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−</sup> + 2 H<sub>3</sub>O<sup>+</sup> ⇌ Cr<sub>2</sub>O<sub>7</sub><sup>2−</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]]