| Carbon dioxide |
|---|
 |
|
|
Carbonic acid gas Carbonic anhydride Carbonic oxide Carbon oxide Carbon(IV) oxide Dry ice (solid phase)
|
| Identifiers |
|---|
| CAS number | 124-38-9  Y |
| PubChem | 280 |
| ChemSpider | 274 Y |
| UNII | 142M471B3J Y |
| EC number | 204-696-9 |
| UN number | 1013 |
| KEGG | D00004 Y |
| MeSH | Carbon+dioxide |
| ChEBI | CHEBI:16526 Y |
| ChEMBL | CHEMBL1231871  N |
| RTECS number | FF6400000 |
| ATC code | V03AN02 |
| Beilstein Reference | 1900390 |
| Gmelin Reference | 989 |
| 3DMet | B01131 |
| Jmol-3D images | Image 1
Image 2 |
|
|
InChI=1S/CO2/c2-1-3 YKey: CURLTUGMZLYLDI-UHFFFAOYSA-N Y
InChI=1/CO2/c2-1-3
Key: CURLTUGMZLYLDI-UHFFFAOYAO
|
| Properties |
|---|
| Molecular formula | 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 point at 5.1 atm) |
| Sublimation conditions | −78.5 °C; −109.2 °F; 194.7 K (1 atm) |
| Solubility in water | 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 |
|---|
Specific
heat capacity C | 37.135 J/K mol |
Std molar
entropy So298 | 214 J·mol−1·K−1 |
Std enthalpy of
formation ΔfHo298 | −393.5 kJ·mol−1 |
| Hazards |
|---|
| MSDS | External MSDS |
| NFPA 704 |
|
| Related compounds |
|---|
| Other anions | Carbon disulfide
Carbon diselenide |
| Other cations | Silicon dioxide
Germanium dioxide
Tin dioxide
Lead dioxide |
| Related carbon oxides | Carbon monoxide
Carbon suboxide
Dicarbon monoxide
Carbon trioxide |
| Related compounds | Carbonic acid
Carbonyl sulfide |
| Supplementary data page |
|---|
Structure and
properties | n, εr, etc. |
Thermodynamic
data | Phase behaviour
Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa) |
 N (verify) (what is:  Y/ N?) |
| Infobox references |
| |
Carbon dioxide (
chemical formula CO2) is a naturally-occurring
chemical compound composed of 2
oxygen atoms each
covalently double bonded to a single
carbon atom. It is a
gas at
standard temperature and pressure and exists in
Earth's atmosphere in this state, as a
trace gas at a concentration of 0.04 per cent (400
ppm) by volume, as of 2014.
[1]
As part of the
carbon cycle,
plants,
algae, and
cyanobacteria use
light energy to
photosynthesize carbohydrate from carbon dioxide and
water, with
oxygen produced as a waste product.
[2] However, photosynthesis cannot occur in darkness and at night some carbon dioxide is produced by plants during
respiration.
[3] It is produced during the respiration of all other
aerobic organisms and is exhaled in the breath of air-breathing land animals, including humans. Carbon dioxide is produced during the processes of
decay of organic materials and the
fermentation of sugars in
beer and
winemaking. It is produced by
combustion of
wood,
carbohydrates and major
carbon- and
hydrocarbon-rich
fossil fuels such as coal, peat, petroleum and natural gas. It is emitted from
volcanoes,
hot springs and
geysers and is freed from
carbonate rocks by
dissolution in water and acids. CO
2 is found in lakes, at depth under the sea and commingled with oil and gas deposits.
[4]
The environmental effects of carbon dioxide are of significant interest. Atmospheric carbon dioxide is the primary source of carbon in
life on Earth and its concentration in Earth's pre-industrial atmosphere since late in the
Precambrian eon was regulated by
photosynthetic organisms. Carbon dioxide is an important
greenhouse gas and burning of carbon-based fuels since the
industrial revolution has rapidly increased its concentration in the atmosphere, leading to
global warming. It is also a major source of
ocean acidification since it dissolves in water to form
carbonic acid.
[5]
Chemical and physical properties
Structure and bonding
The carbon dioxide molecule is linear and
centrosymmetric. The two C=O bonds are equivalent and are short (116.3
pm), consistent with double bonding.
[10] Since it is centrosymmetric, the molecule has no electrical
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 a symmetric 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
H
2CO
3 (
carbonic acid), which is a
weak acid since its ionization in water is incomplete.
- CO
2 + H
2O 
H
2CO
3
The
hydration equilibrium constant of carbonic acid is
![K_{\mathrm h}=\frac{\rm{[H_2CO_3]}}{\rm{[CO_2(aq)]}}=1.70\times 10^{-3}](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_vtDm2VdIDzfYvtPAN7Mvn5wyZ0ALDHcqKtLyQUU90qgY_mW4b1gNmJe7xIMJy_K74Kz5riYeYd8fNbwxsYGj1WsLtuESqvDQxiHrwH5RphMH_Ph4BtRmUzPJ7m_VIQF4A_gmKa-oJL65xnHqc8bg=s0-d)
(at 25 °C). Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as CO
2 molecules not affecting the pH.
The relative concentrations of
CO
2, H
2CO
3, 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 pH of 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 into the
bicarbonate (also called hydrogen carbonate) ion (HCO
3−):
- 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
![K_{a1}=\frac{\rm{[HCO_3^-] [H^+]}}{\rm{[H_2CO_3]}}](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_uZlwcUHIARafKVVIZZQjxH9ljUfK9QbWqXpY9xcjjhGZ2ksLQ9Mvs8NwyBk3idfycoTgQLbnpGyczOywsv4Ldl3O6Z8Hx9CMytZKYjGeyLWIac32vvTm9CHjXrdGQ18d_7GRaAV1Z9MccfuNVfbQ=s0-d)
, where the denominator includes only covalently bound H
2CO
3 and excludes hydrated CO
2(aq). The much smaller and often-quoted value near
4.16×10−7 is an
apparent value calculated on the (incorrect) assumption that all dissolved CO
2 is present as carbonic acid, so that
![K_{\mathrm{a1}}{\rm{(apparent)}}=\frac{\rm{[HCO_3^-] [H^+]}}{\rm{[H_2CO_3] + [CO_2(aq)]}}](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_u1q4VMyXgP4sFbV3jOMecE7Ii0SXylZ-i4hoLfO2dMZnrFV-Z1sbWrtrjhTx3ncDXoIiShrm-ltPVTQNRathMlFxjPMfzyrD-QBuQ_vG0CkRUYyKgR1-S_4A8YqbnAb196cD1bjzhId0wJa7Jy=s0-d)
. Since most of the dissolved CO
2 remains as CO
2 molecules,
Ka1(apparent) has a much larger denominator and a much smaller value than the true
Ka1.
[11]
The
bicarbonate ion is an
amphoteric species that can act as an acid or as a base, depending on pH of the solution. At high
pH, it dissociates significantly into the
carbonate ion (CO
32−):
- HCO3− CO32− + 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
CO
2 is a weak
electrophile. Its reaction with basic water illustrates this property, in which case
hydroxide is the
nucleophile. Other nucleophiles react as well. For example,
carbanions as provided by
Grignard reagents and
organolithium compounds react with CO
2 to give carboxylates:
- MR + CO2 → RCO2M
- where M = Li or MgBr and R = alkyl or aryl.
In
metal carbon dioxide complexes, CO
2 serves as a ligand, which can facilitate the conversion of CO
2 to other chemicals.
[12]
The reduction of CO
2 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
Sample of solid carbon dioxide or "dry ice" pellets
Carbon dioxide is colorless. At low concentrations, the gas is odorless. At higher concentrations it has a sharp, acidic odor. At
standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m
3, about 1.67 times that of
air.
Carbon dioxide has no liquid state at pressures below 5.1
standard atmospheres (520 kPa). At 1 atmosphere (near mean sea level pressure), the gas
deposits directly to a solid 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 about 518
kPa at −56.6 °C (see phase diagram, above). The
critical point is 7.38 MPa at 31.1 °C.
[14] Another form of solid carbon dioxide observed at high pressure is an
amorphous glass-like solid.
[15] This form of glass, called
carbonia, is produced by
supercooling heated CO
2 at extreme pressure (40–48
GPa or about 400,000 atmospheres) in a
diamond anvil. This discovery confirmed the theory that carbon dioxide could exist in a glass state similar to other members of its elemental family, like
silicon (
silica glass) and
germanium dioxide. Unlike silica and germania glasses, however, carbonia glass is not stable at normal pressures and reverts to gas when pressure is released.
At temperatures and pressures above the critical point, carbon dioxide behaves as a
supercritical fluid known as
supercritical carbon dioxide.
Isolation and production
Carbon dioxide is mainly produced as an unrecovered side product of four technologies: combustion of fossil fuels, production of hydrogen by
steam reforming, ammonia synthesis, and fermentation. It can be obtained 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 and wood produces carbon dioxide and, in most cases, water. As an example the chemical reaction between methane and oxygen is given below.
- CH
4+ 2 O
2→ CO
2+ 2 H
2O
Quicklime (CaO), a compound that has many industrial uses, is produced by driving off
CO
2 from limestone by heating (
calcining) at about 850 °C:
- CaCO
3→ CaO + CO
2
Iron is reduced from its oxides with
coke in a
blast furnace, producing
pig iron and carbon dioxide:
[16]
- Fe
2O
3+ 3 CO → 2 Fe + 3 CO
2
Yeast 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:
- C
6H
12O
6 → 2 CO
2+ 2 C
2H
5OH
All
aerobic organisms produce
CO
2 when they oxidize
carbohydrates,
fatty acids, and proteins in the mitochondria of cells. The large number of reactions involved are exceedingly complex and not described easily. Refer to (
cellular respiration,
anaerobic respiration and
photosynthesis). The equation for the respiration of glucose and other
monosaccharides is:
- C
6H
12O
6 + 6 O
2 → 6 CO
2 + 6 H
2O
Photoautotrophs (i.e. plants and
cyanobacteria) use the energy contained in sunlight to photosynthesize simple sugars from
CO
2 absorbed from the air and water:
- nCO2 + nH
2O → (CH
2O)n + nO
2
Laboratory methods
A variety of chemical routes to carbon dioxide are known, such as the reaction between most acids and most metal carbonates. For example, the reaction between
hydrochloric acid and calcium carbonate (limestone or chalk) is depicted below:
- 2 HCl+ CaCO
3→ CaCl
2+ H
2CO
3
The
carbonic acid (H
2CO
3) then decomposes to water and CO
2. Such reactions are accompanied by foaming or bubbling, or both. In industry such reactions are widespread 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 various scales.
[17] In its dominant route, carbon dioxide is produced as a side product of the industrial production of
ammonia and
hydrogen. These processes begin with the reaction of water and natural gas (mainly methane).
[18]
Although carbon dioxide is not often recovered, carbon dioxide results from combustion of
fossil fuels and
wood as well
fermentation of
sugar in the
brewing of
beer,
whisky and other
alcoholic beverages. It also results from thermal decomposition of limestone,
CaCO
3, in the manufacture of lime (
calcium oxide,
CaO). It may be obtained directly from natural carbon dioxide
springs, where it is produced by the action of acidified water on
limestone or
dolomite.
Uses
Carbon dioxide bubbles in a soft drink.
Carbon dioxide is used by the food industry, the oil industry, and the chemical industry.
[17]
Precursor to chemicals[edit]
In the chemical industry, carbon dioxide is mainly consumed as an ingredient in the production of
urea and
methanol.
[citation needed] Metal
carbonates and
bicarbonates, as well as some carboxylic acids derivatives (e.g.,
sodium salicylate) are prepared using CO
2.
[citation needed]
Foods
Carbon dioxide is a
food additive used as a propellant and acidity regulator in the food industry. It is approved 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 10
6 Pa (40 bar, 580 psi). When placed in the mouth, it dissolves (just like other hard candy) and releases the gas bubbles with an audible pop.
Leavening agents cause dough to rise by producing carbon dioxide.
Baker's yeast produces carbon dioxide by fermentation of sugars within the dough, while chemical leaveners such as
baking powder and
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 carbonation in beer and sparkling wine came about through natural fermentation, but many manufacturers carbonate these drinks with carbon dioxide recovered from the fermentation process. In the case of bottled and kegged beer, the most common method used is carbonation with recycled carbon dioxide. With the exception of British
Real Ale, draught beer is usually transferred 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
grapes quickly after picking to help prevent spontaneous
fermentation by wild
yeast. The main advantage of using dry ice over regular water ice is that it cools the grapes without adding any additional water that may decrease the
sugar concentration 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 dioxide gas that results from the sublimation of the dry ice tends to settle to the bottom of tanks because it is denser than air. The settled carbon dioxide gas creates a hypoxic environment which helps to prevent bacteria from growing on the grapes 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 produce
Beaujolais 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 reason, other gases such as
nitrogen or
argon are preferred for this process by professional wine makers.
Inert gas
It is one of the most commonly used compressed gases for pneumatic (pressurized gas) systems in portable pressure tools. Carbon dioxide is also used as an atmosphere for
welding, although in the welding arc, it reacts to
oxidize most metals. Use in the automotive industry is common despite significant evidence that welds made in carbon dioxide are more
brittle than those made in more inert atmospheres, and that such weld joints deteriorate over time because of the formation of carbonic acid. It is used as a welding gas primarily because it is much less expensive than more inert gases such as
argon or
helium. When used for
MIG welding, CO
2 use is sometimes referred to as MAG welding, for Metal Active Gas, as CO
2 can react at these high temperatures. It tends to produce a hotter puddle than truly inert atmospheres, improving the flow characteristics. Although, this may be due to atmospheric reactions occurring at the puddle site. This is usually the opposite of the desired effect when welding, as it tends to embrittle the site, but may not be a problem for general mild steel welding, where ultimate ductility is not a major concern.
It is used in many consumer products that require pressurized gas because it is inexpensive and nonflammable, and because it undergoes a phase transition from gas to liquid at room temperature at an attainable pressure of approximately 60
bar (870 psi, 59 atm), allowing far more carbon dioxide to fit in a given container than otherwise would. Life jackets often contain canisters of pressured carbon dioxide for quick inflation. Aluminium capsules of CO
2 are also sold as supplies of compressed gas for
airguns,
paintball markers, inflating bicycle tires, and for making
carbonated water. Rapid vaporization of liquid carbon dioxide is used for blasting in coal mines. High concentrations of carbon dioxide can also be used to kill pests. Liquid carbon dioxide is used in
supercritical drying of some food products and technological materials, in the preparation of specimens for
scanning electron microscopy and in the
decaffeination of
coffee beans.
Fire extinguisher
Carbon 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 and electrical fires, but not on ordinary combustible fires, because although it excludes oxygen, it does not cool the burning substances significantly and when the carbon dioxide disperses they are free to catch fire upon exposure to atmospheric oxygen. Carbon dioxide has also been widely used as an extinguishing agent in fixed fire protection systems for local application of specific hazards and total flooding of a protected space.
[22] International Maritime
Organization standards also recognize carbon dioxide systems for fire protection of ship holds and engine rooms. Carbon dioxide based fire protection systems have been linked to several deaths, because it can cause suffocation in sufficiently high concentrations. A review of CO
2 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 remove
caffeine from
coffee. Carbon dioxide has attracted attention in the
pharmaceutical and other chemical processing industries as a less toxic alternative to more traditional solvents such as
organochlorides. It is used 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) enrich their atmospheres with additional CO
2 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 compartment would kill green plants, or, at least, completely stop their growth. At very high concentrations (100 times atmospheric concentration, or greater), 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.
[26] Carbon dioxide is used in greenhouses as the main carbon source for
Spirulina algae.
[citation needed]
In medicine, up to 5% carbon dioxide (130 times atmospheric concentration) is added to
oxygen for stimulation of breathing after
apnea and to stabilize the
O
2/CO
2 balance in blood.
It has been proposed that carbon dioxide from power generation be bubbled into ponds to grow algae that 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 oil wells, usually under
supercritical conditions, when it becomes miscible with the oil. This approach can increase original oil recovery by reducing residual oil saturation by between 7 per cent to 23 per cent additional to
primary extraction.
[28] It acts as both a pressurizing agent and, when dissolved into the underground
crude oil, significantly reduces its viscosity, and changing surface chemistry enabling the oil to flow more rapidly through the reservoir to the removal well.
[29] In mature oil fields, extensive pipe networks 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 to produce the fuels
isobutyraldehyde and
isobutanol from
CO
2 using photosynthesis.
[30]
Refrigerant
Comparison of phase diagrams of carbon dioxide (red) and water (blue) as a log-lin chart with phase transitions points at 1 atmosphere
Liquid and solid carbon dioxide are important
refrigerants, especially in the food industry, where they are employed during 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 −78.5 °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 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 a high volumetric cooling capacity. Due to its operation at pressures of up to 130
bar (1880
psi), CO
2 systems require highly resistant components that have already been developed for mass production in many sectors. In automobile air conditioning, in more than 90% of all driving conditions for latitudes higher 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 replace current HFCs in cars, supermarkets, hot water heat pumps, among others.
Coca-Cola has fielded CO
2-based beverage coolers and the
U.S. Army is interested in CO
2 refrigeration and heating technology.
[31][32]
The global automobile industry is expected to decide on the next-generation refrigerant in car air conditioning. CO
2 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 displace methane, as opposed to current methods which primarily use water 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 what might be called niche uses. For example it is used in the
carbon dioxide laser, which is one of the earliest type of lasers.
Carbon dioxide can be used as a means of controlling the
pH of swimming pools, by continuously adding gas to the water, thus keeping the pH level from rising. Among the advantages of this is the avoidance of handling (more hazardous) acids. Similarly, it is also used in the maintaining
reef aquaria, where it is commonly used in
calcium reactors to temporarily lower the pH of water being passed over
calcium carbonate in order to allow the calcium carbonate to dissolve into the water more freely where it is used by some
corals to build their skeleton. It is also used as the primary coolant in
advanced gas-cooled reactors in the nuclear power generation industry.
Carbon dioxide induction is commonly used for the euthanasia of laboratory research animals. Methods to administer CO
2 include placing animals directly into a closed, prefilled chamber containing CO
2, or exposure to a gradually increasing concentration of CO
2. In 2013, the
American Veterinary Medical Association 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 average concentration of about 400 parts per million by volume
[1] (or 591 parts per million by mass). The total mass of atmospheric carbon dioxide is 3.16×10
15 kg (about 3,000 gigatonnes).
[citation needed] Its concentration varies seasonally (see graph at right) and also considerably on a regional basis, especially
near the ground. In urban areas concentrations are generally higher and indoors they can reach 10 times background levels. Carbon dioxide is a
greenhouse gas.
Yearly increase of atmospheric CO
2: In the 1960s, the average annual increase was 37% of the 2000–2007 average.
[35]
As of March 2014
[update],
carbon dioxide in the Earth's atmosphere is at a concentration of approximately 400
ppm by
volume.
[1] Atmospheric concentrations of carbon dioxide fluctuate slightly with the change of the seasons, driven primarily by seasonal plant growth in the
Northern Hemisphere. Concentrations of carbon dioxide fall during the northern spring and summer as plants consume the gas, and rise during the northern autumn and winter as plants go dormant, die and decay. Taking all this into account, the concentration of CO
2 grew by about 2 ppm in 2009.
[36] "The main cause of the current global warming trend is human expansion of the "greenhouse effect" warming that results when the atmosphere traps heat radiating from Earth toward space."
[37] Carbon dioxide is a greenhouse gas as it is transparent to incoming visible light from the sun, but absorbs outgoing infrared radiation from the ground at its two infrared-active vibrational frequencies (see Structure and bonding above). As for all gases, the absorbed energy can be redistributed by molecular collisions which heat the atmosphere.
[38]
Before the advent of release of carbon dioxide to the atmosphere by humans, concentrations tended to increase with increasing global temperatures, acting as a
positive feedback for changes induced by other processes such as
orbital cycles.
[39] There is a seasonal cycle in CO
2 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 occurring 49 million years ago.
[41][42] Human activities such as the combustion of
fossil fuels and
deforestation have caused the atmospheric concentration of carbon dioxide to increase by about 35% since the beginning of the
age of industrialization.
[43]
Up to 40% of the gas emitted by some
volcanoes during
subaerial eruptions is carbon dioxide.
[44] It is estimated that volcanoes release about 130–230 million tonnes (145–255 million
short tons) of CO
2 into the atmosphere each year. Carbon dioxide is also produced by hot springs such 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 CO
2 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 high concentrations of CO
2, produced by disturbance of deep lake water saturated with CO
2 are thought to have caused 37 fatalities at
Lake Monoun,
Cameroon in 1984 and 1700 casualties at
Lake Nyos, Cameroon in 1986.
[46] Emissions of CO
2 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 energy production and transportation industries. As of 2011 concrete contributes 7% to global anthropogenic CO
2 emissions.
[48]
In the oceans
Carbon dioxide dissolves in the ocean to form
carbonic acid (H
2CO
3),
bicarbonate (HCO
3−) and
carbonate (CO
32−), 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 third of CO
2 emitted by human activity.
[49]
As the concentration of carbon dioxide increases in the atmosphere, the increased uptake of carbon dioxide into the oceans is causing a measurable decrease in the pH of the oceans which is referred to as
ocean acidification. Although the
natural absorption of CO
2 by the world's oceans helps mitigate the
climatic effects of anthropogenic emissions of
CO
2, 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
calcifying organisms. These impacts span the
food chain from
autotrophs to
heterotrophs and include organisms such as
coccolithophores,
corals,
foraminifera,
echinoderms,
crustaceans and
molluscs.
Under normal conditions, calcite and aragonite are stable in surface waters since the carbonate ion is at
supersaturating concentrations. However, as ocean pH falls, so does the concentration of this ion, and when carbonate becomes undersaturated, structures made of calcium carbonate are vulnerable to dissolution. Even if there is no change in the rate of calcification, therefore, the rate of dissolution of calcareous material increases.
[50]
Corals,
[51][52][53] coccolithophore algae,
[54][55][56][57] coralline algae,
[58] foraminifera,
[59] shellfish[60] and
pteropods[61] experience reduced calcification or enhanced dissolution when exposed to elevated
CO
2.
Gas solubility decreases as the temperature of water increases (except when both pressure exceeds 300 bar and temperature exceeds 393 K, only found near deep geothermal vents)
[62] and therefore the rate of uptake from the atmosphere decreases as ocean temperatures rise.
Most of the CO
2 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 by photosynthetic organisms, that remove carbon from the cycle. Increased CO
2 in the atmosphere has led to decreasing
alkalinity of seawater, and there is concern that this may adversely affect organisms living in the water. In particular, with decreasing alkalinity, the availability of carbonates for forming shells decreases,
[64] although there's evidence of increased shell production by certain species under increased CO
2 content.
[65]
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 (CO
2) 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 about 30% since the start of the industrial age through a process known as "ocean acidification." 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 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:
[66]
"The uptake of anthropogenic carbon since 1750 has led to the ocean becoming more acidic with an average decrease in pH of 0.1 units. Increasing atmospheric CO
2 concentrations lead to further acidification ... While the effects of observed ocean acidification on the marine biosphere are as yet undocumented, the progressive acidification of oceans is expected to have negative impacts on marine shell-forming organisms (e.g. corals) and their 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 that ocean acidification should be expected to impact them negatively.
[67]
Carbon dioxide is also introduced into the oceans through hydrothermal vents. The
Champagne hydrothermal vent, found at the Northwest Eifuku volcano at
Marianas Trench Marine National Monument, 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 their shells.
[69]
Biological role
Carbon dioxide is an end product of
cellular respiration in organisms that obtain energy by breaking down 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 blood from the body's tissues to the skin (e.g.,
amphibians) or the gills (e.g.,
fish), from where it dissolves in the water, or to the lungs from where it is exhaled. During active photosynthesis,
plants can absorb more carbon dioxide from the atmosphere than they use in respiration.
Photosynthesis and carbon fixation
Overview of photosynthesis and respiration. Carbon dioxide (at right), together with water, form oxygen and organic compounds (at left) by
photosynthesis, which can be
respired to water and (CO
2).
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 own food by
photosynthesis. Photosynthesis uses carbon dioxide and
water to produce
sugars from which other
organic compounds can be constructed, and
oxygen is produced as a by-product.
Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly abbreviated to RuBisCO, is the
enzyme involved in the first major step of
carbon fixation, the production of two molecules of
3-phosphoglycerate from
CO
2 and
ribulose bisphosphate, as shown in the diagram at left.
RuBisCo is thought to be the single most abundant protein on Earth.
[70]
Phototrophs use the products of their photosynthesis as internal food sources and as raw material for the
biosynthesis of more complex organic molecules, such as
polysaccharides,
nucleic acids and
proteins. These are used for their own growth, 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. A globally significant species of coccolithophore is
Emiliania huxleyi whose
calcite scales have formed the basis of many
sedimentary rocks such as
limestone, where what was previously atmospheric carbon can remain fixed for geological timescales.
Plants can grow up to 50 percent faster in concentrations of 1,000 ppm CO
2 when compared with ambient conditions, though this assumes no change in climate and no limitation on other nutrients.
[71] Elevated CO
2 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 CO
2 in FACE experiments.
[72][73]
Increased atmospheric CO
2 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 CO
2 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 amount of protein.
[77]
The concentration of secondary
metabolites such as phenylpropanoids and flavonoids can also be altered in plants exposed to high concentrations of CO
2.
[78][79]
Plants also emit CO
2 during respiration, and so the majority of plants and algae, which use
C3 photosynthesis, are only net absorbers during the day. Though a growing forest will absorb many tons of CO
2 each year, a mature forest will produce as much CO
2 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] and remain 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 CO
2 in the upper ocean and thereby promotes the absorption of CO
2 from the atmosphere.
[82]
Toxicity
Carbon dioxide content in fresh air (averaged between sea-level and 10 kPa level, i.e., about 30 km altitude) varies between 0.036% (360 ppm) and 0.039% (390 ppm), depending on the location.
[84][clarification needed]
CO
2 is an
asphyxiant gas and not classified as toxic or harmful in accordance with
Globally Harmonized System of Classification and Labelling of Chemicals standards of
United Nations Economic Commission for Europe by using the
OECD Guidelines for the Testing of Chemicals. In concentrations up to 1% (10,000 ppm), it will make some people feel drowsy.
[83] Concentrations of 7% to 10% may cause suffocation, even in the presence of sufficient oxygen, manifesting as dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour.
[85] The physiological effects of acute carbon dioxide exposure are grouped together under the term
hypercapnia, a subset of
asphyxiation.
Because it is heavier than air, in locations where the gas seeps from the ground (due to sub-surface volcanic 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 located therein to be suffocated. Carrion feeders attracted to the carcasses are then also killed.
Children have been killed in the same way near the city of
Goma by
CO
2 emissions from the nearby volcano
Mt. Nyiragongo.
[86] The
Swahili term for this phenomenon is '
mazuku'.
Adaptation to increased concentrations of CO
2 occurs in humans. Continuous inhalation of CO
2 can be tolerated at three percent inspired concentrations for at least one month and four percent inspired concentrations for over a week. It was suggested
[by whom?] that 2.0 percent inspired concentrations could be used for closed air spaces (e.g. a
submarine) since the adaptation is physiological and reversible. Decrement in performance or in normal physical activity does not happen at this level.
[87][88] However, submarines have
carbon dioxide scrubbers which reduce a significant amount of the CO
2 present.
[89]
Miners, who are particularly vulnerable to gas exposure, referred to mixtures of carbon dioxide and nitrogen 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 bringing a caged
canary with them as they worked. The canary is more sensitive to asphyxiant gases than humans, and as it became unconscious would stop singing and fall off its perch. The
Davy lamp could also detect high levels of blackdamp (which sinks, and collects near the floor) by burning less brightly, while
methane, another suffocating gas and explosion risk, would make the lamp burn more brightly.
Carbon dioxide differential above outdoor concentrations at steady state conditions (when the occupancy and ventilation system operation are sufficiently long that CO
2 concentration has stabilized) are sometimes used to estimate ventilation rates per person.
[citation needed] CO
2 is considered
[by whom?] to be a surrogate for human bio-effluents and may correlate with other indoor pollutants. Higher CO
2 concentrations are associated with occupant health, comfort and performance degradation.
ASHRAE Standard 62.1–2007 ventilation rates may result in indoor levels up to 2,100 ppm above ambient outdoor conditions. Thus if the outdoor ambient is 400 ppm, indoor concentrations may reach 2,500 ppm with ventilation rates that meet this industry consensus standard.
Concentrations in poorly ventilated spaces can be found even higher than this (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 the lungs. Therefore, the carbon dioxide content in the body is high in the
venous system, and decreases in the
respiratory system, resulting in lower concentrations along any
arterial system. Carbon dioxide content of the blood is often given as the
partial pressure, which is the pressure which carbon dioxide would have had if it alone occupied the volume.
[91]
In humans, the carbon dioxide contents are as follows:
Transport in the blood[edit]
CO
2 is carried in blood in three different ways. (The exact percentages vary depending whether it is arterial or venous blood).
Hemoglobin, the main oxygen-carrying molecule in
red blood cells, carries both oxygen and carbon dioxide. However, the CO
2 bound to hemoglobin does not bind to the same site as oxygen. Instead, it combines with the N-terminal groups on the four globin chains. However, because of
allosteric effects on the hemoglobin molecule, the binding of CO
2 decreases the amount of oxygen that is bound for a given partial pressure of oxygen. The decreased binding to carbon dioxide in the blood due to increased oxygen levels is known as the
Haldane Effect, and is important in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure of CO
2 or a lower pH will cause offloading 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, the
capillaries 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 of CO
2 in their blood. Breathing that is too slow or shallow causes
respiratory acidosis, while breathing that is too rapid leads to
hyperventilation, 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 with no oxygen at all (such as pure nitrogen) can lead to loss of consciousness without ever experiencing
air hunger. This is especially perilous for high-altitude fighter pilots. It is also why flight attendants instruct passengers, in case of loss of cabin pressure, to apply the
oxygen mask to themselves first before helping others; otherwise, one risks losing consciousness.
[94]
The respiratory centers try to maintain an arterial CO
2 pressure of 40 mm Hg. With intentional hyperventilation, the CO
2 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 than without hyperventilating. This carries the risk that unconsciousness may result before the need to breathe becomes overwhelming, which is why hyperventilation is particularly dangerous before free diving.
