Calcium hydroxide

"Slaked lime" redirects here. For (Ca(OH)2)(aq), see limewater. For the application of slaked lime as a wall colorant, see limewash.
Calcium hydroxide
Names
IUPAC name
Calcium hydroxide
Other names
Slaked lime
Milk of lime
Calcium(II) hydroxide
Pickling lime
Hydrated lime
Portlandite
Calcium hydrate
Identifiers
1305-62-0 Yes
ChEBI CHEBI:31341 Yes
ChemSpider 14094 Yes
EC number 215-137-3
Jmol-3D images Image
Image
KEGG D01083 Yes
PubChem 14777
RTECS number EW2800000
UNII PF5DZW74VN Yes
Properties
Ca(OH)2
Molar mass 74.093 g/mol
Appearance white powder
Odor odorless
Density 2.211 g/cm3, solid
Melting point 580 °C (1,076 °F; 853 K) (loses water, decomposes)
0.189 g/100 mL (0 °C)
0.173 g/100 mL (20 °C)
0.066 g/100 mL (100 °C)
5.5×10−6
Solubility Soluble in glycerol and acids.
Insoluble in alcohol.
Acidity (pKa) 12.4
Basicity (pKb) 2.37
1.574
Thermochemistry
83 J·mol−1·K−1[1]
Std enthalpy of
formation (ΔfHo298)
 −987 kJ·mol−1[1]
Hazards
MSDS External MSDS
EU Index C (C), Xi (Xi)
R-phrases R22, R34
S-phrases (S2), S24
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
0
3
0
Flash point Non-flammable
7340 mg/kg (oral, rat)
7300 mg/kg (mouse)
US health exposure limits (NIOSH):
TWA 15 mg/m3 (total) 5 mg/m3 (resp)[2]
TWA 5 mg/m3[2]
N.D.[2]
Related compounds
Other cations
 Magnesium hydroxide
Strontium hydroxide
Barium hydroxide
Related bases
Calcium oxide
Supplementary data page
Refractive index (n),
Dielectric constant (εr), etc.
Thermodynamic
data
 Phase behaviour
solid–liquid–gas
UV, IR, NMR, MS
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Calcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca(OH)2. It is a colorless crystal or white powder and is obtained when calcium oxide (called lime or quicklime) is mixed, or "slaked" with water. It has many names including hydrated lime, builders' lime, slack lime, cal, or pickling lime. Calcium hydroxide is used in many applications, including food preparation. Limewater is the common name for a saturated solution of calcium hydroxide.

Properties

When heated to 512 °C, the partial pressure of water in equilibrium with calcium hydroxide reaches 101 kPa (normal atmospheric pressure), which decomposes calcium hydroxide into calcium oxide and water.[3]

Ca(OH)2 → CaO + H2O

A suspension of fine calcium hydroxide particles in water is called milk of lime. The solution is called limewater and is a medium strength base that reacts with acids and attacks many metals. Limewater turns milky in the presence of carbon dioxide due to formation of calcium carbonate, a process called carbonatation:

Ca(OH)2 + CO2 → CaCO3 + H2O

Structure, preparation, occurrence

SEM image of fractured hardened cement paste, showing plates of calcium hydroxide and needles of ettringite (micron scale)

Calcium hydroxide adopts a polymeric structure, as do the related hydroxides of the alkaline earth metals. The packing resembles the cadmium iodide motif with layers of octahedral Ca centres. Strong hydrogen bonds exist between the layers.[4]

The structure of calcium hydroxide, with the hydrogen atoms omitted (purple = O centres).

Calcium hydroxide is produced commercially by treating lime with water:

CaO + H2O → Ca(OH)2

In the laboratory it can be prepared by mixing aqueous solutions of calcium chloride and sodium hydroxide. The mineral form, portlandite, is relatively rare but can be found in some volcanic, plutonic, and metamorphic rocks. It has also been known to arise in burning coal dumps.

Uses

One significant application of calcium hydroxide is as a flocculant, in water and sewage treatment. It forms a fluffy charged solid that aids in the removal of smaller particles from water, resulting in a clearer product. This application is enabled by the low cost and low toxicity of calcium hydroxide. It is also used in fresh water treatment for raising the pH of the water so pipes will not corrode where the base water is acidic because it is self-regulating and does not raise the pH too much.

Another large application is in the paper industry, where it is used in the production of sodium hydroxide. This conversion is a component of the Kraft process.[4]

Niche uses

Calcium hydroxide is produced on a large scale, is easily handled and is generally inexpensive. Numerous niche applications are in use. A partial listing follows:

Ancient Celtic use

According to Diodorus Siculus:

"The Gauls are tall of body with rippling muscles and white of skin and their hair is blond, and not only naturally so for they also make it their practice by artificial means to increase the distinguishing colour which nature has given it. For they are always washing their hair in limewater and they pull it back from the forehead to the nape of the neck, with the result that their appearance is like that of Satyrs and Pans since the treatment of their hair makes it so heavy and coarse that it differs in no respect from the mane of horses."

Food industry

Because of its low toxicity and the mildness of its basic properties, slaked lime is widely used in the food industry to:

Native American uses

Dry treated corn (left), and untreated corn (right) after boiling in water with calcium hydroxide (1 Tbsp lime for 500 g of corn) for 15 minutes

In Spanish, calcium hydroxide is called cal. Corn cooked with cal (nixtamalization) becomes hominy (nixtamal), which significantly increases the bioavailability of niacin, and it is also considered tastier and easier to digest.

In chewing areca nut or coca leaves, calcium hydroxide is usually chewed alongside to keep the alkaloid stimulants chemically available for absorption by the body. Similarly, Native Americans traditionally chewed tobacco leaves with calcium hydroxide derived from burnt mollusk shells to enhance the effects. It has also been used by some indigenous American tribes as an ingredient in yopo, a psychedelic snuff prepared from the beans of some Anadenanthera species.[6]

Afghan uses

It is used in making naswar (also known as nass or niswar), a type of dipping tobacco made from fresh tobacco leaves, calcium hydroxide (chuna), and wood ash. It is consumed most in the Pathan diaspora, Afghanistan, Pakistan, India, Bangladesh and also in Sweden and Norway. Villagers also use calcium hydroxide to paint their mud houses in Afghanistan and Pakistan.

Health risks

Unprotected exposure to Ca(OH)2 can pose health risks, so it should be limited. It can cause severe skin irritation, chemical burns, blindness, or lung damage. See MSDS.[7]

See also

References

  1. ↑ 1.0 1.1 Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A21. ISBN 0-618-94690-X.
  2. ↑ 2.0 2.1 2.2 "NIOSH Pocket Guide to Chemical Hazards #0092". National Institute for Occupational Safety and Health (NIOSH).
  3. ↑ Halstead, P.E.; Moore, A.E. (1957). "The Thermal Dissociation Of Calcium Hydroxide". Journal of the Chemical Society 769: 3873. doi:10.1039/JR9570003873.
  4. ↑ 4.0 4.1 Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
  5. ↑ O,Driscoll, Catherine (21 July 2008). "A dash of lime -- a new twist that may cut CO2 levels back to pre-industrial levels". Chemistry. PhysOrg.com. Retrieved 20 November 2010.
  6. ↑ A multidisciplinary overview of intoxicating snuff rituals in the Western Hemisphere, Peter A. G. M. de Smet, doi:10.1016/0378-8741(85)90060-1
  7. ↑

External links