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Turkey9

Fri Dec 09 2011, 08:05AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I use the hydrogen peroxide and muriatic acid method for etching circuit boards. I had finished etching a board and instead of flushing the leftovers down the toilet, I decided to stick a penny in. After a couple days the penny almost completely dissolved and was removed for the coolness of having a partially dissolved penny. I left the solution to sit while I went off home over thanksgiving break. Here is where the mystery comes in... My roommate stayed back and said that after about a week, the solution crystallized over night. The crystals were now blue instead of the dark green color of the original solution. What happened? My first thought was that the crystals were CuCl2. Is this correct? Thanks!

klugesmith

Fri Dec 09 2011, 08:33AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Did the partly etched penny look like it was high-copper bronze to begin with, not copper-clad zinc? How much copper did you etch from circuit boards? Hint: why is copper foil thickness often given in "ounces" ?

Here is a well-known and practical warning. Leaving containers of hydrochloric acid (or, I expect, your etchant mixture) open for days is a good way to corrode unprotected ferrous surfaces nearby.

Check out the scary pictures in this thread. The culprit appears to be an uncovered tray of green etchant:

magnet18

Fri Dec 09 2011, 11:26AM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location:

Posts: 624

dark blue would probably indicate copper sulfate, sulfur possibly from the water.
Copper chloride crystals are green
Since this it real world chemistry, t's probably a mixture. is it possible to break open a crystal to see if it's green on the inside?

Forty

Fri Dec 09 2011, 05:47PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

the muriatic acid might of had other junk in it as buffers (if it was from a hardware store, etc.) maybe the copper (or zinc if it was a post 1982 penny) oxidized something in the mix and precipitated out an insoluble salt. or if the solution was sitting around long enough to evaporate a bit, the solution would of become over saturated and crystals would form (possibly of the buffers.)

and also, don't pour copper chloride into the toilet unless you live in a desert. copper solutions are pretty bad for aquatic life. if you end up with too much solution you can either crystallize some of it by evaporation for easier storage, or you could plate out the copper with electricity and remelt it/sell it.

Turkey9

Sat Dec 10 2011, 04:40AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Yeah I've had my tools rust from being in the vicinity of my etchant even just during the etching time. The crystals are very thin and light blue and dissolve easily in water.

The acid did indeed have buffers in it.

I made sure the penny was pre 1982, I think it was mid 70's. I did put another penny in there that was newer to see what would happen when the zinc was exposed. Turns out it bubbled quite a bit and I assume the bubbles were hydrogen.

I new that you shouldn't pour the solution down the drain, but I though it was ok to flush it. This is because black water goes to the treatment plant and is not released into the water table until processed. If this is not correct, I will stop immediately.

klugesmith

Sat Dec 10 2011, 07:19AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Turkey9 wrote ...
I new that you shouldn't pour the solution down the drain, but I though it was ok to flush it. This is because black water goes to the treatment plant and is not released into the water table until processed. If this is not correct, I will stop immediately.

It's good that you thought about the issue. Personally, I'm sure there's no problem with well-diluted acids down the drain. Heck, they sell H2SO4 to digest plugs of hair and grease when the drain runs slow. Metals are another story. Municipal waste-water treatment processes don't involve nuclear reactions, so copper in is copper out. Some is adsorbed onto solid particles that end up in the sludge, and the rest goes through as copper ions in the treated water stream.

With that said, I wouldn't hesitate to pour used etchant down the drain. The quantity is inconsequential, compared to other sources of metals entering the treatment plant.

In many places, you can buy a 2-pound dose of copper sulfate labeled for killing tree roots growing into buried sewer pipes.

magnet18

Sat Dec 10 2011, 01:41PM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location:

Posts: 624

Turkey9 wrote ...

Yeah I've had my tools rust from being in the vicinity of my etchant even just during the etching time. The crystals are very thin and light blue and dissolve easily in water.

The acid did indeed have buffers in it.

I made sure the penny was pre 1982, I think it was mid 70's. I did put another penny in there that was newer to see what would happen when the zinc was exposed. Turns out it bubbled quite a bit and I assume the bubbles were hydrogen.

correct
did the crystals look like this?

Turkey9

Sat Dec 10 2011, 06:42PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Huh yes they did! Guess it was copper sulfate!

GluD

Sat Dec 10 2011, 08:07PM

Registered Member #1221 Joined: Wed Jan 09 2008, 06:17PM
Location: Odense, Denmark
Posts: 196

It is probably not of any signifigance, but where did the sulfate come from if it really is copper sulfate?
Neither hydrogen peroxide or muriatic acid should contain sulfates.

Could your roommate perhaps have added the sulfate to pull your leg ?

Maybe I have missed something but I dont understand where the sulfate could have come from.

Turkey9

Sat Dec 10 2011, 09:52PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

That's what I was wondering too... My roommate knows nothing about chemistry so if he added something it was by accident.

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