I have a standard copper etching solution of 1 part $\pu{10M}$ $\ce{HCL}$ and 2 parts 3% $\ce{H2O2}$ (as described in numerous places online).

I previously had this stored in a tightly-sealed glass bottle, as described in the instructions I followed and general acid storage instructions.

However, my solution seems to be outgassing hydrogen or chlorine gas quite significantly, and when I opened the sealed container a few days later it violently liberated much of its contents into my face. (I was wearing a partial face shield and immediately went under the shower, so no ill effects were observed).

I know that for some chemicals that self-pressurize, such as cryogenic nitrogen, the storage method is essentially constant venting - is that what I'll have to resort to?

  • 1
    $\begingroup$ It's hydrogen peroxide related. It slowly decomposes, especially if iron cations or some other catalysts are present. Cheap hydrochloric acid contains some. Any hydrogen peroxide solution should be stored in a fridge. Any highly outgassing solution should not be stored at all. $\endgroup$
    – permeakra
    Apr 5, 2018 at 5:26
  • $\begingroup$ @permeakra This was over the course of only two days, and it built up a significant amount of pressure...would peroxide decomposition be that fast? My working theory was that it was either some remnants of copper left in solution reacting to form chlorine gas, or hydrogen outgassing as some others using this solution have described... $\endgroup$
    – 0xDBFB7
    Apr 5, 2018 at 13:50
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    $\begingroup$ It can be really fast, up to and including explosions in borderline cases. 2days is a lot of time for something like this. If you want a safe solution for etching copper, consider iron chloride based solutions. They look dirty, but they are (reasonably) safe, efficient and (to some extent) reusable. Do care to find absolutely airtight container for solid iron chloride, though. $\endgroup$
    – permeakra
    Apr 5, 2018 at 15:14
  • $\begingroup$ @permeakra Yeah, I've tried ferric chloride, I've just found it pretty slow. $\endgroup$
    – 0xDBFB7
    Apr 5, 2018 at 16:02
  • $\begingroup$ In some cases, electrolytic etching also works. $\endgroup$
    – ikrase
    Nov 27, 2019 at 5:53

1 Answer 1


Bottles for 30% or stronger $\ce{H2O2}$ usually have vented caps to prevent sudden out-gassing, splashing and boil-over that you experienced, as well as to prevent bursting the container. The vent may be as simple as thinned spot, which may be scored or slitted, or it may have a polythelene, PTFE or other sponge or "paper" separator that helps retain liquids while allowing gas to escape.

vented bottle cap

In either case, I would not suggest storing a mix of $\ce{H2O2}$ and $\ce{HCl}$ for more than a few hours for a few reasons.

  • As gases evolve, the solution becomes weaker and less effective an etchant.
  • Stored on a shelf, the gases will corrode the shelving surprisingly quickly. You would not want shelving with caustic or toxic chemicals to collapse.
  • If gases given off are other than $\ce{O2}$, you probably don't want to breathe them for long.

Ferric chloride is a slow etchant at room temperature, but gentle warming speeds etching greatly (PDF). At 54 °C, $\ce{FeCl3}$ removes ~35 um of copper per minute, so standard PC boards ("0.5 oz" - "1 oz") should be etched in ~1 - 2 minutes.

The biggest nuisance I find using $\ce{FeCl3}$ is that it tends to stain containers, tables and hands, but Iron Out and similar bisulfite or hydrosulfite stain removers can be used on cleanup.

  • $\begingroup$ > stain removers can be used on cleanup || Citric acid doesn't work? $\endgroup$
    – permeakra
    Apr 6, 2018 at 6:08
  • $\begingroup$ Also, links are broken $\endgroup$
    – permeakra
    Apr 6, 2018 at 7:54
  • $\begingroup$ @permeakra, thanks, link fixed (hard to copy PDF URL from Google). As far as citric acid, it might work on fresh stains on an impermeable surface, but once the reduced iron gets into things, it's hard to remove. $\endgroup$ Apr 8, 2018 at 4:20

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