Chromate and dichromate

Potassium-chromate-sample.jpg|Potassium chromate

Potassium-dichromate-sample.jpg|Potassium dichromate

Chromates react with hydrogen peroxide, giving products in which peroxide, {{chem2|O2(2−)}}, replaces one or more oxygen atoms. In acid solution the unstable blue peroxo complex Chromium(VI) oxide peroxide, {{Chem2|CrO(O2)2}}, is formed; it is an uncharged covalent molecule, which may be extracted into ether. Addition of pyridine results in the formation of the more stable complex {{Chem2|CrO(O2)2(pyridine)}}.{{Greenwood&Earnshaw2nd|page=637}}

In aqueous solution, chromate and dichromate anions exist in a chemical equilibrium.

:{{Chem2|2 CrO4(2-) + 2 H+ ⇌ Cr2O7(2-) + H2O}}

The predominance diagram shows that the position of the equilibrium depends on both pH and the analytical concentration of chromium.pCr is equal to the negative of the decimal logarithm of the molar concentration of chromium. Thus, when pCr = 2, the chromium concentration is 10⁻² mol/L.

File:Predominance diagram Cr.png for chromate]]

The chromate ion is the predominant species in alkaline solutions, but dichromate can become the predominant ion in acidic solutions.

Further condensation reactions can occur in strongly acidic solution with the formation of trichromates, {{chem2|Cr3O10(2−)}}, and tetrachromates, {{chem2|Cr4O13(2−)}}.{{Cite journal |last1=Nazarchuk |first1=Evgeny V. |last2=Siidra |first2=Oleg I. |last3=Charkin |first3=Dmitry O. |last4=Kalmykov |first4=Stepan N. |last5=Kotova |first5=Elena L. |date=2021-02-01 |title=Effect of solution acidity on the crystallization of polychromates in uranyl-bearing systems: synthesis and crystal structures of Rb2[(UO2)(Cr2O7)(NO3)2] and two new polymorphs of Rb2Cr3O10 |url=https://www.degruyter.com/document/doi/10.1515/zkri-2020-0078/html |journal=Zeitschrift für Kristallographie - Crystalline Materials |language=en |volume=236 |issue=1–2 |pages=11–21 |doi=10.1515/zkri-2020-0078 |s2cid=231808339 |issn=2196-7105|url-access=subscription }} All polyoxyanions of chromium(VI) have structures made up of tetrahedral {{chem2|CrO4}} units sharing corners.{{Greenwood&Earnshaw2nd|page = 1009}}

The hydrogen chromate ion, {{chem2|HCrO4−}}, is a weak acid:

:{{chem2|HCrO4− ⇌ CrO4(2−) + H+}};{{spaces|5}} pK_a ≈ 5.9

It is also in equilibrium with the dichromate ion:

:{{chem2|2 HCrO4− ⇌ Cr2O7(2−) + H2O}}

This equilibrium does not involve a change in hydrogen ion concentration, which would predict that the equilibrium is independent of pH. The red line on the predominance diagram is not quite horizontal due to the simultaneous equilibrium with the chromate ion. The hydrogen chromate ion may be protonated, with the formation of molecular chromic acid, {{chem2|H2CrO4}}, but the pK_a for the equilibrium

:{{Chem2|H2CrO4 ⇌ HCrO4- + H+}}

is not well characterized. Reported values vary between about −0.8 and 1.6.[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.

The dichromate ion is a somewhat weaker base than the chromate ion:{{cite journal |last=Brito |first=F. |author2=Ascanioa, J. |author3=Mateoa, S. |author4=Hernándeza, C. |author5=Araujoa, L. |author6=Gili, P. |author7=Martín-Zarzab, P. |author8=Domínguez, S. |author9=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}}

:{{Chem2|HCr2O7- ⇌ Cr2O7(2-) + H+}},{{spaces|5}} pK_a = 1.18

The pK_a value for this reaction shows that it can be ignored at pH > 4.

The chromate and dichromate ions are fairly strong oxidizing agents. Commonly three electrons are added to a chromium atom, reducing it to oxidation state +3. In acid solution the aquated {{chem2|Cr(3+)}} ion is produced.

:{{chem2|Cr2O7(2−) + 14 H+ + 6 e− → 2 Cr(3+) + 7 H2O}}{{spaces|5}} 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.{{Holleman&Wiberg}}.

:{{chem2|CrO4(2−) + 4 H2O + 3 e− → Cr(OH)3 + 5 OH-}}{{spaces|5}} ε₀ = −0.13 V

File:Laidlaw school bus.jpg painted in Chrome yellow{{cite book | title = Toxic Substances Controls Guide: Federal Regulation of Chemicals in the Environment | first = Mary Devine | last = Worobec |author2=Hogue, Cheryl| page = 13 | publisher=BNA Books | year = 1992 | isbn = 978-0-87179-752-0 | url =https://books.google.com/books?id=CjWQ6_7AnI4C&pg=PA13}}]]

Approximately {{convert|136000|tonne}} of hexavalent chromium, mainly sodium dichromate, were produced in 1985.{{cite book |last1=Anger |first1=Gerd |last2=Halstenberg |first2=Jost |last3=Hochgeschwender |first3=Klaus |author4=Scherhag, Christoph |author5=Korallus, Ulrich |author6=Knopf, Herbert |author7=Schmidt, Peter |author8=Ohlinger, Manfred |title=Ullmann's Encyclopedia of Industrial Chemistry |year=2005 |publisher=Wiley-VCH |location=Weinheim |doi=10.1002/14356007.a07_067 |chapter=Chromium Compounds|isbn=3527306730}} 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. When used as oxidizing agents or titrants in a redox chemical reaction, chromates and dichromates convert into trivalent chromium, {{chem2|Cr(3+)}}, salts of which typically have a distinctively different blue-green color.

File:Crocoite from Tasmania.jpg, Australia]]

The primary chromium ore is the mixed metal oxide chromite, {{chem2|FeCr2O4}}, found as brittle metallic black crystals or granules. 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, {{chem2|Fe2O3}}:

:{{chem2|4 FeCr2O4 + 8 Na2CO3 + 7 O2 -> 8 Na2CrO4 + 2 Fe2O3 + 8 CO2}}

Subsequent leaching of this material at higher temperatures dissolves the chromates, leaving a residue of 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.{{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 |author2=Lipin Bruce R. | chapter-url = https://books.google.com/books?id=zNicdkuulE4C&pg=PA309}}

Chromate containing minerals are rare. Crocoite, {{chem2|PbCrO4}}, which can occur as spectacular long red crystals, is the most commonly found chromate mineral. Rare potassium chromate minerals and related compounds are found in the Atacama Desert. Among them is lópezite – the only known dichromate mineral.{{cite web |url=http://www.mindat.org |title=Mines, Minerals and More |website=www.mindat.org}}{{page needed|date=February 2020}}

As chromate is isostructural to sulfate, sulfate and chromate minerals can form solid solutions such as hashemite, and chromate minerals are often listed alongside sulfate minerals in mineral classification schemes such as Nickel-Strunz classification.

{{Further|Hexavalent chromium#Toxicity}}

Hexavalent chromium compounds can be toxic and carcinogenic (IARC Group 1). Inhaling particles of hexavalent chromium compounds can cause lung cancer. Also positive associations have been observed between exposure to chromium (VI) compounds and cancer of the nose and nasal sinuses.{{cite book

|author = IARC

|author-link = International Agency for Research on Cancer

|title = Volume 100C: Arsenic, Metals, Fibres, and Dusts

|orig-year = 17–24 March 2009

|url = https://publications.iarc.fr/_publications/media/download/3026/50ed50733f7d1152d91b30a803619022ef098d59.pdf

|access-date = 2020-01-05

|date = 2012

|isbn = 978-92-832-0135-9

|quote = There is sufficient evidence in humans for the carcinogenicity of chromium (VI) compounds. Chromium (VI) compounds cause cancer of the lung. Also positive associations have been observed between exposure to chromium (VI) compounds and cancer of the nose and nasal sinuses. There is sufficient evidence in experimental animals for the carcinogenicity of chromium (VI) compounds. Chromium (VI) compounds are carcinogenic to humans (Group 1).

|publisher = International Agency for Research on Cancer

|location = Lyon

|archive-date = 2020-03-17

|archive-url = https://web.archive.org/web/20200317095517/https://publications.iarc.fr/_publications/media/download/3026/50ed50733f7d1152d91b30a803619022ef098d59.pdf

|url-status = dead

}} 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 on the Restriction of Hazardous Substances (RoHS) Directive (2002/95/EC).

• Chromate conversion coating

{{Reflist|group=notes}}

{{reflist|30em}}

• [http://www.npi.gov.au/substances/chromium-vi/index.html National Pollutant Inventory - Chromium(VI) and compounds fact sheet]
• [https://www.youtube.com/watch?v=zP9qEiaL4kQ Demonstration of chromate-dichromate equilibrium]

{{Chromium compounds}}

{{Chromates and dichromates}}

Category:Oxidizing agents

Category:Transition metal oxyanions

Category:Oxometallates