Is there a chemist in the house? - Etchants

[pSimon]

I do a certain amount of electronics at home, including simple PCB manufacture. Now my A-level chemistry is >25 years ago, so rusty does not even begin to describe it...

At the moment, I'm using an etchant which is specified as:

200ml 35% HCl
30ml 30% H2O2
770ml water

Has anyone got a clue how long the mix will keep? By this, I mean "on the shelf", not in use.

I seem to recall that H2O2 is quite unstable, and somewhat photo-sensitive, so I'm keeping the mix in the cellar (cool & dark).

On a similar note, how long can I expect the H2O2 itself to "keep"?

Finally, I have some old (largely spent) FeCl3 etchant (So it's probably more Fe2+ than Fe3+ now, with a dollop of Cu2+/Cu+ for good measure). I was considering disposing of this by mixing with Lime, which I think should get me an insoluble (so safe for diposal) copper hydroxide?

Ideas and comments?

[Ray Moscow]

That's a lot of peroxide!

Yes, it's reactive, and heat and UV light can initiate undesirable reactions. Check the MSDS.

Iron chloride (either form) is only slightly soluble, but the hydroxide is pretty much completely insoluble. Check the MSDS again for disposal options.

[pSimon]

Heh.

I've 2.5 l of the peroxide. (100 volumes)

I didn't think I'd be able to get it after the recent peroxide/flour incidents, but there doesn't seem to be all that much of a problem. :dunno:


Are you sure about Iron (III) Chloride solubility? 250g/500ml is the suggested solution, and that's not what I'd consider slightly soluble.

See http://www.rapidonline.com/Tools-Fasteners-Production-Equipment/PCB-Equipment/Etching/PCB-Etchant/29447

It's horrid muck though, stains everything, and worse, you can't see what's going on in it, so you /have/ to keep messing with it to see if it's done yet - hence the HCl/H2O2 mixture, which is clear, and also pretty cheap.

[Ray Moscow]

You're right -- Wikipedia gives its solubility at 92 g/100 ml water at 20 C (http://en.wikipedia.org/wiki/Iron(III)_chloride).

I just mis-recalled the old solubility rules (http://www.ausetute.com.au/solrules.html) in that ferric chloride was slightly soluble, whereas actually it's quite soluble.

[ofro]

The only trick about Iron (III) Chloride is that the solution has to be at least slightly acidic. If you neutralize it, you'll have a mucky precipitation of iron hydroxide, which isvery insoluble. If you wait a bit longer and some water is remnoved, you'll end up with a substance commonly known as rust.
It's horrid muck though, stains everything, and worse, you can't see what's going on in it .....
And we all know how to cause stains with rusty material.

The good news is that once you neutralized with lime, it's ugly but not toxic, and you can get rid of it in the garbage. Also, don't worry about still having some divalent iron around; it'll oxidize to rust in its time.

[pSimon]

Any ideas as to the HCl/H2O2 mix stability?

It seems OK after 24 hrs

[ofro]

I am afraid I don't know.
I think that it should be reasonably stable because the final H2O2 concentration is only 1%. However, you don't want to breathe any fumes. It might develop chlorine gas (by H2O2 oxidizing chloride to chlorine). Although, if I remember correctly, H2O2 is not as strong an oxidant as chlorine, so it shouldn't produce too much of the gas.

No matter what, you better always wear safety goggles. I nearly lost an eye once in a teenager chemistry experiment.

[pSimon]

There is a bit of a smell, chlorine and oxy-chlorides, I think.

I wear gloves and glasses when I do the mixing (over the bath), with the window open.

Not something to be careless with.

Thanks for the info!

[VoxRat]

I do a certain amount of electronics at home, including simple PCB manufacture. Now my A-level chemistry is >25 years ago, so rusty does not even begin to describe it...

At the moment, I'm using an etchant which is specified as:

200ml 35% HCl
30ml 30% H2O2
770ml water


What is the surface being etched? (I know a fair amount of chemistry, but not much about electronics).
Has anyone got a clue how long the mix will keep? By this, I mean "on the shelf", not in use.

I seem to recall that H2O2 is quite unstable, and somewhat photo-sensitive, so I'm keeping the mix in the cellar (cool & dark).
I don't know exactly. H2O2 is most stable at ~ neutral pH, but acid isn't as destabilizing as alkali. I'm guessing weeks not years. On a similar note, how long can I expect the H2O2 itself to "keep"? I've had bottles of 30% H2O2 on the shelf, room temperature, for years. I've assayed it, so I know it's still good as new.

Finally, I have some old (largely spent) FeCl3 etchant (So it's probably more Fe2+ than Fe3+ now, with a dollop of Cu2+/Cu+ for good measure).What would
Fe3+ react with to make Fe2+? I'd be more worried about going the other direction.

I was considering disposing of this by mixing with Lime, which I think should get me an insoluble (so safe for diposal) copper hydroxide?

I think that would work. As would NaOH. What kind of volumes and concentrations are we talking about?

[pSimon]

The surface is etched is fairly pure copper on a PCB.

So I can expect weeks - fine. I guess I can keep the resulting Cu2+/HCl mixture and perk it up with a few more ml of H2O2 when I need to, as long as the peroxide keeps well, and it seems like it will.

A site out there even recommends this - it seems acid CuCl2 (supposedly) makes a reasonable etchant. (You then bubble air through to turn Cu+ -> Cu2+ - I'm not sure I can be arsed with that much messing about though).

I've got maybe 500ml of the Ferric solution left, but it will have a fairly high admix of Cu by now, so I want to be very sure it's stable. I did consider NaOH as a neutralising agent, but I guess Ca(OH)2 will get me the hydroxides just as well, and should set in a solid block once done, especially if I add in excess. I could use both, I suppose.

Copper is very toxic in solution, I hear, so the hydroxide seemed a good bet for disposal.

Dunno why but the reaction *is* roughly

2.Fe3+ + Cu -> 2.Fe2+ + Cu2+

I always thought Fe2+ was less stable than Fe3+ but it seems not here.

[Ray Moscow]

I would think that in an oxidising environment Fe3+ would predominate.

But (digging out an old chemistry book), the reduction potential for Fe3+ is pretty high:

Fe3+ (aq) + e- ==> Fe2+ (aq) for +0.77V

which means that Fe3+ is easily reduced if conditions allow.

[VoxRat]

Now that I understand what you're using it for, it's clear that metallic Cu is the electron donor responsible for Fe3+ -> Fe2+.

[ofro]

That really surprised me. I didn't realize that Fe3+ can be such a strong oxidant that it could oxidize metallic copper. I guess the freshmen chemistry years are fading in memory.

[pSimon]

Thanks for the input, all.

[Coragyps]

I just found this thread.....

H2O2 and hydrochloric acid will, indeed, make chlorine gas, and if you put 30% peroxide into 15% hydrochloric acid in a 20,000-gallon tank like some folks at my former employer once did, you can mobilize a terrorist response team at the Navy base that is next to where you dock the boat that holds the tank. I'm pretty sure that the bit of ferric iron in the acid speeds that reaction up, though.

I always thought Fe2+ was less stable than Fe3+ but it seems not here.

Not out in the open air. I agree that lime is the best choice to drop everything out of solution.

At your concentrations, pSimon, it's obvious that you have some stability, but I wouldn't trust the mix to last all that long - maybe a week or two. And, just as important as keeping it in the dark, never screw the cap down tight on anything containing peroxides. The bottles that H2O2 are sold in have a vent built into the lid to keep them from exploding due to oxygen buildup - though commercial 30% solution is stable for years in cool, dark storage.

-Coragyps the oilfield chemist

[pSimon]

I drilled a 0.3mm hole in the lid of my working H2O2 bottle and stuck a bit of masking tape over it as a makeshift vent.