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Chromium

Chromium is a chemical element; it has symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.

Chromium metal is valued for its high corrosion resistance and hardness. A major development in steel production was the discovery that steel could be made highly resistant to corrosion and discoloration by adding metallic chromium to form stainless steel. Stainless steel and chrome plating (electroplating with chromium) together comprise 85% of the commercial use. Chromium is also greatly valued as a metal that is able to be highly polished while resisting tarnishing. Polished chromium reflects almost 70% of the visible spectrum, and almost 90% of infrared light. The name of the element is derived from the Greek word χρῶμα, chrōma, meaning color, because many chromium compounds are intensely colored.

Industrial production of chromium proceeds from chromite ore (mostly FeCr2O4) to produce ferrochromium, an iron-chromium alloy, by means of aluminothermic or silicothermic reactions. Ferrochromium is then used to produce alloys such as stainless steel. Pure chromium metal is produced by a different process: roasting and leaching of chromite to separate it from iron, followed by reduction with carbon and then aluminium.

In the United States, trivalent chromium (Cr(III)) ion is considered an essential nutrient in humans for insulin, sugar, and lipid metabolism. However, in 2014, the European Food Safety Authority, acting for the European Union, concluded that there was insufficient evidence for chromium to be recognized as essential.

While chromium metal and Cr(III) ions are considered non-toxic, chromate and its derivatives, often called "hexavalent chromium", is toxic and carcinogenic. According to the European Chemicals Agency (ECHA), chromium trioxide that is used in industrial electroplating processes is a "substance of very high concern" (SVHC).

Atomic
Gaseous chromium has a ground-state electron configuration of [ Ar] 3d5 4s1. It is the first element in the periodic table whose configuration violates the Aufbau principle. Exceptions to the principle also occur later in the periodic table for elements such as copper, niobium and molybdenum.

Chromium is the first element in the 3d series where the 3d electrons start to sink into the core; they thus contribute less to metallic bonding, and hence the melting and boiling points and the enthalpy of atomisation of chromium are lower than those of the preceding element vanadium. Chromium(VI) is a strong oxidising agent in contrast to the molybdenum(VI) and tungsten(VI) oxides.

Bulk
Chromium is the third hardest element after carbon (diamond) and boron. Its Mohs hardness is 8.5, which means that it can scratch samples of quartz and topaz, but can be scratched by corundum. Chromium is highly resistant to tarnishing, which makes it useful as a metal that preserves its outermost layer from corroding, unlike other metals such as copper, magnesium, and aluminium.

Chromium has a melting point of 1907 °C (3465 °F), which is relatively low compared to the majority of transition metals. However, it still has the second highest melting point out of all the period 4 elements, being topped by vanadium by 3 °C (5 °F) at 1910 °C (3470 °F). The boiling point of 2671 °C (4840 °F), however, is comparatively lower, having the fourth lowest boiling point out of the Period 4 transition metals alone behind copper, manganese and zinc. The electrical resistivity of chromium at 20 °C is 125 nanoohm-meters.

Chromium has a high specular reflection in comparison to other transition metals. In infrared, at 425 μm, chromium has a maximum reflectance of about 72%, reducing to a minimum of 62% at 750 μm before rising again to 90% at 4000 μm. When chromium is used in stainless steel alloys and polished, the specular reflection decreases with the inclusion of additional metals, yet is still high in comparison with other alloys. Between 40% and 60% of the visible spectrum is reflected from polished stainless steel. The explanation on why chromium displays such a high turnout of reflected photon waves in general, especially the 90% in infrared, can be attributed to chromium's magnetic properties. Chromium has unique magnetic properties; it is the only elemental solid that shows antiferromagnetic ordering at room temperature and below. Above 38 °C, its magnetic ordering becomes paramagnetic. The antiferromagnetic properties, which cause the chromium atoms to temporarily ionize and bond with themselves, are present because the body-centric cubic's magnetic properties are disproportionate to the lattice periodicity. This is due to the magnetic moments at the cube's corners and the unequal, but antiparallel, cube centers. From here, the frequency-dependent relative permittivity of chromium, deriving from Maxwell's equations and chromium's antiferromagnetism, leaves chromium with a high infrared and visible light reflectance.

Passivation
Chromium metal in air is passivated: it forms a thin, protective, surface layer of oxide. This layer has a spinel structure a few atomic layers thick; it is very dense and inhibits the diffusion of oxygen into the underlying metal. In contrast, iron forms a more porous oxide through which oxygen can migrate, causing continued rusting. Passivation can be enhanced by short contact with oxidizing acids like nitric acid. Passivated chromium is stable against acids. Passivation can be removed with a strong reducing agent that destroys the protective oxide layer on the metal. Chromium metal treated in this way readily dissolves in weak acids.

Chromium, unlike iron and nickel, does not suffer from hydrogen embrittlement. However, it does suffer from nitrogen embrittlement, reacting with nitrogen from air and forming brittle nitrides at the high temperatures necessary to work the metal parts.

Isotopes
Naturally occurring chromium is composed of four stable isotopes; 50Cr, 52Cr, 53Cr and 54Cr, with 52Cr being the most abundant (83.789% natural abundance). 50Cr is observationally stable, as it is theoretically capable of decaying to 50Ti via double electron capture with a half-life of no less than 1.3 years. Twenty-five radioisotopes have been characterized, ranging from 42Cr to 70Cr; the most stable radioisotope is 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 less than 1 minute. Chromium also has two metastable nuclear isomers. The primary decay mode before the most abundant stable isotope, 52Cr, is electron capture and the primary mode after is beta decay.

53Cr is the radiogenic decay product of 53Mn (half-life 3.74 million years). Chromium isotopes are typically collocated (and compounded) with manganese isotopes. This circumstance is useful in isotope geology. Manganese-chromium isotope ratios reinforce the evidence from 26Al and 107Pd concerning 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 isotopic composition 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. 53Cr has been posited as a proxy for atmospheric oxygen concentration.

Chemistry and compounds
Chromium is a member of group 6, of the transition metals. The +3 and +6 states occur most commonly within chromium compounds, followed by +2; charges of +1, +4 and +5 for chromium are rare, but do nevertheless occasionally exist.

Chromium(0)
Many Cr(0) complexes are known. Bis(benzene)chromium and chromium hexacarbonyl are highlights in organochromium chemistry.

Chromium(II)
Chromium(II) compounds are uncommon, in part because they readily oxidize to chromium(III) derivatives in air. Water-stable chromium(II) chloride that can be made by reducing chromium(III) chloride with zinc. The resulting bright blue solution created from dissolving chromium(II) chloride is stable at neutral pH. Some other notable chromium(II) compounds include chromium(II) oxide, and chromium(II) sulfate. Many chromium(II) carboxylates are known. The red chromium(II) acetate (Cr2(O2CCH3)4) is somewhat famous. It features a Cr-Cr quadruple bond.

Chromium(III)
A large number of chromium(III) compounds are known, such as chromium(III) nitrate, chromium(III) acetate, and chromium(III) oxide. Chromium(III) can be obtained by dissolving elemental chromium in acids like hydrochloric acid or sulfuric acid, but it can also be formed through the reduction of chromium(VI) by cytochrome c7. The ion has a similar radius (63 pm) to  (radius 50 pm), and they can replace each other in some compounds, such as in chrome alum and alum.

Chromium(III) tends to form octahedral complexes. Commercially available chromium(III) chloride hydrate is the dark green complex [CrCl2(H2O)4]Cl. Closely related compounds are the pale green [CrCl(H2O)5]Cl2 and violet [Cr(H2O)6]Cl3. If anhydrous violet chromium(III) chloride is dissolved in water, the violet solution turns green after some time as the chloride in the inner coordination sphere is replaced by water. This kind of reaction is also observed with solutions of chrome alum and other water-soluble chromium(III) salts. A tetrahedral coordination of chromium(III) has been reported for the Cr-centered Keggin anion [α-CrW12O40]5–.

Chromium(III) hydroxide (Cr(OH)3) is amphoteric, dissolving in acidic solutions to form [Cr(H2O)6]3+, and in basic solutions to form. It is dehydrated by heating to form the green chromium(III) oxide (Cr2O3), a stable oxide with a crystal structure identical to that of corundum.

Chromium(VI)
Chromium(VI) compounds are oxidants at low or neutral pH. Chromate anions and dichromate (Cr2O72−) anions are the principal ions at this oxidation state. They exist at an equilibrium, determined by pH:
 * 2 [CrO4]2− + 2 H+ [Cr2O7]2− + H2O

Chromium(VI) oxyhalides are known also and include chromyl fluoride (CrO2F2) and chromyl chloride. However, despite several erroneous claims, chromium hexafluoride (as well as all higher hexahalides) remains unknown, as of 2020.

Sodium chromate is produced industrially by the oxidative roasting of chromite ore with sodium carbonate. The change in equilibrium is visible by a change from yellow (chromate) to orange (dichromate), such as when an acid is added to a neutral solution of potassium chromate. At yet lower pH values, further condensation to more complex oxyanions of chromium is possible.

Both the chromate and dichromate anions are strong oxidizing reagents at low pH:
 * + 14 + 6 e− → 2  + 21  (ε0 = 1.33 V)

They are, however, only moderately oxidizing at high pH:
 * + 4 + 3 e− →  + 5  (ε0 = −0.13 V)

Chromium(VI) compounds in solution can be detected by adding an acidic hydrogen peroxide solution. The unstable dark blue chromium(VI) peroxide (CrO5) is formed, which can be stabilized as an ether adduct.

Chromic acid has the hypothetical formula. It is a vaguely described chemical, despite many well-defined chromates and dichromates being known. The dark red chromium(VI) oxide, the acid anhydride of chromic acid, is sold industrially as "chromic acid". It can be produced by mixing sulfuric acid with dichromate and is a strong oxidizing agent.

Other oxidation states
Compounds of chromium(V) are rather rare; the oxidation state +5 is only realized in few compounds but are intermediates in many reactions involving oxidations by chromate. The only binary compound is the volatile chromium(V) fluoride (CrF5). This red solid has a melting point of 30 °C and a boiling point of 117 °C. It can be prepared by treating chromium metal with fluorine at 400 °C and 200 bar pressure. The peroxochromate(V) is another example of the +5 oxidation state. Potassium peroxochromate (K3[Cr(O2)4]) is made by reacting potassium chromate with hydrogen peroxide at low temperatures. This red brown compound is stable at room temperature but decomposes spontaneously at 150–170 °C.

Compounds of chromium(IV) are slightly more common than those of chromium(V). The tetrahalides, CrF4, CrCl4, and CrBr4, can be produced by treating the trihalides with the corresponding halogen at elevated temperatures. Such compounds are susceptible to disproportionation reactions and are not stable in water. Organic compounds containing Cr(IV) state such as chromium tetra t-butoxide are also known.

Most chromium(I) compounds are obtained solely by oxidation of electron-rich, octahedral chromium(0) complexes. Other chromium(I) complexes contain cyclopentadienyl ligands. As verified by X-ray diffraction, a Cr-Cr quintuple bond (length 183.51(4) pm) has also been described. Extremely bulky monodentate ligands stabilize this compound by shielding the quintuple bond from further reactions.

Occurrence
Chromium is the 21st most abundant element in Earth's crust with an average concentration of 100 ppm. It is also the seventh most abundant element on earth. Chromium compounds are found in the environment from the erosion of chromium-containing rocks, and can be redistributed by volcanic eruptions. Typical background concentrations of chromium in environmental media are: atmosphere <10 ng/m3; soil <500 mg/kg; vegetation <0.5 mg/kg; freshwater <10 μg/L; seawater <1 μg/L; sediment <80 mg/kg. Chromium is mined as chromite (FeCr2O4) ore.

About two-fifths of the chromite ores and concentrates in the world are produced in South Africa, about a third in Kazakhstan, while India, Russia, and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa. Although rare, deposits of native chromium exist. 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 diamonds.

The relation between Cr(III) and Cr(VI) strongly depends on pH and oxidative properties of the location. In most cases, Cr(III) is the dominating species, but in some areas, the ground water can contain up to 39 μg/L of total chromium, of which 30 μg/L is Cr(VI).

Early applications
Chromium minerals as pigments came to the attention of the west in the eighteenth century. On 26 July 1761, Johann Gottlob Lehmann found an orange-red mineral in the Beryozovskoye mines in the Ural Mountains which he named Siberian red lead. Though misidentified as a lead compound with selenium and iron components, the mineral was in fact crocoite with a formula of PbCrO4. In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red lead mineral that was discovered to possess useful properties as a pigment in paints. After Pallas, the use of Siberian red lead as a paint pigment began to develop rapidly throughout the region. Crocoite would be the principal source of chromium in pigments until the discovery of chromite many years later.

In 1794, Louis Nicolas Vauquelin received samples of crocoite ore. He produced chromium trioxide (CrO3) by mixing crocoite with hydrochloric acid. In 1797, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven, for which he is credited as the one who truly discovered the element. Vauquelin was also able to detect traces of chromium in precious gemstones, such as ruby and emerald.

During the nineteenth century, chromium was primarily used not only as a component of paints, but in tanning salts as well. For quite some time, the crocoite found in Russia was the main source for such tanning materials. In 1827, a larger chromite deposit was discovered near Baltimore, United States, which quickly met the demand for tanning salts much more adequately than the crocoite that had been used previously. This made the United States the largest producer of chromium products until the year 1848, when larger deposits of chromite were uncovered near the city of Bursa, Turkey. With the development of metallurgy and chemical industries in the Western world, the need for chromium increased.

Chromium is also famous for its reflective, metallic luster when polished. It is used as a protective and decorative coating on car parts, plumbing fixtures, furniture parts and many other items, usually applied by electroplating. Chromium was used for electroplating as early as 1848, but this use only became widespread with the development of an improved process in 1924.

Production
Approximately 28.8 million metric tons (Mt) of marketable chromite ore was produced in 2013, and converted into 7.5 Mt of ferrochromium. According to John F. Papp, writing for the USGS, "Ferrochromium is the leading end use of chromite ore, [and] stainless steel is the leading end use of ferrochromium."

The largest producers of chromium ore in 2013 have been South Africa (48%), Kazakhstan (13%), Turkey (11%), and India (10%), with several other countries producing the rest of about 18% of the world production.

The two main products of chromium ore refining are ferrochromium and metallic chromium. For those products the ore smelter process differs considerably. For the production of ferrochromium, the chromite ore (FeCr2O4) is reduced in large scale in electric arc furnace or in smaller smelters with either aluminium or silicon in an aluminothermic reaction.

For the production of pure chromium, the iron must be separated from the chromium in a two step roasting and leaching process. The chromite ore is heated with a mixture of calcium carbonate and sodium carbonate in the presence of air. The chromium is oxidized to the hexavalent form, while the iron forms the stable Fe2O3. The subsequent leaching at higher elevated temperatures dissolves the chromates and leaves the insoluble iron oxide. The chromate is converted by sulfuric acid into the dichromate.

 * 4 FeCr2O4 + 8 Na2CO3 + 7 O2 → 8 Na2CrO4 + 2 Fe2O3 + 8 CO2

 * 2 Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O

The dichromate is converted to the chromium(III) oxide by reduction with carbon and then reduced in an aluminothermic reaction to chromium.
 * Na2Cr2O7 + 2 C → Cr2O3 + Na2CO3 + CO
 * Cr2O3 + 2 Al → Al2O3 + 2 Cr

Applications
The creation of metal alloys account for 85% of the available chromium's usage. The remainder of chromium is used in the chemical, refractory, and foundry industries.

Metallurgy

The strengthening effect of forming stable metal carbides at grain boundaries, and the strong increase in corrosion resistance made chromium an important alloying material for steel. High-speed tool steels contain 3–5% chromium. Stainless steel, the primary corrosion-resistant metal alloy, is formed when chromium is introduced to iron in concentrations above 11%. For stainless steel's formation, ferrochromium is added to the molten iron. Also, nickel-based alloys have increased strength due to the formation of discrete, stable, metal, carbide particles at the grain boundaries. For example, Inconel 718 contains 18.6% chromium. Because of the excellent high-temperature properties of these nickel superalloys, they are used in jet engines and gas turbines in lieu of common structural materials. ASTM B163 relies on chromium for condenser and heat-exchanger tubes, while castings with high strength at elevated temperatures that contain chromium are standardised with ASTM A567. AISI type 332 is used where high temperature would normally cause carburization, oxidation or corrosion. Incoloy 800 "is capable of remaining stable and maintaining its austenitic structure even after long time exposures to high temperatures". Nichrome is used as resistance wire for heating elements in things like toasters and space heaters. These uses make chromium a strategic material. Consequently, during World War II, U.S. road engineers were instructed to avoid chromium in yellow road paint, as it "may become a critical material during the emergency". The United States likewise considered chromium "essential for the German war industry" and made intense diplomatic efforts to keep it out of the hands of Nazi Germany.

The high hardness and corrosion resistance of unalloyed chromium makes it a reliable metal for surface coating; it is still the most popular metal for sheet coating, with its above-average durability, compared to other coating metals. A layer of chromium is deposited on pretreated metallic surfaces by electroplating techniques. There are two deposition methods: thin, and thick. Thin deposition involves a layer of chromium below 1 μm thickness deposited by chrome plating, and is used for decorative surfaces. Thicker chromium layers are deposited if wear-resistant surfaces are needed. Both methods use acidic chromate or dichromate solutions. To prevent the energy-consuming change in oxidation state, the use of chromium(III) sulfate is under development; for most applications of chromium, the previously established process is used.

In the chromate conversion coating process, the strong oxidative properties of chromates are used to deposit a protective oxide layer on metals like aluminium, zinc, and cadmium. This passivation and the self-healing properties of the chromate stored in the chromate conversion coating, which is able to migrate to local defects, are the benefits of this coating method. Because of environmental and health regulations on chromates, alternative coating methods are under development.

Chromic acid anodizing (or Type I anodizing) of aluminium is another electrochemical process that does not lead to the deposition of chromium, but uses chromic acid as an electrolyte in the solution. During anodization, an oxide layer is formed on the aluminium. The use of chromic acid, instead of the normally used sulfuric acid, leads to a slight difference of these oxide layers. The high toxicity of Cr(VI) compounds, used in the established chromium electroplating process, and the strengthening of safety and environmental regulations demand a search for substitutes for chromium, or at least a change to less toxic chromium(III) compounds.

Pigment
The mineral crocoite (which is also 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 photodegrade, but it tends to darken due to the formation of chromium(III) oxide. It has a strong color, and was used for school buses in the United States and for the postal services (for example, the Deutsche Post) in Europe. The use of chrome yellow has since declined due to environmental and safety concerns and was replaced by organic pigments or other alternatives that are free from lead and chromium. Other pigments that are based around chromium are, for example, the deep shade of red pigment chrome red, which is simply lead chromate with lead(II) hydroxide (PbCrO4·Pb(OH)2). A very important chromate pigment, which was used widely in metal primer formulations, was zinc chromate, now replaced by zinc phosphate. A wash primer was formulated to replace the dangerous practice of pre-treating aluminium aircraft bodies with a phosphoric acid solution. This used zinc tetroxychromate dispersed in a solution of polyvinyl butyral. An 8% solution of phosphoric acid in solvent was added just before application. It was found that an easily oxidized alcohol was an essential ingredient. A thin layer of about 10–15 μm was applied, which turned from yellow to dark green when it was cured. There is still a question as to the correct mechanism. Chrome green is a mixture of Prussian blue and chrome yellow, while the chrome oxide green is chromium(III) oxide.

Chromium oxides are also used as a green pigment in the field of glassmaking and also as a glaze for ceramics. Green chromium oxide is extremely lightfast and as such is used in cladding coatings. It is also the main ingredient in infrared reflecting paints, used by the armed forces to paint vehicles and to give them the same infrared reflectance as green leaves.

Other uses
Chromium(III) ions present in corundum crystals (aluminium oxide) cause them to be colored red; when corundum appears as such, it is known as a ruby. If the corundum is lacking in chromium(III) ions, it is known as a sapphire. A red-colored artificial ruby may also be achieved by doping chromium(III) into artificial corundum crystals, thus making chromium a requirement for making synthetic rubies. Such a synthetic ruby crystal was the basis for the first laser, produced in 1960, which relied on stimulated emission of light from the chromium atoms in such a crystal. Ruby has a laser transition at 694.3 nanometers, in a deep red color.

Chromium(VI) salts are used for the preservation of wood. For example, chromated copper arsenate (CCA) is used in timber treatment to protect wood from decay fungi, wood-attacking insects, including termites, and marine borers. The formulations contain chromium based on the oxide CrO3 between 35.3% and 65.5%. In the United States, 65,300 metric tons of CCA solution were used in 1996.

Chromium(III) salts, especially chrome alum and chromium(III) sulfate, are used in the tanning of leather. The chromium(III) stabilizes the leather by cross linking the collagen fibers. Chromium tanned leather can contain 4–5% of chromium, which is tightly bound to the proteins. Although the form of chromium used for tanning is not the toxic hexavalent variety, there remains interest in management of chromium in the tanning industry. Recovery and reuse, direct/indirect recycling, and "chrome-less" or "chrome-free" tanning are practiced to better manage chromium usage.

The high heat resistivity and high melting point makes chromite and chromium(III) oxide a material for high temperature refractory applications, like blast furnaces, cement kilns, molds for the firing of bricks and as foundry sands for the casting of metals. In these applications, the refractory materials are made from mixtures of chromite and magnesite. The use is declining because of the environmental regulations due to the possibility of the formation of chromium(VI).

Several chromium compounds are used as catalysts for processing hydrocarbons. For example, the Phillips catalyst, prepared from chromium oxides, is used for the production of about half the world's polyethylene. Fe-Cr mixed oxides are employed as high-temperature catalysts for the water gas shift reaction. Copper chromite is a useful hydrogenation catalyst.

Uses of compounds

 * Chromium(IV) oxide (CrO2) is a magnetic compound. Its ideal shape anisotropy, which imparts high coercivity and remnant magnetization, made it a compound superior to γ-Fe2O3. Chromium(IV) oxide is used to manufacture magnetic tape used in high-performance audio tape and standard audio cassettes.
 * Chromium(III) oxide (Cr2O3) is a metal polish known as green rouge.
 * Chromic acid is a powerful oxidizing agent and is a useful compound for cleaning laboratory glassware of any trace of organic compounds. It is prepared by dissolving potassium dichromate in concentrated sulfuric acid, which is then used to wash the apparatus. Sodium dichromate is sometimes used because of its higher solubility (50 g/L versus 200 g/L respectively). The use of dichromate cleaning solutions is now phased out due to the high toxicity and environmental concerns. Modern cleaning solutions are highly effective and chromium free.
 * Potassium dichromate is a chemical reagent, used as a titrating agent.
 * Chromates are added to drilling muds to prevent corrosion of steel under wet conditions.
 * Chrome alum is Chromium(III) potassium sulfate and is used as a mordant (i.e., a fixing agent) for dyes in fabric and in tanning.

Biological role
The biologically beneficial effects of chromium(III) are debated. Chromium is accepted by the U.S. National Institutes of Health as a trace element for its roles in the action of insulin, a hormone that mediates the metabolism and storage of carbohydrate, fat, and protein. The mechanism of its actions in the body, however, have not been defined, leaving in question the essentiality of chromium.

In contrast, hexavalent chromium (Cr(VI) or Cr6+) is highly toxic and mutagenic. Ingestion of chromium(VI) in water has been linked to stomach tumors, and it may also cause allergic contact dermatitis (ACD).

"Chromium deficiency", involving a lack of Cr(III) in the body, or perhaps some complex of it, such as glucose tolerance factor, is controversial. Some studies suggest that the biologically active form of chromium(III) is transported in the body via an oligopeptide called low-molecular-weight chromium-binding substance (LMWCr), which might play a role in the insulin signaling pathway.

The chromium content of common foods is generally low (1–13 micrograms per serving). The chromium content of food varies widely, due to differences in soil mineral content, growing season, plant cultivar, and contamination during processing. Chromium (and nickel) leach into food cooked in stainless steel, with the effect being largest when the cookware is new. Acidic foods that are cooked for many hours also exacerbate this effect.

Dietary recommendations
There is disagreement on chromium's status as an essential nutrient. Governmental departments from Australia, New Zealand, India, Japan, and the United States consider chromium essential  while the European Food Safety Authority (EFSA) of the European Union does not.

The U.S. National Academy of Medicine (NAM) updated the Estimated Average Requirements (EARs) and the Recommended Dietary Allowances (RDAs) for chromium in 2001. For chromium, there was insufficient information to set EARs and RDAs, so its needs are described as estimates for Adequate Intakes (AIs). The current AIs of chromium for women ages 14 through 50 is 25 μg/day, and the AIs for women ages 50 and above is 20 μg/day. The AIs for women who are pregnant are 30 μg/day, and for women who are lactating, the set AIs are 45 μg/day. The AIs for men ages 14 through 50 are 35 μg/day, and the AIs for men ages 50 and above are 30 μg/day. For children ages 1 through 13, the AIs increase with age from 0.2 μg/day up to 25 μg/day. As for safety, the NAM sets Tolerable Upper Intake Levels (ULs) for vitamins and minerals when the evidence is sufficient. In the case of chromium, there is not yet enough information, hence no UL has been established. Collectively, the EARs, RDAs, AIs, and ULs are the parameters for the nutrition recommendation system known as Dietary Reference Intake (DRI). Australia and New Zealand consider chromium to be an essential nutrient, with an AI of 35 μg/day for men, 25 μg/day for women, 30 μg/day for women who are pregnant, and 45 μg/day for women who are lactating. A UL has not been set due to the lack of sufficient data. India considers chromium to be an essential nutrient, with an adult recommended intake of 33 μg/day. Japan also considers chromium to be an essential nutrient, with an AI of 10 μg/day for adults, including women who are pregnant or lactating. A UL has not been set. The EFSA of the European Union however, does not consider chromium to be an essential nutrient; chromium is the only mineral for which the United States and the European Union disagree.

Labeling
For U.S. food and dietary supplement labeling purposes, the amount of the substance in a serving is expressed as a percent of the Daily Value (%DV). For chromium labeling purposes, 100% of the Daily Value was 120 μg. As of 27 May 2016 the percentage of daily value was revised to 35 μg to bring the chromium intake into a consensus with the official Recommended Dietary Allowance. A table of the old and new adult daily values is provided at Reference Daily Intake.

Food sources
Food composition databases such as those maintained by the U.S. Department of Agriculture do not contain information on the chromium content of foods. A wide variety of animal and vegetable foods contain chromium. Content per serving is influenced by the chromium content of the soil in which the plants are grown, by foodstuffs fed to animals, and by processing methods, as chromium is leached into foods if processed or cooked in stainless steel equipment. One diet analysis study conducted in Mexico reported an average daily chromium intake of 30 micrograms. An estimated 31% of adults in the United States consume multi-vitamin/mineral dietary supplements, which often contain 25 to 60 micrograms of chromium.

Supplementation
Chromium is an ingredient in total parenteral nutrition (TPN), because deficiency can occur after months of intravenous feeding with chromium-free TPN. It is also added to nutritional products for preterm infants. Although the mechanism of action in biological roles for chromium is unclear, in the United States chromium-containing products are sold as non-prescription dietary supplements in amounts ranging from 50 to 1,000 μg. Lower amounts of chromium are also often incorporated into multi-vitamin/mineral supplements consumed by an estimated 31% of adults in the United States. Chemical compounds used in dietary supplements include chromium chloride, chromium citrate, chromium(III) picolinate, chromium(III) polynicotinate, and other chemical compositions. The benefit of supplements has not been proven.

Approved and disapproved health claims
In 2005, the U.S. Food and Drug Administration had approved a qualified health claim for chromium picolinate with a requirement for very specific label wording: "One small study suggests that chromium picolinate may reduce the risk of insulin resistance, and therefore possibly may reduce the risk of type 2 diabetes. FDA concludes, however, that the existence of such a relationship between chromium picolinate and either insulin resistance or type 2 diabetes is highly uncertain." At the same time, in answer to other parts of the petition, the FDA rejected claims for chromium picolinate and cardiovascular disease, retinopathy or kidney disease caused by abnormally high blood sugar levels. In 2010, chromium(III) picolinate was approved by Health Canada to be used in dietary supplements. Approved labeling statements include: a factor in the maintenance of good health, provides support for healthy glucose metabolism, helps the body to metabolize carbohydrates and helps the body to metabolize fats. The European Food Safety Authority (EFSA) approved claims in 2010 that chromium contributed to normal macronutrient metabolism and maintenance of normal blood glucose concentration, but rejected claims for maintenance or achievement of a normal body weight, or reduction of tiredness or fatigue.

Given the evidence for chromium deficiency causing problems with glucose management in the context of intravenous nutrition products formulated without chromium, research interest turned to whether chromium supplementation would benefit people who have type 2 diabetes but are not chromium deficient. Looking at the results from four meta-analyses, one reported a statistically significant decrease in fasting plasma glucose levels (FPG) and a non-significant trend in lower hemoglobin A1C. A second reported the same, a third reported significant decreases for both measures, while a fourth reported no benefit for either. A review published in 2016 listed 53 randomized clinical trials that were included in one or more of six meta-analyses. It concluded that whereas there may be modest decreases in FPG and/or HbA1C that achieve statistical significance in some of these meta-analyses, few of the trials achieved decreases large enough to be expected to be relevant to clinical outcome.

Two systematic reviews looked at chromium supplements as a mean of managing body weight in overweight and obese people. One, limited to chromium picolinate, a popular supplement ingredient, reported a statistically significant −1.1 kg (2.4 lb) weight loss in trials longer than 12 weeks. The other included all chromium compounds and reported a statistically significant −0.50 kg (1.1 lb) weight change. Change in percent body fat did not reach statistical significance. Authors of both reviews considered the clinical relevance of this modest weight loss as uncertain/unreliable. The European Food Safety Authority reviewed the literature and concluded that there was insufficient evidence to support a claim.

Chromium is promoted as a sports performance dietary supplement, based on the theory that it potentiates insulin activity, with anticipated results of increased muscle mass, and faster recovery of glycogen storage during post-exercise recovery. A review of clinical trials reported that chromium supplementation did not improve exercise performance or increase muscle strength. The International Olympic Committee reviewed dietary supplements for high-performance athletes in 2018 and concluded there was no need to increase chromium intake for athletes, nor support for claims of losing body fat.

Fresh-water fish
Irrigation water standards for chromium are 0.1 mg/L, but some rivers in Bangladesh are more than five times that amount. The standard for fish for human consumption is less than 1 mg/kg, but many tested samples were more than five times that amount. Chromium, especially hexavalent chromium, is highly toxic to fish because it is easily absorbed across the gills, readily enters blood circulation, crosses cell membranes and bioconcentrates up the food chain. In contrast, the toxicity of trivalent chromium is very low, attributed to poor membrane permeability and little biomagnification.

Acute and chronic exposure to chromium(VI) affects fish behavior, physiology, reproduction and survival. Hyperactivity and erratic swimming have been reported in contaminated environments. Egg hatching and fingerling survival are affected. In adult fish there are reports of histopathological damage to liver, kidney, muscle, intestines, and gills. Mechanisms include mutagenic gene damage and disruptions of enzyme functions.

There is evidence that fish may not require chromium, but benefit from a measured amount in diet. In one study, juvenile fish gained weight on a zero chromium diet, but the addition of 500 μg of chromium in the form of chromium chloride or other supplement types, per kilogram of food (dry weight), increased weight gain. At 2,000 μg/kg the weight gain was no better than with the zero chromium diet, and there were increased DNA strand breaks.

Precautions
Water-insoluble chromium(III) compounds and chromium metal are not considered a health hazard, while the toxicity and carcinogenic properties of chromium(VI) have been known for a long time. Because of the specific transport mechanisms, only limited amounts of chromium(III) enter the cells. Acute oral toxicity ranges between 50 and 150 mg/kg. A 2008 review suggested that moderate uptake of chromium(III) through dietary supplements poses no genetic-toxic risk. In the US, the Occupational Safety and Health Administration (OSHA) has designated an air permissible exposure limit (PEL) in the workplace as a time-weighted average (TWA) of 1 mg/m3. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 0.5 mg/m3, time-weighted average. The IDLH (immediately dangerous to life and health) value is 250 mg/m3.

Chromium(VI) toxicity
The acute oral toxicity for chromium(VI) ranges between 1.5 and 3.3 mg/kg. In the body, chromium(VI) is reduced by several mechanisms to chromium(III) already in the blood before it enters the cells. The chromium(III) is excreted from the body, whereas the chromate ion is transferred into the cell by a transport mechanism, by which also sulfate and phosphate ions enter the cell. The acute toxicity of chromium(VI) is due to its strong oxidant properties. After it reaches the blood stream, it damages the kidneys, the liver and blood cells through oxidation reactions. Hemolysis, renal, and liver failure result. Aggressive dialysis can be therapeutic.

The carcinogenity of chromate dust has been known for a long time, and in 1890 the first publication described the elevated cancer risk of workers in a chromate dye company. Three mechanisms have been proposed to describe the genotoxicity of chromium(VI). The first mechanism includes highly reactive hydroxyl radicals and other reactive radicals which are by products of the reduction of chromium(VI) to chromium(III). The second process includes the direct binding of chromium(V), produced by reduction in the cell, and chromium(IV) compounds to the DNA. The last mechanism attributed the genotoxicity to the binding to the DNA of the end product of the chromium(III) reduction.

Chromium salts (chromates) are also the cause of allergic reactions in some people. Chromates are often used to manufacture, amongst other things, leather products, paints, cement, mortar and anti-corrosives. Contact with products containing chromates can lead to allergic contact dermatitis and irritant dermatitis, resulting in ulceration of the skin, sometimes referred to as "chrome ulcers". This condition is often found in workers that have been exposed to strong chromate solutions in electroplating, tanning and chrome-producing manufacturers.

Environmental issues
Because chromium compounds were used in dyes, paints, and leather tanning compounds, these compounds are often found in soil and groundwater at active and abandoned industrial sites, needing environmental cleanup and remediation. Primer paint containing hexavalent chromium is still widely used for aerospace and automobile refinishing applications.

In 2010, the Environmental Working Group studied the drinking water in 35 American cities in the first nationwide study. The study found measurable hexavalent chromium in the tap water of 31 of the cities sampled, with Norman, Oklahoma, at the top of list; 25 cities had levels that exceeded California's proposed limit.

The more toxic hexavalent chromium form can be reduced to the less soluble trivalent oxidation state in soils by organic matter, ferrous iron, sulfides, and other reducing agents, with the rates of such reduction being faster under more acidic conditions than under more alkaline ones. In contrast, trivalent chromium can be oxidized to hexavalent chromium in soils by manganese oxides, such as Mn(III) and Mn(IV) compounds. Since the solubility and toxicity of chromium (VI) are greater that those of chromium (III), the oxidation-reduction conversions between the two oxidation states have implications for movement and bioavailability of chromium in soils, groundwater, and plants.

title: Chromium


text:
Cymbal

A cymbal is a common percussion instrument. Often used in pairs, cymbals consist of thin, normally round plates of various alloys. The majority of cymbals are of indefinite pitch, although small disc-shaped cymbals based on ancient designs sound a definite note (such as crotales). Cymbals are used in many ensembles ranging from the orchestra, percussion ensembles, jazz bands, heavy metal bands, and marching groups. Drum kits usually incorporate at least a crash, ride, or crash/ride, and a pair of hi-hat cymbals. A player of cymbals is known as a cymbalist.

Etymology and names
The word cymbal is derived from the Latin cymbalum, which is the latinisation, which in turn derives.

In orchestral scores, cymbals may be indicated by the French cymbales; German Becken, Schellbecken, Teller, or Tschinellen; Italian piatti or cinelli; and Spanish platillos. Many of these derive from the word for plates.

History
Cymbals have existed since ancient times. Representations of cymbals may be found in reliefs and paintings from Armenian Highlands (7th century BC ), Larsa, Babylon, Assyria, ancient Egypt, ancient Greece, and ancient Rome. References to cymbals also appear throughout the Bible, through many Psalms and songs of praise to God. Cymbals may have been introduced to China from Central Asia in the 3rd or 4th century AD.

India
In India, cymbals have been in use since ancient times and are still used across almost all major temples and Buddhist sites. Gigantic aartis along the Ganges, which are revered by Hindus all over the world, are incomplete without large cymbals.

Central Asia and Iran

The Shahnameh (circa 977 and 1010 CE) mentions the use of cymbals at least 14 times in its text, most in the context of creating a loud din in war, to frighten the enemy or to celebrate. The Persian word is sanj or senj (سنج), but the Shahnameh does not claim these to be Persian in origin. Several times it calls then "Indian cymbals." Other adjectives to describe them include "golden" and "brass," and to play them is to "clash" them.

A different form is called sanj angshati (سنج انگشتی), these are zill.

Ashura ceremony
Besides the original use in war, another use in Persian culture was the Ashura ceremony. Originally in the ceremony, two pieces of stone were beaten on the sides of the mourner with special movements accompanied by a lamentation song. This has been replaced by beating Karbzani or Karebzani and playing sanj and ratchets. Cities where this has been performed include Lahijan and Aran of Kashan, as well as Semnan and Sabzevar.

Etymology

 * See Zang

All theories about the etymology of the word Sanj, identify it as a Pahlavi word. By some accounts means weight; and it is possible that the original term was sanjkūb meaning ”striking weights” [against each other]. By some accounts the word is reform version of "Zang" (bell), referring to its bell-shaped plate.

Turkey
Cymbals were employed by Turkish janissaries in the 14th century or earlier. By the 17th century, such cymbals were used in European music, and more commonly played in military bands and orchestras by the mid 18th century. Since the 19th century, some composers have called for larger roles for cymbals in musical works, and a variety of cymbal shapes, techniques, and hardware have been developed in response.

Anatomy
The anatomy of the cymbal plays a large part in the sound it creates. A hole is drilled in the center of the cymbal, which is used to either mount the cymbal on a stand or for tying straps through (for hand playing). The bell, dome, or cup is the raised section immediately surrounding the hole. The bell produces a higher "pinging" pitch than the rest of the cymbal. The bow is the rest of the surface surrounding the bell. The bow is sometimes described in two areas: the ride and crash area. The ride area is the thicker section closer to the bell while the crash area is the thinner tapering section near the edge. The edge or rim is the immediate circumference of the cymbal.

Cymbals are measured by their diameter either in inches or centimeters. The size of the cymbal affects its sound, larger cymbals usually being louder and having longer sustain. The weight describes how thick the cymbal is. Cymbal weights are important to the sound they produce and how they play. Heavier cymbals have a louder volume, more cut, and better stick articulation (when using drum sticks). Thin cymbals have a fuller sound, lower pitch, and faster response.

The profile of the cymbal is the vertical distance of the bow from the bottom of the bell to the cymbal edge (higher profile cymbals are more bowl-shaped). The profile affects the pitch of the cymbal: higher profile cymbals have higher pitch.

Orchestral cymbals
Cymbals offer a composer nearly endless amounts of color and effect. Their unique timbre allows them to project even against a full orchestra and through the heaviest of orchestrations and enhance articulation and nearly any dynamic. Cymbals have been utilized historically to suggest frenzy, fury or bacchanalian revels, as seen in the Venus music in Wagner's Tannhäuser, Grieg's Peer Gynt suite, and Osmin's aria "O wie will ich triumphieren" from Mozart's Die Entführung aus dem Serail.

Clash cymbals

Orchestral clash cymbals are traditionally used in pairs, each one having a strap set in the bell of the cymbal by which they are held. Such a pair is known as clash cymbals, crash cymbals, hand cymbals, or plates. Certain sounds can be obtained by rubbing their edges together in a sliding movement for a "sizzle", striking them against each other in what is called a "crash", tapping the edge of one against the body of the other in what is called a "tap-crash", scraping the edge of one from the inside of the bell to the edge for a "scrape" or "zischen", or shutting the cymbals together and choking the sound in what is called a "hi-hat" or "crush". A skilled percussionist can obtain an enormous dynamic range from such cymbals. For example, in Beethoven's Symphony No. 9, the percussionist is employed to first play cymbals pianissimo, adding a touch of colour rather than loud crash.

Crash cymbals are usually damped by pressing them against the percussionist's body. A composer may write laissez vibrer, or, "let vibrate" (usually abbreviated l.v.), secco (dry), or equivalent indications on the score; more usually, the percussionist must judge when to damp based on the written duration of a crash and the context in which it occurs. Crash cymbals have traditionally been accompanied by the bass drum playing an identical part. This combination, played loudly, is an effective way to accentuate a note since it contributes to both very low and very high-frequency ranges and provides a satisfying "crash-bang-wallop". In older music the composer sometimes provided one part for this pair of instruments, writing senza piatti or piatti soli ("without cymbals" or "cymbals only") if only one is needed. This came from the common practice of having one percussionist play using one cymbal mounted to the shell of the bass drum. The percussionist would crash the cymbals with the left hand and use a mallet to strike the bass drum with the right. This method is nowadays often employed in pit orchestras and called for specifically by composers who desire a certain effect. Stravinsky calls for this in his ballet Petrushka, and Mahler calls for this in his Titan Symphony. The modern convention is for the instruments to have independent parts. However, in kit drumming, a cymbal crash is still most often accompanied by a simultaneous kick to the bass drum, which provides a musical effect and support to the crash.

Hi hats
Crash cymbals evolved into the low-sock and from this to the modern hi-hat. Even in a modern drum kit, they remain paired with the bass drum as the two instruments which are played with the player's feet. However, hi-hat cymbals tend to be heavy with little taper, more similar to a ride cymbal than to a clash cymbal as found in a drum kit, and perform a ride rather than a crash function.

Suspended cymbal
Another use of cymbals is the suspended cymbal. This instrument takes its name from the traditional method of suspending the cymbal by means of a leather strap or rope, thus allowing the cymbal to vibrate as freely as possible for maximum musical effect. Early jazz drumming pioneers borrowed this style of cymbal mounting during the early 1900s and later drummers further developed this instrument into the mounted horizontal or nearly horizontally mounted "crash" cymbals of a modern drum kit instead of a leather strap suspension system. Many modern drum kits use a mount with felt or otherwise dampening fabric to act as a barrier to hold the cymbals between metal clamps: thus forming the modern-day ride cymbal. Suspended cymbals can be played with yarn-, sponge-, or cord wrapped mallets. The first known instance of using a sponge-headed mallet on a cymbal is the final chord of Hector Berlioz' Symphonie Fantastique. Composers sometimes specifically request other types of mallets like felt mallets or timpani mallets for different attack and sustain qualities. Suspended cymbals can produce bright and slicing tones when forcefully struck, and give an eerie transparent "windy" sound when played quietly. A tremolo, or roll (played with two mallets alternately striking on opposing sides of the cymbal) can build in volume from almost inaudible to an overwhelming climax in a satisfyingly smooth manner (as in Humperdinck's Mother Goose Suite). The edge of a suspended cymbal may be hit with the shoulder of a drum stick to obtain a sound somewhat akin to that of clash cymbals. Other methods of playing include scraping a coin or triangle beater rapidly across the ridges on the top of the cymbal, giving a "zing" sound (as some percussionists do in the fourth movement of Dvořák's Symphony No. 9). Other effects that can be used include drawing a bass bow across the edge of the cymbal for a sound like squealing car brakes.

Ancient cymbals
Ancient, antique or tuned cymbals are much more rarely called for. Their timbre is entirely different, more like that of small hand-bells or of the notes of the keyed harmonica. They are not struck full against each other, but by one of their edges, and the note given in by them is higher in proportion as they are thicker and smaller. Berlioz's Romeo and Juliet calls for two pairs of cymbals, modeled on some old Pompeian instruments no larger than the hand (some are no larger than a large coin), and tuned to F and B flat. The modern instruments descended from this line are the crotales.

List of cymbal types
Cymbal types include:
 * Bell cymbal
 * China cymbal
 * Clash cymbal
 * Crash cymbal
 * Crash/ride cymbal
 * Finger cymbal
 * Flat ride cymbal
 * Hi-hat
 * Ride cymbal
 * Sizzle cymbal
 * Splash cymbal
 * Swish cymbal
 * Suspended cymbal
 * Taal – Indian cymbal (clash cymbal)

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Cymbal


text:
Cadmium

Cadmium is a chemical element; it has symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of its compounds, and like mercury, it has a lower melting point than the transition metals in groups 3 through 11. Cadmium and its congeners in group 12 are often not considered transition metals, in that they do not have partly filled d or f electron shells in the elemental or common oxidation states. The average concentration of cadmium in Earth's crust is between 0.1 and 0.5 parts per million (ppm). It was discovered in 1817 simultaneously by Stromeyer and Hermann, both in Germany, as an impurity in zinc carbonate.

Cadmium occurs as a minor component in most zinc ores and is a byproduct of zinc production. Cadmium was used for a long time as a corrosion-resistant plating on steel, and cadmium compounds are used as red, orange, and yellow pigments, to color glass, and to stabilize plastic. Cadmium use is generally decreasing because it is toxic (it is specifically listed in the European Restriction of Hazardous Substances Directive ) and nickel–cadmium batteries have been replaced with nickel–metal hydride and lithium-ion batteries. Due to it being a neutron poison, cadmium is also used as a component of control rods in nuclear fission reactors. One of its few new uses is in cadmium telluride solar panels.

Although cadmium has no known biological function in higher organisms, a cadmium-dependent carbonic anhydrase has been found in marine diatoms.

Physical properties
Cadmium is a soft, malleable, ductile, silvery-white divalent metal. It is similar in many respects to zinc but forms complex compounds. Unlike most other metals, cadmium is resistant to corrosion and is used as a protective plate on other metals. As a bulk metal, cadmium is insoluble in water and is not flammable; however, in its powdered form it may burn and release toxic fumes.

Chemical properties
Although cadmium usually has an oxidation state of +2, it also exists in the +1 state. Cadmium and its congeners are not always considered transition metals, in that they do not have partly filled d or f electron shells in the elemental or common oxidation states. Cadmium burns in air to form brown amorphous cadmium oxide (CdO); the crystalline form of this compound is a dark red which changes color when heated, similar to zinc oxide. Hydrochloric acid, sulfuric acid, and nitric acid dissolve cadmium by forming cadmium chloride (CdCl2), cadmium sulfate (CdSO4), or cadmium nitrate (Cd(NO3)2). The oxidation state +1 can be produced by dissolving cadmium in a mixture of cadmium chloride and aluminium chloride, forming the Cd22+ cation, which is similar to the Hg22+ cation in mercury(I) chloride.
 * Cd + CdCl2 + 2 AlCl3 → Cd2(AlCl4)2

The structures of many cadmium complexes with nucleobases, amino acids, and vitamins have been determined.

Isotopes

Naturally occurring cadmium is composed of eight isotopes. Two of them are radioactive, and three are expected to decay but have not measurably done so under laboratory conditions. The two natural radioactive isotopes are 113Cd (beta decay, half-life is $7.7 y$) and 116Cd (two-neutrino double beta decay, half-life is $2.9 y$). The other three are 106Cd, 108Cd (both double electron capture), and 114Cd (double beta decay); only lower limits on these half-lives have been determined. At least three isotopes – 110Cd, 111Cd, and 112Cd – are stable. Among the isotopes that do not occur naturally, the most long-lived are 109Cd with a half-life of 462.6 days, and 115Cd with a half-life of 53.46 hours. All of the remaining radioactive isotopes have half-lives of less than 2.5 hours, and the majority have half-lives of less than 5 minutes. Cadmium has 8 known meta states, with the most stable being 113mCd (t1⁄2 = 14.1 years), 115mCd (t1⁄2 = 44.6 days), and 117mCd (t1⁄2 = 3.36 hours).

The known isotopes of cadmium range in atomic mass from 94.950 u (95Cd) to 131.946 u (132Cd). For isotopes lighter than 112 u, the primary decay mode is electron capture and the dominant decay product is element 47 (silver). Heavier isotopes decay mostly through beta emission producing element 49 (indium).

One isotope of cadmium, 113Cd, absorbs neutrons with high selectivity: With very high probability, neutrons with energy below the cadmium cut-off will be absorbed; those higher than the cut-off will be transmitted. The cadmium cut-off is about 0.5 eV, and neutrons below that level are deemed slow neutrons, distinct from intermediate and fast neutrons.

Cadmium is created via the s-process in low- to medium-mass stars with masses of 0.6 to 10 solar masses, over thousands of years. In that process, a silver atom captures a neutron and then undergoes beta decay.

History
Cadmium (Latin cadmia, Greek καδμεία meaning "calamine", a cadmium-bearing mixture of minerals that was named after the Greek mythological character Κάδμος, Cadmus, the founder of Thebes) was discovered in contaminated zinc compounds sold in pharmacies in Germany in 1817 by Friedrich Stromeyer. Karl Samuel Leberecht Hermann simultaneously investigated the discoloration in zinc oxide and found an impurity, first suspected to be arsenic, because of the yellow precipitate with hydrogen sulfide. Additionally Stromeyer discovered that one supplier sold zinc carbonate instead of zinc oxide. Stromeyer found the new element as an impurity in zinc carbonate (calamine), and, for 100 years, Germany remained the only important producer of the metal. The metal was named after the Latin word for calamine, because it was found in this zinc ore. Stromeyer noted that some impure samples of calamine changed color when heated but pure calamine did not. He was persistent in studying these results and eventually isolated cadmium metal by roasting and reducing the sulfide. The potential for cadmium yellow as pigment was recognized in the 1840s, but the lack of cadmium limited this application.

Even though cadmium and its compounds are toxic in certain forms and concentrations, the British Pharmaceutical Codex from 1907 states that cadmium iodide was used as a medication to treat "enlarged joints, scrofulous glands, and chilblains".

In 1907, the International Astronomical Union defined the international ångström in terms of a red cadmium spectral line (1 wavelength = 6438.46963 Å). This was adopted by the 7th General Conference on Weights and Measures in 1927. In 1960, the definitions of both the metre and ångström were changed to use krypton.

After the industrial scale production of cadmium started in the 1930s and 1940s, the major application of cadmium was the coating of iron and steel to prevent corrosion; in 1944, 62% and in 1956, 59% of the cadmium in the United States was used for plating. In 1956, 24% of the cadmium in the United States was used for a second application in red, orange and yellow pigments from sulfides and selenides of cadmium.

The stabilizing effect of cadmium chemicals like the carboxylates cadmium laurate and cadmium stearate on PVC led to an increased use of those compounds in the 1970s and 1980s. The demand for cadmium in pigments, coatings, stabilizers, and alloys declined as a result of environmental and health regulations in the 1980s and 1990s; in 2006, only 7% of total cadmium consumption was used for plating, and only 10% was used for pigments. At the same time, these decreases in consumption were compensated by a growing demand for cadmium for nickel–cadmium batteries, which accounted for 81% of the cadmium consumption in the United States in 2006.

Occurrence

Cadmium makes up about 0.1 ppm of Earth's crust and is the 65th most abundant element. It is much rarer than zinc, which makes up about 65 ppm. No significant deposits of cadmium-containing ores are known. The only cadmium mineral of importance, greenockite (CdS), is nearly always associated with sphalerite (ZnS). This association is caused by geochemical similarity between zinc and cadmium, with no geological process likely to separate them. Thus, cadmium is produced mainly as a byproduct of mining, smelting, and refining sulfidic ores of zinc, and, to a lesser degree, lead and copper. Small amounts of cadmium, about 10% of consumption, are produced from secondary sources, mainly from dust generated by recycling iron and steel scrap. Production in the United States began in 1907, but wide use began after World War I.

Metallic cadmium can be found in the Vilyuy River basin in Siberia.

Rocks mined for phosphate fertilizers contain varying amounts of cadmium, resulting in a cadmium concentration of as much as 300 mg/kg in the fertilizers and a high cadmium content in agricultural soils. Coal can contain significant amounts of cadmium, which ends up mostly in coal fly ash.

Cadmium in soil can be absorbed by crops such as rice and cocoa. In 2002, the Chinese ministry of agriculture measured that 28% of rice it sampled had excess lead and 10% had excess cadmium above limits defined by law. Consumer Reports tested 28 brands of dark chocolate sold in the United States in 2022, and found cadmium in all of them, with 13 exceeding the California Maximum Allowable Dose level.

Some plants such as willow trees and poplars have been found to clean both lead and cadmium from soil.

Typical background concentrations of cadmium do not exceed 5 ng/m3 in the atmosphere; 2 mg/kg in soil; 1 μg/L in freshwater and 50 ng/L in seawater. Concentrations of cadmium above 10 μg/L may be stable in water having low total solute concentrations and p H and can be difficult to remove by conventional water treatment processes.

Production
Cadmium is a common impurity in zinc ores, and it is most often isolated during the production of zinc. Some zinc ores concentrates from zinc sulfate ores contain up to 1.4% of cadmium. In the 1970s, the output of cadmium was 6.5 lb per ton of zinc. Zinc sulfide ores are roasted in the presence of oxygen, converting the zinc sulfide to the oxide. Zinc metal is produced either by smelting the oxide with carbon or by electrolysis in sulfuric acid. Cadmium is isolated from the zinc metal by vacuum distillation if the zinc is smelted, or cadmium sulfate is precipitated from the electrolysis solution.

The British Geological Survey reports that in 2001, China was the top producer of cadmium with almost one-sixth of the world's production, closely followed by South Korea and Japan.

Applications
Cadmium is a common component of electric batteries, pigments, coatings, and electroplating.

Batteries
In 2009, 86% of cadmium was used in batteries, predominantly in rechargeable nickel–cadmium batteries. Nickel–cadmium cells have a nominal cell potential of 1.2 V. The cell consists of a positive nickel hydroxide electrode and a negative cadmium electrode plate separated by an alkaline electrolyte (potassium hydroxide). The European Union put a limit on cadmium in electronics in 2004 of 0.01%, with some exceptions, and in 2006 reduced the limit on cadmium content to 0.002%. Another type of battery based on cadmium is the silver–cadmium battery.

Electroplating
Cadmium electroplating, consuming 6% of the global production, is used in the aircraft industry to reduce corrosion of steel components. This coating is passivated by chromate salts. A limitation of cadmium plating is hydrogen embrittlement of high-strength steels from the electroplating process. Therefore, steel parts heat-treated to tensile strength above 1300 MPa (200 ksi) should be coated by an alternative method (such as special low-embrittlement cadmium electroplating processes or physical vapor deposition).

Titanium embrittlement from cadmium-plated tool residues resulted in banishment of those tools (and the implementation of routine tool testing to detect cadmium contamination) in the A-12/SR-71, U-2, and subsequent aircraft programs that use titanium.

Nuclear fission
Cadmium is used in the control rods of nuclear reactors, acting as a very effective neutron poison to control neutron flux in nuclear fission. When cadmium rods are inserted in the core of a nuclear reactor, cadmium absorbs neutrons, preventing them from creating additional fission events, thus controlling the amount of reactivity. The pressurized water reactor designed by Westinghouse Electric Company uses an alloy consisting of 80% silver, 15% indium, and 5% cadmium.

Televisions
QLED TVs have been starting to include cadmium in construction. Some companies have been looking to reduce the environmental impact of human exposure and pollution of the material in televisions during production.

Anticancer drugs
Complexes based on cadmium and other heavy metals have potential for the treatment of cancer, but their use is often limited due to toxic side effects.

Compounds
Cadmium oxide was used in black and white television phosphors and in the blue and green phosphors of color television cathode ray tubes. Cadmium sulfide (CdS) is used as a photoconductive surface coating for photocopier drums.

Various cadmium salts are used in paint pigments, with CdS as a yellow pigment being the most common. Cadmium selenide is a red pigment, commonly called cadmium red. To painters who work with the pigment, cadmium provides the most brilliant and durable yellows, oranges, and reds – so much so that during production, these colors are significantly toned down before they are ground with oils and binders or blended into watercolors, gouaches, acrylics, and other paint and pigment formulations. Because these pigments are potentially toxic, users should use a barrier cream on the hands to prevent absorption through the skin even though the amount of cadmium absorbed into the body through the skin is reported to be less than 1%.

In PVC, cadmium was used as heat, light, and weathering stabilizers. Currently, cadmium stabilizers have been completely replaced with barium-zinc, calcium-zinc and organo-tin stabilizers. Cadmium is used in many kinds of solder and bearing alloys, because it has a low coefficient of friction and fatigue resistance. It is also found in some of the lowest-melting alloys, such as Wood's metal.

Semiconductors
Cadmium is an element in some semiconductor materials. Cadmium sulfide, cadmium selenide, and cadmium telluride are used in some photodetectors and solar cells. HgCdTe detectors are sensitive to mid-infrared light and used in some motion detectors.

Laboratory uses
Helium–cadmium lasers are a common source of blue or ultraviolet laser light. Lasers at wavelengths of 325, 354 and 442 nm are made using this gain medium; some models can switch between these wavelengths. They are notably used in fluorescence microscopy as well as various laboratory uses requiring laser light at these wavelengths.

Cadmium selenide quantum dots emit bright luminescence under UV excitation (He–Cd laser, for example). The color of this luminescence can be green, yellow or red depending on the particle size. Colloidal solutions of those particles are used for imaging of biological tissues and solutions with a fluorescence microscope.

In molecular biology, cadmium is used to block voltage-dependent calcium channels from fluxing calcium ions, as well as in hypoxia research to stimulate proteasome-dependent degradation of Hif-1α.

Cadmium-selective sensors based on the fluorophore BODIPY have been developed for imaging and sensing of cadmium in cells. One powerful method for monitoring cadmium in aqueous environments involves electrochemistry. By employing a self-assembled monolayer one can obtain a cadmium selective electrode with a ppt-level sensitivity.

Biological role
Cadmium has no known function in higher organisms and is considered toxic. Cadmium is considered an environmental pollutant hazardous to living organisms. A cadmium-dependent carbonic anhydrase has been found in some marine diatoms, which live in environments with low zinc concentrations.

Cadmium is preferentially absorbed in the kidneys of humans. Up to about 30 mg of cadmium is commonly inhaled throughout human childhood and adolescence.

Cadmium is under research for its potential toxicity to increase the risk of cancer, cardiovascular disease, and osteoporosis.

Environmental impact
The biogeochemistry of cadmium and its release to the environment is under research.

Safety
Individuals and organizations have been reviewing cadmium's bioinorganic aspects for its toxicity. The most dangerous form of occupational exposure to cadmium is inhalation of fine dust and fumes, or ingestion of highly soluble cadmium compounds. Inhalation of cadmium fumes can result initially in metal fume fever, but may progress to chemical pneumonitis, pulmonary edema, and death.

Cadmium is also an environmental hazard. Human exposure is primarily from fossil fuel combustion, phosphate fertilizers, natural sources, iron and steel production, cement production and related activities, nonferrous metals production, and municipal solid waste incineration. Other sources of cadmium include bread, root crops, and vegetables.

There have been a few instances of general population poisoning as the result of long-term exposure to cadmium in contaminated food and water. Research into an estrogen mimicry that may induce breast cancer is ongoing,. In the decades leading up to World War II, mining operations contaminated the Jinzū River in Japan with cadmium and traces of other toxic metals. As a consequence, cadmium accumulated in the rice crops along the riverbanks downstream of the mines. Some members of the local agricultural communities consumed the contaminated rice and developed itai-itai disease and renal abnormalities, including proteinuria and glucosuria. The victims of this poisoning were almost exclusively post-menopausal women with low iron and low body stores of other minerals. Similar general population cadmium exposures in other parts of the world have not resulted in the same health problems because the populations maintained sufficient iron and other mineral levels. Thus, although cadmium is a major factor in the itai-itai disease in Japan, most researchers have concluded that it was one of several factors.

Cadmium is one of ten substances banned by the European Union's Restriction of Hazardous Substances (RoHS) directive, which regulates hazardous substances in electrical and electronic equipment, but allows for certain exemptions and exclusions from the scope of the law.

The International Agency for Research on Cancer has classified cadmium and cadmium compounds as carcinogenic to humans. Although occupational exposure to cadmium is linked to lung and prostate cancer, there is still uncertainty about the carcinogenicity of cadmium in low environmental exposure. Recent data from epidemiological studies suggest that intake of cadmium through diet is associated with a higher risk of endometrial, breast, and prostate cancer as well as with osteoporosis in humans. A recent study has demonstrated that endometrial tissue is characterized by higher levels of cadmium in current and former smoking females.

Cadmium exposure is associated with a large number of illnesses including kidney disease, early atherosclerosis, hypertension, and cardiovascular diseases. Although studies show a significant correlation between cadmium exposure and occurrence of disease in human populations, a molecular mechanism has not yet been identified. One hypothesis holds that cadmium is an endocrine disruptor and some experimental studies have shown that it can interact with different hormonal signaling pathways. For example, cadmium can bind to the estrogen receptor alpha, and affect signal transduction along the estrogen and MAPK signaling pathways at low doses.

The tobacco plant absorbs and accumulates heavy metals such as cadmium from the surrounding soil into its leaves. Following tobacco smoke inhalation, these are readily absorbed into the body of users. Tobacco smoking is the most important single source of cadmium exposure in the general population. An estimated 10% of the cadmium content of a cigarette is inhaled through smoking. Absorption of cadmium through the lungs is more effective than through the gut. As much as 50% of the cadmium inhaled in cigarette smoke may be absorbed. On average, cadmium concentrations in the blood of smokers is 4 to 5 times greater than non-smokers and in the kidney, 2–3 times greater than in non-smokers. Despite the high cadmium content in cigarette smoke, there seems to be little exposure to cadmium from passive smoking.

In a non-smoking population, food is the greatest source of exposure. High quantities of cadmium can be found in crustaceans, mollusks, offal, frog legs, cocoa solids, bitter and semi-bitter chocolate, seaweed, fungi and algae products. However, grains, vegetables, and starchy roots and tubers are consumed in much greater quantity in the U.S., and are the source of the greatest dietary exposure there. Most plants bio-accumulate metal toxins such as cadmium and when composted to form organic fertilizers, yield a product that often can contain high amounts (e.g., over 0.5 mg) of metal toxins for every kilogram of fertilizer. Fertilizers made from animal dung (e.g., cow dung) or urban waste can contain similar amounts of cadmium. The cadmium added to the soil from fertilizers (rock phosphates or organic fertilizers) become bio-available and toxic only if the soil pH is low (i.e., acidic soils). In the European Union, an analysis of almost 22,000 topsoil samples with LUCAS survey concluded that 5.5% of samples have concentrations higher than 1 mg kg-1.

Zinc, copper, calcium, and iron ions, and selenium with vitamin C are used to treat cadmium intoxication, though it is not easily reversed.

Regulations
Because of the adverse effects of cadmium on the environment and human health, the supply and use of cadmium is restricted in Europe under the REACH Regulation.

The EFSA Panel on Contaminants in the Food Chain specifies that 2.5 μg/kg body weight is a tolerable weekly intake for humans. The Joint FAO/WHO Expert Committee on Food Additives has declared 7 μg/kg body weight to be the provisional tolerable weekly intake level. The state of California requires a food label to carry a warning about potential exposure to cadmium on products such as cocoa powder. The European Commission has put in place the EU regulation (2019/1009) on fertilizing products (EU, 2019), adopted in June 2019 and fully applicable as of July 2022, sets a Cd limit value in phosphate fertilizers to 60 mg kg-1 of P2O5.

The U.S. Occupational Safety and Health Administration (OSHA) has set the permissible exposure limit (PEL) for cadmium at a time-weighted average (TWA) of 0.005 ppm. The National Institute for Occupational Safety and Health (NIOSH) has not set a recommended exposure limit (REL) and has designated cadmium as a known human carcinogen. The IDLH (immediately dangerous to life and health) level for cadmium is 9 mg/m3.

In addition to mercury, the presence of cadmium in some batteries has led to the requirement of proper disposal (or recycling) of batteries.

Product recalls
In May 2006, a sale of the seats from Arsenal F.C.'s old stadium, Highbury in London, England was cancelled when the seats were discovered to contain trace amounts of cadmium. Reports of high levels of cadmium use in children's jewelry in 2010 led to a US Consumer Product Safety Commission investigation. The U.S. CPSC issued specific recall notices for cadmium content in jewelry sold by Claire's and Wal-Mart stores.

In June 2010, McDonald's voluntarily recalled more than 12 million promotional Shrek Forever After 3D Collectible Drinking Glasses because of the cadmium levels in paint pigments on the glassware. The glasses were manufactured by Arc International, of Millville, New Jersey, USA.

title: Cadmium