Chromate

Chromate and dichromate

Names
Systematic IUPAC name Chromate and dichromate
Properties
Chemical formula	CrO2− 4 and Cr 2O2− 7
Molar mass	115.994 g mol−1 and 215.988 g mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Chromate salts contain the chromate anion, CrO2−
4. Dichromate salts contain the dichromate anion, Cr
2O2−
7. They are oxoanions of chromium in the oxidation state +6. They are moderately strong oxidizing agents. In an aqueous solution, chromate and dichromate ions can be interconvertible.

potassium chromate

potassium 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.

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

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.