Chromates and dichromates are salts of chromic acid and dichromic acid, respectively. Chromate salts contain the chromate anion, CrO 2−4, and usually have an intense yellow color. Dichromate salts contain the dichromate anion, Cr2O 2−7, and usually have an intense orange color. The chromates and dichromates of heavy metals (such as silver and lead) often have a red color.
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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.
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]
but only moderate ones at high pH:[1]
Chromate and dichromate salts are widely used in chemical industry, most significantly in the isolation of chromium from its ores.[2]
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]
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]
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]
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 nature. The most commonly met is crocoite. Potassium-bearing chromates and related compounds are known from the Atacama desert, but are very rare minerals.
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