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Carbon dioxide

January 13, 2023

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

Carbon dioxide (chemical formula CO2) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transparent to visible light but absorbs infrared radiation, acting as a greenhouse gas. It is a trace gas in Earth's atmosphere at 421 parts per million (ppm), or about 0.04% by volume (as of May 2022), having risen from pre-industrial levels of 280 ppm.[9][10] Burning fossil fuels is the primary cause of these increased CO2 concentrations and also the primary cause of climate change.[11] Carbon dioxide is soluble in water and is found in groundwater, lakes, ice caps, and seawater. When carbon dioxide dissolves in water, it forms carbonate and mainly bicarbonate (HCO−
3), which causes ocean acidification as atmospheric CO2 levels increase.[12]

Carbon dioxide
Names
Other names
  • Carbonic acid gas
  • Carbonic anhydride
  • Carbonic dioxide
  • Carbon(IV) oxide
  • R-744 (refrigerant)
  • R744 (refrigerant alternative spelling)
  • Dry ice (solid phase)
Identifiers
  • 124-38-9 Y
3D model (JSmol)
  • Interactive image
  • Interactive image
3DMet
  • B01131
1900390
ChEBI
  • CHEBI:16526 Y
ChEMBL
  • ChEMBL1231871 N
ChemSpider
  • 274 Y
ECHA InfoCard 100.004.271
EC Number
  • 204-696-9
E number E290 (preservatives)
989
KEGG
  • D00004 Y
MeSH Carbon+dioxide
  • 280
RTECS number
  • FF6400000
UNII
  • 142M471B3J Y
UN number 1013 (gas), 1845 (solid)
  • DTXSID4027028
  • InChI=1S/CO2/c2-1-3 Y
    Key: CURLTUGMZLYLDI-UHFFFAOYSA-N Y
  • InChI=1/CO2/c2-1-3
    Key: CURLTUGMZLYLDI-UHFFFAOYAO
  • O=C=O
  • C(=O)=O
Properties
CO2
Molar mass 44.009 g·mol−1
Appearance Colorless gas
Odor
  • Low concentrations: none
  • High concentrations: sharp; acidic[1]
Density
  • 1562 kg/m3 (solid at 1 atm (100 kPa) and −78.5 °C (−109.3 °F))
  • 1101 kg/m3 (liquid at saturation −37 °C (−35 °F))
  • 1.977 kg/m3 (gas at 1 atm (100 kPa) and 0 °C (32 °F))
Critical point (T, P) 304.128(15) K[2] (30.978(15) °C), 7.3773(30) MPa[2] (72.808(30) atm)
194.6855(30) K (−78.4645(30) °C) at 1 atm (0.101325 MPa)
1.45 g/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) 6.35, 10.33
−20.5·10−6 cm3/mol
Thermal conductivity 0.01662 W·m−1·K−1 (300 K (27 °C; 80 °F))[3]
1.00045
Viscosity
  • 14.90 μPa·s at 25 °C (298 K)[4]
  • 70 μPa·s at −78.5 °C (194.7 K)
0 D
Structure
Trigonal
Linear
Thermochemistry
37.135 J/K·mol
214 J·mol−1·K−1
−393.5 kJ·mol−1
Pharmacology
V03AN02 (WHO)
Hazards
NFPA 704 (fire diamond)
Lethal dose or concentration (LD, LC):
90,000 ppm (human, 5 min)[6]
NIOSH (US health exposure limits):
PEL (Permissible)
 TWA 5000 ppm (9000 mg/m3)[5]
REL (Recommended)
 TWA 5000 ppm (9000 mg/m3), ST 30,000 ppm (54,000 mg/m3)[5]
IDLH (Immediate danger)
 40,000 ppm[5]
Safety data sheet (SDS) Sigma-Aldrich
Related compounds
Other anions
Other cations
Related carbon oxides
Related compounds
Supplementary data page
Carbon dioxide (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)

As the source of available carbon in the carbon cycle, atmospheric CO2 is the primary carbon source for life on Earth. Its concentration in Earth's pre-industrial atmosphere since late in the Precambrian has been regulated by organisms and geological phenomena. Plants, algae and cyanobacteria use energy from sunlight to synthesize carbohydrates from carbon dioxide and water in a process called photosynthesis, which produces oxygen as a waste product.[13] In turn, oxygen is consumed and CO2 is released as waste by all aerobic organisms when they metabolize organic compounds to produce energy by respiration.[14] CO2 is released from organic materials when they decay or combust, such as in forest fires. Since plants require CO2 for photosynthesis, and humans and animals depend on plants for food, CO2 is 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 CO2 emissions to the atmosphere are absorbed by land and ocean carbon sinks.[15] These sinks can become saturated and are volatile, as decay and wildfires result in the CO2 being released back into the atmosphere.[16] CO2 is eventually sequestered (stored for the long term) in rocks and organic deposits like coal, petroleum and natural gas. Sequestered CO2 is released into the atmosphere through burning fossil fuels or naturally by volcanoes, hot springs, geysers, and when carbonate rocks dissolve in water or react with acids.

CO2 is a versatile industrial material, used, for example, as an inert gas in welding and fire extinguishers, as a pressurizing gas in air guns and oil recovery, and as a supercritical fluid solvent in decaffeination of coffee and supercritical drying.[17] It is a byproduct of fermentation of sugars in bread, beer and wine making, and is added to carbonated beverages like seltzer and beer for effervescence. It has a sharp and acidic odor and generates the taste of soda water in the mouth,[18] but at normally encountered concentrations it is odorless.[1]

Chemical and physical properties

Structure, bonding and molecular vibrations

See also: Molecular orbital diagram § Carbon dioxide

The symmetry of a carbon dioxide molecule is linear and centrosymmetric at its equilibrium geometry. The length of the carbon-oxygen bond in carbon dioxide is 116.3 pm, noticeably shorter than the roughly 140-pm length of a typical single C–O bond, and shorter than most other C–O multiply-bonded functional groups such as carbonyls.[19] Since it is centrosymmetric, the molecule has no electric dipole moment.

 
Stretching and bending oscillations of the CO2 carbon dioxide molecule. Upper left: symmetric stretching. Upper right: antisymmetric stretching. Lower line: degenerate pair of bending modes.

As a linear triatomic molecule, CO2 has four vibrational modes as 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 are degenerate, 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 the infrared (IR) spectrum: the antisymmetric stretching mode at wavenumber 2349 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 in by Raman spectroscopy at 1388 cm−1 (wavelength 7.2 μm).[20]

In the gas phase, carbon dioxide molecules undergo significant vibrational motions and do not keep a fixed structure. However, in a Coulomb explosion imaging experiment, an instantaneous image of the molecular structure can be deduced. Such an experiment[21] has been performed for carbon dioxide. The result of this experiment, and the conclusion of theoretical calculations[22] based on an ab initio potential energy surface of 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 motion volume element vanishes for linear geometries.[22] This is so for all molecules (except diatomics!).

In aqueous solution

See also: Carbonic acid

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

 

The hydration equilibrium constant of carbonic acid is, at 25 °C:

 

Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as CO2 molecules, not affecting the pH.

The relative concentrations of CO2, H2CO3, and the deprotonated forms HCO−3 (bicarbonate) and CO2−3(carbonate) depend on the pH. As shown in a Bjerrum plot, in neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the 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):

 
Ka1 = 2.5×10−4 mol/L; pKa1 = 3.6 at 25 °C.[19]

This is the true first acid dissociation constant, defined as

 

where the denominator includes only covalently bound H2CO3 and does not include hydrated CO2(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 CO2 is present as carbonic acid, so that

 

Since most of the dissolved CO2remains as CO2 molecules, Ka1(apparent) has a much larger denominator and a much smaller value than the true Ka1.[23]

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 (CO2−3):

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

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

Chemical reactions of CO2

CO2 is a potent electrophile having an electrophilic reactivity that is comparable to benzaldehyde or strong α,β-unsaturated carbonyl compounds. However, unlike electrophiles of similar reactivity, the reactions of nucleophiles with CO2 are thermodynamically less favored and are often found to be highly reversible.[24] Only very strong nucleophiles, like the carbanions provided by Grignard reagents and organolithium compounds react with CO2 to give carboxylates:

 
where M = Li or Mg Br and R = alkyl or aryl.

In metal carbon dioxide complexes, CO2 serves as a ligand, which can facilitate the conversion of CO2 to other chemicals.[25]

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

 

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

 

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.[26]

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.[1] At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m3, about 1.53 times that of air.[27]

Carbon dioxide has no liquid state at pressures below 0.51795(10) MPa[2] (5.11177(99) atm). At a pressure of 1 atm (0.101325 MPa), the gas deposits directly to a solid at temperatures below 194.6855(30) K[2] (−78.4645(30) °C) and the solid sublimes directly to a gas above this temperature. In its solid state, carbon dioxide is commonly called dry ice.

 
Pressure–temperature phase diagram of carbon dioxide. Note that it is a log-lin chart.

Liquid carbon dioxide forms only at pressures above 0.51795(10) MPa[2] (5.11177(99) atm); the triple point of carbon dioxide is 216.592(3) K[2] (−56.558(3) °C) at 0.51795(10) MPa[2] (5.11177(99) atm) (see phase diagram). The critical point is 304.128(15) K[2] (30.978(15) °C) at 7.3773(30) MPa[2] (72.808(30) atm). Another form of solid carbon dioxide observed at high pressure is an amorphous glass-like solid.[28] This form of glass, called carbonia, is produced by supercooling heated CO2 at extreme pressures (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 dioxide (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.

Table of thermal and physical properties of saturated liquid carbon dioxide:[29][30]

Temperature (°C) Density (kg/m^3) Specific heat (kJ/kg K) Kinematic viscosity (m^2/s) Conductivity (W/m K) Thermal diffusivity (m^2/s) Prandtl Number Bulk modulus (K^-1)
-50 1156.34 1.84 1.19E-07 0.0855 4.02E-08 2.96 -
-40 1117.77 1.88 1.18E-07 0.1011 4.81E-08 2.46 -
-30 1076.76 1.97 1.17E-07 0.1116 5.27E-08 2.22 -
-20 1032.39 2.05 1.15E-07 0.1151 5.45E-08 2.12 -
-10 983.38 2.18 1.13E-07 0.1099 5.13E-08 2.2 -
0 926.99 2.47 1.08E-07 0.1045 4.58E-08 2.38 -
10 860.03 3.14 1.01E-07 0.0971 3.61E-08 2.8 -
20 772.57 5 9.10E-08 0.0872 2.22E-08 4.1 1.40E-02
30 597.81 36.4 8.00E-08 0.0703 0.279E-08 28.7 -

Table of thermal and physical properties of carbon dioxide (CO2) at atmospheric pressure:[29][30]

Temperature (K) Density (kg/m^3) Specific heat (kJ/kg °C) Dynamic viscosity (kg/m s) Kinematic viscosity (m^2/s) Thermal conductivity (W/m °C) Thermal diffusivity (m^2/s) Prandtl Number
220 2.4733 0.783 1.11E-05 4.49E-06 0.010805 5.92E-06 0.818
250 2.1657 0.804 1.26E-05 5.81E-06 0.012884 7.40E-06 0.793
300 1.7973 0.871 1.50E-05 8.32E-06 0.016572 1.06E-05 0.77
350 1.5362 0.9 1.72E-05 1.12E-05 0.02047 1.48E-05 0.755
400 1.3424 0.942 1.93E-05 1.44E-05 0.02461 1.95E-05 0.738
450 1.1918 0.98 2.13E-05 1.79E-05 0.02897 2.48E-05 0.721
500 1.0732 1.013 2.33E-05 2.17E-05 0.03352 3.08E-05 0.702
550 0.9739 1.047 2.51E-05 2.57E-05 0.03821 3.75E-05 0.685
600 0.8938 1.076 2.68E-05 3.00E-05 0.04311 4.48E-05 0.668
650 0.8143 1.1 2.88E-05 3.54E-05 0.0445 4.97E-05 0.712
700 0.7564 1.13E+00 3.05E-05 4.03E-05 0.0481 5.63E-05 0.717
750 0.7057 1.15 3.21E-05 4.55E-05 0.0517 6.37E-05 0.714
800 0.6614 1.17E+00 3.37E-05 5.10E-05 0.0551 7.12E-05 0.716

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 release in respiration.

Photosynthesis and carbon fixation

 
Overview of the Calvin cycle and carbon fixation

Carbon fixation is a biochemical process by which atmospheric carbon dioxide is incorporated by plants, algae and (cyanobacteria) into energy-rich organic molecules such as glucose, thus creating their 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 CO2 and ribulose bisphosphate, as shown in the diagram at left.

RuBisCO is thought to be the single most abundant protein on Earth.[31]

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.[32] 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.

 
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 (CO2).

Plants can grow as much as 50 percent faster in concentrations of 1,000 ppm CO2 when compared with ambient conditions, though this assumes no change in climate and no limitation on other nutrients.[33] Elevated CO2 levels cause increased growth reflected in the harvestable yield of crops, with wheat, rice and soybean all showing increases in yield of 12–14% under elevated CO2 in FACE experiments.[34][35]

Increased atmospheric CO2 concentrations result in fewer stomata developing on plants[36] which leads to reduced water usage and increased water-use efficiency.[37] Studies using FACE have shown that CO2 enrichment leads to decreased concentrations of micronutrients in crop plants.[38] 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.[39]

The concentration of secondary metabolites such as phenylpropanoids and flavonoids can also be altered in plants exposed to high concentrations of CO2.[40][41]

Plants also emit CO2 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 CO2 each year, a mature forest will produce as much CO2 from respiration and decomposition of dead specimens (e.g., fallen branches) as is used in photosynthesis in growing plants.[42] Contrary to the long-standing view that they are carbon neutral, mature forests can continue to accumulate carbon[43] and remain valuable carbon sinks, helping to maintain the carbon balance of Earth's atmosphere. Additionally, and crucially to life on earth, photosynthesis by phytoplankton consumes dissolved CO2 in the upper ocean and thereby promotes the absorption of CO2 from the atmosphere.[44]

Toxicity

See also: Carbon dioxide poisoning
 
Symptoms of carbon dioxide toxicity, by increasing volume percent in air[45]

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.[46][clarification needed]

CO2 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 and give the lungs a stuffy feeling.[45] 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.[47] 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 CO2 emissions from the nearby volcano Mount Nyiragongo.[48] The Swahili term for this phenomenon is mazuku.

 
Rising levels of CO2 threatened the Apollo 13 astronauts who had to adapt cartridges from the command module to supply the carbon dioxide scrubber in the Lunar Module, which they used as a lifeboat.

Adaptation to increased concentrations of CO2 occurs in humans, including modified breathing and 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. a submarine) 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.[49][50] Yet, other studies show a decrease in cognitive function even at much lower levels.[51][52] Also, with ongoing respiratory acidosis, adaptation or compensatory mechanisms will be unable to reverse such condition.

Below 1%

There are few studies of the health effects of long-term continuous CO2 exposure on humans and animals at levels below 1%. Occupational CO2 exposure limits have been set in the United States at 0.5% (5000 ppm) for an eight-hour period.[53] At this CO2 concentration, International Space Station crew experienced headaches, lethargy, mental slowness, emotional irritation, and sleep disruption.[54] Studies in animals at 0.5% CO2 have demonstrated kidney calcification and bone loss after eight weeks of exposure.[55] A study of humans exposed in 2.5 hour sessions demonstrated significant negative effects on cognitive abilities at concentrations as low as 0.1% (1000 ppm) CO2 likely due to CO2 induced increases in cerebral blood flow.[51] Another study observed a decline in basic activity level and information usage at 1000 ppm, when compared to 500 ppm.[52] However a review of the literature found that most studies on the phenomenon of carbon dioxide induced cognitive impairment to have a small effect on high-level decision making and most of the studies were confounded by inadequate study designs, environmental comfort, uncertainties in exposure doses and differing cognitive assessments used.[56] Similarly a study on the effects of the concentration of CO2 in 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 CO2 declined to safe levels (0.2%).[57][58]

Ventilation

 
A carbon dioxide sensor that measures CO2 concentration using a nondispersive infrared sensor

Poor ventilation is one of the main causes of excessive CO2 concentrations in closed spaces, leading to poor indoor air quality. Carbon dioxide differential above outdoor concentrations at steady state conditions (when the occupancy and ventilation system operation are sufficiently long that CO2 concentration has stabilized) are sometimes used to estimate ventilation rates per person.[citation needed] Higher CO2 concentrations are associated with occupant health, comfort and performance degradation.[59][60] ASHRAE Standard 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, 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.

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.[61] A similar accident occurred in 2018 when a woman died from CO2 fumes emanating from the large amount of dry ice she was transporting in her car.[62]

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 near Rapolano Terme in Tuscany, Italy, situated in a bowl-shaped depression about 100 m (330 ft) in diameter, concentrations of CO2 rise to above 75% overnight, sufficient to kill insects and small animals. After sunrise the gas is dispersed by convection.[63] High concentrations of CO2 produced by disturbance of deep lake water saturated with CO2 are thought to have caused 37 fatalities at Lake Monoun, Cameroon in 1984 and 1700 casualties at Lake Nyos, Cameroon in 1986.[64]

Human physiology

Content

Reference ranges or averages for partial pressures of carbon dioxide (abbreviated pCO2)
Blood compartment (kPa) (mm Hg)
Venous blood carbon dioxide 5.5–6.8 41–51[65] 41–51[65]
Alveolar pulmonary
gas pressures 		4.8 36 36
Arterial blood carbon dioxide 4.7–6.0 35–45[65] 35–45[65]

The body produces approximately 2.3 pounds (1.0 kg) of carbon dioxide per day per person,[66] 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, resulting in lower concentrations in the arteries. The 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.[67] In humans, the blood carbon dioxide contents is shown in the adjacent table.

Transport in the blood

CO2 is carried in blood in three different ways. (Exact percentages vary between arterial and venous blood).

Hemoglobin, the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbon dioxide. However, the CO2 bound to hemoglobin does not bind to the same site as oxygen. Instead, it combines with the N-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, the binding of CO2 decreases the amount of oxygen that is bound for a given partial pressure of oxygen. This 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 CO2 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 concentration is high, the capillaries expand to allow a greater blood flow to that tissue.[69]

Bicarbonate ions are crucial for regulating blood pH. A person's breathing rate influences the level of CO2 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.[70]

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.[68]

The respiratory centers try to maintain an arterial CO2 pressure of 40 mm Hg. With intentional hyperventilation, the CO2 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.[71]

Concentrations and role in the environment

Atmosphere

Further information: Carbon cycle
This section is an excerpt from Carbon dioxide in Earth's atmosphere.[edit]
 
Atmospheric CO2 concentrations measured at Mauna Loa Observatory from 1958 to 2022 (also called the Keeling Curve). Carbon dioxide concentrations have varied widely over the Earth's 4.54 billion year history. However, in 2013 the daily mean concentration of CO2 in the atmosphere surpassed 400 parts per million (ppmv)[72] - this level has never been reached since the mid-Pliocene, 2 to 4 million years ago.[73]

Carbon dioxide in Earth's atmosphere is a trace gas that plays an integral part in the greenhouse effect, carbon cycle, photosynthesis and oceanic carbon cycle. It is one of several greenhouse gases in Earth's atmosphere that are contributing to climate change due to increasing emissions of greenhouse gases from human activities. The current global average concentration of CO2 in the atmosphere is 421 ppm as of May 2022.[74] This is an increase of 50% since the start of the Industrial Revolution, up from 280 ppm during the 10,000 years prior to the mid-18th century.[75][74][76] The increase is due to human activity.[77] Burning fossil fuels is the main cause of these increased CO2 concentrations and also the main cause of climate change.[78] Other large anthropogenic sources include cement production, deforestation, and biomass burning.

While transparent to visible light, carbon dioxide is a greenhouse gas, absorbing and emitting infrared radiation at its two infrared-active vibrational frequencies. CO2 absorbs and emits infrared radiation at wavelengths of 4.26 μm (2,347 cm−1) (asymmetric stretching vibrational mode) and 14.99 μm (667 cm−1) (bending vibrational mode). It plays a significant role in influencing Earth's surface temperature through the greenhouse effect.[79] Light emission from the Earth's surface is most intense in the infrared region between 200 and 2500 cm−1,[80] as opposed to light emission from the much hotter Sun which is most intense in the visible region. Absorption of infrared light at the vibrational frequencies of atmospheric CO2 traps energy near the surface, warming the surface and the lower atmosphere. Less energy reaches the upper atmosphere, which is therefore cooler because of this absorption.[81]

Increases in atmospheric concentrations of CO2 and other long-lived greenhouse gases such as methane, nitrous oxide and ozone increase the absorption and emission of infrared radiation by the atmosphere, causing the observed rise in average global temperature and ocean acidification. Another direct effect is the CO2 fertilization effect. These changes cause a range of indirect effects of climate change on the physical environment, ecosystems and human societies. Carbon dioxide exerts a larger overall warming influence than all of the other greenhouse gases combined.[76] It has an atmospheric lifetime that increases with the cumulative amount of fossil carbon extracted and burned, due to the imbalance that this activity has imposed on Earth's fast carbon cycle.[82] This means that some fraction (a projected 20-35%) of the fossil carbon transferred thus far will persist in the atmosphere as elevated CO2 levels for many thousands of years after these carbon transfer activities begin to subside.[83][84][85] The carbon cycle is a biogeochemical cycle in which carbon is exchanged between the Earth's oceans, soil, rocks and the biosphere. Plants and other photoautotrophs use solar energy to produce carbohydrate from atmospheric carbon dioxide and water by photosynthesis. Almost all other organisms depend on carbohydrate derived from photosynthesis as their primary source of energy and carbon compounds.

The present atmospheric concentration of CO2 is the highest for 14 million years.[86] Concentrations of CO2 in the atmosphere were as high as 4,000 ppm during the Cambrian period about 500 million years ago, when the concentration was 20 times greater than today, and as low as 180 ppm during the Quaternary glaciation of the last two million years.[75] Reconstructed temperature records for the last 420 million years indicate that atmospheric CO2 concentrations peaked at ~2,000 ppm during the Devonian (~400 Ma) period, and again in the Triassic (220–200 Ma) period and was four times current levels during the Jurassic period (201-145 Ma).[87][88]
 
Annual CO2 flows from anthropogenic sources (left) into Earth's atmosphere, land, and ocean sinks (right) since the 1960s. Units in equivalent gigatonnes carbon per year.[89]

Oceans

Main articles: Carbon cycle and Ocean acidification

Ocean acidification

Carbon dioxide dissolves in the ocean to form carbonic acid (H2CO3), bicarbonate (HCO3−) and carbonate (CO32−). There is about fifty times as much carbon dioxide dissolved in the oceans as exists in the atmosphere. The oceans act as an enormous carbon sink, and have taken up about a third of CO2 emitted by human activity.[90]

This section is an excerpt from Ocean acidification.[edit]
Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14.[91] The root cause of ocean acidification is carbon dioxide emissions from human activities which have led to atmospheric carbon dioxide (CO2) levels of more than 410 ppm (in 2020). The oceans absorb CO2 from the atmosphere. This leads to the formation of carbonic acid (H2CO3) which dissociates into a bicarbonate ion (HCO−3) and a hydrogen ion (H+). The free hydrogen ions (H+) decrease the pH of the ocean, therefore increasing acidity (this does not mean that seawater is acidic yet; it is still alkaline, with a pH higher than 8). A decrease in pH corresponds to a decrease in the concentration of carbonate ions, which are the main building block for calcium carbonate (CaCO3) shells and skeletons. Marine calcifying organisms, like mollusks, oysters and corals, are particularly affected by this as they rely on calcium carbonate to build shells and skeletons.[92]
 
Pterapod shell dissolved in seawater adjusted to an ocean chemistry projected for the year 2100
This section is an excerpt from Ocean 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 of calcium carbonate (CaCO
3).[92] This process is called calcification and is important to the biology and survival of a wide range of marine organisms. Calcification involves the precipitation of dissolved ions into solid CaCO
3 structures, structures for many marine organisms, such as coccolithophores, foraminifera, crustaceans, mollusks, etc. After they are formed, these CaCO
3 structures are vulnerable to dissolution unless the surrounding seawater contains saturating concentrations of carbonate ions (CO32−).

Given the current pH of the ocean (around 8.14), of the extra carbon dioxide added into the ocean, very little 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,[93] creating an imbalance in the reaction HCO3− ⇌ CO32− + 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+ + CO32− ⇌ CaCO3.


Hydrothermal vents

Carbon dioxide is also introduced into the oceans through hydrothermal vents. The Champagne hydrothermal vent, found at the Northwest Eifuku volcano in the Mariana Trench, produces almost pure liquid carbon dioxide, one of only two known sites in the world as of 2004, the other being in the Okinawa Trough.[94] The finding of a submarine lake of liquid carbon dioxide in the Okinawa Trough was reported in 2006.[95]

Production

Biological processes

Carbon dioxide is a by-product of the fermentation of sugar in the brewing of beer, whisky and other alcoholic beverages and in the production of bioethanol. Yeast metabolizes sugar to produce CO2 and ethanol, also known as alcohol, as follows:

 

All aerobic organisms produce CO2 when they oxidize carbohydrates, fatty acids, and proteins. 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:

 

Anaerobic organisms decompose organic material producing methane and carbon dioxide together with traces of other compounds.[96] Regardless of the type of organic material, the production of gases follows well defined kinetic 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.[97]

Industrial processes

Carbon dioxide can be obtained by distillation from 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.[98]

Combustion

The combustion of all carbon-based fuels, such as methane (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 and oxygen:

 

Iron is reduced from its oxides with coke in a blast furnace, producing pig iron and carbon dioxide:[99]

By-product from hydrogen production

Carbon dioxide is a byproduct of the industrial production of hydrogen by steam reforming and the water gas shift reaction in ammonia production. These processes begin with the reaction of water and natural gas (mainly methane).[100] This is a major source of food-grade carbon dioxide for use in carbonation of beer and soft drinks, and is also used for stunning animals such as poultry. 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.[101]

Thermal decomposition of limestone

It is produced by thermal decomposition of limestone, CaCO3 by heating (calcining) at about 850 °C (1,560 °F), in the manufacture of quicklime (calcium oxide, CaO), a compound that has many industrial uses:

 

Acids liberate CO2 from most metal carbonates. Consequently, it may be obtained directly from natural carbon dioxide springs, where it is produced by the action of acidified water on limestone or dolomite. The reaction between hydrochloric acid and calcium carbonate (limestone or chalk) is shown below:

 

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

 

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.[98] 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

In the chemical industry, carbon dioxide is mainly consumed as an ingredient in the production of urea, with a smaller fraction being used to produce methanol and a range of other products.[102] Some carboxylic acid derivatives such as sodium salicylate are prepared using CO2 by the Kolbe–Schmitt reaction.[103]

In addition to conventional processes using CO2 for 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 CO2 emissions.[104]

Agriculture

Plants require carbon dioxide to conduct photosynthesis. The atmospheres of greenhouses may (if of large size, must) be enriched with additional CO2 to sustain and increase the rate of plant growth.[105][106] 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.[107]

Foods

 
Carbon dioxide bubbles in a soft drink

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[108] (listed as E number E290), US[109] and Australia and New Zealand[110] (listed by its INS number 290).

A candy called Pop Rocks is pressurized with carbon dioxide gas[111] at about 4,000 kPa (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.[112] 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 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 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.

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 4 converts to carbonic acid leading to a sour taste, and also the dissolved carbon dioxide induces a somatosensory response.[113]

Winemaking

 
Dry ice used to preserve grapes after harvest

Carbon dioxide in the form of dry ice is often used during the cold soak phase in winemaking to cool clusters of grapes quickly after picking to help prevent spontaneous fermentation by wild yeast. 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 the grape must, and thus the alcohol concentration in the finished wine. 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.

Stunning animals

Carbon dioxide is often used to "stun" animals before slaughter.[114] "Stunning" may be a misnomer, as the animals are not knocked out immediately and may suffer distress.[115][116]

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 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.[citation needed] When used for MIG welding, CO2 use is sometimes referred to as MAG welding, for Metal Active Gas, as CO2 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.

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 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 CO2 are also sold as supplies of compressed gas for air guns, paintball markers/guns, inflating bicycle tires, and for making carbonated water. 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[117] and in the decaffeination of coffee beans.

Fire extinguisher

 
Use of a CO2 fire 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. 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 they do not cool the burning substances significantly, and when the carbon dioxide disperses, they can catch fire upon exposure to atmospheric oxygen. They are mainly used in server rooms.[118]

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.[119] International Maritime Organization standards 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 CO2 systems identified 51 incidents between 1975 and the date of the report (2000), causing 72 deaths and 145 injuries.[120]

Supercritical CO2 as solvent

See also: Supercritical carbon dioxide and Green chemistry

Liquid carbon dioxide is a good solvent for many lipophilic organic compounds and is used to remove caffeine from coffee.[17] 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 also used by some dry cleaners for this reason. It is used in the preparation of some aerogels because 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 after apnea and to stabilize the O2/CO2 balance in blood.

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

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

Energy

Supercritical CO2 is used as the working fluid in the Allam power cycle engine.

Fossil fuel 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 7–23% additional to primary extraction.[121] 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.[122] In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points.

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 rely on the removal of water (to reduce pressure) to make the coal seam release its trapped methane.[123]

Bio transformation into fuel

Main article: Carbon capture and utilization

It has been proposed that CO2 from power generation be bubbled into ponds to stimulate growth of algae that could then be converted into biodiesel fuel.[124] A strain of the cyanobacterium Synechococcus elongatus has been genetically engineered to produce the fuels isobutyraldehyde and isobutanol from CO2 using photosynthesis.[125]

Researchers have developed a process called electrolysis, using enzymes isolated from bacteria to power the chemical reactions which convert CO2 into fuels.[126][127][128]

Refrigerant
See also: Refrigerant and Sustainable 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 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 (−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[citation needed] of dichlorodifluoromethane (R12, a chlorofluorocarbon (CFC) compound). CO2 might enjoy a renaissance because one of the main substitutes to CFCs, 1,1,1,2-tetrafluoroethane (R134a, a hydrofluorocarbon (HFC) compound) contributes to climate change more than CO2 does. CO2 physical 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), CO2 systems 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 (GWP of 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-Cola has fielded CO2-based beverage coolers and the U.S. Army is interested in CO2 refrigeration and heating technology.[129][130]

Minor uses

Carbon dioxide is the lasing medium in a 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,[131] 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 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.

Used as the primary coolant in the British advanced gas-cooled reactor for nuclear power generation.

Carbon dioxide induction is commonly used for the euthanasia of laboratory research animals. Methods to administer CO2 include placing animals directly into a closed, prefilled chamber containing CO2, or exposure to a gradually increasing concentration of CO2. The American 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.[132]: 5, 31  Percentages of CO2 vary for different species, based on identified optimal percentages to minimize distress.[132]: 22 

Carbon dioxide is also used in several related cleaning and surface-preparation techniques.

History of discovery

 
Crystal structure of dry ice

Carbon dioxide was the first gas to be described as a discrete substance. In about 1640,[133] the Flemish chemist Jan Baptist van Helmont observed that when he burned charcoal in a closed vessel, the mass of the resulting ash was 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" or "wild spirit" (spiritus sylvestris).[134]

The properties of carbon dioxide were further studied in the 1750s by the Scottish physician Joseph Black. He found that limestone (calcium carbonate) could be heated or treated with acids to 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 through limewater (a saturated aqueous solution of calcium hydroxide), it would precipitate calcium carbonate. He used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, English chemist Joseph Priestley published a paper entitled Impregnating Water with Fixed Air in which he described a process of dripping sulfuric acid (or oil of vitriol as 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.[135]

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and Michael Faraday.[136] The earliest description of solid carbon dioxide (dry ice) was given by the French inventor Adrien-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.[137][138]

See also

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External links

 
Wikimedia Commons has media related to Carbon dioxide.
  • Current global map of carbon dioxide concentration
  • CDC – NIOSH Pocket Guide to Chemical Hazards – Carbon Dioxide
  • Trends in Atmospheric Carbon Dioxide (NOAA)

January 13, 2023
carbon, dioxide, redirects, here, other, uses, disambiguation, chemical, formula, chemical, compound, made, molecules, that, each, have, carbon, atom, covalently, double, bonded, oxygen, atoms, found, state, room, temperature, carbon, dioxide, transparent, vis. CO2 redirects here For other uses see CO2 disambiguation Carbon dioxide chemical formula CO2 is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms It is found in the gas state at room temperature In the air carbon dioxide is transparent to visible light but absorbs infrared radiation acting as a greenhouse gas It is a trace gas in Earth s atmosphere at 421 parts per million ppm or about 0 04 by volume as of May 2022 having risen from pre industrial levels of 280 ppm 9 10 Burning fossil fuels is the primary cause of these increased CO2 concentrations and also the primary cause of climate change 11 Carbon dioxide is soluble in water and is found in groundwater lakes ice caps and seawater When carbon dioxide dissolves in water it forms carbonate and mainly bicarbonate HCO 3 which causes ocean acidification as atmospheric CO2 levels increase 12 Carbon dioxide NamesOther names Carbonic acid gas Carbonic anhydride Carbonic dioxide Carbon IV oxide R 744 refrigerant R744 refrigerant alternative spelling Dry ice solid phase IdentifiersCAS Number 124 38 9 Y3D model JSmol Interactive imageInteractive image3DMet B01131Beilstein Reference 1900390ChEBI CHEBI 16526 YChEMBL ChEMBL1231871 NChemSpider 274 YECHA InfoCard 100 004 271EC Number 204 696 9E number E290 preservatives Gmelin Reference 989KEGG D00004 YMeSH Carbon dioxidePubChem CID 280RTECS number FF6400000UNII 142M471B3J YUN number 1013 gas 1845 solid CompTox Dashboard EPA DTXSID4027028InChI InChI 1S CO2 c2 1 3 YKey CURLTUGMZLYLDI UHFFFAOYSA N YInChI 1 CO2 c2 1 3Key CURLTUGMZLYLDI UHFFFAOYAOSMILES O C OC O OPropertiesChemical formula C O 2Molar mass 44 009 g mol 1Appearance Colorless gasOdor Low concentrations none High concentrations sharp acidic 1 Density 1562 kg m3 solid at 1 atm 100 kPa and 78 5 C 109 3 F 1101 kg m3 liquid at saturation 37 C 35 F 1 977 kg m3 gas at 1 atm 100 kPa and 0 C 32 F Critical point T P 304 128 15 K 2 30 978 15 C 7 3773 30 MPa 2 72 808 30 atm Sublimationconditions 194 6855 30 K 78 4645 30 C at 1 atm 0 101325 MPa Solubility in water 1 45 g 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 6 35 10 33Magnetic susceptibility x 20 5 10 6 cm3 molThermal conductivity 0 01662 W m 1 K 1 300 K 27 C 80 F 3 Refractive index nD 1 00045Viscosity 14 90 mPa s at 25 C 298 K 4 70 mPa s at 78 5 C 194 7 K Dipole moment 0 DStructureCrystal structure TrigonalMolecular shape LinearThermochemistryHeat capacity C 37 135 J K molStd molarentropy S 298 214 J mol 1 K 1Std enthalpy offormation DfH 298 393 5 kJ mol 1PharmacologyATC code V03AN02 WHO HazardsNFPA 704 fire diamond 7 8 200SALethal dose or concentration LD LC LCLo lowest published 90 000 ppm human 5 min 6 NIOSH US health exposure limits PEL Permissible TWA 5000 ppm 9000 mg m3 5 REL Recommended TWA 5000 ppm 9000 mg m3 ST 30 000 ppm 54 000 mg m3 5 IDLH Immediate danger 40 000 ppm 5 Safety data sheet SDS Sigma AldrichRelated compoundsOther anions Carbon disulfide Carbon diselenide Carbon ditellurideOther cations Silicon dioxide Germanium dioxide Tin dioxide Lead dioxideRelated carbon oxides Carbon monoxide Carbon suboxide Dicarbon monoxide Carbon trioxideRelated compounds Carbonic acid Carbonyl sulfideSupplementary data pageCarbon dioxide data page Except where otherwise noted data are given for materials in their standard state at 25 C 77 F 100 kPa N verify what is Y N Infobox references As the source of available carbon in the carbon cycle atmospheric CO2 is the primary carbon source for life on Earth Its concentration in Earth s pre industrial atmosphere since late in the Precambrian has been regulated by organisms and geological phenomena Plants algae and cyanobacteria use energy from sunlight to synthesize carbohydrates from carbon dioxide and water in a process called photosynthesis which produces oxygen as a waste product 13 In turn oxygen is consumed and CO2 is released as waste by all aerobic organisms when they metabolize organic compounds to produce energy by respiration 14 CO2 is released from organic materials when they decay or combust such as in forest fires Since plants require CO2 for photosynthesis and humans and animals depend on plants for food CO2 is 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 CO2 emissions to the atmosphere are absorbed by land and ocean carbon sinks 15 These sinks can become saturated and are volatile as decay and wildfires result in the CO2 being released back into the atmosphere 16 CO2 is eventually sequestered stored for the long term in rocks and organic deposits like coal petroleum and natural gas Sequestered CO2 is released into the atmosphere through burning fossil fuels or naturally by volcanoes hot springs geysers and when carbonate rocks dissolve in water or react with acids CO2 is a versatile industrial material used for example as an inert gas in welding and fire extinguishers as a pressurizing gas in air guns and oil recovery and as a supercritical fluid solvent in decaffeination of coffee and supercritical drying 17 It is a byproduct of fermentation of sugars in bread beer and wine making and is added to carbonated beverages like seltzer and beer for effervescence It has a sharp and acidic odor and generates the taste of soda water in the mouth 18 but at normally encountered concentrations it is odorless 1 Contents 1 Chemical and physical properties 1 1 Structure bonding and molecular vibrations 1 2 In aqueous solution 1 3 Chemical reactions of CO2 1 4 Physical properties 2 Biological role 2 1 Photosynthesis and carbon fixation 2 2 Toxicity 2 2 1 Below 1 2 2 2 Ventilation 2 2 3 Outdoor areas with elevated concentrations 3 Human physiology 3 1 Content 3 2 Transport in the blood 3 3 Regulation of respiration 4 Concentrations and role in the environment 4 1 Atmosphere 4 2 Oceans 4 2 1 Ocean acidification 4 2 2 Hydrothermal vents 5 Production 5 1 Biological processes 5 2 Industrial processes 5 2 1 Combustion 5 2 2 By product from hydrogen production 5 2 3 Thermal decomposition of limestone 6 Commercial uses 6 1 Precursor to chemicals 6 2 Agriculture 6 3 Foods 6 3 1 Beverages 6 3 2 Winemaking 6 3 3 Stunning animals 6 4 Inert gas 6 5 Fire extinguisher 6 6 Supercritical CO2 as solvent 6 7 Medical and pharmacological uses 6 8 Energy 6 8 1 Fossil fuel recovery 6 8 2 Bio transformation into fuel 6 8 2 1 Refrigerant 6 9 Minor uses 7 History of discovery 8 See also 9 References 10 External linksChemical and physical propertiesStructure bonding and molecular vibrations See also Molecular orbital diagram Carbon dioxide The symmetry of a carbon dioxide molecule is linear and centrosymmetric at its equilibrium geometry The length of the carbon oxygen bond in carbon dioxide is 116 3 pm noticeably shorter than the roughly 140 pm length of a typical single C O bond and shorter than most other C O multiply bonded functional groups such as carbonyls 19 Since it is centrosymmetric the molecule has no electric dipole moment Stretching and bending oscillations of the CO2 carbon dioxide molecule Upper left symmetric stretching Upper right antisymmetric stretching Lower line degenerate pair of bending modes As a linear triatomic molecule CO2 has four vibrational modes as 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 are degenerate 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 the infrared IR spectrum the antisymmetric stretching mode at wavenumber 2349 cm 1 wavelength 4 25 mm and the degenerate pair of bending modes at 667 cm 1 wavelength 15 mm The symmetric stretching mode does not create an electric dipole so is not observed in IR spectroscopy but it is detected in by Raman spectroscopy at 1388 cm 1 wavelength 7 2 mm 20 In the gas phase carbon dioxide molecules undergo significant vibrational motions and do not keep a fixed structure However in a Coulomb explosion imaging experiment an instantaneous image of the molecular structure can be deduced Such an experiment 21 has been performed for carbon dioxide The result of this experiment and the conclusion of theoretical calculations 22 based on an ab initio potential energy surface of 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 motion volume element vanishes for linear geometries 22 This is so for all molecules except diatomics In aqueous solution See also Carbonic acid Carbon dioxide is soluble in water in which it reversibly forms H2CO3 carbonic acid which is a weak acid since its ionization in water is incomplete CO 2 H 2 O H 2 CO 3 displaystyle ce CO2 H2O lt gt H2CO3 The hydration equilibrium constant of carbonic acid is at 25 C K h H 2 CO 3 CO 2 aq 1 70 10 3 displaystyle 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 CO2 molecules not affecting the pH The relative concentrations of CO2 H2CO3 and the deprotonated forms HCO 3 bicarbonate and CO2 3 carbonate depend on the pH As shown in a Bjerrum plot in neutral or slightly alkaline water pH gt 6 5 the bicarbonate form predominates gt 50 becoming the most prevalent gt 95 at the pH of seawater In very alkaline water pH gt 10 4 the predominant gt 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 H 2 CO 3 HCO 3 H displaystyle ce H2CO3 lt gt HCO3 H Ka1 2 5 10 4 mol L pKa1 3 6 at 25 C 19 This is the true first acid dissociation constant defined as K a 1 HCO 3 H H 2 CO 3 displaystyle K mathrm a1 frac ce HCO3 H ce H2CO3 where the denominator includes only covalently bound H2CO3 and does not include hydrated CO2 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 CO2 is present as carbonic acid so that K a 1 a p p a r e n t HCO 3 H H 2 CO 3 CO 2 aq displaystyle K mathrm a1 rm apparent frac ce HCO3 H ce H2CO3 CO2 aq Since most of the dissolved CO2remains as CO2 molecules Ka1 apparent has a much larger denominator and a much smaller value than the true Ka1 23 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 CO2 3 HCO 3 CO 3 2 H displaystyle ce HCO3 lt gt CO3 2 H Ka2 4 69 10 11 mol L pKa2 10 329In organisms carbonic acid production is catalysed by the enzyme carbonic anhydrase Chemical reactions of CO2 CO2 is a potent electrophile having an electrophilic reactivity that is comparable to benzaldehyde or strong a b unsaturated carbonyl compounds However unlike electrophiles of similar reactivity the reactions of nucleophiles with CO2 are thermodynamically less favored and are often found to be highly reversible 24 Only very strong nucleophiles like the carbanions provided by Grignard reagents and organolithium compounds react with CO2 to give carboxylates MR CO 2 RCO 2 M displaystyle ce MR CO2 gt RCO2M where M Li or Mg Br and R alkyl or aryl In metal carbon dioxide complexes CO2 serves as a ligand which can facilitate the conversion of CO2 to other chemicals 25 The reduction of CO2 to CO is ordinarily a difficult and slow reaction CO 2 2 e 2 H CO H 2 O displaystyle ce CO2 2 e 2H gt CO H2O Photoautotrophs i e plants and cyanobacteria use the energy contained in sunlight to photosynthesize simple sugars from CO2 absorbed from the air and water n CO 2 n H 2 O CH 2 O n n O 2 displaystyle ce mathit n CO2 mathit n H2O gt CH2O mathit n mathit n O2 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 26 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 1 At standard temperature and pressure the density of carbon dioxide is around 1 98 kg m3 about 1 53 times that of air 27 Carbon dioxide has no liquid state at pressures below 0 51795 10 MPa 2 5 11177 99 atm At a pressure of 1 atm 0 101325 MPa the gas deposits directly to a solid at temperatures below 194 6855 30 K 2 78 4645 30 C and the solid sublimes directly to a gas above this temperature In its solid state carbon dioxide is commonly called dry ice Pressure temperature phase diagram of carbon dioxide Note that it is a log lin chart Liquid carbon dioxide forms only at pressures above 0 51795 10 MPa 2 5 11177 99 atm the triple point of carbon dioxide is 216 592 3 K 2 56 558 3 C at 0 51795 10 MPa 2 5 11177 99 atm see phase diagram The critical point is 304 128 15 K 2 30 978 15 C at 7 3773 30 MPa 2 72 808 30 atm Another form of solid carbon dioxide observed at high pressure is an amorphous glass like solid 28 This form of glass called carbonia is produced by supercooling heated CO2 at extreme pressures 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 dioxide 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 Table of thermal and physical properties of saturated liquid carbon dioxide 29 30 Temperature C Density kg m 3 Specific heat kJ kg K Kinematic viscosity m 2 s Conductivity W m K Thermal diffusivity m 2 s Prandtl Number Bulk modulus K 1 50 1156 34 1 84 1 19E 07 0 0855 4 02E 08 2 96 40 1117 77 1 88 1 18E 07 0 1011 4 81E 08 2 46 30 1076 76 1 97 1 17E 07 0 1116 5 27E 08 2 22 20 1032 39 2 05 1 15E 07 0 1151 5 45E 08 2 12 10 983 38 2 18 1 13E 07 0 1099 5 13E 08 2 2 0 926 99 2 47 1 08E 07 0 1045 4 58E 08 2 38 10 860 03 3 14 1 01E 07 0 0971 3 61E 08 2 8 20 772 57 5 9 10E 08 0 0872 2 22E 08 4 1 1 40E 0230 597 81 36 4 8 00E 08 0 0703 0 279E 08 28 7 Table of thermal and physical properties of carbon dioxide CO2 at atmospheric pressure 29 30 Temperature K Density kg m 3 Specific heat kJ kg C Dynamic viscosity kg m s Kinematic viscosity m 2 s Thermal conductivity W m C Thermal diffusivity m 2 s Prandtl Number220 2 4733 0 783 1 11E 05 4 49E 06 0 010805 5 92E 06 0 818250 2 1657 0 804 1 26E 05 5 81E 06 0 012884 7 40E 06 0 793300 1 7973 0 871 1 50E 05 8 32E 06 0 016572 1 06E 05 0 77350 1 5362 0 9 1 72E 05 1 12E 05 0 02047 1 48E 05 0 755400 1 3424 0 942 1 93E 05 1 44E 05 0 02461 1 95E 05 0 738450 1 1918 0 98 2 13E 05 1 79E 05 0 02897 2 48E 05 0 721500 1 0732 1 013 2 33E 05 2 17E 05 0 03352 3 08E 05 0 702550 0 9739 1 047 2 51E 05 2 57E 05 0 03821 3 75E 05 0 685600 0 8938 1 076 2 68E 05 3 00E 05 0 04311 4 48E 05 0 668650 0 8143 1 1 2 88E 05 3 54E 05 0 0445 4 97E 05 0 712700 0 7564 1 13E 00 3 05E 05 4 03E 05 0 0481 5 63E 05 0 717750 0 7057 1 15 3 21E 05 4 55E 05 0 0517 6 37E 05 0 714800 0 6614 1 17E 00 3 37E 05 5 10E 05 0 0551 7 12E 05 0 716Biological roleCarbon 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 release in respiration Photosynthesis and carbon fixation Overview of the Calvin cycle and carbon fixation Carbon fixation is a biochemical process by which atmospheric carbon dioxide is incorporated by plants algae and cyanobacteria into energy rich organic molecules such as glucose thus creating their 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 CO2 and ribulose bisphosphate as shown in the diagram at left RuBisCO is thought to be the single most abundant protein on Earth 31 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 32 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 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 CO2 Plants can grow as much as 50 percent faster in concentrations of 1 000 ppm CO2 when compared with ambient conditions though this assumes no change in climate and no limitation on other nutrients 33 Elevated CO2 levels cause increased growth reflected in the harvestable yield of crops with wheat rice and soybean all showing increases in yield of 12 14 under elevated CO2 in FACE experiments 34 35 Increased atmospheric CO2 concentrations result in fewer stomata developing on plants 36 which leads to reduced water usage and increased water use efficiency 37 Studies using FACE have shown that CO2 enrichment leads to decreased concentrations of micronutrients in crop plants 38 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 39 The concentration of secondary metabolites such as phenylpropanoids and flavonoids can also be altered in plants exposed to high concentrations of CO2 40 41 Plants also emit CO2 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 CO2 each year a mature forest will produce as much CO2 from respiration and decomposition of dead specimens e g fallen branches as is used in photosynthesis in growing plants 42 Contrary to the long standing view that they are carbon neutral mature forests can continue to accumulate carbon 43 and remain valuable carbon sinks helping to maintain the carbon balance of Earth s atmosphere Additionally and crucially to life on earth photosynthesis by phytoplankton consumes dissolved CO2 in the upper ocean and thereby promotes the absorption of CO2 from the atmosphere 44 Toxicity See also Carbon dioxide poisoning Symptoms of carbon dioxide toxicity by increasing volume percent in air 45 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 46 clarification needed CO2 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 and give the lungs a stuffy feeling 45 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 47 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 CO2 emissions from the nearby volcano Mount Nyiragongo 48 The Swahili term for this phenomenon is mazuku Rising levels of CO2 threatened the Apollo 13 astronauts who had to adapt cartridges from the command module to supply the carbon dioxide scrubber in the Lunar Module which they used as a lifeboat Adaptation to increased concentrations of CO2 occurs in humans including modified breathing and 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 a submarine 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 49 50 Yet other studies show a decrease in cognitive function even at much lower levels 51 52 Also with ongoing respiratory acidosis adaptation or compensatory mechanisms will be unable to reverse such condition Below 1 There are few studies of the health effects of long term continuous CO2 exposure on humans and animals at levels below 1 Occupational CO2 exposure limits have been set in the United States at 0 5 5000 ppm for an eight hour period 53 At this CO2 concentration International Space Station crew experienced headaches lethargy mental slowness emotional irritation and sleep disruption 54 Studies in animals at 0 5 CO2 have demonstrated kidney calcification and bone loss after eight weeks of exposure 55 A study of humans exposed in 2 5 hour sessions demonstrated significant negative effects on cognitive abilities at concentrations as low as 0 1 1000 ppm CO2 likely due to CO2 induced increases in cerebral blood flow 51 Another study observed a decline in basic activity level and information usage at 1000 ppm when compared to 500 ppm 52 However a review of the literature found that most studies on the phenomenon of carbon dioxide induced cognitive impairment to have a small effect on high level decision making and most of the studies were confounded by inadequate study designs environmental comfort uncertainties in exposure doses and differing cognitive assessments used 56 Similarly a study on the effects of the concentration of CO2 in 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 CO2 declined to safe levels 0 2 57 58 Ventilation A carbon dioxide sensor that measures CO2 concentration using a nondispersive infrared sensor Poor ventilation is one of the main causes of excessive CO2 concentrations in closed spaces leading to poor indoor air quality Carbon dioxide differential above outdoor concentrations at steady state conditions when the occupancy and ventilation system operation are sufficiently long that CO2 concentration has stabilized are sometimes used to estimate ventilation rates per person citation needed Higher CO2 concentrations are associated with occupant health comfort and performance degradation 59 60 ASHRAE Standard 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 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 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 61 A similar accident occurred in 2018 when a woman died from CO2 fumes emanating from the large amount of dry ice she was transporting in her car 62 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 near Rapolano Terme in Tuscany Italy situated in a bowl shaped depression about 100 m 330 ft in diameter concentrations of CO2 rise to above 75 overnight sufficient to kill insects and small animals After sunrise the gas is dispersed by convection 63 High concentrations of CO2 produced by disturbance of deep lake water saturated with CO2 are thought to have caused 37 fatalities at Lake Monoun Cameroon in 1984 and 1700 casualties at Lake Nyos Cameroon in 1986 64 Human physiologyContent Reference ranges or averages for partial pressures of carbon dioxide abbreviated pCO2 Blood compartment kPa mm Hg Venous blood carbon dioxide 5 5 6 8 41 51 65 41 51 65 Alveolar pulmonarygas pressures 4 8 36 36Arterial blood carbon dioxide 4 7 6 0 35 45 65 35 45 65 The body produces approximately 2 3 pounds 1 0 kg of carbon dioxide per day per person 66 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 resulting in lower concentrations in the arteries The 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 67 In humans the blood carbon dioxide contents is shown in the adjacent table Transport in the blood CO2 is carried in blood in three different ways Exact percentages vary between arterial and venous blood Majority about 70 to 80 is converted to bicarbonate ions HCO 3 by the enzyme carbonic anhydrase in the red blood cells 68 by the reaction CO2 H2O H2CO3 H HCO 3 5 10 is dissolved in blood plasma 68 5 10 is bound to hemoglobin as carbamino compounds 68 Hemoglobin the main oxygen carrying molecule in red blood cells carries both oxygen and carbon dioxide However the CO2 bound to hemoglobin does not bind to the same site as oxygen Instead it combines with the N terminal groups on the four globin chains However because of allosteric effects on the hemoglobin molecule the binding of CO2 decreases the amount of oxygen that is bound for a given partial pressure of oxygen This 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 CO2 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 concentration is high the capillaries expand to allow a greater blood flow to that tissue 69 Bicarbonate ions are crucial for regulating blood pH A person s breathing rate influences the level of CO2 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 70 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 68 The respiratory centers try to maintain an arterial CO2 pressure of 40 mm Hg With intentional hyperventilation the CO2 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 71 Concentrations and role in the environmentAtmosphere Further information Carbon cycle This section is an excerpt from Carbon dioxide in Earth s atmosphere edit Atmospheric CO2 concentrations measured at Mauna Loa Observatory from 1958 to 2022 also called the Keeling Curve Carbon dioxide concentrations have varied widely over the Earth s 4 54 billion year history However in 2013 the daily mean concentration of CO2 in the atmosphere surpassed 400 parts per million ppmv 72 this level has never been reached since the mid Pliocene 2 to 4 million years ago 73 Carbon dioxide in Earth s atmosphere is a trace gas that plays an integral part in the greenhouse effect carbon cycle photosynthesis and oceanic carbon cycle It is one of several greenhouse gases in Earth s atmosphere that are contributing to climate change due to increasing emissions of greenhouse gases from human activities The current global average concentration of CO2 in the atmosphere is 421 ppm as of May 2022 74 This is an increase of 50 since the start of the Industrial Revolution up from 280 ppm during the 10 000 years prior to the mid 18th century 75 74 76 The increase is due to human activity 77 Burning fossil fuels is the main cause of these increased CO2 concentrations and also the main cause of climate change 78 Other large anthropogenic sources include cement production deforestation and biomass burning While transparent to visible light carbon dioxide is a greenhouse gas absorbing and emitting infrared radiation at its two infrared active vibrational frequencies CO2 absorbs and emits infrared radiation at wavelengths of 4 26 mm 2 347 cm 1 asymmetric stretching vibrational mode and 14 99 mm 667 cm 1 bending vibrational mode It plays a significant role in influencing Earth s surface temperature through the greenhouse effect 79 Light emission from the Earth s surface is most intense in the infrared region between 200 and 2500 cm 1 80 as opposed to light emission from the much hotter Sun which is most intense in the visible region Absorption of infrared light at the vibrational frequencies of atmospheric CO2 traps energy near the surface warming the surface and the lower atmosphere Less energy reaches the upper atmosphere which is therefore cooler because of this absorption 81 Increases in atmospheric concentrations of CO2 and other long lived greenhouse gases such as methane nitrous oxide and ozone increase the absorption and emission of infrared radiation by the atmosphere causing the observed rise in average global temperature and ocean acidification Another direct effect is the CO2 fertilization effect These changes cause a range of indirect effects of climate change on the physical environment ecosystems and human societies Carbon dioxide exerts a larger overall warming influence than all of the other greenhouse gases combined 76 It has an atmospheric lifetime that increases with the cumulative amount of fossil carbon extracted and burned due to the imbalance that this activity has imposed on Earth s fast carbon cycle 82 This means that some fraction a projected 20 35 of the fossil carbon transferred thus far will persist in the atmosphere as elevated CO2 levels for many thousands of years after these carbon transfer activities begin to subside 83 84 85 The carbon cycle is a biogeochemical cycle in which carbon is exchanged between the Earth s oceans soil rocks and the biosphere Plants and other photoautotrophs use solar energy to produce carbohydrate from atmospheric carbon dioxide and water by photosynthesis Almost all other organisms depend on carbohydrate derived from photosynthesis as their primary source of energy and carbon compounds The present atmospheric concentration of CO2 is the highest for 14 million years 86 Concentrations of CO2 in the atmosphere were as high as 4 000 ppm during the Cambrian period about 500 million years ago when the concentration was 20 times greater than today and as low as 180 ppm during the Quaternary glaciation of the last two million years 75 Reconstructed temperature records for the last 420 million years indicate that atmospheric CO2 concentrations peaked at 2 000 ppm during the Devonian 400 Ma period and again in the Triassic 220 200 Ma period and was four times current levels during the Jurassic period 201 145 Ma 87 88 Annual CO2 flows from anthropogenic sources left into Earth s atmosphere land and ocean sinks right since the 1960s Units in equivalent gigatonnes carbon per year 89 Oceans Main articles Carbon cycle and Ocean acidification Ocean acidification Carbon dioxide dissolves in the ocean to form carbonic acid H2CO3 bicarbonate HCO3 and carbonate CO32 There is about fifty times as much carbon dioxide dissolved in the oceans as exists in the atmosphere The oceans act as an enormous carbon sink and have taken up about a third of CO2 emitted by human activity 90 This section is an excerpt from Ocean acidification edit Ocean acidification is the reduction in the pH value of the Earth s ocean Between 1751 and 2021 the average pH value of the ocean surface has decreased from approximately 8 25 to 8 14 91 The root cause of ocean acidification is carbon dioxide emissions from human activities which have led to atmospheric carbon dioxide CO2 levels of more than 410 ppm in 2020 The oceans absorb CO2 from the atmosphere This leads to the formation of carbonic acid H2CO3 which dissociates into a bicarbonate ion HCO 3 and a hydrogen ion H The free hydrogen ions H decrease the pH of the ocean therefore increasing acidity this does not mean that seawater is acidic yet it is still alkaline with a pH higher than 8 A decrease in pH corresponds to a decrease in the concentration of carbonate ions which are the main building block for calcium carbonate CaCO3 shells and skeletons Marine calcifying organisms like mollusks oysters and corals are particularly affected by this as they rely on calcium carbonate to build shells and skeletons 92 Pterapod shell dissolved in seawater adjusted to an ocean chemistry projected for the year 2100 This section is an excerpt from Ocean 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 of calcium carbonate CaCO3 92 This process is called calcification and is important to the biology and survival of a wide range of marine organisms Calcification involves the precipitation of dissolved ions into solid CaCO3 structures structures for many marine organisms such as coccolithophores foraminifera crustaceans mollusks etc After they are formed these CaCO3 structures are vulnerable to dissolution unless the surrounding seawater contains saturating concentrations of carbonate ions CO32 Given the current pH of the ocean around 8 14 of the extra carbon dioxide added into the ocean very little 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 93 creating an imbalance in the reaction HCO3 CO32 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 CO32 CaCO3 Hydrothermal vents Carbon dioxide is also introduced into the oceans through hydrothermal vents The Champagne hydrothermal vent found at the Northwest Eifuku volcano in the Mariana Trench produces almost pure liquid carbon dioxide one of only two known sites in the world as of 2004 the other being in the Okinawa Trough 94 The finding of a submarine lake of liquid carbon dioxide in the Okinawa Trough was reported in 2006 95 ProductionBiological processes Carbon dioxide is a by product of the fermentation of sugar in the brewing of beer whisky and other alcoholic beverages and in the production of bioethanol Yeast metabolizes sugar to produce CO2 and ethanol also known as alcohol as follows C 6 H 12 O 6 2 CO 2 2 C 2 H 5 OH displaystyle ce C6H12O6 gt 2 CO2 2 C2H5OH All aerobic organisms produce CO2 when they oxidize carbohydrates fatty acids and proteins 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 6 H 12 O 6 6 O 2 6 CO 2 6 H 2 O displaystyle ce C6H12O6 6 O2 gt 6 CO2 6 H2O Anaerobic organisms decompose organic material producing methane and carbon dioxide together with traces of other compounds 96 Regardless of the type of organic material the production of gases follows well defined kinetic 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 97 Industrial processes Carbon dioxide can be obtained by distillation from 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 98 Combustion The combustion of all carbon based fuels such as methane 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 and oxygen CH 4 2 O 2 CO 2 2 H 2 O displaystyle ce CH4 2 O2 gt CO2 2 H2O Iron is reduced from its oxides with coke in a blast furnace producing pig iron and carbon dioxide 99 By product from hydrogen production Carbon dioxide is a byproduct of the industrial production of hydrogen by steam reforming and the water gas shift reaction in ammonia production These processes begin with the reaction of water and natural gas mainly methane 100 This is a major source of food grade carbon dioxide for use in carbonation of beer and soft drinks and is also used for stunning animals such as poultry 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 101 Thermal decomposition of limestone It is produced by thermal decomposition of limestone CaCO3 by heating calcining at about 850 C 1 560 F in the manufacture of quicklime calcium oxide CaO a compound that has many industrial uses CaCO 3 CaO CO 2 displaystyle ce CaCO3 gt CaO CO2 Acids liberate CO2 from most metal carbonates Consequently it may be obtained directly from natural carbon dioxide springs where it is produced by the action of acidified water on limestone or dolomite The reaction between hydrochloric acid and calcium carbonate limestone or chalk is shown below CaCO 3 2 HCl CaCl 2 H 2 CO 3 displaystyle ce CaCO3 2HCl gt CaCl2 H2CO3 The carbonic acid H2CO3 then decomposes to water and CO2 H 2 CO 3 CO 2 H 2 O displaystyle ce H2CO3 gt 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 usesCarbon dioxide is used by the food industry the oil industry and the chemical industry 98 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 This section needs expansion You can help by adding to it July 2014 In the chemical industry carbon dioxide is mainly consumed as an ingredient in the production of urea with a smaller fraction being used to produce methanol and a range of other products 102 Some carboxylic acid derivatives such as sodium salicylate are prepared using CO2 by the Kolbe Schmitt reaction 103 In addition to conventional processes using CO2 for 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 CO2 emissions 104 Agriculture Plants require carbon dioxide to conduct photosynthesis The atmospheres of greenhouses may if of large size must be enriched with additional CO2 to sustain and increase the rate of plant growth 105 106 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 107 Foods Carbon dioxide bubbles in a soft drink 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 108 listed as E number E290 US 109 and Australia and New Zealand 110 listed by its INS number 290 A candy called Pop Rocks is pressurized with carbon dioxide gas 111 at about 4 000 kPa 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 112 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 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 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 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 4 converts to carbonic acid leading to a sour taste and also the dissolved carbon dioxide induces a somatosensory response 113 Winemaking Dry ice used to preserve grapes after harvest Carbon dioxide in the form of dry ice is often used during the cold soak phase in winemaking to cool clusters of grapes quickly after picking to help prevent spontaneous fermentation by wild yeast 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 the grape must and thus the alcohol concentration in the finished wine 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 Stunning animals Carbon dioxide is often used to stun animals before slaughter 114 Stunning may be a misnomer as the animals are not knocked out immediately and may suffer distress 115 116 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 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 citation needed When used for MIG welding CO2 use is sometimes referred to as MAG welding for Metal Active Gas as CO2 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 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 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 CO2 are also sold as supplies of compressed gas for air guns paintball markers guns inflating bicycle tires and for making carbonated water 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 117 and in the decaffeination of coffee beans Fire extinguisher Use of a CO2 fire 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 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 they do not cool the burning substances significantly and when the carbon dioxide disperses they can catch fire upon exposure to atmospheric oxygen They are mainly used in server rooms 118 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 119 International Maritime Organization standards 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 CO2 systems identified 51 incidents between 1975 and the date of the report 2000 causing 72 deaths and 145 injuries 120 Supercritical CO2 as solvent See also Supercritical carbon dioxide and Green chemistry Liquid carbon dioxide is a good solvent for many lipophilic organic compounds and is used to remove caffeine from coffee 17 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 also used by some dry cleaners for this reason It is used in the preparation of some aerogels because 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 after apnea and to stabilize the O2 CO2 balance in blood Carbon dioxide can be mixed with up to 50 oxygen forming an inhalable gas this is known as Carbogen and has a variety of medical and research uses Another medical use are the mofette dry spas that use carbon dioxide from post volcanic discharge for therapeutic purposes Energy Supercritical CO2 is used as the working fluid in the Allam power cycle engine Fossil fuel 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 7 23 additional to primary extraction 121 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 122 In mature oil fields extensive pipe networks are used to carry the carbon dioxide to the injection points 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 rely on the removal of water to reduce pressure to make the coal seam release its trapped methane 123 Bio transformation into fuel Main article Carbon capture and utilization It has been proposed that CO2 from power generation be bubbled into ponds to stimulate growth of algae that could then be converted into biodiesel fuel 124 A strain of the cyanobacterium Synechococcus elongatus has been genetically engineered to produce the fuels isobutyraldehyde and isobutanol from CO2 using photosynthesis 125 Researchers have developed a process called electrolysis using enzymes isolated from bacteria to power the chemical reactions which convert CO2 into fuels 126 127 128 Refrigerant See also Refrigerant and Sustainable 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 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 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 citation needed of dichlorodifluoromethane R12 a chlorofluorocarbon CFC compound CO2 might enjoy a renaissance because one of the main substitutes to CFCs 1 1 1 2 tetrafluoroethane R134a a hydrofluorocarbon HFC compound contributes to climate change more than CO2 does CO2 physical 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 CO2 systems 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 GWP of 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 Cola has fielded CO2 based beverage coolers and the U S Army is interested in CO2 refrigeration and heating technology 129 130 Minor uses A carbon dioxide laser Carbon dioxide is the lasing medium in a 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 131 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 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 Used as the primary coolant in the British advanced gas cooled reactor for nuclear power generation Carbon dioxide induction is commonly used for the euthanasia of laboratory research animals Methods to administer CO2 include placing animals directly into a closed prefilled chamber containing CO2 or exposure to a gradually increasing concentration of CO2 The American 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 132 5 31 Percentages of CO2 vary for different species based on identified optimal percentages to minimize distress 132 22 Carbon dioxide is also used in several related cleaning and surface preparation techniques History of discovery Crystal structure of dry ice Carbon dioxide was the first gas to be described as a discrete substance In about 1640 133 the Flemish chemist Jan Baptist van Helmont observed that when he burned charcoal in a closed vessel the mass of the resulting ash was 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 or wild spirit spiritus sylvestris 134 The properties of carbon dioxide were further studied in the 1750s by the Scottish physician Joseph Black He found that limestone calcium carbonate could be heated or treated with acids to 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 through limewater a saturated aqueous solution of calcium hydroxide it would precipitate calcium carbonate He used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation In 1772 English chemist Joseph Priestley published a paper entitled Impregnating Water with Fixed Air in which he described a process of dripping sulfuric acid or oil of vitriol as 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 135 Carbon dioxide was first liquefied at elevated pressures in 1823 by Humphry Davy and Michael Faraday 136 The earliest description of solid carbon dioxide dry ice was given by the French inventor Adrien 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 137 138 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