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| Systematic IUPAC name Chromate and dichromate | |||
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| Properties | |||
| CrO2−4 andCr2O2−7 | |||
| 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 theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Chromate salts contain the chromate anion,CrO2−4.Dichromate salts contain the dichromate anion,Cr2O2−7. They areoxyanions ofchromium in the +6oxidation state and are moderately strongoxidizing agents. In anaqueoussolution, chromate and dichromate ions can be interconvertible.
Chromates react withhydrogen peroxide, giving products in whichperoxide,O2−2, replaces one or more oxygen atoms. In acid solution the unstable blue peroxo complexChromium(VI) oxide peroxide,CrO(O2)2, is formed; it is an unchargedcovalent molecule, which may be extracted intoether. Addition ofpyridine results in the formation of the more stable complexCrO(O2)2(pyridine).[1]
In aqueous solution, chromate and dichromate anions exist in achemical equilibrium.
Thepredominance diagram shows that the position of the equilibrium depends on bothpH 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 oftrichromates,Cr3O2−10, andtetrachromates,Cr4O2−13.[2] All polyoxyanions of chromium(VI) have structures made up of tetrahedralCrO4 units sharing corners.[3]
The hydrogen chromate ion,HCrO−4, is aweak acid:
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 molecularchromic acid,H2CrO4, but thepKa for the equilibrium
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]
The pKa value for this reaction shows that it can be ignored at pH > 4.
The chromate and dichromate ions are fairly strongoxidizing agents. Commonly three electrons are added to a chromium atom,reducing it to oxidation state +3. In acid solution theaquatedCr3+ ion is produced.
In alkaline solutionchromium(III) hydroxide is produced. Theredox potential shows that chromates are weakeroxidizing agent inalkaline solution than inacid solution.[6]
Approximately 136,000 tonnes (150,000 tons) ofhexavalent chromium, mainly sodium dichromate, were produced in 1985.[8] Chromates and dichromates are used inchrome plating to protect metals from corrosion and to improve paint adhesion. Chromate and dichromate salts ofheavy metals,lanthanides andalkaline earth metals are only very slightly soluble in water and are thus used as pigments. The lead-containing pigmentchrome yellow was used for a very long time before environmental regulations discouraged its use.[7] When used as oxidizing agents ortitrants in aredoxchemical reaction, chromates and dichromates convert into trivalent chromium,Cr3+, salts of which typically have a distinctively different blue-green color.[8]
The primary chromium ore is the mixed metal oxidechromite,FeCr2O4, found as brittle metallic black crystals or granules. Chromite ore is heated with a mixture ofcalcium carbonate andsodium carbonate in the presence of air. The chromium is oxidized to the hexavalent form, while the iron forms iron(III) oxide,Fe2O3:
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.Crocoite,PbCrO4, 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 theAtacama Desert. Among them islópezite – the only known dichromate mineral.[10]
As chromate isisostructural tosulfate, sulfate and chromate minerals can formsolid solutions such ashashemite, and chromate minerals are often listed alongsidesulfate minerals in mineral classification schemes such asNickel-Strunz classification.
Hexavalent chromium compounds can betoxic andcarcinogenic (IARC Group 1). Inhaling particles of hexavalent chromium compounds can causelung cancer. Also positive associations have been observed between exposure tochromium (VI) compounds andcancer of thenose andnasal 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 theRestriction of Hazardous Substances (RoHS) Directive (2002/95/EC).
There issufficient evidence in humans for thecarcinogenicity 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 issufficient evidence in experimental animals for the carcinogenicity of chromium (VI) compounds. Chromium (VI) compounds arecarcinogenic to humans (Group 1).
