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In chemistry, carbonic acid is a dibasic acid with the
chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas and ...
. The pure compound decomposes at temperatures greater than ca. −80 °C. In biochemistry, the name "carbonic acid" is often applied to
aqueous solution An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute an ...
s of
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
, which play an important role in the
bicarbonate buffer system The bicarbonate buffer system is an acid-base homeostatic mechanism involving the balance of carbonic acid (H2CO3), bicarbonate ion (HCO), and carbon dioxide Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% highe ...
, used to maintain
acid–base homeostasis Acid–base homeostasis is the homeostatic In biology, homeostasis is the state of steady internal, physics, physical, and chemistry, chemical conditions maintained by organism, living systems. This is the condition of optimal functioning for the ...
.


Chemical equilibria


Equilibrium constant values

In aqueous solution carbonic acid behaves as a dibasic acid. The
Bjerrum plotBjerrum may refer to: ;People * Niels Janniksen Bjerrum (1879 – 1958), was a Danish chemist (son of Jannik Petersen Bjerrum and father of Jannik Bjerrum). * Jannik Bjerrum (1909 – 1992), was a Danish chemist (son of Niels Janniksen Bjerrum). * ...
shows typical equilibrium concentrations,in solution, in
seawater Seawater, or salt water, is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for al ...

seawater
, of carbon dioxide and the various species derived from it, as a function of pH.
Expected change refers to predicted effect of continued ocean acidification. The acidification of natural waters is caused by the increasing concentration of carbon dioxide in the atmosphere, which is believed to be caused by the burning of increasing amounts of
coal Coal is a combustible , Germany ) , image_map = , map_caption = , map_width = 250px , capital = Berlin , coordinates = , largest_city = capital , languages_type = Official language , languages = German language, German , ...

coal
and
hydrocarbon In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prop ...
s.
It has been estimated that the increase in dissolved carbon dioxide has caused the ocean's average surface pH to decrease by about 0.1 from pre-industrial levels. This phenomenon is called
ocean acidification Ocean acidification is the ongoing decrease in the pH value of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of cont ...
. Th
stability constants database
contains 136 entries with values for the overall protonation constants, β1 and β2, of the carbonate ion. In the following expressions +represents the concentration, at equilibrium, of the chemical species H+, etc. :CO3^ <=> HCO3^- : \beta_1 = \frac The value of log β1 decreases with increasing
ionic strength The ionic strength of a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute and the ...
, ''I''. At 25°C: :log β1 = 0.54 ''I''2 −0.96 ''I'' + 9.93 (selected data from SC-database) The value of log β2 also decreases with increasing ionic strength. :CO3^ <=> H2CO3 : \beta_2 = \frac At 25°C, log β2 = −2.5 ''I''2 −0.043 ''I'' + 16.07 At ''I''=0 and 25°C the pK values of the stepwise dissociation constants are 6.77 and 9.93. When pH = pK the two chemical species in equilibrium with each other have the same concentration. Note 1: There are apparently conflicting values in the literature for pKa. Pines ''et al.'' cite a value for "pKapp" of 6.35, consistent with the value 6.77, mentioned above. They also give a value for "pKa" of 3.49 and state that :pKa = pKapp − log KD (eqn. 5) where KD= O2 2CO3 (eqn. 3) The situation arises from the way that the dissociation constants are named and defined, which is clearly stated in the text of the Pines paper, but not in the abstract. Note 2: The numbering of dissociation constants is the reverse of the numbering of the numbering of association constants, so pK2 (dissociation)= log β1 (association). The value of the stepwise constant for the equilibrium :HCO3- <=> CO3^ is given by :pK1(dissociation)1 = log β2 − log β1 (association)


In non-biological solutions

The hydration
equilibrium constant The equilibrium constant of a chemical reaction is the value of its reaction quotient at chemical equilibrium, a state approached by a dynamic chemical system after sufficient time has elapsed at which its composition has no measurable tendency ...
at 25 °C is called Kh, which in the case of carbonic acid is 2CO3 O2≈ 1.7×10−3 in pure waterHousecroft and Sharpe, ''Inorganic Chemistry'', 2nd ed, Prentice-Pearson-Hall 2005, p. 368. and ≈ 1.2×10−3 in
seawater Seawater, or salt water, is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for al ...

seawater
. Hence, the majority of the carbon dioxide is not converted into carbonic acid, remaining as CO2 molecules. In the absence of a
catalyst Catalysis () is the process of increasing the rate of a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In cla ...

catalyst
, the equilibrium is reached quite slowly. The
rate constantIn chemical kinetics a reaction rate constant or reaction rate coefficient, ''k'', quantifies the rate and direction of a chemical reaction. For a reaction between reactants A and B to form product C :''a'' A + ''b'' B → ''c'' C th ...
s are 0.039 s−1 for the forward reaction and 23 s−1 for the reverse reaction. In nature,
limestone Limestone is a common type of carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its na ...

limestone
may react with
rainwater Rain is liquid water in the form of Drop (liquid), droplets that have condensation, condensed from atmosphere, atmospheric water vapor and then precipitation (meteorology), become heavy enough to fall under gravity. Rain is a major component ...
, forming a solution of
calcium bicarbonate Calcium bicarbonate, also called calcium hydrogen carbonate, has a chemical formula Ca(HCO3)2. The term does not refer to a known solid compound; it exists only in aqueous solution containing the calcium (Ca2+), bicarbonate (), and carbonate () io ...

calcium bicarbonate
; evaporation of such a solution will result re-formation of solid calcium carbonate. These processes occur in the formation of
stalactite A stalactite (, ; from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population ...

stalactite
s and
stalagmite A stalagmite (, ; from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its populati ...

stalagmite
s.


In biological solutions

When the enzyme
carbonic anhydrase The carbonic anhydrases (or carbonate dehydratases) form a family of enzymes that catalyst, catalyze the interconversion between carbon dioxide and water and the Dissociation (chemistry), dissociated ions of carbonic acid (i.e. bicarbonate and ...

carbonic anhydrase
is also present in the solution the following reaction takes precedence. :HCO3^- H^+ <=> CO2 H2O When the amount of carbon dioxide created by the forward reaction exceeds its solubility, gas is evolved and a third equilibrium :CO_2 (soln) <=> CO_2 (g) must also be taken into consideration. The equilibrium constant for this reaction is defined by
Henry's law In physical chemistry, Henry's law is a Gas laws, gas law that states that the amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid. The proportionality factor is called Henry's law constant. It was formulate ...
. The two reactions can be combined for the equilibrium in solution. :HCO3^- H^ <=> CO_2(soln) H2O :    K_3 = \frac When Henry's law is used to calculate the value of the term in the denominator care is needed with regard to dimensionality. In
physiology Physiology (; ) is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. ...
, carbon dioxide excreted by the lungs may be called ''volatile acid'' or ''respiratory acid''.


Use of the term carbonic acid

Strictly speaking the term "carbonic acid" refers to the
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by chemical bonds. A homonuclear molecule, m ...
with the formula H2CO3. Since pKa1 has a value of ca. 6.8 , at equilibrium carbonic acid will be almost 50% dissociated in the
extracellular fluid In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
(
cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a s ...
) which has a pH of ca.7.2. Note that dissolved carbon dioxide in extracellular fluid is often called as "carbonic acid" in biochemistry literature, for historical reasons. The reaction in which it is produced :HCO3 + H+ CO2 + H2O is fast in biological systems. Carbon dioxide can be described as the ''anhydride'' of carbonic acid.


Pure carbonic acid

Carbonic acid, H2CO3, is stable at ambient temperatures in strictly
anhydrous A substance is anhydrous if it contains no water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vi ...
conditions. It decomposes to form carbon dioxide in the presence of any water molecules. Carbonic acid forms as a by-product of CO2/H2O irradiation, in addition to carbon monoxide and radical species (HCO and CO3). Another route to form carbonic acid is protonation of bicarbonates (HCO3) with aqueous HCl or HBr. This has to be done at
cryogenic A medium-sized dewar is being filled with liquid nitrogen by a larger cryogenic storage tank In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motio ...
conditions to avoid immediate decomposition of H2CO3 to CO2 and H2O. Amorphous H2CO3 forms above 120 K, and crystallization takes place above 200 K to give "β-H2CO3", as determined by
infrared spectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption spectroscopy, absorption, emission spectrum, emission, or reflection (physics), reflection. ...

infrared spectroscopy
. The spectrum of β-H2CO3 agrees very well with the by-product after CO2/H2O irradiation. β-H2CO3 sublimes at 230–260 K largely without decomposition. Matrix-isolation infared spectroscopy allows for the recording of single molecules of H2CO3.{{cite journal , last1=Bernard , first1=Jürgen , last2=Huber , first2=Roland G. , last3=Liedl , first3=Klaus R. , last4=Grothe , first4=Hinrich , last5=Loerting , first5=Thomas , title=Matrix Isolation Studies of Carbonic Acid—The Vapor Phase above the β-Polymorph , journal=Journal of the American Chemical Society , date=14 May 2013 , volume=135 , issue=20 , pages=7732–7737 , doi=10.1021/ja4020925, pmid=23631554 , pmc=3663070 , doi-access=free The fact that the carbonic acid may form by irradiating a solid H2O + CO2 mixture or even by proton-implantation of
dry ice Dry ice is the solid Solid is one of the four fundamental states of matter 4 (four) is a number A number is a mathematical object A mathematical object is an abstract concept arising in mathematics. In the usual language of mathemat ...

dry ice
alone has given rise to suggestions that H2CO3 might be found in outer space or on
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
, where frozen ices of H2O and CO2 are found, as well as
cosmic ray Cosmic rays are high-energy proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approx ...
s.{{cite journal , last1=Loerting , first1=Thomas , last2=Tautermann , first2=Christofer , last3=Kroemer , first3=Romano T. , last4=Kohl , first4=Ingrid , last5=Hallbrucker , first5=Andreas , last6=Mayer , first6=Erwin , last7=Liedl , first7=Klaus R. , title=On the Surprising Kinetic Stability of Carbonic Acid (H2CO3) , journal=Angewandte Chemie International Edition , date=3 March 2000 , volume=39 , issue=5 , pages=891–894 , doi=10.1002/(SICI)1521-3773(20000303)39:5<891::AID-ANIE891>3.0.CO;2-E The surprising stability of sublimed H2CO3 up to rather high-temperatures of 260 K even allows for gas-phase H2CO3, e.g., above the pole caps of Mars.
Ab initio ''Ab initio'' ( ) is a Latin term meaning "from the beginning" and is derived from the Latin ''ab'' ("from") + ''initio'', ablative singular of ''initium'' ("beginning"). Etymology , from Latin, , from ablative case of ''initium'' "entrance, begi ...
calculations showed that a single molecule of water catalyzes the decomposition of a gas-phase carbonic acid molecule to carbon dioxide and water. In the absence of water, the dissociation of gaseous carbonic acid is predicted to be very slow, with a
half-life Half-life (symbol ''t''1⁄2) is the time required for a quantity to reduce to half of its initial value. The term is commonly used in nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies ...
in the gas-phase of 180,000 years at 300 K. This only applies if the molecules are few and far apart, because it has also been predicted that gas-phase carbonic acid will catalyze its own decomposition by forming dimers, which then break apart into two molecules each of water and carbon dioxide. Solid "α-carbonic acid" was claimed to be generated by a
cryogenic A medium-sized dewar is being filled with liquid nitrogen by a larger cryogenic storage tank In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motio ...
reaction of
potassium bicarbonate Potassium bicarbonate (also known as potassium hydrogen carbonate or potassium acid carbonate) is the inorganic compound In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, comp ...

potassium bicarbonate
and a solution of
HCl The compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with ...
in
methanol Methanol, also known as methyl alcohol, amongst other names, is a chemical A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. ...

methanol
.{{cite journal, last1 = Hage , first1 = W. , last2 = Hallbrucker , first2 = A. , last3 = Mayer , first3 = E. , title = Carbonic Acid: Synthesis by Protonation of Bicarbonate and FTIR Spectroscopic Characterization Via a New Cryogenic Technique , journal = J. Am. Chem. Soc. , volume = 115 , pages = 8427–8431 , doi = 10.1021/ja00071a061 , year = 1993, issue = 18, bibcode = 1993JAChS.115.8427H This claim was disputed in a PhD thesis submitted in January 2014. Instead, isotope labeling experiments point to the involvement of (CAME). Furthermore, the sublimed solid was suggested to contain CAME monomers and dimers, not H2CO3 monomers and dimers as previously claimed. Subsequent matrix-isolation infrared spectra confirmed that CAME rather than carbonic acid is found in the gas-phase above "α-carbonic acid". The assignment as CAME is further corroborated by matrix-isolation of the substance prepared in gas-phase by pyrolysis.{{cite journal , last1 = Reisenauer , first1 = H. P. , last2 = Wagner , first2 = J. P. , last3 = Schreiner , first3 = P. R. , title = Gas-Phase Preparation of Carbonic Acid and Its Monomethyl Ester , journal = Angew. Chem. Int. Ed. , volume = 53 , pages = 11766–11771 , year = 2014 , doi = 10.1002/anie.201406969 , pmid = 25196920 , issue = 44 Despite its complicated history, carbonic acid may still appear as distinct polymorphs. Carbonic acid forms upon oxidization of CO with OH-radicals. It is not clear whether carbonic acid prepared in this way needs to be considered as γ-H2CO3. The structures of β-H2CO3 and γ-H2CO3 have not been characterized crystallographically.


At high pressure

Although molecules of H2CO3 do not constitute a significant portion of the dissolved carbon in aqueous "carbonic acid" under ambient conditions, significant amounts of molecular H2CO3 can exist in aqueous solutions subjected to pressures of multiple gigapascals (tens of thousands of atmospheres), such as can occur in planetary interiors.{{cite journal , last1=Stolte , first1=Nore , last2=Pan , first2=Ding , title=Large presence of carbonic acid in CO2-rich aqueous fluids under Earth's mantle conditions , journal= The Journal of Physical Chemistry Letters, date=4 July 2019 , volume=10 , issue=17 , pages=5135–5141 , doi=10.1021/acs.jpclett.9b01919 , pmid=31411889 , arxiv=1907.01833 , s2cid=195791860 , url=https://arxiv.org/pdf/1907.01833.pdf , access-date=24 March 2021


References

{{Reflist


Further reading

*{{wsPSM2, Climate and Carbonic Acid, 59, July 1901 *{{cite journal , last1 = Welch , first1 = M. J. , last2 = Lifton , first2 = J. F. , last3 = Seck , first3 = J. A. , journal = J. Phys. Chem. , volume = 73 , issue = 335 , year = 1969 , doi = 10.1021/j100844a033 , title = Tracer studies with radioactive oxygen-15. Exchange between carbon dioxide and water , pages = 3351 *{{cite book , author=Jolly, W. L. , title=Modern Inorganic Chemistry (2nd Edn.) , location=New York , publisher=McGraw-Hill , year=1991 , isbn=978-0-07-112651-9 *{{cite journal , last1 = Moore , first1 = M. H. , last2 = Khanna , first2 = R. , title = Infrared and Mass Spectral Studies of Proton Irradiated H2O+Co2 Ice: Evidence for Carbonic Acid Ice: Evidence for Carbonic Acid , journal = Spectrochimica Acta , volume = 47A , doi =10.1016/0584-8539(91)80097-3 , pages = 255–262 , year = 1991 , issue = 2, bibcode = 1991AcSpA..47..255M , url = https://zenodo.org/record/1258549 * {{cite journal , last1 = W. Hage , first1 = K. R. Liedl , last2 = Liedl , first2 = E. , title = Carbonic Acid in the Gas Phase and Its Astrophysical Relevance , journal =
Science Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is something that is truth, true. The usual test for a statement of ...
, volume = 279 , pages = 1332–1335 , year = 1998 , doi = 10.1126/science.279.5355.1332 , pmid = 9478889 , last3 = Hallbrucker , first3 = A , last4 = Mayer , first4 = E , issue = 5355, bibcode = 1998Sci...279.1332H * {{cite journal , last1 = Hage , first1 = W. , last2 = Hallbrucker , first2 = A. , last3 = Mayer , first3 = E. , title = A Polymorph of Carbonic Acid and Its Possible Astrophysical Relevance , journal = J. Chem. Soc. Faraday Trans. , volume = 91 , pages = 2823–2826 , doi = 10.1039/ft9959102823 , year = 1995 , issue = 17, bibcode = 1995JCSFT..91.2823H


External links


Carbonic acid/bicarbonate/carbonate equilibrium in water: pH of solutions, buffer capacity, titration and species distribution vs. pH computed with a free spreadsheet
{{Hydrogen compounds {{Oxides of carbon {{carbon compounds {{Authority control {{DEFAULTSORT:Carbonic Acid Alkanoic acids Carbonates Mineral acids