Calcium hydroxide

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 produced when quicklime (calcium oxide) is mixed with water. Annually, approximately 125 million tons of calcium hydroxide are produced worldwide.<ref name=Ullmann/>

Calcium hydroxide has many names including hydrated lime, caustic lime, builders' lime, slaked lime, cal, and pickling lime. Calcium hydroxide is used in many applications, including food preparation, where it has been identified as E number E526. Limewater, also called milk of lime, is the common name for a saturated solution of calcium hydroxide.

Solubility

Calcium hydroxide is moderately soluble in water, as seen for many dihydroxides. Its solubility increases from 0.66 g/L at 100 °C to 1.89 g/L at 0 °C.<ref name=Ullmann/> Its solubility product K_sp of 5.02Template:E at 25 °C,<ref name="crc" /> its dissociation in water is large enough that its solutions are basic according to the following dissolution reaction: Template:Block indent The solubility is affected by the common-ion effect. Its solubility drastically decreases upon addition of hydroxide or calcium sources.

Reactions

When heated to 512 °C, the partial pressure of water in equilibrium with calcium hydroxide reaches 101Template:NbspkPa (normal atmospheric pressure), which decomposes calcium hydroxide into calcium oxide and water:<ref>Template:Cite journal</ref> Template:Block indent

When carbon dioxide is passed through limewater, the solution takes on a milky appearance due to precipitation of insoluble calcium carbonate: Template:Block indent If excess CO₂ is added: the following reaction takes place: Template:Block indent The milkiness disappears since calcium bicarbonate is water-soluble.

Calcium hydroxide reacts with aluminium. This reaction is the basis of aerated concrete.<ref name=Ullmann/> It does not corrode iron and steel, owing to passivation of their surface.

Calcium hydroxide reacts with hydrochloric acid to give calcium hydroxychloride and then calcium chloride.

In a process called sulfation, sulphur dioxide reacts with limewater: Template:Block indent

Limewater is used in a process known as lime softening to reduce water hardness. It is also used as a neutralizing agent in municipal waste water treatment.

Structure and preparation

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 all metal hydroxides. The structure is identical to that of Mg(OH)Template:Sub (brucite structure); i.e., the cadmium iodide motif. Strong hydrogen bonds exist between the layers.<ref name=G&W/>

Calcium hydroxide is produced commercially by treating (slaking) quicklime with water: Template:Block indent Alongside the production of quicklime from limestone by calcination, this is one of the oldest known chemical reactions; evidence of prehistoric production dates back to at least 7000 BCE.<ref>Template:Cite web</ref>

Uses

Calcium hydroxide is commonly used to prepare lime mortar.

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 that pipes will not corrode where the base water is acidic, because it is self-regulating and does not raise the pH too much.Template:Cn

Another large application is in the paper industry, where it is an intermediate in the reaction in the production of sodium hydroxide. This conversion is part of the causticizing step in the Kraft process for making pulp. In the causticizing operation, burned lime is added to green liquor, which is a solution primarily of sodium carbonate and sodium sulfate produced by dissolving smelt, which is the molten form of these chemicals from the recovery furnace.<ref name=G&W>Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. Template:ISBN.</ref>

In orchard crops, calcium hydroxide is used as a fungicide. Applications of 'lime water' prevent the development of cankers caused by the fungal pathogen Neonectria galligena. The trees are sprayed when they are dormant in winter to prevent toxic burns from the highly reactive calcium hydroxide. This use is authorised in the European Union and the United Kingdom under Basic Substance regulations.<ref>Template:Cite web</ref>

Calcium hydroxide is used in dentistry, primarily in the specialty of endodontics due to its antibacterial properties and induction of hard-tissue deposition.<ref>Template:Cite journal</ref>

Food industry

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

• In USDA certified food production in plants and livestock<ref>Template:Cite web</ref>
• To clarify raw juice from sugarcane or sugar beets in the sugar industry (see carbonatation)
• To process water for alcoholic beverages and soft drinks
• To increase the rate of Maillard reactions (pretzels)<ref name="nordicfoodlab">Template:Cite web</ref>
• Pickle cucumbers and other foods
• To make Chinese century eggs
• In maize preparation: removes the cellulose hull of maize kernels (see nixtamalization)
• To clear a brine of carbonates of calcium and magnesium in the manufacture of salt for food and pharmaceutical uses
• In fortifying (Ca supplement) fruit drinks, such as orange juice, and infant formula
• As a substitute for baking soda in making papadam
• In the removal of carbon dioxide from controlled atmosphere produce storage rooms
• In the preparation of mushroom growing substrates<ref>Template:Cite web</ref>

Native American uses

Dry untreated maize (left), and treated maize (right) after boiling in water with calcium hydroxide (15Template:Nbspml, or 1Template:Nbsptbsp, lime for 500Template:Nbspg of corn) for 15 minutes

In Nahuatl, the language of the Aztecs, the word for calcium hydroxide is Template:Lang. In a process called nixtamalization, maize is cooked with nextli to become nixtamal, also known as hominy. Nixtamalization significantly increases the bioavailability of niacin (vitamin B₃), preventing pellagra.<ref>Template:Cite journal</ref> Nixtamal is also considered tastier and easier to digest. It is often ground into a flour, known as masa, which is used to make tortillas and tamales, which cannot be done with non-nixtamalized maize.Template:Cn

In chewing 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 mollusc shells to enhance the effects. It has also been used by some indigenous South American tribes as an ingredient in yopo, a psychedelic snuff prepared from the beans of some Anadenanthera species.<ref>Template:Cite journal</ref>

Asian uses

Calcium hydroxide, locally known as chuna, choona or soon, is typically added to a bundle of areca nut and betel leaf called "paan" to keep the alkaloid stimulants chemically available to enter the bloodstream via sublingual absorption.

Choona is a key ingredient in Petha, contributing to its characteristic crunchy and firm texture.

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

Traditionally lime water was used in Taiwan and China to preserve persimmon and to remove astringency.<ref>Template:Cite book</ref>Template:Rp

Hobby uses

In buon fresco painting, limewater is used as the colour solvent to apply on fresh plaster. Historically, it is known as the paint whitewash.

Closeup of cast of The Dying Gaul, showing distinctive hairstyle, supposedly derived from washing in limewater

Limewater is widely used by marine aquarists as a primary supplement of calcium and alkalinity for reef aquariums. Corals of order Scleractinia build their endoskeletons from aragonite (a polymorph of calcium carbonate). When used for this purpose, limewater is usually referred to as Kalkwasser. It is also used in tanning and making parchment. The lime is used as a dehairing agent based on its alkaline properties.<ref>The Nature and Making of Parchment by Ronald Reed Template:ISBN?</ref>

Personal care and adornment

Treating one's hair with limewater causes it to stiffen and bleach, with the added benefit of killing any lice or mites living there. Diodorus Siculus described the Celts as follows: "The Gauls are tall of body, with rippling muscles, and white of skin, and their hair is blond, and not only naturally so, but 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 lime-water, and they pull it back from the forehead to the top of the head and back 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."<ref>Template:Cite book</ref>

Calcium hydroxide is also applied in a leather process called liming.

In stars

The ion CaOH⁺ has been detected in the atmosphere of S-type stars.<ref>Template:Cite book</ref>

Limewater

Limewater is a saturated aqueous solution of calcium hydroxide. Calcium hydroxide is sparsely soluble at room temperature in water (1.5 g/L at 25 °C<ref>'Solubility of Inorganic and Metalorganic Compounds – A Compilation of Solubility Data from the Periodical Literature', A. Seidell, W. F. Linke, Van Nostrand (Publisher), 1953 Template:ISBN?</ref>). "Pure" (i.e. less than or fully saturated) limewater is clear and colorless, with a slight earthy smell and an astringent/bitter taste. It is basic in nature with a pH of 12.4. Limewater is named after limestone, not the lime fruit. Limewater may be prepared by mixing calcium hydroxide (Ca(OH)₂) with water and removing excess undissolved solute (e.g. by filtration). When excess calcium hydroxide is added (or when environmental conditions are altered, e.g. when its temperature is raised sufficiently), there results a milky solution due to the homogeneous suspension of excess calcium hydroxide. This liquid has been known traditionally as milk of lime.

Health risks

Unprotected exposure to Ca(OH)₂, as with any strong base, can cause skin burns, but it is not acutely toxic.<ref name=Ullmann>Template:Cite book</ref>

See also

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External links

• Template:Cite web
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• CDC – NIOSH Pocket Guide to Chemical Hazards – Calcium Hydroxide
• MSDS Data Sheet