Calcium (, ) is the
chemical element with the symbol
Ca and
atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray
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 Ca
2+ 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
thumb|upright|left|Flame test. Orange color originates from calcium.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 gray-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, with a specific mass of 1.55 g/cm
3, is the lightest of the alkali earth metals; magnesium (1.74) and beryllium (1.84) are heavier although they are lighter in atomic mass. From strontium on, the alkali earth metals get heavier along with the atomic mass.
Calcium has a higher resistivity than
copper or
aluminium. Yet, weight for weight, allowing for its much lower density, it is a rather better conductor than either. However, its use in terrestrial applications is usually limited by its high reactivity with air.
Calcium salts are colorless from any contribution of the calcium, and ionic solutions of calcium (Ca
2+) 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:
- as a deoxidizer, desulfurizer, or decarbonizer for various ferrous and nonferrous alloys.
Calcium compounds
- Calcium carbonate (CaCO3) used in manufacturing cement and mortar, lime, limestone (usually used in the steel industry); aids in production in the glass industry, also has chemical and optical uses as mineral specimens in toothpastes, for example.
- Calcium hydroxide solution (Ca(OH)2) (also known as limewater) is used to detect the presence of carbon dioxide by being bubbled through a solution. It turns cloudy where CO2 is present.
- Calcium cyclamate (Ca(C6H11NHSO3)2) was used as a sweetening agent but is no longer permitted for use because of suspected cancer-causing properties.
- Calcium permanganate (Ca(MnO4)2) is used in liquid rocket propellant, textile production, as a water sterilizing agent and in dental procedures.
- Calcium sulfate (CaSO4·2H2O) is used as common blackboard chalk, as well as, in its hemihydrate form better known as Plaster of Paris.
- Hydroxylapatite (Ca5(PO4)3(OH), but is usually written Ca10(PO4)6(OH)2) makes up seventy percent of bone. Also carbonated-calcium deficient hydroxylapatite is the main mineral of which dental enamel and dentin are comprised.
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 word meaning "lime") was known as early as the first century when the
Ancient Romans prepared lime as
calcium oxide. Literature dating back to 975 AD notes that
plaster of paris (calcium sulphate), is useful for setting broken bones. 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. Calcium metal was not available in large scale until the beginning of the 20th century.
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 hydroxide (slaked lime) is used in many chemical refinery processes and is made by heating
limestone at high temperature (above 825 °C) and then carefully adding water to it. 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 (CaCO
3) is one of the common compounds of calcium. It is heated to form quicklime (CaO), which is then added to water (H
2O). 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
Calcium is an important component of a
healthy diet and a
mineral necessary for life. The
National Osteoporosis Foundation says, "Calcium plays an important role in building stronger, denser bones early in life and keeping bones strong and healthy later in life." Approximately ninety-nine percent of the body's calcium is stored in the bones and teeth. The rest of the calcium in the body has other important uses, such as some
exocytosis, especially
neurotransmitter release, and
muscle contraction. In the
electrical conduction system of the heart, calcium replaces sodium as the mineral that depolarizes the cell, proliferating the action potential. In cardiac muscle, sodium influx commences an action potential, but during potassium efflux, the cardiac myocyte experiences calcium influx, prolonging the action potential and creating a plateau phase of dynamic equilibrium. 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, a high calcium intake has been associated with a lower risk for kidney stones in more recent research.
is needed to absorb calcium.
Dairy products, such as milk and cheese, are a well-known source of calcium. 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. Research has found an association between diets high in animal protein and increased urinary calcium loss from the bones. A diet high in fruit, vegetables and cereals was demonstrated to result in greater femoral bone mineral density in older men, in comparison to a range of other diets. Diets high in candy were found to result in lower bone density in both men and women . 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. 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.
- 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. It depends on low pH levels for proper absorption in the intestine. Some studies suggests that 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.
- 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.
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.
Possible cancer prevention
A
meta-analysis by the international
Cochrane Collaboration of two
randomized controlled trials found 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) .
- 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 and calcium metabolism disorder, as well as kidney stones.
See also
