carbonate

Ball-and-stick model of the carbonate ion, CO2−3

In chemistry, a carbonate is a salt or ester of carbonic acid, characterized by the presence of the carbonate ion, CO2−3 or a carbonate functional group O=C(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 beverages — either by the addition carbon dioxide gas under pressure, or by dissolving carbonate or bicarbonate salts into the water.

Carbonate minerals are extremely varied and ubiquitous. The most common are calcite or calcium carbonate, CaCO₃, the chief constituent of limestone (as well as the main component of mollusk 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, e.g. 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 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 D_3h molecular symmetry. The carbonate ion carries a negative two formal charge and is the conjugate base of the hydrogen carbonate ion, HCO₃⁻, which is the conjugate base of H₂CO₃, 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:

Like the isoelectronic nitrate ion, resonance structures can be used to depict the carbonate ion:

In reality, CO₃²⁻ has three equally long C-O bonds:

Chemical properties

Metal carbonates generally decompose on heating, liberating carbon dioxide 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, M⁺, attaches to the negatively charged oxygen atoms of the ion, forming an ionic compound:

2 M⁺ + CO₃²⁻ → M₂CO₃

M²⁺ + CO₃²⁻ → MCO₃

2 M³⁺ + 3 CO₃²⁻ → 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.

Acid-base chemistry

The carbonate ion (CO₃²⁻) is a moderately strong base. It is a conjugate base of the weakly acidic bicarbonate (IUPAC name hydrogen carbonate HCO₃⁻), 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 ions (protons).

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.

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 toxic triphosgene.

Biological significance

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.

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 shows evidence for carbonates in space,^[1] 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. Groundwater may have existed at both Gusev^[2] and Meridiani Planum.^[3]

References

1. ^ Kemper, F., Molster, F.J., Jager, C. and Waters, L.B.F.M. (2002) The mineral composition and spatial distribution of the dust ejecta of NGC 6302. Astronomy & Astrophysics 394, 679-690.
2. ^ Squyres et al., (2007) doi 10.1126/science.1139045
3. ^ Squyres et al., (2006) doi 10.1029/2006JE002771

See also

• Bicarbonate