Chromate and dichromate

Source: Wikipedia, the free encyclopedia.
This article is about the salts of the chromium(VI) anion. For other uses, see Chromate. See also for disambiguation of derived terms.
Not to be confused with
tetrachromat
.
Chromate and dichromate
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
3D model (
JSmol
)
ChEBI
DrugBank
UNII
  • chromate: InChI=1S/Cr.4O/q;;;2*-1
    Key: ZCDOYSPFYFSLEW-UHFFFAOYSA-N
  • dichromate: InChI=1S/2Cr.7O/q;;;;;;;2*-1
    Key: SOCTUWSJJQCPFX-UHFFFAOYSA-N
  • chromate: [O-][Cr](=O)(=O)[O-]
  • dichromate: O=[Cr](=O)([O-])O[Cr](=O)(=O)[O-]
Properties
CrO2−
4 and Cr
2O2−
7
Molar mass 115.994 g mol−1 and 215.988 g mol−1
Conjugate acid
 Chromic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Chromate salts contain the chromate anion, CrO2−
4. Dichromate salts contain the dichromate anion, Cr
2O2−
7. They are

aqueous solution
, chromate and dichromate ions can be interconvertible.

Chemical properties

Chromates react with

ether. Addition of pyridine results in the formation of the more stable complex CrO(O2)2py.[1]

Acid–base properties

Predominance diagram for chromate

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.[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
3O2−
10, and tetrachromates, Cr
4O2−
13.

oxyanions of chromium(VI) have structures made up of tetrahedral CrO4 units sharing corners.[3]

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:

2 HCrO−
4 ⇌ Cr
2O2−
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

H2CrO4 ⇌ HCrO−
4 + H+

is not well characterized. Reported values vary between about −0.8 and 1.6.[4]

The dichromate ion is a somewhat weaker base than the chromate ion:[5]

HCr
2O−
7 ⇌ Cr
2O2−
7 + H+,      pK = 1.18

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

reducing
it to oxidation state +3. In acid solution the aquated Cr3+ ion is produced.

Cr
2O2−
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.[6]

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

Applications

School bus painted in Chrome yellow[7]

Approximately 136,000 tonnes (150,000 tons) of hexavalent chromium, mainly sodium dichromate, were produced in 1985.[8] 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.[7] 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.[8]

Natural occurrence and production

Crocoite specimen from the Red Lead Mine, Tasmania, Australia

The primary chromium ore is the mixed metal oxide chromite, FeCr2O4, found as brittle metallic black crystals or granules. 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

Subsequent leaching of this material at higher temperatures dissolves the chromates, leaving a residue of 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.[9]

Chromate containing minerals are rare.

Atacama desert. Among them is lópezite – the only known dichromate mineral.[10]

Toxicity

Further information: Hexavalent chromium § Toxicity

nasal sinuses.[11] 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)
.

See also

Notes

  1. ^ pCr is equal to the negative of the decimal logarithm of the molar concentration of chromium. Thus, when pCr = 2, the chromium concentration is 10−2 mol/L.

References

  1. ISBN 978-0-08-037941-8
    .
  2. S2CID 231808339
    .
  3. ISBN 978-0-08-037941-8
    .
  4. ^ IUPAC SC-Database. A comprehensive database of published data on equilibrium constants of metal complexes and ligands.
  5. doi:10.1016/S0277-5387(97)00128-9
    .
  6. ISBN 0-12-352651-5
    .
  7. ^
    ISBN 978-0-87179-752-0
    .
  8. ^
    ISBN 3527306730
    .
  9. ISBN 978-0-87335-233-8
    .
  10. ^ "Mines, Minerals and More". www.mindat.org.[page needed]
  11. carcinogenicity
    of chromium (VI) compounds. Chromium (VI) compounds cause cancer of the lung. Also positive associations have been observed between exposure to chromium (VI) compounds and cancer of the nose and nasal sinuses. There is sufficient evidence in experimental animals for the carcinogenicity of chromium (VI) compounds. Chromium (VI) compounds are carcinogenic to humans (Group 1).

External links

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