Chromate

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Chromates and dichromates are salts of chromic acid and dichromic acid, respectively. Chromate salts contain the chromate ion, CrO₄²⁻, and have an intense yellow color. Dichromate salts contain the dichromate ion, Cr₂O₇²⁻, and have an intense orange color.

Characteristics

• The chromium atoms are in oxidation state +6 in both, and the chromate and dichromate ions are fairly strong oxidizing agents. Chromium in the +6 (or VI) oxidation state is often referred to as hexavalent chromium.
• In an aqueous solution, chromate and dichromate anions are in a chemical equilibrium.

2 CrO₄²⁻ + 2 H₃O⁺ ⇌ Cr₂O₇²⁻ + 3 H₂O

This equilibrium can be pushed towards 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.

• They are used in environmental analysis to measure chemical oxygen demand (COD).
• They are carcinogenic. All hexavalent chromium compounds are considered toxic and carcinogenic.
• When used as oxidizing agents or titrants in a redox chemical reaction, they will turn into trivalent chromium, Cr³⁺, which has a distinctively different blue-green color.
• The sodium (Na⁺), potassium (K⁺), and ammonium (NH₄⁺) salts are water soluble granular solids and are the most commonly used chromate or dichromate chemical reagents. Most chromate and dichromate salts of heavy metals, lanthanides or alkaline earth metals are only very slightly soluble in water and are thus of much less usefulness.
• Chromate conversion coatings are applied to metals for corrosion protection, and to improve paint adhesion.
• 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

Structures

See also

• Potassium dichromate - K₂Cr₂O₇