Hexavalent chromium

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).

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.

Chromic acid should first be diluted in lots of water, before neutralization, to limit splashing an aerosolization.

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