Carbon dioxide solution; Dihydrogen carbonate; acid of air; Aerial acid; Hydroxymethanoic acid
is the chemical compound with the formula H2
). It is also a name sometimes given to solutions of carbon dioxide in water (carbonated water), because such solutions contain small amounts of H2
. Carbonic acid, which is a weak acid, forms two kinds of salts, the carbonates and the bicarbonates.
When carbon dioxide dissolves in water it exists in chemical equilibrium producing carbonic acid:
The hydration equilibrium constant at 25°C is called Kh
, which in the case of carbonic acid is [H2
] ≈ 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 (CO2
O → H2
) and 23 s−1 for the reverse reaction (H2
O). 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
, 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.
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.
The oceans of the world have absorbed almost half of the CO2
emitted by humans from the burning of fossil fuels. 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 process is known as ocean acidification.
Carbonic acid is one of the polyprotic acids: It 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
Care must be taken when quoting and using the first dissociation constant of carbonic acid. In aqueous solution, carbonic acid exists in equilibrium with carbon dioxide, and the concentration of H2
is much lower than the concentration of CO2
. In many analyses, H2
includes dissolved CO2
(referred to as CO2
* is used to represent the two species when writing the aqueous chemical equilibrium equation. The equation may be rewritten as follows:
Whereas this apparent pKa
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. A similar situation applies to sulfurous acid (H2
), which exists in equilibrium with substantial amounts of unhydrated sulfur dioxide.
The second constant is for the dissociation of the bicarbonate ion into the carbonate ion CO3
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 (H2
− and CO3
2−) as well as of the hydration equilibrium between dissolved CO2
(see above) and of the following equilibrium between the dissolved CO2
and the gaseous CO2
above the solution:
The corresponding equilibrium equations together with the
relation and the charge neutrality condition
result in six equations for the six unknowns [CO2
], [H+], [OH−], [HCO3
−] and [CO3
2−], 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:
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.
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 H2
O + CO2
mixture may suggest that H2
might be found in outer space, where frozen ices of H2
O 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 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.