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Concept
Chromate and dichromate
Summary
Chromate salts contain the chromate anion, CrO42−. Dichromate salts contain the dichromate anion, Cr2O72−. They are oxyanions of chromium in the +6 oxidation state and are moderately strong oxidizing agents. In an aqueous solution, chromate and dichromate ions can be interconvertible.
Potassium-chromate-sample.jpg|[[potassium chromate]]
Potassium-dichromate-sample.jpg|[[potassium dichromate]]
Chromates react with hydrogen peroxide, giving products in which peroxide, O22−, 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. Addition of pyridine results in the formation of the more stable complex CrO(O2)2py.
In aqueous solution, chromate and dichromate anions exist in a chemical equilibrium.
2 CrO42− + 2 H+ Cr2O72− + H2O
The predominance diagram shows that the position of the equilibrium depends on both pH and the analytical concentration of chromium. 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, Cr3O102−, and tetrachromates, Cr4O132−. All polyoxyanions of chromium(VI) have structures made up of tetrahedral CrO4 units sharing corners.
The hydrogen chromate ion, HCrO4−, is a weak acid:
HCrO4− CrO42− + H+; pKa ≈ 5.9
It is also in equilibrium with the dichromate ion:
2 HCrO4− Cr2O72− + 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, H2CrO4, but the pKa for the equilibrium
H2CrO4 HCrO4− + H+
is not well characterized. Reported values vary between about −0.8 and 1.6.
Official source
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