Chromate 577881 222435082 2008-06-29T09:41:15Z 60.242.48.83 [[Image:Potassium-chromate-sample.jpg|thumb|200px|A sample of [[potassium chromate]]]] [[Image:Potassium-dichromate-sample.jpg|thumb|200px|A sample of [[potassium dichromate]]]] '''Chromates''' and '''dichromates''' are [[salt]]s of [[chromic acid]] and dichromic acid, respectively. Chromate salts contain the '''chromate ion''', [[Chromium|Cr]][[Oxygen|O]]<sub>4</sub><sup>2−</sup>, and have an intense yellow color. Dichromate salts contain the '''dichromate ion''', Cr<sub>2</sub>O<sub>7</sub><sup>2−</sup>, and have an intense orange color. == Characteristics == * The [[chromium]] [[atom]]s are in [[oxidation state]] +6 in both, and the chromate and dichromate ions are fairly strong [[oxidizing agent]]s. Chromium in the +6 (or VI) oxidation state is often referred to as '''[[hexavalent chromium]]'''. * In an [[aqueous solution]], chromate and dichromate anions are in a [[chemical equilibrium]]. ::2 CrO<sub>4</sub><sup>2&minus;</sup> + 2 H<sub>3</sub>O<sup>+</sup> ⇌ Cr<sub>2</sub>O<sub>7</sub><sup>2&minus;</sup> + 3 H<sub>2</sub>O :This equilibrium can be pushed towards dichromate by lowering the [[pH]] (making the solution more acidic) or in the other direction towards chromate by raising the pH to [[base (chemistry)|basic]]. This is a classic example of [[Le Chatelier's principle]] at work. This equilibrium is also dependent on concentration of Chromium in solution. * They are used in [[environmental analysis]] to measure [[chemical oxygen demand]] (COD). * They are [[carcinogenic]]. All hexavalent chromium compounds are considered toxic and carcinogenic. * When used as oxidizing agents or titrants in a [[redox]] [[chemical reaction]], they will turn into '''trivalent chromium''', Cr<sup>3+</sup>, which has a distinctively different blue-green color. * The [[sodium]] (Na<sup>+</sup>), [[potassium]] (K<sup>+</sup>), and [[ammonium]] (NH<sub>4</sub><sup>+</sup>) salts are water soluble granular solids and are the most commonly used chromate or dichromate chemical [[reagent]]s. Most chromate and dichromate salts of [[heavy metals]], [[lanthanides]] or [[alkaline earth metals]] are only very slightly soluble in water and are thus of much less usefulness. * [[Chromate conversion coating]]s are applied to metals for corrosion protection, and to improve paint adhesion. * 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 2002/95/EC == Structures == <center> {|align="center" class="wikitable" style="width:500px" |<center>[[Image:Chromate-3D-balls.png|120px]]</center>||<center>[[Image:Chromate-2D-dimensions.png|150px]]</center>||<center>[[Image:Dichromate-3D-balls.png|150px]]</center>||<center>[[Image:Dichromate-2D-dimensions.png|150px]]</center> |- |colspan="2"|<center>the tetrahedral chromate ion, CrO<sub>4</sub><sup>2−</sup></center>||colspan="2"|<center>the dichromate ion, Cr<sub>2</sub>O<sub>7</sub><sup>2−</sup>, consists of two corner-sharing tetrahedra</center> |} </center> == See also == *[[Potassium dichromate]] - K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> *[[Chromate conversion coating]] == External links == *[http://monographs.iarc.fr/ENG/Monographs/vol49/volume49.pdf IARC Monograph "Chromium and Chromium compounds"] * [http://www.npi.gov.au/database/substance-info/profiles/25.html National Pollutant Inventory - Chromium VI and compounds fact sheet] [[Category:Oxoanions]] [[Category:Chromates| ]] [[Category:IARC Group 1 carcinogens]] [[de:Chromate]] [[he:כרומט]] [[nl:Chromateren]] [[pt:Cromato]] [[ru:Хроматы]] [[fi:Kromaatti]] [[zh:铬酸盐]]