Hexavalent chromium

Hexavalent chromium (chromium(VI), Cr(VI), Cr6 or chromium 6) refers to chemical compounds that contain the element chromium in the +6 oxidation state (thus hexavalent).

Properties

Hexavalent chromium compounds are brightly colored, yellow, orange or red. They are soluble in water, though chromium trioxide and chromyl chloride will react exothermically 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/bisulfite/metabisulfite, iron(II) sulfate are very effective, and being readily available and cheap, are excellent neutralization agents. To properly neutralize Cr(VI) compounds like chromates and dichromates, add them to a large volume of water, then add the reducing agent and stir the solution. The solution will turn from the orange/yellow Cr+6 to the green Cr+3. Add a slight excess of reducing agent, then leave the solution to stir for at least a day to completely destroy any trace of hexavalent chromium. Sulfur oxoanions may produce sulfur dioxide as side product, so it's best to do this outside or in a fumehood. For some reducing agents, the pH of the solution needs to be lowered to around 6-5, sometimes even lower, to 2.5. Sugars, like glucose and fructose, can also be used as neutralization agents, and they're even cheaper than the ones mentioned above.

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
• Toxicity of Chromium (VI)
• 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