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'{{about|chromium(VI) cation|chromium(III) cation|chromite (compound)}} [[File:Potassium-chromate-sample.jpg|thumb|200px|A sample of [[potassium chromate]]]] [[File:Potassium-dichromate-sample.jpg|thumb|200px|A sample of [[potassium dichromate]]]] '''Chromate''' salts contain the chromate anion, CrO₄^2&minus;. '''Dichromate''' salts contain the dichromate anion, Cr₂O₇^2&minus;. They are [[oxoanion]]s of [[chromium]] in the [[oxidation state]] +6. They are moderately strong [[oxidizing agent]]s. == Chemical properties == Chromates react with [[hydrogen peroxide]] giving products in which [[peroxide]], O₂^2&minus;, replaces one or more oxygen atoms. In acid solution the unstable blue peroxo complex [[Chromium(VI) oxide peroxide]], CrO(O₂)₂, is formed; it is an uncharged [[covalent]] molecule which may be extracted into [[ether (chemistry)|ether]]. Addition of [[pyridine]], results in the formation of the more stable complex CrO(O₂)₂py.<ref>{{Greenwood&Earnshaw2nd|page=637}}</ref> === Acid-base properties === [[File:Chromate-2D-dimensions.png|thumb|left|120px|chromate ion]] [[File:Dichromate-2D-dimensions.png|thumb|200px| dichromate ion]] [[File:Predominance diagram Cr.png|thumb|200px|[[Predominance diagram]] for chromate]] In [[aqueous]] [[Solution (chemistry)|solution]], chromate and dichromate anions exist in a [[chemical equilibrium]]. :2 CrO₄^2&minus; + 2 H⁺ {{eqm}} Cr₂O₇^2&minus; + H₂O The [[predominance diagram]] shows that the position of the equilibrium depends on both [[pH]] and the analytical concentration of chromium.<ref group=notes>pCr is equal to minus the logarithm of the analytical concentration of chromium. Thus, when pCr=2, the chromium concentration is 10^-2 mol dm^-3</ref> The chromate ion is the predominant species in alkaline solutions, but dichromate can become the predominant ion in acidic solutions. The change in colour with pH from yellow (chromate) to orange (dichromate) and the reversible nature of the equilibrium have been beautifully [http://www.youtube.com/watch?v=zP9qEiaL4kQ illustrated] Further condensation reactions can occur in strongly acidic solution with the formation of trichromates, Cr₃O₁₀^2&minus;, and tetrachromates, Cr₄O₁₃^2&minus;. All poly[[oxyanions]] of chromium(VI) have structures made up of tetrahedral CrO₄ units sharing corners.<ref>{{Greenwood&Earnshaw2nd|page = 1009}}</ref> The chromate ion is a [[weak base]]. :HCrO₄^&minus; {{eqm}} CrO₄^2&minus; + H⁺; pK_a = ca. 5.9 The hydrogen chromate ion, HCrO₄^-, is also in equilibrium with the dichromate ion. :2HCrO₄^&minus; {{eqm}} Cr₂O₇^2&minus; + H₂O This equilibrium does not involve a change in hydrogen ion concentration, so should be independent of pH. The red line on the predominance diagram is not quite horizontal due to the simultaneous equilibrium with the chromate ion. The hydrogenchromate ion may be protonated, with the formation of molecular [[chromic acid]], H₂CrO₄,but the [[acid dissociation constant|pK_a]] for the equilibrium :H₂CrO₄ {{eqm}} [HCrO₄]^&minus; + H⁺ is not well characterized. Reported values vary between about -0.8 to 1.6.<ref>[http://www.acadsoft.co.uk/scdbase/scdbase.htm IUPAC SC-Database] A comprehensive database of published data on equilibrium constants of metal complexes and ligands</ref> The dichromate ion is a somewhat weaker base than the chromate ion. :[HCr₂O₇]^&minus; {{eqm}} [Cr₂O₇]^2&minus; + H⁺, pK = 1.8<ref>{{cite journal|last=Brito|first=F.|coauthors=Ascanioa, J.; Mateoa, S.; Hernándeza, C.; Araujoa, L.; Gili, P.; Martín-Zarzab, P.; Domínguez, S.; Mederos, A.|year=1997|title=Equilibria of chromate(VI) species in acid medium and ab initio studies of these species |journal=Polyhedron|volume=16|issue=21|pages=3835–3846 |doi=10.1016/S0277-5387(97)00128-9}}</ref> The pK value for this reaction shows that is can be ignored at pH > 4. === Oxidation-reduction properties === The chromate and dichromate ions are fairly strong [[oxidizing agent]]s. Commonly three electrons are added to a chromium atom, [[reduction (chemistry)|reducing]] it to oxidation state +3. In acid solution the aquated Cr³⁺ ion is produced. :Cr₂O₇²⁻ + 14 H₃O⁺ + 6 e⁻ → 2 Cr³⁺ + 21 H₂O (ε₀ = 1.33 V) In alkaline solution chromium(III) hydroxide is produced. The [[redox potential]] shows that chromates are weaker oxidizing agent in alkaline solution than in acid solution.<ref name="HollemanAF">{{Holleman&Wiberg}}</ref> :CrO₄^2- + 4 {{chem|H|2|O}} + 3 {{chem|e|-}} → {{chem|Cr(OH)|3}} + 5 {{chem|OH|-}} (ε₀ = −0.13 V) ==Applications== [[File:Laidlaw school bus.jpg|thumb| right| School bus painted in [[Chrome yellow]]<ref name="Yellow">{{cite book | title = Toxic Substances Controls Guide: Federal Regulation of Chemicals in the Environment | first = Mary Devine | last = Worobec | coauthor = Hogue, Cheryl| page = 13 | publisher=BNA Books | year = 1992 | isbn = 978-0-87179-752-0 | url =http://books.google.de/books?id=CjWQ6_7AnI4C&pg=PA13}}</ref>]] Approximately {{convert|136000000|kg|lb}} of hexavalent chromium, mainly sodium dichromate, were produced in 1985.<ref name = Ullmann>{{cite book|last1=Anger|first1=Gerd|last2=Halstenberg|first2=Jost |last3=Hochgeschwender|first3=Klaus |coauthors=Scherhag, Christoph, Korallus, Ulrich; Knopf, Herbert; Schmidt, Peter; Ohlinger, Manfred.|title=Ullmann's Encyclopedia of Industrial Chemistry|year=2005|publisher=Wiley-VCH|location=Weinheim|isbn=10.1002/14356007.a07_067|chapter=Chromium Compounds}}</ref> Chromates and dichromates are used in [[chrome plating]] to protect metals from corrosion and to improve paint adhesion. Chromate and dichromate salts of [[heavy metals]], [[lanthanides]] and [[alkaline earth metals]] are only very slightly soluble in water and are thus used as pigments. The lead containing pigment [[Chrome Yellow]] was used for a very long time before environmental regulations discouraged its use.<ref name="Yellow"/> When used as oxidizing agents or titrants in a [[redox]] [[chemical reaction]], chromates and dichromates convert into trivalent chromium, Cr³⁺, salts of which typically have a distinctively different blue-green color.<ref name=Ullmann/> ==Natural occurrence and production== [[File:Crocoite from Tasmania.jpg|thumb|180px|Crocoite specimen from the Red Lead Mine, [[Tasmania]], [[Australia]]]] The primary chromium ore is the mixed metal oxide [[chromite]], FeCr₂O₄, found as brittle metallic black crystals or granules. The rare mineral [[crocoite]], PbCrO₄, occurs as spectacular long red crystals. Rare potassium chromate minerals and related compounds are found in the [[Atacama desert]]. 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 iron(III) oxide, Fe₂O₃. :4 FeCr₂O₄ + 8 Na₂CO₃ + 7 O₂ → 8 Na₂CrO₄ + 2Fe₂O₃ + 8 CO₂ The subsequent leaching at higher temperatures dissolves the chromates and leaves the insoluble iron oxide. Normally the chromate solution is further processed to make chromium metal, but a chromate salt may be obtained directly from the liquor.<ref name="IndMin">{{cite book | title =Industrial Minerals & Rocks: Commodities, Markets, and Uses | edition = 7th | publisher = SME | year = 2006 | isbn = 978-0-87335-233-8 | chapter = Chromite | first = John F. | last = Papp | coauthor = Lipin Bruce R. | url = http://books.google.de/books?id=zNicdkuulE4C&pg=PA309 }}</ref> ==Safety== All [[hexavalent chromium]] compounds are toxic due to their oxidizing power. They may be [[carcinogenic]], especially when air-borne. The use of chromate compounds in manufactured goods is restricted in the EU (and by market commonality the rest of the world) by EU Parliament directive 2002/95/EC == See also == *[[Chromate conversion coating]] == Notes == {{reflist|group=notes}} ==References== {{reflist}} == External links == *[http://www.npi.gov.au/substances/chromium-vi/index.html National Pollutant Inventory - Chromium VI and compounds fact sheet] [[Category:Chromates| ]] [[Category:Oxidizing agents]] [[Category:Oxoanions]] [[ca:Cromat]] [[de:Chromate]] [[es:Cromato]] [[fr:Dichromate]] [[it:Cromato]] [[he:כרומט]] [[nl:Chromaat en dichromaat]] [[ja:クロム酸塩]] [[pt:Cromato]] [[ru:Хроматы]] [[simple:Chromate]] [[fi:Kromaatti]] [[sv:Kromat]] [[zh:铬酸盐]]'