![]() Natural occurrence and production Crocoite specimen from the Red Lead Mine, Tasmania, Australia When used as oxidizing agents or titrants in a redox chemical reaction, chromates and dichromates convert into trivalent chromium, Cr 3+, salts of which typically have a distinctively different blue-green color. The lead-containing pigment chrome yellow was used for a very long time before environmental regulations discouraged its use. 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. Chromates and dichromates are used in chrome plating to protect metals from corrosion and to improve paint adhesion. Ε 0 = −0.13 V Applications School bus painted in Chrome yellow Īpproximately 136,000 tonnes (150,000 tons) of hexavalent chromium, mainly sodium dichromate, were produced in 1985. The redox potential shows that chromates are weaker oxidizing agent in alkaline solution than in acid solution. In alkaline solution chromium(III) hydroxide is produced. Commonly three electrons are added to a chromium atom, reducing it to oxidation state +3. ![]() The chromate and dichromate ions are fairly strong oxidizing agents. The p K value for this reaction shows that it can be ignored at pH > 4. The dichromate ion is a somewhat weaker base than the chromate ion: HCr Reported values vary between about −0.8 and 1.6. The hydrogen chromate ion may be protonated, with the formation of molecular chromic acid, H 2CrO 4, but the p K a for the equilibrium The red line on the predominance diagram is not quite horizontal due to the simultaneous equilibrium with the chromate ion. This equilibrium does not involve a change in hydrogen ion concentration, which would predict that the equilibrium is independent of pH. It is also in equilibrium with the dichromate ion: The hydrogen chromate ion, HCrO 4 −, is a weak acid: All poly oxyanions of chromium(VI) have structures made up of tetrahedral CrO 4 units sharing corners. The chromate ion is the predominant species in alkaline solutions, but dichromate can become the predominant ion in acidic solutions.įurther condensation reactions can occur in strongly acidic solution with the formation of trichromates, Crġ3. ![]() The predominance diagram shows that the position of the equilibrium depends on both pH and the analytical concentration of chromium. In aqueous solution, chromate and dichromate anions exist in a chemical equilibrium. Acid–base properties Predominance diagram for chromate Addition of pyridine results in the formation of the more stable complex CrO(O 2) 2py. In acid solution the unstable blue peroxo complex Chromium(VI) oxide peroxide, CrO(O 2) 2, is formed it is an uncharged covalent molecule, which may be extracted into ether. Chromates react with hydrogen peroxide, giving products in which peroxide, O 2−Ģ, replaces one or more oxygen atoms. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |