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Author: Subject: Cleaning copper oxides from Native Copper?
bolbol
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[*] posted on 15-3-2019 at 11:10
Cleaning copper oxides from Native Copper?


So I recently acquired some native copper specimens and I want to know how I can clean them without attacking the copper crystals.

The copper is usually coated in a black oxide layer consisting of microcrystaline cuprite(Cu2O) and/or tenorite(CuO). Vinegar soak removed some of it but it takes a long time and requires a LOT of vinegar.

I was wondering if Oxalic acid or its conjugate base would be any helpful. I have some I used to remove hematite(Iron oxide) staining from minerals and it worked great as it both acts as a ligand for iron and reduces it to Fe2+.

Any information would be greatly appreciated.
Thanks
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mayko
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[*] posted on 15-3-2019 at 14:50


Dilute HCl is what comes to mind for me. So long as the solution isn't oxidizing, only Cu2+ will dissolve; the metal is too inert for water alone to oxidize it and so won't be attacked. You might scavenge oxygen from the solution ahead of time by adding some copper and sealing the container.



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bolbol
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[*] posted on 15-3-2019 at 15:19


Quote: Originally posted by mayko  
Dilute HCl is what comes to mind for me. So long as the solution isn't oxidizing, only Cu2+ will dissolve; the metal is too inert for water alone to oxidize it and so won't be attacked. You might scavenge oxygen from the solution ahead of time by adding some copper and sealing the container.


The problem would be to keep the oxygen out of solution. I assume the smallest amount would cause the shiny copper faces to oxidize to some extent.

So I guess this is an option, just not the best. I was wondering if there is any better ways that eliminates the possibility of copper metal being oxidized
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mayko
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[*] posted on 16-3-2019 at 12:59


What about ammonia?



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Deathunter88
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[*] posted on 16-3-2019 at 16:47


Quote: Originally posted by bolbol  
Quote: Originally posted by mayko  
Dilute HCl is what comes to mind for me. So long as the solution isn't oxidizing, only Cu2+ will dissolve; the metal is too inert for water alone to oxidize it and so won't be attacked. You might scavenge oxygen from the solution ahead of time by adding some copper and sealing the container.


The problem would be to keep the oxygen out of solution. I assume the smallest amount would cause the shiny copper faces to oxidize to some extent.

So I guess this is an option, just not the best. I was wondering if there is any better ways that eliminates the possibility of copper metal being oxidized


It's going to oxidize after you remove it from solution due to air anyways. So unless you plan on using spray clear coat to cover the crystals I see no need to worry about the slight amount of oxidation possible due to dissolved oxygen.
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[*] posted on 16-3-2019 at 17:14


In my experience, Cu removed from aq NaOH will oxidize slower than removed from HCl. Hence I think the best way is immerse it in aq NaOH, put some Al into soln and in direct contact with Cu to reduce any Cu oxides, and remove the Cu and rinse with de-i/dist H2O and air dry.



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Solubility table: https://en.wikipedia.org/wiki/Solubility_table
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Abromination
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[*] posted on 18-3-2019 at 14:54


Once cleaned with a non oxidizing acid, the copper can be protected from oxidation with clear nail polish.



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[*] posted on 19-3-2019 at 02:35


dilute sulfuric acid should work too. Don't use concentrated sulfuric acid.



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