Copper

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Copper is a very common metal for refiners, both as a major part of karat gold alloys, as the second metal in sterling silver or in electronics.

Chemical reactions

Copper is resistant against both hydrochloric and sulfuric acid at room temperature while nitric acid readily dissolves copper.

Copper and nitric acid

Depending on the strength of the nitric acid the reaction can take two ways. In diluted acid (typically below 3.9M HNO3) :

3 Cu (s) + 8 HNO3 (aq) = 3 Cu(NO3)2 + 2 NO (g) + 4H2O

If the NO is released to the atmosphere it will react with oxygen and NO2 will be created.

At higher concentrations this reaction is favored :

Cu + 4 HNO3 = 1 Cu(NO3)2 + 2 NO2 + 2 H2O

Translated into weight means that for every pound of copper digested 1.45 lbs of NOx is released into the atmosphere.

References

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