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Context

I have no knowledge in chemistry. I am looking for information about etching circuit boards with citric acid instead of hydrochloric acid because it seems it could be a safer, easier way as citric acid is way safer to handle and not that toxic and harmful.

I am struggling to find much information online about the process since it is not as popular as other etchants.

The process

It seems to work really well if I mix 100ml of 3% hydrogen peroxide solution in water with quite a lot of citric acid powder(4-6 teaspoons) and a bit of salt(around 1 teaspoon), heat everything in a microwave oven to a warmer temperature(not boiling, around 70-80° C) so everything dissolves and then while it's still warm I put the copper clad in it and this seems to work just as good as hydrochloric acid and hydrogen peroxide in room temperature - etches 10x15cm PCB in 15-20 minutes.

Concerns/questions

I have done it a few times in well ventilated room, but I have no knowledge about what exactly happens in the reaction and if it is dangerous in any way, so I would like some insight into what exactly might be happening, what materials are produced and how safe/toxic they are.

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The acids, hydrochloric or citric, have a common property : they produce $\ce{H+}$ in water. These ions are necessary to react with copper and hydrogen peroxide $\ce{H2O2}$. Indeed metallic copper reacts with hydrogen peroxide $\ce{H2O2}$ and an acid in the following way : $$\ce{Cu + H2O2 + 2 H+ -> Cu^{2+} + 2 H2O}$$ So whatever the acid chosen, hydrochloric or citric, the copper will be dissolved the same way. With citric acid, the dissolution should be slower. But it leads to the same final result : metallic copper will be destroyed. The only difference is the final composition of the liquid. With HCl, the final liquid is made of $\ce{Cu^{2+}}$ and $\ce{Cl-}$ ions, which may be partially recombined. With citric acid, the final liquid is made of citrate ions, which will make a complex with $\ce{Cu^{2+}}$. Citral ions from citric acid will make a small change of color, but has no other effect. Both solutions are equally dangerous, or equally non-dangerous if you prefer, because copper solutions are always toxic. You should take care of the obtained copper solutions in the same way, whatever their compositions.

Personally I would recommend using citric acid instead of hydrochloric acid. It is less corrosive, and has nearly the same effect on copper + $\ce{H2O2}$.

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  • $\begingroup$ Thank you very much for the answer. This is indeed what I have observed. Hydrochloric acid is a very nasty thing to have around in comparison with citric acid and I would love to not have to deal with it. The time issue seems to be very easily solved with increased temperature. I have read/heard about adding salt(NaCl) to speed things up in this process and did so as well. Is there any logic in this? Could you comment on the role of salt here? Also any comments on the amounts/proportions of these ingredients? $\endgroup$
    – Martynas Žiemys
    Mar 22, 2021 at 12:44
  • $\begingroup$ Regarding "copper solutions are always toxic", what about Copper Citrate? It is sold as a dietary supplement, just google for "copper citrate supplement". Won't the OP's process produce a copper citrate solution? Does that make it safe(er)? $\endgroup$
    – KJ7LNW
    Mar 25 at 2:45
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    $\begingroup$ @KJ7LNW "The dose makes the poison". $\endgroup$
    – Poutnik
    Mar 25 at 4:55

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