Fun with used Sodium Persulfate etchant.

Thread Starter

Markd77

Joined Sep 7, 2009
2,806
Mods, please feel free to delete if you think this is dangerous, but my thoughts are that anyone who has used etchant knows how to deal with dangerous chemicals and to use gloves and eye protection.
Also the reaction products are less harmful than the reagents.

I found this page:
http://www.phase4c2.com/pic101/pcb/fmc4.htm
and thought I'd give it a try.

If you try this use gloves and eye protection and do it over a plastic tray in case of spills. Have extra water ready in case the reaction goes faster than mine did.

I cut the base from an aluminium can and sanded it to get through the protective coating.
I added about 2ml of water, then 1ml of used sodium persulfate etchant, then a tiny pinch of salt. The reaction started as soon as the salt was added and produced quite a bit of heat, I think it nearly boiled, which is why you need water ready to add to cool and slow the reaction.
I had to scrape the copper from the aluminium with a toothpick after a minute or so to get the reaction to continue.
The amount of aluminium removed from the can is pretty small so there doesn't seem a risk of dissolving through, but that's what the plastic tray is for anyway.

Some equations:
Etching the copper:
Na2S2O8 + Cu = Na2SO4 + CuSO4
Sodium persulfate + copper = sodium sulfate + copper sulfate

Adding the aluminium
3 CuSO4 + 2 Al = Al2(SO4)3 + 3 Cu
copper sulfate + aluminium = aluminium sulfate + copper

adding sodium bicarbonate to neutralise
Al2(SO4)3 + 6 NaHCO3 = 2 Al(OH)3 + 6 CO2 + 3 Na2SO4
aluminium sulfate + sodium bicarbonate = aluminium hydroxide + carbon dioxide + sodium sulfate


On this picture is the copper sludge produced and a drop of the solution afterwards, and before.



The surface of the can afterwards.
 

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nerdegutta

Joined Dec 15, 2009
2,676
So you're saying that you can "clean" the etchant with salt and aluminum, and reuse the etchant?

This was interesting.

I always use gloves.

Safety first!
 

Thread Starter

Markd77

Joined Sep 7, 2009
2,806
Unfortunately the products won't work as an etchant, but are much less toxic and corrosive than at the start.
For me it was just fun to see the process in action, it's a reaction with a good colour and state change, unlike a lot of reactions where you mix two colourless liquids and don't get much to see.
I'll probably keep storing my used etchant until I have enough to make it worth taking to the local waste disposal place for proper disposal and presumably recycling of the copper.
 

loosewire

Joined Apr 25, 2008
1,686
@ 77,If the two drops of liquid in the pan were water,that is a perfect example
of how people walk on hot coals. The smooth drop shows how the
moisture holds together.The drop that is falling apart would burn your feet.
Sorry to get on your post,but you will never see that water demo again,you
can see how hard it would be to explain.
 
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