Chromium

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File:Chromium-1.jpg
Chromium, picture from wikipedia.

Chromium (Cr) is a white metallic element used in stainless steel or as chrome plating to give metal objects a protective and shiny surface. It is the thin oxide layer formed on the surface that gives chromium it's protective qualities.

It is also used as a hardener in gold plated contact surfaces in electronics. Around 1% chromium in the gold plating creates what is called hard gold. This chromium will go into solution when dissolving gold plating from, for example gold fingers.

CAS number : 7440-47-3

Contents

Physical properties

Molecular weight : 51.996 g/mol
Melting point : 1907 °C (3465 °F)
Boiling point : 2671 °C (4840 °F)
Density : 7.19 g/cm3

Acid reactions

Chromium metal reacts with diluted hydrochloric acid to form CrCl2 while releasing hydrogen. In the presence of oxygen in the air the chromium quickly oxidizes to Cr3+. Sulfuric acid can also dissolve chrome in a similar way. Nitric acid on the other hand oxidizes the surface and passivates it, stopping any further reactions.

Chemistry

Chromium have a number of different oxidation states and therefore shows a rich chemistry. It can have oxidation states from -2 to +6 where +3 and +6 is the most common states found in compounds.

Cr6+ can be reduced to the less harmful Cr3+ by mixing it with copperas.

Videos

There are a lot of videos about the chemistry of chromium on youtube.

Chromium in the waste stream

Chromium is one of the RCRA 8 metals so if it is in the waste stream it can inflict higher costs for treatment. Many soluble chromium compounds is both toxic and carcinogenic, especially chromium(VI) compounds.

The EPA Limits for chromium both in water and solid waste is 5ppm (mg/l). For solid waste this is the amount of chromium that can be leached in a test called Toxicity Characteristic Leaching Procedure, TCLP. It simulates landfill conditions and shows if the solids poses a toxic hazard for the environment once it's been deposited. A piece of stainless steel in the solid waste is no problem, but dried up solutions from dissolving stainless would pose a toxic hazard.

It is the responsibility of the producer of the waste to test and mark the waste according to the result.

References

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