Editing Chromium

{{About|the chemical element}} 
{{Use dmy dates|date=November 2018}}
{{Infobox chromium|Vickers hardness=950HV|Vickers hardness ref=https://www.journalmt.com/pdfs/mft/2018/04/16.pdf}}
[[File:Motorcycle Reflections bw edit.jpg|thumb|"Chrome plated" motorcycle parts, plated with a decorative layer of chromium]] 

'''Chromium''' is a [[chemical element]]; it has [[Symbol (chemistry)|symbol]] '''Cr''' and [[atomic number]] 24. It is the first element in [[Group 6 element|group 6]]. It is a steely-grey, [[Luster (mineralogy)|lustrous]], hard, and brittle [[transition metal]].<ref>{{cite journal|last1 = Brandes|first1 = EA|last2 = Greenaway|first2 = HT|last3 = Stone|first3 = HEN|s2cid = 4221048|date = 1956|title = Ductility in Chromium|journal = Nature|volume = 178|issue = 4533|doi = 10.1038/178587a0|page = 587|bibcode = 1956Natur.178..587B|doi-access = free}}</ref>

Chromium is valued for its high [[corrosion]] resistance and [[hardness]]. A major development in steel production was the discovery that steel could be made highly resistant to corrosion and discoloration by adding metallic chromium to form [[stainless steel]]. Stainless steel and [[chrome plating]] ([[electroplating]] with chromium) together comprise 85% of the commercial use. Chromium is also greatly valued as a [[metal]] that is able to be highly [[polishing|polished]] while resisting [[tarnish]]ing. Polished chromium reflects almost 70% of the [[visible spectrum]], and almost 90% of [[infrared|infrared light]].<ref name = "NIST specular reflection">{{cite web|last1 = Coblentz|first1 = WW|last2 = Stair|first2 = R|title = Reflecting power of beryllium, chromium, and several other metals|url = https://nvlpubs.nist.gov/nistpubs/jres/2/jresv2n2p343_A2b.pdf|website = National Institute of Standards and Technology|publisher = NIST Publications|access-date = 11 October 2018|archive-date = 27 April 2020|archive-url = https://web.archive.org/web/20200427201104/https://nvlpubs.nist.gov/nistpubs/jres/2/jresv2n2p343_A2b.pdf|url-status = live}}</ref> The name of the element is derived from the [[Ancient Greek|Greek]] word χρῶμα, ''chrōma'', meaning [[color]],<ref>[https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dxrw%3Dma χρῶμα] {{Webarchive|url=https://web.archive.org/web/20210422204843/http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0057:entry=xrw=ma |date=22 April 2021 }}, Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus</ref> because many chromium compounds are intensely colored.

Industrial production of chromium proceeds from [[chromite]] ore (mostly FeCr<sub>2</sub>O<sub>4</sub>) to produce [[ferrochromium]], an iron-chromium alloy, by means of [[aluminothermic reaction|aluminothermic]] or [[silicothermic reaction]]s. Ferrochromium is then used to produce alloys such as stainless steel. Pure chromium metal is produced by a different process: [[Roasting (metallurgy)|roasting]] and [[Leaching (metallurgy)|leaching]] of chromite to separate it from iron, followed by reduction with [[carbon]] and then [[aluminium]].

[[Trivalent]] chromium (Cr(III)) occurs naturally in many foods and is sold as a [[dietary supplement]], although there is insufficient evidence that dietary chromium provides nutritional benefit to people.<ref name="ods">{{cite web |title=Chromium |url=https://ods.od.nih.gov/factsheets/Chromium-HealthProfessional/ |publisher=Office of Dietary Supplements, US National Institutes of Health |access-date=17 October 2024 |date=2 June 2022}}</ref><ref name="lpi">{{cite web |title=Chromium |url=https://lpi.oregonstate.edu/mic/minerals/chromium |publisher=Micronutrient Information Center, Linus Pauling Institute, Oregon State University, Corvallis |access-date=17 October 2024 |date=January 2024}}</ref> In 2014, the [[European Food Safety Authority]] concluded that research on dietary chromium did not justify it to be recognized as an essential [[nutrient]].<ref name="efsa">{{cite journal|url = https://efsa.onlinelibrary.wiley.com/doi/epdf/10.2903/j.efsa.2014.3845|title = Scientific Opinion on Dietary Reference Values for Chromium| journal=EFSA Journal |date = 18 September 2014| volume=12 | issue=10 | page=3845 |access-date = 17 October 2024|publisher = European Food Safety Authority| doi=10.2903/j.efsa.2014.3845 }}</ref>

While chromium metal and Cr(III) ions are considered non-toxic, chromate and its derivatives, often called "[[hexavalent chromium]]", is toxic and [[carcinogenic]]. According to the European Chemicals Agency (ECHA), [[chromium trioxide]] that is used in industrial electroplating processes is a "substance of very high concern" (SVHC).<ref>{{Cite web|title=Substance Information – ECHA|url=https://echa.europa.eu/substance-information/-/substanceinfo/100.014.189|access-date=2022-01-17|website=echa.europa.eu|language=en-GB|archive-date=23 November 2021|archive-url=https://web.archive.org/web/20211123202221/https://echa.europa.eu/substance-information/-/substanceinfo/100.014.189|url-status=live}}</ref>

== Physical properties ==
=== Atomic ===
Gaseous chromium has a ground-state [[electron configuration]] of [<nowiki/>[[argon|Ar]]] 3d<sup>5</sup> 4s<sup>1</sup>. It is the first element in the periodic table whose configuration violates the [[Aufbau principle]]. Exceptions to the principle also occur later in the periodic table for elements such as [[copper]], [[niobium]] and [[molybdenum]].<ref name = "CasaXPS">{{cite web|title = The Nature of X-Ray Photoelectron Spectra|url = http://www.casaxps.com/help_manual/XPSInformation/IntroductiontoXPS.htm|website = CasaXPS|publisher = Casa Software Ltd.|date = 2005|access-date = 10 March 2019|archive-date = 28 July 2019|archive-url = https://web.archive.org/web/20190728053349/http://www.casaxps.com/help_manual/XPSInformation/IntroductiontoXPS.htm|url-status = live}}</ref>

Chromium is the first element in the 3d series where the 3d electrons start to sink into the core; they thus contribute less to [[metallic bonding]], and hence the melting and boiling points and the [[enthalpy of atomisation]] of chromium are lower than those of the preceding element [[vanadium]]. Chromium(VI) is a strong [[oxidising agent]] in contrast to the [[molybdenum]](VI) and [[tungsten]](VI) oxides.<ref name="Greenwood1004">Greenwood and Earnshaw, pp. 1004–5</ref>

=== Bulk ===
[[File:Chromium.jpg|thumb|left|Sample of chromium metal]]
Chromium is the third hardest element after [[carbon]] ([[diamond]]) and [[boron]]. Its [[Mohs scale of mineral hardness|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 [[tarnish]]ing, which makes it useful as a metal that preserves its outermost layer from [[corrosion|corroding]], unlike other metals such as [[copper]], [[magnesium]], and [[aluminium]].

Chromium has a [[melting point]] of 1907&nbsp;°C (3465&nbsp;°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 element]]s, being topped by [[vanadium]] by 3&nbsp;°C (5&nbsp;°F) at 1910&nbsp;°C (3470&nbsp;°F). The [[boiling point]] of 2671&nbsp;°C (4840&nbsp;°F), however, is comparatively lower, having the fourth lowest boiling point out of the [[Period 4]] [[transition metal]]s alone behind [[copper]], [[manganese]] and [[zinc]].<ref group=note>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</ref> The [[electrical resistivity and conductivity|electrical resistivity]] of chromium at 20&nbsp;°C is 125 [[ohm|nanoohm]]-[[meter]]s.

Chromium has a high [[specular reflection]] in comparison to other transition metals. In [[infrared]], at 425&nbsp;[[micrometre|μm]], chromium has a maximum reflectance of about 72%, reducing to a minimum of 62% at 750&nbsp;μm before rising again to 90% at 4000&nbsp;μm.<ref name = "NIST specular reflection" /> When chromium is used in [[stainless steel]] alloys and [[polishing|polished]], the specular reflection decreases with the inclusion of additional metals, yet is still high in comparison with other alloys. Between 40% and 60% of the visible spectrum is reflected from polished stainless steel.<ref name = "NIST specular reflection" /> The explanation on why chromium displays such a high turnout of reflected [[photon]] waves in general, especially the 90% in infrared, can be attributed to chromium's magnetic properties.<ref name="ISU infrared">{{cite journal|last1 = Lind|first1 = Michael Acton|title = The infrared reflectivity of chromium and chromium-aluminium alloys|url = https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=6216&context=rtd|website = Iowa State University Digital Repository|publisher = Iowa State University|date = 1972|access-date = 4 November 2018|bibcode = 1972PhDT........54L|archive-date = 30 September 2021|archive-url = https://web.archive.org/web/20210930205235/https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=6216&context=rtd|url-status = live}}</ref> Chromium has unique magnetic properties; it is the only elemental solid that shows [[antiferromagnetic]] ordering at room temperature and below. Above 38&nbsp;°C, its magnetic ordering becomes [[paramagnetic]].<ref name="fawcett" /> The antiferromagnetic properties, which cause the chromium atoms to temporarily [[ionization|ionize]] and bond with themselves, are present because the body-centric cubic's magnetic properties are disproportionate to the [[Crystal structure|lattice periodicity]]. This is due to the magnetic moments at the cube's corners and the unequal, but antiparallel, cube centers.<ref name="ISU infrared" /> From here, the frequency-dependent [[relative permittivity]] of chromium, deriving from [[Maxwell's equations]] and chromium's [[antiferromagnetism]], leaves chromium with a high infrared and visible light reflectance.<ref name="ISU optical">{{cite journal|last1 = Bos|first1 = Laurence William|title = Optical properties of chromium-manganese alloys|url = https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=4816&context=rtd|website = Iowa State University Digital Repository|publisher = Iowa State University|date = 1969|access-date = 4 November 2018|bibcode = 1969PhDT.......118B|archive-date = 30 September 2021|archive-url = https://web.archive.org/web/20210930205225/https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=4816&context=rtd|url-status = live}}</ref>

==== Passivation ====
Chromium metal in air is [[passivation (chemistry)|passivated]]: it forms a thin, protective surface layer of chromium oxide with the [[corundum_(structure)|corundum]] structure. Passivation can be enhanced by short contact with [[oxidizing acid]]s 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.<ref name = "HollemanAF" />

The surface [[chromium(III) oxide|chromia]] {{chem2|Cr2O3}} scale, is adherent to the metal. In contrast, iron forms a more porous oxide which is weak and flakes easily and exposes fresh metal to the air, causing continued [[rust]]ing. At room temperature, the chromia scale is a few atomic layers thick, growing in thickness by outward [[Atomic_diffusion|diffusion]] of metal ions across the scale. Above 950 °C volatile [[chromium trioxide]] {{chem2|CrO3}} forms from the chromia scale, limiting the scale thickness and oxidation protection.<ref>{{Cite journal|title = The oxidation of alloys|last = Wallwork|first = GR|date = 1976|journal = Reports on Progress in Physics|volume = 39|pages = 401–485|doi = 10.1088/0034-4885/39/5/001|issue = 5|bibcode = 1976RPPh...39..401W | s2cid=250853920 }}</ref>

Chromium, unlike iron and nickel, does not suffer from [[hydrogen embrittlement]]. However, it does suffer from nitrogen [[embrittlement]], reacting with nitrogen from air and forming brittle nitrides at the high temperatures necessary to work the metal parts.<ref>{{Cite book|url = https://books.google.com/books?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|date = 1970|isbn = 978-0-309-01769-5|access-date = 5 June 2020|archive-date = 10 June 2024|archive-url = https://web.archive.org/web/20240610050309/https://books.google.com/books?id=CGMrAAAAYAAJ|url-status = live}}</ref>

=== Isotopes ===
{{Main|Isotopes of chromium}}

Naturally occurring chromium is composed of four stable [[isotope]]s; <sup>50</sup>Cr, <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]]). <sup>50</sup>Cr is [[Stable nuclide#Still-unobserved decay|observationally stable]], as it is theoretically capable of [[radioactive decay|decaying]] to [[Isotopes of titanium|<sup>50</sup>Ti]] via [[double electron capture]] with a [[half-life]] of no less than 1.3{{e|18}} years. Twenty-five [[radioisotope]]s have been characterized, ranging from <sup>42</sup>Cr to <sup>70</sup>Cr; the most stable radioisotope is <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 less than 1 minute. Chromium also has two [[metastable]] [[nuclear isomer]]s.{{NUBASE2020|ref}} 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]].{{NUBASE2020|ref}}

<sup>53</sup>Cr is the [[radiogenic]] decay product of <sup>53</sup>[[manganese|Mn]] (half-life 3.74&nbsp;million years).<ref>{{cite web|url = https://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html|title = Live Chart of Nuclides|website = International Atomic Energy Agency – Nuclear Data Section|access-date = 18 October 2018|archive-date = 23 March 2019|archive-url = https://web.archive.org/web/20190323230752/https://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html|url-status = live}}</ref> 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 [[aluminium-26|<sup>26</sup>Al]] and <sup>107</sup>[[Palladium|Pd]] concerning 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|date = 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|last1 = Birck|first1 = JL|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}} ---> <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|s2cid = 4373201|title=Fluctuations in Precambrian atmospheric oxygenation recorded by chromium isotopes|journal = Nature |volume = 461|issue = 7261|pages = 250–253|date = 2009|pmid = 19741707|doi = 10.1038/nature08266|bibcode = 2009Natur.461..250F}}</ref>

== Chemistry and compounds ==
{{Main|Chromium compounds}}
[[File:Chromium in water pourbiax diagram.png|thumb|left|The [[Pourbaix diagram]] for chromium in pure water, perchloric acid, or sodium hydroxide<ref name="Crspeci" /><ref name="medusa">Puigdomenech, Ignasi [http://www.kth.se/che/medusa ''Hydra/Medusa Chemical Equilibrium Database and Plotting Software''] {{webarchive |url=https://web.archive.org/web/20130605034847/http://www.kth.se/che/medusa |date=5 June 2013}} (2004) KTH Royal Institute of Technology</ref><!--also part of the Chromium (VI) Handbook of Jacques Guertin on page 73-->]]
Chromium is a member of [[Group 6 element|group 6]], of the [[transition metal]]s. The +3 and +6 states occur most commonly within chromium compounds, followed by +2; charges of +1, +4 and +5 for chromium are rare, but do nevertheless occasionally exist.<ref>{{cite web|last1 = Clark|first1 = Jim|title = Oxidation states (oxidation numbers)|url = https://www.chemguide.co.uk/inorganic/redox/oxidnstates.html|website = Chemguide|access-date = 3 October 2018|archive-date = 27 April 2021|archive-url = https://web.archive.org/web/20210427120058/http://chemguide.co.uk/inorganic/redox/oxidnstates.html|url-status = live}}</ref><ref name="Greenwood" />

=== Common oxidation states ===
{|class="wikitable" style="float:right; margin-left:1em"
|-
! 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>
|-
| −4 (d<sup>10</sup>)
|Na<sub>4</sub>[Cr(CO)<sub>4</sub>]<ref>{{Citation |last1=Theopold |first1=Klaus H. |title=Chromium: Organometallic Chemistry |date=2011-12-15 |encyclopedia=Encyclopedia of Inorganic and Bioinorganic Chemistry |pages=eibc0042 |editor-last=Scott |editor-first=Robert A. |publisher=John Wiley & Sons, Ltd |language=en |doi=10.1002/9781119951438.eibc0042 |isbn=978-1-119-95143-8 |last2=Kucharczyk |first2=Robin R.}}.</ref>
|-
| −2 (d<sup>8</sup>) ||{{chem|Na|2|[Cr(CO)|5|]}}
|-
| −1 (d<sup>7</sup>) ||{{chem|Na|2|[Cr|2|(CO)|10|]}}
|-
| 0 (d<sup>6</sup>) ||[[bis(benzene)chromium|{{chem|Cr(C|6|H|6|)|2}}]]
|-
| +1 (d<sup>5</sup>) ||{{chem| K|3|[Cr(CN)|5|NO]}}
|-
| '''+2 (d<sup>4</sup>)'''||[[Chromium(II) chloride|{{chem|CrCl|2}}]]
|-
| '''+3 (d<sup>3</sup>)''' || [[Chromium(III) chloride|{{chem|CrCl|3}}]]
|-
| +4 (d<sup>2</sup>) ||{{chem|K|2|CrF|6}}
|-
| +5 (d<sup>1</sup>) ||[[Potassium tetraperoxochromate(V)|{{chem|K|3|Cr(O|2|)|4|}}]]
|-
| '''+6 (d<sup>0</sup>)''' || [[Potassium chromate|{{chem|K|2|CrO|4}}]]
|}

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

==== Chromium(II) ====
[[File:Chromium carbide Cr3C2.JPG|thumb|upright|[[Chromium(II) carbide]] (Cr<sub>3</sub>C<sub>2</sub>)]]
Chromium(II) compounds are uncommon, in part because they readily oxidize to chromium(III) derivatives in air.  Water-stable [[chromium(II) chloride]] {{chem|CrCl|2}} that can be made by reducing chromium(III) chloride with zinc. The resulting bright blue solution created from dissolving chromium(II) chloride is stable at neutral [[pH]].<ref name="HollemanAF" /> Some other notable chromium(II) compounds include [[chromium(II) oxide]] {{chem|CrO}}, and [[chromium(II) sulfate]] {{chem|CrSO|4}}. Many chromium(II) carboxylates are known. The red [[chromium(II) acetate]] (Cr<sub>2</sub>(O<sub>2</sub>CCH<sub>3</sub>)<sub>4</sub>) is somewhat famous. It features a Cr-Cr [[quadruple bond]].<ref>{{cite book|last1=Cotton|first1=FA|author-link1=F. Albert Cotton|last2=Walton|first2=RA|title=Multiple Bonds Between Metal Atoms|url=https://archive.org/details/multiplebondsbet0000cott|url-access=registration|publisher=Oxford University Press|location=Oxford|date=1993|isbn=978-0-19-855649-7}}</ref>

==== Chromium(III) ====
[[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, such as [[chromium nitrate|chromium(III) nitrate]], [[chromium(III) acetate]], and [[chromium(III) oxide]].<ref>{{cite web|title = Chromium(III) compounds|url = http://www.npi.gov.au/resource/chromium-iii-compounds|website = National Pollutant Inventory|publisher = Commonwealth of Australia|access-date = 8 November 2018|archive-date = 22 April 2021|archive-url = https://web.archive.org/web/20210422214943/http://www.npi.gov.au/resource/chromium-iii-compounds|url-status = live}}</ref> 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]] [[cytochrome c|c7]].<ref>{{cite journal|last1 = Assfalg|first1 = M|last2 = Banci|first2 = L|last3 = Bertini|first3 = I|last4 = Bruschi|first4 = M|last5 = Michel|first5 = C|last6 = Giudici-Orticoni|first6 = M|last7 = Turano|first7 = P|title = NMR structural characterization of the reduction of chromium(VI) to chromium(III) by cytochrome c7|journal = Protein Data Bank|date = 31 July 2002|issue = 1LM2|doi = 10.2210/pdb1LM2/pdb|url = https://www.rcsb.org/structure/1lm2|access-date = 8 November 2018|archive-date = 2 October 2021|archive-url = https://web.archive.org/web/20211002124813/https://www.rcsb.org/structure/1lm2|url-status = live}}</ref> The {{chem|Cr|3+}} ion has a similar radius (63&nbsp;[[picometer|pm]]) to {{chem|Al|3+}} (radius 50&nbsp;pm), and they can replace each other in some compounds, such as in [[chrome alum]] and [[alum]].

Chromium(III) tends to form [[octahedral molecular geometry|octahedral]] complexes. 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 are the pale green [CrCl(H<sub>2</sub>O)<sub>5</sub>]Cl<sub>2</sub> and violet [Cr(H<sub>2</sub>O)<sub>6</sub>]Cl<sub>3</sub>. If anhydrous violet<ref>{{cite book|first=George W.|last=Luther|date=2016|department=Hydrate (Solvate) Isomers|chapter=Introduction to Transition Metals|chapter-url=https://books.google.com/books?id=Fz7hCgAAQBAJ&pg=PA244|title=Inorganic Chemistry for Geochemistry & Environmental Sciences: Fundamentals & Applications|page=244|isbn=978-1-118-85137-1|publisher=John Wiley & Sons|access-date=2019-08-07|archive-date=10 June 2024|archive-url=https://web.archive.org/web/20240610050327/https://books.google.com/books?id=Fz7hCgAAQBAJ&pg=PA244#v=onepage&q&f=false|url-status=live}}</ref> [[chromium(III) chloride]] is dissolved in water, the violet solution turns green after some time as the chloride in the inner [[coordination sphere]] is replaced by water. This kind of reaction is also observed with solutions of [[chrome alum]] and other water-soluble chromium(III) salts. A [[Tetrahedral molecular geometry|tetrahedral]] coordination of [[chromium(III)]] has been reported for the Cr-centered [[Keggin structure|Keggin]] anion [α-CrW<sub>12</sub>O<sub>40</sub>]<sup>5–</sup>.<ref>{{Cite journal|last1=Gumerova|first1=Nadiia I.|last2=Roller|first2=Alexander|last3=Giester|first3=Gerald|last4=Krzystek|first4=J.|last5=Cano|first5=Joan|last6=Rompel|first6=Annette|date=2020-02-19|title=Incorporation of CrIII into a Keggin Polyoxometalate as a Chemical Strategy to Stabilize a Labile {CrIIIO4} Tetrahedral Conformation and Promote Unattended Single-Ion Magnet Properties|journal=Journal of the American Chemical Society|volume=142|issue=7|pages=3336–3339|doi=10.1021/jacs.9b12797|issn=0002-7863|pmc=7052816|pmid=31967803}}</ref>

[[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>), a stable oxide with a crystal structure identical to that of [[corundum]].<ref name="HollemanAF" />

==== Chromium(VI) ====
{{Main|Hexavalent chromium}}
[[Hexavalent chromium|Chromium(VI) compounds]] are oxidants at low or neutral pH. [[chromate and dichromate|Chromate]] anions ({{chem|CrO|4|2-}}) and [[chromate and dichromate|dichromate]] (Cr<sub>2</sub>O<sub>7</sub><sup>2−</sup>) anions are the principal ions at this oxidation state. They exist at an equilibrium, determined by pH:
:2 [CrO<sub>4</sub>]<sup>2−</sup> + 2 H<sup>+</sup> {{eqm}} [Cr<sub>2</sub>O<sub>7</sub>]<sup>2−</sup> + H<sub>2</sub>O
Chromium(VI) oxyhalides are known also and include [[chromyl fluoride]] (CrO<sub>2</sub>F<sub>2</sub>) and [[chromyl chloride]] ({{chem|CrO|2|Cl|2}}).<ref name="HollemanAF">{{Cite book|publisher = Walter de Gruyter|date = 1985|edition = 91–100|pages = 1081–1095|isbn = 978-3-11-007511-3|title = Lehrbuch der Anorganischen Chemie|last1 = Holleman|first1 = Arnold F|last2 = Wiber|first2 = Egon|last3 = Wiberg|first3 = Nils|chapter = Chromium|language = de}}</ref>  However, despite several erroneous claims, [[chromium hexafluoride]] (as well as all higher hexahalides) remains unknown, as of 2020.<ref>{{Cite journal|last=Seppelt|first=Konrad|date=2015-01-28|title=Molecular Hexafluorides|journal=Chemical Reviews|language=en|volume=115|issue=2|pages=1296–1306|doi=10.1021/cr5001783|pmid=25418862|issn=0009-2665}}</ref>

[[File:Chrom(VI)-oxid.jpg|thumb|right|upright|Chromium(VI) oxide]]
[[Sodium chromate]] is produced industrially by the oxidative roasting of [[chromite]] ore with [[sodium carbonate]]. The change in equilibrium is visible by a change from yellow (chromate) to orange (dichromate), such as when an acid is added to a neutral solution of [[potassium chromate]]. At yet lower pH values, further condensation to more complex [[oxyanion]]s of chromium is possible.

Both the [[chromate and dichromate]] anions are strong oxidizing reagents at low pH:<ref name="HollemanAF" />
:{{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)

[[File:Chroman sodný.JPG|thumb|upright|[[Sodium chromate]] (Na<sub>2</sub>CrO<sub>4</sub>)]]
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.

=== Other oxidation states ===
{{See also|Organochromium chemistry}}
Compounds of chromium(V) are rather rare; the oxidation state +5 is only realized in few compounds but are intermediates in many reactions involving oxidations by chromate. The only binary compound is the volatile [[chromium(V) fluoride]] (CrF<sub>5</sub>). This red solid has a melting point of 30&nbsp;°C and a boiling point of 117&nbsp;°C. It can be prepared by treating chromium metal with fluorine at 400&nbsp;°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 thesis|degree = PhD|url = http://dokumentix.ub.uni-siegen.de/opus/volltexte/2006/52/|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>|date = 2003|first = Gentiana|last = Haxhillazi|publisher = University of Siegen|access-date = 23 August 2013|archive-date = 28 September 2018|archive-url = https://web.archive.org/web/20180928120925/https://dokumentix.ub.uni-siegen.de/opus/volltexte/2006/52/|url-status = live}}</ref>

Compounds of chromium(IV) are slightly more common than those of chromium(V). The tetrahalides, [[chromium(IV) fluoride|CrF<sub>4</sub>]], [[chromium(IV) chloride|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. Organic compounds containing Cr(IV) state such as chromium tetra ''t''-butoxide are also known.<ref>{{Cite journal|last1=Thaler|first1=Eric G.|last2=Rypdal|first2=Kristin|last3=Haaland|first3=Arne|last4=Caulton|first4=Kenneth G.|date=1989-06-01|title=Structure and reactivity of chromium(4+) tert-butoxide|journal=Inorganic Chemistry|volume=28|issue=12|pages=2431–2434|doi=10.1021/ic00311a035|issn=0020-1669}}</ref>

Most chromium(I) compounds are obtained solely by oxidation of electron-rich, [[octahedral molecular geometry|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.<ref>{{cite journal|last1 = Nguyen|first1 = T|last2 = Sutton|first2 = AD|last3 = Brynda|first3 = M|last4 = Fettinger|first4 = JC|last5 = Long|first5 = GJ|last6 = Power|first6 = PP|s2cid = 42853922|title = Synthesis of a stable compound with fivefold bonding between two chromium(I) centers |journal=Science |volume=310 |issue=5749 |pages=844–847 |date=2005 |pmid = 16179432|doi = 10.1126/science.1116789|bibcode = 2005Sci...310..844N|doi-access = free}}</ref> Extremely bulky monodentate ligands stabilize this compound by shielding the quintuple bond from further reactions.
[[File:5-fold chromium.png|thumb|Chromium compound determined experimentally to contain a Cr-Cr quintuple bond]]

== Occurrence ==
{{category see also|Chromium minerals}}
[[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 21st most [[Abundance of elements in Earth's crust|abundant element in Earth's crust]]<ref name="Emsley">{{Cite book |last=Emsley |first=John |title=Nature's Building Blocks: An A–Z Guide to the Elements |date=2001 |publisher=Oxford University Press |isbn=978-0-19-850340-8 |location=Oxford, England, UK |pages=[https://archive.org/details/naturesbuildingb0000emsl/page/495 495–498] |chapter=Chromium |chapter-url=https://archive.org/details/naturesbuildingb0000emsl/page/495}}</ref> with an average concentration of 100&nbsp;ppm. Chromium compounds are found in the environment from the [[erosion]] of chromium-containing rocks, and can be redistributed by volcanic eruptions. Typical background concentrations of chromium in environmental media are: atmosphere <10&nbsp;ng/m<sup>3</sup>; soil <500&nbsp;mg/kg; vegetation <0.5&nbsp;mg/kg; freshwater <10&nbsp;μg/L; seawater <1&nbsp;μg/L; sediment <80&nbsp;mg/kg.<ref name="Rieuwerts">{{cite book |author=John Rieuwerts |title=The Elements of Environmental Pollution |url=https://books.google.com/books?id=XHAGCAAAQBAJ |date=14 July 2017 |publisher=Taylor & Francis |isbn=978-1-135-12679-7 |access-date=9 October 2018 |archive-date=10 June 2024 |archive-url=https://web.archive.org/web/20240610050327/https://books.google.com/books?id=XHAGCAAAQBAJ |url-status=live }}</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 |date = 1974 |isbn = 978-0-309-02217-0 |url = https://books.google.com/books?id=ZZsrAAAAYAAJ |access-date = 11 March 2019 |archive-date = 10 June 2024 |archive-url = https://web.archive.org/web/20240610050311/https://books.google.co.ma/books?id=ZZsrAAAAYAAJ&redir_esc=y |url-status = live }}</ref>

About two-fifths of the chromite ores and concentrates in the world are produced in South Africa, about a third in Kazakhstan,<ref name = "Trump-SoHo-Bayrock">{{cite news |url = https://www.bloomberg.com/news/features/2018-01-11/how-a-trump-soho-partner-ended-up-with-toxic-mining-riches-from-kazakhstan |publisher = [[Bloomberg L.P.]] |date = 11 Jan 2018 |access-date = 21 Jan 2018 |title = How a Trump SoHo Partner Ended Up With Toxic Mining Riches From Kazakhstan |first = Marc |last = Champion |newspaper = Bloomberg.com |archive-date = 14 April 2021 |archive-url = https://web.archive.org/web/20210414114711/https://www.bloomberg.com/news/features/2018-01-11/how-a-trump-soho-partner-ended-up-with-toxic-mining-riches-from-kazakhstan |url-status = live }}</ref> while India, Russia, and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa.<ref name="USGS2015Yb" /> 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 |date = 1982 |first = Michael |last = Fleischer |access-date = 16 February 2009 |archive-date = 26 September 2021 |archive-url = https://web.archive.org/web/20210926205636/http://www.minsocam.org/ammin/AM67/AM67_854.pdf |url-status = live }}</ref><ref>[http://www.mindat.org/min-1037.html Chromium] {{Webarchive|url=https://web.archive.org/web/20210917100820/https://www.mindat.org/min-1037.html |date=17 September 2021 }} (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 diamonds.<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] {{Webarchive|url=https://web.archive.org/web/20210926214304/https://www.mindat.org/locentry-27628.html |date=26 September 2021 }}, Mindat.</ref>

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,<ref name="Crspeci">{{Cite journal |title =Chromium occurrence in the environment and methods of its speciation |volume = 107 |issue = 3 |journal = Environmental Pollution |date = 2000 |pages=263–283 |doi = 10.1016/S0269-7491(99)00168-2 |first1 = J. |last1 = Kotaś |pmid =15092973 |last2 =Stasicka |first2 =Z.}}</ref> but in some areas, the ground water can contain up to 39&nbsp;μg/L of total chromium, of which 30&nbsp;μg/L is 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 |date=2005 |pages = 5505–5511 |doi = 10.1021/es048835n |first1 = A. R. |last1 = Gonzalez |pmid = 16124280 |last2 = Ndung'u |first2 = K. |last3 = Flegal |first3 = A. R. |bibcode = 2005EnST...39.5505G }}</ref>
{{clear left}}

== History ==
=== Early applications ===
The [[History of China|ancient Chinese]] are credited with the first ever use of chromium to prevent [[Rust|rusting]]. Modern archaeologists discovered that bronze-tipped [[crossbow]] bolts at the [[Mausoleum of the First Qin Emperor|tomb of Qin Shi Huang]] showed no sign of corrosion after more than 2,000 years, because they had been coated in chromium.<ref>{{Cite book |last=Cotterell |first=Maurice |title=The Terracotta Warriors: The Secret Codes of the Emperor's Army |publisher=Inner Traditions / Bear & Co |isbn=1-59143-033-X |publication-date=2004 |pages=102}}</ref><ref>{{Cite book |last=Luo |first=Zhewen |title=China's Imperial Tombs and Mausoleums |publisher=Foreign Languages Press |year=1993 |isbn=7-119-01619-9 |pages=44}}</ref> In multiple [[Warring States period]] tombs, sharp ''[[Jian|jians]]'' and other weapons were also found to be coated with 10 to 15 micrometers of chromium oxide, which left them in pristine condition to this day.<ref>{{Cite book |last=J. C. McVeigh |url=https://books.google.com/books?id=QE3bAAAAMAAJ |title=Energy around the world: an introduction to energy studies, global resources, needs, utilization |publisher=Pergamon Press |year=1984 |isbn=0-08-031650-6 |pages=24}}</ref> Chromium was not used anywhere else until the experiments of French pharmacist and chemist [[Louis Nicolas Vauquelin]] (1763–1829) in the late 1790s.<ref>{{Cite book |last1=Jacques Guertin |title=Chromium(VI) Handbook |last2=James A. Jacobs |last3=Cynthia P. Avakian |publisher=CRC Press |isbn=978-1-56670-608-7 |publication-date=2005}}</ref> 

Chromium minerals as pigments came to the attention of the west in the eighteenth century. On 26&nbsp;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''.<ref name="Meyer 1962 p.">{{cite book|last = Meyer|first = RJ|title = Chrom: Teil A – Lieferung 1. Geschichtliches · Vorkommen · Technologie · Element bis Physikalische Eigenschaften|publisher = Springer Berlin Heidelberg Imprint Springer|location = Berlin, Heidelberg|year = 1962| isbn = 978-3-662-11865-8|oclc = 913810356|language = de}}</ref><ref>{{cite book|title = De Nova Minerae Plumbi Specie Crystallina Rubra, Epistola|last1 = Lehmanni|first1 = Iohannis Gottlob|year = 1766|url = http://www.mdz-nbn-resolving.de/urn/resolver.pl?urn=urn:nbn:de:bvb:12-bsb10226162-6|access-date = 7 October 2018|archive-date = 24 July 2020|archive-url = https://web.archive.org/web/20200724004908/http://www.mdz-nbn-resolving.de/urn/resolver.pl?urn=urn:nbn:de:bvb:12-bsb10226162-6|url-status = live}}</ref> Though misidentified as a [[lead]] compound with [[selenium]] and [[iron]] components, the mineral was in fact [[crocoite]] with a formula of PbCrO<sub>4</sub>.<ref name="ChromiumVI">{{Cite book|title = Chromium (VI) Handbook|url = https://archive.org/details/chromiumvihandbo00guer_095|url-access = limited|publisher = CRC Press|date = 2005|isbn = 978-1-56670-608-7|pages = [https://archive.org/details/chromiumvihandbo00guer_095/page/n18 7]–11|author = Guertin, Jacques|author2 = Jacobs, James Alan|author3 = Avakian, Cynthia P.|name-list-style = amp }}</ref> 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 [[paint]]s. After Pallas, the use of Siberian red lead as a paint pigment began to develop rapidly throughout the region.<ref name="Weeks1932">{{cite journal|last1 = Weeks|first1 = Mary Elvira|author-link = Mary Elvira Weeks|title = The discovery of the elements. V. Chromium, molybdenum, tungsten and uranium|journal = Journal of Chemical Education|volume = 9|issue = 3|year = 1932|pages = 459–73|issn = 0021-9584|doi = 10.1021/ed009p459|bibcode = 1932JChEd...9..459W}}</ref> Crocoite would be the principal source of chromium in pigments until the discovery of [[chromite]] many years later.<ref>{{cite web|last1 = Casteran|first1 = Rene|title = Chromite mining|url = https://oregonencyclopedia.org/articles/chromite_mining/#.W7K-TWhKiyJ|website = Oregon Encyclopedia|publisher = Portland State University and the Oregon Historical Society|access-date = 1 October 2018|archive-date = 26 September 2021|archive-url = https://web.archive.org/web/20210926185253/https://www.oregonencyclopedia.org/articles/chromite_mining/#.W7K-TWhKiyJ|url-status = live}}</ref>

[[File:Cut Ruby.jpg|left|thumb|upright|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(VI) oxide|chromium trioxide]] (CrO<sub>3</sub>) by mixing crocoite with [[hydrochloric acid]].<ref name="ChromiumVI" /> 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.<ref>{{Cite journal|url = https://books.google.com/books?id=6dgPAAAAQAAJ&pg=PA145|journal = Journal of Natural Philosophy, Chemistry, and the Arts|date = 1798|pages = 145–146|volume = 3|title = Memoir on a New Metallic Acid which exists in the Red Lead of Siberia|first = Louis Nicolas|last = Vauquelin|access-date = 8 January 2019|archive-date = 10 June 2024|archive-url = https://web.archive.org/web/20240610050310/https://books.google.com/books?id=6dgPAAAAQAAJ&pg=PA145#v=onepage&q&f=false|url-status = live}}</ref><ref>{{Cite journal|url = https://babel.hathitrust.org/cgi/pt?id=uc1.31822008815672;view=1up;seq=542|journal = Transactions of the American Institute of Mining, Metallurgical and Petroleum Engineers|year = 1895|page = 482|volume = 25|title = Chrome in the Southern Appalachian Region|first = William|last = Glenn|access-date = 8 January 2019|archive-date = 26 September 2021|archive-url = https://web.archive.org/web/20210926201003/https://babel.hathitrust.org/cgi/pt?id=uc1.31822008815672;view=1up;seq=542|url-status = live}}</ref> Vauquelin was also able to detect traces of chromium in precious [[gemstone]]s, such as [[ruby]] and [[emerald]].<ref name="ChromiumVI" /><ref>{{cite web|last = van der Krogt|first = Peter|title = Chromium|url = http://elements.vanderkrogt.net/element.php?sym=Cr|access-date = 24 Aug 2008|archive-date = 26 September 2021|archive-url = https://web.archive.org/web/20210926185257/https://elements.vanderkrogt.net/element.php?sym=Cr|url-status = live}}</ref>

During the nineteenth century, chromium was primarily used not only as a component of paints, but in [[Tanning (leather)|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.<ref>{{cite web|last1 = Ortt|first1 = Richard A Jr.|title = Soldier's Delight, Baltimore Country|url = http://www.mgs.md.gov/geology/geology_tour/soldiers_delight.html|website = Maryland Department of Natural Resources|publisher = Maryland Geological Survey|access-date = 13 May 2019|archive-date = 3 May 2021|archive-url = https://web.archive.org/web/20210503161902/http://www.mgs.md.gov/geology/geology_tour/soldiers_delight.html|url-status = live}}</ref> 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.<ref name="NRC" /> With the development of metallurgy and chemical industries in the Western world, the need for chromium increased.<ref>{{Cite book|url=https://www.bkmkitap.com/klasikten-moderne-osmanli-ekonomisi|title=Klasikten Moderne Osmanlı Ekonomisi|publisher=Kronik Kitap|editor-last=Bilgin|editor-first=Arif|location=Turkey|page=240|editor-last2=Çağlar|editor-first2=Burhan|access-date=24 September 2020|archive-date=18 April 2021|archive-url=https://web.archive.org/web/20210418060259/https://www.bkmkitap.com/klasikten-moderne-osmanli-ekonomisi|url-status=live}}</ref>

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.<ref name="Crplating">{{cite book|title = Nickel and Chromium Plating|url = https://archive.org/details/nickelchromiumpl00such|url-access = limited|publisher = Woodhead Publishing|date = 1993|isbn = 978-1-85573-081-6|pages = [https://archive.org/details/nickelchromiumpl00such/page/n22 9]–12|chapter = History of Chromium Plating|last1 = Dennis|first1 = JK|last2 = Such|first2 = TE}}</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-->

== Production ==
[[File:Chrom 1.jpg|thumb|left|Piece of chromium produced with [[aluminothermic reaction]]]]
[[File:Chromium - world production trend.svg|upright=1.4|thumb|World production trend of chromium]]
[[File:Chromium zone refined and 1cm3 cube.jpg|left|thumb|Chromium, remelted in a horizontal arc [[Zone melting|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.<ref name="USGS2015Yb">{{cite web|url = http://minerals.usgs.gov/minerals/pubs/commodity/chromium/mcs-2015-chrom.pdf|publisher = United States Geological Survey|access-date = 3 June 2015|title = Mineral Yearbook 2015: Chromium|first = John F|last = Papp|archive-date = 10 January 2019|archive-url = https://web.archive.org/web/20190110210251/https://minerals.usgs.gov/minerals/pubs/commodity/chromium/mcs-2015-chrom.pdf|url-status = live}}</ref> 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."<ref name="USGS2015Yb" />

The largest producers of chromium ore in 2013 have been South Africa (48%), Kazakhstan (13%), Turkey (11%), and India (10%), with several other countries producing the rest of about 18% of the world production.<ref name="USGS2015Yb" />

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|date = 2006|isbn = 978-0-87335-233-8|chapter = Chromite|author = Papp, John F.|author2 = Lipin, Bruce R.|name-list-style = amp|chapter-url = https://books.google.com/books?id=zNicdkuulE4C&pg=PA309|access-date = 5 June 2020|archive-date = 10 June 2024|archive-url = https://web.archive.org/web/20240610050310/https://books.google.com/books?id=zNicdkuulE4C&pg=PA309#v=onepage&q&f=false|url-status = live}}</ref>
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http://adsabs.harvard.edu/abs/1995EnGeo..25..251G
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http://www.ehponline.org/members/1991/092/92020.PDF
http://journals.tubitak.gov.tr/engineering/issues/muh-06-30-6/muh-30-6-5-0605-5.pdf-->

[[File:World Chromium Production 2002.svg|thumb|upright=1.4|Chromium ore output in 2002<ref name="USGS2002Yb">{{cite web|url = http://minerals.usgs.gov/minerals/pubs/commodity/chromium/chrommyb02.pdf|publisher = United States Geological Survey|access-date = 16 Feb 2009|title = Mineral Yearbook 2002: Chromium|first = John F.|last = Papp|archive-date = 10 January 2019|archive-url = https://web.archive.org/web/20190110130417/https://minerals.usgs.gov/minerals/pubs/commodity/chromium/chrommyb02.pdf|url-status = live}}</ref>]]
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 Fe<sub>2</sub>O<sub>3</sub>. 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.<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 ==
The creation of metal alloys account for 85% of the available chromium's usage. The remainder of chromium is used in the [[chemical industry|chemical]], [[refractory]], and [[foundry]] industries.<ref>{{Cite book|url = https://books.google.com/books?id=k_-0-DWxuRgC&q=metal+alloys+account+for+85%25+of+chromium+use&pg=PA83|title = Environmental Forensics: Contaminant Specific Guide|last1 = Morrison|first1 = RD|last2 = Murphy|first2 = BL|date = 4 Aug 2010|publisher = Academic Press|isbn = 978-0-08-049478-4|language = en|access-date = 30 October 2020|archive-date = 10 June 2024|archive-url = https://web.archive.org/web/20240610050311/https://books.google.com/books?id=k_-0-DWxuRgC&q=metal+alloys+account+for+85%25+of+chromium+use&pg=PA83#v=snippet&q=metal%20alloys%20account%20for%2085%25%20of%20chromium%20use&f=false|url-status = live}}</ref>

=== Metallurgy ===
[[File:Besteck WMF Stockholm ca 1960er.jpg |thumb|right|Stainless steel cutlery made from Cromargan 18/10, containing 18% chromium]]
{{Main|Chrome plating|Stainless steel}}
The strengthening effect of forming stable metal carbides at grain boundaries, and the strong increase in corrosion resistance made chromium an important alloying material for steel. [[High-speed steel|High-speed tool steels]] contain 3–5% chromium. [[Stainless steel]], the primary corrosion-resistant metal alloy, is formed when chromium is introduced to [[iron]] in concentrations above 11%.<ref name="Davis 2000 p.">{{cite book |last=Davis |first=JR |title=Alloy digest sourcebook: stainless steels |publisher=ASM International |location=Materials Park, OH |year=2000 |isbn=978-0-87170-649-2 |oclc=43083287 |language=af |pages=1–5 |url=https://www.asminternational.org/documents/10192/1849770/06940G_Chapter_1.pdf/53f29213-5dd6-4499-9959-841477b385b9 |access-date=5 October 2018 |archive-date=1 July 2021 |archive-url=https://web.archive.org/web/20210701021614/https://www.asminternational.org/documents/10192/1849770/06940G_Chapter_1.pdf/53f29213-5dd6-4499-9959-841477b385b9 |url-status=dead }}</ref> For stainless steel's formation, ferrochromium is added to the molten iron. Also, nickel-based alloys have increased strength due to the formation of discrete, stable, metal, carbide particles at the grain boundaries. For example, [[Inconel]] 718 contains 18.6% chromium. Because of the excellent high-temperature properties of these nickel [[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 = HK|last = Bhadeshia|url = http://www.msm.cam.ac.uk/phase-trans/2003/Superalloys/superalloys.html|access-date = 17 Feb 2009|publisher = University of Cambridge|archive-url = https://web.archive.org/web/20060825053006/http://www.msm.cam.ac.uk/phase-trans/2003/Superalloys/superalloys.html|archive-date = 25 August 2006}}</ref> [[ASTM]] B163 relies on chromium for condenser and heat-exchanger tubes, while [[castings]] with high strength at elevated temperatures that contain chromium are standardised with ASTM A567.<ref name="iarc49">{{cite journal |title=Chromium, Nickel and Welding |journal=IARC Monographs |date=1990 |volume=49 |pages=49–50 |publisher=International Agency for Research on Cancer}}</ref> [[American Iron and Steel Institute|AISI]] type 332 is used where high temperature would normally cause [[carburization]], [[oxidation]] or [[corrosion]].<ref name="azom332">{{cite news |title=Stainless Steel Grade 332 (UNS S33200) |url=https://www.azom.com/article.aspx?ArticleID=8271 |publisher=AZoNetwork |date=5 March 2013 |access-date=1 June 2021 |archive-date=3 June 2021 |archive-url=https://web.archive.org/web/20210603050412/https://www.azom.com/article.aspx?ArticleID=8271 |url-status=live }}</ref> [[Incoloy]] 800 "is capable of remaining stable and maintaining its [[austenitic]] structure even after long time exposures to high temperatures".<ref name="azom800">{{cite news |publisher=AZoNetwork |title=Super Alloy INCOLOY Alloy 800 (UNS N08800) |url=https://www.azom.com/article.aspx?ArticleID=9517 |date=3 July 2013 |access-date=1 June 2021 |archive-date=3 June 2021 |archive-url=https://web.archive.org/web/20210603104203/https://www.azom.com/article.aspx?ArticleID=9517 |url-status=live }}</ref> [[Nichrome]] is used as resistance wire for heating elements in things like [[toasters]] and space heaters. These uses make chromium a [[strategic material]]. Consequently, during World War II, U.S. road engineers were instructed to avoid chromium in yellow road paint, as it "may become a critical material during the emergency".<ref>{{Cite web |url=https://ceprofs.civil.tamu.edu/ghawkins/MUTCD-History_files/1942MUTCD.pdf |title=Manual On Uniform Traffic Control Devices (War Emergency Edition) |page=52 |publisher=American Associan of State Highway Officials |date=November 1942 |location=Washington, DC |access-date=July 8, 2021 |archive-date=29 April 2021 |archive-url=https://web.archive.org/web/20210429061859/https://ceprofs.civil.tamu.edu/ghawkins/MUTCD-History_files/1942MUTCD.pdf |url-status=live }}</ref> The United States likewise considered chromium "essential for the German war industry" and made intense diplomatic efforts to keep it out of the hands of [[Nazi Germany]].<ref>{{Cite web|last=State Department|first=United States|title=Allied Relations and Negotiations with Turkey|url=https://1997-2001.state.gov/regions/eur/rpt_9806_ng_turkey.pdf|url-status=live|archive-url=https://web.archive.org/web/20201109142429/https://1997-2001.state.gov/regions/eur/rpt_9806_ng_turkey.pdf |archive-date=9 November 2020 }}</ref>

[[File:Motorcycle Reflections bw edit.jpg|thumb|left|Decorative chrome plating on a motorcycle]]
The high hardness and corrosion resistance of unalloyed chromium makes it a reliable metal for surface coating; it is still the most popular metal for sheet coating, with its above-average durability, compared to other coating metals.<ref>{{cite web |last1=Breitsameter |first1=M |title=Thermal Spraying versus Hard Chrome Plating |url=https://www.azom.com/article.aspx?ArticleID=1576 |website=Azo Materials |publisher=AZoNetwork |access-date=1 October 2018 |date=2002-08-15 |archive-date=23 April 2021 |archive-url=https://web.archive.org/web/20210423034449/https://www.azom.com/article.aspx?ArticleID=1576 |url-status=live }}</ref> 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&nbsp;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.<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 of 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 = J |title = Coating and Surface Treatment Systems for Metals |publisher = Finishing Publications Ltd. and ASMy International |date = 1997|pages = 66–71 |isbn = 978-0-904477-16-0}}</ref> Because of environmental and health regulations on chromates, alternative coating methods are under development.<ref>{{Cite journal |journal=Surface and Coatings Technology |volume=140 |issue=1 |date=2001 |doi=10.1016/S0257-8972(01)01003-9 |title=Effects of chromate and chromate conversion coatings on corrosion of aluminum alloy 2024-T3 |vauthors= Zhao J, Xia L, Sehgal A, Lu D, McCreery RL, Frankel GS |pages=51–57 |hdl=1811/36519 |hdl-access=free }}</ref>

Chromic acid [[anodizing]] (or Type I anodizing) of aluminium is another electrochemical process that does not lead to the deposition of chromium, but uses [[chromic acid]] as an electrolyte in the solution. During anodization, an oxide layer is formed on the aluminium. The use of chromic acid, instead of the normally used sulfuric acid, leads to a slight difference of these oxide layers.<ref name="surface">{{Cite book| title = ASM Handbook: Surface Engineering|last1 = Cotell|first1 = CM|last2=Sprague|first2 = JA|last3 = Smidt|first3 = FA|url = https://books.google.com/books?id=RGtsPjqUwy0C&pg=PA484|access-date = 17 Feb 2009|publisher = ASM International|isbn = 978-0-87170-384-2|date = 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" />

=== Pigment ===
The mineral [[crocoite]] (which is also [[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 United States and for the postal services (for example, the [[Deutsche Post]]) in Europe. The use of chrome yellow has since declined due to environmental and safety concerns and was replaced by organic pigments or other alternatives that are free from lead and chromium. Other pigments that are based around chromium are, for example, the deep shade of red pigment [[chrome red]], which is simply lead chromate with [[lead(II) hydroxide]] (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 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&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 book|chapter-url = https://books.google.com/books?id=bdQVgKWl3f4C&pg=PA106|title = Painting Materials: A Short Encyclopaedia|first = Rutherford John|last = Gettens|publisher = Courier Dover Publications|date = 1966|isbn = 978-0-486-21597-6|pages = 105–106|chapter =Chrome yellow}}</ref>

Chromium oxides are also used as a green pigment in the field of glassmaking and also as a glaze for 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 [[lightfastness|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.<ref>{{cite book|last1 = Marrion|first1 = Alastair|title = The chemistry and physics of coatings|url = https://books.google.com/books?id=Iz0RQK0oMWUC&pg=PA287|date = 2004|publisher = Royal Society of Chemistry|isbn = 978-0-85404-604-1|pages = 287–}}</ref>

=== Other uses ===
[[File:5 Maiman Laser Components.jpg|thumb|alt=Components of original ruby laser.|Red crystal of a ruby laser]]
Chromium(III) ions present in [[corundum]] crystals (aluminium oxide) cause them to be colored red; when corundum appears as such, it is known as a [[ruby]]. If the corundum is lacking in chromium(III) ions, it is known as a [[sapphire]].<ref group=note>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</ref> 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 group=note>When {{chem|Cr|3+}} replaces {{chem|Al|3+}} in [[corundum]] (aluminium oxide, Al<sub>2</sub>O<sub>3</sub>), [[pink sapphire]] or [[ruby]] is formed, depending on the amount of chromium.</ref><ref>{{Cite journal|doi = 10.1524/zkri.1964.120.4-5.359|journal = Zeitschrift für Kristallographie|volume = 120|pages = 359–363|date = 1964|title = The chromium position in ruby|first2 = RE|last2 = Newnham|first1 = SC|last1 = Moss|url = http://rruff.geo.arizona.edu/doclib/zk/vol120/ZK120_359.pdf|issue = 4–5|bibcode = 1964ZK....120..359M|access-date = 27 April 2009|archive-date = 8 March 2012|archive-url = https://web.archive.org/web/20120308105622/http://rruff.geo.arizona.edu/doclib/zk/vol120/ZK120_359.pdf|url-status = live}}</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. Ruby has a laser transition at 694.3 nanometers, in a deep red color.<ref name="WebbJones2004">{{cite book|last1 = Webb|first1 = Colin E|last2 = Jones|first2 = Julian DC|title = Handbook of Laser Technology and Applications: Laser design and laser systems|url = https://books.google.com/books?id=DEgn44m6OREC&pg=PA323|year = 2004|publisher = CRC Press|isbn = 978-0-7503-0963-9|pages = 323–}}</ref>

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|last1 = Hingston|first1 = J|last2 = Collins|first2 = CD|last3 = Murphy|first3 = RJ|last4 = Lester|first4 = JN|volume = 111|issue = 1|pages = 53–66|date = 2001|doi = 10.1016/S0269-7491(00)00030-0 |pmid = 11202715 }}</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 were used in 1996.<ref name="Hings" />

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.<!-- https://books.google.com/books?id=b1ICltm2IdAC --><ref>{{Cite journal|title = A Conformational Study of Collagen as Affected by Tanning Procedures|last1 = Brown|first1 = EM|journal = Journal of the American Leather Chemists Association |date = 1997|pages = 225–233|volume = 92}}</ref> Chromium tanned leather can contain 4–5% 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. 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|date=2003|doi=10.1016/S0921-3449(02)00151-9|issue=3|last2=Ramasami|first2=T.|bibcode=2003RCR....38..185S }}</ref> and "chrome-less" or "chrome-free" tanning are practiced to better manage chromium usage.<ref>{{cite journal |last1=Qiang |first1=Taotao |last2=Gao |first2=Xin |last3=Ren |first3=Jing |last4=Chen |first4=Xiaoke |last5=Wang |first5=Xuechuan |title=A Chrome-Free and Chrome-Less Tanning System Based on the Hyperbranched Polymer |journal= ACS Sustainable Chemistry & Engineering|date=9 December 2015 |volume=4 |issue=3 |pages=701–707 |doi=10.1021/acssuschemeng.5b00917 }}</ref>

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--><ref name="Barnhart1997">{{cite journal|last1=Barnhart|first1=Joel|title=Occurrences, Uses, and Properties of Chromium|journal=Regulatory Toxicology and Pharmacology|volume=26|issue=1|year=1997|pages=S3–S7|issn=0273-2300|doi=10.1006/rtph.1997.1132|pmid=9380835}}</ref>

Several chromium compounds are used as [[catalyst]]s for processing hydrocarbons. For example, the [[Phillips catalyst]], prepared from chromium oxides, is used for the production of about half the world's [[polyethylene]].<ref>{{Cite journal|journal = Catalysis Today|volume = 51|issue = 2|date = 1999|pages = 215–221|doi = 10.1016/S0920-5861(99)00046-2|title = Olefin polymerization over supported chromium oxide catalysts|first1 = Bert M|last1 = Weckhuysen|last2 = Schoonheydt|first2 = Robert A|url = https://dspace.library.uu.nl/bitstream/handle/1874/21357/weckh_99_olefinpolymerization.pdf|hdl = 1874/21357|s2cid = 98324455|access-date = 5 October 2018|archive-date = 14 April 2021|archive-url = https://web.archive.org/web/20210414090528/https://dspace.library.uu.nl/bitstream/handle/1874/21357/weckh_99_olefinpolymerization.pdf|url-status = live}}</ref> Fe-Cr mixed oxides are employed as high-temperature catalysts for the [[water gas shift reaction]].<ref>{{cite book | chapter-url = https://books.google.com/books?id=YlJRAAAAMAAJ|first1 = MVE|last1 = Twigg|title = Catalyst Handbook|chapter = The Water-Gas Shift Reaction |isbn = 978-0-7234-0857-4 |date = 1989| publisher=Taylor & Francis }}</ref><ref>{{cite journal |doi = 10.1016/0920-5861(94)00135-O | title = Water-gas shift reaction: Finding the mechanistic boundary |date=1995 |last1 = Rhodes|first1 = C|journal = Catalysis Today|volume = 23|pages = 43–58|last2 = Hutchings | first2 = GJ|last3 = Ward|first3 = AM}}</ref> [[Copper chromite]] is a useful [[hydrogenation]] catalyst.<ref>{{OrgSynth |last1 = Lazier|first1 = WA|last2 = Arnold |first2 = HR|name-list-style = amp|year = 1939|title = Copper Chromite Catalyst|volume = 19|pages = 31|collvol = 2|collvolpages = 142|prep = CV2P0142}}</ref>

=== Uses of compounds ===
* [[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 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|chapter-url = https://books.google.com/books?id=rNifWsBxnWkC&pg=PA32| title =The foundations of magnetic recording|first = John C.|last = Mallinson|publisher = Academic Press|date = 1993|isbn = 978-0-12-466626-9|chapter = Chromium Dioxide | page = 32}}</ref>
* [[Chromium(III) oxide]] (Cr<sub>2</sub>O<sub>3</sub>) is a metal polish known as green rouge.<ref name="DoiMarinescu2011">{{cite book|author1=Toshiro Doi|author2=Ioan D. Marinescu|author3=Syuhei Kurokawa|title=Advances in CMP Polishing Technologies|url=https://books.google.com/books?id=LJH9rYdKTZwC&pg=PA60|date=30 November 2011|publisher=William Andrew|isbn=978-1-4377-7860-1|pages=60–}}</ref><ref>{{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|first1 = Anil|last1 = Baral|journal = Environmental Science & Policy|last2 = Engelken|first2 = Robert D.|bibcode = 2002ESPol...5..121B}}</ref> <!--{{doi|10.1016/0026-0576(95)99364-G}} {{doi|10.1016/S0026-0576(02)82003-7}} https://books.google.de/books?id=LJH9rYdKTZwC&pg=PA60 -->
* [[Chromic acid]] is a powerful oxidizing agent and is a useful compound for cleaning laboratory glassware of any trace of organic compounds.<ref>{{cite web |last1=Soderberg |first1=Tim |title=Oxidizing Agents |url=https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_(Smith)/Chapter_12%3A_Oxidation_and_Reduction/12.07_Oxidizing_Agents |website=LibreTexts |publisher=MindTouch |access-date=8 September 2019 |date=3 June 2019 |archive-date=22 April 2021 |archive-url=https://web.archive.org/web/20210422223905/https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map:_Organic_Chemistry_(Smith)/Chapter_12:_Oxidation_and_Reduction/12.07_Oxidizing_Agents |url-status=live }}</ref> It is prepared by dissolving [[potassium dichromate]] in concentrated sulfuric acid, which is then used to wash the apparatus. [[Sodium dichromate]] is sometimes used because of its higher solubility (50&nbsp;g/L versus 200&nbsp;g/L respectively). The use of dichromate cleaning solutions is now phased out due to the high toxicity and environmental concerns. Modern cleaning solutions are highly effective and chromium free.<ref name="Roth1994">{{cite book|last1 = Roth|first1 = Alexander|title = Vacuum Sealing Techniques|url = https://books.google.com/books?id=sdKAPJh5RgQC&pg=PA118|year = 1994|publisher = Springer Science & Business Media|isbn = 978-1-56396-259-2|pages=118–}}</ref>
* [[Potassium dichromate]] is a chemical [[reagent]], used as a titrating agent.<ref>{{cite web |last1=Lancashire |first1=Robert J |title=Determination of iron using potassium dichromate: Redox indicators |url=http://wwwchem.uwimona.edu.jm/lab_manuals/c10expt31.html |publisher=The Department of Chemistry UWI, Jamaica |access-date=8 September 2019 |date=27 October 2008 |archive-date=18 April 2021 |archive-url=https://web.archive.org/web/20210418125750/http://wwwchem.uwimona.edu.jm/lab_manuals/c10expt31.html |url-status=live }}</ref>
* [[Chromate and dichromate|Chromate]]s are added to drilling muds to prevent corrosion of steel under wet conditions.<ref>{{cite book|title = Corrosion in the Petrochemical Industry|first = Linda|last = Garverick|publisher = ASM International|date = 1994|isbn = 978-0-87170-505-1|url = https://books.google.com/books?id=qTfNZZRO758C&pg=PA278|access-date = 5 June 2020|archive-date = 10 June 2024|archive-url = https://web.archive.org/web/20240610045936/https://books.google.com/books?id=qTfNZZRO758C&pg=PA278#v=onepage&q&f=false|url-status = live}}</ref>
* [[Chrome alum]] is [[Chromium(III) potassium sulfate]] and is used as a [[mordant]] (i.e., a fixing agent) for dyes in fabric and in [[tanning (leather)|tanning]].<ref name="Ul-Islam2017">{{cite book|author=Shahid Ul-Islam|title=Plant-Based Natural Products: Derivatives and Applications|url=https://books.google.com/books?id=jYMtDwAAQBAJ&pg=PA74|date=18 July 2017|publisher=Wiley|isbn=978-1-119-42388-1|pages=74–|access-date=17 October 2018|archive-date=10 June 2024|archive-url=https://web.archive.org/web/20240610050514/https://books.google.com/books?id=jYMtDwAAQBAJ&pg=PA74#v=onepage&q&f=false|url-status=live}}</ref>

== Biological role ==

The possible nutritional value of chromium(III) is unproven.<ref>{{cite book|first1=JB|last1=Vincent |chapter=Chromium: Is It Essential, Pharmacologically Relevant, or Toxic? |editor=Astrid Sigel|editor2=Helmut Sigel| editor3=Roland KO Sigel|title=Interrelations between Essential Metal Ions and Human Diseases|series=Metal Ions in Life Sciences |volume=13 |date=2013|publisher=Springer|pages=171–198|doi=10.1007/978-94-007-7500-8_6|pmid=24470092|isbn=978-94-007-7499-5}}</ref><ref>{{cite book |last1=Maret|first1=Wolfgang |editor1-last=Sigel|editor1-first=Astrid |editor2-last=Freisinger|editor2-first=Eva |editor3-last=Sigel|editor3-first=Roland K. O. |editor4-last=Carver|editor4-first=Peggy L. |title=Essential Metals in Medicine:Therapeutic Use and Toxicity of Metal Ions in the Clinic; In: Metal Ions in Life Sciences |volume=19 |date=2019 |publisher=de Gruyter GmbH|location=Berlin|isbn=978-3-11-052691-2 |doi=10.1515/9783110527872-015|pmid=30855110|pages=231–251 |chapter=Chapter 9. Chromium Supplementation in Human Health, Metabolic Syndrome, and Diabetes|journal=Metal Ions in Life Sciences }}</ref> Although chromium is regarded as a trace element and [[mineral (nutrient)|dietary mineral]], its suspected roles in the action of [[insulin]] &ndash; a hormone that mediates the metabolism and storage of carbohydrate, fat, and protein &ndash; have not been adequately established.<ref name="ods" /><ref name=lpi/> The [[mechanism of action|mechanism of its actions]] in the body is undefined, leaving in doubt whether chromium has a biological role in healthy people.<ref name="ods" /><ref name=lpi/><ref name=efsa/> 

In contrast, [[hexavalent chromium]] (Cr(VI) or Cr<sup>6+</sup>) is highly toxic and [[mutagen]]ic.<ref>{{cite journal|pmid=22192535|pmc=4138963|year=2012|last1=Wise|first1=SS |title=Chromium and genomic stability |journal=Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis|volume=733|issue=1–2|pages=78–82|last2=Wise|first2=JP Sr|doi=10.1016/j.mrfmmm.2011.12.002|bibcode=2012MRFMM.733...78W }}</ref> Ingestion of chromium(VI) in water has been linked to stomach tumors, and it may also cause allergic [[contact dermatitis]].<ref>{{cite web|publisher = Agency for Toxic Substances & Disease Registry, [[Centers for Disease Control and Prevention]]|title = ToxFAQs: Chromium|url = https://wwwn.cdc.gov/TSP/ToxFAQs/ToxFAQsLanding.aspx?id=61&tid=17|archive-url = https://web.archive.org/web/20140708162618/http://www.atsdr.cdc.gov/toxfaqs/TF.asp?id=61&tid=17|archive-date = 8 July 2014|date=Feb 2001|url-status = live|access-date = 2 October 2007}}</ref>

"[[Chromium deficiency]]", involving a lack of Cr(III) in the body, or perhaps some complex of it, such as [[glucose tolerance factor]], is not accepted as a medical condition, as it has no symptoms and healthy people do not require chromium supplementation.<ref name="ods" /><ref name=lpi/> 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]] (chromodulin), which might play a role in the insulin signaling pathway.<ref name=ods/><ref name="pharmacological1">{{cite journal|last1=Vincent|first1=JB|title=Is the Pharmacological Mode of Action of Chromium(III) as a Second Messenger?|journal=Biological Trace Element Research|volume=166|issue=1|pages=7–12|date=2015|pmid=25595680|doi=10.1007/s12011-015-0231-9 |bibcode=2015BTER..166....7V |s2cid=16895342}}</ref>

The chromium content of common foods is generally low (1–13 micrograms per serving).<ref name="ods" /><ref name="database">{{cite journal|last1 = Thor|first1 = MY|last2 = Harnack|first2 = L|last3 = King|first3 = D|last4= Jasthi|first4 = B|last5 = Pettit|first5 = J|title = Evaluation of the comprehensiveness and reliability of the chromium composition of foods in the literature|journal=Journal of Food Composition and Analysis|date=2011|volume = 24|issue = 8|pages = 1147–1152|doi = 10.1016/j.jfca.2011.04.006|pmid = 23066174|pmc = 3467697}}</ref> The chromium content of food varies widely, due to differences in soil mineral content, growing season, plant [[cultivar]], and contamination during processing.<ref name=lpi/><ref name="database" /> Chromium (and [[nickel]]) leach into food cooked in stainless steel, with the effect being largest when the cookware is new. Acidic foods that are cooked for many hours also exacerbate this effect.<ref>{{cite journal|author=Kamerud KL|author2= Hobbie KA|author3= Anderson KA|title=Stainless steel leaches nickel and chromium into foods during cooking|journal= Journal of Agricultural and Food Chemistry|date=2013 |doi=10.1021/jf402400v |pmid=23984718|volume=61|issue=39|pages=9495–9501|pmc=4284091|bibcode= 2013JAFC...61.9495K}}</ref><ref>{{cite journal|author=Flint GN|author2= Packirisamy S|title=Purity of food cooked in stainless steel utensils|journal= Food Additives and Contaminants|date=1997|volume=14|issue=2|pages=115–126|doi=10.1080/02652039709374506|pmid=9102344}}</ref>

=== Dietary recommendations ===
{{anchor|Dietary recommendations}}
{{See also|Chromium deficiency}}
There is disagreement on chromium's status as an essential nutrient. Governmental departments from Australia, New Zealand, India, and Japan consider chromium as essential,<ref name="AustraliaNZ" /><ref name="India" /><ref name="Japan" /> while the United States and European Food Safety Authority of the European Union do not.<ref name=ods/><ref name="efsa" />

The U.S. [[National Academy of Medicine]] (NAM) updated the [[Dietary Reference Intake#Parameters|Estimated Average Requirements]] (EARs) and the [[Dietary Reference Intake#Parameters|Recommended Dietary Allowances]] (RDAs) for chromium in 2001. For chromium, there was insufficient information to set EARs and RDAs, so its needs are described as estimates for [[Dietary Reference Intake#Parameters|Adequate Intake]] (AI). From a 2001 assessment, AI of chromium for women ages 14 through 50 is 25&nbsp;μg/day, and the AI for women ages 50 and above is 20&nbsp;μg/day. The AIs for women who are pregnant are 30&nbsp;μg/day, and for women who are lactating, the set AI is 45&nbsp;μg/day. The AI for men ages 14 through 50 is 35&nbsp;μg/day, and the AI for men ages 50 and above is 30&nbsp;μg/day. For children ages 1 through 13, the AI increases with age from 0.2&nbsp;μg/day up to 25&nbsp;μg/day.<ref name=ods/> As for safety, the NAM sets [[Dietary Reference Intake#Parameters|Tolerable Upper Intake Levels]] (ULs) for vitamins and minerals when the evidence is sufficient. In the case of chromium, there is not yet enough information, hence no UL has been established. Collectively, the EARs, RDAs, AIs, and ULs are the parameters for the nutrition recommendation system known as [[Dietary Reference Intake]] (DRI).<ref name="IOM-Chromium">{{cite web |url=https://www.ncbi.nlm.nih.gov/books/NBK222329/ |title=Chromium. IN: Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Chromium, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Chromium |date=2001 |pages=197–223 |website=Institute of Medicine (U.S.) Panel on Micronutrients, National Academy Press |access-date=3 October 2018 |archive-date=24 January 2021 |archive-url=https://web.archive.org/web/20210124005559/https://www.ncbi.nlm.nih.gov/books/NBK222329/ |url-status=live }}</ref> 

Australia and New Zealand consider chromium to be an essential nutrient, with an AI of 35&nbsp;μg/day for men, 25&nbsp;μg/day for women, 30&nbsp;μg/day for women who are pregnant, and 45&nbsp;μg/day for women who are lactating. A UL has not been set due to the lack of sufficient data.<ref name="AustraliaNZ">{{cite web |url=https://www.nrv.gov.au/nutrients/chromium |title=Chromium |date=2014 |website=Nutrient Reference Values for Australia and New Zealand |access-date=4 October 2018 |archive-date=7 October 2021 |archive-url=https://web.archive.org/web/20211007100635/https://www.nrv.gov.au/nutrients/chromium |url-status=live }}</ref> India considers chromium to be an essential nutrient, with an adult recommended intake of 33&nbsp;μg/day.<ref name="India">{{cite web |url=http://icmr.nic.in/final/rda-2010.pdf |title=Nutrient Requirements and Recommended Dietary Allowances for Indians: A Report of the Expert Group of the Indian Council of Medical Research. pp.283–295 (2009)  |archive-url=https://web.archive.org/web/20160615094048/http://icmr.nic.in/final/RDA-2010.pdf |archive-date=15 June 2016 |access-date=3 October 2018 }}</ref> Japan also considers chromium to be an essential nutrient, with an AI of 10&nbsp;μg/day for adults, including women who are pregnant or lactating. A UL has not been set.<ref name="Japan">{{cite web |url=http://www.mhlw.go.jp/file/06-Seisakujouhou-10900000-Kenkoukyoku/Overview.pdf |title=DRIs for Chromium (μg/day) |date=2015 |page=41 |website=Overview of Dietary Reference Intakes for Japanese |access-date=4 October 2018 |archive-date=23 April 2021 |archive-url=https://web.archive.org/web/20210423083531/https://www.mhlw.go.jp/file/06-Seisakujouhou-10900000-Kenkoukyoku/Overview.pdf |url-status=live }}</ref> 

The EFSA does not consider chromium to be an essential nutrient.<ref name=efsa/><ref name="EFSA17">{{cite web|title = Overview on Dietary Reference Values for the EU population as derived by the EFSA Panel on Dietetic Products, Nutrition and Allergies|year = 2017|url = https://www.efsa.europa.eu/sites/default/files/assets/DRV_Summary_tables_jan_17.pdf|access-date = 5 September 2017|archive-date = 28 August 2017|archive-url = https://web.archive.org/web/20170828082247/https://www.efsa.europa.eu/sites/default/files/assets/DRV_Summary_tables_jan_17.pdf|url-status = live}}</ref><ref>{{citation|title = Tolerable Upper Intake Levels For Vitamins And Minerals|publisher = European Food Safety Authority|year = 2006|url = http://www.efsa.europa.eu/sites/default/files/efsa_rep/blobserver_assets/ndatolerableuil.pdf|access-date = 24 July 2016|archive-date = 16 March 2016|archive-url = https://web.archive.org/web/20160316225123/http://www.efsa.europa.eu/sites/default/files/efsa_rep/blobserver_assets/ndatolerableuil.pdf|url-status = live}}</ref>

====Labeling====
For U.S. food and dietary supplement labeling purposes, the amount of the substance in a serving is expressed as a percent of the [[Reference Daily Intake|Daily Value]] (%DV). For chromium labeling purposes, 100% of the Daily Value was 120&nbsp;μg. As of 27&nbsp;May 2016, the percentage of daily value was revised to 35&nbsp;μg to bring the chromium intake into a consensus with the official [[Dietary Reference Intake|Recommended Dietary Allowance]].<ref name="FedReg">{{cite web|url = https://www.gpo.gov/fdsys/pkg/FR-2016-05-27/pdf/2016-11867.pdf|title = Federal Register May 27, 2016 Food Labeling: Revision of the Nutrition and Supplement Facts Labels. FR page 33982|access-date = 31 August 2017|archive-date = 8 August 2016|archive-url = https://web.archive.org/web/20160808164651/https://www.gpo.gov/fdsys/pkg/FR-2016-05-27/pdf/2016-11867.pdf|url-status = live}}</ref><ref>{{cite web | title=Daily Value Reference of the Dietary Supplement Label Database (DSLD) | website=Dietary Supplement Label Database (DSLD) | url=https://www.dsld.nlm.nih.gov/dsld/dailyvalue.jsp | access-date=16 May 2020 | archive-date=7 April 2020 | archive-url=https://web.archive.org/web/20200407073956/https://dsld.nlm.nih.gov/dsld/dailyvalue.jsp  }}</ref> A table of the old and new adult daily values in the United States is provided at [[Reference Daily Intake]].

After evaluation of research on the potential nutritional value of chromium, the European Food Safety Authority concluded that there was no evidence of benefit by dietary chromium in healthy people, thereby declining to establish recommendations in Europe for dietary intake of chromium.<ref name=efsa/>

===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.<ref name="USDA-Database">{{cite web|url = https://fdc.nal.usda.gov/index.html|title = USDA Food Composition Databases|date = April 2018|website = United States Department of Agriculture Agricultural Research Service|access-date = 4 October 2018|archive-date = 3 December 2019|archive-url = https://web.archive.org/web/20191203185131/https://fdc.nal.usda.gov/index.html|url-status = live}}</ref> A wide variety of animal and vegetable foods contain chromium.<ref name=ods/><ref name="IOM-Chromium" /> Content per serving is influenced by the chromium content of the soil in which the plants are grown, by foodstuffs fed to animals, and by processing methods, as chromium is leached into foods if processed or cooked in stainless steel equipment.<ref>
{{cite journal|last1 = Kumpulainen|first1 = JT|title = Chromium content of foods and diets|journal = Biological Trace Element Research|volume = 32|issue = 1–3|pages = 9–18|date = 1992|pmid = 1375091|doi = 10.1007/BF02784582 | bibcode=1992BTER...32....9K |s2cid = 10189109}}</ref> One diet analysis study conducted in Mexico reported an average daily chromium intake of 30 micrograms.<ref>{{cite journal|last1 = Grijalva Haro|first1 = MI|last2 = Ballesteros Vázquez|first2 = MN|last3 = Cabrera Pacheco|first3 = RM|title = Chromium content in foods and dietary intake estimation in the Northwest of Mexico|language = es|journal = Arch Latinoam Nutr|volume = 51|issue = 1|pages = 105–110|date = 2001|pmid = 11515227}}</ref> An estimated 31% of adults in the United States consume multi-vitamin/mineral dietary supplements,<ref name="Kantor2016">{{cite journal|last1 = Kantor|first1 = Elizabeth D|last2 = Rehm|first2 = Colin D|last3 = Du|first3 = Mengmeng|last4 = White|first4 = Emily|last5 = Giovannucci|first5 = Edward L|title = Trends in Dietary Supplement Use Among US Adults From 1999–2012|journal = JAMA|volume = 316|issue = 14|pages = 1464–1474|date = 11 October 2017|pmid = 27727382|pmc = 5540241|doi = 10.1001/jama.2016.14403}}</ref> which often contain 25 to 60 micrograms of chromium.

=== Supplementation ===
Chromium is an ingredient in [[total parenteral nutrition]] (TPN), because deficiency can occur after months of intravenous feeding with chromium-free TPN.<ref name="Stehle2016" /> It is also added to nutritional products for [[preterm infant]]s.<ref>{{cite journal|pmid = 25527182|date = February 2015|last1 = Finch|first1 = Carolyn Weiglein|title = Review of trace mineral requirements for preterm infants: What are the current recommendations for clinical practice?|journal=Nutrition in Clinical Practice|volume = 30|issue = 1|pages = 44–58|doi = 10.1177/0884533614563353}}</ref> Although the mechanism of action in biological roles for chromium is unclear, in the United States chromium-containing products are sold as non-prescription dietary supplements in amounts ranging from 50 to 1,000 μg. Lower amounts of chromium are also often incorporated into multi-vitamin/mineral supplements consumed by an estimated 31% of adults in the United States.<ref name="Kantor2016" /> Chemical compounds used in dietary supplements include chromium chloride, chromium citrate, [[chromium(III) picolinate]], [[Chromium polynicotinate|chromium(III) polynicotinate]], and other chemical compositions.<ref name="ods" /> The benefit of supplements has not been proven.<ref name="ods" /><ref name="Vincent2010">{{cite journal|doi = 10.1039/B920480F|title = Chromium: Celebrating 50 years as an essential element?|date = 2010|last1 = Vincent|first1 = John B|journal = Dalton Transactions|volume = 39|issue = 16|pages = 3787–3794|pmid = 20372701}}</ref>

====Initiation of research on glucose====
The notion of chromium as a potential regulator of glucose metabolism began in the 1950s when scientists performed a series of experiments controlling the diet of rats.<ref name="schwarz">{{Cite journal | doi = 10.1016/0003-9861(59)90479-5 | last1 = Schwarz | first1 = K| last2 = Mertz | first2 = W| title = Chromium(III) and the glucose tolerance factor | journal = Archives of Biochemistry and Biophysics | volume = 85 | pages = 292–95 | year = 1959 | pmid = 14444068}}</ref> The experimenters subjected the rats to a chromium deficient diet, and witnessed an inability to respond effectively to increased levels of blood glucose. A chromium-rich [[yeast|Brewer's yeast]] was provided in the diet, enabling the rats to effectively metabolize glucose, and so giving evidence that chromium may have a role in glucose management.<ref name=schwarz/>

===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 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&nbsp;2 diabetes. FDA concludes, however, that the existence of such a relationship between chromium picolinate and either insulin resistance or type&nbsp;2 diabetes is highly uncertain."'' 

In 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.<ref>[https://wayback.archive-it.org/7993/20171114183739/https://www.fda.gov/Food/IngredientsPackagingLabeling/LabelingNutrition/ucm073017.htm FDA Qualified Health Claims: Letters of Enforcement Discretion, Letters of Denial] U.S. Food and Drug Administration, Docket #2004Q-0144 (August 2005).</ref> As of March 2024, this ruling on chromium remains in effect.<ref name="fda-enforce">{{cite web |title=Qualified Health Claims: Letters of Enforcement Discretion |url=https://www.fda.gov/food/nutrition-food-labeling-and-critical-foods/qualified-health-claims-letters-enforcement-discretion |publisher=US Food and Drug Administration |access-date=19 October 2024 |date=28 March 2024}}</ref>

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.<ref>{{cite web|url = http://webprod.hc-sc.gc.ca/nhpid-bdipsn/monoReq.do?id=65|title = Monograph: Chromium (from Chromium picolinate)|publisher = Health Canada|date = 9 December 2009|access-date = 18 October 2018|archive-date = 9 May 2020|archive-url = https://web.archive.org/web/20200509120406/http://webprod.hc-sc.gc.ca/nhpid-bdipsn/monoReq.do?id=65|url-status = live}}</ref> The European Food Safety Authority 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.<ref name="EFSA2010">[https://efsa.onlinelibrary.wiley.com/doi/epdf/10.2903/j.efsa.2010.1732 Scientific Opinion on the substantiation of health claims related to chromium and contribution to normal macronutrient metabolism (ID 260, 401, 4665, 4666, 4667), maintenance of normal blood glucose concentrations (ID 262, 4667), contribution to the maintenance or achievement of a normal body weight (ID 339, 4665, 4666), and reduction of tiredness and fatigue (ID 261) pursuant to Article 13(1) of Regulation (EC) No 1924/2006] {{Webarchive|url=https://web.archive.org/web/20200421163705/https://efsa.onlinelibrary.wiley.com/doi/epdf/10.2903/j.efsa.2010.1732 |date=21 April 2020 }} European Food Safety Authority EFSA J 2010;8(10)1732.</ref> 

However, in a 2014 reassessment of studies to determine whether a Dietary Reference Intake value could be established for chromium, EFSA stated:<ref name=efsa/>

:''"The Panel concludes that no Average Requirement and no Population Reference Intake for chromium for the performance of physiological functions can be defined."'' and

:''"The Panel considered that there is no evidence of beneficial effects associated with chromium intake in healthy subjects. The Panel concludes that the setting of an Adequate Intake for chromium is also not appropriate."''

====Diabetes====
Given the evidence for chromium deficiency causing problems with glucose management in the context of intravenous nutrition products formulated without chromium,<ref name="Stehle2016">{{cite journal|last1 = Stehle|first1 = P|last2 = Stoffel-Wagner|first2 = B|last3 = Kuh|first3 = KS|title = Parenteral trace element provision: recent clinical research and practical conclusions|journal = European Journal of Clinical Nutrition|volume = 70|issue = 8|pages = 886–893|date = 6 April 2014|pmid = 27049031|pmc = 5399133|doi = 10.1038/ejcn.2016.53}}</ref> research interest turned to whether chromium supplementation would benefit people who have type&nbsp;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 and a non-significant trend in lower [[Glycated hemoglobin|hemoglobin A1C]].<ref name="Mauro">{{cite journal|vauthors=San Mauro-Martin I, Ruiz-León AM, Camina-Martín MA, Garicano-Vilar E, Collado-Yurrita L, Mateo-Silleras B, Redondo P|display-authors=3 |title=[Chromium supplementation in patients with type 2 diabetes and high risk of type 2 diabetes: a meta-analysis of randomized controlled trials] |language = es|journal = Nutr Hosp|volume = 33|issue = 1|page = 27|date = 2016|pmid = 27019254|doi = 10.20960/nh.v33i1.27|doi-broken-date=3 May 2025 }}</ref> A second reported the same,<ref name="Abdoll2013">{{cite journal|last1 = Abdollahi|first1 = M|last2 = Farshchi|first2 = A|last3 = Nikfar|first3 = S|last4 = Seyedifar|first4 = M|title = Effect of chromium on glucose and lipid profiles in patients with type 2 diabetes; a meta-analysis review of randomized trials|journal = J Pharm Pharm Sci|volume = 16|issue = 1|pages = 99–114|date = 2013|pmid = 23683609|doi = 10.18433/J3G022|doi-access = free}}</ref> a third reported significant decreases for both measures,<ref name="Suk2014">{{cite journal |last1 = Suksomboon|first1 = N|last2 = Poolsup|first2 = N|last3 = Yuwanakorn|first3 = A|title = Systematic review and meta-analysis of the efficacy and safety of chromium supplementation in diabetes|journal = J Clin Pharm Ther|volume = 39|issue = 3|pages = 292–306|date = 17 March 2013|pmid = 24635480|doi = 10.1111/jcpt.12147 |s2cid = 22326435|doi-access = free}}</ref> while a fourth reported no benefit for either.<ref>{{cite journal |last1 = Bailey|first1 = Christopher H|title = Improved meta-analytic methods show no effect of chromium supplements on fasting glucose|journal = Biol Trace Elem Res|volume = 157|issue = 1|pages = 1–8|date = January 2014|pmid = 24293356|doi = 10.1007/s12011-013-9863-9 | bibcode=2014BTER..157....1B |s2cid = 2441511}}</ref> A review published in 2016 listed 53 [[randomized clinical trial]]s that were included in one or more of six [[meta-analysis|meta-analyses]]. It concluded that whereas there may be modest decreases in fasting blood glucose 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.<ref name="Costello2016">{{cite journal|last1 = Costello|first1 = Rebecca B|last2 = Dwyer|first2 = Johanna T|last3 = Bailey|first3 = Regan L|title = Chromium supplements for glycemic control in type 2 diabetes: limited evidence of effectiveness|journal = Nutrition Reviews|volume = 74|issue = 7|pages = 455–468|date = 30 May 2016|pmid = 27261273|pmc = 5009459|doi = 10.1093/nutrit/nuw011}}</ref>

====Body weight====
Two [[systematic review]]s looked at chromium supplements as a mean of managing body weight in overweight and obese people. One, limited to [[chromium picolinate]], a common supplement ingredient, reported a statistically significant −1.1&nbsp;kg (2.4&nbsp;lb) weight loss in trials longer than 12 weeks.<ref name="Tian2013">{{cite journal |last1 = Tian|first1 = Honglian|last2 = Guo|first2 = Xiaohu|last3 = Wang|first3 = Xiyu|last4 = He|first4 = Zhiyun|last5 = Sun|first5 = Rao|last6 = Ge|first6 = Sai|last7 = Zhang|first7 = Zongjiu|title = Chromium picolinate supplementation for overweight or obese adults|journal = Cochrane Database Syst Rev |issue = 11|pages = CD010063|date = 2013| volume=2013 |pmid = 24293292 |doi=10.1002/14651858.CD010063.pub2 |pmc = 7433292}}</ref> The other included all chromium compounds and reported a statistically significant −0.50&nbsp;kg (1.1&nbsp;lb) weight change.<ref name = Onakpoya2013>{{cite journal|last1 = Onakpoya|first1 = I|last2 = Posadzki|first2 = P|last3 = Ernst|first3 = E|title = Chromium supplementation in overweight and obesity: a systematic review and meta-analysis of randomized clinical trials|journal = Obes Rev|volume = 14|issue = 6|pages = 496–507|date = 2013|pmid = 23495911|doi = 10.1111/obr.12026 |s2cid = 21832321}}</ref> 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.<ref name="Tian2013" /><ref name="Onakpoya2013" /> The European Food Safety Authority reviewed the literature and concluded that there was insufficient evidence to support a claim.<ref name=efsa/>

====Sports====
Chromium is promoted as a sports performance dietary supplement, based on the theory that it potentiates insulin activity, with anticipated results of increased muscle mass, and faster recovery of glycogen storage during post-exercise recovery.<ref name="Vincent2010" /><ref>{{cite journal |vauthors=Lefavi RG, Anderson RA, Keith RE, Wilson GD, McMillan JL, Stone MH |title=Efficacy of chromium supplementation in athletes: emphasis on anabolism |journal=International Journal of Sport Nutrition |volume=2 |issue=2 |pages=111–122 |date=1992 |pmid=1299487 |doi= 10.1123/ijsn.2.2.111}}</ref><ref name="Vinvent2003">{{cite journal |vauthors=Vincent JB |title=The potential value and toxicity of chromium picolinate as a nutritional supplement, weight loss agent and muscle development agent |journal=Sports Med |volume=33 |issue=3 |pages=213–230 |date=2003 |pmid=12656641 |doi= 10.2165/00007256-200333030-00004 |s2cid=9981172 }}</ref> A review of clinical trials reported that chromium supplementation did not improve exercise performance or increase muscle strength.<ref>{{cite journal |vauthors=Jenkinson DM, Harbert AJ |title=Supplements and sports |journal=Am Fam Physician |volume=78 |issue=9 |pages=1039–1046 |date=2008 |pmid=19007050 }}</ref> 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.<ref>{{cite journal |vauthors=Maughan RJ, Burke LM, et al. |title=IOC Consensus Statement: Dietary Supplements and the High-Performance Athlete |journal=International Journal of Sport Nutrition and Exercise Metabolism |volume=28 |issue=2 |pages=104–125 |date=2018 |pmid=29589768 |pmc=5867441 |doi=10.1123/ijsnem.2018-0020 }}</ref>

=== Fresh-water fish ===
Irrigation water standards for chromium are 0.1&nbsp;mg/L, but some rivers in [[Bangladesh]] are more than five times that amount. The standard for fish for human consumption is less than 1&nbsp;mg/kg, but many tested samples were more than five times that amount.<ref name="Islam2018">{{cite journal |vauthors=Islam MM, Karim MR, Zheng X, Li X |title=Heavy Metal and Metalloid Pollution of Soil, Water and Foods in Bangladesh: A Critical Review |journal=International Journal of Environmental Research and Public Health |volume=15 |issue=12 |page= 2825|date=2018 |pmid=30544988 |pmc=6313774 |doi=10.3390/ijerph15122825 |doi-access=free }}</ref> 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.<ref name="Bakshi2018">{{cite journal |vauthors=Bakshi A, Panigrahi AK |title=A comprehensive review on chromium induced alterations in fresh water fishes |journal=Toxicol Rep |volume=5 |pages=440–447 |date=2018 |pmid=29854615 |pmc=5977408 |doi=10.1016/j.toxrep.2018.03.007 |bibcode=2018ToxR....5..440B }}</ref>

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.<ref name="Bakshi2018" />

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&nbsp;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&nbsp;μg/kg the weight gain was no better than with the zero chromium diet, and there were increased DNA strand breaks.<ref>{{cite journal |vauthors=Ahmed AR, Jha AN, Davies SJ |title=The efficacy of chromium as a growth enhancer for mirror carp (Cyprinus carpio L): an integrated study using biochemical, genetic, and histological responses |journal=Biol Trace Elem Res |volume=148 |issue=2 |pages=187–197 |date=2012 |pmid=22351105 |doi=10.1007/s12011-012-9354-4 |bibcode=2012BTER..148..187A |s2cid=16154712 }}</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|first1 = Donald G|last1 = Barceloux|journal = Clinical Toxicology|volume = 37|issue = 2|pages = 173–194|date = 1999|doi = 10.1081/CLT-100102418|last2 = Barceloux|first2 = Donald|pmid = 10382554}}</ref> Because of the specific [[Transport protein|transport]] mechanisms, only limited amounts of chromium(III) enter the cells. Acute oral toxicity ranges between 50 and 150&nbsp;mg/kg.<ref name="Katz">{{cite journal |title = The toxicology of chromium with respect to its chemical speciation: A review |first1 = SA|last1 = Katz|journal = Journal of Applied Toxicology|volume = 13|issue = 3|pages = 217–224|date = 1992|doi = 10.1002/jat.2550130314|pmid = 8326093|last2 = Salem|first2 = H|s2cid = 31117557}}</ref> A 2008 review suggested that moderate uptake of chromium(III) through dietary supplements poses no genetic-toxic risk.<ref name="Eastmond">{{cite journal
|last1 = Eastmond |first1 = DA |date = 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|s2cid = 21033504 }}</ref> 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&nbsp;mg/m<sup>3</sup>. The [[National Institute for Occupational Safety and Health]] (NIOSH) has set a [[recommended exposure limit]] (REL) of 0.5&nbsp;mg/m<sup>3</sup>, time-weighted average. The [[IDLH]] (immediately dangerous to life and health) value is 250&nbsp;mg/m<sup>3</sup>.<ref>{{PGCH|0141}}</ref>

=== Chromium(VI) toxicity ===
The acute [[mouth|oral]] [[toxicity]] for [[Hexavalent chromium|chromium(VI)]] ranges between 1.5 and 3.3&nbsp;mg/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 [[Redox|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.<ref name="Dayan">{{cite journal |title = Mechanisms of chromium toxicity, carcinogenicity and allergenicity: Review of the literature from 1985 to 2000 |first1 = AD|last1 = Dayan |journal = Human & Experimental Toxicology |volume=20 |issue=9|pages = 439–451 |date=2001 |doi = 10.1191/096032701682693062 |last2 = Paine |first2 = AJ |s2cid = 31351037|pmid=11776406|doi-access = free | bibcode=2001HETox..20..439D }}</ref>

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.<ref>{{cite journal |title = A case of adeno-carcinoma of the left inferior turbinated body, and perforation of the nasal septum, in the person of a worker in chrome pigments |first = D.|last = Newman |journal = Glasgow Medical Journal |volume = 33|pages = 469–470 |date=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 = S |last = Langard |journal = American Journal of Industrial Medicine |volume = 17 |issue = 2|pages = 189–214 |date = 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 |first1 = MD|last1 = Cohen|journal = Critical Reviews in Toxicology |volume=23 |issue = 3|pages=255–281 |date=1993 |doi = 10.3109/10408449309105012 |last2 = Kargacin |first2 = B |last3 = Klein |first3 = CB |last4 = Costa |first4 = M |pmid = 8260068 }}</ref><ref>{{cite book|title=Methods to Develop Inhalation Cancer Risk Estimates for Chromium and Nickel Compounds|date=2011|publisher=U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Health and Environmental Impacts Division|location=Research Triangle Park, NC|url=https://purl.fdlp.gov/GPO/gpo56032|access-date=19 March 2015|archive-date=22 April 2021|archive-url=https://web.archive.org/web/20210422173119/https://permanent.fdlp.gov/gpo56032/ni_cr_methods_final_report_cover.pdf|url-status=live}}</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|last = Ngan|first = V|date = 2002|title = Chrome Allergy|url = http://dermnetnz.org/dermatitis/chrome-allergy.html|access-date = 7 June 2008|archive-date = 7 July 2016|archive-url = https://web.archive.org/web/20160707102208/http://www.dermnetnz.org/dermatitis/chrome-allergy.html|url-status = live}}</ref><ref>{{cite journal|title = Investigation of the threshold for allergic reactivity to chromium|last1 = Basketter|first1 = David|last2 = Horev|first2 = L|last3 = Slodovnik|first3 = D|last4 = Merimes|first4 = S|last5 = Trattner|first5 = A|last6 = Ingber|first6 = A|journal = Contact Dermatitis|volume = 44|issue = 2|pages = 70–74|date = 2000|doi = 10.1034/j.1600-0536.2001.440202.x|pmid = 11205406|s2cid = 45426346}}</ref>

=== Environmental issues ===
Because chromium compounds were used in [[dye]]s, [[paint]]s, and [[leather]] [[Tanning (leather)|tanning]] compounds, these compounds are often found in soil and [[groundwater]] at active and abandoned industrial sites, needing [[environmental cleanup]] and [[Environmental remediation|remediation]]. [[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|date=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 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.<ref>{{cite news|url = https://news.yahoo.com/s/afp/healthusenvironmentpollutionwater|archive-url = https://web.archive.org/web/20101223164916/http://news.yahoo.com/s/afp/healthusenvironmentpollutionwater|archive-date = 23 December 2010|work = [[Yahoo News]]|title = US water has large amounts of likely carcinogen: study|date = 19 December 2010|access-date = 19 December 2010}}</ref>

The more toxic hexavalent chromium form can be reduced to the less soluble trivalent oxidation state in soils by organic matter, ferrous iron, sulfides, and other reducing agents, with the rates of such reduction being faster under more acidic conditions than under more alkaline ones.  In contrast, trivalent chromium can be oxidized to hexavalent chromium in soils by manganese oxides, such as Mn(III) and Mn(IV) compounds. Since the solubility and toxicity of chromium (VI) are greater than those of chromium (III), the oxidation-reduction conversions between the two oxidation states have implications for movement and bioavailability of chromium in soils, groundwater, and plants.<ref>{{Cite journal|last=James|first=Bruce|date=1996|title=The challenge of remediating chromium-contaminated soil|journal=Environmental Science and Technology|volume=30|issue=6|pages=248A–251A|doi=10.1021/es962269h|pmid=21648723}}</ref>
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== Notes ==
<references group="note" />

== References ==
{{reflist}}

== General bibliography ==
* {{Greenwood&Earnshaw2nd}}

== External links ==
{{Commons}}
{{Wiktionary|chromium}}
* [https://www.atsdr.cdc.gov/csem/chromium/cover-page.html ATSDR Case Studies in Environmental Medicine: Chromium Toxicity] U.S. [[Department of Health and Human Services]]
* [https://web.archive.org/web/20040701090041/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.merck.com/mmpe/sec01/ch005/ch005b.html The Merck Manual – Mineral Deficiency and Toxicity]
* [https://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)
* {{cite EB1911|wstitle=Chromium|volume=6|pages=296–298|short=1}}

{{Periodic table (navbox)}}
{{Chromium compounds}}
{{Authority control}}
{{Good article}}

[[Category:Chromium| ]]
[[Category:Chemical elements]]
[[Category:Dietary minerals]]
[[Category:Native element minerals]]
[[Category:Chemical hazards]]
[[Category:Chemical elements with body-centered cubic structure]]