Chemistry of Copper Etchant Solutions

October 19, 2011 1:43 AM

What copper salt is left behind in this neutralized etchant solution? Also, how can I safely dispose of spent etchant in general?

I've been experimenting with etching copper lately, and I have a growing amount of depleted ferric chloride etchant to dispose of. As an experiment, I neutralized the used etchant with baking soda, hoping to precipitate out the metal ions as carbonate and hydroxide so I could dispose of them as dry waste.

I filtered out the resultant brown sludge, and was left with a bright blue solution. I left the solution a little longer and it turned cloudy yellow-green with precipitate. I filtered the solution again and was left with completely transparent blue solution. Evaporating the water left a white solid at the top and a blue/green solid at the bottom. I'm curious what this could be.

It's my understanding that copper carbonate or hydroxide would not be soluble enough to be responsible for the colour of the solution, which would suggest that the resulting salts would be sodium chloride and copper (I)?(II)? chloride, but that would just be a guess.

Finally, how should I dispose of the spent etchant in general? I'm not willing to just put a neutralized solution down the drain because of the copper content. If there is somewhere I could take it (in Vancouver BC) that would dispose of it properly, that would be an option. A method of precipitating out the metal that actually eliminates the copper and leaves behind something non-toxic would also be most welcome.

posted by [expletive deleted] to science & nature (3 answers total)

This isn't a direct answer to your question, but you might want to look at Hydrogen Peroxide + Hydrochloric Acid as your next etchant. The chemicals are a lot easier to dispose of safely.

Maybe you could try contacting these people for advice on where to drop off your tasty chemical soup.
posted by le morte de bea arthur at 2:02 AM on October 19, 2011

To give you even stronger incentive not to go down the drain, I did a lot of circuit board etching in high school and I dumped the spent etchant down the drain of the slop sink in my family's basement and flushed it with a lot of water. Spent etchant isn't as spent as you might think. Even diluted and spent, it ate pinholes through the copper drain pipes. 25 years later, I think my dad has almost forgiven me.

As for your solution, my understanding is what you're doing is using Fe III Chloride which swaps the copper for the iron, so you end up with Cu III Chloride and Iron metal, but you should believe someone who has had a chemistry class more recently than me.
posted by plinth at 3:14 AM on October 19, 2011

You're probably seeing the colour of a bunch of copper II salts. Copper(II) chloride salts, of course, make bright blue solutions. As noted in the wiki article, CuCl₂ turns greenish with excess chloride. The chemistry of the etching process is described here. With excess Fe(III)Cl₃ forcing the Cu(II) oxidation state, I think it's unlikely that you would have much Cu(I) present.

From the CuCl₂ article, you can see that the salts form networks in solution and form insoluble hydroxides Cu(OH)₂, and insoluble carbonates also both blue, when you neutralize. These are probobly both present in your powders.

Cu(OH)₂ will also convert, over time to CuO, a black insoluble powder. Ferric and ferrous salts and oxides are also in the black to brown range. These are probably the colours in your bown sludge.

You don't want to put this down the drain. The allowable limits for copper in drinking water in BC are 500 µg/L fairly high, but for disposal the levels are frequently at the "wildlife levels", 3 µg/L. Above this concentration it's classified as a "deleterious substance" and you can face charges under the federal Fisheries act. People do get charged for even small discharges, though rarely.

The city of Vancouver lists a couple of options for disposal of chemical wastes here. I've dealt with HAZCO before, they're reasonably good. I don't know about Newalta.
posted by bonehead at 9:14 AM on October 19, 2011