Carbonic acid

Carbonic acid
Names
	IUPAC name Carbonic acid
	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 ;
3D model ( JSmol)	Interactive image;
ChEBI	CHEBI:28976 ;
ChEMBL	ChEMBL1161632 ;
ChemSpider	747 ;
DrugBank	DB14531;
ECHA InfoCard	100.133.015
EC Number	610-295-3;
Gmelin Reference	25554
KEGG	C01353 ;
PubChem CID	767;
UNII	142M471B3J ;
CompTox Dashboard (EPA)	DTXSID9043801 ;
	InChI InChI=1S/CH2O3/c2-1(3)4/h(H2,2,3,4) Key: BVKZGUZCCUSVTD-UHFFFAOYSA-N ; 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) (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° (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 [77 °F], 100 kPa). verify (what is ?) Infobox references

Carbonic acid is a chemical compound with the chemical formula H₂C O₃. 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

lungs

may be called volatile acid or respiratory acid.

Anhydrous carbonic acid

At ambient temperatures, pure carbonic acid is a stable gas.

Brønsted acid.^[7]^[8]

Carbonic acid

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

At low temperatures and

X-ray diffraction.^[10]

But at high pressure, carbonic acid crystallizes, and modern analytical spectroscopy can measure its geometry.

According to

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

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]

{\ce {HCO3^- {+}H^+ <=> CO2 {+}H2O}}

When the created carbon dioxide exceeds its solubility, gas evolves and a third equilibrium

{\ce {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:

{\begin{aligned}{\ce {HCO3^{-}{}+ H+{}<=> CO2(soln){}+ H2O}}&&K_{3}={\frac {[{\ce {H+}}][{\ce {HCO3^-}}]}{[{\ce {CO2(soln)}}]}}\end{aligned}}

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

effervesce

.

Significant amounts of molecular H
₂CO
₃ exist in aqueous solutions subjected to pressures of multiple

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

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:

{\begin{aligned}{\ce {CO3^{2-}{}+ H+{}<=> HCO3^-}}&&\beta _{1}={\frac {[{\ce {HCO3^-}}]}{[{\ce {H+}}][{\ce {CO3^{2-}}}]}}\\{\ce {CO3^{2-}{}+ 2H+{}<=> H2CO3}}&&\beta _{2}={\frac {[{\ce {H2CO3}}]}{[{\ce {H+}}]^{2}[{\ce {CO3^{2-}}}]}}\end{aligned}}

where brackets indicate the

specie. At 25 °C, these equilibria empirically satisfy^[18]

{\begin{alignedat}{6}\log(\beta _{1})=&&0&.54&I^{2}-0&.96&I+&&9&.93\\\log(\beta _{2})=&&-2&.5&I^{2}-0&.043&I+&&16&.07\end{alignedat}}

[iupac2013-1] ISBN 978-0-85404-182-4
.

[sublime-2] 
PMID 9478889
.

[neutron-3] 
ISSN 2304-6740
.

[4] ISBN 978-0-08-037941-8
.

[lo-5] 
PMID 10760883
.

[6] Acid-Base Physiology 2.1 – Acid-Base Balance by Kerry Brandis.

[peroxide-7] 
S2CID 240250750
.

[Andersen-8] 
S2CID 17094010
.

[9] :10.1002/cphc.201000220
.

[wi-10] 
PMID 17944463
.

[HS-11] OCLC 56834315
.

[SB-12] :10.1016/S0304-4203(02)00010-5
.

[Lindskog_1997-13] PMID 9336012
.

[14] ISSN 1365-3075
.

[15] PMID 26813580
.

[16] S2CID 195791860
.

[Saleh-Scirep-17] PMID 27580525
.

[18] IUPAC (2006). "Stability constants
" (database).

[19] PMID 26862781
.

[cald03-20] S2CID 4417880
.

[sabine-21] S2CID 5607281. Archived
from the original on 6 July 2008. Retrieved 22 June 2021.

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[13]

[14]

[17]

[18]

[19]

[20]

[21]

v t e Inorganic compounds of carbon and related ions
Compounds	CF CO CO₂ CO₃ CO₄ CO₅ CO₆ COS CS C₂S₂ CS₂ CSe₂ C₃O₂ C₃S₂ SiC
Carbon ions	Carbides [:C≡C:]²⁻, [::C::]⁴⁻, [:C=C=C:]⁴⁻ Cyanide [:C≡N:]⁻ Cyanate [:O−C≡N:]⁻ Thiocyanate [:S−C≡N:]⁻ Fulminate [:C≡N−O:]⁻ Thiofulminate [:C≡N−S:]⁻
Nanostructures	Graphite intercalation compounds Fullerides
Oxides and related	Oxides Nitrides Metal carbonyls Carbonic acid Bicarbonates Carbonates

v t e Oxocarbons
Common oxides	CO CO₂
Exotic oxides	CO₃ CO₄ CO₅ CO₆ C₂O C₂O₂ C₂O₃ C₂O₄ (1,2-Dioxetanedione and 1,3-Dioxetanedione) C₃O C₃O₂ C₃O₃ C₃O₆ C₄O₂ C₄O₄ C₄O₆ C₅O₂ C₅O₅ C₆O₆ (Cyclohexanehexone and Ethylenetetracarboxylic dianhydride) C₈O₈ C₉O₉ C₁₀O₈ C₁₀O₁₀ C₁₂O₆ C₁₂O₉ C₁₂O₁₂
Polymers	Graphite oxide C₃O₂ CO CO₂
Compounds derived from oxides	Metal carbonyls Carbonic acid Bicarbonates Carbonates Polycarbonates (Dicarbonates and Tricarbonates) Peroxydicarbonates

Carbonic acid

Terminology in biochemical literature

Anhydrous carbonic acid

In aqueous solution

In biological solutions

Under high CO₂ partial pressure

Relationship to bicarbonate and carbonate

Ocean acidification

Further reading

References

External links

Terminology in biochemical literature

Anhydrous carbonic acid

In aqueous solution

In biological solutions

Under high CO2 partial pressure

Relationship to bicarbonate and carbonate

Ocean acidification

Further reading

References

External links

Under high CO₂ partial pressure