View source for Chromium

{{About|the chemical element}}
{{Infobox chromium}}
'''Chromium''' is a [[chemical element]] which has the symbol '''Cr''' and [[atomic number]] 24.  It is the first element in [[Group 6 element|Group 6]]. It is a steely-gray, [[Lustre (mineralogy)|lustrous]], [[hardness|hard]] and brittle [[metal]]<ref>{{cite journal |last1=Brandes |first1=E. A. |last2=Greenaway |first2=H. T. |last3=Stone |first3=H. E. N. |date=15 September 1956 |title=Ductility in Chromium |journal=Nature |volume=178 |issue=587 |doi=10.1038/178587a0 |url=http://www.nature.com/nature/journal/v178/n4533/abs/178587a0.html |accessdate=31 March 2013}}</ref> which takes a high polish, resists tarnishing, and has a high melting point. It is also odorless and tasteless. The name of the element is derived from the [[Greek language|Greek]] word "chrōma" (χρώμα), meaning [[colour]],<ref>[http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dxrw%3Dma χρώμα], Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus</ref> because many of its compounds are intensely coloured.

Chromium oxide was used by the Chinese in the [[Qin dynasty]] over 2,000 years ago to coat metal weapons found with the [[Terracotta Army]]. Chromium was discovered as an element after it came to the attention of the western world in the red crystalline mineral [[crocoite]] ([[lead(II) chromate]]), discovered in 1761 and initially used as a [[pigment]]. [[Louis Nicolas Vauquelin]] first isolated chromium metal from this mineral in 1797. Since Vauquelin's first production of metallic chromium, small amounts of native (free) chromium metal have been discovered in rare minerals, but these are not used commercially. Instead, nearly all chromium is commercially extracted from the single commercially viable ore [[chromite]], which is iron chromium oxide (FeCr<sub>2</sub>O<sub>4</sub>). Chromite is also now the chief source of chromium for chromium pigments.

Chromium metal and [[ferrochromium]] alloy are commercially produced from chromite by [[silicothermic]] or [[aluminothermic reaction]]s, or by [[Roasting (metallurgy)|roasting]] and [[Leaching (metallurgy)|leaching]] processes. Chromium metal has proven of high value due to its high [[corrosion]] resistance and [[hardness]]. A major development was the discovery that steel could be made highly resistant to corrosion and discoloration by adding metallic chromium to form [[stainless steel]]. This application, along with [[chrome plating]] ([[electroplating]] with chromium) currently comprise 85% of the commercial use for the element, with applications for chromium compounds forming the remainder.

[[Trivalent]] chromium (Cr(III)) ion is possibly required in trace amounts for [[sugar]] and [[lipid]] [[metabolism]], although the issue remains in debate.<ref name="Cronin"/> In larger amounts and in different forms, chromium can be toxic and [[carcinogenic]]. The most prominent example of toxic chromium is [[hexavalent chromium]] (Cr(VI)). Abandoned chromium production sites often require [[Environmental remediation|environmental cleanup]].

==Characteristics==

===Physical===
Chromium is remarkable for its magnetic properties: it is the only elemental solid which shows [[antiferromagnetic]] ordering at room temperature (and below). Above 38 °C, it transforms into a [[paramagnetic]] state.<ref name="fawcett"/>

====Passivation====
Chromium metal left standing in air is [[Passivation (chemistry)|passivated]] by [[oxygen]], forming a thin protective oxide surface layer. This layer is a [[spinel]] structure only a few atoms thick. It is very dense, and prevents the diffusion of oxygen into the underlying material. This barrier is in contrast to iron or plain carbon steels, where the oxygen migrates into the underlying material and causes [[rust]]ing.<ref>{{Cite journal|title = The oxidation of alloys|url = http://www.iop.org/EJ/article/0034-4885/39/5/001/rpv39i5p401.pdf|last = Wallwork|first = G. R.|year = 1976|journal = Reports on the Progress Physics|volume = 39|pages = 401–485|doi = 10.1088/0034-4885/39/5/001|issue = 5|bibcode = 1976RPPh...39..401W }}</ref> The passivation can be enhanced by short contact with [[oxidizing acid]]s like [[nitric acid]]. [[Passivation (chemistry)|Passivated]] chromium is stable against acids. The opposite effect can be achieved by treatment 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.<ref name = "HollemanAF"/>

Chromium, unlike metals such 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.<ref>{{Cite book|url = http://books.google.com/?id=CGMrAAAAYAAJ|title = High-temperature oxidation-resistant coatings: coatings for protection from oxidation of superalloys, refractory metals, and graphite|author = National Research Council (U.S.). Committee on Coatings|publisher = National Academy of Sciences|year = 1970|isbn = 0-309-01769-6}}</ref>

===Occurrence===
[[File:Crocoite from Tasmania.jpg|left|thumb|upright|[[Crocoite]] (PbCrO<sub>4</sub>)]]
[[File:Chromit 1.jpg|thumb|left|upright|[[Chromite]] ore]]
Chromium is the 24th most [[Abundance of elements in Earth's crust|abundant element in Earth's crust]] with an average concentration of 100 ppm.<ref name="Emsley">{{Cite book|title = Nature's Building Blocks: An A-Z Guide to the Elements|last = Emsley|first=John|publisher = Oxford University Press|year = 2001|location = Oxford, England, UK|isbn = 0-19-850340-7|chapter = Chromium|pages=495–498}}</ref> Chromium compounds are found in the environment, due to [[erosion]] of chromium-containing rocks and can be distributed by volcanic eruptions. The concentrations range in soil is between 1 and 300&nbsp;mg/kg, in sea water 5 to 800&nbsp;µg/liter, and in rivers and lakes 26&nbsp;µg/liter to 5.2&nbsp;mg/liter.<ref name="Crspeci">{{Cite journal|title =Chromium occurrence in the environment and methods of its speciation|volume = 107|issue = 3|journal = Environmental Pollution|year = 2000|pages = 263–283|doi = 10.1016/S0269-7491(99)00168-2|first = J.|last = Kotaś|pmid =15092973|last2 =Stasicka|first2 =Z}}</ref>
Chromium is mined as [[chromite]] (FeCr<sub>2</sub>O<sub>4</sub>) ore.<ref name="NRC">{{Cite book|title = Chromium|author = National Research Council (U.S.). Committee on Biologic Effects of Atmospheric Pollutants|publisher = National Academy of Sciences|year = 1974 |isbn = 978-0-309-02217-0 |url = http://books.google.com/?id=ZZsrAAAAYAAJ|page = 155}}</ref> About two-fifths of the chromite ores and concentrates in the world are produced in [[South Africa]], while [[Kazakhstan]], [[India]], [[Russia]], and [[Turkey]] are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa.<ref name="USGS2002CR">{{cite web|url = http://minerals.usgs.gov/minerals/pubs/commodity/chromium/mcs-2009-chrom.pdf|publisher = United States Geological Survey|accessdate = 2009-03-17|title = Commodity Summary 2009: Chromium|first = John F|last = Papp}}</ref>

Although rare, deposits of [[Native metal|native]] chromium exist.<ref>{{Cite journal|url = http://www.minsocam.org/ammin/AM67/AM67_854.pdf|title = New Mineral Names|journal = American Mineralogist|volume = 67|pages = 854–860|year = 1982|first = Michael|last = Fleischer}}</ref><ref>[http://www.mindat.org/min-1037.html Chromium] (with location data), Mindat</ref> The [[Udachnaya Pipe]] in [[Russia]] produces samples of the native metal. This mine is a [[kimberlite]] pipe, rich in [[diamond]]s, and the [[Redox|reducing environment]] helped produce both elemental chromium and [[diamond]].<ref>[http://www.mindat.org/locentry-27628.html Chromium from Udachnaya-Vostochnaya pipe, Daldyn, Daldyn-Alakit kimberlite field, Saha Republic (Sakha Republic; Yakutia), Eastern-Siberian Region, Russia], Mindat</ref>

The relation between Cr(III) and Cr(VI) strongly depends on [[pH]] and [[oxidative]] properties of the location, but in most cases, the Cr(III) is the dominating species,<ref name="Crspeci"/> although in some areas the ground water can contain up to 39&nbsp;µg/liter of total chromium of which 30&nbsp;µg/liter is present as Cr(VI).<ref>{{Cite journal|title = Natural Occurrence of Hexavalent Chromium in the Aromas Red Sands Aquifer, California|volume = 39|issue = 15|journal = Environmental Science and Technology |year = 2005 |pages = 5505–5511|doi = 10.1021/es048835n|first = A. R.|last = Gonzalez|pmid = 16124280|last2 = Ndung'u|first2 = K|last3 = Flegal|first3 = AR}}</ref>
{{clear left}}

===Isotopes===
{{Main|Isotopes of chromium}}
Naturally occurring chromium is composed of three stable [[isotope]]s; <sup>52</sup>Cr, <sup>53</sup>Cr and <sup>54</sup>Cr with <sup>52</sup>Cr being the most abundant (83.789% [[natural abundance]]). 19 [[radioisotope]]s have been characterized with the most stable being <sup>50</sup>Cr with a [[half-life]] of (more than) 1.8{{e|17}} years, and <sup>51</sup>Cr with a half-life of 27.7 days. All of the remaining [[radioactive]] isotopes have half-lives that are less than 24 hours and the majority of these have half-lives that are less than 1 minute. This element also has 2 [[meta state]]s.<ref name="NUBASE">{{Cite journal| first = Audi| last = Georges|title = The NUBASE Evaluation of Nuclear and Decay Properties| journal = Nuclear Physics A| volume = 729| pages = 3–128| publisher = Atomic Mass Data Center| year = 2003| doi = 10.1016/j.nuclphysa.2003.11.001| bibcode=2003NuPhA.729....3A| last2 = Bersillon| first2 = O.| last3 = Blachot| first3 = J.| last4 = Wapstra| first4 = A.H.}}</ref>

<sup>53</sup>Cr is the [[radiogenic]] decay product of <sup>53</sup>[[manganese|Mn]]. Chromium [[isotope|isotopic]] contents are typically combined with [[manganese]] isotopic contents and have found application in [[isotope geology]]. [[manganese|Mn]]-Cr isotope ratios reinforce the evidence from <sup>26</sup>[[Aluminium|Al]] and <sup>107</sup>[[Palladium|Pd]] for the early history of the [[solar system]]. Variations in <sup>53</sup>Cr/<sup>52</sup>Cr and Mn/Cr ratios from several meteorites indicate an initial <sup>53</sup>Mn/<sup>55</sup>Mn ratio that suggests Mn-Cr isotopic composition must result from in-situ decay of <sup>53</sup>Mn in differentiated planetary bodies. Hence <sup>53</sup>Cr provides additional evidence for [[nucleosynthesis|nucleosynthetic]] processes immediately before coalescence of the solar system.<ref name="53Mn53Cr">{{Cite journal|journal = Geochimica et Cosmochimica Acta|volume = 63|issue = 23–24|year = 1999|pages = 4111–4117|doi = 10.1016/S0016-7037(99)00312-9|title = <sup>53</sup>Mn-<sup>53</sup>Cr evolution of the early solar system|first = J. L.|last = Birck|last2 = Rotaru|first2 = M|last3 = Allegre|first3 = C|bibcode=1999GeCoA..63.4111B}}</ref><!-- {{doi|10.1038/331579a0}} {{doi|10.1016/j.gca.2004.01.008}} {{doi|10.1016/j.epsl.2006.07.036}} --->

The isotopes of chromium range in [[atomic mass]] from 43&nbsp;[[atomic mass unit|u]] (<sup>43</sup>Cr) to 67&nbsp;u (<sup>67</sup>Cr). The primary [[decay mode]] before the most abundant stable isotope, <sup>52</sup>Cr, is [[electron capture]] and the primary mode after is [[beta decay]].<ref name="NUBASE"/> <sup>53</sup>Cr has been posited as a proxy for atmospheric oxygen concentration.<ref>{{Cite journal|last1=Frei|first1=Robert|last2=Gaucher|first2=Claudio|last3=Poulton|first3=Simon W.|last4=Canfield|first4=Don E.|title=Fluctuations in Precambrian atmospheric oxygenation recorded by chromium isotopes|journal=Nature|volume=461|pages=250–253|year=2009|doi=10.1038/nature08266|pmid=19741707|issue=7261|bibcode=2009Natur.461..250F}}</ref>

==Compounds==
{|class="wikitable" style="float:right"
|-
! colspan=2|Oxidation <br />states<ref group=note>Most common oxidation states of chromium are in bold. The right column lists a representative compound for each oxidation state.</ref><ref name=Greenwood>{{Greenwood&Earnshaw2nd}}</ref>
|-
| −2 ||{{chem|Na|2|[Cr(CO)|5|]}}
|-
| −1 ||{{chem|Na|2|[Cr|2|(CO)|10|]}}
|-
| 0 || [[Bis(benzene)chromium|{{chem|Cr(C|6|H|6|)|2}}]]
|-
| +1 ||{{chem| K|3|[Cr(CN)|5|NO]}}
|-
| +2 || [[Chromium(II) chloride|{{chem|CrCl|2}}]]
|-
| '''+3''' || [[Chromium(III) chloride|{{chem|CrCl|3}}]]
|-
| +4 ||{{chem|K|2|CrF|6}}
|-
| +5 ||[[Potassium tetraperoxochromate(V)|{{chem|K|3|CrO|8}}]]
|-
| '''+6''' || [[Potassium chromate|{{chem|K|2|CrO|4}}]]
|}

Chromium is a member of the [[transition metal]]s, in [[Group 6 element|group 6]]. Chromium(0) has an electronic configuration of 4s<sup>1</sup>3d<sup>5</sup>, owing to the lower energy of the [[Spin states (d electrons)|high spin configuration]]. Chromium exhibits a wide range of possible [[oxidation state]]s, where the +3 state is most stable energetically; the +3 and +6 states are most commonly observed in chromium compounds, whereas the +1, +4 and +5 states are rare.<ref name=Greenwood/>

The following is the [[Pourbaix diagram]] for chromium in pure water, perchloric acid or sodium hydroxide:<ref name="Crspeci"/><ref name="medusa">Puigdomenech, Ignasi [http://www.kemi.kth.se/medusa/ ''Hydra/Medusa Chemical Equilibrium Database and Plotting Software''] (2004) KTH Royal Institute of Technology</ref><!--also part of the Chromium (VI) Handbook of Jacques Guertin on page 73-->
[[File:Chromium in water pourbiax diagram.png|350px]]

===Chromium(III)===
[[File:Chlorid chromitý.JPG|thumb|left|upright|Chromium(III) chloride hexahydrate ([CrCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]Cl·2H<sub>2</sub>O)]]
[[File:Chromium(III)-chloride-purple-anhydrous-sunlight.jpg|thumb|upright|Anhydrous chromium(III) chloride (CrCl<sub>3</sub>)]]

A large number of chromium(III) compounds are known. Chromium(III) can be obtained by dissolving elemental chromium in acids like [[hydrochloric acid]] or [[sulfuric acid]]. The {{chem|Cr|3+}} ion has a similar radius (63&nbsp;[[picometre|pm]]) to the {{chem|Al|3+}} ion (radius 50&nbsp;pm), so they can replace each other in some compounds, such as in [[chrome alum]] and [[alum]]. When a trace amount of {{chem|Cr|3+}} replaces {{chem|Al|3+}} in [[corundum]] (aluminium oxide, Al<sub>2</sub>O<sub>3</sub>), the red-colored [[ruby]] is formed.

Chromium(III) ions tend to form [[octahedral molecular geometry|octahedral]] complexes.  The colors of these complexes is determined by the ligands attached to the Cr centre.  The commercially available [[chromium(III) chloride]] hydrate is the dark green complex [CrCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]Cl.  Closely related compounds have different colours: pale green [CrCl(H<sub>2</sub>O)<sub>5</sub>]Cl<sub>2</sub> and the violet [Cr(H<sub>2</sub>O)<sub>6</sub>]Cl<sub>3</sub>. If water-free green [[chromium(III) chloride]] is dissolved in water then the green solution turns violet after some time, due to the substitution of water by chloride in the inner [[coordination sphere]]. This kind of reaction is also observed with solutions of [[chrome alum]] and other water-soluble chromium(III) salts.

[[Chromium(III) hydroxide]] (Cr(OH)<sub>3</sub>) is [[Amphoterism|amphoteric]], dissolving in acidic solutions to form [Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup>, and in basic solutions to form {{chem|[Cr(OH)|6|]|3-}}. It is dehydrated by heating to form the green [[chromium(III) oxide]] (Cr<sub>2</sub>O<sub>3</sub>), which is the stable oxide with a crystal structure identical to that of [[corundum]].<ref name="HollemanAF"/>

===Chromium(VI)===
[[File:Chrom(VI)-oxid.jpg|thumb|left|upright|Chromium(VI) oxide]]
[[Hexavalent chromium|Chromium(VI) compounds]] are powerful oxidants at low or neutral pH.  Most important are [[chromate]] anion ({{chem|CrO|4|2-}}) and [[dichromate]] (Cr<sub>2</sub>O<sub>7</sub><sup>2-</sup>) anions, which exist in equilibrium:
:2 [CrO<sub>4</sub>]<sup>2-</sup>  +  2 H<sup>+</sup>  <math>\overrightarrow{\leftarrow}</math> [Cr<sub>2</sub>O<sub>7</sub>]<sup>2-</sup>  +  H<sub>2</sub>O
Chromium(VI) halides are known also and include the [[hexafluoride]] [[chromium hexafluoride|CrF<sub>6</sub>]] and [[chromyl chloride]] ({{chem|CrO|2|Cl|2}}).<ref name="HollemanAF">{{Cite book|publisher = Walter de Gruyter|year = 1985|edition = 91–100|pages = 1081–1095|isbn = 3-11-007511-3|title = Lehrbuch der Anorganischen Chemie|first = Arnold F.|last = Holleman|coauthors = Wiberg, Egon; Wiberg, Nils;|chapter = Chromium| language = German}}</ref>

[[Sodium chromate]] is produced industrially by the oxidative roasting of [[chromite]] ore with [[calcium carbonate|calcium]] or [[sodium carbonate]]. The dominant species is therefore, by the [[law of mass action]], determined by the pH of the solution. 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 [[oxyanion]]s of chromium is possible.

Both the chromate and dichromate anions are strong oxidizing reagents at low pH:<ref name="HollemanAF"/>
[[File:Chroman sodný.JPG|thumb|upright|[[Sodium chromate]] (Na<sub>2</sub>CrO<sub>4</sub>)]]
:{{chem|Cr|2|O|7|2-}} + 14 {{chem|H|3|O|+}} + 6 e<sup>−</sup> → 2 {{chem|Cr|3+}} + 21 {{chem|H|2|O}} (ε<sub>0</sub> = 1.33&nbsp;V)

They are, however, only moderately oxidizing at high pH:<ref name="HollemanAF"/>
:{{chem|CrO|4|2-}} + 4 {{chem|H|2|O}} + 3 e<sup>−</sup> → {{chem|Cr(OH)|3}} + 5 {{chem|OH|-}} (ε<sub>0</sub> = −0.13&nbsp;V)

Chromium(VI) compounds in solution can be detected by adding an acidic [[hydrogen peroxide]] solution. The unstable dark blue [[chromium(VI) peroxide]] (CrO<sub>5</sub>) is formed, which can be stabilized as an ether adduct {{chem|CrO|5|·OR|2}}.<ref name="HollemanAF"/>

[[Chromic acid]] has the hypothetical formula {{chem|H|2|CrO|4}}. It is a vaguely described chemical, despite many well-defined chromates and dichromates being known. The dark red [[chromium(VI) oxide]] {{chem|CrO|3}}, the acid [[anhydride]] of chromic acid, is sold industrially as "chromic acid".<ref name="HollemanAF"/> It can be produced by mixing sulfuric acid with dichromate, and is a strong oxidizing agent.

===Chromium(V) and chromium(IV)===
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 (CrF<sub>5</sub>). This red solid has a melting point of 30 °C and a boiling point of 117 °C. It can be synthesized 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 tetraperoxochromate(V)|Potassium peroxochromate]] (K<sub>3</sub>[Cr(O<sub>2</sub>)<sub>4</sub>]) 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&nbsp;°C.<ref>{{cite web|url = http://deposit.ddb.de/cgi-bin/dokserv?idn=969592612&dok_var=d1&dok_ext=pdf&fi...|title = Preparation, Structure and Vibrational Spectroscopy of Tetraperoxo Complexes of Cr<sup>V+</sup>, V<sup>V+</sup>, Nb<sup>V+</sup> and Ta<sup>V+</sup> (PhD thesis, University of Siegen, 2003)|first = Gentiana|last = Haxhillazi|publisher= University Siegen}}</ref>

Compounds of chromium(IV) (in the +4 oxidation state) are slightly more common than those of chromium(V). The tetrahalides, CrF<sub>4</sub>, CrCl<sub>4</sub>, and CrBr<sub>4</sub>, can be produced by treating the trihalides ({{chem|CrX|3}}) with the corresponding halogen at elevated temperatures. Such compounds are susceptible to disproportionation reactions and are not stable in water.

===Chromium(II)===
Many chromium(II) compounds are known, including the water-stable [[chromium(II) chloride]], {{chem|CrCl|2}}, which can be made by reduction of chromium(III) chloride with zinc. The resulting bright blue solution is only stable at neutral [[pH]].<ref name="HollemanAF"/> Many chromous carboxylates are also known, most famously, the red [[chromous acetate]] (Cr<sub>2</sub>(O<sub>2</sub>CCH<sub>3</sub>)<sub>4</sub>), which features a quadruple bond.

===Chromium(I)===
Most Cr(I) compounds are obtained by oxidation of electron-rich, octahedral Cr(0) complexes. Other Cr(I) complexes contain [[cyclopentadienyl]] ligands.  As verified by [[X-ray diffraction]], a Cr-Cr [[quintuple bond]] (length 183.51(4) &nbsp;pm) has also been described.<ref>{{Cite journal|author = Nguyen, T.|title = Synthesis of a Stable Compound with Fivefold Bonding Between Two Chromium(I) Centers|year = 2005|journal = [[Science (journal)|Science]]|volume = 310|issue = 5749|pages = 844–847|doi =10.1126/science.1116789|pmid = 16179432|display-authors = 1|last2 = Sutton|first2 = AD|last3 = Brynda|first3 = M|last4 = Fettinger|first4 = JC|last5 = Long|first5 = GJ|last6 = Power|first6 = PP|bibcode = 2005Sci...310..844N }}</ref> Extremely bulky monodentate ligands stabilize this compound by shielding the quintuple bond from further reactions.
[[File:5-fold chromium.png|thumb|upright|Chromium compound determined experimentally to contain a Cr-Cr quintuple bond]]

===Chromium(0)===
{{Main|organochromium chemistry}}
Many chromium(0) compounds are known. Most are derivatives of [[chromium hexacarbonyl]] or [[bis(benzene)chromium]].

==History==
Weapons found in burial pits dating from the late 3rd century B.C. [[Qin Dynasty]] of the [[Terracotta Army]] near [[Xi'an]], [[China]] have been analyzed by archaeologists. Although buried more than 2,000 years ago, the ancient [[bronze]] tips of [[crossbow]] bolts and swords found at the site showed unexpectedly little corrosion, possibly because the bronze was deliberately coated with a thin layer of chromium oxide.<ref>Cotterell, Maurice. (2004). ''The Terracotta Warriors: The Secret Codes of the Emperor's Army''. Rochester: Bear and Company. ISBN 1-59143-033-X. Page 102.</ref> {{dubious|date=October 2012}}<!--this has been discussed in many places. These swords are not chromium coated as we know elemental chromium, but this passage implied that before fixing. [http://www.chinahistoryforum.com/index.php?/topic/17605-qin-bronze-swords%3B-chromium-chroming/ see this]--> However, this oxide layer was not chromium metal or chrome plating as we know it.

Chromium minerals as pigments came to the attention of the west in the 18th century. On 26 July 1761, [[Johann Gottlob Lehmann (scientist)|Johann Gottlob Lehmann]] found an orange-red mineral in the [[Beryozovskoye deposit|Beryozovskoye mines]] in the [[Ural Mountains]] which he named ''Siberian red lead''. Though misidentified as a [[lead]] compound with [[selenium]] and [[iron]] components, the mineral was in fact [[crocoite]] (''[[lead chromate]]'') with a formula of PbCrO<sub>4</sub>.<ref name="ChromiumVI">{{Cite book|title = Chromium (VI) Handbook|publisher = CRC Press|year = 2005|isbn = 978-1-56670-608-7|pages = 7–11|author = Guertin, Jacques; Jacobs, James Alan and Avakian, Cynthia P. }}</ref>

In 1770, [[Peter Simon Pallas]] visited the same site as Lehmann and found a red lead mineral that had useful properties as a [[pigment]] in [[paint]]s. The use of Siberian red lead as a paint pigment then developed rapidly. A bright [[yellow]] pigment made from crocoite also became fashionable.<ref name="ChromiumVI"/>

[[File:Cut Ruby.jpg|left|thumb|upright|The red colour of rubies is from a small amount of chromium.]]

In 1797, [[Louis Nicolas Vauquelin]] received samples of crocoite [[ore]]. He produced [[chromium(VI) oxide|chromium trioxide]] (CrO<sub>3</sub>) by mixing crocoite with [[hydrochloric acid]]. In 1798, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven, making him the discoverer of the element.<ref>{{Cite journal|url = http://books.google.com/?id=6dgPAAAAQAAJ|journal =Journal of Natural Philosophy, Chemistry, and the Art|year = 1798|page = 146|volume =3|title = Memoir on a New Metallic Acid which exists in the Red Lead of Sibiria|first = Louis Nicolas|last = Vauquelin}}</ref> Vauquelin was also able to detect traces of chromium in precious [[gemstone]]s, such as [[ruby]] or [[emerald]].<ref name="ChromiumVI"/><ref>{{cite web|last = van der Krogt|first = Peter|title = Chromium|url = http://elements.vanderkrogt.net/element.php?sym=Cr|accessdate = 2008-08-24}}</ref>

During the 1800s, chromium was primarily used as a component of paints and in [[tanning]] salts. At first, crocoite from [[Russia]] was the main source, but in 1827, a larger chromite deposit was discovered near [[Baltimore]], [[United States]]. This made the United States the largest producer of chromium products till 1848 when large deposits of chromite were found near [[Bursa]], [[Turkey]].<ref name="NRC"/>

Chromium is also known for its 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.<ref name="Crplating">{{Cite book|title = Nickel and Chromium Plating| publisher = Woodhead Publishing|year = 1993| isbn = 978-1-85573-081-6| pages = 9–12|chapter = History of Chromium Plating|author = Dennis, J. K.; Such, T. E.}}</ref>
<!--
* http://visualiseur.bnf.fr/CadresFenetre?O=30000000151765&I=639&M=tdm Ueber die Darstellung von metallischem Chrom auf galvanischem Wege. Aus einem Briefe des Prof. Bunsen Annalen der Physik 1854 (T167 = SER2,T91 619-624
* Electrolytische Versuche (p. 314–333) Anton Geuther {{doi|10.1002/jlac.18560990306}} Volume 99 Issue 3 , Pages 257 – 376 (1856) Justus Liebigs Annalen der Chemie
* Ueber die Electrolyse der Schwefelsäure (p. 129–135) Anton Geuther {{doi|10.1002/jlac.18591090202}} Volume 109 Issue 2 , Pages 129 – 256 (1859)Justus Liebigs Annalen der Chemie
-->

Metal alloys now account for 85% of the use of chromium. The remainder is used in the [[chemical industry]] and [[refractory]] and [[foundry]] industries.

==Production==
[[File:Chrom 1.jpg|thumb|left|upright|Piece of chromium produced with [[aluminothermic reaction]]]]
[[File:Chromium - world production trend.svg|thumb|World production trend of chromium]]
[[File:Chromium zone refined and 1cm3 cube.jpg|left|thumb|chromium, remelted in a horizontal arc zone-refiner, showing large visible crystal grains]]
Approximately 4.4 million metric tons of marketable chromite ore were produced in 2000, and converted into ~3.3 million tons of ferro-chrome with an approximate market value of 2.5 billion [[United States dollar]]s.<ref name="USGS2002Yb">{{cite web|url = http://minerals.usgs.gov/minerals/pubs/commodity/chromium/chrommyb02.pdf|publisher = United States Geological Survey|accessdate = 2009-02-16|title = Mineral Yearbook 2002: Chromium|first = John F|last = Papp}}</ref> The largest producers of chromium ore have been [[South Africa]] (44%) [[India]] (18%), [[Kazakhstan]] (16%) [[Zimbabwe]] (5%), [[Finland]] (4%) [[Iran]] (4%) and [[Brazil]] (2%) with several other countries producing the rest of less than 10% of the world production.<ref name="USGS2002Yb"/>

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 (FeCr<sub>2</sub>O<sub>4</sub>) is reduced in large scale in [[electric arc furnace]] or in smaller smelters with either [[aluminium]] or [[silicon]] in an [[aluminothermic reaction]].<ref name="IndMin">{{Cite book|title =Industrial Minerals & Rocks: Commodities, Markets, and Uses|edition = 7th|publisher = SME|year = 2006|isbn = 978-0-87335-233-8|chapter = Chromite|first = John F|last = Papp |coauthor = Lipin Bruce R.|url = http://books.google.com/?id=zNicdkuulE4C&pg=PA309}}
</ref>
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[[File:World Chromium Production 2002.svg|thumb|300px|Chromium ore output in 2002<ref name="USGS2002Yb"/>]]
For the production of pure chromium, the iron has to 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 Fe<sub>2</sub>O<sub>3</sub>. The subsequent leaching at higher elevated temperatures dissolves the [[chromate]]s and leaves the insoluble iron oxide. The chromate is converted by sulfuric acid into the dichromate.<ref name="IndMin"/>

:4 FeCr<sub>2</sub>O<sub>4</sub> + 8 Na<sub>2</sub>CO<sub>3</sub> + 7 O<sub>2</sub> → 8 Na<sub>2</sub>CrO<sub>4</sub> + 2 Fe<sub>2</sub>O<sub>3</sub> + 8 CO<sub>2</sub>

:2 Na<sub>2</sub>CrO<sub>4</sub> + H<sub>2</sub>SO<sub>4</sub> → Na<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> + Na<sub>2</sub>SO<sub>4</sub> + H<sub>2</sub>O

The dichromate is converted to the chromium(III) oxide by reduction with carbon and then reduced in an aluminothermic reaction to chromium.<ref name="IndMin"/>
:Na<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> + 2 C → Cr<sub>2</sub>O<sub>3</sub> + Na<sub>2</sub>CO<sub>3</sub> + CO
:Cr<sub>2</sub>O<sub>3</sub> + 2 Al → Al<sub>2</sub>O<sub>3</sub> + 2 Cr

==Applications==

===Metallurgy===
[[File:Motorcycle Reflections bw edit.jpg|thumb|Decorative chrome plating on a motorcycle.]]

{{Main|Chrome plating}}

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 steel|high-speed tool steels]] contain between 3 and 5% chromium. [[Stainless steel]], the main corrosion-proof metal alloy, is formed when chromium is added to [[iron]] in sufficient concentrations, usually above 11%. For its 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 [[superalloy]]s, they are used in [[jet engine]]s and [[gas turbine]]s in lieu of common structural materials.<ref name="superal">{{cite web|title = Nickel-Based Superalloys|first = H. k. d. h.|last =Bhadeshia|url = http://www.msm.cam.ac.uk/phase-trans/2003/Superalloys/superalloys.html|accessdate = 2009-02-17|publisher = University of Cambridge}}</ref>

The relative high hardness and corrosion resistance of unalloyed chromium makes it a good surface coating, being still the most "popular" metal coating with unparalleled combined durability. A thin layer of chromium is deposited on pretreated metallic surfaces by [[electroplating]] techniques. There are two deposition methods: Thin, below 1&nbsp;µm thickness, layers are deposited by [[chrome plating]], and are used for decorative surfaces. If wear-resistant surfaces are needed then thicker chromium layers are deposited. Both methods normally use acidic chromate or [[dichromate]] solutions. To prevent the energy-consuming change in oxidation state, the use of chromium(III) sulfate is under development, but for most applications, the established process is used.<ref name="Crplating"/>

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 (chemistry)|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.<ref name="Edwards">{{cite book |last = Edwards |first = Joseph |title = Coating and Surface Treatment Systems for Metals |publisher = Finishing Publications Ltd. and ASMy International |year = 1997|pages = 66–71 |isbn = 0-904477-16-9}}</ref> Because of environmental and health regulations on chromates, alternative coating method are under development.<ref>{{Cite journal|journal = Surface and Coatings Technology|volume = 140|issue = 1| year = 2001|doi = 10.1016/S0257-8972(01)01003-9|title = Effects of chromate and chromate conversion coatings on corrosion of aluminum alloy 2024-T3|first = J.|last = Zhao|url = https://dspace03.it.ohio-state.edu/dspace/bitstream/1811/36519/1/55_FrankelG_EffectsChromateChromateConversion_2001_p51-57.pdf|pages = 51–57|last2 = Xia|first2 = L.|last3 = Sehgal|first3 = A.|last4 = Lu|first4 = D.|last5 = McCreery|first5 = R.L.|last6 = Frankel|first6 = G.S.}}</ref>

[[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.<ref name="surface">{{Cite book| title = ASM Handbook: Surface Engineering|first = J. A.|last = Sprague| coauthor = Smidt, F. A.| url = http://books.google.com/?id=RGtsPjqUwy0C&pg=PA484|accessdate = 2009-02-17|publisher = ASM International|isbn = 978-0-87170-384-2|year = 1994}}</ref>
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.<ref name="Crplating"/>

===Dye and pigment===
[[File:Laidlaw school bus.jpg|thumb|right|School bus painted in [[chrome yellow]]<ref>{{Cite book|title = Toxic Substances Controls Guide: Federal Regulation of Chemicals in the Environment|first = Mary Devine|last = Worobec|coauthor = Hogue, Cheryl| page = 13|year = 1992|isbn = 978-0-87179-752-0|url =http://books.google.com/?id=CjWQ6_7AnI4C&pg=PA13|publisher=BNA Books|location = Washington, D.C.}}</ref>]]
The mineral [[crocoite]] ([[lead chromate]] PbCrO<sub>4</sub>) 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 US and for Postal Service (for example [[Deutsche Post]]) in Europe. The use of chrome yellow declined due to environmental and safety concerns and was replaced by organic pigments or alternatives free from lead and chromium. Other pigments based on chromium are, for example, the bright red pigment chrome red, which is a basic lead chromate (PbCrO<sub>4</sub>·Pb(OH)<sub>2</sub>). 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 pretreating 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&nbsp;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]].<ref name="Cryel">{{Cite journal|url = http://books.google.com/?id=bdQVgKWl3f4C&pg=PA106|title = Painting Materials: A Short Encyclopaedia|first = Rutherford John|last = Gettens|publisher = Courier Dover Publications|year = 1966|isbn = 978-0-486-21597-6|pages = 105–106|chapter =Chrome yellow}}</ref>

Chromium oxides are also used as a green color in glassmaking and as a glaze in ceramics.<ref>Gerd Anger et al. "Chromium Compounds" Ullmann's Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim. {{DOI|10.1002/14356007.a07_067}}</ref> Green chromium oxide is extremely light-fast and as such is used in cladding coatings. It is also the main ingredient in IR reflecting paints, used by the armed forces, to paint vehicles, to give them the same IR reflectance as green leaves.<ref>{{Cite book|author=Marrion, Alastair|title=The chemistry and physics of coatings|url=http://books.google.com/books?id=Iz0RQK0oMWUC&pg=PA287|accessdate=7 January 2012|year=2004|publisher=Royal Society of Chemistry|isbn=978-0-85404-604-1|pages=287–}}</ref>

==== Synthetic ruby and the first laser ====
Natural [[ruby|rubies]] are [[corundum]] (aluminum oxide) crystals that are colored red (the rarest type) due to chromium (III) ions (other colors of corundum gems are termed [[sapphire]]s). 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.<ref>{{Cite journal|doi = 10.1524/zkri.1964.120.4-5.359|journal = Zeitschrift fur Kristallographie|volume = 120|pages = 359–363|year = 1964|title = The chromium position in ruby|first2 = R. E.|last2 = Newnham|first = S. C.|last = Moss| url= http://rruff.geo.arizona.edu/doclib/zk/vol120/ZK120_359.pdf|issue = 4–5|bibcode = 1964ZK....120..359M }}</ref> 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.

===Wood preservative===
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.<ref name="Hings">{{cite journal |title = Leaching of chromated copper arsenate wood preservatives: a review|journal = Environmental Pollution|volume = 111|issue = 1|pages =53–66|year = 2001|doi = 10.1016/S0269-7491(00)00030-0 |author = Hingston, J.|pmid = 11202715 |display-authors = 1 |last2 = Collins |first2 = CD |last3 = Murphy |first3 = RJ |last4 = Lester |first4 = JN }}</ref> The formulations contain chromium based on the oxide CrO<sub>3</sub> between 35.3% and 65.5%. In the United States, 65,300 metric tons of CCA solution have been used in 1996.<ref name="Hings"/>

===Tanning===
{{Main|Tanning }}
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.<!-- http://books.google.com/books?id=b1ICltm2IdAC --><ref>{{Cite journal|title = A Conformational Study of Collagen as Affected by Tanning Procedures|first = E. M.|last = Brown|journal = Journal of the American Leather Chemists Association |year = 1997|pages = 225–233|volume = 92}}</ref> Chromium tanned leather can contain between 4 and 5% of chromium, which is tightly bound to the proteins.<ref name="NRC"/> 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 such as recovery and reuse, direct/indirect recycling,<ref>{{Cite journal|last1=Sreeram|first1=K|title=Sustaining tanning process through conservation, recovery and better utilization of chromium|journal=Resources, Conservation and Recycling|volume=38|pages=185–212|year=2003|doi=10.1016/S0921-3449(02)00151-9|issue=3|last2=Ramasami|first2=T}}</ref> use of less chromium or "chrome-less" tanning are practiced to better manage chromium in tanning.

===Refractory material===
The high heat resistivity and high melting point makes [[chromite]] and chromium(III) oxide a material for high temperature refractory applications, like [[blast furnace]]s, cement [[kiln]]s, molds for the firing of [[brick]]s and as foundry sands for the [[Casting (metalworking)|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).<ref name="IndMin"/> <!--10.1006/rtph.1997.1132 10.1007/BF01285116-->

===Catalysts===
Several chromium compounds are used as [[catalyst]]s for processing hydrocarbons. For example the [[Organochromium chemistry#Ethylene polymerization|Phillips catalysts]] for the production of [[polyethylene]] are mixtures of chromium and [[silicon dioxide]] or mixtures of chromium and [[Titanium dioxide|titanium]] and [[aluminium oxide]].<ref>{{Cite journal|journal = Catalysis Today|volume = 51|issue = 2|year = 1999|pages = 215–221|doi = 10.1016/S0920-5861(99)00046-2|title = Olefin polymerization over supported chromium oxide catalysts|first = Bert M.|last = Weckhuysen|last2 = Schoonheydt|first2 = Robert A}}</ref> Fe-Cr mixed oxides are employed as high-temperature catalysts for the [[water gas shift reaction]].<ref>{{cite book | url = http://books.google.com/books?id=YlJRAAAAMAAJ | first1 = M. V. E. |last1 =Twigg | title =  Catalyst Handbook| chapter = The Water-Gas Shift Reaction | isbn = 978-0-7234-0857-4 | year = 1989}}</ref><ref>{{cite journal | doi = 10.1016/0920-5861(94)00135-O | title = Water-gas shift reaction: Finding the mechanistic boundary | year = 1995 | last1 = Rhodes | first1 = C | journal = Catalysis Today | volume = 23 | pages = 43–58 | last2 = Hutchings | first2 = G.J. | last3 = Ward | first3 = A.M.}}</ref> [[Copper chromite]] is a useful [[hydrogenation]] catalyst.<ref>{{cite web|url=http://www.orgsyn.org/orgsyn/prep.asp?prep=cv2p0142 |title=Organic Syntheses Prep |publisher=Orgsyn.org |date= |accessdate=2012-11-09}}</ref>

===Other use===
* [[Chromium(IV) oxide]] (CrO<sub>2</sub>) is a [[magnetism|magnetic]] compound. Its ideal shape [[anisotropy]], which imparts high [[coercivity]] and remnant magnetization, made it a compound superior to the γ-Fe<sub>2</sub>O<sub>3</sub>. Chromium(IV) oxide is used to manufacture [[magnetic tape]] used in high-performance audio tape and standard [[compact audio cassette|audio cassettes]].<ref>{{Cite book|url = http://books.google.com/?id=rNifWsBxnWkC&pg=PA32| title =The foundations of magnetic recording|first = John C.|last = Mallinson|publisher = Academic Press|year = 1993|isbn = 978-0-12-466626-9|chapter = Chromium Dioxide}}</ref> Chromates can prevent corrosion of steel under wet conditions, and therefore chromates are added to drilling muds.<ref>{{cite book |title = Corrosion in the Petrochemical Industry|first = Linda|last = Garverick|publisher = ASM International|year = 1994|isbn = 978-0-87170-505-1|url = http://books.google.com/?id=qTfNZZRO758C&pg=PA278}}</ref>
* [[Chromium(III) oxide]] is a metal polish known as green rouge.<!--{{cite journal|title = Chromium-based regulations and greening in metal finishing industries in the USA|volume = 5|issue = 2|year = 2002|pages = 121–133|doi = 10.1016/S1462-9011(02)00028-X|first = Anil|last = Baral|journal = Environmental Science & Policy|last2 = Engelken|first2 = Robert D.}} {{doi|10.1016/0026-0576(95)99364-G}} {{doi|10.1016/S0026-0576(02)82003-7}} -->
* [[Chromic acid]] is a powerful oxidizing agent and is a useful compound for cleaning laboratory glassware of any trace of organic compounds. It is prepared ''in situ'' 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). Potassium dichromate is a chemical [[reagent]], used in cleaning [[laboratory glassware]] and as a titrating agent. It is also used as a [[mordant]] (i.e., a fixing agent) for dyes in fabric.

==Biological role==
{{Main|Chromium deficiency}}
Trivalent chromium (Cr(III) or Cr<sup>3+</sup>) occurs in trace amounts in foods and waters, and appears to be benign.<ref>{{Cite journal|title = Chromium in Human Nutrition: A Review|first = Walter|last = Mertz|journal = Journal of Nutrition|pages = 626–33|pmid = 8463863|volume = 123|issue = 4|date=1 April 1993}}</ref> In contrast, [[hexavalent chromium]] (Cr(VI) or Cr<sup>6+</sup>) is very toxic and [[mutagen]]ic when inhaled. Cr(VI) has not been established as a carcinogen when in solution, although it may cause allergic [[contact dermatitis]] (ACD).<ref>{{cite web| publisher = Agency for Toxic Substances & Disease Registry, [[Centers for Disease Control and Prevention]]|title = ToxFAQs: Chromium|url = http://www.atsdr.cdc.gov/tfacts7.html|month = February|year = 2001|accessdate = 2007-10-02}}</ref>

[[Chromium deficiency]], involving a lack of Cr(III) in the body, or perhaps some complex of it, such as [[glucose tolerance factor]] is controversial, or is at least extremely rare. Chromium has no verified biological role and has been classified by some as ''not'' essential for mammals.<ref>{{cite journal | doi = 10.1007/s00775-010-0734-y | title = Chromium is not an essential trace element for mammals: Effects of a "low-chromium" diet | year = 2011 | last1 = Bona | first1 = Kristin R. | last2 = Love | first2 = Sharifa | last3 = Rhodes | first3 = Nicholas R. | last4 = McAdory | first4 = Deana | last5 = Sinha | first5 = Sarmistha Halder | last6 = Kern | first6 = Naomi | last7 = Kent | first7 = Julia | last8 = Strickland | first8 = Jessyln | last9 = Wilson | first9 = Austin |first10=Janis|last10=Beaird|first11=James |last11=Ramage |first12=Jane F. |last12=Rasco |first13=John B. |last13=Vincent |journal = JBIC Journal of Biological Inorganic Chemistry | volume = 16 | issue = 3 | page = 381}}</ref> However, other reviews have regarded it as an essential trace element in humans.<ref>PMID 9380836</ref> 
[[File:Chromax II.JPG|thumb|200px|Watch glass holds two grams of pure chromium (III) picolinate powder manufactured by NUTRITION 21 as a dietary supplement. It is a reddish crystalline powder, 12.5% Cr(III) by weight, very sparingly soluble in water.]]

Chromium deficiency has been attributed to only three people on long-term [[parenteral nutrition]], which is when a patient is fed a liquid diet through [[Intravenous therapy|intravenous drips]] for long periods of time.<ref>{{cite journal |author=Moukarzel A |title=Chromium in parenteral nutrition: too little or too much? |journal=Gastroenterology |volume=137 |issue=5 Suppl |pages=S18–S28 |year=2009 |month=November |pmid=19874946 |doi=10.1053/j.gastro.2009.08.048}}</ref>  

Although no biological role for chromium has ever been demonstrated, dietary supplements for chromium include [[chromium(III) picolinate]], [[Chromium polynicotinate|chromium(III) polynicotinate]], and related materials. The benefit of those supplements is questioned by some studies.<ref>{{cite journal | doi = 10.1039/B920480F | title = Chromium: Celebrating 50 years as an essential element? | year = 2010 | last1 = Vincent | first1 = John B. | journal = Dalton Transactions | volume = 39 | issue = 16 | pages = 3787–94 | pmid = 20372701}}</ref> The use of chromium-containing dietary supplements is controversial, owing to the absence of any verified biological role, the expense of these supplements, and the complex effects of their use.<ref name="Cronin">{{Cite journal|first = Joseph R.|last = Cronin|title = The Chromium Controversy|journal = Alternative and Complementary Therapies|year = 2004|volume = 10|issue = 1|pages = 39–42|doi = 10.1089/107628004772830393}}</ref> The popular dietary supplement [[chromium picolinate]] complex generates chromosome damage in hamster cells (due to the picolinate ligand).<ref>{{Cite journal|title = Chromium(III) picolinate produces chromosome damage in Chinese hamster ovary cells|volume = 9|pages = 1643–8|date=1 December 1995| journal = Federation of American Societies for Experimental Biology|first = D. M.|last = Stearns|pmid = 8529845|issue = 15|author2 = W|author3 = P|author4 = W}}</ref> In the United States the dietary guidelines for daily chromium uptake were lowered in 2001 from 50–200 [[microgram|µg]] for an adult to 35&nbsp;µg (adult male) and to 25&nbsp;µg (adult female).<ref>{{Cite journal|last = Vincent|first = J. B.|year = 2007|title = Recent advances in the nutritional biochemistry of trivalent chromium|journal = Proceedings of the Nutrition Society|volume = 63|issue = 1|pages = 41–47|doi = 10.1079/PNS2003315|pmid = 15070438}}</ref>

==Precautions==
{{Main|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.<ref name="Barceloux">{{cite journal |title = Chromium |first = Donald G. |last = Barceloux |journal = Clinical Toxicology |volume = 37 |issue = 2 |pages = 173–194 |year = 1999 |doi = 10.1081/CLT-100102418 |last2 = Barceloux |first2 = Donald |pmid = 10382554 }}</ref>  Because of the specific transport mechanisms, only limited amounts of '''chromium(III)''' enter the cells. Several ''in vitro'' studies indicated that high concentrations of chromium(III) in the cell can lead to DNA damage.<ref name="Eastmond">{{cite journal
|last = Eastmond |first = David A. |year = 2008|title = Trivalent Chromium: Assessing the Genotoxic Risk of an Essential Trace Element and Widely Used Human and Animal Nutritional Supplement |journal = Critical Reviews in Toxicology |volume = 38 |issue = 3 |pages = 173–190 |doi = 10.1080/10408440701845401 |last2 = MacGregor|first2 = JT|last3 = Slesinski|first3 = RS|pmid = 18324515}}</ref> Acute oral toxicity ranges between 1.5 and 3.3&nbsp;mg/kg.<ref name="Katz">{{cite journal |title = The toxicology of chromium with respect to its chemical speciation: A review |first = Sidney A.|last = Katz|journal = Journal of Applied Toxicology |volume = 13 |issue = 3|pages = 217–224 |year = 1992 |doi = 10.1002/jat.2550130314 |pmid = 8326093 |last2 = Salem |first2 = H }}</ref> The proposed beneficial effects of chromium(III) and the use as dietary supplements yielded some controversial results, but recent reviews suggest that moderate uptake of chromium(III) through dietary supplements poses no risk.<ref name="Eastmond"/>

===Cr(VI)===
The acute [[mouth|oral]] [[toxicity]] for chromium(VI) ranges between 50 and 150&nbsp;µg/kg.<ref name="Katz"/> 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 [[oxidation]]al properties. After it reaches the blood stream, it damages the kidneys, the liver and blood cells through oxidation reactions. [[Hemolysis]], [[renal]] and liver failure are the results of these damages. Aggressive dialysis can improve the situation.<ref name="Dayan">{{cite journal |title = Mechanisms of chromium toxicity, carcinogenicity and allergenicity: Review of the literature from 1985 to 2000 |first = A. D.|last = Dayan |journal = Human & Experimental Toxicology |volume = 20 |issue = 9|pages = 439–451 |year = 2001 |doi = 10.1191/096032701682693062 |last2 = Paine |first2 = A. J. |pmid=11776406}}</ref>

The [[carcinogenity]] of chromate dust is known for a long time, and in 1890 the first publication described the elevated cancer risk of workers in a chromate dye company.<ref>{{cite journal |title = A case of adeno-carcinoma of the left inferior turbinated body, and perforation of thenasal septum, in the person of a worker in chrome pigments |first = D.|last = Newman |journal = Glasgow Medical Journal |volume = 33|pages = 469–470 |year = 1890}}</ref><ref name="Langard">{{cite journal |title = One Hundred Years of Chromium and Cancer: A Review of Epidemiological Evidence and Selected Case Reports |first = Sverre |last = Langard |journal = American Journal of Industrial Medicine |volume = 17 |issue = 2|pages = 189–214 |year = 1990 |doi = 10.1002/ajim.4700170205 |pmid = 2405656}}</ref> Three mechanisms have been proposed to describe the [[genotoxicity]] of chromium(VI). The first mechanism includes highly reactive [[hydroxyl radical]]s 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.<ref name="Cohen">{{cite journal |title = Mechanisms of chromium carcinogenicity and toxicity |first = M. D.|last = Cohen|journal = Critical Reviews in Toxicology |volume = 23 |issue = 3|pages = 255–281 |year = 1993 |doi = 10.3109/10408449309105012 |last2 = Kargacin |first2 = B. |last3 = Klein |first3 = C. B. |last4 = Costa |first4 = M. |pmid = 8260068 }}</ref>

Chromium salts (chromates) are also the cause of [[allergic reaction]]s 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.<ref>{{cite web|publisher = DermNet NZ|title = Chrome Contact Allergy|url = http://dermnetnz.org/dermatitis/chrome-allergy.html}}</ref><ref name=" ">{{cite journal |title = Investigation of the threshold for allergic reactivity to chromium |first = David |last = Basketter |journal = Contact Dermatitis |volume = 44 |issue = 2 |pages = 70–74 |year = 2000 |doi = 10.1034/j.1600-0536.2001.440202.x |last2 = Horev |first2 = L. |last3 = Slodovnik |first3 = D.|last4 = Merimes |first4 = S. |last5 = Trattner |first5 = A.|last6 = Ingber |first6 = A. |pmid = 11205406 }}</ref>

===Environmental issues===
As chromium compounds were used in [[dye]]s and [[paint]]s and the [[tanning]] of [[leather]], these compounds are often found in soil and [[groundwater]] at abandoned industrial sites, now needing [[environmental cleanup]] and [[Environmental remediation|remediation]] per the treatment of [[brownfield land]]. [[Primer (paint)|Primer paint]] containing hexavalent chromium is still widely used for [[aerospace]] and [[automobile]] refinishing applications.<ref>{{Cite book|first = Randall C.|last = Baselt|title = Disposition of Toxic Drugs and Chemicals in Man|edition = 8th|publisher = Biomedical Publications|year = 2008 |pages = 305–307|isbn = 978-0-9626523-7-0|place = Foster City}}</ref>

In 2010, the [[Environmental Working Group]] studied the drinking water in 35 American cities. The study was the first nationwide analysis measuring the presence of the chemical in U.S. water systems. 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.<ref>{{cite news
|url=http://news.yahoo.com/s/afp/healthusenvironmentpollutionwater
|publisher=[[Yahoo News]]
|title=US water has large amounts of likely carcinogen: study
|date=2010-12-19
|accessdate=2010-12-19
}} {{Dead link|date=April 2012|bot=H3llBot}}</ref>
Note: Concentrations of Cr(VI) in US municipal drinking water supplies reported by EWG are within likely, natural background levels for the areas tested and not necessarily indicative of industrial pollution [http://www.waterboards.ca.gov/lahontan/water_issues/projects/pge/docs/pge_background_study_faq2.pdf (CalEPA Fact Sheet)], as asserted by EWG. This factor was not taken into consideration in their report.

==See also==
* [[:Category:Chromium compounds|Chromium compounds]]
* [[:Category:Chromium minerals|Chromium minerals]]

==Notes==
<references group="note" />

==References==
{{Reflist|colwidth=30em}}

==External links==
{{Commons|Chromium}}
{{Wiktionary|chromium}}
* [http://www.atsdr.cdc.gov/csem/chromium ATSDR Case Studies in Environmental Medicine: Chromium Toxicity] U.S. [[Department of Health and Human Services]]
* [http://www-cie.iarc.fr/htdocs/monographs/vol49/chromium.html IARC Monograph "Chromium and Chromium compounds"]
* [http://education.jlab.org/itselemental/ele024.html It's Elemental – The Element Chromium]
* [http://www.npi.gov.au/database/substance-info/profiles/24.html National Pollutant Inventory – Chromium (III) compounds fact sheet]
* [http://www.merck.com/mmpe/sec01/ch005/ch005b.html The Merck Manual – Mineral Deficiency and Toxicity]
* [http://www.cdc.gov/niosh/topics/chromium/ National Institute for Occupational Safety and Health – Chromium Page]
* [http://www.periodicvideos.com/videos/024.htm Chromium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)

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{{Chromium compounds}}

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[[Category:Chemical elements]]
[[Category:Transition metals]]
[[Category:Chromium|*]]
[[Category:Dietary minerals]]
[[Category:Occupational safety and health]]
[[Category:Biology and pharmacology of chemical elements]]