Carbon dioxide

"CO2" redirects here. For other uses, seeCO2 (disambiguation).

Carbon dioxideis achemical compoundwith thechemical formulaCO₂.It is made up ofmoleculesthat each have onecarbonatomcovalentlydouble bondedto twooxygenatoms. It is found in the gas state at room temperature, and as the source of available carbon in thecarbon cycle,atmospheric CO₂is the primarycarbonsource forlifeon Earth. In the air, carbon dioxide is transparent to visible light but absorbsinfrared radiation,acting as agreenhouse gas.Carbon dioxide is soluble inwaterand is found ingroundwater,lakes,ice caps,andseawater.When carbon dioxide dissolves in water, it formscarbonateand mainlybicarbonate(HCO−3), which causesocean acidificationasatmospheric CO₂levels increase.

Carbon dioxide

Ball-and-stick model of carbon dioxide Space-filling model of carbon dioxide
Names
IUPAC name Carbon dioxide
Other names Carbonic acid gas Carbonic anhydride Carbonic dioxide Carbonic oxide Carbon(IV) oxide Methanedione R-744 (refrigerant) R744 (refrigerant alternative spelling) Dry ice(solid phase)
Identifiers
CAS Number	124-38-9Y
3D model (JSmol)	Interactive image Interactive image
3DMet	B01131
Beilstein Reference	1900390
ChEBI	CHEBI:16526Y
ChEMBL	ChEMBL1231871N
ChemSpider	274Y
ECHA InfoCard	100.004.271
EC Number	204-696-9
E number	E290(preservatives)
Gmelin Reference	989
KEGG	D00004Y
MeSH	Carbon+dioxide
PubChemCID	280
RTECS number	FF6400000
UNII	142M471B3JY
UN number	1013 (gas), 1845 (solid)
CompTox Dashboard(EPA)	DTXSID4027028
InChI InChI=1S/CO2/c2-1-3Y Key: CURLTUGMZLYLDI-UHFFFAOYSA-NY InChI=1/CO2/c2-1-3 Key: CURLTUGMZLYLDI-UHFFFAOYAO
SMILES O=C=O C(=O)=O
Properties
Chemical formula	CO2
Molar mass	44.009g·mol−1
Appearance	Colorless gas
Odor	Low concentrations: none High concentrations: sharp; acidic
Density	1562kg/m3(solid at 1 atm (100 kPa) and −78.5 °C (−109.3 °F)) 1101kg/m3(liquid at saturation −37 °C (−35 °F)) 1.977kg/m3(gas at 1 atm (100 kPa) and 0 °C (32 °F))
Critical point(T,P)	304.128(15) K(30.978(15) °C), 7.3773(30) MPa(72.808(30) atm)
Sublimation conditions	194.6855(30) K (−78.4645(30) °C) at 1 atm (0.101325 MPa)
Solubility in water	1.45g/L at 25 °C (77 °F), 100 kPa (0.99 atm)
Vapor pressure	5.7292(30) MPa, 56.54(30) atm (20 °C (293.15 K))
Acidity(pKa)	Carbonic acid: pKa1= 3.6 pKa1(apparent) = 6.35 pKa2= 10.33
Magnetic susceptibility(χ)	−20.5·10−6cm3/mol
Thermal conductivity	0.01662W·m−1·K−1(300 K (27 °C; 80 °F))
Refractive index(nD)	1.00045
Viscosity	14.90 μPa·s at 25 °C (298 K) 70μPa·s at −78.5 °C (194.7 K)
Dipole moment	0D
Structure
Crystal structure	Trigonal
Molecular shape	Linear
Thermochemistry
Heat capacity(C)	37.135J/(K·mol)
Std molar entropy(S⦵298)	214J·mol−1·K−1
Std enthalpy of formation(ΔfH⦵298)	−393.5kJ·mol−1
Pharmacology
ATC code	V03AN02(WHO)
Hazards
NFPA 704(fire diamond)	2 0 0 SA
Lethal doseor concentration (LD, LC):
LCLo(lowest published)	90,000ppm (162,000mg/m3) (human, 5min)
NIOSH(US health exposure limits):
PEL(Permissible)	TWA 5000ppm (9000mg/m3)
REL(Recommended)	TWA 5000ppm (9000mg/m3), ST 30,000ppm (54,000mg/m3)
IDLH(Immediate danger)	40,000ppm (72,000mg/m3)
Safety data sheet(SDS)	Sigma-Aldrich
Related compounds
Otheranions	Carbon disulfide Carbon diselenide Carbon ditelluride
Othercations	Silicon dioxide Germanium dioxide Tin dioxide Lead dioxide Titanium dioxide Zirconium dioxide Hafnium dioxide Cerium dioxide Thorium dioxide
Relatedcarbonoxides	SeeOxocarbon
Related compounds	Carbonic acid Carbonyl sulfide Carbonyl selenide
Supplementary data page
Carbon dioxide (data page)
Except where otherwise noted, data are given for materials in theirstandard state(at 25 °C [77 °F], 100 kPa). Nverify(what isYN?) Infobox references

It is atrace gasin Earth's atmosphereat 421parts per million(ppm),or about 0.04% (as of May 2022) having risen from pre-industrial levels of 280 ppm or about 0.025%.Burningfossil fuelsis the primary cause of these increased CO₂concentrations and also the primary cause ofclimate change.

Itsconcentrationin Earth's pre-industrial atmosphere since late in thePrecambrianwas regulated by organisms and geological phenomena.Plants,algaeandcyanobacteriauseenergyfromsunlightto synthesizecarbohydratesfrom carbon dioxide and water in a process calledphotosynthesis,which produces oxygen as a waste product.In turn, oxygen is consumed and CO₂is released as waste by allaerobic organismswhen they metabolizeorganic compoundsto produce energy byrespiration.CO₂is released from organic materials when theydecayor combust, such as in forest fires. Since plants require CO₂for photosynthesis, and humans and animals depend on plants for food, CO₂is necessary for the survival of life on earth.

Carbon dioxide is 53% more dense than dry air, but is long lived and thoroughly mixes in the atmosphere. About half of excess CO₂emissions to the atmosphere are absorbed bylandand oceancarbon sinks.These sinks can become saturated and are volatile, as decay andwildfiresresult in the CO₂being released back into the atmosphere.CO₂is eventuallysequestered(stored for the long term) in rocks and organic deposits likecoal,petroleumandnatural gas.Sequestered CO₂is released into the atmosphere through burning fossil fuels or naturally byvolcanoes,hot springs,geysers,and whencarbonate rocksdissolvein water or react with acids.

CO₂is a versatile industrial material, used, for example, as an inert gas in welding andfire extinguishers,as a pressurizing gas in air guns and oil recovery, and as a supercritical fluid solvent indecaffeinationandsupercritical drying.It is a byproduct offermentationof sugars inbread,beerandwinemaking, and is added tocarbonated beverageslikeseltzerand beer for effervescence. It has a sharp and acidic odor and generates the taste ofsoda waterin the mouth, but at normally encountered concentrations it is odorless.

Chemical and physical properties

Carbon dioxide cannot beliquefiedat atmospheric pressure. Low-temperature carbon dioxide is commercially used in its solid form, commonly known as "dry ice".The solid-to-gasphase transitionoccurs at 194.7 Kelvin and is calledsublimation.

Structure, bonding and molecular vibrations

See also:Molecular orbital diagram § Carbon dioxide

Thesymmetryof a carbon dioxide molecule is linear andcentrosymmetricat its equilibrium geometry. Thelengthof thecarbon–oxygen bondin carbon dioxide is 116.3pm,noticeably shorter than the roughly 140 pm length of a typical single C–O bond, and shorter than most other C–O multiply bondedfunctional groupssuch ascarbonyls.Since it is centrosymmetric, the molecule has noelectric dipole moment.

Stretching and bending oscillationsof the CO₂molecule. Upper left: symmetric stretching. Upper right: antisymmetric stretching. Lower line: degenerate pair of bending modes.

As a linear triatomic molecule, CO₂has fourvibrational modesas shown in the diagram. In the symmetric and the antisymmetric stretching modes, the atoms move along the axis of the molecule. There are two bending modes, which aredegenerate,meaning that they have the same frequency and same energy, because of the symmetry of the molecule. When a molecule touches a surface or touches another molecule, the two bending modes can differ in frequency because the interaction is different for the two modes. Some of the vibrational modes are observed in theinfrared (IR) spectrum:the antisymmetric stretching mode atwavenumber2349 cm⁻¹(wavelength 4.25 μm) and the degenerate pair of bending modes at 667 cm⁻¹(wavelength 15 μm). The symmetric stretching mode does not create an electric dipole so is not observed in IR spectroscopy, but it is detected inRaman spectroscopyat 1388 cm⁻¹(wavelength 7.2 μm).

In the gas phase, carbon dioxide molecules undergo significant vibrational motions and do not keep a fixed structure. However, in aCoulomb explosion imagingexperiment, an instantaneous image of the molecular structure can be deduced. Such an experimenthas been performed for carbon dioxide. The result of this experiment, and the conclusion of theoretical calculationsbased on anab initiopotential energy surfaceof the molecule, is that none of the molecules in the gas phase are ever exactly linear. This counter-intuitive result is trivially due to the fact that the nuclear motionvolume elementvanishes for linear geometries.This is so for all molecules exceptdiatomic molecules.

In aqueous solution

See also:Carbonic acid

Carbon dioxide issolublein water, in which it reversibly formsH₂CO₃(carbonic acid), which is aweak acid,because its ionization in water is incomplete.

CO₂+ H₂O ⇌ H₂CO₃

Thehydration equilibrium constantof carbonic acid is, at 25 °C:

$K_{\mathrm {h} }={\frac {{\ce {[H2CO3]}}}{{\ce {[CO2_{(aq)}]}}}}=1.70\times 10^{-3}$

Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as CO₂molecules, not affecting the pH.

The relative concentrations of CO₂,H₂CO₃,and thedeprotonatedformsHCO−3(bicarbonate) andCO2−3(carbonate) depend on thepH.As shown in aBjerrum 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.

Beingdiprotic,carbonic acid has twoacid dissociation constants,the first one for the dissociation into the bicarbonate (also called hydrogen carbonate) ion (HCO−3):

H₂CO₃⇌ HCO−3+ H⁺

K_a1= 2.5 × 10⁻⁴mol/L; pK_a1= 3.6 at 25 °C.

This is thetruefirst acid dissociation constant, defined as

$K_{\mathrm {a1} }={\frac {{\ce {[HCO3- ][H+]}}}{{\ce {[H2CO3]}}}}$

where the denominator includes only covalently boundH₂CO₃and does not include hydrated CO₂(aq). The much smaller and often-quoted value near 4.16 × 10⁻⁷(or pK_a1= 6.38) is anapparentvalue calculated on the (incorrect) assumption that all dissolved CO₂is present as carbonic acid, so that

$K_{\mathrm {a1} }{\rm {(apparent)}}={\frac {{\ce {[HCO3- ][H+]}}}{{\ce {[H2CO3] + [CO2_{(aq)}]}}}}$

Since most of the dissolved CO₂remains as CO₂molecules,K_a1(apparent) has a much larger denominator and a much smaller value than the trueK_a1.

The bicarbonate ion is anamphotericspecies that can act as an acid or as a base, depending on pH of the solution. At high pH, it dissociates significantly into thecarbonateion (CO2−3):

HCO−3⇌ CO2−3+ H⁺

K_a2= 4.69 × 10⁻¹¹mol/L; pK_a2= 10.329

In organisms, carbonic acid production is catalysed by theenzymeknown ascarbonic anhydrase.

In addition to altering its acidity, the presence of carbon dioxide in water also affects its electrical properties.

Electrical conductivity of carbondioxide saturated desalinated water when heated from 20 to 98 °C. The shadowed regions indicate the error bars associated with the measurements. Data ongithub.A comparison with the temperature dependence of vented desalinated water can be foundhere.

When carbon dioxide dissolves in desalinated water, the electrical conductivity increases significantly from below 1 μS/cm to nearly 30 μS/cm. When heated, the water begins to gradually lose the conductivity induced by the presence of$\mathrm {CO_{2}}$,especially noticeable as temperatures exceed 30 °C.

Thetemperature dependenceof the electrical conductivity of fully deionized water without$\mathrm {CO_{2}}$saturation is comparably low in relation to these data.

Chemical reactions

CO₂is a potentelectrophilehaving an electrophilic reactivity that is comparable tobenzaldehydeor strongly electrophilicα,β-unsaturated carbonyl compounds.However, unlike electrophiles of similar reactivity, the reactions of nucleophiles with CO₂are thermodynamically less favored and are often found to be highly reversible.The reversible reaction of carbon dioxide withaminesto makecarbamatesis used in CO₂scrubbers and has been suggested as a possible starting point for carbon capture and storage byamine gas treating. Only very strong nucleophiles, like thecarbanionsprovided byGrignard reagentsandorganolithium compoundsreact with CO₂to givecarboxylates:

MR + CO₂→ RCO₂M

where M =LiorMgBrand R =alkyloraryl.

Inmetal carbon dioxide complexes,CO₂serves as aligand,which can facilitate the conversion of CO₂to other chemicals.

The reduction of CO₂toCOis ordinarily a difficult and slow reaction:

CO₂+ 2 e⁻+ 2 H⁺→ CO + H₂O

Theredox potentialfor this reaction near pH 7 is about −0.53 Vversusthestandard hydrogen electrode.The nickel-containing enzymecarbon monoxide dehydrogenasecatalyses this process.

Photoautotrophs(i.e.plantsandcyanobacteria) use the energy contained in sunlight tophotosynthesizesimplesugarsfrom CO₂absorbed from the air and water:

nCO₂+nH₂O → (CH₂O)_n+nO₂

Physical properties

Further information:Carbon dioxide data

Pellets of "dry ice", a common form of solid carbon dioxide

Carbon dioxide is colorless. At low concentrations, the gas is odorless; however, at sufficiently high concentrations, it has a sharp, acidic odor.Atstandard temperature and pressure,the density of carbon dioxide is around 1.98 kg/m³,about 1.53 times that ofair.

Carbon dioxide has no liquid state at pressures below 0.51795(10)MPa(5.11177(99)atm). At a pressure of 1 atm (0.101325 MPa), the gasdepositsdirectly to a solid at temperatures below 194.6855(30) K(−78.4645(30) °C) and the solidsublimesdirectly to a gas above this temperature. In its solid state, carbon dioxide is commonly calleddry ice.

Pressure–temperaturephase diagramof carbon dioxide. Note that it is a log-lin chart.

Liquid carbon dioxideforms only atpressuresabove 0.51795(10) MPa(5.11177(99) atm); thetriple pointof carbon dioxide is 216.592(3) K(−56.558(3) °C) at 0.51795(10) MPa(5.11177(99) atm) (see phase diagram). Thecritical pointis 304.128(15) K(30.978(15) °C) at 7.3773(30) MPa(72.808(30) atm). Another form of solid carbon dioxide observed at high pressure is anamorphousglass-like solid.This form of glass, calledcarbonia,is produced bysupercoolingheated CO₂at extreme pressures (40–48GPa,or about 400,000 atmospheres) in adiamond anvil.This discovery confirmed the theory that carbon dioxide could exist in a glass state similar to other members of its elemental family, likesilicon dioxide(silica glass) andgermanium 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 asupercritical fluidknown assupercritical carbon dioxide.

Table of thermal and physical properties of saturated liquid carbon dioxide:

Temperature(°C)	Density(kg/m3)	Specific heat(kJ/(kg⋅K))	Kinematic viscosity(m2/s)	Thermal conductivity(W/(m⋅K))	Thermal diffusivity(m2/s)	Prandtl Number	Bulk modulus(K^-1)
−50	1156.34	1.84	1.19 × 10−7	0.0855	4.02 × 10−8	2.96
−40	1117.77	1.88	1.18 × 10−7	0.1011	4.81 × 10−8	2.46
−30	1076.76	1.97	1.17 × 10−7	0.1116	5.27 × 10−8	2.22
−20	1032.39	2.05	1.15 × 10−7	0.1151	5.45 × 10−8	2.12
−10	983.38	2.18	1.13 × 10−7	0.1099	5.13 × 10−8	2.2
0	926.99	2.47	1.08 × 10−7	0.1045	4.58 × 10−8	2.38
10	860.03	3.14	1.01 × 10−7	0.0971	3.61 × 10−8	2.8
20	772.57	5	9.10 × 10−8	0.0872	2.22 × 10−8	4.1	1.40 × 10−2
30	597.81	36.4	8.00 × 10−8	0.0703	0.279 × 10−8	28.7

Table of thermal and physical properties of carbon dioxide (CO₂) at atmospheric pressure:

Temperature (K)	Density (kg/m3)	Specific heat (kJ/(kg⋅°C))	Dynamic viscosity(kg/(m⋅s))	Kinematic viscosity (m2/s)	Thermal conductivity (W/(m⋅°C))	Thermal diffusivity (m2/s)	Prandtl Number
220	2.4733	0.783	1.11 × 10−5	4.49 × 10−6	0.010805	5.92 × 10−6	0.818
250	2.1657	0.804	1.26 × 10−5	5.81 × 10−6	0.012884	7.40 × 10−6	0.793
300	1.7973	0.871	1.50 × 10−5	8.32 × 10−6	0.016572	1.06 × 10−5	0.77
350	1.5362	0.9	1.72 × 10−5	1.12 × 10−5	0.02047	1.48 × 10−5	0.755
400	1.3424	0.942	1.93 × 10−5	1.44 × 10−5	0.02461	1.95 × 10−5	0.738
450	1.1918	0.98	2.13 × 10−5	1.79 × 10−5	0.02897	2.48 × 10−5	0.721
500	1.0732	1.013	2.33 × 10−5	2.17 × 10−5	0.03352	3.08 × 10−5	0.702
550	0.9739	1.047	2.51 × 10−5	2.57 × 10−5	0.03821	3.75 × 10−5	0.685
600	0.8938	1.076	2.68 × 10−5	3.00 × 10−5	0.04311	4.48 × 10−5	0.668
650	0.8143	1.1	2.88 × 10−5	3.54 × 10−5	0.0445	4.97 × 10−5	0.712
700	0.7564	1.13	3.05 × 10−5	4.03 × 10−5	0.0481	5.63 × 10−5	0.717
750	0.7057	1.15	3.21 × 10−5	4.55 × 10−5	0.0517	6.37 × 10−5	0.714
800	0.6614	1.17	3.37 × 10−5	5.10 × 10−5	0.0551	7.12 × 10−5	0.716

Biological role

Carbon dioxide is an end product ofcellular respirationin organisms that obtain energy by breaking down sugars, fats andamino acidswith oxygen as part of theirmetabolism.This includes all plants, algae and animals andaerobicfungi and bacteria. Invertebrates,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 releasein respiration.

Photosynthesis and carbon fixation

Overview of theCalvin cycleand carbon fixation

Carbon fixationis a biochemical process by which atmospheric carbon dioxide is incorporated by plants, algae and cyanobacteria intoenergy-richorganic molecules such asglucose,thus creating their own food by photosynthesis. Photosynthesis uses carbon dioxide andwaterto produce sugars from which otherorganic compoundscan be constructed, andoxygenis produced as a by-product.

Ribulose-1,5-bisphosphate carboxylase oxygenase,commonly abbreviated to RuBisCO, is theenzymeinvolved in the first major step of carbon fixation, the production of two molecules of3-phosphoglyceratefrom CO₂andribulose bisphosphate,as shown in the diagram at left.

RuBisCO is thought to be the single most abundant protein on Earth.

Phototrophsuse the products of their photosynthesis as internal food sources and as raw material for thebiosynthesisof more complex organic molecules, such aspolysaccharides,nucleic acids,and proteins. These are used for their own growth, and also as the basis of thefood chainsand webs that feed other organisms, including animals such as ourselves. Some important phototrophs, thecoccolithophoressynthesise hardcalcium carbonatescales.A globally significant species of coccolithophore isEmiliania huxleyiwhosecalcitescales have formed the basis of manysedimentary rockssuch aslimestone,where what was previously atmospheric carbon can remain fixed for geological timescales.

Overview of photosynthesis and respiration. Carbon dioxide (at right), together with water, form oxygen and organic compounds (at left) byphotosynthesis(green), which can berespired(red) to water and CO₂.

Plants can grow as much as 50% faster in concentrations of 1,000 ppm CO₂when compared with ambient conditions, though this assumes no change in climate and no limitation on other nutrients.Elevated CO₂levels 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 CO₂in FACE experiments.

Increased atmospheric CO₂concentrations result in fewer stomata developing on plantswhich leads to reduced water usage and increasedwater-use efficiency.Studies usingFACEhave shown that CO₂enrichment leads to decreased concentrations of micronutrients in crop plants.This may have knock-on effects on other parts ofecosystemsas herbivores will need to eat more food to gain the same amount of protein.

The concentration of secondarymetabolitessuch asphenylpropanoidsandflavonoidscan also be altered in plants exposed to high concentrations of CO₂.

Plants also emit CO₂during respiration, and so the majority of plants and algae, which useC3 photosynthesis,are only net absorbers during the day. Though a growing forest will absorb many tons of CO₂each year, a mature forest will produce as much CO₂from respiration and decomposition of dead specimens (e.g., fallen branches) as is used in photosynthesis in growing plants.Contrary to the long-standing view that they are carbon neutral, mature forests can continue to accumulate carbonand remain valuablecarbon sinks,helping to maintain the carbon balance of Earth's atmosphere. Additionally, and crucially to life on earth, photosynthesis by phytoplankton consumes dissolved CO₂in the upper ocean and thereby promotes the absorption of CO₂from the atmosphere.

Toxicity

See also:Carbon dioxide poisoning

Symptoms of carbon dioxide toxicity, by increasingvolume percentin air

Carbon dioxide content in fresh air (averaged between sea-level and 10 kPa level, i.e., about 30 km (19 mi) altitude) varies between 0.036% (360 ppm) and 0.041% (412 ppm), depending on the location.

CO₂is anasphyxiant gasand not classified as toxic or harmful in accordance withGlobally Harmonized System of Classification and Labelling of Chemicals standardsofUnited Nations Economic Commission for Europeby using theOECD Guidelines for the Testing of Chemicals.In concentrations up to 1% (10,000 ppm), it will make some people feel drowsy and give the lungs a stuffy feeling.Concentrations of 7% to 10% (70,000 to 100,000 ppm) 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.The physiological effects of acute carbon dioxide exposure are grouped together under the termhypercapnia,a subset ofasphyxiation.

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 ofGomaby CO₂emissions from the nearby volcanoMount Nyiragongo.TheSwahiliterm for this phenomenon ismazuku.

Rising levels of CO₂threatened theApollo 13astronauts who had to adapt cartridges from the command module to supply thecarbon dioxide scrubberin theApollo Lunar Module,which they used as a lifeboat.

Adaptation to increased concentrations of CO₂occurs in humans, includingmodified breathingand kidney bicarbonate production, in order to balance the effects of blood acidification (acidosis). Several studies suggested that 2.0 percent inspired concentrations could be used for closed air spaces (e.g. asubmarine) since the adaptation is physiological and reversible, as deterioration in performance or in normal physical activity does not happen at this level of exposure for five days.Yet, other studies show a decrease in cognitive function even at much lower levels.Also, with ongoing respiratoryacidosis,adaptation or compensatory mechanisms will be unable to reverse the condition.

Below 1%

There are few studies of the health effects of long-term continuous CO₂exposure on humans and animals at levels below 1%. Occupational CO₂exposure limits have been set in the United States at 0.5% (5000 ppm) for an eight-hour period.At this CO₂concentration,International Space Stationcrew experienced headaches, lethargy, mental slowness, emotional irritation, and sleep disruption.Studies in animals at 0.5% CO₂have demonstrated kidney calcification and bone loss after eight weeks of exposure.A study of humans exposed in 2.5 hour sessions demonstrated significant negative effects on cognitive abilities at concentrations as low as 0.1% (1000ppm) CO₂likely due to CO₂induced increases in cerebral blood flow.Another study observed a decline in basic activity level and information usage at 1000 ppm, when compared to 500 ppm.

However a review of the literature found that a reliable subset of studies on the phenomenon of carbon dioxide induced cognitive impairment to only show a small effect on high-level decision making (for concentrations below 5000 ppm). Most of the studies were confounded by inadequate study designs, environmental comfort, uncertainties in exposure doses and differing cognitive assessments used.Similarly a study on the effects of the concentration of CO₂in motorcycle helmets has been criticized for having dubious methodology in not noting the self-reports of motorcycle riders and taking measurements using mannequins. Further when normal motorcycle conditions were achieved (such as highway or city speeds) or the visor was raised the concentration of CO₂declined to safe levels (0.2%).

General guidelines on indoor CO₂concentration effects

Concentration	Note
280 ppm	Pre-industrial levels
421 ppm	Current (May 2022) levels
700 ppm	ASHRAErecommendation
1000 ppm	Cognitive impairment, Canada's long term exposure limit
1000-2000 ppm	Drowsiness
2000-5000 ppm	Headaches, sleepiness; poor concentration, loss of attention, slight nausea also possible
5000 ppm	USA 8h exposure limit

Ventilation

Acarbon dioxide sensorthat measures CO₂concentration using anondispersive infrared sensor

Poor ventilation is one of the main causes of excessive CO₂concentrations in closed spaces, leading to poorindoor air quality.Carbon dioxide differential above outdoor concentrations at steady state conditions (when the occupancy and ventilation system operation are sufficiently long that CO₂concentration has stabilized) are sometimes used to estimate ventilation rates per person.Higher CO₂concentrations are associated with occupant health, comfort and performance degradation.ASHRAEStandard 62.1–2007 ventilation rates may result in indoor concentrations up to 2,100 ppm above ambient outdoor conditions. Thus if the outdoor concentration 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 ppm).

Miners, who are particularly vulnerable to gas exposure due to insufficient ventilation, referred to mixtures of carbon dioxide and nitrogen as "blackdamp","choke damp "or" stythe ". Before more effective technologies were developed,minerswould frequently monitor for dangerous levels of blackdamp and other gases in mine shafts by bringing a cagedcanarywith 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. TheDavy lampcould also detect high levels of blackdamp (which sinks, and collects near the floor) by burning less brightly, whilemethane,another suffocating gas and explosion risk, would make the lamp burn more brightly.

In February 2020, three people died from suffocation at a party in Moscow when dry ice (frozen CO₂) was added to a swimming pool to cool it down.A similar accident occurred in 2018 when a woman died from CO₂fumes emanating from the large amount of dry ice she was transporting in her car.

Indoor air

Humans spend more and more time in a confined atmosphere (around 80-90% of the time in a building or vehicle). According to the FrenchAgency for Food, Environmental and Occupational Health & Safety(ANSES) and various actors in France, the CO₂rate in the indoor air of buildings (linked to human or animal occupancy and the presence ofcombustioninstallations), weighted by air renewal, is "usually between about 350 and 2,500 ppm".

In homes, schools, nurseries and offices, there are no systematic relationships between the levels of CO₂and other pollutants, and indoor CO₂is statistically not a good predictor of pollutants linked to outdoor road (or air, etc.) traffic.CO₂is the parameter that changes the fastest (with hygrometry and oxygen levels when humans or animals are gathered in a closed or poorly ventilated room). In poor countries, many open hearths are sources of CO₂and CO emitted directly into the living environment.

Outdoor areas with elevated concentrations

Local concentrations of carbon dioxide can reach high values near strong sources, especially those that are isolated by surrounding terrain. At the Bossoleto hot spring nearRapolano TermeinTuscany,Italy, situated in a bowl-shaped depression about 100 m (330 ft) in diameter, concentrations of CO₂rise to above 75% overnight, sufficient to kill insects and small animals. After sunrise the gas is dispersed by convection.High concentrations of CO₂produced by disturbance of deep lake water saturated with CO₂are thought to have caused 37 fatalities atLake Monoun,Cameroonin 1984 and 1700 casualties atLake Nyos,Cameroon in 1986.

Human physiology

Content

Reference rangesor averages forpartial pressures of carbon dioxide(abbreviatedpCO₂)

Blood compartment	(kPa)	(mm Hg)
Venousblood carbon dioxide	5.5–6.8	41–51
Alveolarpulmonary gas pressures	4.8	36
Arterial blood carbon dioxide	4.7–6.0	35–45

The body produces approximately 2.3 pounds (1.0 kg) of carbon dioxide per day per person,containing 0.63 pounds (290 g) of carbon.In humans, this carbon dioxide is carried through thevenous systemand is breathed out through the lungs, resulting in lower concentrations in thearteries.The carbon dioxide content of the blood is often given as thepartial pressure,which is the pressure which carbon dioxide would have had if it alone occupied the volume.In humans, the blood carbon dioxide contents are shown in the adjacent table.

Transport in the blood

CO₂is carried in blood in three different ways. Exact percentages vary between arterial and venous blood.

• Majority (about 70% to 80%) is converted tobicarbonateionsHCO−3by the enzymecarbonic anhydrasein the red blood cells,by the reaction:

CO₂+ H₂O → H₂CO₃→ H⁺+ HCO−3

• 5–10% is dissolved inblood plasma
• 5–10% is bound tohemoglobinascarbaminocompounds

Hemoglobin,the main oxygen-carrying molecule inred blood cells,carries both oxygen and carbon dioxide. However, the CO₂bound 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 ofallostericeffects on the hemoglobin molecule, the binding of CO₂decreases the amount of oxygen that is bound for a given partial pressure of oxygen. This is known as theHaldane Effect,and is important in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure of CO₂or a lower pH will cause offloading of oxygen from hemoglobin, which is known as theBohr effect.

Regulation of respiration

Carbon dioxide is one of the mediators of localautoregulationof blood supply. If its concentration is high, thecapillariesexpand 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 CO₂in their blood. Breathing that is too slow or shallow causesrespiratory acidosis,while breathing that is too rapid leads tohyperventilation,which can causerespiratory 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 experiencingair 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 theoxygen maskto themselves first before helping others; otherwise, one risks losing consciousness.

The respiratory centers try to maintain an arterial CO₂pressure of 40mmHg.With intentional hyperventilation, the CO₂content of arterial blood may be lowered to 10–20 mmHg (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.

Concentrations and role in the environment

Atmosphere

Further information:Carbon cycle

This section is an excerpt fromCarbon dioxide in Earth's atmosphere.[edit]

Atmospheric CO₂concentration measured atMauna Loa Observatoryin Hawaii from 1958 to 2023 (also called theKeeling Curve). The rise in CO₂over that time period is clearly visible. The concentration is expressed as μmole per mole, orppm.

InEarth's atmosphere,carbon dioxide is atrace gasthat plays an integral part in thegreenhouse effect,carbon cycle,photosynthesisandoceanic carbon cycle.It is one of severalgreenhouse gasesin the atmosphere ofEarth.The current global average concentration of carbon dioxide (CO₂) in the atmosphere is 421ppmas of May 2022 (0.04%).This is an increase of 50% since the start of theIndustrial Revolution,up from 280 ppm during the 10,000 years prior to the mid-18th century.The increaseis due to human activity.

As of March 2024, the monthly average concentration of CO₂reached a new record high of 425.22 parts per million (ppm), marking an increase of 4.7 ppm over March 2023. By the latest measurement, levels had further escalated to 427.48 ppm.This continuous increase in CO₂concentrations is a clear indicator of ongoing global environmental stress, primarily driven by the burning offossil fuels,which is the principal cause of this rise and also a major contributor toclimate change.Other significant human activities that emit CO₂includecementproduction,deforestation,andbiomassburning.

Carbon dioxide is a greenhouse gas. It absorbs and emitsinfrared radiationat its two infrared-active vibrational frequencies. The twowavelengthsare 4.26μm(2,347 cm⁻¹) (asymmetric stretchingvibrational mode) and 14.99 μm (667 cm⁻¹) (bending vibrational mode). CO₂plays a significant role in influencingEarth's surface temperature through the greenhouse effect.Light emission from the Earth's surface is most intense in the infrared region between 200 and 2500 cm⁻¹,as opposed to light emission from the much hotterSunwhich is most intense in the visible region. Absorption of infrared light at the vibrational frequencies of atmospheric CO₂traps energy near the surface, warming the surface of Earth and its lower atmosphere. Less energy reaches the upper atmosphere, which is therefore cooler because of this absorption.

The increase in atmospheric concentrations of CO₂and other long-lived greenhouse gases such asmethaneincrease the absorption and emission of infrared radiation by the atmosphere. This has led to arise in average global temperatureandocean acidification.Another direct effect is theCO₂fertilization effect.The increase in atmospheric concentrations of CO₂causes a range of furthereffects of climate changeon the environment and human living conditions.

The present atmospheric concentration of CO₂is the highest for 14 million years.Concentrations of CO₂in the atmosphere were as high as 4,000 ppm during theCambrian periodabout 500 million years ago, and as low as 180 ppm during theQuaternary glaciationof the last two million years.Reconstructed temperature records for the last 420 million years indicate that atmospheric CO₂concentrations peaked at approximately 2,000 ppm. This peak happened during theDevonianperiod (400 million years ago). Another peak occurred in theTriassicperiod (220–200 million years ago).

Annual CO₂flows from anthropogenic sources (left) into Earth's atmosphere, land, and ocean sinks (right) since the 1960s. Units in equivalent gigatonnes carbon per year.

Oceans

Main articles:Carbon cycleandOcean acidification

Ocean acidification

Carbon dioxide dissolves in the ocean to form carbonic acid (H₂CO₃), bicarbonate (HCO−3), and carbonate (CO2−3). There is about fifty times as much carbon dioxide dissolved in the oceans as exists in the atmosphere. The oceans act as an enormouscarbon sink,and have taken up about a third of CO₂emitted by human activity.

This section is an excerpt fromOcean acidification.[edit]

Ocean acidificationis the ongoing decrease in thepHof the Earth'socean.Over the past 200 years, the rapid increase in anthropogenic CO₂(carbon dioxide) production has led to an increase in the acidity of the Earth’s oceans. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05.Carbon dioxide emissionsfrom human activities are the primary cause of ocean acidification, withatmospheric carbon dioxide (CO₂) levelsexceeding 410 ppm (in 2020). CO₂from theatmosphereis absorbed by the oceans. This chemical reaction producescarbonic acid(H₂CO₃) whichdissociatesinto abicarbonate ion(HCO−3) and ahydrogen ion(H⁺). The presence of free hydrogen ions (H⁺) lowers the pH of the ocean, increasingacidity(this does not mean thatseawateris acidic yet; it is stillalkaline,with a pH higher than 8).Marine calcifying organisms,such asmollusksandcorals,are especially vulnerable because they rely on calcium carbonate to build shells and skeletons.

A change in pH by 0.1 represents a 26% increase in hydrogen ion concentration in the world's oceans (the pH scale is logarithmic, so a change of one in pH units is equivalent to a tenfold change in hydrogen ion concentration). Sea-surface pH and carbonate saturation states vary depending on ocean depth and location. Colder and higher latitude waters are capable of absorbing more CO₂.This can cause acidity to rise, lowering the pH and carbonate saturation levels in these areas. There are several other factors that influence the atmosphere-ocean CO₂exchange, and thus local ocean acidification. These includeocean currentsandupwellingzones, proximity to large continental rivers,sea icecoverage, and atmospheric exchange with nitrogen and sulfur fromfossil fuelburning andagriculture.

Pterapod shell dissolved in seawater adjusted to anocean chemistryprojected for the year 2100

This section is an excerpt fromOcean acidification § Decreased calcification in marine organisms.[edit]

Changes in ocean chemistry can have extensive direct and indirect effects on organisms and their habitats. One of the most important repercussions of increasing ocean acidity relates to the production of shells out ofcalcium carbonate(CaCO₃).This process is called calcification and is important to the biology and survival of a wide range of marine organisms. Calcification involves theprecipitationof dissolved ions into solidCaCO₃structures, structures for many marine organisms, such ascoccolithophores,foraminifera,crustaceans,mollusks,etc. After they are formed, theseCaCO₃structures are vulnerable todissolutionunless the surrounding seawater containssaturatingconcentrations of carbonate ions (CO2−3).

Very little of the extra carbon dioxide that is added into the ocean remains as dissolved carbon dioxide. The majority dissociates into additional bicarbonate and free hydrogen ions. The increase in hydrogen is larger than the increase in bicarbonate,creating an imbalance in the reaction:

HCO−3⇌ CO2−3+ H⁺

To maintain chemical equilibrium, some of the carbonate ions already in the ocean combine with some of the hydrogen ions to make further bicarbonate. Thus the ocean's concentration of carbonate ions is reduced, removing an essential building block for marine organisms to build shells, or calcify:

Ca²⁺+ CO2−3⇌ CaCO₃

Hydrothermal vents

Carbon dioxide is also introduced into the oceans through hydrothermal vents. TheChampagnehydrothermal vent, found at the Northwest Eifuku volcano in theMariana Trench,produces almost pure liquid carbon dioxide, one of only two known sites in the world as of 2004, the other being in theOkinawa Trough.The finding of a submarine lake of liquid carbon dioxide in the Okinawa Trough was reported in 2006.

Production

Biological processes

Carbon dioxide is a by-product of thefermentationof sugar in thebrewingofbeer,whiskyand otheralcoholic beveragesand in the production ofbioethanol.Yeastmetabolizes sugar to produce CO₂andethanol,also known as alcohol, as follows:

C₆H₁₂O₆→ 2 CO₂+ 2 CH₃CH₂OH

Allaerobicorganisms produce CO₂when they oxidizecarbohydrates,fatty acids,andproteins.The large number of reactions involved are exceedingly complex and not described easily. Refer tocellular respiration,anaerobic respirationandphotosynthesis.The equation for the respiration of glucose and othermonosaccharidesis:

C₆H₁₂O₆+ 6 O₂→ 6 CO₂+ 6 H₂O

Anaerobic organismsdecompose organic material producing methane and carbon dioxide together with traces of other compounds.Regardless of the type of organic material, the production of gases follows well definedkinetic pattern.Carbon dioxide comprises about 40–45% of the gas that emanates from decomposition in landfills (termed "landfill gas"). Most of the remaining 50–55% is methane.

Industrial processes

Carbon dioxide can be obtained bydistillationfrom air, but the method is inefficient. Industrially, carbon dioxide is predominantly an unrecovered waste product, produced by several methods which may be practiced at various scales.

Combustion

Thecombustionof allcarbon-based fuels,such asmethane(natural gas), petroleum distillates (gasoline,diesel,kerosene,propane), coal, wood and generic organic matter produces carbon dioxide and, except in the case of pure carbon, water. As an example, the chemical reaction between methane andoxygen:

CH₄+ 2 O₂→ CO₂+ 2 H₂O

Ironis reduced from its oxides withcokein ablast furnace,producingpig ironand carbon dioxide:

Fe₂O₃+ 3 CO → 3 CO₂+ 2 Fe

By-product from hydrogen production

Carbon dioxide is a byproduct of the industrial production of hydrogen bysteam reformingand thewater gas shift reactioninammonia production.These processes begin with the reaction of water and natural gas (mainly methane).This is a major source of food-grade carbon dioxide for use in carbonation ofbeerandsoft drinks,and is also used for stunning animals such aspoultry.In the summer of 2018 a shortage of carbon dioxide for these purposes arose in Europe due to the temporary shut-down of several ammonia plants for maintenance.

Thermal decomposition of limestone

It is produced by thermal decomposition of limestone,CaCO₃by heating (calcining) at about 850 °C (1,560 °F), in the manufacture ofquicklime(calcium oxide,CaO), a compound that has many industrial uses:

CaCO₃→ CaO + CO₂

Acids liberate CO₂from most metal carbonates. Consequently, it may be obtained directly from natural carbon dioxidesprings,where it is produced by the action of acidified water onlimestoneordolomite.The reaction betweenhydrochloric acidand calcium carbonate (limestone or chalk) is shown below:

CaCO₃+ 2 HCl → CaCl₂+ H₂CO₃

Thecarbonic acid(H₂CO₃) then decomposes to water and CO₂:

H₂CO₃→ CO₂+ H₂O

Such reactions are accompanied by foaming or bubbling, or both, as the gas is released. They have widespread uses in industry because they can be used to neutralize waste acid streams.

Commercial uses

Carbon dioxide is used by the food industry, the oil industry, and the chemical industry. The compound has varied commercial uses but one of its greatest uses as a chemical is in the production of carbonated beverages; it provides the sparkle in carbonated beverages such as soda water, beer and sparkling wine.

Precursor to chemicals

See also:Sabatier reaction

In the chemical industry, carbon dioxide is mainly consumed as an ingredient in the production ofurea,with a smaller fraction being used to producemethanoland a range of other products.Some carboxylic acid derivatives such assodium salicylateare prepared using CO₂by theKolbe–Schmitt reaction.

In addition to conventional processes using CO₂for chemical production, electrochemical methods are also being explored at a research level. In particular, the use of renewable energy for production of fuels from CO₂(such as methanol) is attractive as this could result in fuels that could be easily transported and used within conventional combustion technologies but have no net CO₂emissions.

Agriculture

Plants require carbon dioxide to conduct photosynthesis. The atmospheres of greenhouses may (if of large size, must) be enriched with additional CO₂to sustain and increase the rate of plant 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 aswhitefliesandspider mitesin a greenhouse.Some plants respond more favorably to rising carbon dioxide concentrations than others, which can lead to vegetation regime shifts likewoody plant encroachment.

Foods

Carbon dioxide bubbles in a soft drink

Carbon dioxide is afood additiveused as a propellant and acidity regulator in the food industry. It is approved for usage in the EU(listed asE numberE290), US,Australia and New Zealand(listed by itsINS number290).

A candy calledPop Rocksis pressurized with carbon dioxide gasat about 4,000kPa(40bar;580psi). When placed in the mouth, it dissolves (just like other hard candy) and releases the gas bubbles with an audible pop.

Leavening agentscause dough to rise by producing carbon dioxide.Baker's yeastproduces carbon dioxide by fermentation of sugars within the dough, while chemical leaveners such asbaking powderandbaking sodarelease carbon dioxide when heated or if exposed toacids.

Beverages

Carbon dioxide is used to producecarbonatedsoft drinksandsoda water.Traditionally, the carbonation of 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 Britishreal 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.

The taste of soda water (and related taste sensations in other carbonated beverages) is an effect of the dissolved carbon dioxide rather than the bursting bubbles of the gas.Carbonic anhydrase 4converts carbon dioxide tocarbonic acidleading to asourtaste, and also the dissolved carbon dioxide induces asomatosensoryresponse.

Winemaking

Dry ice used to preserve grapes after harvest

Carbon dioxide in the form ofdry iceis often used during thecold soakphase inwinemakingto cool clusters ofgrapesquickly after picking to help prevent spontaneousfermentationby wildyeast.The main advantage of using dry ice over water ice is that it cools the grapes without adding any additional water that might decrease the sugar concentration in thegrape must,and thus thealcoholconcentration in the finished wine. Carbon dioxide is also used to create a hypoxic environment forcarbonic maceration,the process used to produceBeaujolaiswine.

Carbon dioxide is sometimes used to top up wine bottles or otherstoragevessels 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 asnitrogenorargonare preferred for this process by professional wine makers.

Stunning animals

Carbon dioxide is often used to "stun" animals before slaughter."Stunning" may be a misnomer, as the animals are not knocked out immediately and may suffer distress.

Inert gas

Carbon dioxide 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 forwelding,although in the welding arc, it reacts tooxidizemost metals. Use in the automotive industry is common despite significant evidence that welds made in carbon dioxide are morebrittlethan those made in more inert atmospheres.When used forMIG welding,CO₂use is sometimes referred to as MAG welding, for Metal Active Gas, as CO₂can 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.

Carbon dioxide 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 60bar(870psi;59atm), 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.Aluminiumcapsules of CO₂are also sold as supplies of compressed gas forair guns,paintballmarkers/guns, inflating bicycle tires, and for makingcarbonated water.High concentrations of carbon dioxide can also be used to kill pests. Liquid carbon dioxide is used insupercritical dryingof some food products and technological materials, in the preparation of specimens forscanning electron microscopyand in thedecaffeinationofcoffee beans.

Fire extinguisher

Use of a CO₂fire extinguisher

Carbon dioxide can be used to extinguish flames by flooding the environment around the flame with the gas. It does not itself react to extinguish the flame, but starves the flame of oxygen by displacing it. Somefire extinguishers,especially those designed forelectrical 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 they do not cool the burning substances significantly, and when the carbon dioxide disperses, they can catch fire upon exposure toatmospheric oxygen.They are mainly used in server rooms.

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.International Maritime Organizationstandards 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 CO₂systems identified 51 incidents between 1975 and the date of the report (2000), causing 72 deaths and 145 injuries.

Supercritical CO₂as solvent

See also:Supercritical carbon dioxideandGreen chemistry

Liquid carbon dioxide is a goodsolventfor manylipophilicorganic compoundsand is used todecaffeinatecoffee.Carbon dioxide has attracted attention in thepharmaceuticaland other chemical processing industries as a less toxic alternative to more traditional solvents such asorganochlorides.It is also used by somedry cleanersfor this reason. It is used in the preparation of someaerogelsbecause of the properties of supercritical carbon dioxide.

Medical and pharmacological uses

In medicine, up to 5% carbon dioxide (130 times atmospheric concentration) is added to oxygen for stimulation of breathing afterapneaand to stabilize theO₂/CO₂balance in blood.

Carbon dioxide can be mixed with up to 50% oxygen, forming an inhalable gas; this is known asCarbogenand has a variety of medical and research uses.

Another medical use are themofette,dry spas that use carbon dioxide from post-volcanic discharge for therapeutic purposes.

Energy

Supercritical CO₂is used as the working fluid in theAllam power cycleengine.

Fossil fuel recovery

Carbon dioxide is used inenhanced oil recoverywhere it is injected into or adjacent to producing oil wells, usually undersupercriticalconditions, when it becomesmisciblewith the oil. This approach can increase original oil recovery by reducing residual oil saturation by 7–23% additional toprimary extraction.It acts as both a pressurizing agent and, when dissolved into the undergroundcrude oil,significantly reduces its viscosity, and changing surface chemistry enabling the oil to flow more rapidly through the reservoir to the removal well.In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points.

Inenhanced coal bed methane recovery,carbon dioxide would be pumped into the coal seam to displace methane, as opposed to current methods which primarily rely on the removal of water (to reduce pressure) to make the coal seam release its trapped methane.

Bio transformation into fuel

Main article:Carbon capture and utilization

It has been proposed that CO₂from power generation be bubbled into ponds to stimulate growth ofalgaethat could then be converted intobiodieselfuel.A strain of thecyanobacteriumSynechococcus elongatushas been genetically engineered to produce the fuelsisobutyraldehydeandisobutanolfrom CO₂using photosynthesis.

Researchers have developed anelectrocatalytictechnique using enzymes isolated from bacteria to power the chemical reactions which convert CO₂into fuels.

Refrigerant

See also:RefrigerantandSustainable automotive air conditioning

Comparison of the pressure–temperature 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 importantrefrigerants,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 (−109.3 °F) 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 use ofdichlorodifluoromethane(R12, achlorofluorocarbon(CFC) compound).CO₂might enjoy a renaissance because one of the main substitutes to CFCs,1,1,1,2-tetrafluoroethane(R134a,ahydrofluorocarbon(HFC) compound) contributes toclimate changemore than CO₂does. CO₂physical properties are highly favorable for cooling, refrigeration, and heating purposes, having a high volumetric cooling capacity. Due to the need to operate at pressures of up to 130 bars (1,900 psi; 13,000 kPa), CO₂systems require highly mechanically resistant reservoirs and 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°, CO₂(R744) operates more efficiently than systems using HFCs (e.g., R134a). Its environmental advantages (GWPof 1, non-ozone depleting, non-toxic, non-flammable) could make it the future working fluid to replace current HFCs in cars, supermarkets, and heat pump water heaters, among others.Coca-Colahas fielded CO₂-based beverage coolers and theU.S. Armyis interested in CO₂refrigeration and heating technology.

Minor uses

Acarbon-dioxide laser

Carbon dioxide is thelasing mediumin acarbon-dioxide laser,which is one of the earliest type of lasers.

Carbon dioxide can be used as a means of controlling thepHof swimming pools,by continuously adding gas to the water, thus keeping the pH from rising. Among the advantages of this is the avoidance of handling (more hazardous) acids. Similarly, it is also used in the maintainingreef aquaria,where it is commonly used incalcium reactorsto temporarily lower the pH of water being passed overcalcium carbonatein order to allow the calcium carbonate to dissolve into the water more freely, where it is used by somecoralsto build their skeleton.

Used as the primary coolant in the Britishadvanced gas-cooled reactorfor nuclear power generation.

Carbon dioxide induction is commonly used for the euthanasia of laboratory research animals. Methods to administer CO₂include placing animals directly into a closed, prefilled chamber containing CO₂,or exposure to a gradually increasing concentration of CO₂.TheAmerican Veterinary Medical Association's 2020 guidelines for carbon dioxide induction state that a displacement rate of 30–70% of the chamber or cage volume per minute is optimal for the humane euthanasia of small rodents.Percentages of CO₂vary for different species, based on identified optimal percentages to minimize distress.

Carbon dioxide is also used in several relatedcleaning and surface-preparationtechniques.

History of discovery

Crystal structure ofdry ice

Carbon dioxide was the first gas to be described as a discrete substance. In about 1640,theFlemishchemistJan Baptist van Helmontobserved that when he burnedcharcoalin a closed vessel, the mass of the resultingashwas much less than that of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" (from Greek "chaos" ) or "wild spirit" (spiritus sylvestris).

The properties of carbon dioxide were further studied in the 1750s by theScottishphysicianJoseph Black.He found thatlimestone(calcium carbonate) could be heated or treated withacidsto yield a gas he called "fixed air". He observed that the fixed air was denser than air and supported neither flame nor animal life. Black also found that when bubbled throughlimewater(a saturated aqueous solution ofcalcium hydroxide), it wouldprecipitatecalcium carbonate. He used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, English chemistJoseph Priestleypublished a paper entitledImpregnating Water with Fixed Airin which he described a process of drippingsulfuric acid(oroil of vitriolas Priestley knew it) on chalk in order to produce carbon dioxide, and forcing the gas to dissolve by agitating a bowl of water in contact with the gas.

Carbon dioxide was first liquefied (at elevated pressures) in 1823 byHumphry DavyandMichael Faraday.The earliest description of solid carbon dioxide (dry ice) was given by the French inventorAdrien-Jean-Pierre Thilorier,who in 1835 opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid CO₂.

Carbon dioxide in combination with nitrogen was known from earlier times asBlackdamp,stythe or choke damp.Along with the other types ofdampit was encountered in mining operations and well sinking. Slow oxidation of coal and biological processes replaced the oxygen to create asuffocatingmixture of nitrogen and carbon dioxide.

See also

• Arterial blood gas test– A test of blood taken from an artery that measures the amounts of certain dissolved gases
• Bosch reaction– forms elemental carbon from CO2 and hydrogen using a metallic catalystPages displaying wikidata descriptions as a fallback
• Carbon dioxide removal– Removal of atmospheric carbon dioxide through human activity (from the atmosphere)
• Gilbert Plass– Canadian physicist (1920–2004) (early work on CO₂and climate change)
• Greenhouse Gases Observing Satellite– Earth observation satellite
• List of countries by carbon dioxide emissions
• List of least carbon efficient power stations
• Meromictic lake– Permanently stratified lake with layers of water that do not intermix
• NASA'sOrbiting Carbon Observatory 2– NASA climate satellite
• Soil gas– Gases in the air space between soil components

External links

• Current global map of carbon dioxide concentration
• CDC – NIOSH Pocket Guide to Chemical Hazards – Carbon Dioxide
• Trends in Atmospheric Carbon Dioxide(NOAA)
• The rediscovery of CO₂:History, What is Shecco?- asrefrigerant