Chromate and dichromate

Chromate and dichromate
Chromate-2D-dimensions.png
The structure and bonding of the dichromate ion
Ball-and-stick model of the chromate anion
Space-filling model of the dichromate anion
Names
Systematic IUPAC name
Chromate and dichromate
Identifiers
Properties
CrO2−
4
and Cr
2
O2−
7
Molar mass115.994 g mol−1 and 215.988 g mol−1
Conjugate acidChromic acid
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
2
O2−
7
. They are oxoanions of chromium in the 6+ oxidation state and are moderately strong oxidizing agents. In an aqueous solution, chromate and dichromate ions can be interconvertible.

Chemical properties

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.[1]

Acid–base properties

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

CrO2−
4
+ 2 H+Cr
2
O2−
7
+ H2O

The predominance diagram shows that the position of the equilibrium depends on both pH and the analytical concentration of chromium.[notes 1] 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
3
O2−
10
, and tetrachromates, Cr
4
O2−
13
. All polyoxyanions of chromium(VI) have structures made up of tetrahedral CrO4 units sharing corners.[2]

The hydrogen chromate ion, HCrO4, is a weak acid:

HCrO
4
CrO2−
4
+ H+;      pKa ≈ 5.9

It is also in equilibrium with the dichromate ion:

HCrO
4
Cr
2
O2−
7
+ H2O

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

H2CrO4HCrO
4
+ H+

is not well characterized. Reported values vary between about −0.8 and 1.6.[3]

The dichromate ion is a somewhat weaker base than the chromate ion:[4]

HCr
2
O
7
Cr
2
O2−
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
2
O2−
7
+ 14 H+ + 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.[5]

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