Calcium carbonate

Calcium carbonate
General
Systematic name	Calcium carbonate
Other names	Limestone, calcite, aragonite, chalk, marble
Molecular formula	CaCO₃
Molar mass	100.087 g/mol
Appearance	White powder.
CAS number	^*
Properties
Density and phase	2.83 g/cm³, solid.
Solubility in water	Insoluble
Melting point	825°C (1098 K)
Boiling point	Decomposes
Acidity (pK_a)	?
Basicity (pK_b)	?
Thermochemistry
Δ_fH⁰_liquid	-1154 kJ/mol
Δ_fH⁰_solid	-1207 kJ/mol
S⁰_solid	93 J/mol·K
Structure
Molecular shape	Linear
Coordination geometry	Tetrahedral
Dipole moment	? D
Hazards
MSDS	External MSDS
Main hazards	Not hazardous.
NFPA 704
Flash point	Non-flammable.
R/S statement	R: , , S: ,
RTECS number	?
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	Calcium bicarbonate Calcium sulfate
Other cations	Magnesium carbonate (dolomite) Strontium carbonate
Related compounds	Calcium oxide
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Chemical infobox

Calcium carbonate is a chemical compound, with chemical formula Ca C O₃. It is commonly used medicinally as a calcium supplement or as an antacid. Calcium carbonate is the active ingredient in agricultural lime. It is a common substance found as rock in all parts of the world and is the main component of seashells and the shell of snails. It is usually the principle cause of hard water.

Occurrence

Calcium carbonate is found naturally as the following minerals and rocks:

Eggshells are composed of approximately 95% calcium carbonate.

To test whether a mineral or rock contains calcium carbonate, strong acids, like hydrochloric acid, can be dropped with a dropper onto it. If it does contain the chemical, it will fizz and produce carbon dioxide; otherwise, it probably wouldn't react vigorously. For example, all of the rocks/mineral mentioned above will react with acid.

Preparation

The vast majority of calcium carbonate used in industry is extracted by mining or quarrying. Pure calcium carbonate (e.g. for food or pharmaceutical use), can be produced from a pure quarried source (usually marble) or it can be prepared by passing carbon dioxide into a solution of calcium hydroxide: the calcium carbonate precipitates out, and this grade of product is referred to as a precipitate (abbreviated to PCC).

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Chemical properties

See also: Carbonate

Calcium carbonate shares the typical properties of other carbonates. Notably:

it reacts with strong acids, releasing carbon dioxide.

CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O

it releases carbon dioxide on heating (to above 825 °C in the case of CaCO₃), to form calcium oxide.

CaCO₃ → CaO + CO₂

Calcium carbonate will react with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate.

CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂

This reaction is important in the erosion of carbonate rocks, forming caverns, and leads to hard water in many regions.

Uses

The main use of calcium carbonate is in the construction industry, either as a building material in its own right (e.g. marble) or limestone aggregate for roadbuilding or as an ingredient of cement or as the starting material for the preparation of builder's lime by burning in a kiln .

Calcium carbonate is widely used as an extender in paints, in particular matte emulsion paint where typically 30% by weight of the paint is either chalk or marble.

Calcium carbonate is also widely used as a filler in plastics. Some typical examples include around 15 to 20% loading of chalk in uPVC drain pipe, 5 to 15% loading of stearate coated chalk or marble in uPVC window profile. Fine ground calcium carbonate is an essential ingredient in the microporous film used in babies nappies and some building films as the pores are nucleated around the calcium carbonate particles during the manufacture of the film by biaxial stretching.

Calcium carbonate is also used in a wide range of trade and DIY adhesives, sealants and decorating fillers. Ceramic tile adhesives typically contain 70 to 80% limestone. Decorating crack fillers contain similar levels of marble or dolomite. It is also mixed with putty in setting Stained glass windows, and as a resist to prevent glass from sticking to kiln shelves when firing glazes and paints at high temperature.

Calcium carbonate is widely used medicinally as an inexpensive calcium supplement, antacid, and/or phosphate binder. It is also used in the pharmaceutical industry as a base material for tablets of other pharmaceuticals.

Calcium carbonate is known as whiting in ceramics/glazing applications, where it is used as a common ingredient for many glazes in its white powdered form. When a glaze containing this material is fired in a kiln, the whiting acts as a flux material in the glaze.

It is commonly called chalk as it has been a major component of blackboard chalk. Chalk may consist of either calcium carbonate or gypsum, hydrated calcium sulfate CaSO₄·2H₂O.

Recently, calcium carbonate has begun to replace kaolin in the production of glossy paper.

As a food additive, it is used in some soy milk products as a source of dietary calcium.

In 1989, Dr. Simmons introduced CaCO₃ into the Whetstone Brook in Massachusetts. His hope was that the calcium carbonate would counter the acid in the stream from acid rain and save the trout that had ceased to spawn. Although his experiment was a success, it did increase the amounts of aluminum ions in the area of the brook that was not treated with the limestone. This shows that CaCO₃ can be added to neutralize the effects of acid rain in river ecosystems. Nowadays, calcium carbonate is used to neutralise acidic conditions in both soil and water.

Solubility of calcium carbonate in water

Calcium carbonate is not rigorously insoluble in water. For the following equilibrium reaction

CaC0₃(solid) ↔ Ca²⁺ + CO₃²⁻, we take a solubility product $\scriptstyle K_{sp}=$ ^*^*=4.47\times 10^{-9} at 25°C (K_sp=3.8 x 10⁻⁹ is given in ⁽¹⁾)

Considering a saturated pure CaCO₃ solution, the calculation of the Ca²⁺ concentration must take into account the equilibria between the three different carbonate forms (H₂CO₃, HCO₃⁻ and CO₃²⁻) as well as the equilibrium between H₂CO₃ and the dissolved CO₂ and the equilibrium between the dissolved CO₂ and the gaseous CO₂ above the solution. The reactions involved are the following (see carbonic acid):

CO₂(gas) ↔ CO₂(dissolved) with $\scriptstyle \frac{$ ^*}{p_{CO_2}}=\frac{1}{k'_c} where k'_c=29.76 atm/(mol/L) at 25°C (Henry constant), $\scriptstyle p_{CO_2}$ being the CO₂ partial pressure.

CO₂(dissolved) + H₂O ↔ H₂CO₃ with $\scriptstyle K_h=\frac{$ ^*}{^*}=1.70 \times 10^{-3} at 25°C

H₂CO₃ ↔ H⁺ + HCO₃⁻ with $\scriptstyle K_{a1}=\frac{$ ^*^*}{^*}=2.5 \times 10^{-4} at 25°C

HCO₃⁻ ↔ H⁺ + CO₃²⁻ with $\scriptstyle K_{a2}=\frac{$ ^*^*}{^*}=5.61 \times 10^{-11} at 25°C

The above relations (together with the $\scriptstyle$ ^*=10^{-14}" target="_blank" >relation and the neutrality condition $\scriptstyle2$ ^*=^*+2^*+^*, i.e. 7 equations for 7 unknowns) allow the numerical calculation of the pH and of the Ca^{2+ concentration as a function of $\scriptstyle p_{CO_2}$ . The result is given in the table below:}

\scriptstyle p_{CO_2}

(atm)

^* (mol/L)

10⁻¹²

12.0

5.19 x 10⁻³

10⁻¹⁰

11.3

1.12 x 10⁻³

10⁻⁸

10.7

2.55 x 10⁻⁴

10⁻⁶

9.83

1.20 x 10⁻⁴

10⁻⁴

8.62

3.16 x 10⁻⁴

3.5 x 10⁻⁴

8.27

4.70 x 10⁻⁴

10⁻³

7.96

6.62 x 10⁻⁴

10⁻²

7.30

1.42 x 10⁻³

10⁻¹

6.63

3.05 x 10⁻³

5.96

6.58 x 10⁻³

5.30

1.42 x 10⁻²

We see that for normal atmospheric conditions ( $\scriptstyle p_{CO_2}=3.5\times 10^{-4}$ atm), we get a slightly basic solution (pH = 8.3) with a low Ca^{2+ concentration (4.7 x 10⁻⁴ mol/L i.e. 0.019 g/L of Ca). Increasing the CO₂ pressure makes the solution slightly acid with a better Ca solubility (0.57 g/L of Ca at 10 atm). For decreasing CO₂ pressure values, the solubility goes to a minimum for $\scriptstyle p_{CO_2}= 10^{-6}$ atm and then increases again as the solution gets strongly basic.}

Remark: For $\scriptstyle p_{CO_2}$ > 10⁻⁴ atm, CO₃²⁻, H⁺ and OH⁻ concentrations can be neglected in the neutrality condition. This means physically that we have essentially a calcium bicarbonate solution. In this case, the system can be solved analytically, giving (with a very good precision)

$\simeq \left(\frac{K_{a1}K_{sp}K_h}{4K_{a2}k_c^\prime}\right)^{1/3}p_{CO_2}^{1/3}$

References

⁽¹⁾CSUDH

External links

ATC codes: and

Calcium compounds | Carbonates | Limestone | Phosphate binders

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