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Chromium,  24Cr
Chromium crystals and 1cm3 cube.jpg
Appearancesilvery metallic
Standard atomic weight Ar, std(Cr)51.9961(6) [1]
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


Atomic number (Z)24
Group group 6
Period period 4
Block d-block
Element category   Transition metal
Electron configuration [ Ar ] 3d5 4s1
Electrons per shell
2, 8, 13, 1
Physical properties
Phase at  STP solid
Melting point 2180  K (1907 °C,3465 °F)
Boiling point 2944 K(2671 °C,4840 °F)
Density (near r.t.)7.19 g/cm3
when liquid (at m.p.)6.3 g/cm3
Heat of fusion 21.0  kJ/mol
Heat of vaporization 347 kJ/mol
Molar heat capacity 23.35 J/(mol·K)
Vapor pressure
P (Pa)1101001 k10 k100 k
at T (K)165618071991222325302942
Atomic properties
Oxidation states −4, −2, −1, +1, +2, +3, +4, +5, +6 (depending on the oxidation state, an acidic, basic, or amphoteric oxide)
Electronegativity Pauling scale: 1.66
Ionization energies
  • 1st: 652.9 kJ/mol
  • 2nd: 1590.6 kJ/mol
  • 3rd: 2987 kJ/mol
  • (more)
Atomic radius empirical:128  pm
Covalent radius 139±5 pm
Color lines in a spectral range Chromium spectrum visible.png
Color lines in a spectral range
Spectral lines of chromium
Other properties
Natural occurrence primordial
Crystal structure body-centered cubic (bcc)
Speed of sound thin rod5940 m/s(at 20 °C)
Thermal expansion 4.9 µm/(m·K)(at 25 °C)
Thermal conductivity 93.9 W/(m·K)
Electrical resistivity 125 nΩ·m(at 20 °C)
Magnetic ordering antiferromagnetic (rather: SDW) [2]
Magnetic susceptibility +280.0·10−6 cm3/mol(273 K) [3]
Young's modulus 279 GPa
Shear modulus 115 GPa
Bulk modulus 160 GPa
Poisson ratio 0.21
Mohs hardness 8.5
Vickers hardness 1060 MPa
Brinell hardness 687–6500 MPa
CAS Number 7440-47-3
Discovery and first isolation Louis Nicolas Vauquelin (1794,1797)
Main isotopes of chromium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
50Cr4.345% stable
51Cr syn 27.7025 d ε 51V
| references

Chromium is a chemical element with the 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. [4] Chromium is the main additive in stainless steel, to which it adds anti-corrosive properties. Chromium is also highly valued as a metal that is able to be highly polished while resisting tarnishing. Polished chromium reflects almost 70% of the visible spectrum, with almost 90% of infrared light being reflected. [5] The name of the element is derived from the Greek word χρῶμα, chrōma, meaning color, [6] because many chromium compounds are intensely colored.

Chemical element a species of atoms having the same number of protons in the atomic nucleus

A chemical element is a species of atom having the same number of protons in their atomic nuclei. For example, the atomic number of oxygen is 8, so the element oxygen consists of all atoms which have 8 protons.

Symbol (chemistry) an arbitrary or conventional sign used in chemical science to represent a chemical element

In chemistry, a symbol is an abbreviation for a chemical element. Symbols for chemical elements normally consist of one or two letters from the Latin alphabet and are written with the first letter capitalised.

Atomic number number of protons found in the nucleus of an atom

The atomic number or proton number of a chemical element is the number of protons found in the nucleus of every atom of that element. The atomic number uniquely identifies a chemical element. It is identical to the charge number of the nucleus. In an uncharged atom, the atomic number is also equal to the number of electrons.


Ferrochromium alloy is commercially produced from chromite by silicothermic or aluminothermic reactions and chromium metal by roasting and leaching processes followed by reduction with carbon and then aluminium. Chromium metal is of high value 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.

Chromite spinel, oxide mineral

Chromite is a mineral that is an iron chromium oxide. It has a chemical formula of FeCr2O4. It is an oxide mineral belonging to the spinel group. The element magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite(MgCr2O4). A substitution of the element aluminium can also occur, leading to hercynite (FeAl2O4). Chromite today is mined particularly to make stainless steel through the production of ferrochrome (FeCr), which is an iron-chromium alloy.

Aluminothermic reaction

Aluminothermic reactions are exothermic chemical reactions using aluminium as the reducing agent at high temperature. The process is industrially useful for production of alloys of iron. The most prominent example is the thermite reaction between iron oxides and aluminium to produce iron itself:

Roasting (metallurgy) step of the processing of certain ores

Roasting is a process of heating of sulfide ore to a high temperature in presence of air. It is a step of the processing of certain ores. More specifically, roasting is a metallurgical process involving gas–solid reactions at elevated temperatures with the goal of purifying the metal component(s). Often before roasting, the ore has already been partially purified, e.g. by froth flotation. The concentrate is mixed with other materials to facilitate the process. The technology is useful but is also a serious source of air pollution.

In the United States, trivalent chromium (Cr(III)) ion is considered an essential nutrient in humans for insulin, sugar and lipid metabolism. [7] However, in 2014, the European Food Safety Authority, acting for the European Union, concluded that there was not sufficient evidence for chromium to be recognized as essential. [8]

An ion is an atom or molecule that has a net electrical charge. Since the charge of the electron is equal and opposite to that of the proton, the net charge of an ion is non-zero due to its total number of electrons being unequal to its total number of protons. A cation is a positively charged ion, with fewer electrons than protons, while an anion is negatively charged, with more electrons than protons. Because of their opposite electric charges, cations and anions attract each other and readily form ionic compounds.

Insulin mammalian protein found in Homo sapiens

Insulin is a peptide hormone produced by beta cells of the pancreatic islets; it is considered to be the main anabolic hormone of the body. It regulates the metabolism of carbohydrates, fats and protein by promoting the absorption of carbohydrates, especially glucose from the blood into liver, fat and skeletal muscle cells. In these tissues the absorbed glucose is converted into either glycogen via glycogenesis or fats (triglycerides) via lipogenesis, or, in the case of the liver, into both. Glucose production and secretion by the liver is strongly inhibited by high concentrations of insulin in the blood. Circulating insulin also affects the synthesis of proteins in a wide variety of tissues. It is therefore an anabolic hormone, promoting the conversion of small molecules in the blood into large molecules inside the cells. Low insulin levels in the blood have the opposite effect by promoting widespread catabolism, especially of reserve body fat.

Sugar Generic name for sweet-tasting, soluble carbohydrates

Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double sugars, are molecules composed of two monosaccharides joined by a glycosidic bond. Common examples are sucrose, lactose, and maltose. In the body, compound sugars are hydrolysed into simple sugars. Table sugar, granulated sugar or regular sugar refers to sucrose, a disaccharide composed of glucose and fructose.

While chromium metal and Cr(III) ions are not considered toxic, hexavalent chromium (Cr(VI)) is both toxic and carcinogenic. Abandoned chromium production sites often require environmental cleanup. [9]

Hexavalent chromium chemical compounds that contain the element chromium in the +6 oxidation state

Hexavalent chromium is any chemical compound that contains the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via hexavalent chromium, specifically the salt sodium dichromate. Approximately 136,000 tonnes (300,000,000 lb) of hexavalent chromium were produced in 1985. Additional hexavalent chromium compounds are chromium trioxide and various salts of chromate and dichromate, among others. Hexavalent chromium is used in textile dyes, wood preservation, anti-corrosion products, chromate conversion coatings, and a variety of niche uses. Industrial uses of hexavalent chromium compounds include chromate pigments in dyes, paints, inks, and plastics; chromates added as anticorrosive agents to paints, primers, and other surface coatings; and chromic acid electroplated onto metal parts to provide a decorative or protective coating. Hexavalent chromium can be formed when performing "hot work" such as welding on stainless steel or melting chromium metal. In these situations the chromium is not originally hexavalent, but the high temperatures involved in the process result in oxidation that converts the chromium to a hexavalent state. Hexavalent chromium can also be found in drinking water and public water systems.

Environmental remediation deals with the removal of pollution or contaminants from environmental media such as soil, groundwater, sediment, or surface water. This would mean that once requested by the government or a land remediation authority, immediate action should be taken as this can impact negatively on human health and the environment.

Physical properties


Chromium is the fourth transition metal found on the periodic table, and has an electron configuration of [ Ar] 3d5 4s1. It is also the first element in the periodic table whose ground-state electron configuration violates the Aufbau principle. This occurs again later in the periodic table with other elements and their electron configurations, such as copper, niobium, and molybdenum. [10] This occurs because electrons in the same orbital repel each other due to their like charges. In the previous elements, the energetic cost of promoting an electron to the next higher energy level is too great to compensate for that released by lessening inter-electronic repulsion. However, in the 3d transition metals, the energy gap between the 3d and the next-higher 4s subshell is very small, and because the 3d subshell is more compact than the 4s subshell, inter-electron repulsion is smaller between 4s electrons than between 3d electrons. This lowers the energetic cost of promotion and increases the energy released by it, so that the promotion becomes energetically feasible and one or even two electrons are always promoted to the 4s subshell. (Similar promotions happen for every transition metal atom but one, palladium.) [11]

In chemistry, the term transition metal has three possible meanings:

Argon Chemical element with atomic number 18

Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in the Earth's atmosphere, at 0.934%. It is more than twice as abundant as water vapor, 23 times as abundant as carbon dioxide, and more than 500 times as abundant as neon. Argon is the most abundant noble gas in Earth's crust, comprising 0.00015% of the crust.

Aufbau principle fundamental rule for filling electrons in a orbital

The aufbau principle, from the German Aufbauprinzip, “to build” states that in the ground state of an atom or ion, electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels. For example, the 1s subshell is filled before the 2s subshell is occupied. In this way, the electrons of an atom or ion form the most stable electron configuration possible. An example is the configuration 1s2 2s2 2p6 3s2 3p3 for the phosphorus atom, meaning that the 1s subshell has 2 electrons, and so on.

Chromium is the first element in the 3d series where the 3d electrons start to sink into the inert 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. [12]

Atomic nucleus core of the atom; composed of bound nucleons (protons and neutrons)

The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickly developed by Dmitri Ivanenko and Werner Heisenberg. An atom is composed of a positively-charged nucleus, with a cloud of negatively-charged electrons surrounding it, bound together by electrostatic force. Almost all of the mass of an atom is located in the nucleus, with a very small contribution from the electron cloud. Protons and neutrons are bound together to form a nucleus by the nuclear force.

Metallic bonding is a type of chemical bonding that rises from the electrostatic attractive force between conduction electrons and positively charged metal ions. It may be described as the sharing of free electrons among a structure of positively charged ions (cations). Metallic bonding accounts for many physical properties of metals, such as strength, ductility, thermal and electrical resistivity and conductivity, opacity, and luster.

Vanadium Chemical element with atomic number 23

Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, ductile, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer (passivation) somewhat stabilizes the free metal against further oxidation.


Sample of pure chromium metal Chromium.jpg
Sample of pure chromium metal

Chromium is extremely hard, and is the third hardest element behind 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 third lowest boiling point out of the Period 4 transition metals alone behind manganese and zinc. [note 1] The electrical resistivity of chromium at 20 °C is 125 nanoohm-meters.

Chromium has an unusually high specular reflection in comparison to that of other transition metals. At 425 μm, chromium was found to have a relative maximum reflection of about 72% reflectance, before entering a depression in reflectivity, reaching a minimum of 62% reflectance at 750 μm before rising again to reflecting roughly 90% of 4000 μm of infrared waves. [5] When chromium is formed into a stainless steel alloy and polished, the specular reflection decreases with the inclusion of additional metals, yet is still rather high in comparison with other alloys. Between 40% and 60% of the visible spectrum is reflected from polished stainless steel. [5] The explanation on why chromium displays such a high turnout of reflected photon waves in general, especially the 90% of infrared waves that were reflected, can be attributed to chromium's magnetic properties. [13] Chromium has unique magnetic properties in the sense that chromium is the only elemental solid which shows antiferromagnetic ordering at room temperature (and below). Above 38 °C, its magnetic ordering changes to paramagnetic. [2] 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 fact that the magnetic moments at the cube's corners and the cube centers are not equal, but are still antiparallel. [13] From here, the frequency-dependent relative permittivity of chromium, deriving from Maxwell's equations in conjunction with chromium's antiferromagnetivity, leaves chromium with a high infrared and visible light reflectance. [14]


Chromium metal left standing in air is passivated by oxidation, forming a thin, protective, surface layer. This layer is a spinel structure only a few molecules thick. It is very dense, and prevents the diffusion of oxygen into the underlying metal. This is different from the spongy oxide that forms on iron and carbon steel, through which elemental oxygen continues to migrate, reaching the underlying material to cause continued further rusting. [15] 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. [16]

Chromium, unlike such metals as 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. [17]


Naturally occurring chromium is composed of three stable isotopes; 52Cr, 53Cr and 54Cr, with 52Cr being the most abundant (83.789% natural abundance). 19 radioisotopes have been characterized, with the most stable being 50Cr with a half-life of (more than) 1.8×1017 years, and 51Cr with a half-life of 27.7 days. All of the remaining radioactive isotopes have half-lives that are less than 24 hours and the majority less than 1 minute. Chromium also has two metastable nuclear isomers. [18]

53Cr is the radiogenic decay product of 53 Mn (half-life = 3.74 million years). [19] 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 26 Al and 107 Pd 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. [20]

The isotopes of chromium range in atomic mass from 43  u (43Cr) to 67 u (67Cr). The primary decay mode before the most abundant stable isotope, 52Cr, is electron capture and the primary mode after is beta decay. [18] 53Cr has been posited as a proxy for atmospheric oxygen concentration. [21]

Chemistry and compounds

Chemical properties

The Pourbaix diagram for chromium in pure water, perchloric acid, or sodium hydroxide Chromium in water pourbiax diagram.png
The Pourbaix diagram for chromium in pure water, perchloric acid, or sodium hydroxide

Chromium is a member of group 6, of the transition metals. Chromium(0) has an electron configuration of [Ar]3d54s1, owing to the lower energy of the high spin configuration. Chromium exhibits a wide range of oxidation states, but chromium being ionized into a cation with a positive 3 charge serves as chromium's most stable ionic state. [24] The +3 and +6 states occur the most commonly within chromium compounds; charges of +1, +4 and +5 for chromium are rare, but do nevertheless occasionally exist. [25]

Common oxidation states

states [note 2] [25]
0 Cr(C
+2 CrCl
+3 CrCl
+5 K
+6 K


Anhydrous chromium(III) chloride (CrCl3) Chromium(III)-chloride-purple-anhydrous-sunlight.jpg
Anhydrous chromium(III) chloride (CrCl3)

A large number of chromium(III) compounds are known, such as chromium(III) nitrate, chromium(III) acetate, and chromium(III) oxide. [26] 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. [27] The Cr3+
ion has a similar radius (63  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 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 water-free green [28] chromium(III) chloride is dissolved in water, the green solution turns violet 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.

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)
. 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. [16]


Chromium(VI) compounds are oxidants at low or neutral pH. Chromate anions (CrO2−
) 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) halides are known also and include the hexafluoride CrF6 and chromyl chloride (CrO
). [16]

Chromium(VI) oxide Chrom(VI)-oxid.jpg
Chromium(VI) oxide

Sodium chromate is produced industrially by the oxidative roasting of chromite ore with calcium or 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: [16]

+ 14 H
+ 6 e → 2 Cr3+
+ 21 H
0 = 1.33 V)

They are, however, only moderately oxidizing at high pH: [16]

+ 4 H
+ 3 eCr(OH)
+ 5 OH
0 = −0.13 V)
Sodium chromate (Na2CrO4) Chroman sodny.JPG
Sodium chromate (Na2CrO4)

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 CrO
. [16]

Chromic acid has the hypothetical formula H
. It is a vaguely described chemical, despite many well-defined chromates and dichromates being known. The dark red chromium(VI) oxide CrO
, the acid anhydride of chromic acid, is sold industrially as "chromic acid". [16] 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. [29]

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
) with the corresponding halogen at elevated temperatures. Such compounds are susceptible to disproportionation reactions and are not stable in water.

Many chromium(II) compounds are known, such as the water-stable chromium(II) chloride CrCl
that can be made by reducing chromium(III) chloride with zinc. The resulting bright blue solution created from dissolving chromium(II) chloride is only stable at neutral pH. [16] Some other notable chromium(II) compounds include chromium(II) oxide CrO, and chromium(II) sulfate CrSO
. Many chromous carboxylates are known as well, the most famous of these being the red chromium(II) acetate (Cr2(O2CCH3)4) that features a quadruple bond. [30]

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. [31] 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 5-fold chromium.png
Chromium compound determined experimentally to contain a Cr-Cr quintuple bond

Most chromium(0) compounds are derivatives of the compounds chromium hexacarbonyl or bis(benzene)chromium. [32]


Crocoite (PbCrO4) Crocoite from Tasmania.jpg
Crocoite (PbCrO4)
Chromite ore Chromit 1.jpg
Chromite ore

Chromium is the 13th most abundant element in Earth's crust with an average concentration of 100 ppm. [33] 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 m−3; soil <500 mg kg−1; vegetation <0.5 mg kg−1; freshwater <10 μg L−1; seawater <1 μg L−1; sediment <80 mg kg−1. [34] Chromium is mined as chromite (FeCr2O4) ore. [35]

About two-fifths of the chromite ores and concentrates in the world are produced in South Africa, about a third in Kazakhstan, [36] while India, Russia, and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa. [37] Although rare, deposits of native chromium exist. [38] [39] 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. [40]

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, [22] but in some areas, the ground water can contain up to 39 µg/liter of total chromium of which 30 µg/liter is Cr(VI). [41]


Ancient uses

The bronze tips of swords and crossbow bolts found in burial pits dating from the late third century B.C. Qin Dynasty of the Terracotta Army near Xi'an, China show unexpectedly little corrosion, possibly because the bronze was deliberately coated with a thin layer of chromium oxide. [42] The oxide layer that was found on the weapons was not from pure chromium metal or chrome plating as is commonly produced today, but a mere 10-15 μm layer of chromium oxide molecules at up to 2% chromium, apparently sufficient to protect the bronze from corroding. [43] Recent studies indicate that the limited corrosion was not necessarily caused by the deliberate application of chromium coating, but that the paint applied on the wooden pieces may have been a leading factor in slowing the corrosion. [44]

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. [45] [46] Though misidentified as a lead compound with selenium and iron components, the mineral was in fact crocoite with a formula of PbCrO4. [47] 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. [48] Crocoite would be the principal source of chromium in pigments until the discovery of chromite many years later. [49]

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

In 1794, Louis Nicolas Vauquelin received samples of crocoite ore. He produced chromium trioxide (CrO3) by mixing crocoite with hydrochloric acid. [47] 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. [50] [51] Vauquelin was also able to detect traces of chromium in precious gemstones, such as ruby and emerald. [47] [52]

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. [53] 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. [35]

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. [54]


Piece of chromium produced with aluminothermic reaction Chrom 1.jpg
Piece of chromium produced with aluminothermic reaction
World production trend of chromium Chromium - world production trend.svg
World production trend of chromium
Chromium, remelted in a horizontal arc zone-refiner, showing large visible crystal grains Chromium zone refined and 1cm3 cube.jpg
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. [37] 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." [37]

The largest producers of chromium ore in 2013 have been South Africa (48%), Kazakhstan (13%), Turkey (11%), India (10%) with several other countries producing the rest of about 18% of the world production. [37]

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. [55]

Chromium ore output in 2002 World Chromium Production 2002.svg
Chromium ore output in 2002

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. [55]

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. [55]

Na2Cr2O7 + 2 C → Cr2O3 + Na2CO3 + CO
Cr2O3 + 2 Al → Al2O3 + 2 Cr


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. [57]


Stainless steel cutlery made from Cromargan 18/10, containing 18% chromium. Besteck WMF Stockholm ca 1960er.jpg
Stainless steel cutlery made from Cromargan 18/10, containing 18% chromium.

The strengthening effect of forming stable metal carbides at the grain boundaries and the strong increase in corrosion resistance made chromium an important alloying material for steel. The high-speed tool steels contain between 3 and 5% chromium. Stainless steel, the primary corrosion-resistant metal alloy, is formed when chromium is introduced to iron in sufficient concentrations, usually where the chromium concentration is above 11%. [58] For stainless steel's formation, ferrochromium is added to the molten iron. Also, nickel-based alloys increase in 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. [59]

Decorative chrome plating on a motorcycle. Motorcycle Reflections bw edit.jpg
Decorative chrome plating on a motorcycle.

The relative high hardness and corrosion resistance of unalloyed chromium makes chrome 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. [60] 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. [54]

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 by the chromate stored in the chromate conversion coating, which is able to migrate to local defects, are the benefits of this coating method. [61] Because of environmental and health regulations on chromates, alternative coating methods are under development. [62]

Chromic acid anodizing (or Type I anodizing) of aluminium is another electrochemical process, which does not lead to the deposition of chromium, but uses chromic acid as 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. [63] 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. [54]


School bus painted in chrome yellow Laidlaw school bus.jpg
School bus painted in chrome yellow

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 Service (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. [65]

Chromium oxides are also used as a green pigment in the field of glassmaking and also as a glaze for ceramics. [66] 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. [67]

Other uses

Red crystal of a ruby laser 5 Maiman Laser Components.jpg
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] [68] 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. A ruby laser is lasing at 694.3 nanometers, in a deep red color. [69]

Because of their toxicity, 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. [70] 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. [70]

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. [71] Chromium tanned leather can contain between 4 and 5% of chromium, which is tightly bound to the proteins. [35] 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, [72] and "chrome-less" or "chrome-free" tanning are practiced to better manage chromium usage. [73]

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). [55] [74]

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. [75] Fe-Cr mixed oxides are employed as high-temperature catalysts for the water gas shift reaction. [76] [77] Copper chromite is a useful hydrogenation catalyst. [78]

Uses of compounds

Biological role

The biologically beneficial effects of Chromium(III) continue to be debated. Some experts believe that they reflect pharmacological rather than nutritional responses, while others suggest that they are side effects of a toxic metal. The discussion is marred by elements of negativity and occasionally becomes acrimonious. [87] [88] 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 critical to the metabolism and storage of carbohydrate, fat and protein. [7] The precise mechanism of its actions in the body, however, have not been fully defined, leaving in question whether chromium is essential for healthy people. [89] [90]

In contrast, hexavalent chromium (Cr(VI) or Cr6+) is highly toxic and mutagenic when inhaled. [91] Ingestion of chromium(VI) in water has been linked to stomach tumors, and it may also cause allergic contact dermatitis (ACD). [92]

Chromium deficiency, involving a lack of Cr(III) in the body, or perhaps some complex of it, such as glucose tolerance factor is controversial. [7] 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. [93]

Chromium content of common foods is generally low (1-13 micrograms per serving). [7] [94] Chromium content of food varies widely due to differences in soil mineral content, growing season, plant cultivar, and contamination during processing. [94] In addition, chromium (and nickel) leach into food cooked in stainless steel, with the effect largest when the cookware is new. Acidic foods such as tomato sauce which are cooked for many hours also exacerbate this effect. [95] [96]

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 [97] [98] [99] [100] while the European Food Safety Authority (EFSA), representing the European Union, does not. [101]

The 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 not sufficient 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 and 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). [100] 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. [97] India considers chromium to be an essential nutrient, with an adult recommended intake of 33 μg/day. [98] 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. [99] 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. [101] [102]

For the United States' 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 May 27, 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. [103] The original deadline to be in compliance was July 28, 2018, but on September 29, 2017 the Food and Drug Administration released a proposed rule that extended the deadline to January 1, 2020 for large companies and January 1, 2021 for small companies. [104]

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. [105] A wide variety of animal-sourced and vegetable-sourced foods contain chromium. [100] Content per serving is influenced by the chromium content of the soil in which the plants are grown and by feedstuffs fed to animals; also by processing methods, as chromium is leached into foods if processed or cooked in chromium-containing stainless steel equipment. [106] One diet analysis study conducted in Mexico reported an average daily chromium intake of 30 micrograms. [107] An estimated 31% of adults in the United States consume multi-vitamin/mineral dietary supplements [108] which often contain 25 to 60 micrograms of chromium.


Chromium is an ingredient in total parenteral nutrition (TPN) because deficiency can occur after months of intravenous feeding with chromium-free TPN. For this reason, chromium is added to TPN solutions, along with other trace minerals. [109] It is also in nutritional products for preterm infants. [110] Although the mechanism 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. [108] Chemical compounds used in dietary supplements include chromium chloride, chromium citrate, chromium(III) picolinate, chromium(III) polynicotinate, and other chemical compositions. [7] The benefit of supplements has not been proven. [7] [111]

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. [112] 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. [113] 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. [114]

Given the evidence for chromium deficiency causing problems with glucose management in the context of intravenous nutrition products formulated without chromium, [109] 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. [115] A second reported the same, [116] a third reported significant decreases for both measures, [117] while a fourth reported no benefit for either. [118] 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. [119]

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. [120] The other included all chromium compounds and reported a statistically significant -0.50 kg (1.1 lb) weight change. [121] 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. [120] [121] The European Food Safety Authority reviewed the literature and concluded that there was insufficient evidence to support a claim. [114]

Chromium is promoted as a sports performance dietary supplement, based on the theory that it potentiated insulin activity, with anticipated results of increased muscle mass, and faster recovery of glycogen storage during post-exercise recovery. [111] [122] [123] A review of clinical trials reported that chromium supplementation did not improve exercise performance or increase muscle strength. [124] 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. [125]

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. [126] 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. [127]

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. [127]

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. [128]


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. [129] Because of the specific transport mechanisms, only limited amounts of chromium(III) enter the cells. Acute oral toxicity ranges between 1.5 and 3.3 mg/kg. [130] A 2008 review suggested that moderate uptake of chromium(III) through dietary supplements poses no genetic-toxic risk. [131] 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. [132]

Chromium(VI) toxicity

The acute oral toxicity for chromium(VI) ranges between 50 and 150 mg/kg. [130] 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. [133]

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. [134] [135] 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. [136] [137]

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. [138] [139]

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. [140]

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. [141]

See also


  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. When Cr3+
    replaces Al3+
    in corundum (aluminium oxide, Al2O3), pink sapphire or ruby is formed, depending on the amount of chromium.

Related Research Articles

The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the molecular species, H2CrO4 of which the trioxide is the anhydride. Chromic acid features chromium in an oxidation state of +6 (or VI). It is a strong and corrosive oxidising agent.

Chromate and dichromate anion, CrO₄²⁻, a moderately strong oxidizing agent

Chromate salts contain the chromate anion, CrO2−
. Dichromate salts contain the dichromate anion, Cr
. They are oxoanions of chromium in the 6+ oxidation state and are moderately strong oxidizing agents. In an aqueous solution, chromate and dichromate ions can be interconvertible.

Potassium dichromate chemical compound

Potassium dichromate, K2Cr2O7, is a common inorganic chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is acutely and chronically harmful to health. It is a crystalline ionic solid with a very bright, red-orange color. The salt is popular in the laboratory because it is not deliquescent, in contrast to the more industrially relevant salt sodium dichromate.

Chromium trioxide chemical compound, highly toxic, corrosive, and carcinogenic

Chromium trioxide is an inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name. This compound is a dark-purple solid under anhydrous conditions, bright orange when wet and which dissolves in water concomitant with hydrolysis. Millions of kilograms are produced annually, mainly for electroplating. Chromium trioxide is a powerful oxidiser and a suspected carcinogen.

Chromium oxide may refer to:

Chromium(III) oxide chemical compound

Chromium(III) oxide is an inorganic compound with the formula Cr
. It is one of the principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.

Chromium(III) picolinate chemical compound

Chromium(III) picolinate (CrPic3) is a chemical compound sold as a nutritional supplement to treat type 2 diabetes and promote weight loss. This bright-red coordination compound is derived from chromium(III) and picolinic acid. Small quantities of chromium are needed for glucose utilization by insulin in normal health, but deficiency is extremely rare and has only been observed in people receiving 100% of their nutrient needs intravenously, i.e., total parenteral nutrition diets. Chromium has been identified as regulating insulin by increasing the sensitivity of the insulin receptor. As such, chromium(III) picolinate has been proposed as a treatment for type 2 diabetes, although its effectiveness remains controversial due to conflicting evidence from human trials.

Chromium deficiency is described as the consequence of an insufficient dietary intake of the mineral chromium. Chromium was first proposed as an essential element for normal glucose metabolism in 1959, and was widely accepted as being such by the 1990s. Cases of deficiency were described in people who received all of their nutrition intravenously for long periods of time.

Sodium dichromate Inorganic compound

Sodium dichromate is the inorganic compound with the formula Na2Cr2O7. Usually, however, the salt is handled as its dihydrate Na2Cr2O7·2H2O. Virtually all chromium ore is processed via conversion to sodium dichromate and virtually all compounds and materials based on chromium are prepared from this salt. In terms of reactivity and appearance, sodium dichromate and potassium dichromate are very similar. The sodium salt is, however, around twenty times more soluble in water than the potassium salt (49 g/L at 0 °C) and its equivalent weight is also lower, which is often desirable.

Chromyl chloride chemical compound

Chromyl chloride is a chemical compound with the formula CrO2Cl2. The molecule is tetrahedral, like most of the commonly encountered chromium(VI) derivatives such as chromate, [CrO4]2−. In terms of physical properties and structure, it resembles sulfuryl chloride. It is a hygroscopic dark red liquid, that in terms of color and volatility has also been described as resembling bromine.

Ammonium dichromate chemical compound

Ammonium dichromate is an inorganic compound with the formula (NH4)2Cr2O7. In this compound, as in all chromates and dichromates, chromium is in a +6 oxidation state, commonly known as hexavalent chromium. It is a salt consisting of ammonium ions and dichromate ions.

Chromium(III) sulfate chemical compound

Chromium(III) sulfate usually refers to the inorganic compounds with the formula Cr2(SO4)3.x(H2O), where x can range from 0 to 18. Additionally, ill-defined but commercially important "basic chromium sulfates" are known. These salts are usually either violet or green solids that are soluble in water. It is commonly used in tanning leather.

Sodium chromate chemical compound

Sodium chromate is the inorganic compound with the formula Na2CrO4. It exists as a yellow hygroscopic solid, which can form tetra-, hexa-, and decahydrates. It is an intermediate in the extraction of chromium from its ores. Sodium chromate, like other hexavalent chromium compounds, is toxic and carcinogenic.

Chromate or chromat, and their derived terms, may refer to:

Chromium toxicity refers to any poisonous toxic effect in an organism or cell that results from exposure to specific forms of chromium—especially hexavalent chromium. Hexavalent chromium and its compounds are toxic when inhaled or ingested. Trivalent chromium is a trace mineral that is essential to human nutrition. There is a hypothetical risk of genotoxicity in humans if large amounts of trivalent chromium were somehow able to enter living cells, but normal metabolism and cell function prevent this.

Chromium(VI) oxide peroxide chemical compound

Chromium(VI) peroxide (CrO5) or chromium oxide peroxide is an unstable compound formed by the addition of acidified hydrogen peroxide solutions to solutions of metal chromates or dichromates, such as sodium chromate or potassium dichromate. The generally yellow chromates or orange dichromates turn to dark blue as chromium(VI) peroxide is formed. Chromate or dichromate reacts with hydrogen peroxide and an acid to give chromium peroxide and water.

Chromite (compound) chromite (chemistry) refers to compounds formally containing the anion (CrO₂)⁻

In chemistry the term chromite has been used in two contexts. Under IUPAC naming conventions, chromate(III) is preferred to chromite.

  1. For compounds containing an oxyanion of chromium in oxidation state 3
  2. For other compounds of chromium(III) as a means of distinguishing a chemical species such as hexacyanochromite(III). [Cr(CN)6]3− from an analogous compound in which chromium is a different oxidation state.
Jones oxidation

The Jones oxidation is an organic reaction for the oxidation of primary and secondary alcohols to carboxylic acids and ketones, respectively. It is named after its discoverer, Sir Ewart Jones.

A chromate ester is a chemical structure that contains a chromium atom (symbol Cr) in a +6 oxidation state that is connected via an oxygen (O) linkage to a carbon (C) atom. The Cr itself is in its chromate form, with several oxygens attached, and the Cr–O–C attachment makes this chemical group structurally similar to other ester functional groups. They can be synthesized from various chromium(VI) metal compounds, such as CrO3, chromium chloride complexes, and aqueous chromate ions, and tend to react via redox reactions to liberate chromium(IV).


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