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Bjerrum plot for carbonate speciation in seawater (ionic strength 0.7 mol dm-3)
In aqueous solution carbonic acid behaves as a dibasic acid. 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. The acidification of natural waters is caused by the increasing concentration of carbon dioxide in the atmosphere, which is caused by the burning of increasing amounts of coal and hydrocarbons.
Expected change refers to predicted effect of continued ocean acidification. 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.
The stability constants database contains 136 entries with values for the overall protonation constants, ?1 and ?2, of the carbonate ion. In the following expressions [H+] represents the concentration, at equilibrium, of the chemical species H+, etc.
The value of log ?1 decreases with increasing ionic strength, . At 25 °C:
(selected data from SC-database)
The value of log ?2 also decreases with increasing ionic strength.
At =0 and 25 °C the pK values of the stepwise dissociation constants are
pK1 = log?2 - log?1 = 6.77.
pK2 = log?1 = 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=[CO2]/[H2CO3]. (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 ?1 (association). The value of the stepwise constant for the equilibrium
The hydrationequilibrium constant at 25 °C is called Kh, which in the case of carbonic acid is [H2CO3]/[CO2] ? 1.7×10-3 in pure water and ? 1.2×10-3 in seawater. Hence, the majority of the carbon dioxide is not converted into carbonic acid, remaining as CO2 molecules. In the absence of a catalyst, the equilibrium is reached quite slowly. The rate constants are 0.039 s-1 for the forward reaction and 23 s-1 for the reverse reaction.
When the enzyme carbonic anhydrase is also present in the solution the following reaction takes precedence.
When the amount of carbon dioxide created by the forward reaction exceeds its solubility, gas is evolved 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 value of the term in the denominator care is needed with regard to dimensionality.
In physiology, carbon dioxide excreted by the lungs may be called volatile acid or respiratory acid.
Use of the term carbonic acid
Speciation for a monoprotic acid, AH as a function of pH.
Strictly speaking the term "carbonic acid" refers to the chemical compound with the formula .
Since pKa1 has a value of ca. 6.8 , at equilibrium carbonic acid will be almost 50% dissociated in the extracellular fluid (cytosol) 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 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 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 "?-H2CO3", as determined by infrared spectroscopy. The spectrum of ?-H2CO3 agrees very well with the by-product after CO2/H2O irradiation. ?-H2CO3 sublimes at 230-260 K largely without decomposition. Matrix-isolation infared spectroscopy allows for the recording of single molecules of H2CO3.
The fact that the carbonic acid may form by irradiating a solid H2O + CO2 mixture or even by proton-implantation of dry ice alone has given rise to suggestions that H2CO3 might be found in outer space or on Mars, where frozen ices of H2O and CO2 are found, as well as cosmic rays. 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 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 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 "?-carbonic acid" was claimed to be generated by a cryogenic reaction of potassium bicarbonate and a solution of HCl in methanol. This claim was disputed in a PhD thesis submitted in January 2014. Instead, isotope labeling experiments point to the involvement of carbonic acid monomethyl ester (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 "?-carbonic acid". The assignment as CAME is further corroborated by matrix-isolation of the substance prepared in gas-phase by pyrolysis.
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 ?-H2CO3. The structures of ?-H2CO3 and ?-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.
Carbonic acid should be stabilized under pressures of 0.6-1.6 GPa at 100 K, and 0.75-1.75 GPa at 300 K. These pressures 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, would then sink under the ice layers and separate them from the rocky cores of these moons.
^Housecroft and Sharpe, Inorganic Chemistry, 2nd ed, Prentice-Pearson-Hall 2005, p. 368.
^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.
^ abcLoerting, 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): 891-894. doi:10.1002/(SICI)1521-3773(20000303)39:5<891::AID-ANIE891>3.0.CO;2-E.
^Hage, Wolfgang; Hallbrucker, Andreas; Mayer, Erwin (1995). "A polymorph of carbonic acid and its possible astrophysical relevance". Journal of the Chemical Society, Faraday Transactions. 91 (17): 2823. Bibcode:1995JCSFT..91.2823H. doi:10.1039/ft9959102823.
^Bernard, Jürgen; Seidl, Markus; Kohl, Ingrid; Liedl, Klaus R.; Mayer, Erwin; Gálvez, Óscar; Grothe, Hinrich; Loerting, Thomas (18 February 2011). "Spectroscopic Observation of Matrix-Isolated Carbonic Acid Trapped from the Gas Phase". Angewandte Chemie International Edition. 50 (8): 1939-1943. doi:10.1002/anie.201004729. PMID21328675.