I did copper etching (to make a PCB) using a solution of vinegar + H202 + NaCl and now I need to dispose the solution. There is no waste collection for this kind of stuff where I live, so after some research I saw that the cleanest dispose process seams to be to make electrolysis.

I used a copper electrode at the negative pole (cathode), a nail on the positive (anode) and plugged a 9V power supply. I was expecting some bubbling on the nail (Cl2 gas) and the Cu2+ to accumulate on the copper electrode, but the bubbling happened only around the copper wire and after 15 min the solution started to get really dark brown.

My goal is to get the copper out of the solution, so that I can throw the solution away in the drain and the copper in trash.

  1. Is this what is supposed to happen, or did I do something wrong? How do I know when it's done taking the copper from the solution? when done, can I just filtrate and then pour the solution in the drain?

  2. Why the bubbling happened at the cathode and not the anode and why the solution turned brown?


the result after some 2-3 hours. how to know if it is done? enter image description here enter image description here

  • $\begingroup$ >My goal is to get the copper out of the solution || Make a solution of baking soda, mix it with your remains, leave it for a few days, decant clean water, filter off the precipitate, dry it. The result is basic copper carbonate which is mostly harmless, and the solution would be sodium chloride with minor impurities, reasonably safe to go down the drain || You CAN try to reduce it to get metallic copper out, but it is quite slow and isn't easy to do right (I tried). $\endgroup$ – permeakra Jan 10 at 0:13
  • $\begingroup$ thanks for reply. but what is mostly harmless? can I just throw it in the trash or is there a "cleaner" option? and how to know how much baking soda is necessary? only by the color? $\endgroup$ – some_user Jan 10 at 2:15
  • $\begingroup$ You should not eat it. Otherwise, it's safe. In my country I would wash it down the drain. If I wanted to be particularly responsible, I would dry and heat the solids until it becomes completely black before washing it down the drain. As for amount of backing soda, you need need one molecule of soda per one $HCl$ molecule in the the etching solution and then a slight excess. Assuming ~35% $HCl$, this should be around 90-100 g of solid baking soda per 100 ml of $HCl$ solution used for the etching solution. $\endgroup$ – permeakra Jan 10 at 15:33
  • $\begingroup$ looking at density table Actually, to be safe, I would go for about 110 g. I didn't check the solution density. $\endgroup$ – permeakra Jan 10 at 15:53
  • $\begingroup$ A simple test to know whether a solution does contain copper ions is to filtrate it and then to add some milliliters of ammonia solution. If the solution contains copper ions, it will take a deep blue color, and no precipitate. If not it will make a brown precipitate, the solution contains Fe ions. $\endgroup$ – Maurice Jan 10 at 17:51

Mix with sodium hydroxide drain opener (outside!) The metal ions (copper and iron now) will combine with the OH- ions and drop out of solution as harmless iron and copper hydroxide and can be thrown out. The Na+ ions will combine with the Cl- ions and form aqueous NaCl and can safely be put down the drain.

Even if you put too much sodium hydroxide in your solution, it’s still safe to put down the drain because it’s less caustic than it started as when it was drain opener.

If you’re still uneasy about disposing of the copper and iron hydroxide, burn it and the hydroxides will turn into metal oxides.

  • $\begingroup$ interesting. should the copper and iron hydroxides formed be filtered out of the solution? or can I throw them together down the drain? $\endgroup$ – some_user Jan 10 at 14:28
  • $\begingroup$ I wouldn't put the hydroxides down the drain - there's no need to add additional metals to your wastewater for your local water treatment facility to deal with. Also, the inside of most people's plumbing isn't as smooth as people expect it to be. Put solids in the drain and it's going to cause you problems when it will be most inconvenient (i.e. when you cannot get in touch with a plumber!) $\endgroup$ – Jack Spencer Jan 11 at 0:03

The best thing to do is to put some nails in the copper solution, or anything made of iron. You will soon see that the nails ge. covered by metallic copper, which is red-brown, After maybe one hour, the solution is not blue any more. So it does not contain copper ions anymore. You can throw this solution down the drain : it will contain iron instead of copper, and it looks like rust. And you can throw away the nails covered with copper. The equation of the chemical reaction is : $$\ce{Fe + Cu^{2+}-> Fe^{2+} + Cu(s)}$$ Quite often, these $\ce{Fe^{2+}}$ions are then oxidized in the air to produce a brown deposit of $\ce{Fe(OH)3}$ according to $$\ce{4 Fe^{2+} + 3 O2 + 6 H2O -> 4 Fe(OH)3}$$ It is a typical chemistry demonstration that probably all chemistry teachers in the world present in their high school classes. It never fails.

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    $\begingroup$ Even better could be a galvanic cell , formed by connected iron and copper plates. This would have advantage iron would not get covered by copper. $\endgroup$ – Poutnik Jan 9 at 20:26
  • $\begingroup$ @Poutnik. You are right. But my method is quickly done, and does not require any equipment. $\endgroup$ – Maurice Jan 9 at 20:36
  • $\begingroup$ Iron +3 is very good in masking copper coloration. BTW, copper-chloride solutions are often green. $\endgroup$ – permeakra Jan 10 at 0:17

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