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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

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 ⇦ this on eBay or Amazon [affil links] solution, preferably hot for a duration that will depend on oxide thickness.

Ram Singh
Sunnyvale, California
1999