carbonate#Biological significance

The carbonate ion is the simplest oxocarbon anion. It consists of one carbon atom surrounded by three oxygen atoms, in a trigonal planar arrangement, with D_3h molecular symmetry. It has a molecular mass of 60.01 g/mol and carries a total formal charge of −2. It is the conjugate base of the hydrogencarbonate (bicarbonate){{Citation |url = http://old.iupac.org/publications/books/rbook/Red_Book_2005.pdf |title = Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005 |publisher = IUPAC |page = 137 |url-status = live |archive-url = https://web.archive.org/web/20170518230415/http://old.iupac.org/publications/books/rbook/Red_Book_2005.pdf |archive-date = 2017-05-18}} ion, {{chem2|HCO3-}}, which is the conjugate base of {{chem2|H2CO3}}, carbonic acid.

The Lewis structure of the carbonate ion has two (long) single bonds to negative oxygen atoms, and one short double bond to a neutral oxygen atom.

:File:Carbonate-ion-localised-2D.png

This structure is incompatible with the observed symmetry of the ion, which implies that the three bonds are the same length and that the three oxygen atoms are equivalent. As in the case of the isoelectronic nitrate ion, the symmetry can be achieved by a resonance among three structures:

:File:Carbonate-ion-resonance-2D.png

This resonance can be summarized by a model with fractional bonds and delocalized charges:

:File:Carbonate-ion-delocalised-partial-charges-2D.png File:Carbonate-3D-vdW.png

image:Labeled speleothems (cropped).jpg

Metal carbonates generally decompose on heating, liberating carbon dioxide leaving behind an oxide of the metal. This process is called calcination, after calx, the Latin name of quicklime or calcium oxide, CaO, which is obtained by roasting limestone in a lime kiln:

:{{chem2|CaCO3 → CaO + CO2}}

As illustrated by its affinity for {{chem2|Ca(2+)}}, carbonate is a ligand for many metal cations. Transition metal carbonate and bicarbonate complexes feature metal ions covalently bonded to carbonate in a variety of bonding modes.

Lithium, sodium, potassium, rubidium, caesium, and ammonium carbonates are water-soluble salts, but carbonates of 2+ and 3+ ions are often poorly soluble in water. Of the insoluble metal carbonates, calcium carbonate is important because, in the form of scale, it accumulates in and impedes flow through pipes. Hard water is rich in this material, giving rise to the need for infrastructural water softening.

Acidification of carbonates generally liberates carbon dioxide:

:{{chem2|CaCO3 + 2 HCl → CaCl2 + CO2 + H2O}}

Thus, scale can be removed with acid.

In solution the equilibrium between carbonate, bicarbonate, carbon dioxide and carbonic acid is sensitive to pH, temperature, and pressure. Although di- and trivalent carbonates have low solubility, bicarbonate salts are far more soluble. This difference is related to the disparate lattice energies of solids composed of mono- vs dianions, as well as mono- vs dications.

In aqueous solution, carbonate, bicarbonate, carbon dioxide, and carbonic acid participate in a dynamic equilibrium. In strongly basic conditions, the carbonate ion predominates, while in weakly basic conditions, the bicarbonate ion is prevalent. In more acid conditions, aqueous carbon dioxide, {{chem2|CO2(aq)}}, is the main form, which, with water, {{chem2|H2O}}, is in equilibrium with carbonic acid{{snd}}the equilibrium lies strongly towards carbon dioxide. Thus sodium carbonate is basic, sodium bicarbonate is weakly basic, while carbon dioxide itself is a weak acid.

{{main|Carbonate ester}}

In organic chemistry a carbonate can also refer to a functional group within a larger molecule that contains a carbon atom bound to three oxygen atoms, one of which is double bonded. These compounds are also known as organocarbonates or carbonate esters, and have the general formula {{chem2|R\sO\sC(\dO)\sO\sR′}}, or {{chem2|RR′CO3}}. Important organocarbonates include dimethyl carbonate, the cyclic compounds ethylene carbonate and propylene carbonate, and the phosgene replacement, triphosgene.

Three reversible reactions control the pH balance of blood and act as a buffer to stabilise it in the range 7.37–7.43:{{cite web |url=http://www.scifun.org/chemweek/BioBuff/BioBuffers.html |title=Chemical of the Week -- Biological Buffers |access-date=2010-09-05 |url-status=dead |archive-url=https://web.archive.org/web/20110721174437/http://www.scifun.org/chemweek/BioBuff/BioBuffers.html |archive-date=2011-07-21}}{{MerckManual|12|157|a||Acid–Base Regulation and Disorders}}

1. {{chem2|H+ + HCO3- ⇌ H2CO3}}
2. {{chem2|H2CO3 ⇌ CO2(aq) + H2O}}
3. {{chem2|CO2(aq) ⇌ CO2(g)}}

Exhaled {{chem2|CO2(g)}} depletes {{chem2|CO2(aq)}}, which in turn consumes {{chem2|H2CO3}}, causing the equilibrium of the first reaction to try to restore the level of carbonic acid by reacting bicarbonate with a hydrogen ion, an example of Le Châtelier's principle. The result is to make the blood more alkaline (raise pH). By the same principle, when the pH is too high, the kidneys excrete bicarbonate ({{chem2|HCO3-}}) into urine as urea via the urea cycle (or Krebs–Henseleit ornithine cycle). By removing the bicarbonate, more {{chem2|H+}} is generated from carbonic acid ({{chem2|H2CO3}}), which comes from {{chem2|CO2(g)}} produced by cellular respiration.{{cite book |last1= Silverthorn |first1= Dee Unglaub |title= Human physiology. An integrated approach |edition= Seventh, Global |year= 2016 |publisher= Pearson |location=Harlow, England |isbn=978-1-292-09493-9 |pages= 607–608, 666–673}}

Crucially, a similar buffer operates in the oceans. It is a major factor in climate change and the long-term carbon cycle, due to the large number of marine organisms (especially coral) which are made of calcium carbonate. Increased solubility of carbonate through increased temperatures results in lower production of marine calcite and increased concentration of atmospheric carbon dioxide. This, in turn, increases Earth temperature. The amount of {{chem2|CO3(2-)}} available is on a geological scale and substantial quantities may eventually be redissolved into the sea and released to the atmosphere, increasing {{chem2|CO2}} levels even more.{{cite book |ref={{harvid|IPCC SROCC Summary for Policymakers|2019}} |chapter=Summary for Policymakers |chapter-url=https://www.ipcc.ch/site/assets/uploads/sites/3/2019/11/03_SROCC_SPM_FINAL.pdf |author=IPCC |author-link=IPCC |year=2019 |title=IPCC Special Report on the Ocean and Cryosphere in a Changing Climate |pages=3–35}}

{{see also|Category:Carbonates}}

• Carbonate overview:

{{Carbonates}}

It is generally thought that the presence of carbonates in rock is strong evidence for the presence of liquid water. Recent observations of the planetary nebula NGC 6302 show evidence for carbonates in space,Kemper, F., Molster, F.J., Jager, C. and Waters, L.B.F.M. (2001) The mineral composition and spatial distribution of the dust ejecta of NGC 6302. Astronomy & Astrophysics 394, 679–690. where aqueous alteration similar to that on Earth is unlikely. Other minerals have been proposed which would fit the observations.

Small amounts of carbonate deposits have been found on Mars via spectral imaging {{Cite journal |last1=Fairén |first1=Alberto G. |last2=Fernández-Remolar |first2=David |last3=Dohm |first3=James M. |last4=Baker |first4=Victor R. |last5=Amils |first5=Ricardo |date=September 2004 |title=Inhibition of carbonate synthesis in acidic oceans on early Mars |url=https://www.nature.com/articles/nature02911 |journal=Nature |volume=431 |issue=7007 |pages=423–426 |doi=10.1038/nature02911 |pmid=15386004 |bibcode=2004Natur.431..423F |issn=1476-4687|url-access=subscription }} and Martian meteorites also contain small amounts. Groundwater may have existed at Gusev{{Cite journal |last1=Squyres |first1=S. W. |last2=Aharonson |first2=O. |last3=Clark |first3=B.C. |last4=Cohen |first4=B. A. |last5=Crumpler |first5=L. |last6=De Souza |first6=P. A. |last7=Farrand |first7=W. H. |last8=Gellert |first8=R. |last9=Grant |first9=J. |display-authors=1 |year=2007 |title=Pyroclastic Activity at Home Plate in Gusev Crater, Mars |journal=Science |volume=316 |issue=5825 |pages=738–742 |bibcode=2007Sci...316..738S |doi=10.1126/science.1139045 |pmid=17478719 |last10=Grotzinger |first10=J. P. |last11=Haldemann |first11=A. F. C. |last12=Johnson |first12=J. R. |last13=Klingelhofer |first13=G. |last14=Lewis |first14=K. W. |last15=Li |first15=R. |last16=McCoy |first16=T. |last17=McEwen |first17=A. S. |last18=McSween |first18=H. Y. |last19=Ming |first19=D. W. |last20=Moore |first20=J. M. |last21=Morris |first21=R. V. |last22=Parker |first22=T. J. |last23=Rice |first23=J. W. |last24=Ruff |first24=S. |last25=Schmidt |first25=M. |last26=Schroder |first26=C. |last27=Soderblom |first27=L. A. |last28=Yen |first28=A.|url=https://authors.library.caltech.edu/36849/7/Squyres.SOM.pdf |archive-url=https://web.archive.org/web/20170922134035/http://authors.library.caltech.edu/36849/7/Squyres.SOM.pdf |archive-date=2017-09-22 |url-status=live |hdl=2060/20070016011 |s2cid=9687521 |hdl-access=free}} and Meridiani Planum.{{Cite journal |last1=Squyres |first1=S. W. |last2=Arvidson |first2=R. E. |last3=Bollen |first3=D. |last4=Bell |first4=J. F. |last5=Brückner |first5=J. |last6=Cabrol |first6=N. A. |last7=Calvin |first7=W. M. |last8=Carr |first8=M. H. |last9=Christensen |first9=P. R. |display-authors=1 |year=2006 |title=Overview of the Opportunity Mars Exploration Rover Mission to Meridiani Planum: Eagle Crater to Purgatory Ripple |journal=Journal of Geophysical Research: Planets |volume=111 |issue=E12 |bibcode=2006JGRE..11112S12S |doi=10.1029/2006JE002771 |last10=Clark |first10=B. C. |last11=Crumpler |first11=L. |last12=Des Marais |first12=D. J. |last13=d'Uston |first13=C. |last14=Economou |first14=T. |last15=Farmer |first15=J. |last16=Farrand |first16=W. H. |last17=Folkner |first17=W. |last18=Gellert |first18=R. |last19=Glotch |first19=T. D. |last20=Golombek |first20=M. |last21=Gorevan |first21=S. |last22=Grant |first22=J. A. |last23=Greeley |first23=R. |last24=Grotzinger |first24=J. |last25=Herkenhoff |first25=K. E. |last26=Hviid |first26=S. |last27=Johnson |first27=J. R. |last28=Klingelhöfer |first28=G. |last29=Knoll |first29=A. H. |last30=Landis |first30=G.|pages=n/a |url=http://dspace.stir.ac.uk/bitstream/1893/17165/1/Squyres2006_Overview_of_the_Opportunity_Mars_Exploration_Rover_Mission_to_Meridiani_Planum-Eagle_Crater_to_Purgatory_Ripple.pdf |archive-url=https://web.archive.org/web/20170808163157/http://dspace.stir.ac.uk/bitstream/1893/17165/1/Squyres2006_Overview_of_the_Opportunity_Mars_Exploration_Rover_Mission_to_Meridiani_Planum-Eagle_Crater_to_Purgatory_Ripple.pdf |archive-date=2017-08-08 |url-status=live |hdl=1893/17165 |doi-access=free}}

• Cap carbonates
• Carbon trioxide
• Orthocarbonic acid, {{chem2|H4CO4}}, or {{chem2|C(OH)4}}, a hypothetic unstable molecule
• Oxalate
• Peroxocarbonate
• Sodium percarbonate

{{reflist}}

{{Commons category|Carbonates}}

• [http://www2.iq.usp.br/docente/gutz/Curtipot_.html Carbonate/bicarbonate/carbonic acid equilibrium in water: pH of solutions, buffer capacity, titration and species distribution vs. pH computed with a free spreadsheet]
• {{cite encyclopedia|title=Carbonate|url=http://dictionary.reference.com/browse/carbonate|dictionary=Dictionary.com|access-date=5 April 2014}}

{{Oxides of carbon}}

{{Carbon compounds}}

{{Authority control}}

Category:Carbon oxyanions

Category:Oxocarbon anions