calcium

calcium

[kal-see-uhm]
calcium [Lat.,=lime], metallic chemical element; symbol Ca; at. no. 20; at. wt. 40.08; m.p. about 839°C;; b.p. 1,484°C;; sp. gr. 1.55 at 20°C;; valence +2. Calcium is a malleable, ductile, silver-white, relatively soft metal with face-centered, cubic crystalline structure. Chemically it resembles strontium and barium; it is classed with them as an alkaline-earth metal in Group 2 of the periodic table. Calcium is chemically active; it tarnishes rapidly when exposed to air and burns with a bright yellow-red flame when heated, mainly forming the nitride. It reacts directly with water, forming the hydroxide. It combines with other elements, e.g., with oxygen, carbon, hydrogen, chlorine, fluorine, arsenic, phosphorus, and sulfur, forming many compounds.

Although lime (calcium oxide) has been known since ancient times, elemental calcium was first isolated by Sir Humphry Davy in 1808. Today, calcium metal is usually prepared by electrolysis of fused calcium chloride to which a little calcium fluoride has been added. It is used in alloys with other metals, such as aluminum, lead, or copper; in preparation of other metals, such as thorium and uranium, by reduction; and (like barium) in the manufacture of vacuum tubes to remove residual gases.

The metal is of little commercial importance compared to its compounds, which are widely and diversely used. The element is a constituent of lime (see calcium oxide), chloride of lime (bleaching powder), mortar, plaster, cement (see cement, concrete, whiting, putty, precipitated chalk, gypsum, and plaster of Paris. Tremolite, a form of asbestos, is a naturally occurring compound of calcium, magnesium, silicon, and oxygen. Calcium carbide reacts with water to form acetylene gas; it is also used to prepare calcium cyanamide, which is used as a fertilizer. The phosphate is a major constituent of bone ash. The arsenate and the cyanide are used as insecticides. Calcium bicarbonate causes temporary hardness in water; calcium sulfate causes permanent hardness. Generally, calcium compounds show an orange or yellow-red color when held in the Bunsen burner flame.

Although calcium is the fifth most abundant element in the earth's crust, of which it constitutes about 3.6%, it is not found uncombined. It is found widely distributed in its compounds, e.g., Iceland spar, marble, limestone, feldspar, apatite, calcite, dolomite, fluorite, garnet, and labradorite. It is a constituent of most plant and animal matter.

Calcium is essential to the formation and maintenance of strong bones and teeth. In the human adult the bone calcium is chiefly in the form of the phosphate and carbonate salts. A sufficient store of vitamin D in the body is necessary for the proper utilization of calcium. Calcium also functions in the regulation of the heartbeat and in the conversion of prothrombin to thrombin, a necessary step in the clotting of blood.

Inadequate supply or metabolism of calcium, the main structural element of bones and teeth. Its metabolism is regulated by vitamin D, phosphorus, and hormones (see parathyroid gland). Calcium in the blood has roles in muscle contraction, nerve-impulse transmission, blood clotting, milk production, hormone secretion, and enzyme function, for which calcium is pulled from the bones if deficiency develops. Chronic deficiency may be a factor in the development of osteoporosis or osteomalacia (softening of bone) and may contribute to hypertension and colon cancer. Severe calcium deficiency (hypocalcemia), usually the result of a metabolic problem rather than a dietary deficiency, causes numbness, tingling, and painful muscle aches and spasms.

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Chemical element, one of the alkaline earth metals, chemical symbol Ca, atomic number 20. The most abundant metallic element in the human body, it is an essential part of bones and teeth and has many physiological functions (see calcium deficiency; tooth). It is the fifth most abundant element in Earth's crust but does not occur naturally in the free state. In its compounds calcium has valence 2. It occurs in limestone, chalk, marble, dolomite, eggshells, pearls, coral, and many marine shells as calcium carbonate, or calcite; in apatite as calcium phosphate; in gypsum as calcium sulfate; and in many other minerals. It is used as an alloying agent and in other metallurgical applications; its alloy with lead is used in cable sheathing and grids for batteries. Calcite is used as a lime source, a filler, a neutralizer, and an extender; in pure form it is used in baking powder and as an antacid and calcium supplement. Calcium oxide (lime) and its product after water addition, calcium hydroxide (slaked lime), are important industrially. Other significant compounds are calcium chloride (a drying agent), calcium hypochlorite (a bleach), calcium sulfate (gypsum and plaster of paris), and calcium phosphate (a plant food and stabilizer for plastics).

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Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust. Calcium is also the fifth most abundant dissolved ion in seawater by both molarity and mass, after sodium, chloride, magnesium, and sulfate.

Calcium is essential for living organisms, particularly in cell physiology, where movement of the calcium ion Ca2+ into and out of the cytoplasm functions as a signal for many cellular processes. As a major material used in mineralization of bones and shells, calcium is the most abundant metal by mass in many animals.

Notable characteristics

Chemically calcium is reactive and soft for a metal (though harder than lead, it can be cut with a knife with difficulty). It is a silvery metallic element that must be extracted by electrolysis from a fused salt like calcium chloride. Once produced, it rapidly forms a grey-white oxide and nitride coating when exposed to air. It is somewhat difficult to ignite, unlike magnesium, but when lit, the metal burns in air with a brilliant high-intensity red light. Calcium metal reacts with water, evolving hydrogen gas at a rate rapid enough to be noticeable, but not fast enough at room temperature to generate much heat. In powdered form, however, the reaction with water is extremely rapid, as the increased surface area of the powder accelerates the reaction with the water. Part of the slowness of the calcium-water reaction results from the metal being partly protected by insoluble white calcium hydroxide. In water solutions of acids where the salt is water soluble, calcium reacts vigorously.

Calcium, though it has a higher resistivity than copper or aluminium, weight for weight, allowing for its much lower density calcium is a rather better conductor than either. However, its use in terrestrial applications is usually limited by its high reactivity with air. In vacuum use, calcium tends to sublime unless plated.

Calcium salts are colorless from any contribution of the calcium, and ionic solutions of calcium (Ca2+) are colorless as well. Many calcium salts are not soluble in water. When in solution, the calcium ion to the human taste varies remarkably, being reported as mildly salty, sour, "mineral like" or even "soothing." It is apparent that many animals can taste, or develop a taste, for calcium, and use this sense to detect the mineral in salt licks or other sources. In human nutrition, soluble calcium salts may be added to tart juices without much effect to the average palate.

Calcium is the fifth most abundant element by mass in the human body, where it is a common cellular ionic messenger with many functions, and serves also as a structural element in bone. It is the relatively high atomic-numbered calcium in the skeleton which causes bone to be radio-opaque. Of the human body's solid components after drying (as for example, after cremation), about a third of the total mass is the approximately one kilogram of calcium which composes the average skeleton (the remainder being mostly phosphorus and oxygen).

Occurrence

Calcium is not naturally found in its elemental state. Calcium occurs most commonly in sedimentary rocks in the minerals calcite, dolomite and gypsum. It also occurs in igneous and metamorphic rocks chiefly in the silicate minerals: plagioclase, amphiboles, pyroxenes and garnets.

See also Calcium minerals.

Applications

Some uses are:

Calcium compounds

H and K lines

In the visible portion of the spectrum of many stars, including the Sun, strong absorption lines of singly-ionized calcium are shown. Prominent among these are the H-line at 3968.5 Å and the K line at 3933.7 Å of singly-ionized calcium, or Ca II. For the Sun and stars with low temperatures, the prominence of the H and K lines can be an indication of strong magnetic activity in the chromosphere. Measurement of periodic variations of these active regions can also be used to deduce the rotation periods of these stars.

History

Calcium (Latin calx, meaning "limestone") was known as early as the first century when the Ancient Romans prepared lime as calcium oxide. It was not isolated until 1808 in England when Sir Humphry Davy electrolyzed a mixture of lime and mercuric oxide. Davy was trying to isolate calcium; when he heard that Swedish chemist Jöns Jakob Berzelius and Pontin prepared calcium amalgam by electrolyzing lime in mercury, he tried it himself. He worked with electrolysis throughout his life and also discovered/isolated sodium, potassium, magnesium, boron and barium.

Compounds

Calcium, combined with phosphate to form hydroxylapatite, is the mineral portion of human and animal bones and teeth. The mineral portion of some corals can also be transformed into hydroxylapatite.

Calcium oxide (lime) is used in many chemical refinery processes and is made by heating and carefully adding water to limestone. When lime is mixed with sand, it hardens into a mortar and is turned into plaster by carbon dioxide uptake. Mixed with other compounds, lime forms an important part of Portland cement.

Calcium carbonate (CaCO3) is one of the common compounds of calcium. It is heated to form quicklime (CaO), which is then added to water (H2O). This forms another material known as slaked lime (Ca(OH)2), which is an inexpensive base material used throughout the chemical industry. Chalk, marble, and limestone are all forms of calcium carbonate.

When water percolates through limestone or other soluble carbonate rocks, it partially dissolves the rock and causes cave formation and characteristic stalactites and stalagmites and also forms hard water. Other important calcium compounds are calcium nitrate, calcium sulfide, calcium chloride, calcium carbide, calcium cyanamide and calcium hypochlorite.

Isotopes

Calcium has four stable isotopes (40Ca and 42Ca through 44Ca), plus two more isotopes (46Ca and 48Ca) that have such long half-lives that for all practical purposes they can be considered stable. It also has a cosmogenic isotope, radioactive 41Ca, which has a half-life of 103,000 years. Unlike cosmogenic isotopes that are produced in the atmosphere, 41Ca is produced by neutron activation of 40Ca. Most of its production is in the upper metre or so of the soil column, where the cosmogenic neutron flux is still sufficiently strong. 41Ca has received much attention in stellar studies because it decays to 41K, a critical indicator of solar-system anomalies.

97% of naturally occurring calcium is in the form of 40Ca. 40Ca is one of the daughter products of 40K decay, along with 40Ar. While K-Ar dating has been used extensively in the geological sciences, the prevalence of 40Ca in nature has impeded its use in dating. Techniques using mass spectrometry and a double spike isotope dilution have been used for K-Ca age dating.

The most abundant isotope, 40Ca, has a nucleus of 20 protons and 20 neutrons. This is the heaviest stable isotope of any element which has equal numbers of protons and neutrons. In supernova explosions, calcium is formed from the reaction of carbon with various numbers of alpha particles (helium nuclei), until the most common calcium isotope (containing 10 helium nuclei) has been synthesized.

Nutrition

Recommended Adequate Intake by the IOM for Calcium:
Age Calcium (mg/day)
0–6 months 210
7–12 months 270
1–3 years 500
4–8 years 800
9–18 years 1300
19–50 years 1000
51+ years 1200

Calcium is an important component of a healthy diet. Calcium is essential for the normal growth and maintenance of bones and teeth, and calcium requirements must be met throughout life. Long-term calcium deficiency can lead to rickets and poor blood clotting and in case of a menopausal woman, it can lead to osteoporosis, in which the bone deteriorates and there is an increased risk of fractures. While a lifelong deficit can affect bone and tooth formation, over-retention can cause hypercalcemia (elevated levels of calcium in the blood), impaired kidney function and decreased absorption of other minerals. High calcium intakes or high calcium absorption were previously thought to contribute to the development of kidney stones. However, more recent studies show that high dietary calcium intakes actually decrease the risk for kidney stones. Vitamin D is needed to absorb calcium.

Dairy products, such as milk and cheese, are a well-known source of calcium. However, some individuals are allergic to dairy products and even more people, particularly those of non Indo-European descent, are lactose-intolerant, leaving them unable to consume non-fermented dairy products in quantities larger than about half a liter per serving. Others, such as vegans, avoid dairy products for ethical and health reasons. Fortunately, many good sources of calcium exist. These include seaweeds such as kelp, wakame and hijiki; nuts and seeds (like almonds and sesame); blackstrap molasses; beans; oranges; figs; quinoa; amaranth; collard greens; okra; rutabaga; broccoli; dandelion leaves; kale; and fortified products such as orange juice and soy milk. (However, calcium fortified orange juice often contains vitamin D3 derived from lanolin, and is thus unacceptable for vegans. ) An overlooked source of calcium is eggshell, which can be ground into a powder and mixed into food or a glass of water. Cultivated vegetables generally have less calcium than wild plants.

The calcium content of most foods can be found in the USDA National Nutrient Database.

Dietary calcium supplements

Calcium supplements are used to prevent and to treat calcium deficiencies. Most experts recommend that supplements be taken with food and that no more than 600 mg should be taken at a time because the percent of calcium absorbed decreases as the amount of calcium in the supplement increases. It is recommended to spread doses throughout the day. Recommended daily calcium intake for adults ranges from 1000 to 1500 mg. It is recommended to take supplements with food to aid in absorption.

Vitamin D is added to some calcium supplements. Vitamin D is not necessary, but it might be beneficial if the person has low vitamin D status. Proper vitamin D status is important because vitamin D is converted to a hormone in the body which then induces the synthesis of intestinal proteins responsible for calcium absorption.

  • The absorption of calcium from most food and commonly-used dietary supplements is very similar. This is contrary to what many calcium supplement manufacturers claim in their promotional materials.
  • Milk is an excellent source of dietary calcium because it has a high concentration of calcium and the calcium in milk is excellently absorbed.
  • Calcium carbonate is the most common and least expensive calcium supplement. It should be taken with food. The absorption of calcium from calcium carbonate is similar to the absorption of calcium from milk. While most people digest calcium carbonate very well, some might develop gastrointestinal discomfort or gas. Taking magnesium with it can help to avoid constipation. Calcium carbonate is 40% elemental calcium. 1000 mg will provide 400 mg of calcium. However, supplement labels will usually indicate how much calcium is present in each serving, not how much calcium carbonate is present.
  • Antacids, such as Tums, frequently contain calcium carbonate, and are a very commonly-used, inexpensive calcium supplement.
  • Coral Calcium is a salt of calcium derived from fossilized coral reefs. Coral calcium is composed of calcium carbonate and trace minerals.
  • Calcium citrate can be taken without food and is the supplement of choice for individuals with achlorhydria or who are taking histamine-2 blockers or proton-pump inhibitors. It is more easily digested and absorbed than calcium carbonate if taken on empty stomach and less likely to cause constipation and gas than calcium carbonate. It also has a lower risk of contributing to the formation of kidney stones. Calcium citrate is about 21% elemental calcium. 1000 mg will provide 210 mg of calcium. It is more expensive than calcium carbonate and more of it must be taken to get the same amount of calcium.
  • Calcium phosphate costs more than calcium carbonate, but less than calcium citrate. It is easily absorbed and is less likely to cause constipation and gas than either.
  • Calcium lactate has similar absorption as calcium carbonate, but is more expensive. Calcium lactate and calcium gluconate are less concentrated forms of calcium and are not practical oral supplements.
  • Calcium chelates are synthetic calcium compounds, with calcium bound to an organic molecule, such as malate, aspartate, or fumarate. These forms of calcium may be better absorbed on an empty stomach. However, in general they are absorbed similarly to calcium carbonate and other common calcium supplements when taken with food. The 'chelate' mimics the action that natural food performs by keeping the calcium soluble in the intestine. Thus, on an empty stomach, in some individuals, chelates might theoretically be absorbed better.
  • Microcrystalline hydroxyapatite (MH) is marketed as a calcium supplement, and has in some randomized trials been found to be more effective than calcium carbonate.
  • Orange juice with calcium added is a good dietary source for persons who have lactose intolerance.

The National Nutritional Food Association — NNFA (Newport Beach, Calif.) defines a chelate very specifically, and several criteria must be met in order for chelation to actually occur. Some of the claimed "chelates" on the market are the various Krebs (Citric Acid) Cycle chelates, such as citrate, malate, and aspartate. Dicalcium malate (chelated with malic acid) is a newer form of a true calcium chelate. It contains a high amount of elemental calcium (30%).

In July 2006, a report citing research from Fred Hutchinson Cancer Research Center in Seattle, Washington claimed that women in their 50s gained 5 pounds less in a period of 10 years by taking more than 500 mg of calcium supplements than those who did not. However, the doctor in charge of the study, Dr. Alejandro J. Gonzalez also noted it would be "going out on a limb" to suggest calcium supplements as a weight-limiting aid.

Prevention of fractures due to osteoporosis

Such studies often do not test calcium alone, but rather combinations of calcium and vitamin D. Randomized controlled trials found both positive and negative effects. The different results may be explained by doses of calcium and underlying rates of calcium supplementation in the control groups. However, it is clear that increasing the intake of calcium promotes deposition of calcium in the bones, where it is of more benefit in preventing the compression fractures resulting from the osteoporotic thinning of the dendritic web of the bodies of the vertebrae, than it is at preventing the more serious cortical bone fractures which happen at hip and wrist.

Prevention of cancer?

A meta-analysis by the international Cochrane Collaboration of two randomized controlled trialsfound that calcium "might contribute to a moderate degree to the prevention of adenomatous colonic polyps".

More recent studies were conflicting, and one which was positive for effect (Lappe, et al.) did control for a possible anti-carcinogenic effect of vitamin D, which was found to be an independent positive influence from calcium-alone on cancer risk (see second study below) .

  • A randomized controlled trial found that 1000 mg of elemental calcium and 400 IU of vitamin D3 had no effect on colorectal cancer
  • A randomized controlled trial found that 1400–1500 mg supplemental calcium and 1100 IU vitamin D3 reduced aggregated cancers with a relative risk of 0.402.
  • An observational cohort study found that high calcium and vitamin D intake was associated with "lower risk of developing premenopausal breast cancer."

Overdose

Exceeding the recommended daily calcium intake for an extended period of time can result in hypercalcemia.

See also

Notes

References

  • Rebecca J. Donatelle. Health, The Basics. 6th ed. San Francisco: Pearson Education, Inc. 2005.

External links

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