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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.
Speciation for a monoprotic acid, AH as a function of pH.
In aqueous solution carbonic acid behaves as a dibasic acid. pKa1, the cologarithm of the first dissociation constant has a value of ca. 3.6 at 25 C. Like all dissociation constants the precise value varies with the ionic strength of the solution.
Bjerrum plot for carbonate speciation in seawater.
Undissociated carbonic acid will only be present (in significant concentration) in solutions that are mildly acidic. In geology, limestone may react with rainwater, which is mildly acidic, to form a solution of calcium bicarbonate; evaporation of such solutions may result in the formation of stalactites and stalagmites.
The cologarithm of the first dissociation constant, pKa1, has a value of ca. 6.35 at 25 C. It follows that the concentration of the bicarbonate ion will be more than 1% in solutions in the pH range ca. 4 - 8. Above this pH range the bicarbonate ion dissociates into the carbonate ion CO32- and the hydronium ion.
The Bjerrum plot on the right shows the calculated equilibrium concentrations of the various species in seawater as a function of pH. Due to the many orders of magnitude spanned by the concentrations, the scale of the vertical axis is logarithmic. 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 and hydrocarbons. It has been estimated that the extra dissolved carbon dioxide has caused the ocean's average surface pH to shift by about −0.1 unit from pre-industrial levels. This is known as ocean acidification, even though the ocean remains basic.
In biological solutions
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
Strictly speaking the term "carbonic acid" refers to the chemical compound with the formula . However, at biological pH, the concentration of this compound is less than 0.01% of the total concentration of the acid. This follows from the fact that its pKa value is ca. 3.6, whereas the pH of the extracellular fluid is ca.7.2. A species distribution calculation shows that the proportion is already less than 1% at pH > 5.6. Therefore there is effectively no present in biological solutions; there is ca. 99% bicarbonate and ca 1% carbonate, as shown in the diagram above.
Nevertheless, dissolved carbon dioxide is generally described as carbonic acid in biochemistry literature, for historical reasons. Technically, carbon dioxide is the anhydride of carbonic acid (c.f. sulfur trioxide, the anhydride of sulfuric acid).
Pure carbonic acid
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.
^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.
^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.