| Description | Chromium (pronounced /?kro?mi?m/, KROH-mee-?m) is a chemical element which has the symbol Cr and atomic number 24, first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable. The name of the element is derived from the Greek word "chr?ma" (?????), meaning color, because many of its compounds are intensely colored. It was discovered by Louis Nicolas Vauquelin in the mineral crocoite (lead chromate) in 1797. Crocoite was used as a pigment, and after the discovery that the mineral chromite also contains chromium this latter mineral was used to produce pigments as well.; 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 is a naturally occurring heavy metal found in the environment commonly in trivalent, Cr(III), and hexavalent, Cr(VI), forms. The reduction of Cr(VI) to Cr(III) results in the formation of reactive intermediates that contribute to the cytotoxicity, genotoxicity and carcinogenicity of Cr(VI)-containing compounds. The major non-occupational source of chromium for humans is food such as vegetables, meat, urban air, hip or knee prostheses and cigarettes. Cr(VI) is a widely used in industrial chemicals, extensively used in paints, metal finishes, steel including stainless steel manufacturing, alloy cast irons, chrome and wood treatment. On the contrary, Cr(III) salts such as chromium polynicotinate, chromium chloride and chromium picolinate (CrP) are used as micronutrients and nutritional supplements and have been demonstrated to exhibit a significant number of health benefits in animals and humans. Physiologically, it exists as an ion in the body.; Chromium enters the body through the lungs, gastro-intestinal tract and to a lesser extent through skin. Inhalation is the most important route for occupational exposure, whereas non-occupational exposure occurs via ingestion of chromium-containing food and water. Regardless of route of exposure Cr(III) is poorly absorbed whereas Cr(VI) is more readily absorbed. Further, absorption of Cr(VI) is poorer by oral route, it is thus not very toxic when introduced by the oral route. But chromium is very toxic by dermal and inhalation routes and causes lung cancer, nasal irritation, nasal ulcer, hypersensitivity reactions and contact dermatitis. All the ingested Cr(VI) is reduced to Cr(III) before entering in the blood stream. The main routes for the excretion of chromium are via kidney/urine and the bile/feces. Cr(III) is unable to enter into the cells but Cr(VI) enters through membrane anionic transporters. Intracellular Cr(VI) is metabolically reduced to Cr(III). Cr(VI) does not react with macromolecules such as DNA, RNA, proteins and lipids. However, both Cr(III) and the reductional intermediate Cr(V) are capable of co-ordinate, covalent interactions with macromolecules. Chromium is an essential nutrient required by the human body to promote the action of insulin for the utilization of sugars, proteins and fats. CrP has been used as nutritional supplement; Chromium is passivated by oxygen, forming a thin protective oxide surface layer. This layer is a spinel structure only a few atoms thick. It is very dense, preventing diffusion of oxygen into the underlying material. (In iron or plain carbon steels the oxygen migrates into the underlying material.) Chromium is usually plated on top of a nickel layer which may first have been copper plated. Chromium, unlike metals such as iron and nickel, does not suffer from hydrogen embrittlement. It does suffer from nitrogen embrittlement - chromium reacts with nitrogen from air and forms brittle nitrides at temperatures necessary to work the metal parts. The Pourbaix diagram can be seen above. 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 passivation can be increased by short contact with oxidizing acids like nitric acid. The passivated chromium is stable against acids. The contrary effect can be achieved if a strong reducing reactant destroys the oxide protection layer on the metal, a metal treated in this way readily dissolves in weak acids.; Chromium(VI) compounds in solution can be detected by adding acidic hydrogen peroxide solution. A dark blue unstable chromium(VI) peroxide (CrO5) is formed which can be stabilized as an ether adduct CrO5 ? OR2. 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, the chromates and dichromates. chromium(VI) oxide CrO3, the acid anhydride of chromic acid, is sold industrially as "chromic acid". The dark red chromium(VI) oxide can be produced by mixing sulfuric acid with dichromate, and is an extremely strong oxidizing agent.; In some parts of Russia, pentavalent chromium was reported as one of the causes of premature dementia.; Several chromium compounds are used as catalyst. For example the Phillips catalysts for the production polyethylene are mixtures of chromium and silicon dioxide or mixtures of chromium and titanium and aluminium oxide. Chromium(IV) oxide (CrO2) is a magnetic compound. Its ideal shape anisotropy, which imparted high coercivity and remanent magnetization, made it a compound superior to the ?-Fe2O3. Chromium(IV) oxide is used to manufacture magnetic tape used in high performance audio tape and standard audio cassette. Chromates can prevent corrosion of steel under wet conditions, and therefore chromates are added to the drilling muds. The long known influence of chromium uptake on diabetes conditions suggested the positive influence of dietary supplement containing chromium(III) also on healthy persons. For this reason, dietary supplement or slimming aid usually contain chromium(III) chloride, chromium(III) picolinate, chromium(III) polynicotinate or amino acid chelate, such as chromium(III) D-phenylalanine. The benefit of those supplements is still under investigation and is questioned by some studies.; The mineral crocoite (lead chromate PbCrO4) was used as a yellow pigment shortly after its discovery. After a synthesis method became available starting from the more abundant chromite, Chrome yellow was, together with cadmium yellow, one of the most used yellow pigments. The pigment does not degrade in the light and has a strong color. The signaling effect of yellow was used for school buses in the United States and for Postal Service (for example Deutsche Post) in Europe. The use of chrome yellow declined due to environmental and safety concerns and was substituted by organic pigments or other lead-free alternatives. Other pigments based on chromium are, for example, the bright red pigment Chrome red, which is a basic lead chromate (PbCrO4?Pb(OH)2). Chrome green is a mixture of Prussian blue and chrome yellow, while the Chrome oxide green is Chromium(III) oxide.; Though native chromium deposits are rare, some native chromium metal has been discovered. The Udachnaya Pipe in Russia produces samples of the native metal. This mine is a kimberlite pipe rich in diamonds, and the reducing environment helped produce both elemental chromium and diamond.; 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.; it controls blood sugar in diabetes and may reduce cholesterol and blood pressure levels. Chromium increases insulin binding to cells, insulin receptor number and activates insulin receptor kinase leading to increased insulin sensitivity. ; But high doses of chromium and long term exposure of it can give rise to various, cytotoxic and genotoxic reactions that affect the immune system of the body. However, the mechanism of the Cr(VI)-induced cytotoxicity is not entirely understood. A series of in vitro and in vivo studies have demonstrated that Cr(VI) induces oxidative stress through enhanced production of reactive oxygen species (ROS) leading to genomic DNA damage and oxidative deterioration of lipids and proteins. A cascade of cellular events occur following Cr(VI)-induced oxidative stress including enhanced production of superoxide anion and hydroxyl radicals, increased lipid peroxidation and genomic DNA fragmentation, modulation of intracellular oxidized states, activation of protein kinase C, apoptotic cell death and altered gene expression. Some of the factors in determining the biological outcome of chromium exposure include the bioavailability, solubility of chromium compounds and chemical speciation, intracellular reduction and interaction with DNA. The chromium genotoxicity manifests as several types of DNA lesions, gene mutations and inhibition of macromolecular synthesis. Further, chromium exposure may lead to apoptosis, premature terminal growth arrest or neoplastic transformation. Chromium-induced tumor suppressor gene p53 and oxidative processes are some of the major factors that may determine the cellular outcome. Studies have utilized these approaches to understand the interrelationship between chromium-induced genotoxicity, apoptosis and effects on immune response. (PMID: 12208600). |
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