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Oxygen DemandsSandeep Badarla

ContentsArticles

Oxygen 1Oxygen saturation 23Chemical oxygen demand 24Biochemical oxygen demand 27Theoretical oxygen demand 32Carbonaceous biochemical oxygen demand 33

ReferencesArticle Sources and Contributors 37Image Sources, Licenses and Contributors 38

Article LicensesLicense 39

Oxygen 1

Oxygen

Oxygen

Appearance

Colorless gas; pale blue liquid. Oxygen bubbles rise in this photo of liquidoxygen.

Spectral lines of oxygen

General properties

Name, symbol, number oxygen, O, 8

Pronunciation /ˈɒksɪdʒɪn/ OK-si-jin

Element category nonmetal, chalcogen

Group, period, block 16, 2, p

Standard atomic weight 15.9994(3) g·mol−1

Electron configuration 1s2 2s2 2p4

Electrons per shell 2, 6 (Image)

Physical properties

Phase gas

Density (0 °C, 101.325 kPa)1.429 g/L

Melting point 54.36 K,-218.79 °C,-361.82 °F

Boiling point 90.20 K,-182.95 °C,-297.31 °F

Critical point 154.59 K, 5.043 MPa

Heat of fusion (O2) 0.444 kJ·mol−1

Heat of vaporization (O2) 6.82 kJ·mol−1

Specific heat capacity (25 °C) (O2)29.378 J·mol−1·K−1

Vapor pressure

P/Pa 1 10 100 1 k 10 k 100 k

at T/K 61 73 90

Atomic properties

Oxidation states 2, 1, −1, −2

Oxygen 2

Electronegativity 3.44 (Pauling scale)

Ionization energies(more)

1st: 1313.9 kJ·mol−1

2nd: 3388.3 kJ·mol−1

3rd: 5300.5 kJ·mol−1

Covalent radius 66±2 pm

Van der Waals radius 152 pm

Miscellanea

Crystal structure cubic

Magnetic ordering paramagnetic

Thermal conductivity (300 K) 26.58x10-3  W·m−1·K−1

Speed of sound (gas, 27 °C) 330 m/s

CAS registry number 7782-44-7

Most stable isotopes

iso NA half-life DM DE (MeV) DP

16O 99.76% 16O is stable with 8 neutron

17O 0.039% 17O is stable with 9 neutron

18O 0.201% 18O is stable with 10 neutron

Oxygen (  /ˈɒksɪdʒɪn/ OK-si-jin) is the element with atomic number 8 and represented by the symbol O. Its namederives from the Greek roots ὀξύς (oxys) ("acid", literally "sharp", referring to the sour taste of acids) and -γενής(-genēs) ("producer", literally "begetter"), because at the time of naming, it was mistakenly thought that all acidsrequired oxygen in their composition. At standard temperature and pressure, two atoms of the element bind to formdioxygen, a very pale blue, odorless, tasteless diatomic gas with the formula O2.Oxygen is a member of the chalcogen group on the periodic table and is a highly reactive nonmetallic element thatreadily forms compounds (notably oxides) with almost all other elements. Oxygen is a strong oxidizing agent andhas the second-highest electronegativity of all the elements (only fluorine has a higher electronegativity).[1] By mass,oxygen is the third-most abundant element in the universe, after hydrogen and helium[2] and the most abundantelement by mass in the Earth's crust, making up almost half of the crust's mass.[3] Free oxygen is too chemicallyreactive to appear on Earth without the photosynthetic action of living organisms, which use the energy of sunlight toproduce elemental oxygen from water. Elemental O2 only began to accumulate in the atmosphere after theevolutionary appearance of these organisms, roughly 2.5 billion years ago.[4] Diatomic oxygen gas constitutes 20.8%of the volume of air.[5]

Because it comprises most of the mass in water, oxygen comprises most of the mass of living organisms (forexample, about two-thirds of the human body's mass). All major classes of structural molecules in living organisms,such as proteins, carbohydrates, and fats, contain oxygen, as do the major inorganic compounds that comprise animalshells, teeth, and bone. Elemental oxygen is produced by cyanobacteria, algae and plants, and is used in cellularrespiration for all complex life. Oxygen is toxic to obligately anaerobic organisms, which were the dominant form ofearly life on Earth until O2 began to accumulate in the atmosphere. Another form (allotrope) of oxygen, ozone (O3),helps protect the biosphere from ultraviolet radiation with the high-altitude ozone layer, but is a pollutant near thesurface where it is a by-product of smog. At even higher low earth orbit altitudes atomic oxygen is a significantpresence and a cause of erosion for spacecraft.[6]

Oxygen 3

Oxygen was independently discovered by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, and Joseph Priestleyin Wiltshire, in 1774, but Priestley is often given priority because his work was published first. The name oxygenwas coined in 1777 by Antoine Lavoisier,[7] whose experiments with oxygen helped to discredit the then-popularphlogiston theory of combustion and corrosion. Oxygen is produced industrially by fractional distillation of liquefiedair, use of zeolites with pressure-cycling to concentrate oxygen from air, electrolysis of water and other means. Usesof oxygen include the production of steel, plastics and textiles; rocket propellant; oxygen therapy; and life support inaircraft, submarines, spaceflight and diving.

Characteristics

Structure

Oxygen discharge (spectrum) tube

At standard temperature and pressure, oxygen is a very pale blue, odorless gaswith the molecular formula O2, in which the two oxygen atoms are chemicallybonded to each other with a spin triplet electron configuration. This bond has abond order of two, and is often simplified in description as a double bond[8] or asa combination of one two-electron bond and two three-electron bonds.[9]

Triplet oxygen (not to be confused with ozone, O3) is the ground state of the O2 molecule.[10] The electronconfiguration of the molecule has two unpaired electrons occupying two degenerate molecular orbitals.[11] Theseorbitals are classified as antibonding (weakening the bond order from three to two), so the diatomic oxygen bond isweaker than the diatomic nitrogen triple bond in which all bonding molecular orbitals are filled, but someantibonding orbitals are not.[10]

A trickle of liquid oxygen isdeflected by a magnetic field,

illustrating its paramagnetic property

In normal triplet form, O2 molecules are paramagnetic. That is, they form amagnet in the presence of a magnetic field—because of the spin magneticmoments of the unpaired electrons in the molecule, and the negative exchangeenergy between neighboring O2 molecules.[12] Liquid oxygen is attracted to amagnet to a sufficient extent that, in laboratory demonstrations, a bridge of liquidoxygen may be supported against its own weight between the poles of a powerfulmagnet.[13][14]

Singlet oxygen is a name given to several higher-energy species of molecular O2in which all the electron spins are paired. It is much more reactive towardscommon organic molecules than is molecular oxygen per se. In nature, singlet

oxygen is commonly formed from water during photosynthesis, using the energy of sunlight.[15] It is also producedin the troposphere by the photolysis of ozone by light of short wavelength,[16] and by the immune system as a sourceof active oxygen.[17] Carotenoids in photosynthetic organisms (and possibly also in animals) play a major role inabsorbing energy from singlet oxygen and converting it to the unexcited ground state before it can cause harm totissues.[18]

Oxygen 4

Allotropes

Ozone is a rare gas on Earth foundmostly in the stratosphere.

The common allotrope of elemental oxygen on Earth is called dioxygen, O2. Ithas a bond length of 121 pm and a bond energy of 498 kJ·mol−1.[19] This is theform that is used by complex forms of life, such as animals, in cellularrespiration (see Biological role) and is the form that is a major part of the Earth'satmosphere (see Occurrence). Other aspects of O2 are covered in the remainderof this article.

Trioxygen (O3) is usually known as ozone and is a very reactive allotrope ofoxygen that is damaging to lung tissue.[20] Ozone is produced in the upper

atmosphere when O2 combines with atomic oxygen made by the splitting of O2 by ultraviolet (UV) radiation.[7]

Since ozone absorbs strongly in the UV region of the spectrum, the ozone layer of the upper atmosphere functions asa protective radiation shield for the planet.[7] Near the Earth's surface, however, it is a pollutant formed as aby-product of automobile exhaust.[20] The metastable molecule tetraoxygen (O4) was discovered in 2001,[21][22] andwas assumed to exist in one of the six phases of solid oxygen. It was proven in 2006 that this phase, created bypressurizing O2 to 20 GPa, is in fact a rhombohedral O8 cluster.[23] This cluster has the potential to be a much morepowerful oxidizer than either O2 or O3 and may therefore be used in rocket fuel.[21][22] A metallic phase wasdiscovered in 1990 when solid oxygen is subjected to a pressure of above 96 GPa[24] and it was shown in 1998 thatat very low temperatures, this phase becomes superconducting.[25]

Physical propertiesOxygen is more soluble in water than nitrogen is; water contains approximately 1 molecule of O2 for every 2molecules of N2, compared to an atmospheric ratio of approximately 1:4. The solubility of oxygen in water istemperature-dependent, and about twice as much (14.6 mg·L−1) dissolves at 0 °C than at 20 °C (7.6 mg·L−1).[26][27]

At 25 °C and 1 standard atmosphere (unknown operator: u'strong' kPa) of air, freshwater contains about6.04 milliliters (mL) of oxygen per liter, whereas seawater contains about 4.95 mL per liter.[28] At 5 °C the solubilityincreases to 9.0 mL (50% more than at 25 °C) per liter for water and 7.2 mL (45% more) per liter for sea water.Oxygen condenses at 90.20 K (−182.95 °C, −297.31 °F), and freezes at 54.36 K (−218.79 °C, −361.82 °F).[29] Bothliquid and solid O2 are clear substances with a light sky-blue color caused by absorption in the red (in contrast withthe blue color of the sky, which is due to Rayleigh scattering of blue light). High-purity liquid O2 is usually obtainedby the fractional distillation of liquefied air.[30] Liquid oxygen may also be produced by condensation out of air,using liquid nitrogen as a coolant. It is a highly reactive substance and must be segregated from combustiblematerials.[31]

Oxygen 5

Isotopes and stellar origin

Late in a massive star's life, 16O concentrates inthe O-shell, 17O in the H-shell and 18O in the

He-shell.

Naturally occurring oxygen is composed of three stable isotopes, 16O,17O, and 18O, with 16O being the most abundant (99.762% naturalabundance).[32]

Most 16O is synthesized at the end of the helium fusion process inmassive stars but some is made in the neon burning process.[33] 17O isprimarily made by the burning of hydrogen into helium during theCNO cycle, making it a common isotope in the hydrogen burningzones of stars.[33] Most 18O is produced when 14N (made abundantfrom CNO burning) captures a 4He nucleus, making 18O common inthe helium-rich zones of evolved, massive stars.[33]

Fourteen radioisotopes have been characterized. The most stable are15O with a half-life of 122.24 seconds and 14O with a half-life of70.606 seconds.[32] All of the remaining radioactive isotopes havehalf-lives that are less than 27 s and the majority of these havehalf-lives that are less than 83 milliseconds.[32] The most common decay mode of the isotopes lighter than 16O is β+

decay[34][35][36] to yield nitrogen, and the most common mode for the isotopes heavier than 18O is beta decay toyield fluorine.[32]

Occurrence

Ten most common elements in the Milky Way Galaxy estimated spectroscopically[37]

Z Element Mass fraction in parts per million

1 Hydrogen 739,000 71 × mass of Oxygen (red bar)

2 Helium 240,000 23 × mass of Oxygen (red bar)

8 Oxygen 10,400 unknown operator: u','

6 Carbon 4,600 unknown operator: u','

10 Neon 1,340 unknown operator: u','

26 Iron 1,090 unknown operator: u','

7 Nitrogen 960

14 Silicon 650

12 Magnesium 580

16 Sulfur 440

Oxygen is the most abundant chemical element, by mass, in our biosphere, air, sea and land. Oxygen is the thirdmost abundant chemical element in the universe, after hydrogen and helium.[2] About 0.9% of the Sun's mass isoxygen.[5] Oxygen constitutes 49.2% of the Earth's crust by mass[3] and is the major component of the world'soceans (88.8% by mass).[5] Oxygen gas is the second most common component of the Earth's atmosphere, taking up20.8% of its volume and 23.1% of its mass (some 1015 tonnes).[5][38][39] Earth is unusual among the planets of theSolar System in having such a high concentration of oxygen gas in its atmosphere: Mars (with 0.1% O2 by volume)and Venus have far lower concentrations. However, the O2 surrounding these other planets is produced solely byultraviolet radiation impacting oxygen-containing molecules such as carbon dioxide.

Oxygen 6

The unusually high concentration of oxygen gas on Earth is the result of the oxygen cycle. This biogeochemicalcycle describes the movement of oxygen within and between its three main reservoirs on Earth: the atmosphere, thebiosphere, and the lithosphere. The main driving factor of the oxygen cycle is photosynthesis, which is responsiblefor modern Earth's atmosphere. Photosynthesis releases oxygen into the atmosphere, while respiration and decayremove it from the atmosphere. In the present equilibrium, production and consumption occur at the same rate ofroughly 1/2000th of the entire atmospheric oxygen per year.

Cold water holds more dissolved O2.

Free oxygen also occurs in solution in the world's water bodies. Theincreased solubility of O2 at lower temperatures (see Physicalproperties) has important implications for ocean life, as polar oceanssupport a much higher density of life due to their higher oxygencontent.[40] Polluted water may have reduced amounts of O2 in it,depleted by decaying algae and other biomaterials through a processcalled eutrophication. Scientists assess this aspect of water quality bymeasuring the water's biochemical oxygen demand, or the amount ofO2 needed to restore it to a normal concentration.[41]

Biological role

Photosynthesis and respiration

Photosynthesis splits water to liberate O2 andfixes CO2 into sugar.

In nature, free oxygen is produced by the light-driven splitting of waterduring oxygenic photosynthesis. According to some estimates, Greenalgae and cyanobacteria in marine environments provide about 70% ofthe free oxygen produced on earth and the rest is produced byterrestrial plants.[42] Other estimates of the oceanic contribution toatmospheric oxygen are higher, while some estimates are lower,suggesting oceans produce ~45% of Earth's atmospheric oxygen eachyear.[43]

A simplified overall formula for photosynthesis is:[44]

6 CO2 + 6 H2O + photons → C6H12O6 + 6 O2 (or simplycarbon dioxide + water + sunlight → glucose + dioxygen)

Photolytic oxygen evolution occurs in the thylakoid membranes ofphotosynthetic organisms and requires the energy of four photons.[45]

Many steps are involved, but the result is the formation of a protongradient across the thylakoid membrane, which is used to synthesizeATP via photophosphorylation.[46] The O2 remaining after oxidation of

the water molecule is released into the atmosphere.[47]

Molecular dioxygen, O2, is essential for cellular respiration in all aerobic organisms. Oxygen is used in mitochondriato help generate adenosine triphosphate (ATP) during oxidative phosphorylation. The reaction for aerobic respirationis essentially the reverse of photosynthesis and is simplified as:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + 2880 kJ·mol−1

In vertebrates, O2 diffuses through membranes in the lungs and into red blood cells. Hemoglobin binds O2, changing its color from bluish red to bright red[20] (CO2 is released from another part of hemoglobin through the Bohr effect). Other animals use hemocyanin (molluscs and some arthropods) or hemerythrin (spiders and lobsters).[38] A liter of

Oxygen 7

blood can dissolve 200 cm3 of O2.[38]

Reactive oxygen species, such as superoxide ion (O) and hydrogen peroxide (H2O2), are dangerous by-products ofoxygen use in organisms.[38] Parts of the immune system of higher organisms, however, create peroxide, superoxide,and singlet oxygen to destroy invading microbes. Reactive oxygen species also play an important role in thehypersensitive response of plants against pathogen attack.[46]

An adult human in rest inhales 1.8 to 2.4 grams of oxygen per minute.[48] This amounts to more than 6 billion tonnesof oxygen inhaled by humanity per year.[49]

Content in body

Partial pressures of oxygen in the human body (PO2)

Unit Alveolarpulmonary

gas pressures

Arterial blood oxygen Venous blood gas

kPa 14.2 11[50]-13[50] 4.0[50]-5.3[50]

mmHg 107 75[51]-100[51] 30[52]-40[52]

The oxygen content in the body of a living organism is usually highest in the respiratory system, and decreases alongany arterial system, peripheral tissues and venous system, respectively. Oxygen content in this sense is often given asthe partial pressure, which is the pressure which oxygen would have if it alone occupied the volume.[53]

Build-up in the atmosphere

O2 build-up in Earth's atmosphere: 1) no O2 produced; 2) O2 produced, butabsorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is

absorbed by land surfaces and formation of ozone layer; 4–5) O2 sinks filled andthe gas accumulates

Free oxygen gas was almost nonexistent inEarth's atmosphere before photosyntheticarchaea and bacteria evolved. Free oxygenfirst appeared in significant quantitiesduring the Paleoproterozoic eon (between2.5 and 1.6 billion years ago). At first, theoxygen combined with dissolved iron in theoceans to form banded iron formations. Freeoxygen started to outgas from the oceans2.7 billion years ago, reaching 10% of itspresent level around 1.7 billion yearsago.[54]

The presence of large amounts of dissolvedand free oxygen in the oceans and atmosphere may have driven most of the anaerobic organisms then living toextinction during the Great Oxygenation Event(oxygen catastrophe) about 2.4 billion years ago. However, cellularrespiration using O2 enables aerobic organisms to produce much more ATP than anaerobic organisms, helping theformer to dominate Earth's biosphere.[55] Photosynthesis and cellular respiration of O2 allowed for the evolution ofeukaryotic cells and ultimately complex multicellular organisms such as plants and animals.

Since the beginning of the Cambrian period 540 million years ago, O2 levels have fluctuated between 15% and 30% by volume.[56] Towards the end of the Carboniferous period (about 300 million years ago) atmospheric O2 levels reached a maximum of 35% by volume,[56] which may have contributed to the large size of insects and amphibians at this time.[57] 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] At the current rate of photosynthesis it would take

Oxygen 8

about 2,000 years to regenerate the entire O2 in the present atmosphere.[58]

History

Early experiments

Philo's experiment inspired laterinvestigators.

One of the first known experiments on the relationship between combustion andair was conducted by the 2nd century BCE Greek writer on mechanics, Philo ofByzantium. In his work Pneumatica, Philo observed that inverting a vessel over aburning candle and surrounding the vessel's neck with water resulted in somewater rising into the neck.[59] Philo incorrectly surmised that parts of the air inthe vessel were converted into the classical element fire and thus were able toescape through pores in the glass. Many centuries later Leonardo da Vinci builton Philo's work by observing that a portion of air is consumed during combustionand respiration.[60]

In the late 17th century, Robert Boyle proved that air is necessary forcombustion. English chemist John Mayow (1641–1679) refined this work byshowing that fire requires only a part of air that he called spiritus nitroaereus orjust nitroaereus.[61] In one experiment he found that placing either a mouse or alit candle in a closed container over water caused the water to rise and replaceone-fourteenth of the air's volume before extinguishing the subjects.[62] From thishe surmised that nitroaereus is consumed in both respiration and combustion.

Mayow observed that antimony increased in weight when heated, and inferredthat the nitroaereus must have combined with it.[61] He also thought that thelungs separate nitroaereus from air and pass it into the blood and that animal heatand muscle movement result from the reaction of nitroaereus with certain substances in the body.[61] Accounts ofthese and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract "Derespiratione".[62]

Phlogiston theory

Stahl helped develop and popularizethe phlogiston theory.

Robert Hooke, Ole Borch, Mikhail Lomonosov, and Pierre Bayen all producedoxygen in experiments in the 17th and the 18th century but none of themrecognized it as an element.[26] This may have been in part due to the prevalenceof the philosophy of combustion and corrosion called the phlogiston theory,which was then the favored explanation of those processes.

Established in 1667 by the German alchemist J. J. Becher, and modified by thechemist Georg Ernst Stahl by 1731,[63] phlogiston theory stated that allcombustible materials were made of two parts. One part, called phlogiston, wasgiven off when the substance containing it was burned, while thedephlogisticated part was thought to be its true form, or calx.[60]

Highly combustible materials that leave little residue, such as wood or coal, werethought to be made mostly of phlogiston; whereas non-combustible substancesthat corrode, such as iron, contained very little. Air did not play a role inphlogiston theory, nor were any initial quantitative experiments conducted to test

Oxygen 9

the idea; instead, it was based on observations of what happens when something burns, that most common objectsappear to become lighter and seem to lose something in the process.[60] The fact that a substance like wood actuallygains overall weight in burning was hidden by the buoyancy of the gaseous combustion products. Indeed one of thefirst clues that the phlogiston theory was incorrect was that metals, too, gain weight in rusting (when they weresupposedly losing phlogiston).

Discovery

Carl Wilhelm Scheele beat Priestleyto the discovery but published

afterwards.

Oxygen was first discovered by Swedish pharmacist Carl Wilhelm Scheele. Hehad produced oxygen gas by heating mercuric oxide and various nitrates byabout 1772.[5][60] Scheele called the gas "fire air" because it was the only knownsupporter of combustion, and wrote an account of this discovery in a manuscripthe titled Treatise on Air and Fire, which he sent to his publisher in 1775.However, that document was not published until 1777.[64]

Joseph Priestley is usually givenpriority in the discovery.

In the meantime, on August 1, 1774, an experiment conducted by the Britishclergyman Joseph Priestley focused sunlight on mercuric oxide (HgO) inside aglass tube, which liberated a gas he named "dephlogisticated air".[5] He notedthat candles burned brighter in the gas and that a mouse was more active andlived longer while breathing it. After breathing the gas himself, he wrote: "Thefeeling of it to my lungs was not sensibly different from that of common air, but Ifancied that my breast felt peculiarly light and easy for some timeafterwards."[26] Priestley published his findings in 1775 in a paper titled "AnAccount of Further Discoveries in Air" which was included in the second volumeof his book titled Experiments and Observations on Different Kinds of Air.[60][65]

Because he published his findings first, Priestley is usually given priority in thediscovery.

The noted French chemist Antoine Laurent Lavoisier later claimed to havediscovered the new substance independently. However, Priestley visited Lavoisier in October 1774 and told himabout his experiment and how he liberated the new gas. Scheele also posted a letter to Lavoisier on September 30,1774 that described his own discovery of the previously unknown substance, but Lavoisier never acknowledgedreceiving it (a copy of the letter was found in Scheele's belongings after his death).[64]

Oxygen 10

Lavoisier's contributionWhat Lavoisier did indisputably do (although this was disputed at the time) was to conduct the first adequatequantitative experiments on oxidation and give the first correct explanation of how combustion works.[5] He usedthese and similar experiments, all started in 1774, to discredit the phlogiston theory and to prove that the substancediscovered by Priestley and Scheele was a chemical element.

Antoine Lavoisier discredited thePhlogiston theory.

In one experiment, Lavoisier observed that there was no overall increase inweight when tin and air were heated in a closed container.[5] He noted that airrushed in when he opened the container, which indicated that part of the trappedair had been consumed. He also noted that the tin had increased in weight andthat increase was the same as the weight of the air that rushed back in. This andother experiments on combustion were documented in his book Sur lacombustion en général, which was published in 1777.[5] In that work, he provedthat air is a mixture of two gases; 'vital air', which is essential to combustion andrespiration, and azote (Gk. ἄζωτον "lifeless"), which did not support either.Azote later became nitrogen in English, although it has kept the name in Frenchand several other European languages.[5]

Lavoisier renamed 'vital air' to oxygène in 1777 from the Greek roots ὀξύς (oxys)(acid, literally "sharp," from the taste of acids) and -γενής (-genēs) (producer, literally begetter), because hemistakenly believed that oxygen was a constituent of all acids.[7] Chemists (notably Sir Humphrey Davy in 1812)eventually determined that Lavoisier was wrong in this regard (it is in fact hydrogen that forms the basis for acidchemistry), but by that time it was too late, the name had taken.

Oxygen entered the English language despite opposition by English scientists and the fact that the EnglishmanPriestley had first isolated the gas and written about it. This is partly due to a poem praising the gas titled "Oxygen"in the popular book The Botanic Garden (1791) by Erasmus Darwin, grandfather of Charles Darwin.[64]

Later history

Robert H. Goddard and a liquid oxygen-gasolinerocket

John Dalton's original atomic hypothesis assumed that all elementswere monoatomic and that the atoms in compounds would normallyhave the simplest atomic ratios with respect to one another. Forexample, Dalton assumed that water's formula was HO, giving theatomic mass of oxygen as 8 times that of hydrogen, instead of themodern value of about 16.[66] In 1805, Joseph Louis Gay-Lussac andAlexander von Humboldt showed that water is formed of two volumesof hydrogen and one volume of oxygen; and by 1811 AmedeoAvogadro had arrived at the correct interpretation of water'scomposition, based on what is now called Avogadro's law and theassumption of diatomic elemental molecules.[67][68]

By the late 19th century scientists realized that air could be liquefied,and its components isolated, by compressing and cooling it. Using acascade method, Swiss chemist and physicist Raoul Pierre Pictetevaporated liquid sulfur dioxide in order to liquefy carbon dioxide,which in turn was evaporated to cool oxygen gas enough to liquefy it.He sent a telegram on December 22, 1877 to the French Academy of Sciences in Paris announcing his discovery of

liquid oxygen.[69] Just two days later, French physicist Louis Paul Cailletet announced his own method of liquefying molecular oxygen.[69] Only a few drops of the liquid were produced in either case so no meaningful analysis could

Oxygen 11

be conducted. Oxygen was liquified in stable state for the first time on March 29, 1883 by Polish scientists fromJagiellonian University, Zygmunt Wróblewski and Karol Olszewski.[70]

In 1891 Scottish chemist James Dewar was able to produce enough liquid oxygen to study.[12] The firstcommercially viable process for producing liquid oxygen was independently developed in 1895 by German engineerCarl von Linde and British engineer William Hampson. Both men lowered the temperature of air until it liquefiedand then distilled the component gases by boiling them off one at a time and capturing them.[71] Later, in 1901,oxyacetylene welding was demonstrated for the first time by burning a mixture of acetylene and compressed O2.This method of welding and cutting metal later became common.[71]

In 1923 the American scientist Robert H. Goddard became the first person to develop a rocket engine; the engineused gasoline for fuel and liquid oxygen as the oxidizer. Goddard successfully flew a small liquid-fueled rocket 56 mat 97 km/h on March 16, 1926 in Auburn, Massachusetts, USA.[71][72]

Industrial productionTwo major methods are employed to produce 100 million tonnes of O2 extracted from air for industrial usesannually.[64] The most common method is to fractionally distill liquefied air into its various components, with N2distilling as a vapor while O2 is left as a liquid.[64]

Hofmann electrolysis apparatus used inelectrolysis of water.

The other major method of producing O2 gas involves passing a streamof clean, dry air through one bed of a pair of identical zeolite molecularsieves, which absorbs the nitrogen and delivers a gas stream that is90% to 93% O2.[64] Simultaneously, nitrogen gas is released from theother nitrogen-saturated zeolite bed, by reducing the chamber operatingpressure and diverting part of the oxygen gas from the producer bedthrough it, in the reverse direction of flow. After a set cycle time theoperation of the two beds is interchanged, thereby allowing for acontinuous supply of gaseous oxygen to be pumped through a pipeline.This is known as pressure swing adsorption. Oxygen gas isincreasingly obtained by these non-cryogenic technologies (see alsothe related vacuum swing adsorption).[73]

Oxygen gas can also be produced through electrolysis of water intomolecular oxygen and hydrogen. DC electricity must be used: if AC isused, the gases in each limb consist of hydrogen and oxygen in theexplosive ratio 2:1. Contrary to popular belief, the 2:1 ratio observed inthe DC electrolysis of acidified water does not prove that the empiricalformula of water is H2O unless certain assumptions are made about themolecular formulae of hydrogen and oxygen themselves. A similarmethod is the electrocatalytic O2 evolution from oxides and oxoacids.

Chemical catalysts can be used as well, such as in chemical oxygen generators or oxygen candles that are used aspart of the life-support equipment on submarines, and are still part of standard equipment on commercial airliners incase of depressurization emergencies. Another air separation technology involves forcing air to dissolve throughceramic membranes based on zirconium dioxide by either high pressure or an electric current, to produce nearly pureO2 gas.[41]

In large quantities, the price of liquid oxygen in 2001 was approximately $0.21/kg.[74] Since the primary cost ofproduction is the energy cost of liquefying the air, the production cost will change as energy cost varies.For reasons of economy, oxygen is often transported in bulk as a liquid in specially insulated tankers, since one litre of liquefied oxygen is equivalent to 840 liters of gaseous oxygen at atmospheric pressure and 20 °C (unknown

Oxygen 12

operator: u'strong' °F).[64] Such tankers are used to refill bulk liquid oxygen storage containers, which standoutside hospitals and other institutions with a need for large volumes of pure oxygen gas. Liquid oxygen is passedthrough heat exchangers, which convert the cryogenic liquid into gas before it enters the building. Oxygen is alsostored and shipped in smaller cylinders containing the compressed gas; a form that is useful in certain portablemedical applications and oxy-fuel welding and cutting.[64]

Applications

Medical

An oxygen concentrator in anemphysema patient's house

Uptake of O2 from the air is the essential purpose of respiration, so oxygensupplementation is used in medicine. Treatment not only increases oxygen levelsin the patient's blood, but has the secondary effect of decreasing resistance toblood flow in many types of diseased lungs, easing work load on the heart.Oxygen therapy is used to treat emphysema, pneumonia, some heart disorders(congestive heart failure), some disorders that cause increased pulmonary arterypressure, and any disease that impairs the body's ability to take up and usegaseous oxygen.[75]

Treatments are flexible enough to be used in hospitals, the patient's home, orincreasingly by portable devices. Oxygen tents were once commonly used inoxygen supplementation, but have since been replaced mostly by the use ofoxygen masks or nasal cannulas.[76]

Hyperbaric (high-pressure) medicine uses special oxygen chambers to increasethe partial pressure of O2 around the patient and, when needed, the medical

staff.[77] Carbon monoxide poisoning, gas gangrene, and decompression sickness (the 'bends') are sometimes treatedusing these devices.[78] Increased O2 concentration in the lungs helps to displace carbon monoxide from the hemegroup of hemoglobin.[79][80] Oxygen gas is poisonous to the anaerobic bacteria that cause gas gangrene, soincreasing its partial pressure helps kill them.[81][82] Decompression sickness occurs in divers who decompress tooquickly after a dive, resulting in bubbles of inert gas, mostly nitrogen and helium, forming in their blood. Increasingthe pressure of O2 as soon as possible is part of the treatment.[75][83][84]

Oxygen is also used medically for patients who require mechanical ventilation, often at concentrations above the21% found in ambient air.

Oxygen 13

Life support and recreational use

Low pressure pure O2 is used in space suits.

A notable application of O2 as a low-pressure breathing gas is inmodern space suits, which surround their occupant's body withpressurized air. These devices use nearly pure oxygen at about onethird normal pressure, resulting in a normal blood partial pressure ofO2.[85][86] This trade-off of higher oxygen concentration for lowerpressure is needed to maintain flexible spacesuits.

Scuba divers and submariners also rely on artificially delivered O2, butmost often use normal pressure, and/or mixtures of oxygen and air.Pure or nearly pure O2 use in diving at higher-than-sea-level pressuresis usually limited to rebreather, decompression, or emergencytreatment use at relatively shallow depths (~6 meters depth, orless).[87][88] Deeper diving requires significant dilution of O2 withother gases, such as nitrogen or helium, to help prevent oxygentoxicity.[87]

People who climb mountains or fly in non-pressurized fixed-wing aircraft sometimes have supplemental O2supplies.[89] Passengers traveling in (pressurized) commercial airplanes have an emergency supply of O2automatically supplied to them in case of cabin depressurization. Sudden cabin pressure loss activates chemicaloxygen generators above each seat, causing oxygen masks to drop. Pulling on the masks "to start the flow ofoxygen" as cabin safety instructions dictate, forces iron filings into the sodium chlorate inside the canister.[41] Asteady stream of oxygen gas is then produced by the exothermic reaction.

Oxygen, as a supposed mild euphoric, has a history of recreational use in oxygen bars and in sports. Oxygen bars areestablishments, found in Japan, California, and Las Vegas, Nevada since the late 1990s that offer higher than normalO2 exposure for a fee.[90] Professional athletes, especially in American football, also sometimes go off field betweenplays to wear oxygen masks in order to get a "boost" in performance. The pharmacological effect is doubtful; aplacebo effect is a more likely explanation.[90] Available studies support a performance boost from enriched O2mixtures only if they are breathed during aerobic exercise.[91]

Other recreational uses that do not involve breathing the gas include pyrotechnic applications, such as GeorgeGoble's five-second ignition of barbecue grills.[92]

Industrial

Most commercially produced O2 is used to smeltiron into steel.

Smelting of iron ore into steel consumes 55% of commerciallyproduced oxygen.[41] In this process, O2 is injected through ahigh-pressure lance into molten iron, which removes sulfur impuritiesand excess carbon as the respective oxides, SO2 and CO2. Thereactions are exothermic, so the temperature increases to 1,700 °C.[41]

Another 25% of commercially produced oxygen is used by thechemical industry.[41] Ethylene is reacted with O2 to create ethyleneoxide, which, in turn, is converted into ethylene glycol; the primaryfeeder material used to manufacture a host of products, includingantifreeze and polyester polymers (the precursors of many plastics andfabrics).[41]

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.[41] Oxygen is used in oxyacetylene welding burning

Oxygen 14

acetylene with O2 to produce a very hot flame. In this process, metal up to 60 cm thick is first heated with a smalloxy-acetylene flame and then quickly cut by a large stream of O2.[93] Larger rockets use liquid oxygen as theiroxidizer, which is mixed and ignited with the fuel for propulsion.

Scientific

500 million years of climate change vs 18O

Paleoclimatologists measure the ratio of oxygen-18 andoxygen-16 in the shells and skeletons of marineorganisms to determine what the climate was likemillions of years ago (see oxygen isotope ratio cycle).Seawater molecules that contain the lighter isotope,oxygen-16, evaporate at a slightly faster rate than watermolecules containing the 12% heavier oxygen-18; thisdisparity increases at lower temperatures.[94] Duringperiods of lower global temperatures, snow and rainfrom that evaporated water tends to be higher inoxygen-16, and the seawater left behind tends to behigher in oxygen-18. Marine organisms thenincorporate more oxygen-18 into their skeletons and

shells than they would in a warmer climate.[94] Paleoclimatologists also directly measure this ratio in the watermolecules of ice core samples that are up to several hundreds of thousands of years old.

Planetary geologists have measured different abundances of oxygen isotopes in samples from the Earth, the Moon,Mars, and meteorites, but were long unable to obtain reference values for the isotope ratios in the Sun, believed to bethe same as those of the primordial solar nebula. However, analysis of a silicon wafer exposed to the solar wind inspace and returned by the crashed Genesis spacecraft has shown that the Sun has a higher proportion of oxygen-16than does the Earth. The measurement implies that an unknown process depleted oxygen-16 from the Sun's disk ofprotoplanetary material prior to the coalescence of dust grains that formed the Earth.[95]

Oxygen presents two spectrophotometric absorption bands peaking at the wavelengths 687 and 760 nm. Someremote sensing scientists have proposed using the measurement of the radiance coming from vegetation canopies inthose bands to characterize plant health status from a satellite platform.[96] This approach exploits the fact that inthose bands it is possible to discriminate the vegetation's reflectance from its fluorescence, which is much weaker.The measurement is technically difficult owing to the low signal-to-noise ratio and the physical structure ofvegetation; but it has been proposed as a possible method of monitoring the carbon cycle from satellites on a globalscale.

Oxygen 15

Compounds

Water (H2O) is the most familiaroxygen compound.

The oxidation state of oxygen is −2 in almost all known compounds of oxygen.The oxidation state −1 is found in a few compounds such as peroxides.[97]

Compounds containing oxygen in other oxidation states are very uncommon:−1/2 (superoxides), −1/3 (ozonides), 0 (elemental, hypofluorous acid), +1/2(dioxygenyl), +1 (dioxygen difluoride), and +2 (oxygen difluoride).

Oxides and other inorganic compounds

Water (H2O) is the oxide of hydrogen and the most familiar oxygen compound.Hydrogen atoms are covalently bonded to oxygen in a water molecule but alsohave an additional attraction (about 23.3 kJ·mol−1 per hydrogen atom) to anadjacent oxygen atom in a separate molecule.[98] These hydrogen bonds betweenwater molecules hold them approximately 15% closer than what would beexpected in a simple liquid with just van der Waals forces.[99][100]

Oxides, such as iron oxide or rust form whenoxygen combines with other elements.

Due to its electronegativity, oxygen forms chemical bonds with almostall other elements at elevated temperatures to give correspondingoxides. However, some elements readily form oxides at standardconditions for temperature and pressure; the rusting of iron is anexample. The surface of metals like aluminium and titanium areoxidized in the presence of air and become coated with a thin film ofoxide that passivates the metal and slows further corrosion. Some ofthe transition metal oxides are found in nature as non-stoichiometriccompounds, with a slightly less metal than the chemical formula wouldshow. For example, the natural occurring FeO (wüstite) is actuallywritten as Fe1 − xO, where x is usually around 0.05.[101]

Oxygen as a compound is present in the atmosphere in trace quantities in the form of carbon dioxide (CO2). Theearth's crustal rock is composed in large part of oxides of silicon (silica SiO2, found in granite and sand), aluminium(aluminium oxide Al2O3, in bauxite and corundum), iron (iron(III) oxide Fe2O3, in hematite and rust), and calciumcarbonate (in limestone). The rest of the Earth's crust is also made of oxygen compounds, in particular variouscomplex silicates (in silicate minerals). The Earth's mantle, of much larger mass than the crust, is largely composedof silicates of magnesium and iron.

Water-soluble silicates in the form of Na4SiO4, Na2SiO3, and Na2Si2O5 are used as detergents and adhesives.[102]

Oxygen also acts as a ligand for transition metals, forming metal–O2 bonds with the iridium atom in Vaska'scomplex,[103] with the platinum in PtF6,[104] and with the iron center of the heme group of hemoglobin.

Oxygen 16

Organic compounds and biomolecules

Acetone is an important feeder material in thechemical industry.

  Oxygen  Carbon  Hydrogen|alt=A ball structureof a molecule. Its backbone is a zig-zag chain ofthree carbon atoms connected in the center to an

oxygen atom and on the end to 6 hydrogens.

Oxygen represents more than 40% of themolecular mass of the ATP molecule.

Among the most important classes of organic compounds that containoxygen are (where "R" is an organic group): alcohols (R-OH); ethers(R-O-R); ketones (R-CO-R); aldehydes (R-CO-H); carboxylic acids(R-COOH); esters (R-COO-R); acid anhydrides (R-CO-O-CO-R); andamides (R-C(O)-NR2). There are many important organic solvents thatcontain oxygen, including: acetone, methanol, ethanol, isopropanol,furan, THF, diethyl ether, dioxane, ethyl acetate, DMF, DMSO, aceticacid, and formic acid. Acetone ((CH3)2CO) and phenol (C6H5OH) areused as feeder materials in the synthesis of many different substances.Other important organic compounds that contain oxygen are: glycerol,formaldehyde, glutaraldehyde, citric acid, acetic anhydride, andacetamide. Epoxides are ethers in which the oxygen atom is part of aring of three atoms.

Oxygen reacts spontaneously with many organic compounds at orbelow room temperature in a process called autoxidation.[105] Most ofthe organic compounds that contain oxygen are not made by directaction of O2. Organic compounds important in industry and commercethat are made by direct oxidation of a precursor include ethylene oxideand peracetic acid.[102]

The element is found in almost all biomolecules that are important to(or generated by) life. Only a few common complex biomolecules,such as squalene and the carotenes, contain no oxygen. Of the organiccompounds with biological relevance, carbohydrates contain thelargest proportion by mass of oxygen. All fats, fatty acids, amino acids,and proteins contain oxygen (due to the presence of carbonyl groups inthese acids and their ester residues). Oxygen also occurs in phosphate (PO) groups in the biologically importantenergy-carrying molecules ATP and ADP, in the backbone and the purines (except adenine) and pyrimidines ofRNA and DNA, and in bones as calcium phosphate and hydroxylapatite.

Oxygen 17

Safety and precautions

Toxicity

Main symptoms of oxygen toxicity[106]

Oxygen toxicity occurs when the lungs take in ahigher than normal O2 partial pressure, which can

occur in deep scuba diving.

Oxygen gas (O2) can be toxic at elevatedpartial pressures, leading to convulsions andother health problems.[87][107][108] Oxygentoxicity usually begins to occur at partialpressures more than 50 kilopascals (kPa), or2.5 times the normal sea-level O2 partialpressure of about 21 kPa (equal to about50% oxygen composition at standardpressure). This is not a problem except forpatients on mechanical ventilators, since gassupplied through oxygen masks in medicalapplications is typically composed of only30%–50% O2 by volume (about 30 kPa atstandard pressure).[26] (although this figurealso is subject to wide variation, dependingon type of mask).

At one time, premature babies were placedin incubators containing O2-rich air, but thispractice was discontinued after some babieswere blinded by it.[26][109]

Breathing pure O2 in space applications,such as in some modern space suits, or inearly spacecraft such as Apollo, causes nodamage due to the low total pressuresused.[85][110] In the case of spacesuits, theO2 partial pressure in the breathing gas is, ingeneral, about 30 kPa (1.4 times normal),and the resulting O2 partial pressure in theastronaut's arterial blood is only marginallymore than normal sea-level O2 partialpressure (for more information on this, seespace suit and arterial blood gas).

Oxygen toxicity to the lungs and centralnervous system can also occur in deep scuba

diving and surface supplied diving.[26][87] Prolonged breathing of an air mixture with an O2 partial pressure morethan 60 kPa can eventually lead to permanent pulmonary fibrosis.[111] Exposure to a O2 partial pressures greater than160 kPa (about 1.6 atm) may lead to convulsions (normally fatal for divers). Acute oxygen toxicity (causingseizures, its most feared effect for divers) can occur by breathing an air mixture with 21% O2 at 66 m or more ofdepth; the same thing can occur by breathing 100% O2 at only 6 m.[111][112][113][114]

Oxygen 18

Combustion and other hazards

The interior of the Apollo 1 Command Module.Pure O2 at higher than normal pressure and aspark led to a fire and the loss of the Apollo 1

crew.

Highly concentrated sources of oxygen promote rapid combustion. Fireand explosion hazards exist when concentrated oxidants and fuels arebrought into close proximity; however, an ignition event, such as heator a spark, is needed to trigger combustion.[115] Oxygen itself is not thefuel, but the oxidant. Combustion hazards also apply to compounds ofoxygen with a high oxidative potential, such as peroxides, chlorates,nitrates, perchlorates, and dichromates because they can donate oxygento a fire.

Concentrated O2 will allow combustion to proceed rapidly andenergetically.[115] Steel pipes and storage vessels used to store andtransmit both gaseous and liquid oxygen will act as a fuel; andtherefore the design and manufacture of O2 systems requires specialtraining to ensure that ignition sources are minimized.[115] The fire thatkilled the Apollo 1 crew in a launch pad test spread so rapidly because the capsule was pressurized with pure O2 butat slightly more than atmospheric pressure, instead of the 1⁄3 normal pressure that would be used in amission.[116][117]

Liquid oxygen spills, if allowed to soak into organic matter, such as wood, petrochemicals, and asphalt can causethese materials to detonate unpredictably on subsequent mechanical impact.[115] As with other cryogenic liquids, oncontact with the human body it can cause frostbites to the skin and the eyes.

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[9] Pauling, L. (1960). The nature of the chemical bond and the structure of molecules and crystals : an introduction to modern structuralchemistry (3rd ed.). Ithaca, N.Y.: Cornell University Press. ISBN 0-8014-0333-2.

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[11] An orbital is a concept from quantum mechanics that models an electron as a wave-like particle that has a spacial distribution about an atomor molecule.

[12][12] Emsley 2001, p.303[13] "Demonstration of a bridge of liquid oxygen supported against its own weight between the poles of a powerful magnet" (http:/ / web.

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Oxygen 19

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[41][41] Emsley 2001, p.301[42] Fenical, William (September 1983). "Marine Plants: A Unique and Unexplored Resource" (http:/ / books. google. com/

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In fact, chloroplasts are thought to have evolved from cyanobacteria that were once symbiotic partners with the progenerators of plants andalgae.

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Oxygen 20

[47] Water oxidation is catalyzed by a manganese-containing enzyme complex known as the oxygen evolving complex (OEC) or water-splittingcomplex found associated with the lumenal side of thylakoid membranes. Manganese is an important cofactor, and calcium and chloride arealso required for the reaction to occur.(Raven 2005)

[48] "For humans, the normal volume is 6–8 liters per minute." (http:/ / www. patentstorm. us/ patents/ 6224560-description. html)[49][49] (1.8 grams/min/person)×(60 min/h)×(24 h/day)×(365 days/year)×(6.6 billion people)/1,000,000 g/t=6.24 billion tonnes[50][50] Derived from mmHg values using 0.133322 kPa/mmHg[51] Normal Reference Range Table (http:/ / pathcuric1. swmed. edu/ PathDemo/ nrrt. htm) from The University of Texas Southwestern Medical

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20080117230939/ http:/ / www. physics. upenn. edu/ courses/ gladney/ mathphys/ subsubsection1_1_3_2. html). University of Pennsylvania.Archived from the original (http:/ / www. physics. upenn. edu/ courses/ gladney/ mathphys/ Contents. html) on January 17, 2008. . Retrieved2008-01-28.

[67] Roscoe, Henry Enfield; Schorlemmer, Carl (1883). A Treatise on Chemistry. D. Appleton and Co.. p. 38.[68] However, these results were mostly ignored until 1860. Part of this rejection was due to the belief that atoms of one element would have no

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[84] Acott, C. (1999). "A brief history of diving and decompression illness" (http:/ / archive. rubicon-foundation. org/ 6004). South PacificUnderwater Medicine Society Journal 29 (2). ISSN 0813-1988. OCLC 16986801. . Retrieved 2008-09-22.

[85] Morgenthaler GW, Fester DA, Cooley CG (1994). "As assessment of habitat pressure, oxygen fraction, and EVA suit design for spaceoperations". Acta Astronaut 32 (1): 39–49. doi:10.1016/0094-5765(94)90146-5. PMID 11541018.

[86] Webb JT, Olson RM, Krutz RW, Dixon G, Barnicott PT (1989). "Human tolerance to 100% oxygen at 9.5 psia during five daily simulated8-hour EVA exposures". Aviat Space Environ Med 60 (5): 415–21. PMID 2730484.

[87] Acott, C. (1999). "Oxygen toxicity: A brief history of oxygen in diving" (http:/ / archive. rubicon-foundation. org/ 6014). South PacificUnderwater Medicine Society Journal 29 (3). ISSN 0813-1988. OCLC 16986801. . Retrieved 2008-09-21.

[88] Longphre, J. M. et al.; Denoble, PJ; Moon, RE; Vann, RD; Freiberger, JJ (2007). "First aid normobaric oxygen for the treatment ofrecreational diving injuries" (http:/ / archive. rubicon-foundation. org/ 5514). Undersea Hyperb Med. 34 (1): 43–49. ISSN 1066-2936.OCLC 26915585. PMID 17393938. . Retrieved 2008-09-21.

[89] The reason is that increasing the proportion of oxygen in the breathing gas at low pressure acts to augment the inspired O2 partial pressurenearer to that found at sea-level.

[90] Bren, Linda (November–December 2002). "Oxygen Bars: Is a Breath of Fresh Air Worth It?" (http:/ / web. archive. org/ web/20071018041754/ http:/ / www. fda. gov/ Fdac/ features/ 2002/ 602_air. html). FDA Consumer magazine. U.S. Food and DrugAdministration. Archived from the original (http:/ / www. fda. gov/ Fdac/ features/ 2002/ 602_air. html) on October 18, 2007. . Retrieved2007-12-23.

[91] "Ergogenic Aids" (http:/ / web. archive. org/ web/ 20070928051412/ http:/ / www. pponline. co. uk/ encyc/ 1008. htm). Peak PerformanceOnline. Archived from the original (http:/ / www. pponline. co. uk/ encyc/ 1008. htm) on 2007-09-28. . Retrieved 2008-01-04.

[92] "George Goble's extended home page (mirror)" (http:/ / www. bkinzel. de/ misc/ ghg/ index. html). .[93] Cook & Lauer 1968, p.508[94][94] Emsley 2001, p.304[95] Hand, Eric (2008-03-13). "The Solar System's first breath" (http:/ / www. nature. com/ news/ 2008/ 080313/ full/ 452259a. html). Nature

452 (7185): 259. Bibcode 2008Natur.452..259H. doi:10.1038/452259a. PMID 18354437. . Retrieved 2009-03-18.[96] Miller, J.R.; Berger, M.; Alonso, L.; Cerovic, Z.; Goulas, Y.; Jacquemoud, S.; Louis, J.; Mohammed, G.; Moya, I.; Pedros, R.; Moreno, J.F.;

Verhoef, W.; Zarco-Tejada, P.J.. "Progress on the development of an integrated canopy fluorescence model" (http:/ / ieeexplore. ieee. org/ xpl/freeabs_all. jsp?tp=& arnumber=1293855& isnumber=28601). Geoscience and Remote Sensing Symposium, 2003. IGARSS '03. Proceedings.2003 IEEE International. . Retrieved 2008-01-22.

[97] Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements (2nd ed.). Butterworth–Heinemann. ISBN 0080379419.,p. 28[98] Maksyutenko, P.; T. R. Rizzo, and O. V. Boyarkin (2006). "A direct measurement of the dissociation energy of water". J. Chem. Phys. 125

(18): 181101. Bibcode 2006JChPh.125r1101M. doi:10.1063/1.2387163. PMID 17115729.[99] Chaplin, Martin (2008-01-04). "Water Hydrogen Bonding" (http:/ / www. lsbu. ac. uk/ water/ hbond. html). . Retrieved 2008-01-06.[100] Also, since oxygen has a higher electronegativity than hydrogen, the charge difference makes it a polar molecule. The interactions between

the different dipoles of each molecule cause a net attraction force.[101] Smart, Lesley E.; Moore, Elaine A. (2005). Solid State Chemistry: An Introduction (3rd ed.). CRC Press. p. 214. ISBN 978-0-7487-7516-3.[102] Cook & Lauer 1968, p.507[103] Crabtree, R. (2001). The Organometallic Chemistry of the Transition Metals (3rd ed.). John Wiley & Sons. p. 152.

ISBN 978-0-471-18423-2.[104] Cook & Lauer 1968, p.505[105] Cook & Lauer 1968, p.506[106] Dharmeshkumar N Patel, Ashish Goel, SB Agarwal, Praveenkumar Garg, Krishna K Lakhani (2003). "Oxygen Toxicity" (http:/ / medind.

nic. in/ jac/ t03/ i3/ jact03i3p234. pdf). Indian Academy of Clinical Medicine 4 (3): 234. .[107] Since O2's partial pressure is the fraction of O2 times the total pressure, elevated partial pressures can occur either from high O2 fraction in

breathing gas or from high breathing gas pressure, or a combination of both.[108] Cook & Lauer 1968, p.511[109] Drack AV (1998). "Preventing blindness in premature infants". N. Engl. J. Med. 338 (22): 1620–1. doi:10.1056/NEJM199805283382210.

PMID 9603802.

Oxygen 22

[110] Wade, Mark (2007). "Space Suits" (http:/ / web. archive. org/ web/ 20071213122134/ http:/ / www. astronautix. com/ craftfam/ spasuits.htm). Encyclopedia Astronautica. Archived from the original (http:/ / www. astronautix. com/ craftfam/ spasuits. htm) on December 13, 2007.. Retrieved 2007-12-16.

[111] Wilmshurst P (1998). "Diving and oxygen". BMJ 317 (7164): 996–9. doi:10.1136/bmj.317.7164.996. PMC 1114047. PMID 9765173.[112] Donald, Kenneth (1992). Oxygen and the Diver. England: SPA in conjunction with K. Donald. ISBN 1-85421-176-5.[113] Donald K. W. (1947). "Oxygen Poisoning in Man: Part I". Br Med J 1 (4506): 667–72. doi:10.1136/bmj.1.4506.667. PMC 2053251.

PMID 20248086.[114] Donald K. W. (1947). "Oxygen Poisoning in Man: Part II". Br Med J 1 (4507): 712–7. doi:10.1136/bmj.1.4507.712. PMC 2053400.

PMID 20248096.[115] Werley, Barry L. (Edtr.) (1991). "Fire Hazards in Oxygen Systems". ASTM Technical Professional training. Philadelphia: ASTM

International Subcommittee G-4.05.[116][116] No single ignition source of the fire was conclusively identified, although some evidence points to arc from an electrical spark). (Report of

Apollo 204 Review Board NASA Historical Reference Collection, NASA History Office, NASA HQ, Washington, DC)[117] Chiles, James R. (2001). Inviting Disaster: Lessons from the edge of Technology: An inside look at catastrophes and why they happen.

New York: HarperCollins Publishers Inc.. ISBN 0-06-662082-1.

References• Cook, Gerhard A.; Lauer, Carol M. (1968). "Oxygen". In Clifford A. Hampel. The Encyclopedia of the Chemical

Elements. New York: Reinhold Book Corporation. pp. 499–512. LCCN 68-29938.• Emsley, John (2001). "Oxygen". Nature's Building Blocks: An A-Z Guide to the Elements. Oxford, England, UK:

Oxford University Press. pp. 297–304. ISBN 0-19-850340-7.• Raven, Peter H.; Ray F. Evert, Susan E. Eichhorn (2005). Biology of Plants, 7th Edition. New York: W.H.

Freeman and Company Publishers. pp. 115–27. ISBN 0-7167-1007-2.

Further reading• Walker, J. (1980). "The oxygen cycle". In Hutzinger O.. Handbook of Environmental Chemistry. Volume 1. Part

A: The natural environment and the biogeochemical cycles. Berlin; Heidelberg; New York: Springer-Verlag.p. 258. ISBN 0-387-09688-4.

External links• The Periodic Table of Videos video of Oxygen (http:/ / www. youtube. com/ ?v=WuG5WTId-IY) at YouTube• Oxidizing Agents > Oxygen (http:/ / www. organic-chemistry. org/ chemicals/ oxidations/ oxygen. shtm)• Oxygen (O2) Properties, Uses, Applications (http:/ / www. uigi. com/ oxygen. html)• Roald Hoffmann article on "The Story of O" (http:/ / www. americanscientist. org/ template/ AssetDetail/ assetid/

29647/ page/ 1)• WebElements.com – Oxygen (http:/ / www. webelements. com/ webelements/ elements/ text/ O/ index. html)• Chemistry in its element podcast (http:/ / www. rsc. org/ chemistryworld/ podcast/ element. asp) (MP3) from the

Royal Society of Chemistry's Chemistry World: Oxygen (http:/ / www. rsc. org/ images/CIIE_oxygen_48k_tcm18-117681. mp3)

• Oxygen (http:/ / www. bbc. co. uk/ programmes/ b0088nql) on In Our Time at the BBC. ( listen now (http:/ /www. bbc. co. uk/ iplayer/ console/ b0088nql/ In_Our_Time_Oxygen))

• Scripps Institute: Atmospheric Oxygen has been dropping for 20 years (http:/ / scrippso2. ucsd. edu/ )

Oxygen saturation 23

Oxygen saturationOxygen saturation or dissolved oxygen (DO) is a relative measure of the amount of oxygen that is dissolved orcarried in a given medium. It can be measured with a dissolved oxygen probe such as an oxygen sensor or an optodein liquid media, usually water.Oxygen saturation can be measured regionally and non-invasively. Arterial oxygenation is commonly measuredusing pulse oximetry. Tissue saturation at peripheral scale can be measured using NIRS. This technique can beapplied on both muscle as brain.

Oxygen in medicineIn medicine, oxygen saturation refers to oxygenation, or when oxygen molecules (O2) enter the tissues of the body.In this case blood is oxygenated in the lungs, where oxygen molecules travel from the air and into the blood. Oxygensaturation, or O2 sats measure the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen.Fish, invertebrates, plants, and aerobic bacteria all require oxygen for respiration. Blood is also vital to the bodysystem. The optimal levels in an estuary for Dissolved Oxygen (DO) is higher than 6 ppm.

Environmental oxygen saturationOxygen saturation in the environment generally refers to the amount of oxygen dissolved in the soil or bodies ofwater. Environmental oxygenation can be important to the sustainability of a particular ecosystem. Insufficientoxygen (environmental hypoxia) may occur in bodies of water such as ponds and rivers, tending to suppress thepresence of aerobic organisms such as fish. Deoxygenation increases the relative population of anaerobic organismssuch as plants and some bacteria, resulting in fish kills and other adverse events. The net effect is to alter the balanceof nature by increasing the concentration of anaerobic over aerobic species.

Chemical oxygen demand 24

Chemical oxygen demandIn environmental chemistry, the chemical oxygen demand (COD) test is commonly used to indirectly measure theamount of organic compounds in water. Most applications of COD determine the amount of organic pollutants foundin surface water (e.g. lakes and rivers) or wastewater, making COD a useful measure of water quality. It is expressedin milligrams per liter (mg/L), which indicates the mass of oxygen consumed per liter of solution. Older referencesmay express the units as parts per million (ppm).

OverviewThe basis for the COD test is that nearly all organic compounds can be fully oxidized to carbon dioxide with a strongoxidizing agent under acidic conditions. The amount of oxygen required to oxidize an organic compound to carbondioxide, ammonia, and water is given by:

This expression does not include the oxygen demand caused by the oxidation of ammonia into nitrate. The process ofammonia being converted into nitrate is referred to as nitrification. The following is the correct equation for theoxidation of ammonia into nitrate.

It is applied after the oxidation due to nitrification if the oxygen demand from nitrification must be known.Dichromate does not oxidize ammonia into nitrate, so this nitrification can be safely ignored in the standard chemicaloxygen demand test.The International Organization for Standardization describes a standard method for measuring chemical oxygendemand in ISO 6060 [1].

HistoryFor many years, the strong oxidizing agent potassium permanganate (KMnO4) was used for measuring chemicaloxygen demand. Measurements were called oxygen consumed from permanganate, rather than the oxygen demand oforganic substances. Potassium permanganate's effectiveness at oxidizing organic compounds varied widely, and inmany cases biochemical oxygen demand (BOD) measurements were often much greater than results from CODmeasurements. This indicated that potassium permanganate was not able to effectively oxidize all organiccompounds in water, rendering it a relatively poor oxidizing agent for determining COD.Since then, other oxidizing agents such as ceric sulphate, potassium iodate, and potassium dichromate have beenused to determine COD. Of these, potassium dichromate (K2Cr2O7) has been shown to be the most effective: it isrelatively cheap, easy to purify, and is able to nearly completely oxidize almost all organic compounds.In these methods, a fixed volume with a known excess amount of the oxidant is added to a sample of the solutionbeing analyzed. After a refluxing digestion step, the initial concentration of organic substances in the sample iscalculated from a titrimetric or spectrophotometric determination of the oxidant still remaining in the sample.

Chemical oxygen demand 25

Using potassium dichromatePotassium dichromate is a strong oxidizing agent under acidic conditions. (Acidity is usually achieved by theaddition of sulfuric acid.) The reaction of potassium dichromate with organic compounds is given by:

where d = 2n/3 + a/6 - b/3 - c/2. Most commonly, a 0.25 N solution of potassium dichromate is used for CODdetermination, although for samples with COD below 50 mg/L, a lower concentration of potassium dichromate ispreferred.In the process of oxidizing the organic substances found in the water sample, potassium dichromate is reduced (sincein all redox reactions, one reagent is oxidized and the other is reduced), forming Cr3+. The amount of Cr3+ isdetermined after oxidization is complete, and is used as an indirect measure of the organic contents of the watersample.

BlanksBecause COD measures the oxygen demand of organic compounds in a sample of water, it is important that nooutside organic material be accidentally added to the sample to be measured. To control for this, a so-called blanksample is required in the determination of COD (and BOD -biochemical oxygen demand - for that matter). A blanksample is created by adding all reagents (e.g. acid and oxidizing agent) to a volume of distilled water. COD ismeasured for both the water and blank samples, and the two are compared. The oxygen demand in the blank sampleis subtracted from the COD for the original sample to ensure a true measurement of organic matter.

Measurement of excessFor all organic matter to be completely oxidized, an excess amount of potassium dichromate (or any oxidizing agent)must be present. Once oxidation is complete, the amount of excess potassium dichromate must be measured toensure that the amount of Cr3+ can be determined with accuracy. To do so, the excess potassium dichromate istitrated with ferrous ammonium sulfate (FAS) until all of the excess oxidizing agent has been reduced to Cr3+.Typically, the oxidation-reduction indicator Ferroin is added during this titration step as well. Once all the excessdichromate has been reduced, the Ferroin indicator changes from blue-green to reddish-brown. The amount offerrous ammonium sulfate added is equivalent to the amount of excess potassium dichromate added to the originalsample. and also we can determine COD by boiling the water sample and we can determine CO2 ratio by theinfra-red analyzer

Preparation Ferroin Indicator reagentA solution of 1.485 g 1,10-phenanthroline monohydrate is added to a solution of 695 mg FeSO4·7H2O in water, andthe resulting red solution is diluted to 100 mL.

CalculationsThe following formula is used to calculate COD:

where b is the volume of FAS used in the blank sample, s is the volume of FAS in the original sample, and n is thenormality of FAS. If milliliters are used consistently for volume measurements, the result of the COD calculation isgiven in mg/L.The COD can also be estimated from the concentration of oxidizable compound in the sample, based on its stoichiometric reaction with oxygen to yield CO2 (assume all C goes to CO2), H2O (assume all H goes to H2O), and

Chemical oxygen demand 26

NH3 (assume all N goes to NH3), using the following formula:COD = (C/FW)(RMO)(32)

Where C = Concentration of oxidizable compound in the sample,FW = Formula weight of the oxidizable compound in the sample,RMO = Ratio of the # of moles of oxygen to # of moles of oxidizable compound in their reaction to CO2,water, and ammonia

For example, if a sample has 500 wppm of phenol:C6H5OH + 7O2 → 6CO2 + 3H2OCOD = (500/94)(7)(32) = 1191 wppm

Inorganic interferenceSome samples of water contain high levels of oxidizable inorganic materials which may interfere with thedetermination of COD. Because of its high concentration in most wastewater, chloride is often the most serioussource of interference. Its reaction with potassium dichromate follows the equation:

Prior to the addition of other reagents, mercuric sulfate can be added to the sample to eliminate chloride interference.The following table lists a number of other inorganic substances that may cause interference. The table also listschemicals that may be used to eliminate such interference, and the compounds formed when the inorganic moleculeis eliminated.

Inorganic molecule Eliminated by Elimination forms

Chloride Mercuric sulfate Mercuric chloride complex

Nitrite Sulfamic acid N2 gas

Ferrous iron - -

Sulfides - -

Government regulationMany governments impose strict regulations regarding the maximum chemical oxygen demand allowed inwastewater before they can be returned to the environment. For example, in Switzerland, a maximum oxygendemand between 200 and 1000 mg/L must be reached before wastewater or industrial water can be returned to theenvironment [2].

References• Clair N. Sawyer, Perry L. McCarty, Gene F. Parkin (2003). Chemistry for Environmental Engineering and

Science (5th ed.). New York: McGraw-Hill. ISBN 0-07-248066-1.• Lenore S. Clescerl, Arnold E. Greenberg, Andrew D. Eaton. Standard Methods for Examination of Water &

Wastewater (20th ed.). Washington, DC: American Public Health Association. ISBN 0-87553-235-7.

Chemical oxygen demand 27

External links• ISO 6060: Water quality - Determination of the chemical oxygen demand [1]

• Water chemical oxygen demand [3] (Food and Agriculture Organization of the United Nations)

References[1] http:/ / www. iso. org/ iso/ en/ CatalogueDetailPage. CatalogueDetail?CSNUMBER=12260& ICS1=13& ICS2=60& ICS3=50[2] http:/ / www. csem. ch/ corporate/ Report2002/ pdf/ p56. pdf[3] http:/ / www. fao. org/ gtos/ tems/ variable_show. jsp?VARIABLE_ID=123

Biochemical oxygen demandBiochemical oxygen demand or B.O.D. is the amount of dissolved oxygen needed by aerobic biological organismsin a body of water to break down organic material present in a given water sample at certain temperature over aspecific time period. The term also refers to a chemical procedure for determining this amount. This is not a precisequantitative test, although it is widely used as an indication of the organic quality of water.[1] The BOD value is mostcommonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20 °C andis often used as a robust surrogate of the degree of organic pollution of water.BOD can be used as a gauge of the effectiveness of wastewater treatment plants. It is listed as a conventionalpollutant in the U.S. Clean Water Act.

BackgroundMost natural waters contain small quantities of organic compounds. Aquatic microorganisms have evolved to usesome of these compounds as food. Microorganisms living in oxygenated waters use dissolved oxygen to convert theorganic compounds into energy for growth and reproduction. Populations of these microorganisms tend to increasein proportion to the amount of food available. This microbial metabolism creates an oxygen demand proportional tothe amount of organic compounds useful as food. Under some circumstances, microbial metabolism can consumedissolved oxygen faster than atmospheric oxygen can dissolve into the water. Fish and aquatic insects may die whenoxygen is depleted by microbial metabolism.[2]

Biochemical oxygen demand is the amount of oxygen required for microbial metabolism of organic compounds inwater. This demand occurs over some variable period of time depending on temperature, nutrient concentrations, andthe enzymes available to indigenous microbial populations. The amount of oxygen required to completely oxidizethe organic compounds to carbon dioxide and water through generations of microbial growth, death, decay, andcannibalism is total biochemical oxygen demand (total BOD). Total BOD is of more significance to food webs thanto water quality. Dissolved oxygen depletion is most likely to become evident during the initial aquatic microbialpopulation explosion in response to a large amount of organic material. If the microbial population deoxygenates thewater, however, that lack of oxygen imposes a limit on population growth of aerobic aquatic microbial organismsresulting in a longer term food surplus and oxygen deficit.[3]

A standard temperature at which BOD testing should be carried out was first proposed by the Royal Commission onSewage Disposal in its eighth report in 1912:

" (c) An effluent in order to comply with the general standard must not contain as discharged more than3 parts per 100,000 of suspended matter, and with its suspended matters included must not take up at65°F (18-3°C.) more than 2.0 parts per 100,000 of dissolved oxygen in 5 days. This general standardshould be prescribed either by Statute or by order of the Central Authority, and should be subject tomodifications by that Authority after an interval of not less than ten years.

Biochemical oxygen demand 28

This was later standardised at 68°F and then 20°C. This temperature may be significantly different from thetemperature of the natural environment of the water being tested. Investigators also decided to eliminate anaerobicconditions.Although the Royal Commission on Sewage Disposal proposed 5 days as an adequate test period for rivers of theUnited Kingdom of Great Britain and Ireland, longer periods were investigated for North American rivers.Incubation periods of 1, 2, 5, 10 and 20 days were being used into the mid-20th century.[4] Keeping dissolvedoxygen available at their chosen temperature, investigators found up to 99 percent of total BOD was exerted within20 days, 90 percent within 10 days, and approximately 68 percent within 5 days.[5] Variable microbial populationshifts to nitrifying bacteria limit test reproducibility for periods greater than 5 days. The 5-day test protocol withacceptably reproducible results emphasizing carbonaceous BOD has been endorsed by the United StatesEnvironmental Protection Agency. This 5-day BOD test result may be described as the amount of oxygen requiredfor aquatic microorganisms to stabilize decomposable organic matter under aerobic conditions.[6] Stabilization, inthis context, may be perceived in general terms as the conversion of food to living aquatic fauna. Although thesefauna will continue to exert biochemical oxygen demand as they die, that tends to occur within a more stable evolvedecosystem including higher trophic levels.[3]

The BOD5 testThere are two commonly recognized methods for the measurement of BOD.

Dilution methodTo ensure that all other conditions are equal, a very small amount of micro-organism seed is added to each samplebeing tested. This seed is typically generated by diluting organisms with buffered dilution water. The BOD test iscarried out by diluting the sample with oxygen saturated dilution water, inoculating it with a fixed aliquot of seed,measuring the dissolved oxygen (DO) and then sealing the sample to prevent further oxygen dissolving in. Thesample is kept at 20 °C in the dark to prevent photosynthesis (and thereby the addition of oxygen) for five days, andthe dissolved oxygen is measured again. The difference between the final DO and initial DO is the BOD.The loss of dissolved oxygen in the sample, once corrections have been made for the degree of dilution, is called theBOD5. For measurement of carbonaceous BOD (cBOD), a nitrification inhibitor is added after the dilution waterhas been added to the sample. The inhibitor hinders the oxidation of ammonia nitrogen.BOD can be calculated by:•• Undiluted: Initial DO - Final DO = BOD•• Diluted: ((Initial DO - Final DO)- BOD of Seed) x Dilution FactorBOD is similar in function to chemical oxygen demand (COD), in that both measure the amount of organiccompounds in water. However, COD is less specific, since it measures everything that can be chemically oxidized,rather than just levels of biologically active organic matter.

Manometric methodThis method is limited to the measurement of the oxygen consumption due only to carbonaceous oxidation.Ammonia oxidation is inhibited.The sample is kept in a sealed container fitted with a pressure sensor. A substance that absorbs carbon dioxide(typically lithium hydroxide) is added in the container above the sample level. The sample is stored in conditionsidentical to the dilution method. Oxygen is consumed and, as ammonia oxidation is inhibited, carbon dioxide isreleased. The total amount of gas, and thus the pressure, decreases because carbon dioxide is absorbed. From thedrop of pressure, the sensor electronics computes and displays the consumed quantity of oxygen.The main advantages of this method compared to the dilution method are:

Biochemical oxygen demand 29

•• simplicity: no dilution of sample required, no seeding, no blank sample.•• direct reading of BOD value.•• continuous display of BOD value at the current incubation time.

Dissolved Oxygen Probes: Membrane and LuminescenceSince the publication of a simple, accurate and direct dissolved oxygen analytical procedure by Winkler,[7] theanalysis of dissolved oxygen levels for water have been key to the determination of surface water purity andecological wellness. The Winkler method is still one of only two analytical techniques used to calibrate oxygenelectrode meters, the other procedure based on oxygen solubility at saturation as per Henry's law. Though manyresearchers have refined the Winkler analysis to dissolved oxygen levels in the low PPB range the method does notlend itself to automation.The development of an analytical instrument that utilizes the reduction-oxidation (redox) chemistry of oxygen in thepresence of dissimilar metal electrodes was introduced during the 1950s.[8] This redox electrode utilized an oxygenpermeable membrane to allow the diffusion of the gas into an electrochemical cell and its concentration determinedby polarographic or galvanic electrodes. This analytical method is sensitive and accurate to down to levels of ±0.1 mg/l dissolved oxygen. Calibration of the redox electrode of this membrane electrode still requires the use of theHenry’s law table or the Winkler test for dissolved oxygen.During the last two decades, a new form of electrode was developed based on the luminescence emission of a photoactive chemical compound and the quenching of that emission by oxygen. This quenching photophysics mechanismis described by the Stern-Volmer equation for dissolved oxygen in a solution[9]:

The determination of oxygen concentration by luminescence quenching has a linear response over a broad range ofoxygen concentrations and has excellent accuracy and reproducibility.[10] There are several recognized EPA methodsfor the measurement of Dissolved Oxygen for BOD, including the following methods:• Standard Methods for the Examination of Water and Wastewater, Method 4500 O[11]

• In-Situ Inc. Method 1003-8-2009 Biochemical Oxygen Demand (BOD) Measurement by Optical Probe.[12]

Test LimitationsThe test method involves variables limiting reproducibility. Tests normally show observations varying plus or minusten to twenty percent around the mean.[13]:82

ToxicitySome wastes contain chemicals capable of suppressing microbiological growth or activity. Potential sources includeindustrial wastes, antibiotics in pharmaceutical or medical wastes, sanitizers in food processing or commercialcleaning facilities, chlorination disinfection used following conventional sewage treatment, and odor-controlformulations used in sanitary waste holding tanks in passenger vehicles or portable toilets. Suppression of themicrobial community oxidizing the waste will lower the test result.[13]:85

Biochemical oxygen demand 30

Appropriate Microbial PopulationThe test relies upon a microbial ecosystem with enzymes capable of oxidizing the available organic material. Somewaste waters, such as those from biological secondary sewage treatment, will already contain a large population ofmicroorganisms acclimated to the water being tested. An appreciable portion of the waste may be utilized during theholding period prior to commencement of the test procedure. On the other hand, organic wastes from industrialsources may require specialized enzymes. Microbial populations from standard seed sources may take some time toproduce those enzymes. A specialized seed culture may be appropriate to reflect conditions of an evolved ecosystemin the receiving waters.[13]:85-87

History of the use of BODThe Royal Commission on River Pollution, which was established in 1865 and the formation of the RoyalCommission on Sewage Disposal in 1898 led to the selection in 1908 of BOD5 as the definitive test for organicpollution of rivers. Five days was chosen as an appropriate test period because this is supposedly the longest timethat river water takes to travel from source to estuary in the U.K.. In its sixth report the Royal Commissionrecommended that the standard set should be 15 parts by weight per million of water.[14] However in the Ninthreport the commission had revised the recommended standard :

" An effluent taking up 2-0 parts dissolved oxygen per 100,000 would be found by a simple calculationto require dilution with at least 8 volumes of river water taking up 0.2 part if the resulting mixture wasnot to take up more than 0.4 part. Our experience indicated that in a large majority of cases the volumeof river water would exceed 8 times the volume of effluent, and that the figure of 2-0 parts dissolvedoxygen per 100,000, which had been shown to be practicable, would be a safe figure to adopt for thepurposes of a general standard, taken in conjunction with the condition that the effluent should notcontain more than 3-0 parts per 100,000 of suspended solids.

[14] This was the cornerstone 20:30 (BOD:Suspended Solids) + full nitrification standard which was used as ayardstick in the U.K. up to the 1970s for sewage works effluent quality.The United States includes BOD effluent limitations in its secondary treatment regulations. Secondary sewagetreatment is generally expected to remove 85 percent of the BOD measured in sewage and produce effluent BODconcentrations with a 30-day average of less than 30 mg/L and a 7-day average of less than 45 mg/L. The regulationsalso describe "treatment equivalent to secondary treatment" as removing 65 percent of the BOD and producingeffluent BOD concentrations with a 30-day average less than 45 mg/L and a 7-day average less than 65 mg/L.[15]

Typical BOD valuesMost pristine rivers will have a 5-day carbonaceous BOD below 1 mg/L. Moderately polluted rivers may have aBOD value in the range of 2 to 8 mg/L. Municipal sewage that is efficiently treated by a three-stage process wouldhave a value of about 20 mg/L or less. Untreated sewage varies, but averages around 600 mg/L in Europe and as lowas 200 mg/L in the U.S., or where there is severe groundwater or surface water Infiltration/Inflow. (The generallylower values in the U.S. derive from the much greater water use per capita than in other parts of the world.)[1]

BOD BiosensorAn alternative to measure BOD is the development of biosensors, which are devices for the detection of an analyte that combines a biological component with a physicochemical detector component. Biosensors can be used to indirectly measure BOD via a fast (usually <30 min) to be determined BOD substitute and a corresponding calibration curve method (pioneered by Karube et al., 1977). Consequently, biosensors are now commercially available, but they do have several limitations such as their high maintenance costs, limited run lengths due to the need for reactivation, and the inability to respond to changing quality characteristics as would normally occur in

Biochemical oxygen demand 31

wastewater treatment streams; e.g. diffusion processes of the biodegradable organic matter into the membrane anddifferent responses by different microbial species which lead to problems with the reproducibility of result (Praet etal., 1995). Another important limitation is the uncertainty associated with the calibration function for translating theBOD substitute into the real BOD (Rustum et al., 2008).

BOD Software sensorRustum et al. (2008) proposed the use the KSOM to develop intelligent models for making rapid inferences aboutBOD using other easy to measure water quality parameters, which, unlike BOD, can be obtained directly andreliably using on-line hardware sensors. This will make the use of BOD for on-line process monitoring and control amore plausible proposition. In comparison to other data-driven modeling paradigms such as multi-layer perceptronsartificial neural networks (MLP ANN) and classical multi-variate regression analysis, the KSOM is not negativelyaffected by missing data. Moreover, time sequencing of data is not a problem when compared to classical time seriesanalysis.

References• Lenore S. Clescerl, Arnold E. Greenberg, Andrew D. Eaton (1999). Standard Methods for Examination of Water

& Wastewater (20th ed.). Washington, DC: American Public Health Association. ISBN 0-87553-235-7. Alsoavailable by online subscription at www.standardmethods.org [16]

• Rustum R., A. J. Adeloye, and M. Scholz (2008) Applying Kohonen Self-organizing Map as a Software Sensor toPredict the Biochemical Oxygen Demand, Water Environment Research, 80 (1), 32 – 40.

Notes[1] Clair N. Sawyer, Perry L. McCarty, Gene F. Parkin (2003). Chemistry for Environmental Engineering and Science (5th ed.). New York:

McGraw-Hill. ISBN 0-07-248066-1.[2] Goldman, Charles R. & Horne, Alexander J. Limnology (1983) McGraw-Hill ISBN 0-07-023651-8 pp.88&267[3] Reid, George K. Ecology of Inland Waters and Estuaries (1961) Van Nostrand Reinhold pp.317-320[4] Norton, John F. Standard Methods for the Examination of Water and Sewage 9th Ed. (1946) American Public Health Association p.139[5] Urquhart, Leonard Church Civil Engineering Handbook 4th Ed. (1959) McGraw-Hill p.9-40[6] Sawyer, Clair N. & McCarty, Perry L. Chemistry for Sanitary Engineers 2nd Ed. (1967) McGraw-Hill pp.394-399[7][7] Winkler, L. W. (1888). "Die zur Bestimmung des in Wasser gelösten Sauerstoffes " Berichte der Deutschen Chemischen Gesellschaft 21(2):

2843-2854.[8][8] Kemula, W. and S. Siekierski (1950). "Polarometric determination of oxygen." Collect. Czech. Chem. Commun. 15: 1069-75.[9][9] Garcia-Fresnadillo, D., M. D. Marazuela, et al. (1999). "Luminescent Nafion Membranes Dyed with Ruthenium(II) Complexes as Sensing

Materials for Dissolved Oxygen." Langmuir 15(19): 6451-6459.[10][10] Titze, J., H. Walter, et al. (2008). "Evaluation of a new optical sensor for measuring dissolved oxygen by comparison with standard

analytical methods." Monatsschr. Brauwiss.(Mar./Apr.): 66-80.[11] Lenore S. Clescerl, Andrew D. Eaton, Eugene W. Rice (2005). Standard Methods for Examination of Water & Wastewater (21st ed.).

Washington, DC: American Public Health Association, American Water Works Association, and the Water Environment Association ISBN0-84553-047-8 Also available by online subscription at http:/ / www. standardmethods. org

[12] In-Situ Inc. Method 1002-8-2009 Dissolved Oxygen Measurement by Optical Probe, In-Situ Inc., 221 E Lincoln Ave., Ft. Collins, CO80524 http:/ / www. in-situ. com/ RDO_EPA_Approval

[13] Hammer, Mark J. (1975). Water and Waste-Water Technology. John Wiley & Sons. ISBN 0-471-34726-4.[14] FINAL REPORT OF THE COMMISSIONERS APPOINTED TO INQUIRE AND REPORT WHAT METHODS OF Treating and

Disposing of Sewage. 1912 (http:/ / ia700404. us. archive. org/ 35/ items/ cu31924003641929/ cu31924003641929. pdf)[15] U.S. Environmental Protection Agency (EPA). Washington, DC. "Secondary Treatment Regulation." (http:/ / www. access. gpo. gov/ nara/

cfr/ waisidx_07/ 40cfr133_07. html) Code of Federal Regulations, 40 CFR Part 133.[16] http:/ / www. standardmethods. org

Biochemical oxygen demand 32

External links• BOD Doctor (http:/ / www. boddoctor. com/ wiki/ index. php?title=Main_Page) - a troubleshooting wiki for this

problematic test

Theoretical oxygen demandTheoretical Oxygen Demand (ThOD) is the calculated amount of oxygen required to oxidize a compound to itsfinal oxidation products. However, there are some differences between standard methods that can influence theresults obtained: for example, some calculations assume that nitrogen released from organic compounds is generatedas ammonia, whereas others allow for ammonia oxidation to nitrate. Therefore in expressing results, the calculationassumptions should always be stated.

ExampleIn order to determine the ThOD for glycine (CH2(NH2)COOH) using the following assumptions:

1. In the first step, the organic carbon and nitrogen are converted

to carbon dioxide (CO2) and ammonia (NH

3), respectively.

2. In the second and third steps, the ammonia is oxidized

sequentially to nitrite and nitrate.

3. The ThOD is the sum of the oxygen required for all three steps.

We can calculate by following steps:

1. Write balanced reaction for the carbonaceous oxygen demand.

CH2(NH

2)COOH + 1.5 0

2 ->NH

3 + 2CO

2 + H

20

2. Write balanced reactions for the nitrogenous oxygen demand.

NH3 + 1.5 0

2 -> HNO

2 + H

20

HNO2 + 0.5 O

2 -> HNO

3 NH

3 + 2 O

2 -> HNO

3 + H

2O

3. Determine the ThOD.

ThOD= (1.5 + 2) mol O2/mol glycine

= 3.5 mol O2/mol glycine x 32 g/mol O

2 / 75 g/mol glycine

= 1.49 g O2/g glycine

References

Carbonaceous biochemical oxygen demand 33

Carbonaceous biochemical oxygen demandCarbonaceous biochemical oxygen demand or CBOD is a method defined test measured by the depletion ofdissolved oxygen by biological organisms in a body of water in which the contribution from nitrogenous bacteria hasbeen suppressed. CBOD is a method defined parameter is widely used as an indication of the pollutant removal fromwastewater. It is listed as a conventional pollutant in the U.S. Clean Water Act.

The CBOD5 testThe CBOD tests have the widest application in measuring waste loadings to treatment plants and in evaluating theCBOD-removal efficiency of such treatment systems. The test measures the molecular oxygen utilized during aspecified incubation period for the biochemical degradation of organic material (carbonaceous demand) and theoxygen used to oxidize inorganic material such as sulfides and ferrous iron. It also may measure the amount ofoxygen used to oxidize reduced forms of nitrogen (nitrogenous demand) unless their oxidation is prevented by aninhibitor. The seeding and dilution procedures provide an estimate of the CBOD at pH 6.5 to 7.5.There are two recognized EPA methods for the measurement of CBOD:• Standard Methods for the Examination of Water and Wastewater, Method 5210B[1]

• In-Situ Inc. Method 1004-8-2009 Carbonaceous Biochemical Oxygen Demand (CBOD) Measurement by OpticalProbe.[2]

Dissolved Oxygen Probes: Membrane and LuminescenceSince the publication of a simple, accurate and direct dissolved oxygen analytical procedure by Winkler [3], theanalysis of dissolved oxygen levels for water have been key to the determination of surface water purity andecological wellness. The Winkler method is still one of only two analytical techniques used to calibrate oxygenelectrode meters, the other procedure based on oxygen solubility at saturation as per Henry's law. Though manyresearchers have refined the Winkler analysis to dissolved oxygen levels in the low PPB range the method does notlend itself to automation.The development of an analytical instrument that utilizes the reduction-oxidation (redox) chemistry of oxygen in thepresence of dissimilar metal electrodes was introduced during the 1950s[4]. This redox electrode utilized an oxygenpermeable membrane to allow the diffusion of the gas into an electrochemical cell and its concentration determinedby polarographic or galvanic electrodes. This analytical method is sensitive and accurate to down to levels of ±0.1 mg/l dissolved oxygen. Calibration of the redox electrode of this membrane electrode still requires the use of theHenry’s law table or the Winkler test for dissolved oxygen.During the last two decades, a new form of electrode was developed based on the luminescence emission of a photoactive chemical compound and the quenching of that emission by oxygen. This quenching photophysics mechanismis described by the Stern-Volmer equation for dissolved oxygen in a solution[5]:

The determination of oxygen concentration by luminescence quenching has a linear response over a broad range ofoxygen concentrations and has excellent accuracy and reproducibility[6]. There are two recognized EPA methods forthe measurement of Dissolved Oxygen for CBOD

Carbonaceous biochemical oxygen demand 34

• Standard Methods for the Examination of Water and Wastewater, Method 4500 O[7]

• In-Situ Inc. Method 1002-8-2009 Dissolved Oxygen Measurement by Optical Probe.[8]

CBOD Method SummaryBring the sample to ambient room temperature. If pH of sample is <6.5 or >7.5 neutralize the sample toapproximately a pH of 7.0 using either sulfuric acid or sodium hydroxide. Aliquots of the neutralized sample aretransferred to 300 mL CBOD bottles. These CBOD samples must be at concentrations that will deplete by at least2 mg/L dissolved oxygen (DO) and have at least 1 mg/L DO left after five days of incubation. Therefore makeenough dilutions (minimum of 3) of the prepared sample to bracket the predicted CBOD.The minimum aliquot volume transferred to a 300 mL CBOD bottle will be 3 mL as set by Standard Methods. If asmaller volume is needed to meet the DO depletion requirements, then you must make dilutions to the sample. Addapproximately 0.1 g of Nitrification Inhibitor (2-chloro-6-(trichloro-methyl) pyridine) to each 300mL CBOD bottlebefore adding CBOD dilution water. If the sample is being prepared as a seeded sample, add enough prepared seedto the sample to achieve acceptable dissolved oxygen depletion. Add CBOD Dilution water to each CBOD samplebottle so as to completely fill the bottle with no air spaces or bubbles when the stopper is placed in the bottle.Place the dissolved oxygen probe in the bottle and allow the dissolved oxygen meter to come to equilibrium. Allowthe meter to come to equilibrium prior to accepting dissolved oxygen value. Record the DO of the sample, stopperthe bottle, add DI water to the water seal if needed, cap the water seal, and incubate for 5 days at 20°C ± 1°C.Exclude light to avoid growth of algae in the bottles during incubation.Upon completion of the 5-day incubation± 6 hours, record the DO of the depleted samples with a calibrated DOmeter. Allow the meter to come to equilibrium prior to accepting dissolved oxygen value. Calculate the CBODs fromthe formula below. Only bottles, including seed controls, giving a minimum DO depletion of 2.0 mg/L and a residualDO of at least 1.0 mg/L after 5 days of incubation are considered to produce valid data, because at least 2.0 mgoxygen uptake per L is required to give a meaningful measure of oxygen uptake and at least 1.0 mg/L must remainthroughout the test to ensure that insufficient DO does not affect the rate of oxidation of waste constituents.

Bacterial Seed CBOD CorrectionSeed CBOD Uptake: Typically a 10, 20, and 30 mL sample of seed added to 3 separate CBOD bottles withapproximately 0.1 g Nitrification Inhibitor and diluted with CBOD dilution water. Run these QC samples with eachbatch of seeded CBOD. Calculate the DO uptake per mL of seed added to each bottle using either the slope methodor the ratio method.For the slope method, plot DO depletion in milligrams per liter versus mLs of seed for all seed control bottles havinga 2.0 mg/L depletion and 1.0 minimum residual DO. The plot should present a straight line for which the slopeindicates DO depletion per mL of seed. The DO-axis intercept is oxygen depletion caused by the dilution water andshould be less than 0.20 mg/L.For the ratio method, divide the DO depletion by the volume of seed in mLs for each seed control bottle having a2.0 mg/L depletion and greater than 1.0 mg/L minimum residual DO and average the results.

Carbonaceous biochemical oxygen demand 35

CBOD SeedThe CBOD test is method defined. Factors such as bacterial seed viability, anoxic stress during the 5 days, andnitrogenous inhibition efficacy will produce method variability between duplicates, analysts and laboratories. Clearquality assurance and quality control limits must be developed to produce valid results.

Sample ToxicityWastewater by definition may contain pollutants that inhibit bacterial seed metabolisms or are toxic to the seed. Inthese cases, all samples should be seeded with a known amount of viable bacteria for the CBOD analysis. Toxicity orinhibition is observed in CBOD analysis when the calculated CBOD increases with progressive dilutions of thesample.

Appropriate Microbial PopulationSelection of a viable microbial population for the CBOD analysis is key in obtaining valid results. The bacterialpopulation needs both carbonaceous and nitrogenous strains present. Sources of viable bacterial seed can be primaryclarifier effluent, non-disinfected secondary clarifier effluent or a commercial seed preparation. Each source shouldhave clear quality assurance and quality control requirements set by the glucose-glutamic acid check sample.

Glucose-Glutamic Acid Check SampleTransfer a known amount of glucose-glutamic acid solution to a CBOD bottle and add sufficient seed to achieveacceptable dissolved oxygen depletion. Fill CBOD bottle with CBOD dilution water and Nitrification Inhibitor.Determine the 5 Day CBOD. Passing results will have a CBOD of 198 (+ 30.5) mg/L. Run these check samples witheach batch of CBOD samples. It is important to realize that glucose-glutamic acid is not intended to be an accuracycheck in the test. Its sole purpose is to demonstrate that the seed is viable and metabolizing in the proper range ofactivity under the conditions of the test.

Regulatory use of CBODIn order to reduce a wastewater plants BOD5 values to meet regulatory compliance requirements, some plantoperators try to suppress nitrification when they are not required to meet ammonia limits. This practice usuallyresults in increased effluent toxicity and oxygen demand on the receiving waters. Therefore, to eliminate thissituation and because the BOD5 test is not reflective of effluent quality under nitrifying conditions, the wastewaterplant should:1. Perform parallel CBOD5 and BOD5 tests to indicate whether there is a problem with BOD5 compliance due tonitrification in the BOD5 test results and that the CBOD5 is not directly correlated with the BOD5 test results, and2. Baseline wastewater plant influent and effluent ammonia, nitrite and nitrate data (same frequency and duration asthe parallel CBOD5 and BOD5 data) have been provided to perform mass balances for nitrification inhibition. Theresults of these analysis can show that CBOD5 should be utilized for regulatory compliance with wastewaterdischarge requirements.

Carbonaceous biochemical oxygen demand 36

References[1] Lenore S. Clesceri, Andrew D. Eaton, Eugene W. Rice (2005). Standard Methods for Examination of Water & Wastewater Method 5210B.

Washington, DC: American Public Health Association, American Water Works Association, and the Water Environment Association. Alsoavailable by online subscription at http:/ / www. standardmethods. org .

[2] In-Situ Inc. Method 1004-8-2009 Carbonaceous Biochemical Oxygen Demand (CBOD) Measurement by Optical Probe, In-Situ Inc., 221 ELincoln Ave., Ft. Collins, CO 80524 http:/ / www. in-situ. com/ RDO_EPA_Approval .

[3][3] Winkler, L. W. (1888). "Die zur Bestimmung des in Wasser gelösten Sauerstoffes " Berichte der Deutschen Chemischen Gesellschaft 21(2):2843-2854.

[4][4] Kemula, W. and S. Siekierski (1950). "Polarometric determination of oxygen." Collect. Czech. Chem. Commun. 15: 1069-75.[5][5] Garcia-Fresnadillo, D., M. D. Marazuela, et al. (1999). "Luminescent Nafion Membranes Dyed with Ruthenium(II) Complexes as Sensing

Materials for Dissolved Oxygen." Langmuir 15(19): 6451-6459.[6][6] Titze, J., H. Walter, et al. (2008). "Evaluation of a new optical sensor for measuring dissolved oxygen by comparison with standard analytical

methods." Monatsschr. Brauwiss.(Mar./Apr.): 66-80.[7] Lenore S. Clescerl, Andrew D. Eaton, Eugene W. Rice (2005). Standard Methods for Examination of Water & Wastewater (21st ed.).

Washington, DC: American Public Health Association, American Water Works Association, and the Water Environment Association ISBN0-8455-3047-8 Also available by online subscription at http:/ / www. standardmethods. org

[8] In-Situ Inc. Method 1002-8-2009 Dissolved Oxygen Measurement by Optical Probe, In-Situ Inc., 221 E Lincoln Ave., Ft. Collins, CO 80524http:/ / www. in-situ. com/ RDO_EPA_Approval

External links• National Pollutant Discharge Elimination System (NPDES) (http:/ / cfpub. epa. gov/ npdes/ )• Summary of the Clean Water Act (http:/ / www. epa. gov/ regulations/ laws/ cwa. html)• U.S. Geological Survey TWRI Book 9 Chapter A7.2 Five-day Biochemical Oxygen Demand (http:/ / water. usgs.

gov/ owq/ FieldManual/ Chapter7-Archive/ chapter7. 2/ pdf/ 7. 2. pdf)

Article Sources and Contributors 37

Article Sources and ContributorsOxygen  Source: http://en.wikipedia.org/w/index.php?oldid=491359895  Contributors: 0612, 123qwe, 1266asdsdjapg, 1297, 1Martin33, 84user, A2569875, Abarenbo, Abu Amal Bahraini,Acalamari, Acroterion, Acs4b, Adambiswanger1, Adashiel, Addshore, Adrian, AdultSwim, Aeros320, AgainErick, Ahoerstemeier, AidepikiW kcuF, Airconswitch, Aitias, Alex.muller, AlexG,Alexeymorgunov, Alexf, AlexiusHoratius, Algont, Alias Flood, Alison, All Is One, Alsandro, Amphetamine Analogue, Ancheta Wis, Andonic, Andre Engels, Andres, Andrewlp1991, Andros1337, AngelOfSadness, AngryParsley, Angusmclellan, Ann Stouter, Anna512, AnonMoos, Antandrus, Anthony Appleyard, Arcadian, Archimerged, Ardric47, Aristotle28, Arjun01, Arkuat,ArnoldReinhold, Arnon Chaffin, Art LaPella, Atemperman, Attarparn, AuburnPilot, AvicAWB, Axlq, AzaToth, BANZ111, BHS Sux, Bachrach44, Badocter, Ballsonyourwalls, Balthazarduju,Bandn, BanyanTree, Bart133, Bbatsell, Bbi5291, Bboy14, Beetstra, Beland, Benbest, Bender235, Benjah-bmm27, Benjiboi, Bensderbest, Bevo, Bggoldie, Bhadani, BigCow, Bigbuck, Billcurtis,Blackjack3, BlueMoonlet, Bluezy, Bobak, Bobo192, Bomac, Bongwarrior, Bonjour amis, Bornfury, Borovy3488, Bradkittenbrink, British Ben, Bryan Derksen, Buchanan-Hermit,Buckthebronco, Burntsauce, Burzmali, Buzzgrav08, Bwrs, C.Fred, C777, CJLL Wright, CWii, CYD, Cactus.man, Caesura, Calabe1992, Caltas, CambridgeBayWeather, Can't sleep, clown willeat me, CanadianCaesar, Candlewicke, CanisRufus, Captain-n00dle, Cardiffchestnut, Carel.jonkhout, Carlo.milanesi, Carloseduardo, Carnildo, Casliber, Catslash, CattleGirl, Causesobad,Cd12holden, Cdc, CelticJobber, Cfw Master, Chameleon, CharlesGillingham, Charleythegodfather, Chcknpie04, Chemicalinterest, Chilisauce2727, Cholmes75, Chowbok, Chris Dybala, Christhe speller, Chrislk02, Christian List, Christian75, Christopherlin, Chubbyzson, Ck lostsword, Ckatz, Clivegrey, Cmapm, Cnaude, Colbuckshot, ColdFeet, Cometstyles, CommonsDelinker, Conn,Kit, Conversion script, Corpx, Cosmium, Crazycomputers, Cryptic C62, Crystallina, Ctbolt, Curb Chain, Curps, Cybercobra, D, DARTH SIDIOUS 2, DJ Clayworth, DRosenbach, DVD R W,DVdm, Dac107, Damieng, Dan56, Dana boomer, DancingPenguin, Daniel Case, DanielCD, Danny, Dantheman531, Danyg, Dark Mage, Darrien, Dauno, Dave3457, David Latapie, Davidj1991,Davumaya, Dawn Bard, Ddday-z, DeadEyeArrow, Deflective, Deglr6328, Deli nk, Delta G, Demoscn, Denisarona, DerHexer, Derek Ross, Derek.cashman, Deryck Chan, Devl2666, Digitalme,Dillard421, Dirac1933, Dirac66, DirectEdge, Discospinster, Dmz5, Doct.proloy, Dolive21, DomCleal, Dominus, DonSiano, Donarreiskoffer, Donilredd, Dr bab, Dr. Whooves, Dragonmaster84,Dravick, Dreadstar, Droll, Drphilharmonic, Dsyzdek, Dwmyers, Dycedarg, Dysepsion, EEMIV, EL Willy, Ecophreek, EdC, EdChem, Eddideigel, Edgar181, Edsanville, Edward, Egabisstranded,Egil, Eilatybartfast, El C, Eldin raigmore, Eleassar, Eliz81, Elkman, Emperorbma, Enemyunknown, Eng02019, Enigmaman, Eric Forste, Eric Kvaalen, Eric119, Erik Zachte, Eshabat, Etaoin,Ethel Aardvark, Eu.stefan, Euyyn, Evercat, Everyking, Evil Monkey, Ewen, Ex nihil, Exarion, Excirial, Eye.earth, FF2010, FTGHSmith, Fabartus, Faithlessthewonderboy, Femto, Finell,Firestarterrulz, FisherQueen, Foobaz, FrancoGG, Francs2000, Freakofnurture, Fredrik, Frencheigh, FreplySpang, Frymaster, Funandtrvl, Funky Monkey, Fvw, G. Campbell, GEWilker, Gadfium,Gaff, Galoubet, Garfield226, Gary King, Gene Hobbs, Gene Nygaard, Geneb1955, Geoffrey.landis, Geopersona, Georgewilliamherbert, Ghakko, Giftlite, Gingekerr, GiollaUidir, Gjm867,Glacialfox, Gman124, Gmcole, Gobonobo, Gogo Dodo, GoodSirJava, Goodnightmush, Gotgame, GraemeL, Graham87, Grandia01, Gravitan, Green caterpillar, Greensburger, GregorB,Grendelkhan, Grim23, GrouchyDan, Grunkhead, Guest9999, Gurch, Gyrobo, HJ Mitchell, HYC, Haakon, Hadal, Hak-kâ-ngìn, Hallows AG, Harland1, Haskellguy, Hdt83, Headbomb, Heimstern,Henrik, Herbee, Herk1955, Heron, Heyheynomaybe, Heytherebuddy, Hhi192837465, HiEv, Hij54, Hmains, Hokanomono, Honette, HorsePunchKid, Howcheng, Hvn0413, Hyper year,Iamcool234, Iamsam478, Iamsonoob, Iantresman, Icairns, Icek, Icestorm815, Ida Shaw, Ike9898, Illuminattile, Ilovepowerpufgirls., Imasleepviking, Insanephantom, Intelati, Ionlyputrealfacts,Irfanh, Irishguy, IronGargoyle, Isaac, Itub, Ixfd64, J.delanoy, JDG, JEN9841, JForget, JFreeman, JaGa, Jagpreet sant, Jagun, James pic, Jaraalbe, Jarble, Jaxl, Jaydosbros, Jecar, Jedidan747,Jeff3000, Jellyandjam, Jfdwolff, Jim Swenson, Jim-gagnon, Jim1138, Jimfbleak, JimmyH260, Jimp, Jjron, Jlawniczak, JoanneB, Jobe6, JoeBlogsDord, Joefromrandb, John, John Cardinal, Jojitfb, Jooler, Joriki, Jose77, Joshua BishopRoby, Joshua Issac, Jpk, Jrockley, Jschnur, Junglecat, Jusjih, Justiceslayer, JustinTime55, Jww2, Kafziel, Kailahascootis, Kaini, Kalin1344, Kamandor,Kandar, Karada, Karl-Henner, Karositoasdfdf, Kaszeta, Kazvorpal, Ke5skw, Keegan, Keilana, Kenb215, Kerotan, Kev923, Kieff, Kikumbob, Kilo-Lima, Kils, King Vegita, Kiwiboy471, Kizor,KnowledgeOfSelf, Knutux, Koliri, Kosebamse, Kozuch, Kpjas, Kragen, Krashlandon, Krich, Krinsky, Kukini, Kungfuadam, Kuru, Kwame Nkrumah, Kwamikagami, Kyoko, LMB,LSCProductions, La goutte de pluie, Lake.lamp98, LambaJan, Lankiveil, Lanthanum-138, Lapinski, Latka, Laudaka, Laundrypowder, Lefty, Lfh, Lightmouse, Lights, LinguistAtLarge,LizardWizard, 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Oxygen saturation  Source: http://en.wikipedia.org/w/index.php?oldid=488658839  Contributors: AB, Alqadri86, Arcadian, Auntof6, BradyDale, Bryan Derksen, Can't sleep, clown will eat me,Choij, Chriswaterguy, Coar, CommonsDelinker, Correctaboot, Danaut, Davehi1, Davidruben, DerHexer, Dionyziz, Dlodge, Drk7, Drunken Pirate, El C, Fibrosis, Gene Nygaard, Headbomb,Hede2000, Helmoony, Hu12, Immunize, Ja 62, JamesBWatson, Jfdwolff, Jmeppley, Julesd, Kaarel, Karthickbala, Kauczuk, Kenneth Charles, Kils, Lipothymia, Lou.weird, Luna Santin,Marshman, Mbeychok, Mikael Häggström, Mnd, Musiphil, Neilc, Northumbrian, Ohthelameness, Paleorthid, PamD, Perfusion guy, Philip Trueman, Pinethicket, Plumbago, Press9761,Prestonmag, Raise lkblr, Robert K S, RobertHannah89, RobinHood70, Shniken, Silenceisgod, Smalljim, Spiffy sperry, The FD, Tiddly Tom, Tide rolls, Unioneagle, Wiki Jibiki, Wwheaton, 153anonymous edits

Chemical oxygen demand  Source: http://en.wikipedia.org/w/index.php?oldid=484678676  Contributors: Aksi great, Annabel, Aperea, Biscuittin, Bry9000, Christopherfair, Diberri, DjCapricorn, Dr.Soft, Enviroboy, Erud, Gene Nygaard, Kaarel, Karthickbala, LOL, Lamro, Langbein Rise, Margoz, Mbeychok, Michael Devore, Millermk90, Mintleaf, Moreau1, MorningRazor,Morwen, Nick Number, Nono64, Pengo, Peruvianllama, Pkr1980, Quadell, RSido, Remota, Roboa1983, Rucharahul, Schewek, Spellmi, Spiral5800, Srleffler, Thewellman, VIGNERON, Velella,Vina, Vortexrealm, Vuong Ngan Ha, WCFrancis, Welsh, Yadavbasti, Yonatan, 85 anonymous edits

Biochemical oxygen demand  Source: http://en.wikipedia.org/w/index.php?oldid=486605795  Contributors: Alan Liefting, Alansohn, Alqadri86, Amitauti, Anandsince, Andrea105, Aperea,Arthena, Azwaldo, Biker Biker, Biscuittin, BradyDale, Chris the speller, Christopherfair, Chriswaterguy, Cmdrjameson, ConCompS, Diberri, Dj Capricorn, Euchiasmus, Exir Kamalabadi,Fanghong, Floria L, FramingArmageddon, Fullerenedream, Funnyfarmofdoom, Gas Panic42, Gene Nygaard, GhaziKakany, Giggs for Temporary, GrDn, Gurch, HighKing, Ike9898,InSituFortCollins, Jeffq, Jfeucht82, Joriki, Kenneth Charles, KudzuVine, LFaraone, Lamro, Lights, Lord Roem, Mandarax, Materialscientist, Mateus Hidalgo, Mbeychok, Metamagician3000,MightyWarrior, Mnhb2948, Moreau1, MrSomeone, Nono64, Pablowerk, Paleorthid, Panchhee, Parmesan, PeN, Phgao, Pinethicket, Quadell, Rui Silva, SSpiffy, Sampo Torgo, Samw, Sandip90,Savh, Sercan K., Smack, Stemonitis, Tbhotch, Thewellman, Thue, Tide rolls, Van helsing, Vanisaac, Velella, Vortexrealm, Vuong Ngan Ha, WCFrancis, Yaluen, Yilloslime, Yohan Duminda,Zarathushtra1111, 199 anonymous edits

Theoretical oxygen demand  Source: http://en.wikipedia.org/w/index.php?oldid=337432824  Contributors: Aperea, Biscuittin, Fabrictramp, Facts707, Grika, Itub, Katharineamy, Malcolmxl5,Mr. Vernon, Nithin aneesh, 3 anonymous edits

Carbonaceous biochemical oxygen demand  Source: http://en.wikipedia.org/w/index.php?oldid=490890335  Contributors: Aperea, Ben Ben, Crazysane, ErikHaugen, Robyvecchio, 2anonymous edits

Image Sources, Licenses and Contributors 38

Image Sources, Licenses and Contributorsfile:Liquid Oxygen.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Liquid_Oxygen.gif  License: GNU General Public License  Contributors: Dr. Warwick Hillierfile:Oxygen spectre.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Oxygen_spectre.jpg  License: Public Domain  Contributors: TeravoltFile:Loudspeaker.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Loudspeaker.svg  License: Public Domain  Contributors: Bayo, Gmaxwell, Husky, Iamunknown, Mirithing,Myself488, Nethac DIU, Omegatron, Rocket000, The Evil IP address, Wouterhagens, 18 anonymous editsFile:Oxygen discharge tube.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Oxygen_discharge_tube.jpg  License: Free Art License  Contributors: Alchemist-hp (talk) (www.pse-mendelejew.de)File:Paramagnetism of liquid oxygen.jpeg  Source: http://en.wikipedia.org/w/index.php?title=File:Paramagnetism_of_liquid_oxygen.jpeg  License: Public Domain  Contributors: Pieter KuiperFile:Ozone-1,3-dipole.png  Source: http://en.wikipedia.org/w/index.php?title=File:Ozone-1,3-dipole.png  License: Public Domain  Contributors: Benjah-bmm27, Cwbm (commons), Kerina yin,ZapyonFile:Evolved star fusion shells.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Evolved_star_fusion_shells.svg  License: Creative Commons Attribution-ShareAlike 3.0 Unported Contributors: User:RursusFile:WOA05 sea-surf O2 AYool.png  Source: http://en.wikipedia.org/w/index.php?title=File:WOA05_sea-surf_O2_AYool.png  License: Creative Commons Attribution-Sharealike 3.0 Contributors: PlumbagoFile:Simple photosynthesis overview.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Simple_photosynthesis_overview.svg  License: Creative Commons Attribution-Share Alike Contributors: Daniel Mayer (mav) - original imageVector version by YerpoFile:Oxygenation-atm.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Oxygenation-atm.svg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors:Heinrich D. HollandFile:Philos experiment of the burning candle.PNG  Source: http://en.wikipedia.org/w/index.php?title=File:Philos_experiment_of_the_burning_candle.PNG  License: Public Domain Contributors: Wilhelm SchmidtFile:Georg Ernst Stahl.png  Source: http://en.wikipedia.org/w/index.php?title=File:Georg_Ernst_Stahl.png  License: Public Domain  Contributors: PolarlysFile:Carl Wilhelm Scheele from Familj-Journalen1874.png  Source: http://en.wikipedia.org/w/index.php?title=File:Carl_Wilhelm_Scheele_from_Familj-Journalen1874.png  License: PublicDomain  Contributors: Celsius, Crux, Den fjättrade ankan, JMCC1, Sanao, Vogler, 1 anonymous editsFile:PriestleyFuseli.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:PriestleyFuseli.jpg  License: Public Domain  Contributors: Turner, Charles , 1774 - 1857 (Engraver); Fuseli,Henry, 1741 - 1825 (Painter)File:Antoine lavoisier.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Antoine_lavoisier.jpg  License: Public Domain  Contributors: Ecummenic, Kilom691, Matanya (usurped),Pieter Kuiper, Siebrand, Sir GawainFile:Goddard and Rocket.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Goddard_and_Rocket.jpg  License: Public Domain  Contributors: Esther C. GoddardFile:Hofmann voltameter fr.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Hofmann_voltameter_fr.svg  License: GNU Free Documentation License  Contributors: IIVQFile:Home oxygen concentrator.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Home_oxygen_concentrator.jpg  License: Creative Commons Attribution-Sharealike 2.0 Contributors: Original uploader was GiollaUidir at en.wikipediaFile:Wisoff on the Arm - GPN-2000-001069.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Wisoff_on_the_Arm_-_GPN-2000-001069.jpg  License: Public Domain  Contributors:NASAFile:Clabecq JPG01.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Clabecq_JPG01.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Jean-PolGRANDMONTFile:Phanerozoic Climate Change.png  Source: http://en.wikipedia.org/w/index.php?title=File:Phanerozoic_Climate_Change.png  License: unknown  Contributors: Royer, Dana L., Robert A.Berner, Isabel P. Montañez, Neil J. Tabor, and David J. BeerlingFile:Stilles Mineralwasser.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Stilles_Mineralwasser.jpg  License: GNU Free Documentation License  Contributors: Walter J. Pilsak,Waldsassen, GermanyFile:Rust screw.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Rust_screw.jpg  License: Creative Commons Attribution 2.0  Contributors: User:Paulnasca. Original uploader wasPaulnasca at en.wikipediaFile:Acetone-3D-vdW.png  Source: http://en.wikipedia.org/w/index.php?title=File:Acetone-3D-vdW.png  License: Public Domain  Contributors: Ben MillsFile:ATP structure.svg  Source: http://en.wikipedia.org/w/index.php?title=File:ATP_structure.svg  License: Public Domain  Contributors: User:MysidFile:Symptoms of oxygen toxicity.png  Source: http://en.wikipedia.org/w/index.php?title=File:Symptoms_of_oxygen_toxicity.png  License: Public Domain  Contributors: Mikael HäggströmFile:Scuba-diving.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Scuba-diving.jpg  License: Creative Commons Attribution-Sharealike 2.5  Contributors: BLueFiSH.as, Civertan,Diwas, Fschoenm, Man vyi, Wikipeder, 3 anonymous editsFile:Apollo 1 fire.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Apollo_1_fire.jpg  License: Public Domain  Contributors: NASA

License 39

LicenseCreative Commons Attribution-Share Alike 3.0 Unported//creativecommons.org/licenses/by-sa/3.0/