Carbonic acid

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Carbonic acid
Carbonic-acid-2D.svg
Carbonic-acid-3D-balls.png
Names
IUPAC name
Carbonic acid [1]
Other names
  • Oxidocarboxylic acid
  • Hydroxyformic acid
  • Hydroxymethanoic acid
  • Carbonylic acid
  • Hydroxycarboxylic acid
  • Dihydroxycarbonyl
  • Carbon dioxide solution
  • Aerial acid
  • Metacarbonic acid
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.133.015 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 610-295-3
25554
KEGG
PubChem CID
UNII
  • InChI=1S/CH2O3/c2-1(3)4/h(H2,2,3,4) Yes check.svgY
    Key: BVKZGUZCCUSVTD-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/H2O3/c2-1(3)4/h(H2,2,3,4)
    Key: BVKZGUZCCUSVTD-UHFFFAOYAU
  • O=C(O)O
Properties
H
2CO
3
AppearanceColorless gas
Melting point −53 °C (−63 °F; 220 K) [2] (sublimes)
Boiling point 127 °C (261 °F; 400 K) (decomposes)
Reacts to form carbon dioxide and water
Acidity (pKa)
  • pKa1 = 3.75 (25 °C; anhydrous) [3]
  • pKa1 = 6.35 (hydrous) [3]
  • pKa2 = 10.33 [3]
Conjugate base Bicarbonate, carbonate
Hazards
NFPA 704 (fire diamond)
NFPA 704.svgHealth 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g. sodium chlorideFlammability 0: Will not burn. E.g. waterInstability 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calciumSpecial hazards (white): no code
0
0
1
Structure
monoclinic
p21/c, No. 14
-
a = 5.392 Å, b = 6.661 Å, c = 5.690 Å
α = 90°, β = 92.66°, γ = 90° [4]
(D
2CO
3 at 1.85 GPa, 298 K)
204.12 Å3
4 formula per cell
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

Carbonic acid is a chemical compound with the chemical formula H 2 C O 3. The molecule rapidly converts to water and carbon dioxide in the presence of water. However, in the absence of water, it is quite stable at room temperature. [5] [6] The interconversion of carbon dioxide and carbonic acid is related to the breathing cycle of animals and the acidification of natural waters. [4]

In biochemistry and physiology, the name "carbonic acid" is sometimes applied to aqueous solutions of carbon dioxide. These chemical species play an important role in the bicarbonate buffer system, used to maintain acid–base homeostasis. [7]

Terminology in biochemical literature

In chemistry, the term "carbonic acid" strictly refers to the chemical compound with the formula H
2CO
3. Some biochemistry literature effaces the distinction between carbonic acid and carbon dioxide dissolved in extracellular fluid.

In physiology, carbon dioxide excreted by the lungs may be called volatile acid or respiratory acid.

Anhydrous carbonic acid

At ambient temperatures, pure carbonic acid is a stable gas. [6] There are two main methods to produce anhydrous carbonic acid: reaction of hydrogen chloride and potassium bicarbonate at 100 K in methanol and proton irradiation of pure solid carbon dioxide. [2] Chemically, it behaves as a diprotic Brønsted acid. [8] [9]

Carbonic acid monomers exhibit three conformational isomers: cis–cis, cis–trans, and trans–trans. [10]

At low temperatures and atmospheric pressure, solid carbonic acid is amorphous and lacks Bragg peaks in X-ray diffraction. [11] But at high pressure, carbonic acid crystallizes, and modern analytical spectroscopy can measure its geometry.

According to neutron diffraction of dideuterated carbonic acid (D
2CO
3) in a hybrid clamped cell (Russian alloy/copper-beryllium) at 1.85 GPa, the molecules are planar and form dimers joined by pairs of hydrogen bonds. All three C-O bonds are nearly equidistant at 1.34  Å, intermediate between typical C-O and C=O distances (respectively 1.43 and 1.23 Å). The unusual C-O bond lengths are attributed to delocalized π bonding in the molecule's center and extraordinarily strong hydrogen bonds. The same effects also induce a very short O—O separation (2.13 Å), through the 136° O-H-O angle imposed by the doubly hydrogen-bonded 8-membered rings. [4] Longer O—O distances are observed in strong intramolecular hydrogen bonds, e.g. in oxalic acid, where the distances exceed 2.4 Å. [11]

In aqueous solution

In even a slight presence of water, carbonic acid dehydrates to carbon dioxide and water, which then catalyzes further decomposition. [6] For this reason, carbon dioxide can be considered the carbonic acid anhydride.

The hydration equilibrium constant at 25 °C is [H
2CO
3]/[CO2] ≈ 1.7×10−3 in pure water [12] and ≈ 1.2×10−3 in seawater. [13] Hence the majority of carbon dioxide at geophysical or biological air-water interfaces does not convert to carbonic acid, remaining dissolved CO2 gas. However, the uncatalyzed equilibrium is reached quite slowly: the rate constants are 0.039  s −1 for hydration and 23 s−1 for dehydration.

In biological solutions

In the presence of the enzyme carbonic anhydrase, equilibrium is instead reached rapidly, and the following reaction takes precedence: [14]

When the created carbon dioxide exceeds its solubility, gas evolves and a third equilibrium must also be taken into consideration. The equilibrium constant for this reaction is defined by Henry's law.

The two reactions can be combined for the equilibrium in solution: When Henry's law is used to calculate the denominator care is needed with regard to units since Henry's law constant can be commonly expressed with 8 different dimensionalities. [15]

In water pH control

In wastewater treatment and agriculture irrigation, carbonic acid is used to acidify the water similar to sulfuric acid and sulfurous acid produced by sulfur burners. [16]

Under high CO2 partial pressure

In the beverage industry, sparkling or "fizzy water" is usually referred to as carbonated water. It is made by dissolving carbon dioxide under a small positive pressure in water. Many soft drinks treated the same way effervesce.

Significant amounts of molecular H
2CO
3 exist in aqueous solutions subjected to pressures of multiple gigapascals (tens of thousands of atmospheres) in planetary interiors. [17] [18] Pressures of 0.6–1.6  GPa at 100  K, and 0.75–1.75 GPa at 300 K are attained in the cores of large icy satellites such as Ganymede, Callisto, and Titan, where water and carbon dioxide are present. Pure carbonic acid, being denser, is expected to have sunk under the ice layers and separate them from the rocky cores of these moons. [19]

Relationship to bicarbonate and carbonate

Bjerrum plot of speciation for a hypothetical monoprotic acid: AH concentration as a function of the difference between pK and pH Weak acid speciation.svg
Bjerrum plot of speciation for a hypothetical monoprotic acid: AH concentration as a function of the difference between pK and pH

Carbonic acid is the formal Brønsted–Lowry conjugate acid of the bicarbonate anion, stable in alkaline solution. The protonation constants have been measured to great precision, but depend on overall ionic strength I. The two equilibria most easily measured are as follows: where brackets indicate the concentration of species. At 25 °C, these equilibria empirically satisfy [20] log(β1) decreases with increasing I, as does log(β2). In a solution absent other ions (e.g. I = 0), these curves imply the following stepwise dissociation constants: Direct values for these constants in the literature include pK1 = 6.35 and pK2 - pK1 = 3.49. [21]

To interpret these numbers, note that two chemical species in an acid equilibrium are equiconcentrated when pK = pH. In particular, the extracellular fluid (cytosol) in biological systems exhibits pH ≈ 7.2, so that carbonic acid will be almost 50%-dissociated at equilibrium.

Ocean acidification

Carbonate speciation in seawater (ionic strength 0.7 mol/dm ). The expected change shown is due to the current anthropogenic increase in atmospheric carbon dioxide concentration. Carbonate system of seawater.svg
Carbonate speciation in seawater (ionic strength 0.7 mol/dm ). The expected change shown is due to the current anthropogenic increase in atmospheric carbon dioxide concentration.

The Bjerrum plot shows typical equilibrium concentrations, in solution, in seawater, of carbon dioxide and the various species derived from it, as a function of pH. [8] [9] As human industrialization has increased the proportion of carbon dioxide in Earth's atmosphere, the proportion of carbon dioxide dissolved in sea- and freshwater as carbonic acid is also expected to increase. This rise in dissolved acid is also expected to acidify those waters, generating a decrease in pH. [22] [23] It has been estimated that the increase in dissolved carbon dioxide has already caused the ocean's average surface pH to decrease by about 0.1 from pre-industrial levels.

Further reading

Related Research Articles

<span class="mw-page-title-main">Bicarbonate</span> Polyatomic anion

In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula HCO−
3.

<span class="mw-page-title-main">Carbon dioxide</span> Chemical compound with formula CO₂

Carbon dioxide is a chemical compound with the chemical formula CO2. It is 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 and at normally-encountered concentrations it is odorless. As the source of carbon in the carbon cycle, atmospheric CO2 is the primary carbon source for life on Earth. In the air, carbon dioxide is transparent to visible light but absorbs infrared radiation, acting as a greenhouse gas. Carbon dioxide is soluble in water and is found in groundwater, lakes, ice caps, and seawater.

<span class="mw-page-title-main">Carbonate</span> Salt or ester of carbonic acid

A carbonate is a salt of carbonic acid,, characterized by the presence of the carbonate ion, a polyatomic ion with the formula CO2−3. The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate groupO=C(−O−)2.

<span class="mw-page-title-main">Hydroxide</span> Chemical compound

Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, a ligand, a nucleophile, and a catalyst. The hydroxide ion forms salts, some of which dissociate in aqueous solution, liberating solvated hydroxide ions. Sodium hydroxide is a multi-million-ton per annum commodity chemical. The corresponding electrically neutral compound HO• is the hydroxyl radical. The corresponding covalently bound group –OH of atoms is the hydroxy group. Both the hydroxide ion and hydroxy group are nucleophiles and can act as catalysts in organic chemistry.

<span class="mw-page-title-main">Calcium carbonate</span> Chemical compound

Calcium carbonate is a chemical compound with the chemical formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skeletons and pearls. Materials containing much calcium carbonate or resembling it are described as calcareous. Calcium carbonate is the active ingredient in agricultural lime and is produced when calcium ions in hard water react with carbonate ions to form limescale. It has medical use as a calcium supplement or as an antacid, but excessive consumption can be hazardous and cause hypercalcemia and digestive issues.

In chemistry, an acid dissociation constant is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction

<span class="mw-page-title-main">Arterial blood gas test</span> A test of blood taken from an artery that measures the amounts of certain dissolved gases

An arterial blood gas (ABG) test, or arterial blood gas analysis (ABGA) measures the amounts of arterial gases, such as oxygen and carbon dioxide. An ABG test requires that a small volume of blood be drawn from the radial artery with a syringe and a thin needle, but sometimes the femoral artery in the groin or another site is used. The blood can also be drawn from an arterial catheter.

The self-ionization of water (also autoionization of water, autoprotolysis of water, autodissociation of water, or simply dissociation of water) is an ionization reaction in pure water or in an aqueous solution, in which a water molecule, H2O, deprotonates (loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH−. The hydrogen nucleus, H+, immediately protonates another water molecule to form a hydronium cation, H3O+. It is an example of autoprotolysis, and exemplifies the amphoteric nature of water.

<span class="mw-page-title-main">Ammonium bicarbonate</span> Chemical compound

Ammonium bicarbonate is an inorganic compound with formula (NH4)HCO3. The compound has many names, reflecting its long history. Chemically speaking, it is the bicarbonate salt of the ammonium ion. It is a colourless solid that degrades readily to carbon dioxide, water and ammonia.

In chemistry and biochemistry, the Henderson–Hasselbalch equation relates the pH of a chemical solution of a weak acid to the numerical value of the acid dissociation constant, Ka, of acid and the ratio of the concentrations, of the acid and its conjugate base in an equilibrium.

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. The pedosphere is the skin of the Earth and only develops when there is a dynamic interaction between the atmosphere, biosphere, lithosphere and the hydrosphere. The pedosphere is the foundation of terrestrial life on Earth.

<span class="mw-page-title-main">Alkalinity</span> Capacity of water to resist changes in pH that would make the water more acidic

Alkalinity (from Arabic: القلوية, romanized: al-qaly, lit. 'ashes of the saltwort') is the capacity of water to resist acidification. It should not be confused with basicity, which is an absolute measurement on the pH scale. Alkalinity is the strength of a buffer solution composed of weak acids and their conjugate bases. It is measured by titrating the solution with an acid such as HCl until its pH changes abruptly, or it reaches a known endpoint where that happens. Alkalinity is expressed in units of concentration, such as meq/L (milliequivalents per liter), μeq/kg (microequivalents per kilogram), or mg/L CaCO3 (milligrams per liter of calcium carbonate). Each of these measurements corresponds to an amount of acid added as a titrant.

<span class="mw-page-title-main">Grotthuss mechanism</span> Protons hopping across hydrogen bonds between hydronium ions and water molecules

The Grotthuss mechanism is a model for the process by which an 'excess' proton or proton defect diffuses through the hydrogen bond network of water molecules or other hydrogen-bonded liquids through the formation and concomitant cleavage of covalent bonds involving neighboring molecules.

Acid–base homeostasis is the homeostatic regulation of the pH of the body's extracellular fluid (ECF). The proper balance between the acids and bases in the ECF is crucial for the normal physiology of the body—and for cellular metabolism. The pH of the intracellular fluid and the extracellular fluid need to be maintained at a constant level.

<span class="mw-page-title-main">Bicarbonate buffer system</span> Buffer system that maintains pH balance in humans

The bicarbonate buffer system is an acid-base homeostatic mechanism involving the balance of carbonic acid (H2CO3), bicarbonate ion (HCO−
3), and carbon dioxide (CO2) in order to maintain pH in the blood and duodenum, among other tissues, to support proper metabolic function. Catalyzed by carbonic anhydrase, carbon dioxide (CO2) reacts with water (H2O) to form carbonic acid (H2CO3), which in turn rapidly dissociates to form a bicarbonate ion (HCO−
3 ) and a hydrogen ion (H+) as shown in the following reaction:

<span class="mw-page-title-main">Acid–base disorder</span> Abnormality of the human bodys normal balance of acids and bases

Acid–base imbalance is an abnormality of the human body's normal balance of acids and bases that causes the plasma pH to deviate out of the normal range. In the fetus, the normal range differs based on which umbilical vessel is sampled. It can exist in varying levels of severity, some life-threatening.

<span class="mw-page-title-main">Bjerrum plot</span> Graph of polyprotic acid concentration compared to pH

A Bjerrum plot (named after Niels Bjerrum), sometimes also known as a Sillén diagram (after Lars Gunnar Sillén), or a Hägg diagram (after Gunnar Hägg) is a graph of the concentrations of the different species of a polyprotic acid in a solution, as a function of pH, when the solution is at equilibrium. Due to the many orders of magnitude spanned by the concentrations, they are commonly plotted on a logarithmic scale. Sometimes the ratios of the concentrations are plotted rather than the actual concentrations. Occasionally H+ and OH− are also plotted.

<span class="mw-page-title-main">Carbonic anhydrase</span> Class of enzymes

The carbonic anhydrases form a family of enzymes that catalyze the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid. The active site of most carbonic anhydrases contains a zinc ion. They are therefore classified as metalloenzymes. The enzyme maintains acid-base balance and helps transport carbon dioxide.

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<span class="mw-page-title-main">Thomas Loerting</span> Austrian chemist

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References

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  3. 1 2 3 Perrin, D. D., ed. (1982) [1969]. Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution. IUPAC Chemical Data (2nd ed.). Oxford: Pergamon (published 1984). "Carbonic Acid, H2CO3" entry. ISBN   0-08-029214-3. LCCN   82-16524.
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  5. ↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 310. ISBN   978-0-08-037941-8.
  6. 1 2 3 Loerting, Thomas; Tautermann, Christofer; Kroemer, Romano T.; Kohl, Ingrid; Hallbrucker, Andreas; Mayer, Erwin; Liedl, Klaus R.; Loerting, Thomas; Tautermann, Christofer; Kohl, Ingrid; Hallbrucker, Andreas; Erwin, Mayer; Liedl, Klaus R. (2000). "On the Surprising Kinetic Stability of Carbonic Acid (H2CO3)". Angewandte Chemie International Edition. 39 (5): 891–4. doi:10.1002/(SICI)1521-3773(20000303)39:5<891::AID-ANIE891>3.0.CO;2-E. PMID   10760883.
  7. ↑ Acid-Base Physiology 2.1 – Acid-Base Balance by Kerry Brandis.
  8. 1 2 Pangotra, Dhananjai; Csepei, Lénárd-István; Roth, Arne; Ponce de León, Carlos; Sieber, Volker; Vieira, Luciana (2022). "Anodic production of hydrogen peroxide using commercial carbon materials". Applied Catalysis B: Environmental. 303: 120848. doi:10.1016/j.apcatb.2021.120848. S2CID   240250750.
  9. 1 2 Andersen, C. B. (2002). "Understanding carbonate equilibria by measuring alkalinity in experimental and natural systems". Journal of Geoscience Education. 50 (4): 389–403. Bibcode:2002JGeEd..50..389A. doi:10.5408/1089-9995-50.4.389. S2CID   17094010.
  10. ↑ Loerting, Thomas; Bernard, Juergen (2010). "Aqueous Carbonic Acid (H2CO3)". ChemPhysChem (11): 2305–9. doi:10.1002/cphc.201000220.
  11. 1 2 Winkel, Katrin; Hage, Wolfgang; Loerting, Thomas; Price, Sarah L.; Mayer, Erwin (2007). "Carbonic Acid: From Polyamorphism to Polymorphism". Journal of the American Chemical Society. 129 (45): 13863–71. doi:10.1021/ja073594f. PMID   17944463.
  12. ↑ Housecroft, C.E.; Sharpe, A.G. (2005). Inorganic Chemistry (2nd ed.). Prentice-Pearson-Hall. p. 368. ISBN   0-13-039913-2. OCLC   56834315.
  13. ↑ Soli, A. L.; R. H. Byrne (2002). "CO2 system hydration and dehydration kinetics and the equilibrium CO2/H2CO3 ratio in aqueous NaCl solution". Marine Chemistry. 78 (2–3): 65–73. doi:10.1016/S0304-4203(02)00010-5.
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  15. ↑ Sander, Rolf; Acree, William E.; Visscher, Alex De; Schwartz, Stephen E.; Wallington, Timothy J. (1 January 2022). "Henry's law constants (IUPAC Recommendations 2021)". Pure and Applied Chemistry. 94 (1): 71–85. doi: 10.1515/pac-2020-0302 . ISSN   1365-3075.
  16. ↑ Meneses, Adolfo (19 November 2024). "Irrigation water acidification using captured CO2; An option to traditional acidification systems" (PDF). World Ag Expo. Retrieved 19 November 2024.
  17. ↑ Wang, Hongbo; Zeuschner, Janek; Eremets, Mikhail; Troyan, Ivan; Williams, Jonathon (27 January 2016). "Stable solid and aqueous H2CO3 from CO2 and H2O at high pressure and high temperature". Scientific Reports. 6 (1): 19902. Bibcode:2016NatSR...619902W. doi: 10.1038/srep19902 . PMC   4728613 . PMID   26813580.
  18. ↑ Stolte, Nore; Pan, Ding (4 July 2019). "Large presence of carbonic acid in CO2-rich aqueous fluids under Earth's mantle conditions". The Journal of Physical Chemistry Letters. 10 (17): 5135–41. arXiv: 1907.01833 . doi:10.1021/acs.jpclett.9b01919. PMID   31411889. S2CID   195791860.
  19. ↑ G. Saleh; A. R. Oganov (2016). "Novel Stable Compounds in the C-H-O Ternary System at High Pressure". Scientific Reports. 6: 32486. Bibcode:2016NatSR...632486S. doi:10.1038/srep32486. PMC   5007508 . PMID   27580525.
  20. ↑ IUPAC (2006). "Stability constants" (database).
  21. ↑ Pines, Dina; Ditkovich, Julia; Mukra, Tzach; Miller, Yifat; Kiefer, Philip M.; Daschakraborty, Snehasis; Hynes, James T.; Pines, Ehud (2016). "How Acidic Is Carbonic Acid?". J Phys Chem B. 120 (9): 2440–51. doi:10.1021/acs.jpcb.5b12428. PMC   5747581 . PMID   26862781.
  22. ↑ Caldeira, K.; Wickett, M. E. (2003). "Anthropogenic carbon and ocean pH". Nature . 425 (6956): 365. Bibcode:2001AGUFMOS11C0385C. doi: 10.1038/425365a . PMID   14508477. S2CID   4417880.
  23. ↑ Sabine, C. L. (2004). "The Oceanic Sink for Anthropogenic CO2". Science. 305 (5682): 367–371. Bibcode:2004Sci...305..367S. doi:10.1126/science.1097403. hdl: 10261/52596 . PMID   15256665. S2CID   5607281. Archived from the original on 6 July 2008. Retrieved 22 June 2021.
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