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I may be at the wrong forum with such a basic question but trying to ferret out the answer on Google has not been productive. Apologies in advance. I do art metal, working almost exclusively in copper. As copper hardens quickly when worked I frequently anneal it by heating the workpiece to 1200° F and quenching. Following annealing the copper is placed in a citric acid bath of approximate PH 2.0 which removes all of the oxides on the surface. Over time the acid bath becomes heavily laden with the dissolved copper oxides, turning the solution blue. As I've seen on the internet, this dissolved copper is harmful to fish and other aquatic life so removing it is necessary before disposal. One technique I've found states that by neutralizing the acid/copper solution with baking soda the soluble copper will precipitate out in a non-soluble form which can then be disposed of. So my question is: Is this true? Is this all that is necessary to both neutralize the spent acid bath and change the dissolved soluble copper to a non-soluble form which will render the remaining liquid safe for the drain and the precipitated copper safe for disposal in the trash? Thank you.

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Yes, basic copper carbonate is fairly insoluble, so it should work.

Citric acid itself should be environmentally safe, if diluted with enough water (don't let it sit full strength in the P- or S-Trap of a sink, though, or it might eventually corrode a copper waste pipe). The reason it must be neutralized with baking soda is to precipitate the copper.

BTW, though insoluble in water, the salt produced in this reaction is soluble in ammonia solution, the basis for making cuprammonium rayon (an experiment you can try with the waste solution).

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