Sciencemadness Discussion Board

Cleaning copper oxides from Native Copper?

bolbol - 15-3-2019 at 11:10

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

mayko - 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.

bolbol - 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

mayko - 16-3-2019 at 12:59

What about ammonia?

Deathunter88 - 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.

fusso - 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.

Abromination - 18-3-2019 at 14:54

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

phlogiston - 19-3-2019 at 02:35

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

Abromination - 6-4-2019 at 13:31

A mix of vinegar and table salt works well, especially if you dont have mineral acids

stamasd - 7-5-2019 at 19:04

Quote: Originally posted by mayko  
What about ammonia?


Ammonia is indeed very effective and my preferred way of cleaning copper of oxides. Does not attack the metal at all.

[Edited on 8-5-2019 by stamasd]

Amos - 7-5-2019 at 19:25

mechanical abrasion with very fine, comparatively soft materials over a long time will give you your best result. Any harsh and instantaneous chemical method is going to pit the surface of your copper and make it look most unimpressive. At least that's my experience.

AJKOER - 22-6-2019 at 03:58

Quote: Originally posted by stamasd  
Quote: Originally posted by mayko  
What about ammonia?


Ammonia is indeed very effective and my preferred way of cleaning copper of oxides. Does not attack the metal at all.

[Edited on 8-5-2019 by stamasd]


Well, yes and no. Avoid an oxygen source and an electrolyte (NaCl), and the reaction will be very slow, which may be of value here. Otherwise, a mix of an electrochemical and standard chemical reactions actually employed in the commercial leaching of copper ores (the ammonium carbonate mention below). Interestingly, the electrochemical part has an inception period (a delayed start) typical of electrochemical reactions. See my extract below from a prior thread comment:
Quote: Originally posted by AJKOER  
The action of an oxygen source (O2, H2O2 where dilute is fine,..) on aqueous NH3 (where the ammonia is first added to the copper metal, thereby saving H2O2 from a decomposition reaction with copper surface oxides), proceeds mainly by an electro-chemical reaction and some side reactions.

As such, add a small amount of sea salt to function as an electrolyte. I recommend jump starting the reaction for 30 seconds in a microwave.

Some of the underlying chemistry, which is, in part, electro-chemical in nature (which is why I recommend adding a good electrolyte to promote the reaction):

2 Cu + 4 NH3 + 1/2 O2 (or H2O2) + H2O --> 2 [Cu(NH3)2]OH (see half cells and Cu(l)/Cu(ll) and Cu/Cu(ll) equilibrium reactions at https://onlinelibrary.wiley.com/doi/abs/10.1002/bbpc.1963067... )

2 [Cu(NH3)2]OH + 4 NH3 (aq) + 1/2 O2 + H2O --> 2 [Cu(NH3)4](OH)2

Cu + [Cu(NH3)4](OH)2 <---> 2 [Cu(NH3)2]OH

Additional source: see "Kinetics and Mechanism of Copper Dissolution In Aqueous Ammonia" fully available after signing on to ones Facebook account at http://www.academia.edu/292096/Kinetics_and_Mechanism_of_Cop...

If, as a consequence of a standard chemical side reaction, some NH4NO2/HNO2 is also be formed (per the cited reference) usually within the first 1O minutes of the reaction (per my observation), you can personally observed the decomposition of any formed HNO2/NH4NO2 (well, at least if one uses cheap household ammonia that foams owing to the presence of surfactants, which can actually create a column of foam as the decomposition takes place). Caution, in a sealed vessel, eruption/spillage can occur.
----------------------------------------------------------------------

More details on a leaching solution with added ammonium carbonate, see https://www.researchgate.net/publication/229233363_Dissoluti... .

[Edited on 12-4-2018 by AJKOER]


My prior research on NH4NO2 (a minor side product above, which is unstable and decomposes in minutes in this experiment with a release of much N2 gas) nevertheless noted that ammonium nitrite was potentially (did not detail whether an oral and/or skin contact pathway) a very dangerous toxin, so use gloves or avoid the ammonia cleaning all together, to be safe.
------------------------

Just did a research update on NH4NO2 and to quote:

"The compound is highly toxic to humans and aquatic animals."

Apparently, it is actually used "As a microbiocide, rodenticide and agricultural pesticide [4]."

Link: http://www.chemistrylearner.com/ammonium-nitrite.html

Please take note of the word "highly" which in toxins implies a very powerful agent. If one looks at what else is used to kill rats, you will get the idea.

[Edited on 22-6-2019 by AJKOER]

Hegi - 22-6-2019 at 12:54

I would suggest oxalic acid/potassium oxalate mixture if Cu 2+ oxide is present at higher temperature. With powder CuO this combination at temperature above 80°C reacts realtively fast and gives blue coloured bis-oxalato complex.

Not sure what to do with Cu 1+ oxide, though..

Tsjerk - 22-6-2019 at 13:56

Use acetic acid (couple percent) and table salt (0.5%). I used this on copper coins and those are shiny as if new in seconds.

XeonTheMGPony - 22-6-2019 at 15:42

heat to dull red in a hydrogen atmosphere!

AJKOER - 23-6-2019 at 06:00

Quote: Originally posted by XeonTheMGPony  
heat to dull red in a hydrogen atmosphere!


Wikipedia on 'Native Copper' at https://en.wikipedia.org/wiki/Native_copper to quote:

"Native copper is an uncombined form of copper that occurs as a natural mineral. Copper is one of the few metallic elements to occur in native form, although it most commonly occurs in oxidized states and mixed with other elements."

Native copper sourced from Michigan is noted especially for its purity. Elsewhere less pure (see, for example, "Arsenic Bronze: Dirty Copper or Chosen Alloy? A View from the Americas", 1st page at https://www.jstor.org/stable/530550?seq=1#page_scan_tab_cont... ).

As such, when dealing with a possible mix element copper source, be mindful of a possible Arsenic presence, see 'Copper and arsenic dissolution from chalcopyrite–....' at https://www.sciencedirect.com/science/article/abs/pii/S03043... .

So, heating even a small amount of As in H2 may not be especially safe along with other issues (like a difficult procedure, and hydrogen gas presence, especially in large quantities, is a potential explosion hazard with any air mixing) if not performed with the proper safety measures in place.
-----------------------------------

Be careful on the amount/time of air contact with the cleaning solution!

All reactions treatments mentioned in this thread based on an acid and NaCl may further possess, in my opinion, also an electrochemical (like in a metal-air battery) path employing an acid, air (oxygen and even perhaps some CO2) along with NaCl serving as an electrolyte and a complexing agent. See copper half cell reactions cited in reference above based on Cu/NH3(aq)/O2. Also, more generally:

4 Cu(l) (as a soluble complex) + O2 + 2 H+ → 4 Cu(ll) + 2 OH-

which is interestingly a cited reaction path for the commercial preparation of basic copper chloride (see Eq 7 at https://en.wikipedia.org/wiki/Dicopper_chloride_trihydroxide ). Related reaction with other transitions metals has also been reported, especially well known being with Fe (the process of rust formation) and Fe(ll).

[Edited on 23-6-2019 by AJKOER]