In chemistry, a carbonate is a salt or ester of carbonic acid.
To test for the presence of the carbonate anion in a salt, the addition of dilute mineral acid (e.g. hydrochloric acid
) will yield carbon dioxide gas.
Carbonate-containing salts are industrially and mineralogically ubiquitous. The term "carbonate" is also commonly used to refer to one of these salts or carbonate minerals. Most common is calcite, or calcium carbonate, CaCO3, the chief constituent of limestone. The process of removing carbon dioxide from these salts by heating is called calcination.
The term is also used as a verb, to describe the process of raising carbonate and bicarbonate concentrations in soda, see also carbonated water, either by the introduction under pressure of carbon dioxide gas into the bottle, or by dissolving carbonate or bicarbonate salts into the water.
Structure and bonding
The carbonate ion
is a polyatomic anion
with the empirical formula CO32−
and a molecular mass of 60.01 daltons
; it consists of one central carbon atom
surrounded by three identical oxygen atoms in a trigonal planar
arrangement, and has D3h molecular symmetry
. The carbonate ion carries a negative two formal charge
and is the conjugate base
of the hydrogen carbonate
, which is the conjugate base of H2
, carbonic acid.
The structure and bonding of the carbonate ion cannot be properly represented by its Lewis structure, which depicts CO32− with two long single bonds and one short double bond:
Resonance structures can be used to depict the carbonate ion:
In reality, CO32− has three equally long C-O bonds:
A carbonate salt forms when a positively charged ion attaches to the negatively charged oxygen atoms of the ion, forming an ionic compound
- 2M+ + CO32− → M2CO3
- M2+ + CO32− → MCO3
- 2M3+ + 3CO32− → M2(CO3)3
Most carbonate salts are insoluble in water at standard temperature and pressure, with solubility constants of less than 1×10−8. Exceptions include sodium, potassium and ammonium carbonates.
In aqueous solution, carbonate, bicarbonate, carbon dioxide, and carbonic acid exist together 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, CO2(aq), is the main form, which, with water, H2O, is in equilibrium with carbonic acid - 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.
Carbonated water is formed by dissolving CO2 in water under pressure. When the partial pressure of CO2 is reduced, for example when a can of soda is opened, the equilibrium for each of the forms of carbonate (carbonate, bicarbonate, carbon dioxide, and carbonic acid) shifts until the concentration of CO2 in the solution is equal to the solubility of CO2 at that temperature and pressure. In living systems an enzyme, carbonic anhydrase, speeds the interconversion of CO2 and carbonic acid.
The carbonate ion (CO32−
) is a moderately strong base. It is a conjugate base of the weakly acidic bicarbonate
name hydrogen carbonate
), itself a moderately strong conjugate base of the still weakly acidic carbonic acid
. As such in aqueous solution, the carbonate ion seeks to reclaim hydrogen atoms.
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 which is double bonded. These compounds are also known as organocarbonates or carbonate esters, and have the general formula ROCOOR′, or RR′CO3
. Important organocarbonates include dimethyl carbonate
, the cyclic compounds ethylene carbonate
and propylene carbonate
, and the toxic triphosgene
It works as a buffer in the blood as follows:
when pH is too low, the concentration of hydrogen ions is too high, so you exhale CO2
. This will cause the equation to shift left, essentially decreasing the concentration of H+
ions, causing a more basic pH.
When pH is too high, the concentration of hydrogen ions in the blood is too low, so the kidneys excrete bicarbonate (HCO3−). This causes the equation to shift right, essentially increasing the concentration of hydrogen ions, causing a more acidic pH.
It is generally thought that the presence of carbonates in rock
is unequivocal evidence for the presence of liquid water. Recent observations of the Planetary nebula NGC 6302
shows evidence for carbonates in space, where aqueous alteration similar to that on Earth is unlikely. Other minerals have been proposed which would fit the observations.
Significant carbonate deposits have not been found on Mars via remote sensing or in situ missions, even though Martian meteorites contain small amounts and groundwater may have existed at both Gusev and Meridiani Planum.