Product Code Database
A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group .
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, , the chief constituent of limestone (as well as the main component of mollusc shells and coral skeletons); dolomite, a calcium-magnesium carbonate ; and siderite, or iron(II) carbonate, , 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 , and more. New applications of alkali metal carbonates include: thermal energy storage, catalysis and electrolyte both in fuel cell technology as well as in electrosynthesis of in aqueous media.Anodic generation of hydrogen peroxide in continuous flow, DOI: 10.1039/D2GC02575B (Paper) Green Chem., 2022, 24, 7931–7940
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.
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:
This resonance can be summarized by a model with fractional bonds and delocalization charges:
Lithium, sodium carbonate, 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, is important because, in the form of limescale, 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:
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 energy 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, , is the main form, which, with water, , 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.
Exhaled depletes , which in turn consumes , 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 () into urine as urea via the urea cycle (or Krebs–Henseleit ornithine cycle). By removing the bicarbonate, more is generated from carbonic acid (), which comes from produced by cellular respiration. (2025). 9781292094939, Pearson. ISBN 9781292094939
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 available is on a geological scale and substantial quantities may eventually be redissolved into the sea and released to the atmosphere, increasing levels even more.
Small amounts of carbonate deposits have been found on Mars via spectral imaging and Martian meteorites also contain small amounts. Groundwater may have existed at Gusev crater and Meridiani Planum.
|
|
