Chromate and dichromate Information & Chromate and dichromate Links at HealthHaven.com

Chromates and dichromates are salts of chromic acid and dichromic acid, respectively. Chromate salts contain the chromate anion, Cr O2−4, and usually have an intense yellow color. Dichromate salts contain the dichromate anion, Cr₂O2−7, and usually have an intense orange color. The chromates and dichromates of heavy metals (such as silver and lead) often have a red color.

[edit] Characteristics

The chromium atoms in both of these anions are in oxidation state +6. The chromate and dichromate ions are fairly strong oxidizing agents. In an aqueous solution, chromate and dichromate anions exist in a chemical equilibrium.

2 CrO2−4 + 2 H₃O⁺ ⇌ Cr₂O2−7 + 3 H₂O

This equilibrium can be pushed toward dichromate by lowering the pH (making the solution more acidic) or in the other direction towards chromate by raising the pH to basic. This is a classic example of Le Chatelier's principle at work. This equilibrium is also dependent on concentration of chromium in solution. At lower pH further condensation to more complex chromates is possible. The chromate and dichromate are strong oxidizing reagents at low pH:^[1]

Cr₂O2−7 + 14 H₃O⁺ + 6 e⁻ → 2 Cr³⁺ + 21 H₂O (ε₀ = 1.33 V)

but only moderate ones at high pH:^[1]

CrO2−4 + 4 H₂O + 3 e⁻ → Cr(OH)₃ + 5 OH⁻ (ε₀ = −0.13 V)

[edit] Applications

Chromate and dichromate salts are widely used in chemical industry, most significantly in the isolation of chromium from its ores.^[2]

School bus painted in Chrome yellow^[3]

The sodium dichromate (Na₂Cr₂O₇) and potassium dichromate (K₂Cr₂O₇), as well as other derivatives are water soluble granular solids and are common reagents. 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.^[3]

Chromates and dichromates are used in chrome plating to protect metals for corrosion protection and to improve paint adhesion. 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.^[2]

[edit] Production

Chromates are an intermediate in the production of chromium metal. In the production of chromium from chromite ore (FeCr₂O₄) the iron has to be separated from the chromium in a two step roasting and leaching process. The chromite ore is heated with a mixture of calcium carbonate and sodium carbonate in the presence of air. The chromium is oxidized to the hexavalent form, while the iron forms the stable Fe₂O₃. The subsequent leaching at higher temperatures dissolves the chromates and leaves the unsoluble iron oxide. The chromate is converted by sulfuric acid into the dichromate.^[4]

4 FeCr₂O₄ + 8 Na₂CO₃ + 7 O₂ → 8 Na₂CrO₄ + 2 Fe₂O₃ + 8 CO₂

Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₄ + Na₂SO₄ + H₂O

To get chromium metal the dichromate is converted to the chromium(III) oxide by reduction with carbon and then reduced in an aluminothermic reaction to chromium.^[4]

[edit] Safety

Chromium in the +6 (or VI) oxidation state is often referred to as hexavalent chromium. Such compounds, especially when air-borne, are carcinogenic. All hexavalent chromium compounds are considered toxic. 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

[edit] Structures


The tetrahedral chromate ion, CrO₄²⁻		The dichromate ion, Cr₂O₇²⁻, consists of two corner-sharing tetrahedra

[edit] Natural occurrence

Crocoite specimen from the Red Lead Mine, Tasmania, Australia

Chromate minerals are rarely found in the nature. The most commonly met is crocoite. Potassium-bearing chromates and related compounds are known from the Atacama desert, but are very rare minerals.

[edit] Examples

Ammonium dichromate (NH₄)₂Cr₂O₇
Barium chromate BaCrO₄
Cadmium chromate CdCrO₄
Caesium chromate Cs₂CrO₄
Calcium chromate CaCr₂O₇
Cobalt(II) chromate CoCrO₄
Cobalt(II) dichromate CoCr₂O₇
Cobalt(III) chromate Co₂(CrO₄)₃
Cobalt(III) dichromate Co₂(Cr₂O₇)₃
Copper(I) chromate Cu₂CrO₄
Copper(I) dichromate Cu₂Cr₂O₇
Copper(II) chromate CuCrO₄
Copper(II) dichromate CuCr₂O₇
Iron(III) chromate Fe₂(CrO₄)₃
Iron(III) dichromate Fe₂(Cr₂O₇)₃
Lead(II) chromate PbCrO₄
Lead(II) dichromate PbCr₂O₇
Lead(IV) chromate Pb(CrO₄)₂
Lead(IV) dichromate Pb(Cr₂O₇)₂
Nickel(II) chromate NiCrO₄
Nickel(II) dichromate NiCr₂O₇
Nickel(III) chromate Ni₂(CrO₄)₃
Nickel(III) dichromate Ni₂(Cr₂O₇)₃
Potassium chromate K₂CrO₄
Potassium trioxochlorochromate KCrO₃Cl
Potassium dichromate K₂Cr₂O₇
Pyridinium chlorochromate
Silver chromate Ag₂CrO₄
Silver dichromate Ag₂Cr₂O₇
Sodium chromate Na₂CrO₄
Strontium chromate SrCrO₄
Tin(II) chromate SnCrO₄
Tin(II) dichromate SnCr₂O₇
Tin(IV) chromate Sn(CrO₄)₂
Tin(IV) dichromate Sn(Cr₂O₇)₂
Zinc chromate ZnCrO₄
Zinc dichromate ZnCr₂O₇

[edit] See also

Chromate conversion coating

[edit] References

^ ^a ^b Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils; (1985). "Chromium" (in German). Lehrbuch der Anorganischen Chemie (91–100 ed.). Walter de Gruyter. pp. 1081–1095. ISBN 3110075113.
^ ^a ^b Gerd Anger, Jost Halstenberg, Klaus Hochgeschwender, Christoph Scherhag, Ulrich Korallus, Herbert Knopf, Peter Schmidt, Manfred Ohlinger, "Chromium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.
^ ^a ^b Worobec, Mary Devine; Hogue, Cheryl (1992). Toxic Substances Controls Guide: Federal Regulation of Chemicals in the Environment. p. 13. ISBN 9780871797520. http://books.google.de/books?id=CjWQ6_7AnI4C&pg=PA13.
^ ^a ^b Papp, John F.; Lipin Bruce R. (2006). "Chromite". Industrial Minerals & Rocks: Commodities, Markets, and Uses (7th ed.). SME. ISBN 9780873352338. http://books.google.de/books?id=zNicdkuulE4C&pg=PA309.

[edit] External links

[HollemanAF-0] Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils; (1985). "Chromium" (in German). Lehrbuch der Anorganischen Chemie (91–100 ed.). Walter de Gruyter. pp. 1081–1095. ISBN 3110075113.

[Ullmann-1] Gerd Anger, Jost Halstenberg, Klaus Hochgeschwender, Christoph Scherhag, Ulrich Korallus, Herbert Knopf, Peter Schmidt, Manfred Ohlinger, "Chromium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.

[Yellow-2] Worobec, Mary Devine; Hogue, Cheryl (1992). Toxic Substances Controls Guide: Federal Regulation of Chemicals in the Environment. p. 13. ISBN 9780871797520. http://books.google.de/books?id=CjWQ6_7AnI4C&pg=PA13.

[IndMin-3] Papp, John F.; Lipin Bruce R. (2006). "Chromite". Industrial Minerals & Rocks: Commodities, Markets, and Uses (7th ed.). SME. ISBN 9780873352338. http://books.google.de/books?id=zNicdkuulE4C&pg=PA309.

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