I'm not a chemist - (College Chem101 in 1958).\ufffd That's why I
referenced the article - I'll see if I can find it again - might
even be in the files in this group.\ufffd
As I understand it, the etching is a conversion of metallic copper to copper chloride, which is soluble.\ufffd Therefore, the free chlorine ion is required.\ufffd I don't think the chlorine in the HCl is free to combine.\ufffd The formation of the copper chloride is what turns the solution blue-green (also why ferrous chloride turns greenish as it is used).
Anyway, the HCl + H2O2 works for me, it's cheap, and just about inexhaustible (Bubble air through it\ufffd - converts the CuCL to CuO (or some such) and releases the Cl ions for re-use.\ufffd Adding the small amounts of H2O2 would also add the needed oxygen to regenerate the solution without the bubbles.
Again, not a chemist, but this is what I've read/experienced.
Norm
\ufffdThanks Norm.
\ufffd
Yeah, I\ufffdm just reluctant about using actual HCl for the reasons you mentioned.\ufffd I\ufffdm generally pretty careful but I\ufffdm also a bit accident prone and I don\ufffdt want to wreck something, or myself, messing around with that kind of acid.\ufffd I\ufffdve accidentally splashed this vinegar/salt/peroxide solution on me and not even slight irritation.\ufffd I\ufffdm sure that wouldn\ufffdt be the case with HCl.
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What I\ufffdm curious about though is that supposedly the reaction will still work *without* salt.. just takes longer?