Carbonate

In chemistry, a carbonate is a salt of carbonic acid (H2CO3),[2] characterized by the presence of the carbonate ion, a polyatomic ion with the formula of CO2−
3
. 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 beverages – either 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, CO2−
3
. 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, e.g. in iron smelting, as a raw material for Portland cement and lime manufacture, in the composition of ceramic glazes, and more.

Carbonate
Ball-and-stick model of the carbonate anion
Names
IUPAC name
Carbonate
Systematic IUPAC name
Trioxidocarbonate[1]:127
Identifiers
3D model (JSmol)
ChemSpider
UNII
Properties
CO2−
3
Molar mass 60.008 g·mol−1
Conjugate acid Bicarbonate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

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 D3h 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, HCO
3
, which is the conjugate base of H
2
CO
3
, 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.

Simple, localised Lewis structure of the carbonate ion

Simple, localised Lewis structure of the carbonate ion

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 between three structures:

Resonance structures of the carbonate ion

Resonance structures of the carbonate ion

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

Delocalisation and partial charges on the carbonate ion
Space-filling model of the carbonate ion

Delocalisation and partial charges on the carbonate ion
Space-filling model of the carbonate ion

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.[2] 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+
, M2+
, or M3+
, associates with the negatively charged oxygen atoms of the ion by forming electrostatic attractions with them, forming an ionic compound:

2 M+
+ CO2−
3
M
2
CO
3
M2+
+ CO2−
3
MCO
3
2 M3+
+ 3 CO2−
3
M
2
(CO
3
)
3

Most carbonate salts are insoluble in water at standard temperature and pressure, with solubility constants of less than 1 × 10−8. 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 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.

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, e.g. 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−).[1]:127 Similarly, cyanide anion CN is named nitridocarbonate(1−).[1]:291 Following this logic, e.g. carbonate(4−) in the systematic additive nomenclature would mean carbide anion, but it is not the case.[1]:287

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.

Biological significance

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 (HCO
3
). 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:[3]

  1. H2CO3(aq) ⇌ H+(aq) + HCO
    3
    (aq)
  1. H2CO3(aq) ⇌ CO2(aq) + H2O(l)
  1. 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 (HCO
3
) 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 CO2−
3
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:
Carbonates
H2CO3 He
Li2CO3,
LiHCO3
BeCO3 B C (NH4)2CO3,
NH4HCO3
O F Ne
Na2CO3,
NaHCO3,
Na3H(CO3)2
MgCO3,
Mg(HCO3)2
Al2(CO3)3 Si P S Cl Ar
K2CO3,
KHCO3
CaCO3,
Ca(HCO3)2
Sc Ti V Cr MnCO3 FeCO3 CoCO3 NiCO3 CuCO3 ZnCO3 Ga Ge As Se Br Kr
Rb2CO3 SrCO3 Y Zr Nb Mo Tc Ru Rh Pd Ag2CO3 CdCO3 In Sn Sb Te I Xe
Cs2CO3,
CsHCO3
BaCO3   Hf Ta W Re Os Ir Pt Au Hg Tl2CO3 PbCO3 (BiO)2CO3 Po At Rn
Fr Ra   Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og
La2(CO3)3 Ce2(CO3)3 Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Ac Th Pa UO2CO3 Np Pu Am Cm Bk Cf Es Fm Md No Lr

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,[4] 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 both Gusev[5] and Meridiani Planum.[6]

See also

References

  1. ^ a b c d International Union of Pure and Applied Chemistry (2005). Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005). Cambridge (UK): RSCIUPAC. ISBN 0-85404-438-8. Electronic version.
  2. ^ a b Wikisource Chisholm, Hugh, ed. (1911). "Carbonates" . Encyclopædia Britannica (11th ed.). Cambridge University Press.
  3. ^ "Archived copy". Archived from the original on 2011-07-21. Retrieved 2010-09-05.CS1 maint: Archived copy as title (link)
  4. ^ 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.
  5. ^ Squyres, S. W.; et al. (2007). "Pyroclastic Activity at Home Plate in Gusev Crater, Mars". Science. 316 (5825): 738–742. Bibcode:2007Sci...316..738S. doi:10.1126/science.1139045. PMID 17478719.
  6. ^ Squyres, S. W.; et al. (2006). "Overview of the Opportunity Mars Exploration Rover Mission to Meridiani Planum: Eagle Crater to Purgatory Ripple". Journal of Geophysical Research: Planets. 111 (E12): n/a. Bibcode:2006JGRE..11112S12S. doi:10.1029/2006JE002771.

External links

Bicarbonate

In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula HCO−3.

Bicarbonate serves a crucial biochemical role in the physiological pH buffering system.The term "bicarbonate" was coined in 1814 by the English chemist William Hyde Wollaston. The prefix "bi" in "bicarbonate" comes from an outdated naming system and is based on the observation that there is twice as much carbonate (CO2−3) per sodium ion in sodium bicarbonate (NaHCO3) and other bicarbonates than in sodium carbonate (Na2CO3) and other carbonates. The name lives on as a trivial name.

Calcareous

Calcareous is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.

Calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as "calcite".

Other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite over timescales of days or less at temperatures exceeding 300 °C, and vaterite is even less stable.

Calcium carbonate

Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisting mainly of calcite) and is the main component of pearls and the shells of marine organisms, snails, and eggs. Calcium carbonate is the active ingredient in agricultural lime and is created when calcium ions in hard water react with carbonate ions to create limescale. It is medicinally used as a calcium supplement or as an antacid, but excessive consumption can be hazardous.

Carbonate minerals

Carbonate minerals are those minerals containing the carbonate ion, CO32−.

Carbonate rock

Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3) and dolostone, which is composed of the mineral dolomite (CaMg(CO3)2).

Calcite can be either dissolved by groundwater or precipitated by groundwater, depending on several factors including the water temperature, pH, and dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility in which it becomes less soluble in water as the temperature increases.

When conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together or it can fill fractures.

Karst topography and caves develop in carbonate rocks because of their solubility in dilute acidic groundwater. Cooling groundwater or mixing of different groundwaters will also create conditions suitable for cave formation.

Marble is the metamorphic carbonate rock. Rare igneous carbonate rocks exist as intrusive carbonatites and even rarer volcanic carbonate lava.

Carbonic acid

Not to be confused with carbolic acid, an antiquated name for phenol.Carbonic acid is a chemical compound with the chemical formula H2CO3 (equivalently OC(OH)2). It is also a name sometimes given to solutions of carbon dioxide in water (carbonated water), because such solutions contain small amounts of H2CO3. In physiology, carbonic acid is described as volatile acid or respiratory acid, because it is the only acid excreted as a gas by the lungs. It plays an important role in the bicarbonate buffer system to maintain acid–base homeostasis.

Carbonic acid, which is a weak acid, forms two kinds of salts: the carbonates and the bicarbonates. In geology, carbonic acid causes limestone to dissolve, producing calcium bicarbonate, which leads to many limestone features such as stalactites and stalagmites.

It was long believed that carbonic acid could not exist as a pure compound. However, in 1991 it was reported that NASA scientists had succeeded in making solid H2CO3 samples.

Carbonic anhydrase

The carbonic anhydrases (or carbonate dehydratases) from a family of enzymes that catalyze the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid (i.e. bicarbonate and hydrogen ions). The active site of most carbonic anhydrases contains a zinc ion. They are therefore classified as metalloenzymes.

The enzyme maintains acid-base balance and helps transport carbon dioxide.Carbonic anhydrase helps regulate pH and fluid balance. Depending on its location, the role of the enzyme changes slightly. For example, carbonic anhydrase produces acid in the stomach lining. In the kidney, the control of bicarbonate ions influences the water content of the cell. The control of bicarbonate ions also influences the water content in the eyes, and if the enzyme does not work properly, a buildup of fluid can lead to glaucoma.

Dolomite

Dolomite ( ) is an anhydrous carbonate mineral composed of calcium magnesium carbonate, ideally CaMg(CO3)2. The term is also used for a sedimentary carbonate rock composed mostly of the mineral dolomite. An alternative name sometimes used for the dolomitic rock type is dolostone.

Lake County, Colorado

Lake County is one of the 64 counties in the U.S. state of Colorado. As of the 2010 census, the population was 7,310. The county seat and the only municipality in the county is Leadville. The highest natural point in Colorado and the entire Rocky Mountains is the summit of Mount Elbert in Lake County at 14,440 feet (4401.2 meters) elevation.

Limestone

Limestone is a sedimentary rock which is often composed of the skeletal fragments of marine organisms such as coral, foraminifera, and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3).

About 10% of sedimentary rocks are limestones. The solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock.

Limestone has numerous uses: as a building material, an essential component of concrete (Portland cement), as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.

Lithium carbonate

Lithium carbonate is an inorganic compound, the lithium salt of carbonate with the formula Li2CO3. This white salt is widely used in the processing of metal oxides.

For the treatment of bipolar disorder, it is on the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system.

Magnesium carbonate

Magnesium carbonate, MgCO3 (archaic name magnesia alba), is an inorganic salt that is a white solid. Several hydrated and basic forms of magnesium carbonate also exist as minerals.

Malachite

Malachite is a copper carbonate hydroxide mineral, with the formula Cu2CO3(OH)2. This opaque, green banded mineral crystallizes in the monoclinic crystal system, and most often forms botryoidal, fibrous, or stalagmitic masses, in fractures and spaces, deep underground, where the water table and hydrothermal fluids provide the means for chemical precipitation. Individual crystals are rare but do occur as slender to acicular prisms. Pseudomorphs after more tabular or blocky azurite crystals also occur.

Marl

Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and silt. The dominant carbonate mineral in most marls is calcite, but other carbonate minerals such as aragonite, dolomite, and siderite may be present. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay and calcium carbonate, formed under freshwater conditions; specifically an earthy substance containing 35–65% clay and 65–35% carbonate. It also describes a habit of coralline red alga. The term is today often used to describe indurated marine deposits and lacustrine (lake) sediments which more accurately should be named 'marlstone'. Marlstone is an indurated (resists crumbling or powdering) rock of about the same composition as marl, more correctly called an earthy or impure argillaceous limestone. It has a blocky subconchoidal fracture, and is less fissile than shale. The term 'marl' is widely used in English-language geology, while the terms Mergel and Seekreide (German for "lake chalk") are used in European references.

The lower stratigraphic units of the chalk cliffs of Dover consist of a sequence of glauconitic marls followed by rhythmically banded limestone and marl layers. Upper Cretaceous cyclic sequences in Germany and marl–opal-rich Tortonian-Messinian strata in the Sorbas basin related to multiple sea drawdown have been correlated with Milankovitch orbital forcing.Marl as lacustrine sediment is common in post-glacial lake-bed sediments, often found underlying peat bogs. It has been used as a soil conditioner and acid soil neutralizing agent.

Mudstone

Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.063 millimetres (0.0025 in) with individual grains too small to be distinguished without a microscope. With increased pressure over time, the platy clay minerals may become aligned, with the appearance of fissility or parallel layering. This finely bedded material that splits readily into thin layers is called shale, as distinct from mudstone. The lack of fissility or layering in mudstone may be due to either original texture or the disruption of layering by burrowing organisms in the sediment prior to lithification. Mud rocks such as mudstone and shale account for some 65% of all sedimentary rocks. Mudstone looks like hardened clay and, depending upon the circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit.Mudstone can be separated into these categories:

Siltstone — more than half of the composition is silt-sized particles.

Claystone — more than half of the composition is clay-sized particles.

Mudstone — hardened mud; a mix of silt and clay sized particles. Mudstone can include:

Shale — exhibits lamination or fissility.

Argillite — has undergone low-grade metamorphism.

Potassium carbonate

Potassium carbonate is the inorganic compound with the formula K2CO3. It is a white salt, which is soluble in water. It is deliquescent, often appearing a damp or wet solid. Potassium carbonate is mainly used in the production of soap and glass.

Sodium bicarbonate

Sodium bicarbonate (IUPAC name: sodium hydrogen carbonate), commonly known as baking soda, is a chemical compound with the formula NaHCO3. It is a salt composed of a sodium cation (Na+) and a bicarbonate anion (HCO3−). Sodium bicarbonate is a white solid that is crystalline, but often appears as a fine powder. It has a slightly salty, alkaline taste resembling that of washing soda (sodium carbonate). The natural mineral form is nahcolite. It is a component of the mineral natron and is found dissolved in many mineral springs.

Sodium carbonate

Sodium carbonate, Na2CO3, (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, water-soluble salts. All forms have a strongly alkaline taste and give moderately alkaline solutions in water. Historically it was extracted from the ashes of plants growing in sodium-rich soils. Because the ashes of these sodium-rich plants were noticeably different from ashes of wood (once used to produce potash), sodium carbonate became known as "soda ash". It is produced in large quantities from sodium chloride and limestone by the Solvay process.

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