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I'd like to use 5% sulfuric acid and 3% hydrogen peroxide as an etching solution for copper. (In the past I've used dilute hydrochloric acid and hydrogen peroxide but the generation of chlorine gas has always made me a little nervous.)

I'm planning to use 200ml 5% sulfuric acid and add a 25ml portion of 3% hydrogen peroxide to it whenever the etching stops so that the concentration of hydrogen peroxide in the solution stays low. This solution should not be able to generate any gases other than oxygen (which is harmless as long as I don't store it in a sealed container).

What I'm not sure about, however, is whether this solution will be able to dissolve organic material. As far as I know, acidic piranha solution depends on the dehydrating properties of concentrated sulfuric acid to form the strongly oxidizing Caro's acid and oxygen radicals. I'm not sure whether this reaction can take place in my case. I'd like to avoid dealing with piranha solution when possible.

I've not been able to find any resources on the effects of very dilute piranha solutions (or whether dilute solutions of sulfuric acid and hydrogen peroxide act like piranha at all). The only thing I could find was instructions on how to dispose of piranha, which recommend dilution with water followed by neutralization with a base, for example this page from Princeton.

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    $\begingroup$ Your final solution will be $4.45$% sulfuric acid and $0.33$% hydrogen peroxide. These values are rather low, and will not have any effect on organic material. On the other hand, I suspect these low concentrations may not be able to act on copper metal. $\endgroup$
    – Maurice
    Commented Dec 5, 2021 at 16:30
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    $\begingroup$ Hydrochloric acid has the advantage that it reacts and forms complex ions containing chlorine atoms, like $\ce{[CuCl2]^-}$ or $\ce{[CuCl4]^{2-}}$. This helps accelerate the reaction. Sulfate ions do not make such complexes. So the reaction with copper may be slower. $\endgroup$
    – Maurice
    Commented Dec 5, 2021 at 16:41
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    $\begingroup$ To directly address just the title - no, it cannot. By definition, it is mixture of concentrated sulphuric acid ( like >=95%) and H2O2 30% ( ratio in formulations differ) $\endgroup$
    – Poutnik
    Commented Dec 5, 2021 at 17:20
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    $\begingroup$ I guess even vinegar could be used for etching instead of sulphuric acid, as copper dissolves slowly in vinegar too, oxidized by aerial oxygen. If you leave copper in vinegar overnight, solution gets blue. H2O2 is better that air. $\endgroup$
    – Poutnik
    Commented Dec 6, 2021 at 8:40
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    $\begingroup$ @Poutnik Maybe vinegar and air would be called “guppie solution”? ;-) $\endgroup$
    – Ed V
    Commented Dec 7, 2021 at 19:47

1 Answer 1

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First, per a educational source

copper does not react with dilute sulphuric acid

As such, you may wish to follow a path employed for the commercial leaching of copper ore. The reaction is accomplished with copper metal in aqueous ammonia, optionally an ammonium salt (carbonate), an electrolyte (like $\ce{NaCl}$) and air (or, dilute $\ce{H2O2}$ works just fine).

The underlying chemistry was debated for decades but is now resolved to consist of a mixed reaction involving some spontaneous electrochemistry (and so, for the associated electrolyte, I use a pinch of sea salt). Here is a review article from ResearchGate. Also, this article, "Kinetics and Mechanism of Copper Dissolution In Aqueous Ammonia".

The cited provided chemistry is even a little more complex (and unfortunately, in my opinion, does require an exposition due to safety issues involving implementation and handling) with a mention of a minor secondary radical related reactions resulting in an unstable nitrite ($\ce{NH4NO2}$). My take is that the $\ce{O2/Cu}$ reaction with $\ce{NH3}$ is, in effect, a metal/air battery. The latter can introduce solvated electrons into the mix (as sourced from copper, so it is correct to claim that copper is catalytic to the oxidation of ammonia to nitrite from $\ce{O2/H2O2}$). The battery cell provided $\ce{e-(aq)}$ with $\ce{H+}$ produces the hydrogen atom radical, which can react with $\ce{NH3}$ forming $\ce{•NH2}$ radical, which can further interact with $\ce{O2/H2O2}$ as follows:

$\ce{•NH2 + O2 → NH2O2•}$ (aminylperoxyl radical unstable) → $\ce{NO + H2O }$

As a supporting reference on the above, see "On the aqueous reactions of the aminyl radical with molecular oxygen and the superoxide anion", particularly Page 24, Table 2.1.

where the $\ce{NO}$ is a precursor to nitrite, and in particular, a small amount of unwanted $\ce{NH4NO2}$. The latter has the annoying property (based on a lowing of the pH following ammonia consumption) of suddenly decomposing releasing volumes of $\ce{N2}$ gas, which apparently is sufficient so as to cause leakage and even breakage of sealed reaction vessels (as I have experienced).

Consequentially, don't tightly seal your suitably large reaction vessel (and I would recommend avoiding any skin contact with the solution due to also potentially toxic ammonium nitrite here (unclear as to whether its toxicity extends to a skin ingestion pathway).

Note, the ascribed electrochemical reaction has an inception period (waiting time).

Have problems with ammonia? An alternate electrochemical path is Copper metal, lemon juice, dilute $\ce{H2O2}$, a piece of carbon (large surface area) and some sea salt. Heat (especially for a few minutes in a microwave, if possible, which eliminates the inception period) and you will have a colorful solution of copper citrate/ascorbate.

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  • $\begingroup$ Thanks a lot for the elaborate explanation! Nitrous oxides, nitrites and ammonia are something I'd like to avoid even more than trace amounts of chlorine, so that's pretty much off the table. I just did a test etch with dilute H2SO4 and it does etch when I add a small amount of table salt (as you suggested in your citric acid method). The problem with that is of course that it generates some chlorine again, although way less than what I got when using dilute HCl. I'd imagine that the citric acid (lemon juice) method will also produce small quantities of chlorine as the chloride gets oxidized. $\endgroup$ Commented Dec 7, 2021 at 18:14
  • $\begingroup$ Copper metal, lemon juice, dilute H2O2, a piece of carbon (large surface area) and some sea salt is electrochemical in nature. Basically, as carbon is very noble (a good cathode) and Copper is not as much (so it is the anode here), the Cu undergoes galvanic corrosion releasing Copper ions. The H2O2 also captures electrons forming radicals which assist. In a microwave, the process is surprisingly fast. and never any chlorine. $\endgroup$
    – AJKOER
    Commented Dec 7, 2021 at 19:00
  • $\begingroup$ per a educational source I had a little chuckle and then started crying... the site you have linked is an insult to actual educational sources. $\endgroup$ Commented Dec 10, 2021 at 23:40

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