Chromate and dichromate

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.

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Ion

An ion (ˈaɪ.ɒn,_-ən) is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convention. The net charge of an ion is not zero because its total number of electrons is unequal to its total number of protons. A cation is a positively charged ion with fewer electrons than protons while an anion is a negatively charged ion with more electrons than protons.

Hexavalent chromium

Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is chromium in any chemical compound that contains the element in the +6 oxidation state (thus hexavalent). Virtually all chromium ore is processed via hexavalent chromium, specifically the salt sodium dichromate. Hexavalent chromium is key to all materials made from chromium. Approximately of hexavalent chromium were produced in 1985. Additional hexavalent chromium compounds include chromium trioxide and various salts of chromate and dichromate, among others.

Chromic acid

Chromic acid is an inorganic acid composed of the elements chromium, oxygen, and hydrogen. It is a dark, purplish red, odorless, sand-like solid powder. When dissolved in water, it is a strong acid. There are 2 types of chromic acid, they are: molecular chromic acid with the formula H2CrO4 and dichromic acid with the formula H2Cr2O7. The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide.

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