Hexavalent chromium
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Hexavalent chromium (chromium(VI), Cr(VI) or chromium 6) refers to chemical compounds that contain the element chromium in the +6 oxidation state (thus hexavalent).
Contents
Properties
Hexavalent chromium compounds are brightly colored, yellow, orange or red. They are soluble in water, though chromium trioxide and chromyl chloride will react with water. They are powerful oxidizers, though chromates and dichromates will need an acid to initiate the reaction.
Notable compounds
- Ammonium dichromate
- Chromic acid
- Chromium trioxide
- Chromium(VI) oxide peroxide
- Chromyl chloride
- Potassium dichromate
Preparation
Cr(VI) compounds like dichromates can be prepared by oxidizing an aqueous mixture of chromium(III) hydroxide with hypochlorite or hydrogen peroxide.
Uses
- Jones oxidation
- Cleaning glassware
- Tabletop volcano
Safety
Hexavalent chromium is recognized as a human genotoxic carcinogen. Exposure to hexavalent chromium increases the risk of developing lung cancer, asthma, or damage to the nasal epithelia and skin.
Within the European Union, the use of hexavalent chromium in most applications is regulated by the Restriction of Hazardous Substances Directive.
Neutralization
Chromium(VI) compounds can be converted to the less harmful Cr(III) using reducing agents. Ascorbic acid, sodium/potassium/ammonium sulfite/thiosulfate/metabisulfite are very effective, and being readily available and cheap, are excellent neutralization agents. Sugars, like glucose and fructose, can also be used, and they're even cheaper.[1]
Chromic acid, and chromium trioxide should first be diluted in a large volume of water, before neutralization, to limit splashing and aerosolization of Cr(VI). Chromium(VI) pentoxide is very unstable and will rapidly break down, though it's best to add some reducing agent to the solution to remove any potential traces of Cr(VI).
References
Relevant Sciencemadness threads
- HEXAVALENT CHROMIUM
- Reduction of Hexavalent Chromium
- Disposal of hexavalent chromium
- Easy preparation of hexavalent chromium for electrolysis in chlorate and bromate cells
- recycling hexavalent chromium
- Hexavalent Chromium in Aluminum 6061-T651
- Unknow Chromium VI Compound?
- Preparation of Chrome Yellow and Chrome Orange
