Chromium

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Chromium, 24Cr
Chromium
Appearancesilvery metallic
Standard atomic weight Ar°(Cr)
Chromium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
–
↑
Cr
↓
Mo
vanadium ← chromium → manganese
kJ/mol
Heat of vaporization347 kJ/mol
Molar heat capacity23.35 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 1656 1807 1991 2223 2530 2942
Atomic properties
Discovery and first isolation
Louis Nicolas Vauquelin (1794, 1797)
Isotopes of chromium
Main isotopes[6] Decay
abun­dance half-life (t1/2) mode pro­duct
50Cr 4.34%
stable
51Cr synth 27.7025 d ε
51V
γ
 		–
52Cr 83.8% stable
53Cr 9.50% stable
54Cr 2.37% stable
 Category: Chromium
| references

Chromium is a

lustrous, hard, and brittle transition metal.[7]

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.[8] The name of the element is derived from the Greek word χρῶμα, chrōma, meaning color,[9] because many chromium compounds are intensely colored.

Industrial production of chromium proceeds from

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,

essential nutrient in humans for insulin, sugar, and lipid metabolism.[10] 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.[11]

While chromium metal and Cr(III) ions are considered non-toxic,

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).[12]

Abandoned chromium production sites often require environmental cleanup.[13]

Physical properties

Atomic

Chromium is the fourth transition metal found on the periodic table, and 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.[14]

Chromium is the first element in the 3d series where the 3d electrons start to sink into the core; they thus contribute less to

oxidising agent in contrast to the molybdenum(VI) and tungsten(VI) oxides.[15]

Bulk

Sample of pure chromium metal

Chromium is extremely

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

meters
.

Chromium has a high

paramagnetic.[4] 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.[16] 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.[17]

Passivation

Chromium metal left standing 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.[18] 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.[19]

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.[20]

Isotopes

Main article: Isotopes of chromium

Naturally occurring chromium is composed of four stable

decay mode before the most abundant stable isotope, 52Cr, is electron capture and the primary mode after is beta decay.[6]

53Cr is the

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.[22] 53Cr has been posited as a proxy for atmospheric oxygen concentration.[23]

Chemistry and compounds

Main article: Chromium compounds
The Pourbaix diagram for chromium in pure water, perchloric acid, or sodium hydroxide[24][25]

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.[26][27]

Common oxidation states

Oxidation
states[note 2][27]
−4 (d10) Na4[Cr(CO)4][28]
−2 (d8) Na
2[Cr(CO)
5]
−1 (d7) Na
2[Cr
2(CO)
10]
0 (d6) Cr(C
6H
6)
2
+1 (d5) K
3[Cr(CN)
5NO]
+2 (d4) CrCl
2
+3 (d3) CrCl
3
+4 (d2) K
2CrF
6
+5 (d1) K
3Cr(O
2)
4
+6 (d0) K
2CrO
4

Chromium(0)

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

Chromium(II)

Chromium(II) carbide (Cr3C2)

Chromium(II) compounds are uncommon, in part because they readily oxidize to chromium(III) derivatives in air. Water-stable chromium(II) chloride CrCl
2 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.[19] Some other notable chromium(II) compounds include chromium(II) oxide CrO, and chromium(II) sulfate CrSO
4. Many chromium(II) carboxylates are known. The red chromium(II) acetate (Cr2(O2CCH3)4) is somewhat famous. It features a Cr-Cr quadruple bond.[29]

Chromium(III)

Anhydrous chromium(III) chloride (CrCl3)

A large number of chromium(III) compounds are known, such as

pm) to Al3+
 (radius 50 pm), and they can replace each other in some compounds, such as in chrome alum and alum
.

Chromium(III) tends to form

chromium(III) has been reported for the Cr-centered Keggin anion [α-CrW12O40]5–.[33]

Chromium(III) hydroxide (Cr(OH)3) is amphoteric, dissolving in acidic solutions to form [Cr(H2O)6]3+, and in basic solutions to form [Cr(OH)
6]3−
. 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.[19]

Chromium(VI)

Main article: Hexavalent chromium

Chromium(VI) compounds are oxidants at low or neutral pH. Chromate anions (CrO2−
4) 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 (CrO
2Cl
2).[19] However, despite several erroneous claims, chromium hexafluoride (as well as all higher hexahalides) remains unknown, as of 2020.[34]

Chromium(VI) oxide

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:[19]

Cr
2O2−
7 + 14 H
3O+
 + 6 e− → 2 Cr3+
 + 21 H
2O (ε0 = 1.33 V)

They are, however, only moderately oxidizing at high pH:[19]

CrO2−
4 + 4 H
2O + 3 e− → Cr(OH)
3 + 5 OH−
 (ε0 = −0.13 V)
Sodium chromate (Na2CrO4)

Chromium(VI) compounds in solution can be detected by adding an acidic

chromium(VI) peroxide (CrO5) is formed, which can be stabilized as an ether adduct CrO
5·OR
2.[19]

anhydride of chromic acid, is sold industrially as "chromic acid".[19]
It can be produced by mixing sulfuric acid with dichromate and is a strong oxidizing agent.

Other oxidation states

See also: Organochromium chemistry

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.[35]

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 (CrX
3) 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.[36]

Most chromium(I) compounds are obtained solely by oxidation of electron-rich,

X-ray diffraction, a Cr-Cr quintuple bond (length 183.51(4)  pm) has also been described.[37]
Extremely bulky monodentate ligands stabilize this compound by shielding the quintuple bond from further reactions.

Chromium compound determined experimentally to contain a Cr-Cr quintuple bond

Occurrence

See also: Category:Chromium minerals
Crocoite (PbCrO4)
Chromite ore

Chromium is the 21st[38] most abundant element in Earth's crust with an average concentration of 100 ppm. 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.[39] Chromium is mined as chromite (FeCr2O4) ore.[40]

About two-fifths of the chromite ores and concentrates in the world are produced in South Africa, about a third in Kazakhstan,

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.[45]

The relation between Cr(III) and Cr(VI) strongly depends on

oxidative properties of the location. In most cases, Cr(III) is the dominating species,[24] 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).[46]

History

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.[47][48] Though misidentified as a lead compound with selenium and iron components, the mineral was in fact crocoite with a formula of PbCrO4.[49] 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.[50] Crocoite would be the principal source of chromium in pigments until the discovery of chromite many years later.[51]

The red color of rubies is due to trace amounts of chromium within the corundum.

In 1794,

chromium trioxide (CrO3) by mixing crocoite with hydrochloric acid.[49] 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.[52][53] Vauquelin was also able to detect traces of chromium in precious gemstones, such as ruby and emerald.[49][54]

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.[55] 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.[40] With the development of metallurgy and chemical industries in the Western world, the need for chromium increased.[56]

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.[57]

Production

Piece of chromium produced with aluminothermic reaction
World production trend of chromium
Chromium, remelted in a horizontal arc zone-refiner, showing large visible crystal grains

Approximately 28.8 million metric tons (Mt) of marketable chromite ore was produced in 2013, and converted into 7.5 Mt of ferrochromium.[42] 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."[42]

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.[42]

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.[58]

Chromium ore output in 2002[59]

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

chromates and leaves the insoluble iron oxide. The chromate is converted by sulfuric acid into the dichromate.[58]

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.[58]

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.[60]

Metallurgy

Stainless steel cutlery made from Cromargan 18/10, containing 18% chromium
Main articles: Chrome plating and Stainless steel

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.

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."[66] 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.[67]

Decorative chrome plating on a motorcycle

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.

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.[57]

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.[69] Because of environmental and health regulations on chromates, alternative coating methods are under development.[70]

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.[71] 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.[57]

Pigment

The mineral

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.[72]

Chromium oxides are also used as a green pigment in the field of glassmaking and also as a glaze for ceramics.[73] 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.[74]

Other uses

Components of original ruby laser.
Red crystal of a ruby laser

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.[note 3] 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.[note 4][75] 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.[76]

Because of their toxicity, chromium(VI) salts are used for the preservation of wood. For example,

termites, and marine borers.[77] 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.[77]

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.[78] Chromium tanned leather can contain 4–5% of chromium, which is tightly bound to the proteins.[40] 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,[79] and "chrome-less" or "chrome-free" tanning are practiced to better manage chromium usage.[80]

The high heat resistivity and high melting point makes

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).[58] [81]

Several chromium compounds are used as

water gas shift reaction.[83][84] Copper chromite is a useful hydrogenation catalyst.[85]

Chromates of metals are used in humistor.[86]

Uses of compounds

  • audio cassettes.[87]
  • Chromium(III) oxide (Cr2O3) is a metal polish known as green rouge.[88][89]
  • Chromic acid is a powerful oxidizing agent and is a useful compound for cleaning laboratory glassware of any trace of organic compounds.[90] 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.[91]
  • Potassium dichromate is a chemical reagent, used as a titrating agent.[92]
  • Chromates are added to drilling muds to prevent corrosion of steel under wet conditions.[93]
  • Chromium(III) potassium sulfate and is used as a mordant (i.e., a fixing agent) for dyes in fabric and in tanning.[94]

Biological role

See also: Chromium in glucose metabolism

The biologically beneficial effects of chromium(III) are debated.[95][96] 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.[10] The mechanism of its actions in the body, however, have not been defined, leaving in question the essentiality of chromium.[97][98]

In contrast, hexavalent chromium (Cr(VI) or Cr6+) is highly toxic and mutagenic.[99] Ingestion of chromium(VI) in water has been linked to stomach tumors, and it may also cause allergic contact dermatitis (ACD).[100]

"

glucose tolerance factor, is controversial.[10] 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.[101]

The chromium content of common foods is generally low (1–13 micrograms per serving).[10][102] The chromium content of food varies widely, due to differences in soil mineral content, growing season, plant cultivar, and contamination during processing.[102] 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.[103][104]

Dietary recommendations

See also: Chromium deficiency

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[105][106][107][108] while the European Food Safety Authority (EFSA) of the European Union does not.[109]

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).[108] 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.[105] India considers chromium to be an essential nutrient, with an adult recommended intake of 33 μg/day.[106] 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.[107] 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.[109][110]

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.[111][112] 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.[113] A wide variety of animal and vegetable foods contain chromium.[108] 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.[114] One diet analysis study conducted in Mexico reported an average daily chromium intake of 30 micrograms.[115] An estimated 31% of adults in the United States consume multi-vitamin/mineral dietary supplements,[116] which often contain 25 to 60 micrograms of chromium.

Supplementation

Chromium is an ingredient in

chromium(III) polynicotinate, and other chemical compositions.[10] The benefit of supplements has not been proven.[10][119]

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.[120] 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.[121] 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.[122]

Given the evidence for chromium deficiency causing problems with glucose management in the context of intravenous nutrition products formulated without chromium,

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.[127]

Two

chromium picolinate, a popular supplement ingredient, reported a statistically significant −1.1 kg (2.4 lb) weight loss in trials longer than 12 weeks.[128] The other included all chromium compounds and reported a statistically significant −0.50 kg (1.1 lb) weight change.[129] 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.[128][129] The European Food Safety Authority reviewed the literature and concluded that there was insufficient evidence to support a claim.[122]

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.[119][130][131] A review of clinical trials reported that chromium supplementation did not improve exercise performance or increase muscle strength.[132] 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.[133]

Fresh-water fish

Chromium is naturally present in the environment in trace amounts, but industrial use in rubber and stainless steel manufacturing, chrome plating, dyes for textiles, tanneries and other uses contaminates aquatic systems. In Bangladesh, rivers in or downstream from industrialized areas exhibit heavy metal contamination. Irrigation water standards for chromium are 0.1 mg/L, but some rivers 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.[134] 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.[135]

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.[135]

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.[136]

Precautions

Main article: Chromium toxicity

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.

IDLH (immediately dangerous to life and health) value is 250 mg/m3.[140]

Chromium(VI) toxicity

The acute

renal, and liver failure result. Aggressive dialysis can be therapeutic.[141]

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.[142][143] 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.[144][145]

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.[146][147]

Environmental issues

Because chromium compounds were used in

automobile refinishing applications.[148]

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.[149]

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.[150]

Notes

  1. ^ The melting/boiling point of transition metals are usually higher compared to the alkali metals, alkaline earth metals, and nonmetals, which is why the range of elements compared to chromium differed between comparisons
  2. ^ Most common oxidation states of chromium are in bold. The right column lists a representative compound for each oxidation state.
  3. ^ Any color of corundum (disregarding red) is known as a sapphire. If the corundum is red, then it is a ruby. Sapphires are not required to be blue corundum crystals, as sapphires can be other colors such as yellow and purple
  4. pink sapphire or ruby
    is formed, depending on the amount of chromium.

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General bibliography

External links

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