Carbonic acid
Other names	Carbon dioxide solution
Identifiers
CAS number	463-79-6
SMILES	C(=O)(O)O
Properties
Molecular formula	H2CO3
Molar mass	62.03 g/mol
Density	1.0 g/cm3 (dilute solution)
Solubility in water	exists only in solution
Acidity (pKa)	6.36 (see text) 10.25
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Carbonic acid (ancient name acid of air or aerial acid) has the formula H₂CO₃. It is also a name sometimes given to solutions of carbon dioxide in water, which contain small amounts of H₂CO₃. The salts of carbonic acids are called bicarbonates (or hydrogencarbonates) and carbonates. It is a weak acid. Carbonic acid should not be confused with carbolic acid, an antiquated name for phenol. Image File history File links Download high-resolution version (1100x605, 21 KB) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Carbonic acid ... Image File history File links Download high-resolution version (1100x955, 167 KB) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Carbonic acid ... CAS registry numbers are unique numerical identifiers for chemical compounds, polymers, biological sequences, mixtures and alloys. ... The simplified molecular input line entry specification or SMILES is a specification for unambiguously describing the structure of chemical molecules using short ASCII strings. ... A chemical formula is a concise way of expressing information about the atoms that constitute a particular chemical compound. ... Molar mass is the mass of one mole of a chemical element or chemical compound. ... For other uses, see Density (disambiguation). ... Solubility is a chemical property referring to the ability for a given substance, the solute, to dissolve in a solvent. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... The acid dissociation constant (Ka), also known as the acidity constant or the acid-ionization constant, is a specific equilibrium constant for the reaction of an acid with its conjugate base in aqueous solution [1]. // When an acid dissolves in water, it partly dissociates forming hydronium ions and its conjugate... The plimsoll symbol as used in shipping In chemistry, the standard state of a material is its state at 1 bar (100 kilopascals exactly). ... A chemical formula (also called molecular formula) is a concise way of expressing information about the atoms that constitute a particular chemical compound. ... Making a saline water solution by dissolving table salt (NaCl) in water This article is about chemical solutions. ... Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. ... H2O and HOH redirect here. ... For baking soda, see Sodium bicarbonate In inorganic chemistry, a bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. ... Carbonate is an anion with a charge of -2 and an empirical formula of CO32-. An aqueous solution of carbon dioxide contains a minute amount of H2CO3, called carbonic acid, which dissociates to form hydrogen ions and carbonate ions. ... A weak acid is an acid that does not fully ionize in solution; that is, if the acid was represented by the general formula HA, then in aqueous solution a significant amount of undissolved HA still remains. ... Phenol, also known under an older name of carbolic acid, is a colourless crystalline solid with a typical sweet tarry odor. ...

Carbon dioxide dissolved in water is in equilibrium with carbonic acid: Apparatus for carrying out acid-base titration. ...

CO₂ + H₂O ⇌ H₂CO₃

The equilibrium constant at 25°C is K_h= 1.70×10⁻³: hence, the majority of the carbon dioxide is not converted into carbonic acid and stays 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 (CO₂ + H₂O → H₂CO₃) and 23 s⁻¹ for the reverse reaction (H₂CO₃ → CO₂ + H₂O). In chemistry, the equilibrium constant is a quantity characterizing a chemical equilibrium in a chemical reaction. ... It has been suggested that this article or section be merged into Catalysis. ... In chemical kinetics a reaction rate constant quantifies the speed of a chemical reaction. ...

Role of carbonic acid in blood

Carbonic acid plays a very important role in mammalian blood. When CO₂ enters the blood from various cells, it is combined with water to produce carbonic acid. It then has a H⁺ taken away from it to become bicarbonate (HCO₃^-). In order to transport the bicarbonate that is in the blood stream out of the body, it enters another red blood cell, has H⁺ attached to it to form carbonic acid once again, then has H₂O taken away from it and is expelled from the red blood cell as CO₂. Then the carbon dioxide is permitted to be expelled out of capillaries and into the lungs.

The equilibrium between carbon dioxide and carbonic acid is very important for controlling the acidity of body fluids, and almost all living organisms have an enzyme, carbonic anhydrase, which catalyzes the conversion between the two compounds, increasing the reaction rate by a factor of nearly a billion. Ribbon diagram of the enzyme TIM, surrounded by the space-filling model of the protein. ... Carbonic anhydrase (carbonate dehydratase) is a family of metalloenzymes (enzymes that contain one or more metal atoms as a functional component of the enzyme) that catalyze the rapid interconversion of carbon dioxide and water into carbonic acid, protons, and bicarbonate ions. ...

Acidity of carbonic acid

Carbonic Acid is diprotic, that is it has two hydrogens which disassociate and thus two dissociation constants: A diprotic acid is an acid such as (sulfuric acid) that happens to contain within its molecular structure two hydrogen atoms capable of dissociation (i. ... The acid dissociation constant (Ka), also known as the acidity constant or the acid-ionization constant, is a specific equilibrium constant for the reaction of an acid with its conjugate base in aqueous solution [1]. // When an acid dissolves in water, it partly dissociates forming hydronium ions and its conjugate...

H₂CO₃ ⇌ HCO₃⁻ + H⁺

K_a1 = 2.5×10⁻⁴ mol/L; pK_a1 = 3.60 at 25 °C.

HCO₃⁻ ⇌ CO₃²⁻ + H⁺

K_a2 = 5.61×10⁻¹¹ mol/L; pK_a2 = 10.25 at 25 °C.

Care must be taken when quoting and using the first dissociation constant of carbonic acid. The value quoted above is correct for the H₂CO₃ molecule, and shows that it is a stronger acid than acetic acid or formic acid: this might be expected from the influence of the electronegative oxygen substituent. However, carbonic acid only ever exists in solution in equilibrium with carbon dioxide, and so the concentration of H₂CO₃ is much lower than the concentration of CO₂, reducing the measured acidity. The equation may be rewritten as follows (c.f. sulfurous acid): R-phrases , S-phrases , , , Flash point 43 °C Related Compounds Related carboxylic; acids Formic acid; Propionic acid; Butyric acid Related compounds acetamide; ethyl acetate; acetyl chloride; acetic anhydride; acetonitrile; acetaldehyde; ethanol; thioacetic acid; acetylcholine; acetylcholinesterase Supplementary data page Structure and properties n, εr, etc. ... Formic acid (systematically called methanoic acid) is the simplest carboxylic acid. ... Electronegativity is a measure of the ability of an atom or molecule to attract electrons in the context of a chemical bond. ... Sulfurous acid (or sulphurous acid in British spelling) is a name given to aqueous solutions of sulfur dioxide. ...

CO₂ + H₂O ⇌ HCO₃⁻ + H⁺

K_a = 4.30×10⁻⁷ mol/L; pK_a = 6.36.

This figure is quoted as the dissociation constant of carbonic acid, although this is ambiguous: it might better be referred to as the acidity constant of carbon dioxide, as it is particularly useful for calculating the pH of CO₂ solutions. For other uses, see PH (disambiguation). ...

pH and composition of a carbonic acid solution

At a given temperature, the composition of a pure carbonic acid solution (or of a pure CO₂ solution) is completely determined by the partial pressure of carbon dioxide above the solution. To calculate this composition, account must be taken of the above equilibria between the three different carbonate forms (H₂CO₃, HCO₃⁻ and CO₃²⁻) as well as of the hydratation equilibrium between dissolved CO₂ and H₂CO₃ with constant (see above) and of the following equilibrium between the dissolved CO₂ and the gaseous CO₂ above the solution:

CO₂(gas) ↔ CO₂(dissolved) with where k_H=29.76 atm/(mol/L) at 25°C (Henry constant)

The corresponding equilibrium equations together with the relation and the neutrality condition result in six equations for the six unknowns [CO₂], [H₂CO₃], [H⁺], [OH⁻], [HCO₃⁻] and [CO₃²⁻], showing that the composition of the solution is fully determined by . The equation obtained for [H⁺] is a cubic whose numerical solution yields the following values for the pH and the different species concentrations: In chemistry, Henrys law is one of the gas laws, formulated by William Henry. ...

(atm)	pH	[CO2] (mol/L)	[H2CO3] (mol/L)	[HCO3−] (mol/L)	[CO32−] (mol/L)
10−8	7.00	3.36 × 10-10	5.71 × 10−13	1.42 × 10−9	7.90 × 10−13
10−6	6.81	3.36 × 10−8	5.71 × 10−11	9.16 × 10−8	3.30 × 10−11
10−4	5.92	3.36 × 10−6	5.71 × 10−9	1.19 × 10−6	5.57 × 10−11
3.5 × 10−4	5.65	1.18 × 10−5	2.00 × 10−8	2.23 × 10−6	5.60 × 10−11
10−3	5.42	3.36 × 10−5	5.71 × 10−8	3.78 × 10−6	5.61 × 10−11
10−2	4.92	3.36 × 10−4	5.71 × 10−7	1.19 × 10−5	5.61 × 10−11
10−1	4.42	3.36 × 10−3	5.71 × 10−6	3.78 × 10−5	5.61 × 10−11
1	3.92	3.36 × 10−2	5.71 × 10−5	1.20 × 10−4	5.61 × 10−11
2.5	3.72	8.40 × 10−2	1.43 × 10−4	1.89 × 10−4	5.61 × 10−11
10	3.42	0.336	5.71 × 10−4	3.78 × 10−4	5.61 × 10−11

• We see that in the total range of pressure, the pH is always largely lower than pKa₂ so that the CO₃²⁻ concentration is always negligible with respect to HCO₃⁻ concentration. In fact CO₃²⁻ play no quantitive role in the present calculation (see remark below).

• For vanishing , the pH is close to the one of pure water (pH = 7) and the dissolved carbon is essentially in the HCO₃⁻ form.

• For normal atmospherics conditions ( atm), we get a slightly acid solution (pH = 5.7) and the dissolved carbon is now essentially in the CO₂ form. From this pressure on, [OH⁻] becomes also negligible so that the ionized part of the solution is now an equimolar mixture of H⁺ and HCO₃⁻.

• For a CO₂ pressure typical of the one in soda drinks bottles ( ~ 2.5 atm), we get a relatively acid medium (pH = 3.7) with a high concentration of dissolved CO₂. These features are responsible for the sour and sparkling taste of these drinks.

• Between 2.5 and 10 atm, the pH crosses the pKa₁ value (3.60) giving a dominant H₂CO₃ concentration (with respect to HCO₃⁻) at high pressures.

Remark: As noted above, [CO₃²⁻] may be neglected for this specific problem, resulting in the following very precise analytical expression for [H⁺]:

Instability of carbonic acid

It has long been recognized that it is impossible to obtain pure hydrogen bicarbonate at room temperatures (about 20 °C or about 70 °F). However, in 1991 scientists at NASA's Goddard Space Flight Center (USA) succeeded in making the first pure H₂CO₃ samples. They did so by exposing a frozen mixture of water and carbon dioxide to high-energy radiation, and then warming to remove the excess water. The carbonic acid that remained was characterized by infrared spectroscopy. The fact that the carbonic acid was prepared by irradiating a solid H₂O + CO₂ mixture has given rise to suggestions that H₂CO₃ might be found in outer space, where frozen ices of H₂O and CO₂ are common, as are cosmic rays and ultraviolet light, to help them react. The same carbonic acid polymorph (denoted beta-carbonic acid) was prepared by a cryotechnique at the University of Innsbruck: alternating layers of glassy aqueous solutions of bicarbonate and acid were heated in vacuo, which causes protonation of bicarbonate, and the solvent was subsequently removed. A second polymorph (denoted alpha-carbonic acid) was prepared by the same technique at the University of Innsbruck using methanol rather than water as a solvent. Aerial view of Goddard Space Flight Center. ...

It has since been shown, by theoretical calculations, that the presence of even a single molecule of water causes carbonic acid to revert to carbon dioxide and water fairly quickly. Pure carbonic acid is predicted to be stable in the gas phase, in the absence of water, with a calculated half-life of 180,000 years. Half-Life For a quantity subject to exponential decay, the half-life is the time required for the quantity to fall to half of its initial value. ... A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ...

There is a hypothetical acid orthocarbonic acid which is even more hydrated, being H₄CO₄. A hypothesis (= assumption in ancient Greek) is a proposed explanation for a phenomenon. ... Orthocarbonic acid is the hypothetical acid with the chemical formula H4CO4. ...

References

• Welch, M. J.; Lipton, J. F.; Seck, J. A. (1969). J. Phys. Chem. 73:3351.
• Jolly, W. L. (1991). Modern Inorganic Chemistry (2nd Edn.). New York: MgGraw-Hill. ISBN 0-07-112651-1. 
• M. H. Moore and R. Khanna "Infrared and Mass Spectral Studies of Proton Irradiated H2O+CO2 Ice: Evidence for Carbonic Acid", Spectrochimica Acta, 47A, pp. 255-262 (1991)
• T. Loerting, C. Tautermann, R.T. Kroemer, I. Kohl, E. Mayer, A. Hallbrucker, K. R. Liedl "On the Surprising Kinetic Stability of Carbonic Acid", Angew. Chem. Int. Ed., 39, pp. 891-895 (2001)
• W. Hage, K. R. Liedl, A. Hallbrucker and E. Mayer, "Carbonic acid in the gas phase and its astrophysical relevance", Science, 279, pp. 1332-1335 (1998)
• W. Hage, A. Hallbrucker, E. Mayer, "Carbonic acid: synthesis by protonation of bicarbonate and FTIR spectroscopic characterization via a new cryogenic technique.", J. Am. Chem. Soc., 115, pp. 8427-8431(1993)
• W. Hage, A. Hallbrucker, E. Mayer, "A polymorph of carbonic acid and its possible astrophysical relevance.", J. Chem. Soc. Farad. Trans., 91, 2823-2826 (1995).

See also

• Carbonated water
• Ocean acidification
• Carbon dioxide

Effervescence from soda. ... Change in sea surface pH caused by anthropogenic CO2 between the 1700s and the 1990s Ocean acidification is the name given to the ongoing decrease in the pH of the Earths oceans, caused by their uptake of anthropogenic carbon dioxide from the atmosphere. ... Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. ...

External links

• Ask a Scientist: Carbonic Acid Decomposition
• Why was the existence of carbonic acid unfairly doubted for so long?