chromium: Definition and Much More from Answers.com
- ️Tue Oct 02 2007
n. (Symbol Cr)
A lustrous, hard, steel-gray metallic element, resistant to tarnish and corrosion and found primarily in chromite. It is used in the hardening of steel alloys and the production of stainless steels, in corrosion-resistant decorative platings, and as a pigment in glass. Atomic number 24; atomic weight 51.996; melting point 1,890°C; boiling point 2,482°C; specific gravity 7.18; valence 2, 3, 6.
[From French chrome. See chrome.]
A chemical element, Cr, atomic number 24, and atomic weight 51.996, which is the weighted average for several isotopes weighing 50 (4.31%), 52 (83.76%), 53 (9.55%), and 54 (2.38%). The orbital arrangement of the electrons is 1s2, 2s2, 2p6, 3s2, 3p6, 3d5, 4s1. The stability of the half-filled d shell doubtless accounts for this rather unusual arrangement. In the crust of the Earth, chromium is the twenty-first element in abundance, which ranks it along with vanadium, zinc, nickel, and copper. Traces of chromium are present in the human body; in fact, it is essential to life. See also Periodic table.
The element was discovered in 1797 and isolated the following year by the French chemist L. N. Vauquelin. It was named chromium because of the many colors of its compounds. It occurs in nature largely as the mineral chromite (FeO · Cr2O3), which is a spinel, but the ore is usually contaminated with Al3+, Fe3+, Mn2+, and Mg2+. Smaller quantities are found as the yellow mineral crocoite (PbCrO4).
As a transition metal, chromium exists in all oxidation states from 2− to 6+. The chemistry of its aqueous solutions, at least in the 3+ (chromic) state, is complicated by the fact that the compounds exist in many isomeric forms, which have quite different chemical properties.
Pure chromium metal has a bluish-white color, reflects light well, and takes a high polish. When pure, it is ductile, but even small amounts of impurities render it brittle. The metal melts at about 1900°C (3452°F) and boils at 2642°C (4788°F). Chromium shows a wide range of oxidation states; the compounds in which the metal is in a low oxidation state are powerful reducing agents, whereas those in which it shows a high oxidation state are strong oxidizing agents.
The bright color and resistance to corrosion make chromium highly desirable for plating plumbing fixtures, automobile radiators and bumpers, and other decorative pieces. Unfortunately, chrome plating is difficult and expensive. It must be done by electrolytic reduction of dichromate in sulfuric acid solution. This requires the addition of six electrons per chromium ion. This reduction does not take place in one step, but through a series of steps, most of which are not clearly understood. The current efficiency is low (maybe 12%), and the chromium plate contains microscopic cracks and other flaws, and so it does not adequately protect the metal under it from corrosion. It is customary, therefore, to first plate the object with copper, then with nickel, and finally, with chromium.
In alloys with iron, nickel, and other metals, chromium has many desirable properties. Chrome steel is hard and strong and resists corrosion to a marked degree. Stainless steel contains roughly 18% chromium and 8% nickel. Some chrome steels can be hardened by heat treatment and find use in cutlery; still others are used in jet engines. Nichrome and chromel consist largely of nickel and chromium; they have low electrical conductivity and resist corrosion, even at red heat, so they are used for heating coils in space heaters, toasters, and similar devices. Other important alloys are Hastelloy C (Cr, Mo, W, Fe, Ni), used in chemical equipment which is in contact with HCl, oxidizing acids, and hypochlorite. Stellite [Co, Cr, Ni, C, W (or Mo)], noted for its hardness and abrasion resistance at high temperatures, is used for lathes and engine valves, and Inconel (Cr, Fe, Ni) is used in heat treating and in corrosion-resistant equipment in the chemical industry.
Several chromium compounds are used as paint pigments—chrome oxide green (Cr2O3), chrome yellow (PbCrO4), chrome orange (PbCrO4 · PbO), molybdate orange (a solution of PbSO4, PbCrO4, and PbMoO4), chrome green (a mixture of PbCrO4 and Prussian blue), and zinc yellow (potassium zinc chromate). Several of these, particularly zinc yellow, are used to inhibit corrosion. The gems ruby, emerald, and alexandrite owe their colors to traces of chromium compounds.See also Paint.
Dichromates are widely used as oxidizing agents, as rust inhibitors on steel, and as wood preservatives. In the last application, they kill fungi, termites, and boring insects. The wood can still be painted and glued, and retains its strength. Other chromium compounds find use as catalysts, as drilling muds, and in photochemical reactions. The last are important in the printing industry. A metal plate is coated with a colloidal material (for example, glue, shellac, or casein) containing a dichromate. On exposure to strong light under a negative image, the dichromate is reduced to Cr3+, which reacts with the colloid, hardening it and making it resistant to removal by washing. The unexposed material is washed off, and the metal plate is etched with acid to give a printing plate.
Chromium is essential to life. A deficiency (in rats and monkeys) has been shown to impair glucose tolerance, decrease glycogen reserve, and inhibit the utilization of amino acids. It has also been found that inclusion of chromium in the diet of humans sometimes, but not always, improves glucose tolerance. Certain chromium(III) compounds enhance the action of insulin.
On the other hand, chromates and dichromates are severe irritants to the skin and mucous membranes, so workers who handle large amounts of these materials must be protected against dusts and mists. Continued breathing of the dusts finally leads to ulceration and perforation of the nasal septum. Contact of cuts or abrasions with chromate may lead to serious ulceration. Even on normal skin, dermatitis frequently results. Cases of lung cancer have been observed in plants where chromates are manufactured.
A metallic element that is a dietary essential. It forms an organic complex with nicotinic acid, known as the glucose tolerance factor, which facilitates the interaction of insulin with receptors on cell surfaces. Deficiency results in impaired glucose tolerance.
There is little evidence on which to base estimates of requirements; deficiency has been observed at intakes below 6 μg (0.12 μmol)/day; the US/Canadian adequate intake is 35 μg for men and 25 μg for women. High intakes of inorganic chromium salts (in excess of 1-2 mg/day) are associated with kidney and liver damage.
A metallic element essential in the diet for efficient carbohydrate metabolism. It improves the ability of insulin to convert glucose to glycogen (the main energy store in muscles).
Some body-builders use a salt of chromium (chromium picolinate) as an anabolic agent. They claim it burns fat and increases lean body mass by accelerating protein metabolism. They also claim that it may reduce the risk of cardiovascular diseases by increasing the proportion of high density lipoproteins in the blood. Studies in the USA have recently disproved these claims. Obesity experts state that chromium supplements do not speed up metabolism or burn fat. Nutritionists generally advise that you should get enough chromium from a well-balanced diet. Good sources of chromium include liver, meat, cheese, wholegrains, brewer's yeast, and wine.
n
Cr
A hard, brittle, metallic element with an atomic number of 24 and an atomic weight of 51.996. Chromium strongly resists corrosion and is used extensively to plate other metals and as an alloy to harden steel. Stainless steels are more than 10% chromium.
Brand names: Chroma-Burn®, Chroma-Pak®, Chroma Slim®, Chroma Trim®, Chromax®, Chromium400™
Chemical formula:
- Drug Forms:
- Chromium injection (below)
- Chromium tablets or capsules
Chromium injection
What is chromium injection?
CHROMIUM (Chroma-Pac®) is an essential nutrient. People require very small amounts of chromium in our diet daily to help our bodies use and process dietary sugars properly. Chromium also appears to help the body use and process fats and proteins. Very few people develop true chromium deficiency. People who are being fed by intravenous nutrition solutions called TPN may require chromium injection to be placed in their TPN formula. This injection is only prescribed under the direction of a health care prescriber in order to prevent a chromium deficiency. Generic chromium injection is available.
What should I tell my health care provider before I take this medicine?
They need to know if you have any of these conditions:
• dependent on intravenous nutrition (TPN) or liquid nutrition products for food
• diabetes mellitus or high blood sugar levels
• kidney disease
• an unusual or allergic reaction to chromium or other medicines, foods, dyes, or preservatives
• pregnant or trying to get pregnant
• breast-feeding
How should I use this medicine?
Chromium injection is added into an intravenous solution (TPN or other fluid) by a qualified health care professional, like a pharmacist, as directed by your health-care prescriber. Most people receive a small amount of chromium in their TPN every day along with other vitamins and minerals. Our bodies need just small amounts every day in our diets. The TPN is then infused into a vein either at home or in the hospital, according to your prescribers directions.
Contact your pediatrician or health care professional regarding the use of this medicine in children. Special care may be needed. Chromium is used in TPN or intravenous fluids in children only on prescription by a qualified health care prescriber.
What if I miss a dose?
This does not apply. Your TPN prescription will contain the proper amount of chromium you need based on your prescribers orders; you will not have to worry about missing your needed dose.
What drug(s) may interact with chromium?
• medications used to treat diabetes or to lower blood sugar
Tell your prescriber or health care professional about all other medicines you are taking, including non-prescription medicines, nutritional supplements, or herbal products. Also tell your prescriber or health care professional if you are a frequent user of drinks with caffeine or alcohol, if you smoke, or if you use illegal drugs. These may affect the way your medicine works. Check with your health care professional before stopping or starting any of your medicines.
What should I watch for while taking chromium?
If you notice any changes in your physical or mental health while taking chromium, you should contact your health care provider.
Certain forms of chromium are not for human use. Do not take chromium products that are not labeled for use as nutritional supplements or medicine.
You will need regular checks on your progress and regular blood work to check the levels of sugar and other important contents of the blood while on this medication.
Remember to keep your appointments for these tests.
What side effects may I notice from using chromium?
Side effects that you should report to your prescriber or health care professional as soon as possible:
Symptoms of low blood sugar, for example:
• chills
• cold sweats
• confusion
• headache
• hunger
• irregular heartbeat or palpitations
• nausea or vomiting
• nervousness or anxious
• seizures (convulsions) or unusual movements
• shakiness
• unusual tiredness
• trembling
• vision changes
Other side effects that should be reported to your prescriber or health care professional as soon as possible:
• blurred vision
• drowsiness
• changes in emotions or mood
• change in urine amount, color, or frequency
• fever or chills
• muscle aches, cramps, or pains
• retaining fluid in your legs or abdomen
• skin rash or unusual skin reaction
• unusual weakness or muscle fatigue
Where can I keep my medicine?
Keep out of the reach of children.
Store your TPN or intravenous solution as directed on the label. Many times the solution will need to be refrigerated prior to the time it is needed for use. Do not freeze the solution. Throw away any unused medicine after the expiration date as directed by your health care professional or pharmacist.
Last updated: 7/1/2002
Important Disclaimer: The drug information provided here is for educational purposes only. It is intended to supplement, not substitute for, the diagnosis, treatment and advice of a medical professional. This drug information does not cover all possible uses, precautions, side effects and interactions. It should not be construed to indicate that this or any drug is safe for you. Consult your medical professional for guidance before using any prescription or over the counter drugs.
Description
Chromium is a mineral that is essential to humans. It is found naturally in a variety of foods, and supplements are available in capsules or tablets. Supplements are prepared using a number of formulas, including chromium (III), chromium aspartate, chromium chloride, chromium citrate, chromium nicotinate, chromium picolinate, GTF chromium, and trivalent chromium.
General Use
Chromium supports the normal function of insulin, which is a hormone secreted by the pancreas. Insulin helps transport glucose from the bloodstream into liver, muscle, and fat cells. Once it is inside these cells, the sugar is metabolized into a source of energy. Insulin is also involved in regulating protein, fat, and catalytic enzyme processes. People with diabetes do not produce insulin (or produce very little) or their bodies cannot properly use the insulin that is produced. As a result, sugar builds up in the bloodstream, causing serious health effects. Numerous scientific studies have shown that chromium is useful in treating insulin resistance (metabolic syndrome) and diabetes. Diabetic peripheral neuropathy, a form of nerve damage that is a direct result of diabetes, is indirectly related to a lack of sufficient chromium.
Several studies have shown that chromium supplements may improve insulin sensitivity, and lower blood glucose and elevated body fat. In February 2004, the University of Pennsylvania School of Medicine began a comprehensive study of chromium as a therapy for insulin resistance. This condition occurs when the body fails to respond properly to the insulin it already produces. People who are insulin resistant may have the ability to overcome this problem by producing more insulin. However, if the body cannot produce sufficient amounts of insulin, glucose levels in the bloodstream rise, and type 2 diabetes ultimately occurs. It is estimated that up to 80 million Americans have insulin resistance.
A study conducted by Isala Clinics and University Hospital Groningen in the Netherlands, and released in 2003, showed that a daily dose of 1,000 micrograms of chromium significantly reduced blood sugar levels in people with poorly controlled type 2 diabetes who use insulin.
Chromium has also been used as an effective treatment for polycystic ovarian syndrome (PCOS), a hormonal condition affecting about two million American women. The condition can lead to infertility if untreated, and is associated with insulin resistance and type 2 diabetes. A study released in 2003 by the State University of New York at Stony Brook showed that insulin sensitivity increased an average of 35 percent after two months of daily treatment with 1,000 micrograms (μg) of chromium.
Through its involvement with insulin function, chromium plays an indirect role in lowering blood lipids. Studies suggest, but have not proven, that chromium supplementation can reduce the risk of cardiovascular (heart) disease in men, and may decrease total cholesterol and triglyceride levels. However, several studies contradict these claims. Studies in animals suggest chromium supplementation may reduce hypertension (high blood pressure). Lipid reduction is secondary to insulin regulation and control; therefore, persons whose insulin is well regulated and controlled may not achieve reduced heart disease risk by taking chromium supplements.
Chromium supplements in high doses—1,000 g or more a day—are sometimes used in weight loss and muscle development. However, a number of scientific studies have found that chromium supplements are not effective in these areas. In fact, precautions warn against chromium doses exceeding 1,000 g per day. Weight loss and muscle development are secondary to insulin regulation and control. Therefore, when insulin is well regulated and controlled, chromium may not impact weight loss or muscle development.
Preparations
A complete lack of chromium is rare, and the United States Food and Drug Administration (FDA) has not established recommended dietary allowances (RDA) for the mineral. However, national statistics on the prevalence of diabetes, heart disease, and obesity suggest that chromium deficiencies may be common. Chromium occurs naturally in meat, seafood, dairy products, eggs, whole grains, black pepper, and almonds. According to The PDR Family Guide to Natural Medicines and Healing Therapies, the usual chromium supplement dose for children ages seven and older and adults is 50–200 g a day in tablets or capsules. For persons with type 2 diabetes who are not taking insulin, doses from 200–1,000 g daily may be taken. However, persons should only take doses at these levels after consulting with a physician. Chromium should not be taken in doses exceeding 1,000 g a day. The cost of a bottle of 100 tablets or capsules (200 g) of chromium picolinate ranges from $5 to $10.
Precautions
Doses of 200–1,000 g of chromium should be taken only after consultation with a physician. Pregnant or breastfeeding women are advised to consult a physician before taking chromium supplements. Chromium should not be taken in doses exceeding 1,000 g a day. Increased dietary sugar may be associated with higher urinary excretion of chromium.
Side Effects
Several studies have noted occasional reports of irregular heartbeats with chromium use. Infrequently, chromium has been reported to cause such sleep pattern changes as insomnia and increased dream activity. Irritability has also been reported. In rare instances, persons may be allergic to a chromium formula. The symptoms of an allergic reaction include difficulty breathing, chest pain, hives, rash, and itchy or swollen skin. If this happens, the patient is advised to seek medical care immediately. High doses may also cause liver and kidney damage, or gastric irritation, although these side effects are rare.
Interactions
Persons who are taking antacids are advised to talk with a physician before taking chromium supplements. Studies in animals suggest that antacids, especially those containing calcium carbonate, may reduce the body's ability to absorb chromium. Studies have shown that chromium may enhance the effectiveness of drugs taken by people who have type 2 diabetes or insulin resistance. These drugs include glimepiride, glipizide, glyburide, insulin, and metformin. Individuals taking these drugs should discuss chromium supplementation with a physician because improved insulin function may necessitate medication dosage changes.
Resources
Books
Brown, Donald J. Herbal Prescriptions for Health and Healing: Your Everyday Guide to Using Herbs Safely and Effectively. Twin Lakes, WI: Lotus Press, 2003.
Evans, Gary. All About Chromium Picolinate: Frequently Asked Questions. Garden City Park, NY: Avery, 1999.
Icon Health Publications. Chromium Picolinate: A Medical Dictionary, Bibliography, and Annotated Research Guide to Internet References. San Diego, CA: Icon Health Publications, 2003.
Kamen, Betty. The Chromium Connection: A lesson in Nutrition. Novato, CA: Nutrition Encounter Inc., 1995.
Passwater, Richard A. Chromium Picolinate. New York, NY: McGraw-Hill/Contemporary Books; Reprint Edition, 1995.
Periodicals
Biotech Week Editors and Staff. "Company Seeks FDA Approval of Health Claims for Chromium." Biotech Week (January 28, 2004): 308.
"Chromium." UC Berkeley Wellness Letter (July 2003): 3.
Drug Week Editors and Staff. "Chromium Picolinate May Benefit Patients with Insulin Resistance." Drug Week (December 26, 2003): 121.
Drug Week Editors and Staff. "Research: Lower Chromium Levels Linked to Increased Risk of Disease." Drug Week (April 2, 2004): 263.
Lamson, Davis W., and Steven M. Plaza. "The Safety and Efficacy of High-Dose Chromium. Alternative Medicine Review (June 2002): 218–236.
Lydic, Michael L., et al. "Effects of Chromium Supplementation on Insulin Sensitivity and Reproductive Function in Polycystic Ovarian Syndrome: A Pilot Study." Fertility and Sterility (Supplement 3) (September 2003): 45–46.
Volpe, Stella L., et al. "Effect of Chromium Supplementation and Exercise on Body Composition, Resting Metabolic Rate, and Selected Biochemical Parameters in Moderately Obese Women Following an Exercise Program." Journal of the American College of Nutrition (August 2001): 293–306.
Ward, Elizabeth M. "Top 10 Supplements for Men." Men's Health (December 2003): 106.
[Article by: Ken R. Wells]
Metallic chemical element, one of the transition elements, chemical symbol Cr, atomic number 24. A hard, steel-gray metal that takes a high polish, it is used in alloys (e.g., ferrochromium, steel, stainless steel) to increase strength and corrosion resistance. It usually has valence 2, 3, or 6 and always occurs combined with other elements, especially oxygen; chromite is its only commercial source. Various coloured gemstones (e.g., ruby, emerald, serpentine) owe their colour to chromium. Sodium chromate and dichromate are used in leather tanning, in metal surface treatment, and as catalysts. Chromium trioxide is used in chrome plating and as a colorant for ceramics. Chromium oxide, lead chromate, and various other chromium compounds are used as pigments. Chromium dioxide, strongly magnetic, is used in recording tapes and as a catalyst.
For more information on chromium, visit Britannica.com.
A hard, brittle metal resistant to corrosion, workable when annealed, gray-white in color; used in alloys, esp. steel, and in plating.
A metallic element essential for efficient glucose metabolism. Chromium is readily available in a variety of foods, including liver, meat, cheese, wholegrains, brewer's yeast, and wine. Because chromium deficiency is rare, there is no Reference Nutrient Intake in the UK, but a safe daily intake is set at 25 μg.
(krō'mēəm) [Gr.,=color], metallic chemical element; symbol Cr; at. no. 24; at. wt. 51.996; m.p. about 1,857°C; b.p. 2,672°C; sp. gr. about 7.2 at 20°C; valence +2, +3, +6. Chromium is a silver-gray, lustrous, brittle, hard metal that can be highly polished. It is found in Group 6 of the periodic table. It does not tarnish in air, but burns when heated, forming the green chromic oxide. When combined with oxygen, besides yielding chromic oxide, which is used as a pigment, it forms chromic anhydride (the red trioxide and anhydride of chromic acid). With other metallic elements, e.g., lead and potassium, together with oxygen, it forms the chromates and dichromates. These compounds are salts of chromic acid and are used as pigments in paints, in dyeing, and in the tanning of leather. Chrome yellow, a pigment, consists largely of lead chromate. Other chrome colors are black, red, orange, and green. In the chrome process for tanning leather, a dichromate is used, and chromium hydroxide, a basic compound of chromium, hydrogen, and oxygen, is precipitated and held in the leather. The hydroxide is used also as a mordant in dyeing cloth. A mixture of potassium dichromate and sulfuric acid is used as a powerful agent for cleaning laboratory glassware. Chromium is a comparatively rare element, never occurring by itself in nature but always in compounds. Its chief source is the mineral chromite, which is composed of iron, chromium, and oxygen and is found principally in the nations of the former Soviet Union, South Africa, Zimbabwe, Turkey, and the Philippines. The element, in the form of chromic oxide, gives the greenish tint to the emerald and the aquamarine. Metallic chromium is prepared by reduction of the oxide by aluminum or by carbon. It is used in plating other metals because of its hardness and nontarnishing properties. In alloys with other metals it contributes hardness, strength, and heat resistance. Its most important use is in the steel industry, where it is a constituent of several alloy steels, e.g., chromium steel or chrome steel. Stainless steel contains from 11% to 18% chromium. An alloy of nickel and chromium, often called Nichrome, is widely used as a heating element in electric toasters, coffeepots, and other appliances. Stellite is an extremely hard alloy of cobalt, chromium, and tungsten, with small amounts of iron, silicon, and carbon; it is used in metal cutting tools and for wear-resistant surfaces. A similar alloy, with molybdenum instead of tungsten, is used in surgical tools since it does not react with body fluids. Chromium was discovered in 1797 by L. N. Vauquelin.
A chemical element, atomic number 24, atomic weight 51.996, symbol Cr.
- c.-51 — a radioisotope of chromium having a half-life of 27.8 days; used to label red blood cells to determine red cell volume and red cell survival time. Symbol 51Cr. See also cr51 edta.
- c. nutritional deficiency — possibly causally related to the onset of diabetes mellitus in primates.
- c. trioxide — possibly carcinogenic in humans. See also chromate.
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| General | ||||||||||||||||||||||||||||||
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| Name, symbol, number | chromium, Cr, 24 | |||||||||||||||||||||||||||||
| Chemical series | transition metals | |||||||||||||||||||||||||||||
| Group, period, block | 6, 4, d | |||||||||||||||||||||||||||||
| Appearance | silvery metallic |
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| Standard atomic weight | 51.9961(6) g·mol−1 | |||||||||||||||||||||||||||||
| Electron configuration | [Ar] 3d5 4s1 | |||||||||||||||||||||||||||||
| Electrons per shell | 2, 8, 13, 1 | |||||||||||||||||||||||||||||
| Physical properties | ||||||||||||||||||||||||||||||
| Phase | solid | |||||||||||||||||||||||||||||
| Density (near r.t.) | 7.15 g·cm−3 | |||||||||||||||||||||||||||||
| Liquid density at m.p. | 6.3 g·cm−3 | |||||||||||||||||||||||||||||
| Melting point | 2180 K (1907 °C, 3465 °F) |
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| Boiling point | 2944 K (2671 °C, 4840 °F) |
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| Heat of fusion | 21.0 kJ·mol−1 | |||||||||||||||||||||||||||||
| Heat of vaporization | 339.5 kJ·mol−1 | |||||||||||||||||||||||||||||
| Heat capacity | (25 °C) 23.35 J·mol−1·K−1 | |||||||||||||||||||||||||||||
Vapor pressure
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| Atomic properties | ||||||||||||||||||||||||||||||
| Crystal structure | cubic body centered | |||||||||||||||||||||||||||||
| Oxidation states | 6, 4, 3, 2 (strongly acidic oxide) |
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| Electronegativity | 1.66 (Pauling scale) | |||||||||||||||||||||||||||||
| Ionization energies (more) |
1st: 652.9 kJ·mol−1 | |||||||||||||||||||||||||||||
| 2nd: 1590.6 kJ·mol−1 | ||||||||||||||||||||||||||||||
| 3rd: 2987 kJ·mol−1 | ||||||||||||||||||||||||||||||
| Atomic radius | 140 pm | |||||||||||||||||||||||||||||
| Atomic radius (calc.) | 166 pm | |||||||||||||||||||||||||||||
| Covalent radius | 127 pm | |||||||||||||||||||||||||||||
| Miscellaneous | ||||||||||||||||||||||||||||||
| Magnetic ordering | AFM (rather: SDW) | |||||||||||||||||||||||||||||
| Electrical resistivity | (20 °C) 125 nΩ·m | |||||||||||||||||||||||||||||
| Thermal conductivity | (300 K) 93.9 W·m−1·K−1 | |||||||||||||||||||||||||||||
| Thermal expansion | (25 °C) 4.9 µm·m−1·K−1 | |||||||||||||||||||||||||||||
| Speed of sound (thin rod) | (20 °C) 5940 m/s | |||||||||||||||||||||||||||||
| Young's modulus | 279 GPa | |||||||||||||||||||||||||||||
| Shear modulus | 115 GPa | |||||||||||||||||||||||||||||
| Bulk modulus | 160 GPa | |||||||||||||||||||||||||||||
| Poisson ratio | 0.21 | |||||||||||||||||||||||||||||
| Mohs hardness | 8.5 | |||||||||||||||||||||||||||||
| Vickers hardness | 1060 MPa | |||||||||||||||||||||||||||||
| Brinell hardness | 1120 MPa | |||||||||||||||||||||||||||||
| CAS registry number | 7440-47-3 | |||||||||||||||||||||||||||||
| Selected isotopes | ||||||||||||||||||||||||||||||
Main article: Isotopes of chromium
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Chromium (IPA: /ˈkrəʊmiəm/) is a chemical element in the periodic table that has the symbol Cr and atomic number 24. It is a steel-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable.
History
On 26 July 1761, Johann Gottlob Lehmann found an orange-red mineral in the Ural Mountains which he named Siberian red lead. Though misidentified as a lead compound with selenium and iron components, the material was in fact lead chromate with a formula of PbCrO4, now known as the mineral crocoite.
In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red "lead" mineral that had very useful properties as a pigment in paints. The use of Siberian red lead as a paint pigment developed rapidly. A bright yellow made from crocoite became a color in fashion.
In 1797, Louis Nicolas Vauquelin received samples of crocoite ore. He was able to produce chromium oxide with a chemical formula of CrO3, by mixing crocoite with hydrochloric acid. In 1798, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven. He was also able to detect traces of chromium in precious gemstones, such as ruby, or emerald. Later that year he successfully isolated chromium atoms.
During the 1800s chromium was primarily used as a component of paints and in tanning salts but now metal alloys account for 85% of the use of chromium. The remainder is used in the chemical industry and refractory and foundry industries.
Chromium was named after the Greek word "Chrôma" meaning color, because of the many colorful compounds made from it.
Occurrence and production
World production trend
Chromium is mined as chromite (FeCr2O4) ore. About two-fifths of the chromite ores and concentrates in the world are produced in South Africa. Kazakhstan, India, Russia and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa.
Approximately 15 million tons of marketable chromite ore were produced in 2000, and converted into approximately 4 million tons of ferro-chrome with an approximate market value of 2.5 billion United States dollars.
Though native chromium deposits are rare, some native chromium metal has been discovered. The Udachnaya Mine in Russia produces samples of the native metal. This mine is a kimberlite pipe rich in diamonds, and the reducing environment so provided helped produce both elemental chromium and diamond. (See also chromium minerals)
Chromium is obtained commercially by heating the ore in the presence of aluminium or silicon.
Chemical properties
Chromium
Chromium is a member of the transition metals, in group 6. Chromium(0) has an electronic configuration of 4s13d5, due to the lower energy of the high spin configuration. Chromium exhibits a wide range of possible oxidation states. The most common oxidation states of chromium are +2, +3, and +6, with +3 being the most stable. +1, +4 and +5 are rare. Chromium compounds of oxidation state +6 are powerful oxidants.
Chromium is passivated by oxygen, forming a thin (usually a few atoms thick being transparent because of thickness) protective oxide surface layer with another element such as nickel, and/or iron. It forms a coumpound called a spinal structure which being very dense prevents diffusion of oxygen into the underlying layer. (In iron or plain carbon steels the oxygen actually migrates into the underlying material.) Chromium is usually plated on top of a nickel layer which may or may not have been copper plated first. Chromium as opposite to most other metals such as iron and nickel does not suffer from hydrogen embrittelment. It does though suffer from nitrogen embrittelment and hence no straight chromium alloy has ever been developed. Below the pourbiax diagram can be seen, it is important to understand that the diagram only displays the thermodynamic data and it does not display any details of the rates of reaction.
![The Pourbaix diagram for chromium in pure water, perchloric acid or sodium hydroxide[1]](http://web.archive.org/web/20080407131342im_/http://content.answers.com/main/content/wp/en/thumb/a/a4/240px-Chromium_in_water_pourbiax_diagram.png)
Compounds
See also chromium compounds. Potassium dichromate is a powerful oxidizing agent and is the preferred compound for cleaning laboratory glassware of any trace organics. It is used as a saturated solution in concentrated sulfuric acid for washing the apparatus. For this purpose, however, sodium dichromate is sometimes used because of its higher solubility (5 g/100 ml vs. 20 g/100 ml respectively). Chrome green is the green oxide of chromium, Cr2O3, used in enamel painting, and glass staining. Chrome yellow is a brilliant yellow pigment, PbCrO4, used by painters.
Chromic acid has the hypothetical structure H2CrO4. Neither chromic nor dichromic acid is found in nature, but their anions are found in a variety of compounds. Chromium trioxide, CrO3, the acid anhydride of chromic acid, is sold industrially as "chromic acid".
Chromium and the quintuple bond
The compound synthesized by Nguyen, which was determined experimentally to contain a Cr-Cr quintuple bond
Chromium is notable for its ability to form quintuple covalent bonds. The synthesis of
a compound of chromium(I) and a hydrocarbon radical was shown via X-ray diffraction to
contain a quintuple bond of length 183.51(4) pm (1.835 angstroms) joining the two central
chromium atoms.[2] This was accomplished through the use of
an extremely bulky monodentate ligand which through its sheer size prevents further coordination. Chromium currently remains the
only element for which quintuple bonds have been observed.
Applications
Uses of chromium:
- In metallurgy, to impart corrosion resistance and a shiny finish:
- as an alloy constituent, such as in stainless steel in cutlery
- in chrome plating,
- in anodized aluminium, literally turning the surface of aluminium into ruby.
- As dyes and paints :
- Chromium(III) oxide is a metal polish known as green rouge.
- Chromium salts color glass an emerald green.
- Chromium is what makes a ruby red, and therefore is used in producing synthetic rubies.
- also makes a brilliant yellow for painting
- As a catalyst.
- Chromite is used to make molds for the firing of bricks.
- Chromium salts are used in the tanning of leather.
- Potassium dichromate is a chemical reagent, used in cleaning laboratory glassware and as a titrating agent. It is also used as a mordant (i.e., a fixing agent) for dyes in fabric.
- Chromium(IV) oxide (CrO2) is used to manufacture magnetic tape, where its higher coercivity than iron oxide tapes gives better performance.
- In well drilling muds as an anti-corrosive.
- In medicine, as a dietary supplement or slimming aid, usually as chromium (III) chloride or chromium(III) picolinate.
- Chromium hexacarbonyl (Cr(CO)6) is used as a gasoline additive.
- Chromium boride (CrB) is used as a high-temperature electrical conductor.
- Chromium (III) sulfate (Cr2(SO4)3) is used as a green pigment in paints, in ceramic, varnishes and inks as well as in chrome plating.
- Chromium (VI) is used in the post Ballard preparation of Gravure (rotogravure) printing Forme Cylinders. By electroplating the metal onto the second coat of copper (after the Ballard skin), the longevity of the printing cylinder is increased.
Biological role
Trivalent chromium (Cr(III), or Cr3+) is required in trace amounts for sugar metabolism in humans (Glucose Tolerance Factor) and its deficiency may cause a disease called chromium deficiency. In contrast, hexavalent chromium is very toxic and mutagenic when inhaled as popularized by the film Erin Brockovich. Cr(VI) has not been established as a carcinogen when not inhaled but in solution it is well established as a cause of allergic contact dermatitis (ACD).[3]
Recently it was shown that the popular dietary supplement chromium picolinate complex generates chromosome damage in hamster cells. In the United States the dietary guidelines for daily chromium uptake were lowered from 50-200 µg for an adult to 35 µg (adult male) and to 25 µg (adult female).[4]
Isotopes
Naturally occurring chromium is composed of three stable isotopes; 52Cr, 53Cr, and 54Cr with 52Cr being the most abundant (83.789% natural abundance). Nineteen radioisotopes have been characterized with the most stable being 50Cr with a half-life of (more than) 1.8x1017 years, and 51Cr with a half-life of 27.7 days. All of the remaining radioactive isotopes have half-lives that are less than 24 hours and the majority of these have half-lives that are less than 1 minute. This element also has 2 meta states.
53Cr is the radiogenic decay product of 53Mn. Chromium isotopic contents are typically combined with manganese isotopic contents and have found application in isotope geology. Mn-Cr isotope ratios reinforce the evidence from 26Al and 107Pd for the early history of the solar system. Variations in 53Cr/52Cr and Mn/Cr ratios from several meteorites indicate an initial 53Mn/55Mn ratio that suggests Mn-Cr isotope systematics must result from in-situ decay of 53Mn in differentiated planetary bodies. Hence 53Cr provides additional evidence for nucleosynthetic processes immediately before coalescence of the solar system.
The isotopes of chromium range in atomic weight from 43 u (43Cr) to 67 u (67Cr). The primary decay mode before the most abundant stable isotope, 52Cr, is electron capture and the primary mode after is beta decay.
Precautions
Chromium metal and chromium(III) compounds are not usually considered health hazards; chromium is an essential trace mineral.[5] However, hexavalent chromium (chromium VI) compounds can be toxic if orally ingested or inhaled. The lethal dose of poisonous chromium (VI) compounds is about one half teaspoon of material. Most chromium (VI) compounds are irritating to eyes, skin and mucous membranes. Chronic exposure to chromium (VI) compounds can cause permanent eye injury, unless properly treated. Chromium(VI) is an established human carcinogen. An investigation into hexavalent chromium release into drinking water formed the plot of the motion picture Erin Brockovich.
World Health Organization recommended maximum allowable concentration in drinking water for chromium (VI) is 0.05 milligrams per liter. Hexavalent chromium is also one of the substances whose use is restricted by the European Restriction of Hazardous Substances Directive.
As chromium compounds were used in dyes and paints and the tanning of leather, these compounds are often found in soil and groundwater at abandoned industrial site, now needing environmental cleanup and remediation per the treatment of brownfield land. Primer paint containing hexavalent chromium is still widely used for aerospace and automobile refinishing applications.
See also
- Chromium compounds
- Chromium minerals
- Chromium VI
References
- ^ Ignasi Puigdomenech, Hydra/Medusa Chemical Equilibrium Database and Plotting Software (2004) KTH Royal Institute of Technology, freely downloadable software at [1]
- ^ T. Nguyen, A. D. Sutton, M. Brynda, J. C. Fettinger, G. J. Long and P. P. Power (2005). "Synthesis of a Stable Compound with Fivefold Bonding Between Two Chromium(I) Centers". Science 310 (5749): 844-847. DOI:10.1126/science.1116789.
- ^ ToxFAQs: Chromium. Agency for Toxic Substances & Disease Registry, Centers for Disease Control and Prevention (February 2001). Retrieved on 2007-10-02.
- ^ Vincent, J.B. (2007). "Recent advances in the nutritional biochemistry of trivalent chromium". Proceedings of the Nutrition Society 63 (01): 41-47. DOI:10.1079/PNS2003315. Retrieved on 2007-07-12.
- ^ Chromium. Wellness Letter.
External links
Wikimedia Commons has media related to:
- Case Studies in Environmental Medicine: Chromium Toxicity
- IARC Monograph "Chromium and Chromium compounds"
- International Chromium Development Association
- It's Elemental – The Element Chromium
- National Pollutant Inventory - Chromium (III) compounds fact sheet
- The Merck Manual – Mineral Deficiency and Toxicity
- National Institute for Occupational Safety and Health - Chromium Page
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
Ελληνική (Greek)
n. - (χημ.) χρώμιο
Português (Portuguese)
n. - cromo (m) (Quím.)
中文（简体） (Chinese (Simplified))
铬
中文（繁體） (Chinese (Traditional))
n. - 鉻
العربيه (Arabic)
(الاسم) معدن الكروميوم
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