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Chromates and dichromates are salts of chromic acid and dichromic acid, respectively. Chromate salts contain the chromate anion, CrO42−, and have an intense yellow color. Dichromate salts contain the dichromate anion, Cr2O72−, and have an intense orange color.
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The chromium centers in both of these anions in oxidation state +6. The chromate and dichromate ions are fairly strong oxidizing agents. Chromate and dichromate anions exist in a chemical equilibrium.
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.
Chromate and dichromate salts are widely used in chemical industry, most significantly in the isolation of chromium from its ores.[1]
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.
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 turn into trivalent chromium, Cr3+, salts of which typically have a distinctively different blue-green color.[1]
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
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| The tetrahedral chromate ion, CrO42− | The dichromate ion, Cr2O72−, consists of two corner-sharing tetrahedra | ||
Chromate minerals are rarely found in the nature. The most commonly met is crocoite. Potassium-bearing chromates and related compounds are known from Atacama desert, but are very rare minerals.
