Chromate salts contain the chromate anion, . Dichromate salts contain the dichromate anion, . They are of chromium in the +6 oxidation state and are moderately strong . In an aqueous solution, chromate and dichromate ions can be interconvertible.
Chemical properties
Potassium-chromate-sample.jpg|Potassium chromate
Potassium-dichromate-sample.jpg|Potassium dichromate
Chromates react with hydrogen peroxide, giving products in which
peroxide, , replaces one or more oxygen atoms. In acid solution the unstable blue peroxo complex Chromium(VI) oxide peroxide, , 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 .
Acid–base properties
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.
[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.]
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, , and tetrachromates, .
The hydrogen chromate ion, , is a weak acid:
- ; p Ka ≈ 5.9
It is also in equilibrium with the dichromate ion:
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, , but the p
Ka for the equilibrium
is not well characterized. Reported values vary between about −0.8 and 1.6.
[ IUPAC SC-Database. A comprehensive database of published data on equilibrium constants of metal complexes and ligands.]
The dichromate ion is a somewhat weaker base than the chromate ion:
- , p Ka = 1.18
The p
Ka value for this reaction shows that it can be ignored at pH > 4.
Oxidation–reduction properties
The chromate and dichromate ions are fairly strong
. Commonly three electrons are added to a chromium atom, reducing it to oxidation state +3. In acid solution the
aqueous solution ion is produced.
- 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.
[.]
- ε0 = −0.13 V
Applications
Approximately 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,
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
Titration in a
redox chemical reaction, chromates and dichromates convert into trivalent chromium, , 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, , 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, :
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.
Chromate containing minerals are rare. Crocoite, , which can occur as spectacular long red crystals, is the most commonly found chromate mineral. Rare potassium chromate minerals and related compounds are found in the Atacama Desert. Among them is lópezite – the only known dichromate mineral.
As chromate is isostructural to sulfate, sulfate and chromate minerals can form such as hashemite, and chromate minerals are often listed alongside in mineral classification schemes such as Nickel-Strunz classification.
Toxicity
Hexavalent chromium compounds can be toxic and carcinogenic (IARC Group 1). Inhaling particles of hexavalent chromium compounds can cause lung cancer. Also positive associations have been observed between exposure to chromium (VI) compounds and cancer of the nose and nasal sinuses.
See also
-
Chromate conversion coating
Notes
External links
