Chromate and dichromate

Chromates react with hydrogen peroxide giving products in which peroxide, O2−
2, 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.

Acid–base properties

In aqueous solution, chromate and dichromate anions exist in a chemical equilibrium.

2 CrO2−
4 + 2 H⁺ ⇌ Cr
2O2−
7 + 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, Cr
3O2−
10, and tetrachromates, Cr
4O2−
13. All polyoxyanions of chromium(VI) have structures made up of tetrahedral CrO₄ units sharing corners.

The chromate ion is a weak acid.

HCrO−
4 ⇌ CrO2−
4 + H⁺;      pK_a ≈ 5.9

The hydrogen chromate ion, HCrO₄⁻, is also in equilibrium with the dichromate ion.

2 HCrO−
4 ⇌ Cr
2O2−
7 + H₂O

This equilibrium does not involve a change in hydrogen ion concentration, so should be independent of pH. The red line on the predominance diagram is not quite horizontal due to the simultaneous equilibrium with the chromate ion. The hydrogenchromate ion may be protonated, with the formation of molecular chromic acid, H₂CrO₄,but the pK_a for the equilibrium

H₂CrO₄ ⇌ HCrO−
4 + H⁺

is not well characterized. Reported values vary between about −0.8 to 1.6.

The dichromate ion is a somewhat weaker base than the chromate ion.

HCr
2O−
7 ⇌ Cr
2O2−
7 + H⁺,      pK = 1.8

The pK value for this reaction shows that it can be ignored at pH > 4.

Oxidation–reduction properties

The chromate and dichromate ions are fairly strong oxidizing agents. Commonly three electrons are added to a chromium atom, reducing it to oxidation state +3. In acid solution the aquated Cr³⁺ ion is produced.

Cr
2O2−
7 + 14 H₃O⁺ + 6 e⁻ → 2 Cr³⁺ + 7 H₂O      ε₀ = 1.33 V

In alkaline solution chromium(III) hydroxide is produced. The redox potential shows that chromates are weaker oxidizing agent in alkaline solution than in acid solution.

CrO2−
4 + 4 H
2O + 3 e⁻ → Cr(OH)
3 + 5 OH−
      ε₀ = −0.13 V

Applications

Approximately 136,000 tonnes (150,000 tons) of hexavalent chromium, mainly sodium dichromate, were produced in 1985. Chromates and dichromates are used in chrome plating to protect metals from corrosion and to improve paint adhesion. 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. 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.

Natural occurrence and production

The primary chromium ore is the mixed metal oxide chromite, FeCr₂O₄, found as brittle metallic black crystals or granules. The rare mineral crocoite, PbCrO₄, occurs as spectacular long red crystals. Rare potassium chromate minerals and related compounds are found in the Atacama desert.

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 iron(III) oxide, Fe₂O₃.

4 FeCr₂O₄ + 8 Na₂CO₃ + 7 O₂ → 8 Na₂CrO₄ + 2 Fe₂O₃ + 8 CO₂

The subsequent leaching at higher temperatures dissolves the chromates and leaves the insoluble iron oxide. Normally the chromate solution is further processed to make chromium metal, but a chromate salt may be obtained directly from the liquor.

Safety

All hexavalent chromium compounds are toxic due to their oxidizing power. They may be carcinogenic, especially when airborne. 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 on the Restriction of Hazardous Substances (RoHS) Directive (2002/95/EC).