Carbon Dioxide - Wikipedia, The Free Encyclopedia

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Carbon dioxide

Carbonic acid gasCarbonic anhydride

Carbonic oxideCarbon oxide

Carbon(IV) oxideDry ice (solid phase)

Identifiers

CAS number 124-38-9

PubChem 280

ChemSpider 274

UNII 142M471B3J

EC number 204-696-9

UN number 1013

KEGG D00004

MeSH Carbon+dioxide

ChEBI CHEBI:16526

ChEMBL CHEMBL1231871

RTECS number FF6400000

ATC code V03AN02(http://www.whocc.no/atc_ddd_index/?code=V03AN02)

BeilsteinReference

1900390

GmelinReference

989

3DMet B01131

Jmol-3D images Image 1(http://chemapps.stolaf.edu/jmol/jmol.php?model=O%3DC%3DO)

Carbon dioxideFrom Wikipedia, the free encyclopedia

Carbon dioxide (chemical formula CO2) is anaturally occurring chemical compoundcomposed of 2 oxygen atoms each covalentlydouble bonded to a single carbon atom. It is agas at standard temperature and pressure andexists in Earth's atmosphere in this state, as atrace gas at a concentration of 0.04 per cent (400ppm) by volume, as of 2014.[1]

As part of the carbon cycle, plants, algae, andcyanobacteria use light energy tophotosynthesize carbohydrate from carbondioxide and water, with oxygen produced as awaste product.[2] However, photosynthesiscannot occur in darkness and at night somecarbon dioxide is produced by plants duringrespiration.[3] It is produced during therespiration of all other aerobic organisms and isexhaled in the breath of air-breathing landanimals, including humans. Carbon dioxide isproduced during the processes of decay oforganic materials and the fermentation of sugarsin beer and winemaking. It is produced bycombustion of wood, carbohydrates and majorcarbon- and hydrocarbon-rich fossil fuels suchas coal, peat, petroleum and natural gas. It isemitted from volcanoes, hot springs and geysersand is freed from carbonate rocks by dissolutionin water and acids. CO2 is found in lakes, atdepth under the sea and commingled with oiland gas deposits.[4]

The environmental effects of carbon dioxide areof significant interest. Atmospheric carbondioxide is the primary source of carbon in life onEarth and its concentration in Earth's pre-industrial atmosphere since late in thePrecambrian eon was regulated byphotosynthetic organisms. Carbon dioxide is animportant greenhouse gas and burning ofcarbon-based fuels since the industrialrevolution has rapidly increased itsconcentration in the atmosphere, leading toglobal warming. It is also a major source ofocean acidification since it dissolves in water toform carbonic acid.[5]

Other names

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Image 2(http://chemapps.stolaf.edu/jmol/jmol.php?model=C%28%3DO%29%3DO)

Properties

Molecularformula

CO2

Molar mass 44.01 g mol−1

Appearance Colorless gas

Odor Odorless

Density 1562 kg/m3 (solid at 1 atm and −78.5 °C)

770 kg/m3 (liquid at 56 atm and 20 °C)

1.977 kg/m3 (gas at 1 atm and 0 °C)

Melting point −56.6 °C; −69.8 °F; 216.6 K (Triple pointat 5.1 atm)

Sublimationconditions

−78.5 °C; −109.2 °F; 194.7 K (1 atm)

Solubility inwater

1.45 g/L at 25 °C, 100 kPa

Vapor pressure 5.73 MPa (20 °C)

Acidity (pKa) 6.35, 10.33

Refractive index(nD)

1.1120

Viscosity 0.07 cP at −78.5 °C

Dipole moment 0 D

Structure

Crystal structure trigonal

Molecular shape linear

Thermochemistry

Specificheat capacity C

37.135 J/K mol

Std molar

entropy So298

214 J·mol−1·K−1

Std enthalpy offormationΔfHo

298

−393.5 kJ·mol−1

Hazards

MSDS External MSDS

NFPA 704

Contents

1 History2 Chemical and physical properties

2.1 Structure and bonding2.2 In aqueous solution2.3 Chemical reactions of CO2

2.4 Physical properties3 Isolation and production

3.1 Laboratory methods3.2 Industrial production

4 Uses4.1 Precursor to chemicals4.2 Foods

4.2.1 Beverages4.2.2 Wine making

4.3 Inert gas4.4 Fire extinguisher4.5 Supercritical CO2 as solvent

4.6 Agricultural and biologicalapplications4.7 Oil recovery4.8 Bio transformation into fuel4.9 Refrigerant4.10 Coal bed methane recovery4.11 Niche uses

5 In the Earth's atmosphere6 In the oceans7 Biological role

7.1 Photosynthesis and carbonfixation7.2 Toxicity7.3 Human physiology

7.3.1 Content7.3.2 Transport in the blood7.3.3 Regulation ofrespiration

SMILES

InChI

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Related compounds

Other anions Carbon disulfideCarbon diselenide

Other cations Silicon dioxideGermanium dioxideTin dioxideLead dioxide

Related carbonoxides

Carbon monoxideCarbon suboxideDicarbon monoxideCarbon trioxide

Relatedcompounds

Carbonic acidCarbonyl sulfide

Supplementary data page

Structure andproperties

n, εr, etc.

Thermodynamicdata

Phase behaviourSolid, liquid, gas

Spectral data UV, IR, NMR, MS

Except where noted otherwise, data are given for materialsin their standard state (at 25 °C (77 °F), 100 kPa)

(verify) (what is: / ?)

Infobox references

Crystal structure of dry ice

8 See also9 References10 Further reading11 External links

History

Carbon dioxidewas one of thefirst gases to bedescribed as asubstancedistinct from air.In theseventeenthcentury, theFlemish chemistJan Baptist vanHelmont

observed that when he burned charcoal in aclosed vessel, the mass of the resulting ash wasmuch less than that of the original charcoal. Hisinterpretation was that the rest of the charcoalhad been transmuted into an invisible substancehe termed a "gas" or "wild spirit" (spiritussylvestre).[6]

The properties of carbon dioxide were studiedmore thoroughly in the 1750s by the Scottish physician Joseph Black. He found that limestone (calciumcarbonate) could be heated or treated with acids to yield a gas he called "fixed air." He observed that thefixed air was denser than air and supported neither flame nor animal life. Black also found that whenbubbled through limewater (a saturated aqueous solution of calcium hydroxide), it would precipitatecalcium carbonate. He used this phenomenon to illustrate that carbon dioxide is produced by animalrespiration and microbial fermentation. In 1772, English chemist Joseph Priestley published a paperentitled Impregnating Water with Fixed Air in which he described a process of dripping sulfuric acid (oroil of vitriol as Priestley knew it) on chalk in order to produce carbon dioxide, and forcing the gas todissolve by agitating a bowl of water in contact with the gas.[7]

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and MichaelFaraday.[8] The earliest description of solid carbon dioxide was given by Adrien-Jean-Pierre Thilorier,who in 1835 opened a pressurized container of liquid carbon dioxide, only to find that the coolingproduced by the rapid evaporation of the liquid yielded a "snow" of solid CO2.[9]

Chemical and physical properties

Structure and bonding

01 0

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The carbon dioxide molecule is linear and centrosymmetric. The two C=O bonds are equivalent and areshort (116.3 pm), consistent with double bonding.[10] Since it is centrosymmetric, the molecule has noelectrical dipole. Consistent with this fact, only two vibrational bands are observed in the IR spectrum –an antisymmetric stretching mode at 2349 cm−1 and a bending mode near 666 cm−1. There is also asymmetric stretching mode at 1388 cm−1 which is only observed in the Raman spectrum.

In aqueous solution

Carbon dioxide is soluble in water, in which it reversibly forms H2CO3 (carbonic acid), which is a weakacid since its ionization in water is incomplete.

CO2 + H2O H2CO3

The hydration equilibrium constant of carbonic acid is (at

25 °C). Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains asCO2 molecules, not affecting the pH.

The relative concentrations of CO2, H2CO3, and the deprotonated forms HCO−3 (bicarbonate) and

CO2−3 (carbonate) depend on the pH. As shown in a Bjerrum plot, in neutral or slightly alkaline water

(pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pHof seawater. In very alkaline water (pH > 10.4), the predominant (>50%) form is carbonate. The oceans,being mildly alkaline with typical pH = 8.2–8.5, contain about 120 mg of bicarbonate per liter.

Being diprotic, carbonic acid has two acid dissociation constants, the first one for the dissociation intothe bicarbonate (also called hydrogen carbonate) ion (HCO3

−):

H2CO3 HCO3− + H+

Ka1 = 2.5 × 10−4 mol/litre; pKa1 = 3.6 at 25 °C.[10]

This is the true first acid dissociation constant, defined as , where the

denominator includes only covalently bound H2CO3 and excludes hydrated CO2(aq). The much smaller

and often-quoted value near 4.16 × 10−7 is an apparent value calculated on the (incorrect) assumptionthat all dissolved CO2 is present as carbonic acid, so that

. Since most of the dissolved CO2 remains as CO2

molecules, Ka1(apparent) has a much larger denominator and a much smaller value than the true Ka1.[11]

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Carbon dioxide pressure-temperaturephase diagram showing the triplepoint and critical point of carbondioxide

Sample of solid carbon dioxide or"dry ice" pellets

The bicarbonate ion is an amphoteric species that can act as an acid or as a base, depending on pH of thesolution. At high pH, it dissociates significantly into the carbonate ion (CO3

2−):

HCO3− CO3

2− + H+

Ka2 = 4.69 × 10−11 mol/litre; pKa2 = 10.329

In organisms carbonic acid production is catalysed by the enzyme, carbonic anhydrase.

Chemical reactions of CO2

CO2 is a weak electrophile. Its reaction with basic water illustrates this property, in which casehydroxide is the nucleophile. Other nucleophiles react as well. For example, carbanions as provided byGrignard reagents and organolithium compounds react with CO2 to give carboxylates:

MR + CO2 → RCO2M

where M = Li or MgBr and R = alkyl or aryl.

In metal carbon dioxide complexes, CO2 serves as a ligand, which can facilitate the conversion of CO2

to other chemicals.[12]

The reduction of CO2 to CO is ordinarily a difficult and slow reaction:

CO2 + 2 e− + 2H+ → CO + H2O

The redox potential for this reaction near pH 7 is about −0.53 V versus the standard hydrogen electrode.The nickel-containing enzyme carbon monoxide dehydrogenase catalyses this process.[13]

Physical properties

Carbon dioxide iscolorless. At lowconcentrations, the gas isodorless. At higherconcentrations it has asharp, acidic odor. Atstandard temperature andpressure, the density ofcarbon dioxide is around1.98 kg/m3, about 1.67times that of air.

Carbon dioxide has noliquid state at pressuresbelow 5.1 standard atmospheres (520 kPa). At 1 atmosphere(near mean sea level pressure), the gas deposits directly to a solid

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at temperatures below −78.5 °C (−109.3 °F; 194.7 K) and the solid sublimes directly to a gas above−78.5 °C. In its solid state, carbon dioxide is commonly called dry ice.

Liquid carbon dioxide forms only at pressures above 5.1 atm; the triple point of carbon dioxide is about518 kPa at −56.6 °C (see phase diagram, above). The critical point is 7.38 MPa at 31.1 °C.[14] Anotherform of solid carbon dioxide observed at high pressure is an amorphous glass-like solid.[15] This form ofglass, called carbonia, is produced by supercooling heated CO2 at extreme pressure (40–48 GPa or about400,000 atmospheres) in a diamond anvil. This discovery confirmed the theory that carbon dioxide couldexist in a glass state similar to other members of its elemental family, like silicon (silica glass) andgermanium dioxide. Unlike silica and germania glasses, however, carbonia glass is not stable at normalpressures and reverts to gas when pressure is released.

At temperatures and pressures above the critical point, carbon dioxide behaves as a supercritical fluidknown as supercritical carbon dioxide.

Isolation and production

Carbon dioxide is mainly produced as an unrecovered side product of four technologies: combustion offossil fuels, production of hydrogen by steam reforming, ammonia synthesis, and fermentation. It can beobtained by distillation from air, but this method is inefficient.

The combustion of all carbon-containing fuels, such as methane (natural gas), petroleum distillates(gasoline, diesel, kerosene, propane), coal, wood and generic organic matter produces carbon dioxideand, in most cases, water. As an example the chemical reaction between methane and oxygen is givenbelow.

CH4+ 2 O2→ CO2+ 2 H2O

Quicklime (CaO), a compound that has many industrial uses, is produced by driving off CO2 fromlimestone by heating (calcining) at about 850 °C:

CaCO3→ CaO + CO2

Iron is reduced from its oxides with coke in a blast furnace, producing pig iron and carbon dioxide:[16]

Fe2O3+ 3 CO → 2 Fe + 3 CO2

Yeast metabolizes sugar to produce carbon dioxide and ethanol, also known as alcohol, in the productionof wines, beers and other spirits, but also in the production of bioethanol:

C6H12O6 → 2 CO2+ 2 C2H5OH

All aerobic organisms produce CO2 when they oxidize carbohydrates, fatty acids, and proteins in themitochondria of cells. The large number of reactions involved are exceedingly complex and notdescribed easily. Refer to (cellular respiration, anaerobic respiration and photosynthesis). The equationfor the respiration of glucose and other monosaccharides is:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

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Carbon dioxide bubbles in a soft drink.

Photoautotrophs (i.e. plants and cyanobacteria) use the energy contained in sunlight to photosynthesizesimple sugars from CO2 absorbed from the air and water:

n CO2 + n H2O → (CH2O) + n O2

Laboratory methods

A variety of chemical routes to carbon dioxide are known, such as the reaction between most acids andmost metal carbonates. For example, the reaction between hydrochloric acid and calcium carbonate(limestone or chalk) is depicted below:

CaCO + 2 HCl → CaCl2+ H2CO3

The carbonic acid (H2CO3) then decomposes to water and CO2:

H2CO3→ CO2+ H2O

Such reactions are accompanied by foaming or bubbling, or both. In industry such reactions arewidespread because they can be used to neutralize waste acid streams.

Industrial production

Industrial carbon dioxide can be produced by several methods, many of which are practiced at variousscales.[17] In its dominant route, carbon dioxide is produced as a side product of the industrial productionof ammonia and hydrogen. These processes begin with the reaction of water and natural gas (mainlymethane).[18]

Although carbon dioxide is not often recovered, carbon dioxide results from combustion of fossil fuelsand wood as well fermentation of sugar in the brewing of beer, whisky and other alcoholic beverages. Italso results from thermal decomposition of limestone, CaCO3, in the manufacture of lime (calciumoxide, CaO). It may be obtained directly from natural carbon dioxide springs, where it is produced bythe action of acidified water on limestone or dolomite.

Uses

Carbon dioxide is used by the food industry, the oil industry,and the chemical industry.[17]

Precursor to chemicals

In the chemical industry, carbon dioxide is mainly consumedas an ingredient in the production of urea and methanol.Metal carbonates and bicarbonates, as well as somecarboxylic acids derivatives (e.g., sodium salicylate) areprepared using CO2.

Foods

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Carbon dioxide is a food additive used as a propellant and acidity regulator in the food industry. It isapproved for usage in the EU[19] (listed as E number E290), USA[20] and Australia and New Zealand[21]

(listed by its INS number 290).

A candy called Pop Rocks is pressurized with carbon dioxide gas at about 4 x 106 Pa (40 bar, 580 psi).When placed in the mouth, it dissolves (just like other hard candy) and releases the gas bubbles with anaudible pop.

Leavening agents cause dough to rise by producing carbon dioxide. Baker's yeast produces carbondioxide by fermentation of sugars within the dough, while chemical leaveners such as baking powderand baking soda release carbon dioxide when heated or if exposed to acids.

Beverages

Carbon dioxide is used to produce carbonated soft drinks and soda water. Traditionally, the carbonationin beer and sparkling wine came about through natural fermentation, but many manufacturers carbonatethese drinks with carbon dioxide recovered from the fermentation process. In the case of bottled andkegged beer, the most common method used is carbonation with recycled carbon dioxide. With theexception of British Real Ale, draught beer is usually transferred from kegs in a cold room or cellar todispensing taps on the bar using pressurized carbon dioxide, sometimes mixed with nitrogen.

Wine making

Carbon dioxide in the form of dry ice is often used in the wine making process to cool down bunches ofgrapes quickly after picking to help prevent spontaneous fermentation by wild yeast. The mainadvantage of using dry ice over regular water ice is that it cools the grapes without adding any additionalwater that may decrease the sugar concentration in the grape must, and therefore also decrease thealcohol concentration in the finished wine.

Dry ice is also used during the cold soak phase of the wine making process to keep grapes cool. Thecarbon dioxide gas that results from the sublimation of the dry ice tends to settle to the bottom of tanksbecause it is denser than air. The settled carbon dioxide gas creates a hypoxic environment which helpsto prevent bacteria from growing on the grapes until it is time to start the fermentation with the desiredstrain of yeast.

Carbon dioxide is also used to create a hypoxic environment for carbonic maceration, the process usedto produce Beaujolais wine.

Carbon dioxide is sometimes used to top up wine bottles or other storage vessels such as barrels toprevent oxidation, though it has the problem that it can dissolve into the wine, making a previously stillwine slightly fizzy. For this reason, other gases such as nitrogen or argon are preferred for this processby professional wine makers.

Inert gas

It is one of the most commonly used compressed gases for pneumatic (pressurized gas) systems inportable pressure tools. Carbon dioxide is also used as an atmosphere for welding, although in thewelding arc, it reacts to oxidize most metals. Use in the automotive industry is common despitesignificant evidence that welds made in carbon dioxide are more brittle than those made in more inertatmospheres, and that such weld joints deteriorate over time because of the formation of carbonic acid. Itis used as a welding gas primarily because it is much less expensive than more inert gases such as argon

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or helium. When used for MIG welding, CO2 use is sometimes referred to as MAG welding, for MetalActive Gas, as CO2 can react at these high temperatures. It tends to produce a hotter puddle than trulyinert atmospheres, improving the flow characteristics. Although, this may be due to atmosphericreactions occurring at the puddle site. This is usually the opposite of the desired effect when welding, asit tends to embrittle the site, but may not be a problem for general mild steel welding, where ultimateductility is not a major concern.

It is used in many consumer products that require pressurized gas because it is inexpensive andnonflammable, and because it undergoes a phase transition from gas to liquid at room temperature at anattainable pressure of approximately 60 bar (870 psi, 59 atm), allowing far more carbon dioxide to fit ina given container than otherwise would. Life jackets often contain canisters of pressured carbon dioxidefor quick inflation. Aluminium capsules of CO2 are also sold as supplies of compressed gas for airguns,paintball markers, inflating bicycle tires, and for making carbonated water. Rapid vaporization of liquidcarbon dioxide is used for blasting in coal mines. High concentrations of carbon dioxide can also be usedto kill pests. Liquid carbon dioxide is used in supercritical drying of some food products andtechnological materials, in the preparation of specimens for scanning electron microscopy and in thedecaffeination of coffee beans.

Fire extinguisher

Carbon dioxide extinguishes flames, and some fire extinguishers, especially those designed for electricalfires, contain liquid carbon dioxide under pressure. Carbon dioxide extinguishers work well on smallflammable liquid and electrical fires, but not on ordinary combustible fires, because although it excludesoxygen, it does not cool the burning substances significantly and when the carbon dioxide disperses theyare free to catch fire upon exposure to atmospheric oxygen. Carbon dioxide has also been widely used asan extinguishing agent in fixed fire protection systems for local application of specific hazards and totalflooding of a protected space.[22] International Maritime Organization standards also recognize carbondioxide systems for fire protection of ship holds and engine rooms. Carbon dioxide based fire protectionsystems have been linked to several deaths, because it can cause suffocation in sufficiently highconcentrations. A review of CO2 systems identified 51 incidents between 1975 and the date of the

report, causing 72 deaths and 145 injuries.[23]

Supercritical CO2 as solvent

Liquid carbon dioxide is a good solvent for many lipophilic organic compounds and is used to removecaffeine from coffee. Carbon dioxide has attracted attention in the pharmaceutical and other chemicalprocessing industries as a less toxic alternative to more traditional solvents such as organochlorides. It isused by some dry cleaners for this reason (see green chemistry).

Agricultural and biological applications

Plants require carbon dioxide to conduct photosynthesis. Greenhouses may (if of large size, must) enrichtheir atmospheres with additional CO2 to sustain and increase plant growth.[24][25] A photosynthesis-related drop (by a factor less than two) in carbon dioxide concentration in a greenhouse compartmentwould kill green plants, or, at least, completely stop their growth. At very high concentrations (100 timesatmospheric concentration, or greater), carbon dioxide can be toxic to animal life, so raising the

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Comparison of phase diagrams of carbon dioxide (red) and water(blue) as a log-lin chart with phase transitions points at 1 atmosphere

concentration to 10,000 ppm (1%) or higher for several hours will eliminate pests such as whiteflies andspider mites in a greenhouse.[26] Carbon dioxide is used in greenhouses as the main carbon source forSpirulina algae.

In medicine, up to 5% carbon dioxide (130 times atmospheric concentration) is added to oxygen forstimulation of breathing after apnea and to stabilize the O2/CO2 balance in blood.

It has been proposed that carbon dioxide from power generation be bubbled into ponds to grow algaethat could then be converted into biodiesel fuel.[27]

Oil recovery

Carbon dioxide is used in enhanced oil recovery where it is injected into or adjacent to producing oilwells, usually under supercritical conditions, when it becomes miscible with the oil. This approach canincrease original oil recovery by reducing residual oil saturation by between 7 per cent to 23 per centadditional to primary extraction.[28] It acts as both a pressurizing agent and, when dissolved into theunderground crude oil, significantly reduces its viscosity, and changing surface chemistry enabling theoil to flow more rapidly through the reservoir to the removal well.[29] In mature oil fields, extensive pipenetworks are used to carry the carbon dioxide to the injection points.

Bio transformation into fuel

Researchers have genetically modified a strain of the cyanobacterium Synechococcus elongatus toproduce the fuels isobutyraldehyde and isobutanol from CO2 using photosynthesis.[30]

Refrigerant

Liquid and solid carbon dioxide areimportant refrigerants, especially inthe food industry, where they areemployed during the transportationand storage of ice cream and otherfrozen foods. Solid carbon dioxide iscalled "dry ice" and is used for smallshipments where refrigerationequipment is not practical. Solidcarbon dioxide is always below−78.5 °C at regular atmosphericpressure, regardless of the airtemperature.

Liquid carbon dioxide (industry nomenclature R744 or R-744) was used as a refrigerant prior to thediscovery of R-12 and may enjoy a renaissance due to the fact that R134a contributes to climate change.Its physical properties are highly favorable for cooling, refrigeration, and heating purposes, having ahigh volumetric cooling capacity. Due to its operation at pressures of up to 130 bar (1880 psi), CO2systems require highly resistant components that have already been developed for mass production inmany sectors. In automobile air conditioning, in more than 90% of all driving conditions for latitudeshigher than 50°, R744 operates more efficiently than systems using R134a. Its environmental advantages(GWP of 1, non-ozone depleting, non-toxic, non-flammable) could make it the future working fluid to

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A carbon dioxide laser.

replace current HFCs in cars, supermarkets, hot water heat pumps, among others. Coca-Cola has fieldedCO2-based beverage coolers and the U.S. Army is interested in CO2 refrigeration and heating

technology.[31][32]

The global automobile industry is expected to decide on the next-generation refrigerant in car airconditioning. CO2 is one discussed option.(see Sustainable automotive air conditioning)

Coal bed methane recovery

In enhanced coal bed methane recovery, carbon dioxide would be pumped into the coal seam to displacemethane, as opposed to current methods which primarily rely on the removal of water (to reducepressure) to make the coal seam release its trapped methane.[33]

Niche uses

Carbon dioxide is so inexpensive and so innocuous,that it finds many small uses that represent whatmight be called niche uses. For example it is used inthe carbon dioxide laser, which is one of the earliesttype of lasers.

Carbon dioxide can be used as a means ofcontrolling the pH of swimming pools, bycontinuously adding gas to the water, thus keepingthe pH level from rising. Among the advantages ofthis is the avoidance of handling (more hazardous)acids. Similarly, it is also used in the maintainingreef aquaria, where it is commonly used in calcium reactors to temporarily lower the pH of water beingpassed over calcium carbonate in order to allow the calcium carbonate to dissolve into the water morefreely where it is used by some corals to build their skeleton. It is also used as the primary coolant inadvanced gas-cooled reactors in the nuclear power generation industry.

Carbon dioxide induction is commonly used for the euthanasia of laboratory research animals. Methodsto administer CO2 include placing animals directly into a closed, prefilled chamber containing CO2, orexposure to a gradually increasing concentration of CO2. In 2013, the American Veterinary MedicalAssociation issued new guidelines for carbon dioxide induction, stating that a flow rate of 10% to 30%volume/min is optimal for the humane euthanization of small rodents.[34]

In the Earth's atmosphere

Carbon dioxide in Earth's atmosphere is considered a trace gas currently occurring at an averageconcentration of about 400 parts per million by volume[1] (or 591 parts per million by mass). The totalmass of atmospheric carbon dioxide is 3.16×1015 kg (about 3,000 gigatonnes). Its concentration variesseasonally (see graph at right) and also considerably on a regional basis, especially near the ground. Inurban areas concentrations are generally higher and indoors they can reach 10 times background levels.Carbon dioxide is a greenhouse gas.

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The Keeling Curve of atmospheric CO2

concentrations measured at Mauna LoaObservatory.

Yearly increase of atmospheric CO2:In the 1960s, the average annualincrease was 37% of the 2000–2007average.[35]

As of March 2014, carbon dioxide in the Earth's atmosphere is at a concentration of approximately 400ppm by volume.[1] Atmospheric concentrations of carbon dioxide fluctuate slightly with the change ofthe seasons, driven primarily by seasonal plant growth in the Northern Hemisphere. Concentrations ofcarbon dioxide fall during the northern spring and summer as plants consume the gas, and rise during thenorthern autumn and winter as plants go dormant, die and decay. Taking all this into account, theconcentration of CO2 grew by about 2 ppm in 2009.[36] "The main cause of the current global warmingtrend is human expansion of the "greenhouse effect" warming that results when the atmosphere trapsheat radiating from Earth toward space."[37] Carbon dioxide is a greenhouse gas as it is transparent to

incoming visiblelight from thesun, but absorbsoutgoinginfraredradiation fromthe ground at itstwo infrared-activevibrationalfrequencies (seeStructure andbonding above).As for all gases,the absorbedenergy can beredistributed by molecular collisions which heat the

atmosphere.[38]

Before the advent of release of carbon dioxide to the atmosphere by humans, concentrations tended toincrease with increasing global temperatures, acting as a positive feedback for changes induced by otherprocesses such as orbital cycles.[39] There is a seasonal cycle in CO2 concentration associated primarily

with the Northern Hemisphere growing season.[40]

Five hundred million years ago carbon dioxide was 20 times more prevalent than today, decreasing to 4–5 times during the Jurassic period and then slowly declining with a particularly swift reduction occurring49 million years ago.[41][42] Human activities such as the combustion of fossil fuels and deforestationhave caused the atmospheric concentration of carbon dioxide to increase by about 35% since thebeginning of the age of industrialization.[43]

Up to 40% of the gas emitted by some volcanoes during subaerial eruptions is carbon dioxide.[44] It isestimated that volcanoes release about 130–230 million tonnes (145–255 million short tons) of CO2 intothe atmosphere each year. Carbon dioxide is also produced by hot springs such as those at the Bossoletosite near Rapolano Terme in Tuscany, Italy. Here, in a bowl-shaped depression of about 100 m diameter,local concentrations of CO2 rise to above 75% overnight, sufficient to kill insects and small animals, but

it warms rapidly when sunlit and the gas is dispersed by convection during the day.[45] Locally highconcentrations of CO2, produced by disturbance of deep lake water saturated with CO2 are thought to

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have caused 37 fatalities at Lake Monoun, Cameroon in 1984 and 1700 casualties at Lake Nyos,Cameroon in 1986.[46] Emissions of CO2 by human activities are estimated to be 135 times greater than

the quantity emitted by volcanoes.[47]

The cement industry is one of the three primary producers of carbon dioxide along with the energyproduction and transportation industries. As of 2011 concrete contributes 7% to global anthropogenicCO2 emissions.[48]

In the oceans

Carbon dioxide dissolves in the ocean to form carbonic acid (H2CO3), bicarbonate (HCO3−) and

carbonate (CO32−), and there is about fifty times as much carbon dissolved in the sea water of the oceans

as exists in the atmosphere. The oceans act as an enormous carbon sink, and have taken up about a thirdof CO2 emitted by human activity.[49]

As the concentration of carbon dioxide increases in the atmosphere, the increased uptake of carbondioxide into the oceans is causing a measurable decrease in the pH of the oceans which is referred to asocean acidification. Although the natural absorption of CO2 by the world's oceans helps mitigate the

climatic effects of anthropogenic emissions of CO2, it also results in a decrease in the pH of the oceans.This reduction in pH impacts the biological systems in the oceans, primarily oceanic calcifyingorganisms. These impacts span the food chain from autotrophs to heterotrophs and include organismssuch as coccolithophores, corals, foraminifera, echinoderms, crustaceans and molluscs. Under normalconditions, calcite and aragonite are stable in surface waters since the carbonate ion is at supersaturatingconcentrations. However, as ocean pH falls, so does the concentration of this ion, and when carbonatebecomes undersaturated, structures made of calcium carbonate are vulnerable to dissolution. Even ifthere is no change in the rate of calcification, therefore, the rate of dissolution of calcareous materialincreases.[50]

Corals,[51][52][53] coccolithophore algae,[54][55][56][57] coralline algae,[58] foraminifera,[59] shellfish[60] andpteropods[61] experience reduced calcification or enhanced dissolution when exposed to elevated CO2.

Gas solubility decreases as the temperature of water increases (except when both pressure exceeds 300bar and temperature exceeds 393 K, only found near deep geothermal vents)[62] and therefore the rate ofuptake from the atmosphere decreases as ocean temperatures rise.

Most of the CO2 taken up by the ocean, which is about 30% of the total released into the atmosphere,[63]

forms carbonic acid in equilibrium with bicarbonate. Some of these chemical species are consumed byphotosynthetic organisms, that remove carbon from the cycle. Increased CO2 in the atmosphere has ledto decreasing alkalinity of seawater, and there is concern that this may adversely affect organisms livingin the water. In particular, with decreasing alkalinity, the availability of carbonates for forming shellsdecreases,[64] although there's evidence of increased shell production by certain species under increasedCO2 content.[65]

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NOAA states in their May 2008 "State of the science fact sheet for ocean acidification" that:"The oceans have absorbed about 50% of the carbon dioxide (CO2) released from the burning of fossilfuels, resulting in chemical reactions that lower ocean pH. This has caused an increase in hydrogen ion(acidity) of about 30% since the start of the industrial age through a process known as "oceanacidification." A growing number of studies have demonstrated adverse impacts on marine organisms,including:

The rate at which reef-building corals produce their skeletons decreases, while production ofnumerous varieties of jellyfish increases.The ability of marine algae and free-swimming zooplankton to maintain protective shells isreduced.The survival of larval marine species, including commercial fish and shellfish, is reduced."

Also, the Intergovernmental Panel on Climate Change (IPCC) writes in their Climate Change 2007:Synthesis Report:[66]

"The uptake of anthropogenic carbon since 1750 has led to the ocean becoming more acidic with anaverage decrease in pH of 0.1 units. Increasing atmospheric CO2 concentrations lead to furtheracidification ... While the effects of observed ocean acidification on the marine biosphere are as yetundocumented, the progressive acidification of oceans is expected to have negative impacts on marineshell-forming organisms (e.g. corals) and their dependent species."

Some marine calcifying organisms (including coral reefs) have been singled out by major researchagencies, including NOAA, OSPAR commission, NANOOS and the IPCC, because their most currentresearch shows that ocean acidification should be expected to impact them negatively.[67]

Carbon dioxide is also introduced into the oceans through hydrothermal vents. The Champagnehydrothermal vent, found at the Northwest Eifuku volcano at Marianas Trench Marine NationalMonument, produces almost pure liquid carbon dioxide, one of only two known sites in the world.[68]

Sea urchins have been discovered to be able to convert carbon dioxide into raw material for theirshells.[69]

Biological role

Carbon dioxide is an end product of cellular respiration in organisms that obtain energy by breakingdown sugars, fats and amino acids with oxygen as part of their metabolism. This includes all plants,algae and animals and aerobic fungi and bacteria. In vertebrates, the carbon dioxide travels in the bloodfrom the body's tissues to the skin (e.g., amphibians) or the gills (e.g., fish), from where it dissolves inthe water, or to the lungs from where it is exhaled. During active photosynthesis, plants can absorb morecarbon dioxide from the atmosphere than they use in respiration.

Photosynthesis and carbon fixation

Carbon fixation is a biochemical process by which atmospheric carbon dioxide is incorporated by plants,algae and (cyanobacteria) into energy-rich organic molecules such as glucose, thus creating their ownfood by photosynthesis. Photosynthesis uses carbon dioxide and water to produce sugars from whichother organic compounds can be constructed, and oxygen is produced as a by-product.

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Overview of photosynthesis andrespiration. Carbon dioxide (atright), together with water, formoxygen and organic compounds(at left) by photosynthesis, whichcan be respired to water and(CO2).

Figure 2. Overview of the Calvin cycle and carbonfixation

Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly abbreviated to RuBisCO, is the enzymeinvolved in the first major step of carbon fixation, the production of two molecules of 3-phosphoglycerate from CO2 and ribulose bisphosphate, as shown in the diagram at left.

RuBisCo is thought to be the single most abundant protein on Earth.[70]

Phototrophs usethe products oftheirphotosynthesisas internal foodsources and asraw material forthe biosynthesisof morecomplex organicmolecules, suchaspolysaccharides,nucleic acidsand proteins.These are usedfor their owngrowth, and also

as the basis of the food chains and webs that feed other organisms,including animals such as ourselves. Some important phototrophs,the coccolithophores synthesise hard calcium carbonate scales. Aglobally significant species of coccolithophore is Emiliania huxleyi whose calcite scales have formed thebasis of many sedimentary rocks such as limestone, where what was previously atmospheric carbon canremain fixed for geological timescales.

Plants can grow up to 50 percent faster in concentrations of 1,000 ppm CO2 when compared with

ambient conditions, though this assumes no change in climate and no limitation on other nutrients.[71]

Elevated CO2 levels cause increased growth reflected in the harvestable yield of crops, with wheat, rice

and soybean all showing increases in yield of 12–14% under elevated CO2 in FACE experiments.[72][73]

Increased atmospheric CO2 concentrations result in fewer stomata developing on plants[74] which leads

to reduced water usage and increased water-use efficiency.[75] Studies using FACE have shown that CO2

enrichment leads to decreased concentrations of micronutrients in crop plants.[76] This may have knock-on effects on other parts of ecosystems as herbivores will need to eat more food to gain the same amountof protein.[77]

The concentration of secondary metabolites such as phenylpropanoids and flavonoids can also be alteredin plants exposed to high concentrations of CO2.[78][79]

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Main symptoms of carbon dioxide toxicity, byincreasing volume percent in air.[83]

Plants also emit CO2 during respiration, and so the majority of plants and algae, which use C3photosynthesis, are only net absorbers during the day. Though a growing forest will absorb many tons ofCO2 each year, a mature forest will produce as much CO2 from respiration and decomposition of dead

specimens (e.g., fallen branches) as is used in photosynthesis in growing plants.[80] Contrary to the long-standing view that they are carbon neutral, mature forests can continue to accumulate carbon[81] andremain valuable carbon sinks, helping to maintain the carbon balance of the Earth's atmosphere.Additionally, and crucially to life on earth, photosynthesis by phytoplankton consumes dissolved CO2 in

the upper ocean and thereby promotes the absorption of CO2 from the atmosphere.[82]

Toxicity

Carbon dioxide content in fresh air (averaged betweensea-level and 10 kPa level, i.e., about 30 km altitude)varies between 0.036% (360 ppm) and 0.039% (390ppm), depending on the location.[84]

CO2 is an asphyxiant gas and not classified as toxic orharmful in accordance with Globally Harmonized Systemof Classification and Labelling of Chemicals standards ofUnited Nations Economic Commission for Europe byusing the OECD Guidelines for the Testing of Chemicals.In concentrations up to 1% (10,000 ppm), it will makesome people feel drowsy.[83] Concentrations of 7% to10% may cause suffocation, even in the presence ofsufficient oxygen, manifesting as dizziness, headache,visual and hearing dysfunction, and unconsciousnesswithin a few minutes to an hour.[85] The physiologicaleffects of acute carbon dioxide exposure are grouped together under the term hypercapnia, a subset ofasphyxiation.

Because it is heavier than air, in locations where the gas seeps from the ground (due to sub-surfacevolcanic or geothermal activity) in relatively high concentrations, without the dispersing effects of wind,it can collect in sheltered/pocketed locations below average ground level, causing animals locatedtherein to be suffocated. Carrion feeders attracted to the carcasses are then also killed. Children havebeen killed in the same way near the city of Goma by CO2 emissions from the nearby volcano Mt.Nyiragongo.[86] The Swahili term for this phenomenon is 'mazuku'.

Adaptation to increased concentrations of CO2 occurs in humans. Continuous inhalation of CO2 can betolerated at three percent inspired concentrations for at least one month and four percent inspiredconcentrations for over a week. It was suggested that 2.0 percent inspired concentrations could be usedfor closed air spaces (e.g. a submarine) since the adaptation is physiological and reversible. Decrementin performance or in normal physical activity does not happen at this level.[87][88] However, submarineshave carbon dioxide scrubbers which reduce a significant amount of the CO2 present.[89]

Miners, who are particularly vulnerable to gas exposure, referred to mixtures of carbon dioxide andnitrogen as "blackdamp," "choke damp" or "stythe." Before more effective technologies were developed,miners would frequently monitor for dangerous levels of blackdamp and other gases in mine shafts by

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bringing a caged canary with them as they worked. The canary is more sensitive to asphyxiant gasesthan humans, and as it became unconscious would stop singing and fall off its perch. The Davy lampcould also detect high levels of blackdamp (which sinks, and collects near the floor) by burning lessbrightly, while methane, another suffocating gas and explosion risk, would make the lamp burn morebrightly.

Carbon dioxide differential above outdoor concentrations at steady state conditions (when the occupancyand ventilation system operation are sufficiently long that CO2 concentration has stabilized) aresometimes used to estimate ventilation rates per person. CO2 is considered to be a surrogate for humanbio-effluents and may correlate with other indoor pollutants. Higher CO2 concentrations are associatedwith occupant health, comfort and performance degradation. ASHRAE Standard 62.1–2007 ventilationrates may result in indoor levels up to 2,100 ppm above ambient outdoor conditions. Thus if the outdoorambient is 400 ppm, indoor concentrations may reach 2,500 ppm with ventilation rates that meet thisindustry consensus standard. Concentrations in poorly ventilated spaces can be found even higher thanthis (range of 3,000 or 4,000).

Human physiology

Content

The body produces approximately 2.3 pounds (1.0 kg) of carbon dioxide per day per person,[90]

containing 0.63 pounds (290 g) of carbon.

In humans, this carbon dioxide is carried through the venous system and is breathed out through thelungs. Therefore, the carbon dioxide content in the body is high in the venous system, and decreases inthe respiratory system, resulting in lower concentrations along any arterial system. Carbon dioxidecontent of the blood is often given as the partial pressure, which is the pressure which carbon dioxidewould have had if it alone occupied the volume.[91]

In humans, the carbon dioxide contents are as follows:

Reference ranges or averages for partial pressures of carbon dioxide(abbreviated PCO2)

Unit Venous blood gas Alveolar pulmonarygas pressures Arterial blood carbon dioxide

kPa 5.5[92]-6.8[92] 4.8 4.7[92]-6.0[92]

mmHg 41–51 36 35[93]-45[93]

Transport in the blood

CO2 is carried in blood in three different ways. (The exact percentages vary depending whether it isarterial or venous blood).

Most of it (about 70% to 80%) is converted to bicarbonate ions HCO−3 by the enzyme carbonic

anhydrase in the red blood cells,[94] by the reaction CO2 + H2O → H2CO3 → H+ + HCO−3 .

5% – 10% is dissolved in the plasma[94]

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5% – 10% is bound to hemoglobin as carbamino compounds[94]

Hemoglobin, the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbondioxide. However, the CO2 bound to hemoglobin does not bind to the same site as oxygen. Instead, itcombines with the N-terminal groups on the four globin chains. However, because of allosteric effectson the hemoglobin molecule, the binding of CO2 decreases the amount of oxygen that is bound for agiven partial pressure of oxygen. The decreased binding to carbon dioxide in the blood due to increasedoxygen levels is known as the Haldane Effect, and is important in the transport of carbon dioxide fromthe tissues to the lungs. Conversely, a rise in the partial pressure of CO2 or a lower pH will causeoffloading of oxygen from hemoglobin, which is known as the Bohr Effect.

Regulation of respiration

Carbon dioxide is one of the mediators of local autoregulation of blood supply. If its levels are high, thecapillaries expand to allow a greater blood flow to that tissue.

Bicarbonate ions are crucial for regulating blood pH. A person's breathing rate influences the level ofCO2 in their blood. Breathing that is too slow or shallow causes respiratory acidosis, while breathing thatis too rapid leads to hyperventilation, which can cause respiratory alkalosis.

Although the body requires oxygen for metabolism, low oxygen levels normally do not stimulatebreathing. Rather, breathing is stimulated by higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (such as pure nitrogen) can lead to loss ofconsciousness without ever experiencing air hunger. This is especially perilous for high-altitude fighterpilots. It is also why flight attendants instruct passengers, in case of loss of cabin pressure, to apply theoxygen mask to themselves first before helping others; otherwise, one risks losing consciousness.[94]

The respiratory centers try to maintain an arterial CO2 pressure of 40 mm Hg. With intentionalhyperventilation, the CO2 content of arterial blood may be lowered to 10–20 mm Hg (the oxygen contentof the blood is little affected), and the respiratory drive is diminished. This is why one can hold one'sbreath longer after hyperventilating than without hyperventilating. This carries the risk thatunconsciousness may result before the need to breathe becomes overwhelming, which is whyhyperventilation is particularly dangerous before free diving.

See also

Carbon monoxideBosch reactionBottled gasCarbogenCarbon dioxide sensorCO2 sequestration

EcoCute – As refrigerantsEmission standardsIndustrial gas

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Kaya identityLake KivuList of least carbon efficient power stationsList of countries by carbon dioxide emissionsMeromictic lake

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Further reading

Shendell, Prill, Fisk, Apte1, Blake & Faulkner, Associations between classroom CO2

concentrations and student attendance in Washington and Idaho, Indoor Air 2004.Seppanen, Fisk and Mendell, Association of Ventilation Rates and CO2 Concentrations with

Health and Other Responses in Commercial and Institutional Buildings, Indoor Air 1999.

External links

International Chemical Safety Card 0021(http://www.inchem.org/documents/icsc/icsc/eics0021.htm)CID 280 (http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=280) from PubChemCarbon dioxide MSDS (http://sdsdata.org/68399) by Amerigas in the SDSdata.org database.CDC – NIOSH Pocket Guide to Chemical Hazards – Carbon Dioxide(http://www.cdc.gov/niosh/npg/npgd0103.html)CO2 Carbon Dioxide Properties, Uses, Applications (http://www.uigi.com/carbondioxide.html)

Dry Ice information (http://www.dryiceinfo.com/science.htm)Trends in Atmospheric Carbon Dioxide (http://www.cmdl.noaa.gov/ccgg/trends/) (NOAA)"A War Gas That Saves Lives." (http://books.google.com/books?id=RicDAAAAMBAJ&pg=PA53) Popular Science, June 1942, pp. 53–57.NASA's Orbiting Carbon Observatory (http://oco.jpl.nasa.gov/)The on-line catalogue of CO2 natural emissions in Italy (http://googas.ov.ingv.it/)

Reactions, Thermochemistry, Uses, and Function of Carbon Dioxide (http://www.chemistry-reference.com/q_compounds.asp?CAS=124-38-9)Carbon Dioxide – Part One (http://www.periodicvideos.com/videos/mv_carbon_dioxide_one.htm)and Carbon Dioxide – Part Two(http://www.periodicvideos.com/videos/mv_carbon_dioxide_two.htm) at The Periodic Table ofVideos (University of Nottingham)

Retrieved from "http://en.wikipedia.org/w/index.php?title=Carbon_dioxide&oldid=629715568"

Categories: Acid anhydrides Acidic oxides Carbon dioxide Coolants Fire suppression agentsGreenhouse gases Household chemicals Inorganic solvents Laser gain media

93. ^ a b Normal Reference Range Table(http://web.archive.org/web/20111225185659/http://pathcuric1.swmed.edu/pathdemo/nrrt.htm). University ofTexas Southwestern Medical Center at Dallas. Used in Interactive Case Study Companion to Pathologic basisof disease.

94. ^ a b c d "Carbon dioxide" (http://www.solarnavigator.net/solar_cola/carbon_dioxide.htm). solarnavigator.net.Retrieved 2007-10-12.

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