Carbonic acid

Names
IUPAC name Carbonic acid[1]
Other names Oxidocarboxylic acid Hydroxyformic acid Hydroxymethanoic acid Carbonylic acid Hydroxycarboxylic acid Dihydroxycarbonyl Carbon dioxide solution Aerial acid Metacarbonic acid
Identifiers
CAS Number	463-79-6 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:28976 Y
ChEMBL	ChEMBL1161632 Y
ChemSpider	747 Y
DrugBank	DB14531
ECHA InfoCard	100.133.015
EC Number	610-295-3
Gmelin Reference	25554
KEGG	C01353 Y
PubChem CID	767
UNII	142M471B3J Y
CompTox Dashboard (EPA)	DTXSID9043801
InChI InChI=1S/CH2O3/c2-1(3)4/h(H2,2,3,4) Y Key: BVKZGUZCCUSVTD-UHFFFAOYSA-N Y InChI=1/H2O3/c2-1(3)4/h(H2,2,3,4) Key: BVKZGUZCCUSVTD-UHFFFAOYAU
SMILES O=C(O)O
Properties
Chemical formula	H 2CO 3
Appearance	Colorless gas
Melting point	−53 °C (−63 °F; 220 K)[2] (sublimes)
Boiling point	127 °C (261 °F; 400 K) (decomposes)
Solubility in water	Reacts to form carbon dioxide and water
Acidity (pKa)	pKa1 = 3.6 at 25 °C pKa2 = 10.329
Conjugate base	Bicarbonate, carbonate
Hazards
NFPA 704 (fire diamond)	0 0 1
Structure
Crystal structure	monoclinic
Space group	p21/c, No. 14
Point group	-
Lattice constant	a = 5.392 Å, b = 6.661 Å, c = 5.690 Å α = 90°, β = 92.66°, γ = 90°[3] (D 2CO 3 at 1.85 GPa, 298 K)
Lattice volume (V)	204.12 Å3
Formula units (Z)	4 formula per cell
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). Y verify (what is YN ?) Infobox references

Carbonic acid is a chemical compound with the chemical formula H₂CO₃. The molecule rapidly converts to water and carbon dioxide in the presence of water. However, in the absence of water, it is (contrary to popular belief) quite stable at room temperature.^[4][5] The interconversion of carbon dioxide and carbonic acid is related to the breathing cycle of animals and the acidification of natural waters.^[3]

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.^[6]

Terminology in biochemical literature

In chemistry, the term "carbonic acid" strictly refers to the chemical compound with the formula H
₂CO
₃. 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.^[5] There are two main methods to produce anhydrous carbonic acid: reaction of hydrogen chloride and potassium bicarbonate at 100 K in methanol and proton irradiation of pure solid carbon dioxide.^[2] Chemically, it behaves as a diprotic Brønsted acid.^[7][8]

Carbonic acid monomers exhibit three conformational isomers: cis–cis, cis–trans, and trans–trans.^[9]

At low temperatures and atmospheric pressure, solid carbonic acid is amorphous and lacks Bragg peaks in X-ray diffraction.^[10] But at high pressure, carbonic acid crystallizes, and modern analytical spectroscopy can measure its geometry.

According to neutron diffraction of dideuterated carbonic acid (D
₂CO
₃) in a hybrid clamped cell (Russian alloy/copper-beryllium) at 1.85 GPa, the molecules are planar and form dimers joined by pairs of hydrogen bonds. All three C-O bonds are nearly equidistant at 1.34 Å, 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 π bonding 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.^[3] Longer O—O distances are observed in strong intramolecular hydrogen bonds, e.g. in oxalic acid, where the distances exceed 2.4 Å.^[10]

In aqueous solution

In even a slight presence of water, carbonic acid dehydrates to carbon dioxide and water, which then catalyzes further decomposition.^[5] For this reason, carbon dioxide can be considered the carbonic acid anhydride.

The hydration equilibrium constant at 25 °C is / ≈ 1.7×10⁻³ in pure water^[11] and ≈ 1.2×10⁻³ in seawater.^[12] Hence the majority of carbon dioxide at geophysical or biological air-water interfaces does not convert to carbonic acid, remaining dissolved CO₂ gas. However, the uncatalyzed equilibrium is reached quite slowly: the rate constants are 0.039 s⁻¹ for hydration and 23 s⁻¹ for dehydration.

In biological solutions

In the presence of the enzyme carbonic anhydrase, equilibrium is instead reached rapidly, and the following reaction takes precedence:^[13]

When the created carbon dioxide exceeds its solubility, gas evolves 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 denominator care is needed with regard to units since Henry's law constant can be commonly expressed with 8 different dimensionalities.^[14]

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 soft drinks treated the same way effervesce.

Significant amounts of molecular H
₂CO
₃ exist in aqueous solutions subjected to pressures of multiple gigapascals (tens of thousands of atmospheres) in planetary interiors.^[15][16] Pressures of 0.6–1.6 GPa at 100 K, and 0.75–1.75 GPa at 300 K are attained in the cores of large icy satellites such as Ganymede, Callisto, and Titan, where water and carbon dioxide are present. Pure carbonic acid, being denser, is expected to have sunk under the ice layers and separate them from the rocky cores of these moons.^[17]

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 I. The two equilibria most easily measured are as follows: