Calcium Carbonate
Encyclopedia : C : CA : CAL : 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 | [471-34-1] |
| Properties | |
| Density and phase | 2.83 g/cm3, solid. |
| Solubility in water | Insoluble |
| Melting point | 825°C (1098 K) |
| Boiling point | Decomposes |
| Acidity (pKa) | ? |
| Basicity (pKb) | ? |
| Thermochemistry | |
| ΔfH0liquid | -1154 kJ/mol |
| ΔfH0solid | -1207 kJ/mol |
| S0solid | 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: R36, R37, R38 S: S26, S36 |
| RTECS number | ? |
| Supplementary data page | |
| Structure and properties | n, εr, 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 infoboxInfobox disclaimer and references] | |
Calcium carbonate is a chemical compound, with chemical formula CaCO3. 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
- it reacts with strong acids, releasing carbon dioxide.
- :CaCO3 + 2HCl → CaCl2 + CO2 + H2O
- CaCO3 + CO2 + H2O → Ca(HCO3)2
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_=[Ca^][CO_3^]=4.47\times 10^] at 25°C (Ksp=3.8 x 10−9 is given in (1))
- CO2(gas) ↔ CO2(dissolved) with [\scriptstyle \frac}=\frac] where k'c=29.76 atm/(mol/L) at 25°C (Henry constant), [\scriptstyle p_] being the CO2 partial pressure.
- CO2(dissolved) + H2O ↔ H2CO3 with [\scriptstyle K_h=\frac=1.70 \times 10^] at 25°C
- H2CO3 ↔ H+ + HCO3− with [\scriptstyle K_=\frac=2.5 \times 10^] at 25°C
- HCO3− ↔ H+ + CO32− with [\scriptstyle K_=\frac]}=5.61 \times 10^] at 25°C
| [\scriptstyle p_] (atm) | pH | [Ca2+] (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 |
| 1 | 5.96 | 6.58 x 10−3 |
| 10 | 5.30 | 1.42 x 10−2 |
We see that for normal atmospheric conditions ([\scriptstyle p_=3.5\times 10^] atm), we get a slightly basic solution (pH = 8.3) with a low Ca2+ 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_= 10^] atm and then increases again as the solution gets strongly basic.
Remark: For [\scriptstyle p_] > 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)
[\scriptstyle[H^+] \simeq \left(\frac^2K_K_h^2}k_c^}\right)^p_^\;\;\;,\;\;\;[Ca^] \simeq \left(\fracK_K_h}k_c^\prime}\right)^p_^]
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