Carbonic acid is a chemical compound with the chemical formula . The molecule rapidly converts to water and carbon dioxide in the presence of water. However, in the absence of water, it is quite stable at room temperature.[
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In biochemistry and physiology, the name "carbonic acid" is sometimes applied to aqueous solutions of carbon dioxide. These chemical species play an important role in the bicarbonate buffer system, used to maintain acid–base homeostasis.[ Acid-Base Physiology 2.1 – Acid-Base Balance by Kerry Brandis.]
Terminology in biochemical literature
In chemistry, the term "carbonic acid" strictly refers to the chemical compound with the formula . Some biochemistry literature effaces the distinction between carbonic acid and carbon dioxide dissolved in extracellular fluid.
In physiology, carbon dioxide excreted by the lungs may be called volatile acid or respiratory acid.
Anhydrous carbonic acid
At ambient temperatures, pure carbonic acid is a stable gas.
Carbonic acid monomers exhibit three conformational isomers: cis–cis, cis–trans, and trans–trans.
At low temperatures and atmospheric pressure, solid carbonic acid is Amorphous solid and lacks Bragg peak in X-ray diffraction.
According to neutron diffraction of deuterium carbonic acid () in a hybrid clamped cell (Russian alloy/copper-beryllium) at 1.85 GPa, the molecules are planar and form dimers joined by pairs of . All three C-O bonds are nearly equidistant at 1.34 Angstrom, intermediate between typical C-O and C=O distances (respectively 1.43 and 1.23 Å). The unusual C-O bond lengths are attributed to delocalized Pi bond in the molecule's center and extraordinarily strong hydrogen bonds. The same effects also induce a very short O—O separation (2.13 Å), through the 136° O-H-O angle imposed by the doubly hydrogen-bonded 8-membered rings.
In aqueous solution
In even a slight presence of water, carbonic acid dehydrates to carbon dioxide and water, which then Autocatalysis further decomposition.
The hydrate equilibrium constant at 25 °C is in pure water
In biological solutions
In the presence of the enzyme carbonic anhydrase, equilibrium is instead reached rapidly, and the following reaction takes precedence:
When the created carbon dioxide exceeds its solubility, gas evolves and a third equilibrium CO_2 (soln) <=> CO_2 (g) 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 denominator care is needed with regard to units since Henry's law constant can be commonly expressed with 8 different dimensionalities.
In water pH control
In wastewater treatment and agriculture irrigation, carbonic acid is used to acidify the water similar to sulfuric acid and sulfurous acid produced by sulfur burners.
Under high CO2 partial pressure
In the beverage industry, sparkling or "fizzy water" is usually referred to as carbonated water. It is made by dissolving carbon dioxide under a small positive pressure in water. Many treated the same way effervescent.
Significant amounts of molecular exist in aqueous solutions subjected to pressures of multiple Gigapascal (tens of thousands of atmospheres) in planetary interiors.
Relationship to bicarbonate and carbonate
Carbonic acid is the formal Brønsted–Lowry conjugate acid of the bicarbonate anion, stable in alkaline solution. The protonation constants have been measured to great precision, but depend on overall ionic strength . The two equilibria most easily measured are as follows: where brackets indicate the concentration of species. At 25 °C, these equilibria empirically satisfy[IUPAC (2006). " Stability constants" (database). ] decreases with increasing , as does . In a solution absent other ions (e.g. ), these curves imply the following stepwise dissociation constants: Direct values for these constants in the literature include and .
To interpret these numbers, note that two chemical species in an acid equilibrium are Equinumerosity when . In particular, the extracellular fluid (cytosol) in biological systems exhibits , so that carbonic acid will be almost 50%-dissociated at equilibrium.
Ocean acidification
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.
Further reading
External links
