Chromate and dichromate
| |||
| |||
| Names | |||
|---|---|---|---|
| Systematic IUPAC name
Chromate and dichromate | |||
| Identifiers | |||
| |||
3D model (JSmol)
|
| ||
| ChEBI |
| ||
| DrugBank |
| ||
PubChemCID
|
|||
| UNII |
| ||
CompTox Dashboard(EPA)
|
| ||
| |||
| |||
| Properties | |||
| CrO2− 4andCr 2O2− 7 | |||
| Molar mass | 115.994 g mol−1and 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).
| |||
Chromatesalts contain the chromate anion,CrO2−
4.Dichromatesalts contain the dichromate anion,Cr
2O2−
7.They areoxyanionsofchromiumin the +6oxidation stateand are moderately strongoxidizing agents.In anaqueoussolution,chromate and dichromate ions can be interconvertible.
Chemical properties
[edit]
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 unchargedcovalentmolecule, which may be extracted intoether.Addition ofpyridineresults in the formation of the more stable complex CrO(O2)2py.[1]
Acid–base properties
[edit]
In aqueous solution, chromate and dichromate anions exist in achemical equilibrium.
- 2 CrO2−4+ 2 H+⇌ Cr2O2−7+ H2O
Thepredominance diagramshows that the position of the equilibrium depends on bothpHand 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,Cr
3O2−
10,andtetrachromates,Cr
4O2−
13.[2]All polyoxyanionsof chromium(VI) have structures made up of tetrahedral CrO4units sharing corners.[3]
The hydrogen chromate ion, HCrO4−,is aweak acid:
- HCrO−
4⇌CrO2−
4+ H+;pKa≈ 5.9
It is also in equilibrium with the dichromate ion:
- 2HCrO−
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 molecularchromic acid,H2CrO4,but thepKafor 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]
- HCr2O−7⇌ Cr2O2−7+ H+,pKa= 1.18
The pKavalue for this reaction shows that it can be ignored at pH > 4.
Oxidation–reduction properties
[edit]The chromate and dichromate ions are fairly strongoxidizing agents.Commonly three electrons are added to a chromium atom,reducingit to oxidation state +3. In acid solution the aquated Cr3+ion is produced.
- Cr
2O2−
7+ 14 H++ 6 e−→ 2 Cr3++ 7 H2Oε0= 1.33 V
In alkaline solution chromium(III) hydroxide is produced. Theredox potentialshows that chromates are weaker oxidizing agent in alkaline solution than in acid solution.[6]
- CrO2−
4+ 4H
2O+ 3 e−→Cr(OH)
3+ 5OH−
ε0= −0.13 V
Applications
[edit]
Approximately 136,000 tonnes (150,000 tons) ofhexavalent chromium,mainly sodium dichromate, were produced in 1985.[8]Chromates and dichromates are used inchrome platingto protect metals from corrosion and to improve paint adhesion. Chromate and dichromate salts ofheavy metals,lanthanidesandalkaline earth metalsare only very slightly soluble in water and are thus used as pigments. The lead-containing pigmentchrome yellowwas used for a very long time before environmental regulations discouraged its use.[7]When used as oxidizing agents or titrants in aredoxchemical 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
[edit]
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 carbonateandsodium carbonatein 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.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]
Toxicity
[edit]Hexavalent chromiumcompounds can betoxicandcarcinogenic(IARC Group 1). Inhaling particles of hexavalent chromium compounds can causelung cancer.Also positive associations have been observed between exposure tochromium (VI)compounds andcancerof thenoseandnasal 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).
See also
[edit]Notes
[edit]- ^pCr is equal to the negative of the decimal logarithm of themolar concentrationof chromium. Thus, when pCr = 2, the chromium concentration is 10−2mol/L.
References
[edit]- ^Greenwood, Norman N.;Earnshaw, Alan (1997).Chemistry of the Elements(2nd ed.).Butterworth-Heinemann.p. 637.ISBN978-0-08-037941-8.
- ^Nazarchuk, Evgeny V.; Siidra, Oleg I.; Charkin, Dmitry O.; Kalmykov, Stepan N.; Kotova, Elena L. (2021-02-01)."Effect of solution acidity on the crystallization of polychromates in uranyl-bearing systems: synthesis and crystal structures of Rb2[(UO2)(Cr2O7)(NO3)2] and two new polymorphs of Rb2Cr3O10".Zeitschrift für Kristallographie - Crystalline Materials.236(1–2): 11–21.doi:10.1515/zkri-2020-0078.ISSN2196-7105.S2CID231808339.
- ^Greenwood, Norman N.;Earnshaw, Alan (1997).Chemistry of the Elements(2nd ed.).Butterworth-Heinemann.p. 1009.ISBN978-0-08-037941-8.
- ^IUPAC SC-Database.A comprehensive database of published data on equilibrium constants of metal complexes and ligands.
- ^Brito, F.; Ascanioa, J.; Mateoa, S.; Hernándeza, C.; Araujoa, L.; Gili, P.; Martín-Zarzab, P.; Domínguez, S.; Mederos, A. (1997). "Equilibria of chromate(VI) species in acid medium and ab initio studies of these species".Polyhedron.16(21): 3835–3846.doi:10.1016/S0277-5387(97)00128-9.
- ^Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.),Inorganic Chemistry,translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter,ISBN0-12-352651-5.
- ^abWorobec, Mary Devine; Hogue, Cheryl (1992).Toxic Substances Controls Guide: Federal Regulation of Chemicals in the Environment.BNA Books. p. 13.ISBN978-0-87179-752-0.
- ^abAnger, Gerd; Halstenberg, Jost; Hochgeschwender, Klaus; Scherhag, Christoph; Korallus, Ulrich; Knopf, Herbert; Schmidt, Peter; Ohlinger, Manfred (2005). "Chromium Compounds".Ullmann's Encyclopedia of Industrial Chemistry.Weinheim: Wiley-VCH.doi:10.1002/14356007.a07_067.ISBN3527306730.
- ^Papp, John F.; Lipin Bruce R. (2006)."Chromite".Industrial Minerals & Rocks: Commodities, Markets, and Uses(7th ed.). SME.ISBN978-0-87335-233-8.
- ^"Mines, Minerals and More".mindat.org.[page needed]
- ^IARC(2012) [17–24 March 2009].Volume 100C: Arsenic, Metals, Fibres, and Dusts(PDF).Lyon: International Agency for Research on Cancer.ISBN978-92-832-0135-9.Archived fromthe original(PDF)on 2020-03-17.Retrieved2020-01-05.
There issufficient evidencein humans for thecarcinogenicityof 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 evidencein experimental animals for the carcinogenicity of chromium (VI) compounds. Chromium (VI) compounds arecarcinogenic to humans (Group 1).