Carbonic acid

Not to be confused with carbolic acid, an antiquated name for phenol.

Carbonic acid is also an archaic name for carbon dioxide

Carbonic acid
IUPAC name Carbonic acid
Other names Carbon dioxide solution; Dihydrogen carbonate; acid of air; Aerial acid; Hydroxymethanoic acid
Identifiers
CAS number	463-79-6 Y
ChemSpider	747 Y
KEGG	C01353 Y
ChEBI	CHEBI:28976 Y
ChEMBL	CHEMBL1161632 Y
Jmol-3D images	Image 1
SMILES O=C(O)O
InChI InChI=1S/CH2O3/c2-1(3)4/h(H2,2,3,4) Y Key: BVKZGUZCCUSVTD-UHFFFAOYSA-N Y InChI=1/CH2O3/c2-1(3)4/h(H2,2,3,4) Key: BVKZGUZCCUSVTD-UHFFFAOYAU
Properties
Molecular formula	H2CO3
Molar mass	62.03 g/mol
Density	1.0 g/cm3 (dilute soln.)
Melting point	n/a
Solubility in water	Exists only in solution
Acidity (pKa)	6.352 (pKa1)
Y acid (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Carbonic acid is the inorganic compound with the formula H₂CO₃ (equivalently OC(OH)₂). It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H₂CO₃, and reactions proceed as if the main species was H₂CO₃. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates. It is a weak acid.

Chemical equilibrium

When carbon dioxide dissolves in water it exists in chemical equilibrium producing carbonic acid:

CO₂ + H₂O H₂CO₃

The hydration equilibrium constant at 25 °C is called K_h, which in the case of carbonic acid is [H₂CO₃]/[CO₂] = 1.70×10⁻³: 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 smaller for the forward reaction (CO₂ + H₂O → H₂CO₃) than for the reverse reaction (H₂CO₃ → CO₂ + H₂O). Carbonic acid is used in the making of soft drinks, inexpensive and artificially carbonated sparkling wines, and other bubbly drinks. The addition of two equivalents of water to CO₂ would give orthocarbonic acid, C(OH)₄, which exists only in minute amounts in aqueous solution.

Addition of base to an excess of carbonic acid gives bicarbonate. With excess base, carbonic acid reacts to give carbonate salts.

Role of carbonic acid in blood

Carbonic acid is an intermediate step in the transport of CO₂ out of the body via respiratory gas exchange. The hydration reaction of CO₂ is generally very slow in the absence of a catalyst, but red blood cells contain carbonic anhydrase, which both increases the reaction rate and dissociates a hydrogen ion (H⁺) from the resulting carbonic acid, leaving bicarbonate (HCO₃^-) dissolved in the blood plasma. This catalysed reaction is reversed in the lungs, where it converts the bicarbonate back into CO₂ and allows it to be expelled. This equilibration plays an important role as a buffer in mammalian blood.^[1]

Role of carbonic acid in ocean chemistry

The oceans of the world have absorbed approximately one-third of the CO₂ emitted by humans from the burning of fossil fuels.^[2]  The extra dissolved carbon dioxide has caused the ocean's average surface pH to shift by about 0.1 unit from pre-industrial levels.^[3] This process is known as ocean acidification.

Acidity of carbonic acid

Carbonic acid is diprotic: it has two protons, which may dissociate from the parent molecule. Thus there are two dissociation constants, the first one for the dissociation into the bicarbonate (also called hydrogen carbonate) ion HCO₃⁻:

H₂CO₃ HCO₃⁻ + H⁺

K_a1 = 4.45×10⁻⁷ ; pK_a1 = 6.352 at 25 °C.

With a pK_a1 of 6.352, carbonic acid H₂CO₃ is almost 10x weaker acid than acetic acid.

The second for the dissociation of the bicarbonate ion into the carbonate ion CO₃²⁻:

HCO₃⁻ CO₃²⁻ + H⁺

K_a2 = 4.69×10⁻¹¹ ; pK_a2 = 10.329 at 25 °C and Ionic Strength = 0.0.

Care must be taken when quoting and using the first dissociation constant of carbonic acid. In aqueous solution carbonic acid only exists in equilibrium with carbon dioxide, and the concentration of H₂CO₃ is much lower than the dissolved CO₂ concentration. Since it is not possible to distinguish between H₂CO₃ and dissolved CO₂ (referred to as CO₂(aq)) by conventional methods, H₂CO₃^* is used to represent the two species when writing the aqueous chemical equilibrium equation. The equation may be rewritten as follows (cf. sulfurous acid):

H₂CO₃^* HCO₃⁻ + H⁺

K_a = 4.6×10⁻⁷(General Chemistry: An Integrated Approach Third Edition); pK_a = 6.352 at 25 °C and Ionic Strength = 0.0.(NIST CRITICAL Database)

Whereas this pK_a is quoted as the dissociation constant of carbonic acid, it is ambiguous: it might better be referred to as the acidity constant of dissolved carbon dioxide, as it is particularly useful for calculating the pH of CO₂-containing solutions.

pH and composition of carbonic acid solutions

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 $\scriptstyle p_{CO_2}$ 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 hydration equilibrium between dissolved CO₂ and H₂CO₃ with constant $\scriptstyle K_h=\frac{[H_2CO_3]}{[CO_2]}$ (see above) and of the following equilibrium between the dissolved CO₂ and the gaseous CO₂ above the solution:

CO₂(gas) CO₂(dissolved) with $\scriptstyle \frac{[CO_2]}{p_{CO_2}}=\frac{1}{k_\mathrm{H}}$ where k_H=29.76 atm/(mol/L) at 25°C (Henry constant)

The corresponding equilibrium equations together with the $\scriptstyle[H^+][OH^-]=10^{-14}$ relation and the charge neutrality condition $\scriptstyle[H^+]=[OH^-]+[HCO_3^-]+2[CO_3^{2-}]$ 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 $\scriptstyle p_{CO_2}$. The equation obtained for [H⁺] is a cubic whose numerical solution yields the following values for the pH and the different species concentrations:

$\scriptstyle p_{CO_2}$ (atm)	pH	[CO2] (mol/L)	[H2CO3] (mol/L)	[HCO3−] (mol/L)	[CO32−] (mol/L)
1.0 × 10−8	7.00	3.36 × 10−10	5.71 × 10−13	1.42 × 10−09	7.90 × 10−13
1.0 × 10−7	6.94	3.36 × 10−09	5.71 × 10−12	5.90 × 10−09	1.90 × 10−12
1.0 × 10−6	6.81	3.36 × 10−08	5.71 × 10−11	9.16 × 10−08	3.30 × 10−11
1.0 × 10−5	6.42	3.36 × 10−07	5.71 × 10−09	3.78 × 10−07	4.53 × 10−11
1.0 × 10−4	5.92	3.36 × 10−06	5.71 × 10−09	1.19 × 10−06	5.57 × 10−11
3.5 × 10−4	5.65	1.18 × 10−05	2.00 × 10−08	2.23 × 10−06	5.60 × 10−11
1.0 × 10−3	5.42	3.36 × 10−05	5.71 × 10−08	3.78 × 10−06	5.61 × 10−11
1.0 × 10−2	4.92	3.36 × 10−04	5.71 × 10−07	1.19 × 10−05	5.61 × 10−11
1.0 × 10−1	4.42	3.36 × 10−03	5.71 × 10−06	3.78 × 10−05	5.61 × 10−11
1.0 × 10+0	3.92	3.36 × 10−02	5.71 × 10−05	1.20 × 10−04	5.61 × 10−11
2.5 × 10+0	3.72	8.40 × 10−02	1.43 × 10−04	1.89 × 10−04	5.61 × 10−11
1.0 × 10+1	3.42	3.36 × 10−01	5.71 × 10−04	3.78 × 10−04	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⁻ plays no quantitative role in the present calculation (see remark below).
• For vanishing $\scriptstyle p_{CO_2}$, the pH is close to the one of pure water (pH = 7) and the dissolved carbon is essentially in the HCO₃⁻ form.
• For normal atmospheric conditions ($\scriptstyle p_{CO_2}=3.5\times 10^{-4}$ 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 drink bottles ($\scriptstyle p_{CO_2}$ ~ 2.5 atm), we get a relatively acid medium (pH = 3.7) with a high concentration of dissolved CO₂. These features contribute to 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⁺]:

$\scriptstyle[H^+] \simeq \left( 10^{-14}+\frac {K_hK_{a1}}{k_\mathrm{H}} p_{CO_2}\right)^{1/2}$

Spectroscopic studies of carbonic acid

Theoretical calculations show that the presence of even a single molecule of water causes carbonic acid to revert 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 of 180,000 years.^[4]

It has long been recognized that pure carbonic acid cannot be obtained at room temperatures (about 20 °C or about 70 °F). It can be generated 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 may suggest 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.^[4] The same carbonic acid polymorph (denoted beta-carbonic acid) was prepared by heating alternating layers of glassy aqueous solutions of bicarbonate and acid in vacuo, which causes protonation of bicarbonate, followed by removal of the solvent. Alpha-carbonic acid was prepared by the same technique using methanol rather than water as a solvent.

See also

• Carbonated water
• Ocean acidification
• Carbon dioxide

References

1. ^ "excretion." Encyclopædia Britannica. Encyclopædia Britannica Ultimate Reference Suite. Chicago: Encyclopædia Britannica, 2010.
2. ^ Sabine, C.L.; et al. (2004). " "The Oceanic Sink for Anthropogenic CO₂". Science 305 (5682): 367–371. doi:10.1126/science.1097403. PMID 15256665. http://www.sciencemag.org/cgi/content/short/305/5682/367". ^{[dead link]}
3. ^ "Ocean Acidification Network". http://ioc3.unesco.org/oanet/FAQacidity.html. 
4. ^ ^{a b} Loerting, T.; Tautermann, C.; Kroemer, R.T.; Kohl, I.; Mayer, E.; Hallbrucker, A.; Liedl, K. R. (2001). "On the Surprising Kinetic Stability of Carbonic Acid". Angew. Chem. Int. Ed. 39 (5): 891–895. doi:10.1002/(SICI)1521-3773(20000303)39:5<891::AID-ANIE891>3.0.CO;2-E. PMID 10760883. 

Further reading

• Welch, M. J.; Lipton, J. F.; Seck, J. A. (1969). "Tracer studies with radioactive oxygen-15. Exchange between carbon dioxide and water". J. Phys. Chem. 73 (335): 3351. doi:10.1021/j100844a033. 
• Jolly, W. L. (1991). Modern Inorganic Chemistry (2nd Edn.). New York: McGraw-Hill. ISBN 0-07-112651-1. 
• Moore, M. H.; Khanna, R. (1991). "Infrared and Mass Spectral Studies of Proton Irradiated H2O+Co2 Ice: Evidence for Carbonic Acid Ice: Evidence for Carbonic Acid". Spectrochimica Acta 47A (2): 255–262. doi:10.1016/0584-8539(91)80097-3. 
• W. Hage, K. R. Liedl; Mayer, E.; Hallbrucker, A; Mayer, E (1998). "Carbonic Acid in the Gas Phase and Its Astrophysical Relevance". Science 279 (5355): 1332–1335. doi:10.1126/science.279.5355.1332. PMID 9478889. 
• Hage, W.; Hallbrucker, A.; Mayer, E. (1993). "Carbonic Acid: Synthesis by Protonation of Bicarbonate and Ftir Spectroscopic Characterization Via a New Cryogenic Technique". J. Am. Chem. Soc. 115 (18): 8427–8431. doi:10.1021/ja00071a061. 
• Hage, W.; Hallbrucker, A.; Mayer, E. (1995). "A Polymorph of Carbonic Acid and Its Possible Astrophysical Relevance". J. Chem. Soc. Farad. Trans. 91 (17): 2823–2826. doi:10.1039/ft9959102823. 

External links

• Ask a Scientist: Carbonic Acid Decomposition
• Why was the existence of carbonic acid unfairly doubted for so long?
• Carbonic acid/bicarbonate/carbonate equilibrium in water: pH of solutions, buffer capacity, titration and species distribution vs. pH computed with a free spreadsheet
• How to calculate concentration of Carbonic Acid in Water