Carbon dioxide 1

Carbon dioxide

Carbon dioxide [[Image:Dry Ice Pellets Subliming.jpg Sample of solid carbon dioxide or "dry

ice", pellets]]

[[File:Carbon-dioxide-2D-dimensions.svgStructural formula of carbondioxide with a bond length]]

[[File:Carbon-dioxide-3D-vdW.svg Spacefill modelof carbondioxide]]

[[Image:Carbon_dioxide_structure.png Ball and stick model of carbon dioxide]]

Identifiers

CAS number 124-38-9 [1] 

PubChem 280 [2]

ChemSpider 274 [3] 

UNII 142M471B3J [4] 

EC number 204-696-9 [5]

UN number 1013

KEGG D00004 [6] 

MeSH Carbon+dioxide [7]

ChEBI CHEBI:16526 [8] 

RTECS number FF6400000

ATC code V03 AN02 [9]

Beilstein Reference 1900390

Gmelin Reference 989

3DMet B01131 [10]

Jmol-3D images Image 1 [11]

Image 2 [12]

Properties

Molecular formula CO2

Molar mass 44.01 g mol−1

Exact mass 43.989829244 g mol−1

Appearance Colorless gas

Odor Odorless

Carbon dioxide 2

Density 1.562 g/mL (solid at 1 atmand −78.5 °C)0.770 g/mL (liquid at 56 atmand 20 °C)1.977 g/L (gas at 1 atm and0 °C)

Melting point -78 °C, 194.7 K, -109 °F(subl.)

Boiling point -57 °C, 216.6 K, -70 °F (at5.185 bar)

Solubility in water 1.45 g/L at 25 °C, 100 kPa

Acidity (pKa) 6.35, 10.33

Refractive index (nD) 1.1120

Viscosity 0.07 cP at −78 °C

Dipole moment zero

Structure

Molecular shape linear

Hazards

NFPA 704

Related compounds

Other anions Carbon disulfideCarbon diselenide

Other cations Silicon dioxideGermanium dioxideTin dioxideLead dioxide

Related carbon oxides Carbon monoxideCarbon suboxideDicarbon monoxideCarbon trioxide

Related compounds Carbonic acidCarbonyl sulfide

(what is this?)   (verify) [13]

Except where noted otherwise, data are given for materials in theirstandard state (at 25 °C, 100 kPa)

Infobox references

Carbon dioxide (chemical formula CO2) is a naturally occurring chemical compound composed of two oxygen

atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists inEarth's atmosphere in this state, as a trace gas at a concentration of 0.039% by volume.As part of the carbon cycle known as photosynthesis, plants, algae, and cyanobacteria absorb carbon dioxide, light,and water to produce carbohydrate energy for themselves and oxygen as a waste product.[14] But in darknessphotosynthesis cannot occur, and during the resultant respiration small amounts of carbon dioxide are produced.[15]

Carbon dioxide also is a by-product of combustion; emitted from volcanoes, hot springs, and geysers; and freed fromcarbonate rocks by dissolution.As of October 2010, carbon dioxide in the Earth's atmosphere is at a concentration of 388 ppm by volume.[16]

Atmospheric concentrations of carbon dioxide fluctuate slightly with the change of the seasons, driven primarily by

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seasonal plant growth in the Northern Hemisphere. Concentrations of carbon dioxide fall during the northern springand summer as plants consume the gas, and rise during the northern autumn and winter as plants go dormant, die anddecay. Taking all this into account, the concentration of CO2 grew by about 2 ppm in 2009.[17] Carbon dioxide is agreenhouse gas as it transmits visible light but absorbs strongly in the infrared and near-infrared, before slowlyre-emitting the infrared at the same or longer wavelengths as what was absorbed.Before the advent of human-caused release of carbon dioxide to the atmosphere, concentrations tended to increasewith increasing global temperatures, acting as a positive feedback for changes induced by other processes such asorbital cycles.[18] There is a seasonal cycle in CO2 concentration associated primarily with the Northern Hemispheregrowing season.[19]

Carbon dioxide has no liquid state at pressures below 5.1 standard atmospheres (520 kPa). At 1 atmosphere (nearmean sea level pressure), the gas deposits directly to a solid at temperatures below −78 °C (−108 °F; 195 K) and thesolid sublimes directly to a gas above −78 °C. In its solid state, carbon dioxide is commonly called dry ice.CO2 is an acidic oxide: an aqueous solution turns litmus from blue to pink. It is the anhydride of carbonic acid, anacid which is unstable in aqueous solution, from which it cannot be concentrated. In organisms carbonic acidproduction is catalysed by the enzyme, carbonic anhydrase.

CO2 + H2O H2CO3CO2 is toxic in higher concentrations: 1% (10,000 ppm) will make some people feel drowsy.[20] Concentrations of7% to 10% cause dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes toan hour.[21]

Chemical and physical properties

Carbon dioxide pressure-temperature phase diagramshowing the triple point and critical point of carbon dioxide

Carbon dioxide is colorless. At low concentrations, the gas isodorless. At higher concentrations it has a sharp, acidic odor.It can cause asphyxiation and irritation. When inhaled atconcentrations much higher than usual atmospheric levels, itcan produce a sour taste in the mouth and a stinging sensationin the nose and throat. These effects result from the gasdissolving in the mucous membranes and saliva, forming aweak solution of carbonic acid. This sensation can also occurduring an attempt to stifle a burp after drinking a carbonatedbeverage. Amounts above 5,000 ppm are considered veryunhealthy, and those above about 50,000 ppm (equal to 5% byvolume) are considered dangerous to animal life.[22]

At standard temperature and pressure, the density of carbondioxide is around 1.98 kg/m3, about 1.5 times that of air. Thecarbon dioxide molecule (O=C=O) contains two double bonds

and has a linear shape. It has no electrical dipole, and as it is fully oxidized, it is moderately reactive and isnon-flammable, but will support the combustion of metals such as magnesium.

Above , carbon dioxide changes directly from a solid phase to a gaseous phase through sublimation, or from gaseousto solid through deposition. Solid carbon dioxide is commonly called "dry ice", a generic trademark. It was firstobserved in 1825 by the French chemist Charles Thilorier. Dry ice is commonly used as a cooling agent, and it isrelatively inexpensive. A convenient property for this purpose is that solid carbon dioxide sublimes directly into thegas phase, leaving no liquid. It can often be found in grocery stores and laboratories and is also used in the shippingindustry. The largest non-cooling use for dry ice is blast cleaning.

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Liquid carbon dioxide forms only at pressures above 5.1 atm; the triple point of carbon dioxide is about 518 kPa at−56.6 °C (see phase diagram, above). The critical point is 7.38 MPa at 31.1 °C.[23]

Solid carbon dioxide, an amorphous glass-like solid, is known, although not at atmospheric pressure.[24] This form ofglass, called carbonia, was produced by supercooling heated CO2 at extreme pressure (40–48 GPa or about 400,000atmospheres) in a diamond anvil. This discovery confirmed the theory that carbon dioxide could exist in a glass statesimilar to other members of its elemental family, like silicon (silica glass) and germanium. Unlike silica andgermania glasses, however, carbonia glass is not stable at normal pressures and reverts back to gas when pressure isreleased.

History

Crystal structure of dry ice

Carbon dioxide was one of the first gases to be described as asubstance distinct from air. In the seventeenth century, theFlemish chemist Jan Baptist van Helmont observed that whenhe burned charcoal in a closed vessel, the mass of theresulting ash was much less than that of the original charcoal.His interpretation was that the rest of the charcoal had beentransmuted into an invisible substance he termed a "gas" or"wild spirit" (spiritus sylvestre).

The properties of carbon dioxide were studied morethoroughly in the 1750s by the Scottish physician JosephBlack. He found that limestone (calcium carbonate) could beheated or treated with acids to yield a gas he called "fixed air."He observed that the fixed air was denser than air andsupported neither flame nor animal life. Black also found thatwhen bubbled through an aqueous solution of lime (calcium

hydroxide), it would precipitate calcium carbonate. He used this phenomenon to illustrate that carbon dioxide isproduced by animal respiration and microbial fermentation. In 1772, English chemist Joseph Priestley published apaper entitled Impregnating Water with Fixed Air in which he described a process of dripping sulfuric acid (or oil ofvitriol as Priestley knew it) on chalk in order to produce carbon dioxide, and forcing the gas to dissolve by agitatinga bowl of water in contact with the gas.[25] This was the invention of Soda water.

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and Michael Faraday.[26] Theearliest description of solid carbon dioxide was given by Charles Thilorier, who in 1834 opened a pressurizedcontainer of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquidyielded a "snow" of solid CO2.[27]

Isolation and productionCarbon dioxide can be obtained by or from air distillation, however, this method is inefficient. A variety of chemicalroutes to carbon dioxide are known, such as the reaction between most acids and most metal carbonates. Forexample, the reaction between hydrochloric acid and calcium carbonate (limestone or chalk) is depicted below:

2 HCl + CaCO3 → CaCl2 + H2CO3The carbonic acid (H2CO3) then decomposes to water and CO2. Such reactions are accompanied by foaming orbubbling, or both. In industry such reactions are widespread because they can be used to neutralize waste acidstreams.

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The production of quicklime (CaO), a chemical that has widespread use, from limestone by heating at about 850 °Calso produces CO2:

CaCO3 → CaO + CO2The combustion of all carbon containing fuels, such as methane (natural gas), petroleum distillates (gasoline, diesel,kerosene, propane), but also of coal and wood, will yield carbon dioxide and, in most cases, water. As an examplethe chemical reaction between methane and oxygen is given below.

CH4 + 2 O2 → CO2 + 2 H2OIron is reduced from its oxides with coke in a blast furnace, producing pig iron and carbon dioxide:[28]

Fe2O3 + 3 CO → 2 Fe + 3 CO2Yeast metabolizes sugar to produce carbon dioxide and ethanol, also known as alcohol, in the production of wines,beers and other spirits, but also in the production of bioethanol:

C6H12O6 → 2 CO2 + 2 C2H5OHAll aerobic organisms produce CO2 when they oxidize carbohydrates, fatty acids, and proteins in the mitochondria ofcells. The large number of reactions involved are exceedingly complex and not described easily. Refer to (cellularrespiration, anaerobic respiration and photosynthesis). Photoautotrophs (i.e. plants, cyanobacteria) use anothermodus operandi: Plants absorb CO2 from the air, and, together with water, react it to form carbohydrates:

nCO2 + nH2O → (CH2O)n + nO2Carbon dioxide is soluble in water, in which it spontaneously interconverts between CO2 and H2CO3 (carbonic acid).The relative concentrations of CO2, H2CO3, and the deprotonated forms HCO3− (bicarbonate) andCO32−(carbonate) depend on the pH. In neutral or slightly alkaline water (pH > 6.5), the bicarbonate formpredominates (>50%) becoming the most prevalent (>95%) at the pH of seawater, while in very alkaline water (pH >10.4) the predominant (>50%) form is carbonate. The bicarbonate and carbonate forms are very soluble, such thatair-equilibrated ocean water (mildly alkaline with typical pH = 8.2 – 8.5) contains about 120 mg of bicarbonate perliter.

Industrial productionIndustrial carbon dioxide is produced mainly from six processes:[29]

• Directly from natural carbon dioxide springs, where it is produced by the action of acidified water on limestone ordolomite.

• As a by-product of hydrogen production plants, where methane is converted to CO2;• From combustion of fossil fuels and wood;• As a by-product of fermentation of sugar in the brewing of beer, whisky and other alcoholic beverages;• From thermal decomposition of limestone, CaCO3, in the manufacture of lime (Calcium oxide, CaO);

Carbon dioxide 6

Uses

Carbon dioxide bubbles in a soft drink.

Carbon dioxide is used by the food industry, the oilindustry, and the chemical industry.[29] It is used inmany consumer products that require pressurized gasbecause it is inexpensive and nonflammable, andbecause it undergoes a phase transition from gas toliquid at room temperature at an attainable pressure ofapproximately 60 bar (870 psi, 59 atm), allowing farmore carbon dioxide to fit in a given container thanotherwise would. Life jackets often contain canisters ofpressured carbon dioxide for quick inflation.Aluminum capsules of CO2 are also sold as supplies ofcompressed gas for airguns, paintball markers, inflatingbicycle tires, and for making carbonated water. Rapidvaporization of liquid carbon dioxide is used for blasting in coal mines. High concentrations of carbon dioxide canalso be used to kill pests.

FoodsA candy called Pop Rocks is pressurized with carbon dioxide gas at about 40 bar (580 psi). When placed in themouth, it dissolves (just like other hard candy) and releases the gas bubbles with an audible pop.Leavening agents produce carbon dioxide to cause dough to rise. Baker's yeast produces carbon dioxide byfermentation of sugars within the dough, while chemical leaveners such as baking powder and baking soda releasecarbon dioxide when heated or if exposed to acids.

Beverages

Carbon dioxide is used to produce carbonated soft drinks and soda water. Traditionally, the carbonation in beer andsparkling wine came about through natural fermentation, but many manufacturers carbonate these drinks with carbondioxide recovered from the fermentation process. In the case of bottled and kegged beer, recylcled carbon dioxidecarbonation is the most common method used. With the exception of British Real Ale, draught beer is usuallytransferred from kegs in a cold room or cellar to dispensing 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 of grapesquickly after picking to help prevent spontaneous fermentation by wild yeasts. The main advantage of using dry iceover regular water ice is that it cools the grapes without adding any additional water that may decrease the sugarconcentration in the grape must, and therefore also decrease the alcohol concentration in the finished wine.Dry ice is also used during the cold soak phase of the wine making process to keep grapes cool. The carbon dioxidegas that results from the sublimation of the dry ice tends to settle to the bottom of tanks because it is heavier than air.The settled carbon dioxide gas creates a hypoxic environment which helps to prevent bacteria from growing on thegrapes until it is time to start the fermentation with the desired strain of yeast.Carbon dioxide is also used to create a hypoxic environment for carbonic maceration, the process used to produceBeaujolais wine.Carbon dioxide is sometimes used to top up wine bottles or other storage vessels such as barrels to prevent oxidation, though it has the problem that it can dissolve into the wine, making a previously still wine slightly fizzy. For this

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reason, other gases such as nitrogen or argon are preferred for this process by professional wine makers.

Pneumatic systemsCarbon dioxide is one of the most commonly used compressed gases for pneumatic (pressurized gas) systems inportable pressure tools and combat robots.

Fire extinguisherCarbon dioxide extinguishes flames, and some fire extinguishers, especially those designed for electrical fires,contain liquid carbon dioxide under pressure. Carbon dioxide extinguishers work well on small flammable liquid andelectrical fires, but not on ordinary combustible fires, as it is so dry. Carbon dioxide has also been widely used as anextinguishing agent in fixed fire protection systems for local application of specific hazards and total flooding of aprotected space, (National Fire Protection Association Code 12). International Maritime Organization standards alsorecognize carbon dioxide systems for fire protection of ship holds and engine rooms. Carbon dioxide based fireprotection systems have been linked to several deaths, because it does not support life in the concentrations used toextinguish fire (40% or so), however, it is not considered to be toxic to humans. A review of CO2 systems (CarbonDioxide as a Fire Suppressant: Examining the Risks, US EPA) identified 51 incidents between 1975 and the date ofthe report, causing 72 deaths and 145 injuries.

WeldingCarbon dioxide also finds use as an atmosphere for welding, although in the welding arc, it reacts to oxidize mostmetals. Use in the automotive industry is common despite significant evidence that welds made in carbon dioxide aremore brittle than those made in more inert atmospheres, and that such weld joints deteriorate over time because ofthe formation of carbonic acid. It is used as a welding gas primarily because it is much less expensive than moreinert gases such as argon or helium.When used for MIG welding, CO2 use is sometimes referred to as MAG welding, for Metal Active Gas, as CO2 canreact at these high temperatures. It tends to produce a hotter puddle than truly inert atmospheres, improving the flowcharacteristics. Although, this may be due to atmospheric reactions occurring at the puddle site. This is usually theopposite of the desired effect when welding, as it tends to embrittle the site, but may not be a problem for generalmild steel welding, where ultimate ductility is not a major concern.

Pharmaceutical and other chemical processingLiquid carbon dioxide is a good solvent for many lipophilic organic compounds and is used to remove caffeine fromcoffee. Carbon dioxide has attracted attention in the pharmaceutical and other chemical processing industries as aless toxic alternative to more traditional solvents such as organochlorides. It is used by some dry cleaners for thisreason. (See green chemistry.)Carbon dioxide is used as an ingredient in the production of urea, carbonates and bicarbonates, and sodiumsalicylate.[30] Carbon dioxide is known to react with Grignard reagents to form carboxylic acids. In a metal carbondioxide complexes, CO2 serves as a ligand, which can facilitate the conversion of CO2 to other chemicals.

Agricultural and biological applicationsPlants require carbon dioxide to conduct photosynthesis. Greenhouses may (if of large size, must) enrich their atmospheres with additional CO2 to sustain and increase plant growth.[31] [32] [33] A photosynthesis-related drop (by a factor less than two) in carbon dioxide concentration in a greenhouse compartment would kill green plants, or, at least, completely stop their growth. At very high concentrations (a factor of 100 or more higher than its atmospheric concentration), carbon dioxide can be toxic to animal life, so raising the concentration to 10,000 ppm (1%) or higher for several hours will eliminate pests such as whiteflies and spider mites in a greenhouse.[34] Carbon dioxide is used

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in greenhouses as the main carbon source for Spirulina algae.In medicine, up to 5% carbon dioxide (130 times the atmospheric concentration) is added to oxygen for stimulationof 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 algae that could thenbe converted into biodiesel fuel.[35]

Lasers

A carbon dioxide laser.

A common type of industrial gas laser is thecarbon dioxide laser.

Oil recovery

Carbon dioxide is used in enhanced oilrecovery where it is injected into or adjacentto producing oil wells, usually undersupercritical conditions. It acts as both apressurizing agent and, when dissolved intothe underground crude oil, significantlyreduces its viscosity, enabling the oil to flowmore rapidly through the earth to theremoval well.[36] In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injectionpoints.

RefrigerantLiquid and solid carbon dioxide are important refrigerants, especially in the food industry, where they are employedduring the transportation and storage of ice cream and other frozen foods. Solid carbon dioxide is called "dry ice"and is used for small shipments where refrigeration equipment is not practical. Solid carbon dioxide is always below−78 °C at regular atmospheric pressure, regardless of the air temperature.Liquid carbon dioxide (industry nomenclature R744 or R-744) was used as a refrigerant prior to the discovery ofR-12 and may enjoy a renaissance due to the fact that r134a contributes to climate change. Its physical properties arehighly favorable for cooling, refrigeration, and heating purposes, having a high volumetric cooling capacity. Due toits operation at pressures of up to 130 bar (1880 psi), CO2 systems require highly resistant components that havealready been developed for mass production in many sectors. In automobile air conditioning, in more than 90% of alldriving conditions for latitudes higher than 50°, R744 operates more efficiently than systems using R-134a. Itsenvironmental advantages (GWP of 1, non-ozone depleting, non-toxic, non-flammable) could make it the futureworking fluid to replace current HFCs in cars, supermarkets, hot water heat pumps, among others. Coca-Cola hasfielded CO2-based beverage coolers and the U.S. Army is interested in CO2 refrigeration and heating technology.[37]

[38]

The global automobile industry is expected to decide on the next-generation refrigerant in car air conditioning. CO2is one discussed option.(see The Cool War)

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Coal bed methane recoveryIn enhanced coal bed methane recovery, carbon dioxide is pumped into the coal seam to displace methane.[39]

pH controlCarbon dioxide can be used as a mean of controlling the pH of swimming pools, by continuously adding gas to thewater, thus keeping the pH level from rising. Among the advantages of this is the avoidance of handling (morehazardous) acids.CO2 is also used in the keeping of reef aquaria, where it is commonly used in calcium reactors to temporarily lowerthe pH of water being passed over calcium carbonate in order to allow the calcium carbonate to dissolve into thewater more freely where it is used by some corals to build their skeleton.

In the Earth's atmosphere

The Keeling Curve of atmospheric CO2 concentrations measured at MaunaLoa Observatory.

Carbon dioxide in earth's atmosphere isconsidered a trace gas currently occurring at anaverage concentration of about 390 parts permillion by volume or 591 parts per million bymass.[40] The total mass of atmospheric carbondioxide is 3.16×1015 kg (about 3,000gigatonnes). Its concentration varies seasonally(see graph at right) and also considerably on aregional basis, especially near the ground. Inurban areas concentrations are generally higherand indoors they can reach 10 times backgroundlevels. Carbon dioxide is a greenhouse gas.

Yearly increase of atmospheric CO2: In the1960s, the average annual increase was 37% of

the 2000–2007 average.[41]

Five hundred million years ago carbon dioxide was 20 times moreprevalent than today, decreasing to 4–5 times during the Jurassicperiod and then slowly declining with a particularly swift reductionoccurring 49 million years ago.[42] [43] Human activities such as thecombustion of fossil fuels and deforestation have caused theatmospheric concentration of carbon dioxide to increase by about 35%since the beginning of the age of industrialization.[44]

Up to 40% of the gas emitted by some volcanoes during subaerialeruptions is carbon dioxide.[45] It is estimated that volcanoes releaseabout 130–230 million tonnes (145–255 million tons) of CO2 into theatmosphere each year. Carbon dioxide is also produced by hot springssuch as those at the Bossoleto site 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 warmsrapidly when sunlit and the gas is dispersed by convection during the day.[46] Locally high concentrations of CO2,produced by disturbance of deep lake water saturated with CO2 are thought to have caused 37 fatalities at LakeMonoun, Cameroon in 1984 and 1700 casualties at Lake Nyos, Cameroon in 1986.[47] Emissions of CO2 by human

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activities are currently more than 130 times greater than the quantity emitted by volcanoes, amounting to about 27billion tonnes per year.[48]

In the oceansThere is about fifty times as much carbon dissolved in the sea water of the oceans in the form of CO2 and carbonicacid, bicarbonate and carbonate ions as exists in the atmosphere. The oceans act as an enormous carbon sink, andhave taken up about a third of CO2 emitted by human activity.[49] Gas solubility decreases as the temperature ofwater increases (except when both pressure exceeds 300 bar and temperature exceeds 393 K, only found near deepgeothermal vents)[50] and therefore the rate of uptake 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,[51] formscarbonic acid in equilibrium with bicarbonate and carbonate ions. Some is consumed in photosynthesis by organismsin the water, and a small proportion of that sinks and leaves the carbon cycle. Increased CO2 in the atmosphere hasled to decreasing alkalinity of seawater and there is concern that this may adversely affect organisms living in thewater. In particular, with decreasing alkalinity, the availability of carbonates for forming shells decreases,[52]

although there's evidence of increased shell production by certain species under increased CO2 content.[53]

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 fossil fuels,resulting in chemical reactions that lower ocean pH. This has caused an increase in hydrogen ion (acidity) of about30% since the start of the industrial age through a process known as “ocean acidification.” A growing number ofstudies have demonstrated adverse impacts on marine organisms, including:• The rate at which reef-building corals produce their skeletons decreases, while production of numerous varieties

of jellyfish increases.• The ability of marine algae and free-swimming zooplankton to maintain protective shells is reduced.• 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[54] :"The uptake of anthropogenic carbon since 1750 has led to the ocean becoming more acidic with an averagedecrease in pH of 0.1 units. Increasing atmospheric CO2 concentrations lead [sic] to further acidification [...] Whilethe effects of observed ocean acidification on the marine biosphere are as yet undocumented, the progressiveacidification of oceans is expected to have negative impacts on marine shell-forming organisms (e.g. corals) andtheir dependent species."Some marine calcifying organisms (including coral reefs) have been singled out by major research agencies,including NOAA, OSPAR commission, NANOOS and the IPCC, because their most current research shows thatocean acidification should be expected to impact them negatively.[55]

The Champagne hydrothermal 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.[56]

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Biological roleCarbon dioxide is an end product in organisms that obtain energy from breaking down sugars, fats and amino acidswith oxygen as part of their metabolism, in a process known as cellular respiration. This includes all plants, animals,many fungi and some bacteria. In higher animals, the carbon dioxide travels in the blood from the body's tissues tothe lungs where it is exhaled. In plants using photosynthesis, carbon dioxide is absorbed from the atmosphere.

Photosynthesis and carbon fixation

Overview of photosynthesis and respiration. Carbondioxide (at right), together with water, form oxygenand organic compounds (at left) by photosynthesis,

which can be respired to water and (CO2).

Carbon fixation is the removal of carbon dioxide from the air andits incorporation into solid compounds. Plants, algae, and manyspecies of bacteria (cyanobacteria) fix carbon and create their ownfood by photosynthesis. Photosynthesis uses carbon dioxide andwater to produce sugars and occasionally other organiccompounds, releasing oxygen as a waste product. Thesephototrophs use the products of their photosynthesis as internalfood sources and as raw material for the construction of morecomplex organic molecules, such as polysaccharides, nucleic acidsand proteins. These are used for their own growth, and also as thebasis for the food chains and webs whereby other organisms,including animals such as ourselves, are fed. Some importantphototrophs, the coccolithophores synthesise hard calciumcarbonate scales. A globally significant species of coccolithophoreis Emiliania huxleyi whose calcite scales have formed the basis ofmany sedimentary rocks such as limestone, where what waspreviously atmospheric carbon can remain fixed for geologicaltimescales.

Plants can grow up to 50 percent faster in concentrations of 1,000 ppm CO2 when compared with ambientconditions, though this assumes no change in climate and no limitation on other nutrients.[57] Some people (forexample David Bellamy) believe that as the concentration of CO2 rises in the atmosphere that it will lead to fasterplant growth and therefore increase food production.[58] Recent research supports this position: elevated CO2 levelscause increased growth reflected in the harvestable yield of crops, with wheat, rice and soybean all showingincreases in yield of 12–14% under elevated CO2 in FACE experiments.[59] [60]

Studies have shown that increased CO2 leads to fewer stomata developing on plants[61] which leads to reduced waterusage.[62] Studies using FACE have shown that increases in CO2 lead to decreased concentration of micronutrients incrop plants.[63] This may have knock-on effects on other parts of ecosystems as herbivores will need to eat morefood to gain the same amount of protein.[64]

Plants also emit CO2 during respiration, and so the majority of plants and algae, which use C3 photosynthesis, areonly net absorbers during the day. Though a growing forest will absorb many tons of CO2 each year, the World Bankwrites that a mature forest will produce as much CO2 from respiration and decomposition of dead specimens (e.g.,fallen branches) as is used in biosynthesis in growing plants.[65] However six experts in biochemistry, biogeology,forestry and related areas writing in the science journal Nature that "Our results demonstrate that old-growth forestscan continue to accumulate carbon, contrary to the long-standing view that they are carbon neutral." [66] Matureforests are valuable carbon sinks, helping maintain balance in the Earth's atmosphere. Additionally, and crucially tolife on earth, photosynthesis by phytoplankton consumes dissolved CO2 in the upper ocean and thereby promotes theabsorption of CO2 from the atmosphere.[67]

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Toxicity

Main symptoms of carbon dioxide toxicity, by increasing volume percentin air.[20] [68]

Carbon dioxide content in fresh air (averagedbetween sea-level and 10 kPa level, i.e., about30 km altitude) varies between 0.036% (360 ppm)and 0.039% (390 ppm), depending on thelocation.[69]

Prolonged exposure to moderate concentrationscan cause acidosis and adverse effects on calciumphosphorus metabolism resulting in increasedcalcium deposits in soft tissue. Carbon dioxide istoxic to the heart and causes diminished contractileforce.[68]

Toxicity and its effects increase with theconcentration of CO2, here given in volumepercent of CO2 in the air:

• 1% can cause drowsiness with prolongedexposure.[20]

• At 2% it is mildly narcotic and causes increasedblood pressure and pulse rate, and causes reduced hearing.[68]

• At about 5% it causes stimulation of the respiratory center, dizziness, confusion and difficulty in breathingaccompanied by headache and shortness of breath.[68] Panic attacks may also occur at this concentration.[70] [71]

• At about 8% it causes headache, sweating, dim vision, tremor and loss of consciousness after exposure forbetween five and ten minutes.[68]

Due to the health risks associated with carbon dioxide exposure, the U.S. Occupational Safety and HealthAdministration says that average exposure for healthy adults during an eight-hour work day should not exceed 5,000ppm (0.5%). The maximum safe level for infants, children, the elderly and individuals with cardio-pulmonary healthissues is significantly less. For short-term (under ten minutes) exposure, the U.S. National Institute for OccupationalSafety and Health (NIOSH) and American Conference of Government Industrial Hygienists (ACGIH) limit is 30,000ppm (3%). NIOSH also states that carbon dioxide concentrations exceeding 4% are immediately dangerous to lifeand health[72] although physiological experiments show that such levels can be tolerated for some time.[73]

Adaptation to increased levels of CO2 occurs in humans. Continuous inhalation of CO2 can be tolerated at threepercent inspired concentrations for at least one month and four percent inspired concentrations for over a week. Itwas suggested that 2.0 percent inspired concentrations could be used for closed air spaces (e.g. a submarine) sincethe adaptation is physiological and reversible. Decrement in performance or in normal physical activity does nothappen at this level.[73] [74] However, it should be noted that submarines have carbon dioxide scrubbers which reducea significant amount of the CO2 present.[75]

These figures are valid for pure carbon dioxide. In indoor spaces occupied by people the carbon dioxideconcentration will reach higher levels than in pure outdoor air. Concentrations higher than 1,000 ppm will causediscomfort in more than 20% of occupants, and the discomfort will increase with increasing CO2 concentration. Thediscomfort will be caused by various gases coming from human respiration and perspiration, and not by CO2 itself.At 2,000 ppm the majority of occupants will feel a significant degree of discomfort, and many will develop nauseaand headaches. The CO2 concentration between 300 and 2,500 ppm is used as an indicator of indoor air quality.Acute carbon dioxide toxicity is sometimes known by the names given to it by miners: blackdamp (also called choke damp or stythe). Blackdamp is primarily nitrogen and carbon dioxide and kills via suffocation (having displaced oxygen). Miners would try to alert themselves to dangerous levels of blackdamp and other gasses in a mine shaft by

Carbon dioxide 13

bringing a caged canary with them as they worked. The canary is more sensitive to environmental gasses thanhumans and as it became unconscious would stop singing and fall off its perch. The Davy lamp could also detecthigh levels of blackdamp (which collect near the floor) by burning less brightly, while methane, another suffocatinggas and explosion risk would make the lamp burn more brightly).Carbon dioxide differential above outdoor levels at steady state conditions (when the occupancy and ventilationsystem operation are sufficiently long that CO2 concentration has stabilized) are sometimes used to estimateventilation rates per person. CO2 is considered to be a surrogate for human bio-effluents and may correlate with otherindoor pollutants. Higher CO2 concentrations are associated with occupant health, comfort and performancedegradation. ASHRAE Standard 62.1–2007 ventilation rates may result in indoor levels up to 2,100 ppm aboveambient outdoor conditions. Thus if the outdoor ambient is 400 ppm, indoor levels may reach 2,500 ppm withventilation rates that meet this industry consensus standard. Levels in poorly ventilated spaces can be found evenhigher than this (range of 3,000 or 4,000). [Mendell and Shendell references]

Human physiologyCO2 is carried in blood in three different ways. (The exact percentages vary depending whether it is arterial orvenous blood).• Most of it (about 70% to 80%) is converted to bicarbonate ions HCO3− by the enzyme carbonic anhydrase in the

red blood cells,[76] by the reaction CO2 + H2O → H2CO3 → H+ + HCO3−.• 5% – 10% is dissolved in the plasma[76]

• 5% – 10% is bound to hemoglobin as carbamino compounds[76]

Hemoglobin, the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbon dioxide.However, the CO2 bound to hemoglobin does not bind to the same site as oxygen. Instead, it combines with theN-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, thebinding of CO2 decreases the amount of oxygen that is bound for a given partial pressure of oxygen. The decreasedbinding to carbon dioxide in the blood due to increased oxygen levels is known as the Haldane Effect, and isimportant in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure ofCO2 or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the Bohr Effect.Carbon dioxide is one of the mediators of local autoregulation of blood supply. If its levels are high, the capillariesexpand 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 of CO2 in theirblood. Breathing that is too slow or shallow causes respiratory acidosis, while breathing that is too rapid leads tohyperventilation, which can cause respiratory alkalosis.Although the body requires oxygen for metabolism, low oxygen levels normally do not stimulate breathing. Rather,breathing is stimulated by higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture withno oxygen at all (such as pure nitrogen) can lead to loss of consciousness without ever experiencing air hunger. Thisis especially perilous for high-altitude fighter pilots. It is also why flight attendants instruct passengers, in case ofloss of cabin pressure, to apply the oxygen mask to themselves first before helping others; otherwise, one risks losingconsciousness.[76]

The respiratory centers try to maintain an arterial CO2 pressure of 40 mm Hg. With intentional hyperventilation, theCO2 content of arterial blood may be lowered to 10–20 mm Hg (the oxygen content of the blood is little affected),and the respiratory drive is diminished. This is why one can hold one's breath longer after hyperventilating thanwithout hyperventilating. This carries the risk that unconsciousness may result before the need to breathe becomesoverwhelming, which is why hyperventilation is particularly dangerous before free diving.Breathing produces approximately 2.3 pounds (1 kg) of carbon dioxide per day per person.[77]

Carbon dioxide 14

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Further reading• Tyler Volk (2008), CO2 Rising: The World's Greatest Environmental Challenge, The MIT Press, 223 pages,

ISBN 978-0262220835. A short, balanced primer on CO2's role as a greenhouse gas. Review (http:/ / www.ehponline. org/ docs/ 2009/ 117-2/ newbooks. html) at Environmental Health Perspectives

• Shendell, Prill, Fisk, Apte1, Blake & Faulkner, Associations between classroom CO2 concentrations and studentattendance in Washington and Idaho, Indoor Air 2004.

• Seppanen, Fisk and Mendell, Association of Ventilation Rates and CO2 Concentrations with Health and OtherResponses 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 PubChem• 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)• 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)

Article Sources and Contributors 17

Article Sources and ContributorsCarbon dioxide  Source: http://en.wikipedia.org/w/index.php?oldid=445870659  Contributors: 1clifford1, 2D, 2over0, 2spk, 42istheanswer, 4light, 5ir 3ntropy, 84user, A3RO, AGeorgas,AMalstrom13, AThing, Aboofaheem, AdjustShift, Adrian J. 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Image Sources, Licenses and ContributorsFile:Yes check.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Yes_check.svg  License: Public Domain  Contributors: SVG by Gregory Maxwell (modified by WarX)Image:Equilibrium.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Equilibrium.svg  License: Public Domain  Contributors: L'AquatiqueFile:Carbon dioxide pressure-temperature phase diagram.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Carbon_dioxide_pressure-temperature_phase_diagram.svg  License:Creative Commons Zero  Contributors: Ben Finney Mark JacobsFile:Carbon-dioxide-crystal-3D-vdW.png  Source: http://en.wikipedia.org/w/index.php?title=File:Carbon-dioxide-crystal-3D-vdW.png  License: Public Domain  Contributors: Ben MillsFile:Soda bubbles macro.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Soda_bubbles_macro.jpg  License: Public domain  Contributors: en:User:SpiffFile:Carbon Dioxide Laser At The Laser Effects Test Facility.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Carbon_Dioxide_Laser_At_The_Laser_Effects_Test_Facility.jpg License: Public Domain  Contributors: Avron, Mousy, Pieter KuiperFile:Mauna Loa Carbon Dioxide-en.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Mauna_Loa_Carbon_Dioxide-en.svg  License: Creative Commons Attribution-Sharealike3.0,2.5,2.0,1.0  Contributors: SémhurFile:CO2 increase rate.png  Source: http://en.wikipedia.org/w/index.php?title=File:CO2_increase_rate.png  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors:New Image Uploader 929 (talk). Original uploader was New Image Uploader 929 at en.wikipedia

Image Sources, Licenses and Contributors 18

File:Auto-and heterotrophs.png  Source: http://en.wikipedia.org/w/index.php?title=File:Auto-and_heterotrophs.png  License: Creative Commons Attribution-Sharealike 3.0  Contributors:Mikael HäggströmFile:Main symptoms of carbon dioxide toxicity.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Main_symptoms_of_carbon_dioxide_toxicity.svg  License: Public Domain Contributors: Mikael Häggström

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