Carbonates

In chemistry, a carbonate is a salt of

carbonic acid

(H₂CO₃), characterized by the presence of the carbonate ion, a polyatomic ion with the formula of . The name may also refer to a

carbonate ester

, an organic compound containing the carbonate group C(=O)(O–)₂.

The term is also used as a verb, to describe carbonation: the process of raising the concentrations of carbonate and

bicarbonate

ions in water to produce carbonated water and other carbonated beverageseither by the addition of

carbon dioxide

gas under pressure, or by dissolving carbonate or bicarbonate salts into the water.

In

geology

and

mineralogy

, the term "carbonate" can refer both to carbonate minerals and carbonate rock (which is made of chiefly carbonate minerals), and both are dominated by the carbonate ion, . Carbonate minerals are extremely varied and ubiquitous in chemically precipitated

sedimentary rock

. The most common are

calcite

or

calcium carbonate

, CaCO₃, the chief constituent of

limestone

(as well as the main component of mollusc shells and

coral

skeletons); dolomite, a calcium-magnesium carbonate CaMg(CO₃)₂; and

siderite

, or

iron

(II) carbonate, FeCO₃, an important iron ore.

Sodium carbonate

("soda" or "natron") and

potassium carbonate

("potash") have been used since antiquity for cleaning and preservation, as well as for the manufacture of

glass

. Carbonates are widely used in industry, such as in iron smelting, as a raw material for

Portland cement

and lime manufacture, in the composition of ceramic glazes, and more.

Structure and bonding

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

hydrogen carbonate (bicarbonate)

ion, , which is the conjugate base of ,

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.

:

This structure is incompatible with the observed symmetry of the ion, which implies that the three bonds are equally long 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:

:

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

delocalized

charges:

:

Chemical properties

Metal carbonates generally decompose on heating, liberating carbon dioxide from the long term

carbon cycle

to the short term carbon cycle and 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

.

A carbonate salt forms when a positively charged ion, , , or , associates with the negatively charged oxygen atoms of the ion by forming electrostatic attractions with them, forming an ionic compound:

:2  + →

: + →

:2  + 3  →

Most carbonate salts are

insoluble

in

water

at standard temperature and pressure, with solubility constants of less than . Exceptions include

lithium

,

sodium

,

potassium

and

ammonium

carbonates, as well as many uranium 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 acidthe 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.

Although the carbonate salts of most metals are insoluble in water, the same is not true of the bicarbonate salts. In solution this equilibrium between carbonate, bicarbonate, carbon dioxide and carbonic acid changes constantly to the changing temperature and pressure conditions. In the case of metal ions with insoluble carbonates, such as

CaCO₃

, formation of insoluble compounds results. This is an explanation for the buildup of scale inside pipes caused by hard water.

Carbonate in the inorganic nomenclature

Systematic additive IUPAC name for carbonate anion is trioxidocarbonate(2−). Similarly,

cyanide

anion CN^− is named nitridocarbonate(1−). However, following the same logic for carbonate(4−) (orthocarbonic acid), by similitude to silicate(4−) ( orthosilicic acid), in the systematic additive nomenclature makes no sense as this species has never been identified under normal conditions of temperature and pressure. Orthocarbonic acid is energetically much less stable than orthosilicic acid and cannot exist under normal conditions because of the energetically unfavorable orbital configuration of a single central carbon atom bound to four oxygen atoms.

Organic carbonates

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 ROCOOR′, or RR′CO₃. Important organocarbonates include

dimethyl carbonate

, the cyclic compounds

ethylene carbonate

and propylene carbonate, and the phosgene replacement, triphosgene.

Buffer

It works as a buffer in the blood as follows: when pH is low, the concentration of hydrogen ions is too high, so one exhales 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 (). This causes the equation to shift right, essentially increasing the concentration of hydrogen ions, causing a more acidic pH.

Three important reversible reactions control the above pH balance:

# H₂CO₃ H⁺ +

# H₂CO₃ CO₂ + H₂O

# CO₂(aq) CO₂(g)

Exhaled CO₂(g) depletes CO₂(aq), which in turn consumes H₂CO₃, causing the aforementioned shift left in the first reaction by Le Châtelier's principle. By the same principle, when the pH is too high, the kidneys excrete bicarbonate () into urine as urea via the

urea cycle

(or Krebs–Henseleit ornithine cycle). By removing the bicarbonate, more H⁺ is generated from carbonic acid (H₂CO₃), which comes from CO₂(g) produced by

cellular respiration

.

Crucially, this same 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 formed 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 tonnage of is on a geological scale and may all be redissolved into the sea and released to the atmosphere, increasing CO₂ levels even more.

Carbonate salts

* Carbonate overview:

Presence outside Earth

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, where aqueous alteration similar to that on Earth is unlikely. Other minerals have been proposed which would fit the observations.

Until recently carbonate deposits have not been found on Mars via remote sensing or in situ missions, even though Martian meteorites contain small amounts. Groundwater may have existed at

Gusev

and

Meridiani Planum

.

See also

* Cap carbonates
* Orthocarbonic acid, , or , a hypothetic unstable molecule
* Oxalate
* Peroxocarbonate
*

Sodium percarbonate

References

External links

Carbonate/bicarbonate/carbonic acid equilibrium in water: pH of solutions, buffer capacity, titration and species distribution vs. pH computed with a free spreadsheet
*

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Oxyanions