Editing Chromium (section)

==== 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.