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topic 3363

Removal of copper oxide using cyanide


(1999)
My question relates to the removal of copper oxide from sterling silver parts using a mixture of sodium cyanide, water, and hydrogen peroxide. Sterling silver parts (92.5% silver and 7.5% copper) are soldered using a traditional soldering operation. A copper oxide forms on the parts. The copper oxide is removed using a three bath process.
 
The first bath contains a mixture of:
100 mL dilute sodium cyanide (180 g/L NaCN)
1.5 L H20
100 mL hydrogen peroxide
The second bath contains:
400 mL of dilute sodium cyanide (180 g/L NaCN)
3L H20
The third bath contains only water.
Questions:
1. Exactly what is the chemical reaction that causes the copper oxide to be removed?
2. Does the hydrogen peroxide oxidize the sodium cyanide?
3. Is this process considered "chemical etching?"
4. What alternatives are available to replace the cyanide?

Any information/suggestions much appreciated.

kelley begin
- boston, Massachusetts


(1999)

It appears that you have oxidation-reduction reaction. Hydrogen peroxide is a mild oxidizing agent. So NaCN will be oxidized to NaOH or Na2CO3 and copper oxide is reduced to copper, removing oxide. As an alternative you my try 10% acetic acid solution, preferably hot for a duration that will depend on oxide thickness.

Ram Singh
Sunnyvale, California



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