, then, given their "deliberate" employment as antimicrobial agents in the immune response.[50]
Oxygen was almost nonexistent in earth's atmosphere before the evolution of water oxidation in photosynthetic bacteria. Free oxygen first appeared in significant quantities during the Paleoproterozoic era (between 2.5 billion years ago and 1.6 billion years ago) as a product of the metabolic action of early anaerobes (archaea and bacteria). These organisms developed the mechanism of oxygen evolution between 3.5 and 2.7 billion years ago. At first, the produced oxygen dissolved in the oceans and reacted with iron, creating banded iron formations. It started to gas out of the oxygen-saturated waters about 2.7 billion years ago as evident in the rusting of iron-rich terrestrial rocks starting around that time. The amount of oxygen in the atmosphere increased gradually at first and shot up rapidly around 2.2 to 1.7 billion years ago to about 10% of its present level.[51]

The development of an oxygen-rich atmosphere was one of the most important events in the history of life on earth. The presence of large amounts of dissolved and free oxygen in the oceans and atmosphere may have driven most of the anaerobic organisms then living to extinction during the oxygen catastrophe about 2.4 billion years ago. However, the high electronegativity of O2 creates a large potential energy drop for cellular respiration, thus enabling organisms using aerobic respiration to produce much more ATP than anaerobic organisms. This makes them so efficient that they have come to dominate earth's biosphere.[52] Photosynthesis and cellular respiration of oxygen allowed for the evolution of eukaryotic cells and ultimately complex multicellular organisms such as plants and animals.

The atmospheric abundance of free oxygen in later geological epochs and its gradual increase up to the present has been largely due to synthesis by photosynthetic organisms. Over the past 500 million years, oxygen levels fluctuated between 15 and 35% per volume. Towards the end of the Carboniferous era (coal age) about 300 million years ago, atmospheric oxygen levels reached a maximum of 35% by volume, allowing insects and amphibians with limiting respiratory systems to grow much larger than today's species. Today, oxygen is the second most common component of the earth's atmosphere (about 21% by volume) after nitrogen. Human activities, including the burning of 7 billion tonnes of fossil fuels each year have had very little effect on the amount of free oxygen in the atmosphere.[12]

Oxygen is the most common of all chemical elements on earth, being found in water, air, and most mineral and organic substances, including most compounds in the human body. It combines with almost all other elements, and is so reactive that it was given the Greek name ‘oxygen’, meaning acid-forming. However, most of the compounds it forms are not acids. Its chemical reactions usually form heat (as in the animal body) and sometimes light (as in candles).

It has always been known that animals cannot live without air, but in 1674 Mayow showed that only one part of the air, about one-fifth, is essential for life, and named it ‘vital air’. A hundred years later Priestley isolated this part, oxygen; Lavoisier purified oxygen and its properties began to be studied.

Atmospheric air contains 21% oxygen, at a pressure of about 150 mm Hg varying with barometric pressure and to a small extent with humidity. It enters the lungs during breathing and is absorbed into the blood passively by diffusion, combining with haemoglobin and being carried in the bloodstream to all parts of the body. There it is used to metabolize or ‘burn’ foodstuffs in the cells, especially fats and carbohydrates, providing heat and creating new chemical compounds, water, and the waste product carbon dioxide. Tissues and organs vary in the length of time they can survive without oxygen, according to their provision for anaerobic metabolism. The brain cannot survive without oxygen; the cessation of breathing will cause unconsciousness in a few minutes, and death soon afterwards. Other tissues such as skeletal muscle can continue to work for a limited time, when glycogen stores are broken down without oxygen to provide energy; lactic acid is a by-product that leaks into the blood and makes it more acid, but can be recycled into carbohydrate stores in the liver.

In quiet breathing at rest we absorb about 0.2-0.3 litres/min of oxygen (depending on body size), but in vigorous exercise this can go up to over 2 litres/min. This increase is accomplished by increased breathing (which supplies oxygen to the lungs at a greater rate), increased cardiac output and flow of blood to the muscles, and greater extraction of oxygen from the blood by the muscles. If the oxygen supply to the muscles is inadequate then the anaerobic threshold is passed and anaerobic metabolism takes place, with production of lactic acid. After the exercise additional oxygen is needed to convert the lactic acid back to glycogen, and breathing remains enhanced while the oxygen debt is repaid.

The supply of oxygen to the body depends not on the percentage in the air breathed but on its tension or pressure. At high altitude, say 5000 metres above sea level, the percentage of oxygen is still 21%, but because atmospheric pressure is halved, the oxygen pressure is half that at sea level — 75 mm Hg rather than 150 mm Hg. A person may as a result suffer from hypoxia — a lack of oxygen.

High oxygen pressures can be harmful and cause oxygen poisoning, including lung damage and brain dysfunction. In nature high oxygen pressures only exist in deep water diving, and mankind has not had to evolve ways of combating them. Once scientists had purified oxygen it became possible to administer it to patients; this has life-saving possibilities, but care has to be taken not to exceed the toxic level.

Some compounds rich in oxygen, such as the pollutant ozone (itself a molecular form of oxygen), and hydrogen peroxide, can react with cells to produce strongly reactive forms of oxygen. Superoxide anions and unstable oxygen free radicals (such as hydroxyl and hydroperoxy radicals) can be toxic to cells, by way of excess lipid peroxidation. These are implicated, for example, in damage following the restoration of blood flow (reperfusion) after the blockage which causes heart attacks or strokes, and in a variety of other disease processes. However the body does have inherent enzymatic defences against free radical accumulation, and there are antioxidants, such as uric acid, ascorbate, and glutathione, which can inactivate them. Free radicals are likely to contribute also to the ageing process: the very substance by which we live may itself limit our lifespan. Thus oxygen, like most good things, can be dangerous in excess.

Mankind evolved to live close to sea level. Climbing mountains (causing hypoxia) and deep-sea diving (causing nitrogen narcosis or oxygen poisoning) can both be dangerous, in the absence of the right precautions.

Oxygen is one of the basic chemical elements. In its most common form, oxygen is a colorless gas found in air. It is one of the life-sustaining elements on Earth and is needed by all animals. Oxygen is also used in many industrial, commercial, medical, and scientific applications. It is used in blast furnaces to make steel, and is an important component in the production of many synthetic chemicals, including ammonia, alcohols, and various plastics. Oxygen and acetylene are combusted together to provide the very high temperatures needed for welding and metal cutting. When oxygen is cooled below -297° F (-183° C), it becomes a pale blue liquid that is used as a rocket fuel.

Oxygen is one of the most abundant chemical elements on Earth. About one-half of the earth's crust is made up of chemical compounds containing oxygen, and a fifth of our atmosphere is oxygen gas. The human body is about two-thirds oxygen. Although oxygen has been present since the beginning of scientific investigation, it wasn't discovered and recognized as a separate element until 1774 when Joseph Priestley of England isolated it by heating mercuric oxide in an inverted test tube with the focused rays of the sun. Priestley described his discovery to the French scientist Antoine Lavoisier, who experimented further and determined that it was one of the two main components of air. Lavoisier named the new gas oxygen using the Greek words oxys, meaning sour or acid, and genes, meaning producing or forming, because he believed it was an essential part of all acids.

In 1895, Karl Paul Gottfried von Linde of Germany and William Hampson of England independently developed a process for lowering the temperature of air until it liquefied. By carefully distillation of the liquid air, the various component gases could be boiled off one at a time and captured. This process quickly became the principal source of high quality oxygen, nitrogen, and argon.

In 1901, compressed oxygen gas was burned with acetylene gas in the first demonstration of oxy-acetylene welding. This technique became a common industrial method of welding and cutting metals.

The first use of liquid rocket propellants came in 1923 when Robert Goddard of the United States developed a rocket engine using gasoline as the fuel and liquid oxygen as the oxidizer. In 1926, he successfully flew a small liquid-fueled rocket a distance of 184 ft (56 m) at a speed of about 60 mph (97 kph).

After World War II, new technologies brought significant improvements to the air separation process used to produce oxygen. Production volumes and purity levels increased while costs decreased. In 1991, over 470 billion cubic feet (13.4 billion cubic meters) of oxygen were produced in the United States, making it the second-largest-volume industrial gas in use.

Worldwide the five largest oxygen-producing areas are Western Europe, Russia (formerly the USSR), the United States, Eastern Europe, and Japan.

Raw Materials

Oxygen can be produced from a number of materials, using several different methods. The most common natural method is photo-synthesis, in which plants use sunlight convert carbon dioxide in the air into oxygen. This offsets the respiration process, in which animals convert oxygen in the air back into carbon dioxide.

The most common commercial method for producing oxygen is the separation of air using either a cryogenic distillation process or a vacuum swing adsorption process. Nitrogen and argon are also produced by separating them from air.

Oxygen can also be produced as the result of a chemical reaction in which oxygen is freed from a chemical compound and becomes a gas. This method is used to generate limited quantities of oxygen for life support on submarines, aircraft, and spacecraft.

Hydrogen and oxygen can be generated by passing an electric current through water and collecting the two gases as they bubble off. Hydrogen forms at the negative terminal and oxygen at the positive terminal. This method is called electrolysis and produces very pure hydrogen and oxygen. It uses a large amount of electrical energy, however, and is not economical for large-volume production.

The Manufacturing
Process

Most commercial oxygen is produced using a variation of the cryogenic distillation process originally developed in 1895. This process produces oxygen that is 99+% pure. More recently, the more energy-efficient vacuum swing adsorption process has been used for a limited number of applications that do not require oxygen with more than 90-93% purity.

Here are the steps used to produce commercial-grade oxygen from air using the cryogenic distillation process.

Pretreating

Because this process utilizes an extremely cold cryogenic section to separate the air, all impurities that might solidify—such as water vapor, carbon dioxide, and certain heavy hydrocarbons—must first be removed to prevent them from freezing and plugging the cryogenic piping.

* The air is compressed to about 94 psi (650 kPa or 6.5 atm) in a multi-stage compressor. It then passes through a water-cooled aftercooler to condense any water vapor, and the condensed water is removed in a water separator.
* The air passes through a molecular sieve adsorber. The adsorber contains zeolite and silica gel-type adsorbents, which trap the carbon dioxide, heavier hydrocarbons, and any remaining traces of water vapor. Periodically the adsorber is flushed clean to remove the trapped impurities. This usually requires two adsorbers operating in parallel, so that one can continue to process the air-flow while the other one is flushed.

Separating

Air is separated into its major components—nitrogen, oxygen, and argon—through a distillation process known as fractional distillation. Sometimes this name is shortened to fractionation, and the vertical structures used to perform this separation are called fractionating columns. In the fractional distillation process, the components are gradually separated in several stages. At each stage the level of concentration, or fraction, of each component is increased until the separation is complete.

Because all distillation processes work on the principle of boiling a liquid to separate one or more of the components, a cryogenic section is required to provide the very low temperatures needed to liquefy the gas components.

* The pretreated air stream is split. A small portion of the air is diverted through a compressor, where its pressure is boosted. It is then cooled and allowed to expand to nearly atmospheric pressure. This expansion rapidly cools the air, which is injected into the cryogenic section to provide the required cold temperatures for operation.
* The main stream of air passes through one side of a pair of plate fin heat exchangers operating in series, while very cold oxygen and nitrogen from the cryogenic section pass through the other side. The incoming air stream is cooled, while the oxygen and nitrogen are warmed. In some operations, the air may be cooled by passing it through an expansion valve instead of the second heat exchanger. In either case, the temperature of the air is lowered to the point where the oxygen, which has the highest boiling point, starts to liquefy.
* The air stream—now part liquid and part gas—enters the base of the high-pressure fractionating column. As the air works its way up the column, it loses additional heat. The oxygen continues to liquefy, forming an oxygen-rich mixture in the bottom of the column, while most of the nitrogen and argon flow to the top as a vapor.
* The liquid oxygen mixture, called crude liquid oxygen, is drawn out of the bottom of the lower fractionating column and is cooled further in the subcooler. Part of this stream is allowed to expand to nearly atmospheric pressure and is fed into the low-pressure fractionating column. As the crude liquid oxygen works its way down the column, most of the remaining nitrogen and argon separate, leaving 99.5% pure oxygen at the bottom of the column.
* Meanwhile, the nitrogen/argon vapor from the top of the high-pressure column is cooled further in the subcooler. The mixed vapor is allowed to expand to nearly atmospheric pressure and is fed into the top of the low-pressure fractionating column. The nitrogen, which has the lowest boiling point, turns to gas first and flows out the top of the column as 99.995% pure nitrogen.
* The argon, which has a boiling point between the oxygen and the nitrogen, remains a vapor and begins to sink as the nitrogen boils off. As the argon vapor reaches a point about two-thirds the way down the column, the argon concentration reaches its maximum of about 7-12% and is drawn off into a third fractionating column, where it is further recirculated and refined. The final product is a stream of crude argon containing 93-96% argon, 2-5% oxygen, and the balance nitrogen with traces of other gases.

Purifying

The oxygen at the bottom of the low-pressure column is about 99.5% pure. Newer cryogenic distillation units are designed to recover more of the argon from the low-pressure column, and this improves the oxygen purity to about 99.8%.

* If higher purity is needed, one or more additional fractionating columns may be added in conjunction with the low-pressure column to further refine the oxygen product. In some cases, the oxygen may also be passed over a catalyst to oxidize any hydrocarbons. This process produces carbon dioxide and water vapor, which are then captured and removed.

Distributing

About 80-90% of the oxygen produced in the United States is distributed to the end users in gas pipelines from nearby air separation plants. In some parts of the country, an extensive network of pipelines serves many end users over an area of hundred of miles (kilometers). The gas is compressed to about 500 psi (3.4 MPa or 34 atm) and flows through pipes that are 4-12 in (10-30 cm) in diameter. Most of the remaining oxygen is distributed in insulated tank trailers or railroad tank cars as liquid oxygen.

* If the oxygen is to be liquefied, this process is usually done within the low-pressure fractionating column of the air separation plant. Nitrogen from the top of the low-pressure column is compressed, cooled, and expanded to liquefy the nitrogen. This liquid nitrogen stream is then fed back into the low-pressure column to provide the additional cooling required to liquefy the oxygen as it sinks to the bottom of the column.
* Because liquid oxygen has a high boiling point, it boils off rapidly and is rarely shipped farther than 500 mi (800 km). It is transported in large, insulated tanks. The tank body is constructed of two shells and the air is evacuated between the inner and outer shell to retard heat loss. The vacuum space is filled with a semisolid insulating material to further halt heat flow from the outside.

Quality Control

The Compressed Gas Association establishes grading standards for both gaseous oxygen and liquid oxygen based on the amount and type of impurities present. Gas grades are called Type I and range from A, which is 99.0% pure, to F, which is 99.995% pure. Liquid grades are called Type II and also range from A to F, although the types and amounts of allowable impurities in liquid grades are different than in gas grades. Type I Grade B and Grade C and Type II Grade C are 99.5% pure and are the most commonly produced grades of oxygen. They are used in steel making and in the manufacture of synthetic chemicals.

The operation of cryogenic distillation airseparation units is monitored by automatic instruments and often uses computer controls. As a result, their output is consistent in quality. Periodic sampling and analysis of the final product ensures that the standards of purity are being met.

The Future

In January 1998, the United States launched the Lunar Prospector satellite into orbit around the moon. Among its many tasks, this satellite will be scanning the surface of the moon for indications of water. Scientists hope that if sufficient quantities of water are found, it could be used to produce hydrogen and oxygen gases through electrolysis, using solar power to generate the electricity. The hydrogen could be used as a fuel, and the oxygen could be used to provide life support for lunar colonies. Another plan involves extracting oxygen from chemical compounds in the lunar soil using a solar-powered furnace for heat.