Step 4: Chemistry Break

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(Note: I'm an economist, not a chemist. Please leave a comment if you've got any corrections and/or additions to this stuff!)

I stumbled on this idea when I saw this website: Etching with Air Regenerated Acid Cupric Chloride by Adam Sechelle. Cupric chloride can be re-used indefinately by topping up the acid levels and adding oxygen (bubbled in from the atmosphere). Sounds cheap and environmentally friendly to boot.

The website's got a lot of good chemistry info on cupric chloride etching. His data on etching speeds is great, and his simple titration procedure for maintaining the acidity of the solution is pretty nice.

To make the cupric chloride solution, he dissolves a bunch of copper wire in hydrochloric acid, and mentions maybe using hydrogen peroxide to speed up the oxidation, but doesn't go into detail.

Which got me thinking. You didn't have any cupric chloride yet, but you can make it by dissolving copper. Dissolving copper is the name of the etching game. So we can make one etchant that makes another etchant that's infinitely re-chargeable. Elegant.

Turns out that hydrochloric/peroxide is a common home-brew etchant (and I've re-re-invented the wheel, again) but I guess that people got so used to throwing away their "spent" etchant that they don't think about re-using it. The whole point of this instructable is that you don't throw it away, but use the dissolved copper forevermore as your long-run etchant.

Here's what's going on chemically:

Before there's much copper dissolved in the solution, Cu + 2 HCl + H2O2 -> CuCl2+ 2H2O is the dominant net reaction. That is, the extra oxygen in solution from the peroxide is oxidizing the copper metal, in presence of the acid, to make copper (II) chloride. That's our starter etchant. The resulting CuCl2 shoud be a nice emerald green color.

After you've dissolved a lot of copper into the solution, and used up all the peroxide, the copper chloride does most of the etching for you: CuCl2 + Cu -> 2 CuCl. That's the end etchant.

Eventually you etch so much that you convert all the CuCl2 into CuCl, which doesn't dissolve copper (and is a yucky brown color). As long as you've got enough acid in the solution, you can simply add more oxygen to re-oxidize the copper(I), making more copper(II) chloride and water: 2 CuCl + 2 HCl + O -> 2 CuCl2 + H2O. And then you can etch again.

Bottom Line:

Two things to maintain: CuCl2 levels and acid levels.

CuCl2: After all the peroxide is used up, and the solution starts turning brownish, you'll have to add oxygen to regenerate the solution again: toss in a few more capfuls of peroxide or bubble air through the solution or swirl it around vigorously, or just pour it into an open container and wait. It's easy to tell when you're ready to etch again, because the solution turns green.

It's also impossible to add too much oxygen by adding air, so bubble/swirl to your heart's content. If you're using peroxide to add oxygen, be sparing -- a little goes a long way, and it's mostly water so you're diluting your etchant by adding it.

Acid: Note that HCl is being consumed in the starter etchant and the regeneration reactions. So we're going to have to add a bit more acid as time goes by. If you notice that it's harder to re-green your brown etchant, it's probably time to start thinking acid.

I've tried the titration described on Adam's site a couple times, and it's pretty easy but requires an accurate scale and pure lye (back to the hardware store...). It's easier to just toss in a capful of acid every few batches of boards, which seems to do the trick for me.

Mulemom37 months ago

Will the starter etchant work on brass? I'm using ferric to etch brass, but it stops working after just a few uses. If I can make the etchant cheep, I'm ok with that, but if I can make it cheep AND use it forever, well that sounds perfect! Would it work better if I dissolved some copper wire in it?

apappano1 year ago

On the chem page, Google is detecting malware on the original Adam Sechelle site. Here's a reposting of it:


May want to update the link until it gets fixed.

hmm, wouldn't one of those bubble thingamajigs for fishtanks do the job pretty well?
sh*t you can just use the fishtank as well.
oh... i see some already beat me to it :S
Props to The Ideanator
The regeneration process consists of adding oxygen and HCl to make CuCl turn back into CuCl2. Can't I just add HCl and bubble oxygen to my already worn-out (thus full of CuCl) solution of ferric chlorine, then?
pfred25 years ago
I'm not sure what everyone else uses as a bubbler but I use one of those 12 volt tire inflators connected to a tire valve, with a little piece of hose jambed into that going to another piece of tubing that fits onto a spray can cap. You know, the end that used to go into the spray can. Bubbles nicely for me. That, or I use a pressure regulator off one of my regular air compressors connected to the same tire valve, hoses, spray can cap rig. But the 12V tire inflator works nicely and just about anyone can come across one of those.

Bubbling can make a huge difference in etching speed and agitating is for the birds!
I use an aquarium bubbler hooked up to my bubbler tubes. it works well.
I checked out your Instructable and the nail seems to make a large hole in the tubing. You should try hot gluing a plastic spray can head to the end of a piece of tubing, or maybe even to one of your nail holes and see if you like the smaller outlet. It'd make for a finer jet bubble pattern.
Minifig6665 years ago
 Great trick, I will have to try this.
legionlabs6 years ago
Another way to make CuCl2 is to react CuSO4 with NaOH, and then HCl. Cu(OH)2 precipitates out as a solid, and you also get some NaCl that obviously remains in solution. I found that taking the Cu(OH)2 and reacting it with HCl (muriatic acid) at the same concentration you use would give me a nice batch of CuCl2. One or more of these reactions may have been exothermic, I cannot recall. This method would let you quickly achieve high levels of CuCl2 if you have large amounts of copper sulphate lying around (like I do). For more information about these chemicals, please check the material safety data sheet for each (MSDS).
matchett8086 years ago
if you made an airtight tank with a couple of electrodes and with your aqua pump pumped the atmosphere above the soln back in you would regenerate your acid and end up with a copper electrode!