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Speeding up the electrodeposition of copper

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Silence
#1
May28-09, 11:37 PM
P: 5
Hi all,

I'd eventually like to form a trumpet receiver and bell out of pure copper, and the standard method to do that, from what I know, is electrodeposition. This is done using a mandrel made of iron or something else with a lower reduction potential than copper's. I've calculated (initially ad-hoc, and just now using Faraday's law of electrolysis) that it would take 383 amp-hours per pound of copper, and the bell will likely be around a pound in weight, so this could take a good number of days.

I haven't done any advanced electrochemistry yet so I only now discovered how Faraday's law can be used. I rearranged it (see the attached png) and used Faraday's constant in Ah/mole instead.

The minimum voltage for electrolysis is the potential difference between the reduction potentials of the plating material (in this case, +0.34 volts) and the mandrel (for iron, −0.041 volts). More significantly, there's also a vague upper limit – when the electricity plates copper on the cathode faster than the acid dissolves it from the anode, the following happens, somehow producing smut on the surface of the cathode:
2e⁻ + 2H₂O → H₂ + 2OH⁻ (E° is −0.83 volts)

The question is, how do I find the maximum voltage that I can pump through, without producing smut? This depends on the concentration of aqueous copper, which I don't know how to change.

Why doesn't more voltage speed up oxidation in addition to speeding up reduction? Does current aid oxidation at all, or is acidity the only factor in dissolving the copper? If that's the case, what's the relationship between the acid's pH/pKₐ and the dissolution of the metal? All the electrolysis examples on the net are lab demos for kids, so they use vinegar, which has a relatively tame pH of 2.4. I'd rather use diluted nitric acid in order to increase the molarity of CuČ⁺ and hence the maximum rate of electrolysis.

Thanks in advance for any help with this. I'm a little stumped. :)

Edit: Looks like copper reacts with some of the nitrate in nitric acid, and this is a redox reaction in itself – no reduction potential. Maybe that's why current is irrelevant in oxidizing the copper. It also means I would need a substantial supply of nitric acid.
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MATLABdude
#2
May29-09, 01:04 AM
Sci Advisor
P: 1,724
I won't be approaching your question from a chemistry point of view. I'm only a lowly (electrical) engineer, so take what I say with a grain of salt. I believe that electrodeposition is how you form INGOTS of very pure metals, not finished (or near finished products). These are then melted down and rolled / cast / forged into various sheets, pipe, bar stock, etc. that you can buy from a (specialty) metal wholesaler.

From that point of view, you'd be better off buying the appropriate bar stock and machining (using a lathe, mill, CNC machine, etc.) your pieces out of it (and buying copper pipe and bending it into the desired shapes). Our science is not yet at the point where we can locally control electrodeposition to the point that you can mix some copper solution in a tank, program a few button, and come back a few days later with a fully functional copper instrument. Selective laser sintering might come close, but I think you'd still need to machine it down to smooth it out. Also, it's really, really expensive (the machines, or even time on the machines):
http://en.wikipedia.org/wiki/Selective_laser_sintering

Lastly, I'm curious: why copper? I think it'd be more easily tarnished, and much easier to ding / scratch / dent than brass.
Borek
#3
May29-09, 02:28 AM
Admin
Borek's Avatar
P: 23,600
1. Around 383 amp-hours assuming 100% efficiency, in practice longer.

2. Maximum voltage - start with a low one, then go up till you see evolving gas.

3. Changing copper concentration - just add solid copper sulate to the solution, once it will dissolve, concentration of copper went up. However, you want copper concentration to be between some limits, electroplating bath have been experimentally fine tuned to give the best results.

4. Don't use nitric acid, I think good recipes call for sulfuric acid. And don't try to reinvent the wheel, look for ready copper electroplating recipes. No, I don't know where (other then visiting library, then google is your friend).

5. It is not acid that dissolves copper on the anode, it is the same current that deposits it on the other end.


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