…still messing with forces I don’t understand – the formula.

by Stephen Hobley on March 2, 2011

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NEWSFLASH : Just when I thought this story was done with – I just tried adding a sprinkle of salt to the (already) potent mix of Hydrogen Peroxide and Muriatic Acid – Dang! It etched the whole board in under a minute… Wow! Definitely some kind of catalyst. Resulting solution is bright green (CuCl2?), not the usual blue.

Now back to the regularly scheduled program…


I’ve done more trials since I posted about the etching solution yesterday. Some of them have failed, so I thought it best to present the recipe as a movie – so you get to see how the chemical process should work.

[OMG, he's not going to make me sit through a movie is he?]

Don’t panic – I’ll reveal the recipe within the first 30 seconds. I would recommend you watch all the way through, as it’s easy to get the formula wrong and see no reaction. At least now you’ll know what you’re looking for:



The important thing is that you get the copper to precipitate out, as the dark “rust” – if the liquid is just turning blue then this is not the reaction involving the salt – just the other two ingredients. This works too, but takes a long time.

Vinegar – Distilled White Vinegar – diluted with water to 5% acidity (Meijer)
Peroxide – 3% solution (Meijer)
Salt – to taste. (Just keep adding it until the “fizzing” continues all by itself).

Right now, I still don’t know what’s going on exactly – the fumes are slightly noxious – but only in a “OMG I just inhaled vinegar” kind of way. Repeat the experiment at your own risk.

…and dispose of the solution like you would Ferric Chloride.

Super Safe Etchant results

UPDATE:

My sister-in-law, a Doctor of Chemistry, provided the following explanation :

“Hydrogen peroxide is what we call an oxidizing agent (a mild one), meaning that it easily accepts electrons from other species to form H2O (hydrogen peroxide itself is electron deficient). What happens when it “dissolves” copper metal is that a neutral copper metal atom releases two electrons, to form a Cu2+ ion in solution. All metals tend to release electrons to form positively charged species….we refer to anything that DONATES electrons as a reducing agents. The strength of metals as reducing agents varies and copper is a fairly weak reducing agent. I did a demonstration in my general chemistry class recently where I dissolved a piece of copper metal in concentrated nitric acid….again copper loses electrons to form Cu2+ and H+ ions from the acid gain electrons to produce hydrogen gas. If you see a bluish color to your solution after the copper is dissolved, it is due to the presence of the Cu2+ ions.

The other components HC2H3O2 is acetic acid, and yes it gives the characteristic taste and smell to vinegar. It is a relatively weak acid, but aids the oxidation-reduction reaction between the copper and the hydrogen peroxide by providing a source of H+ ions to solution that are used to form two H2O molecules from one H2O2. I am not sure why it is necessary to have to NaCl in the solution, unless as a source of counterions. I will try to dig more into this if I have time.

Since H2O2 is a mild oxidizing agent (and can sometimes be a reducing agent if combined with a very strong oxidizing agent), it should only be dangerous to other weak or strong reducing agents. It may do this much more slowly depending on the metal. I wouldn’t leave it in contact with aluminum, nickel, or other common metals for long periods of time.

It is not toxic to humans (used an antiseptic). The main safety concern is the long term storage. Hydrogen peroxide decomposes (thermodynamically favorable) to water and oxygen gas, so if stored over time, you have a potentially dangerous product. We are very careful about compounds that are peroxides or can form peroxides in lab. If you are working at a low concentration (not sure of the concentration that you are using) this is obviously not a problem. The peroxide that you buy in the store is low enough concentration that we are not concerned about storage (I think less than 10%).”

So I believe that means it’s relatively safe. :-)

UPDATE UPDATE : The gunky stuff is almost certainly Copper Chloride, so it’s taking that from the salt and leaving the Sodium to fight it out with whatever else is swimming around.

UPDATE III : More from Allyson (Dr. of Chem) on the Science Desk:

“So, I have had a possible thought about the role of NaCl…

It has to do with equilibrium.

As the reaction proceeds to form Cu2+ we reach a point where the system reaches equilibrium (the rate of formation of Cu2+ is equal to the back rate of formation of Cu metal). At this point, no more Cu2+ can be formed. My guess is that the NaCl provides Cl- ions that will react with Cu2+ to form CuCl2. This removes Cu2+ ions from solution and drives the reaction
forward toward the formation of more Cu2+ ions. If the NaCl is in excess (as it sounds by dumping large amounts around the reaction), Cu2+ can be continuously formed.

Does this make sense? It’s just a guess…”

Next week : How to fold space using Henderson’s Relish

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{ 88 comments… read them below or add one }

Phill March 3, 2011 at 12:19 pm

“Does this make sense? It’s just a guess…”

Having a braniac sister who’s a p.h.d. in chemistry doesn’t make sense…

Michael March 3, 2011 at 5:00 pm

Wow, science is COOL. It’s been a long, long time since I’ve done anything chemistry related. Great tip. Will have to try this next time I etch a board!

M Bison March 3, 2011 at 5:06 pm

@Phill
What doesn’t make sense about it? Sounds like you’re a sexist jerk!

Anon March 3, 2011 at 5:31 pm

Jesus. H2O2 is NOT electron deficient. What school did you sister buy her PhD from anyways? I’m sorry but I couldn’t read past that glaring error.

Attrezzo March 3, 2011 at 5:37 pm

The free sodium snaps up some H and O then?
Sodium superoxide and Sodium hydride?

MrRight March 3, 2011 at 5:52 pm

better question is, is she single?

Stephen Hobley March 3, 2011 at 5:53 pm

Alas no – she married my brother in law…

dade March 3, 2011 at 5:56 pm

why not using muriatic acid (HCl) and hydrogen peroxide (H2O2) in a 70/30 solution? It etches through a board in just under 3 minutes, although it generates nasty Cl gases, so you have to do it outside.
You should have your sister check wether you’d better do this reaction outside as well, the reaction principle is the same and you’re adding Cl via the salt, which may be producing nasty stuff too! Be safe.

David Tomaschik March 3, 2011 at 6:03 pm

This is pretty cool. I’ve been looking for a way to play around with PCB etching for a while now, but I’ve always been pretty concerned about the safety profile of the chemicals involved. Looks like this is a safe(r) way to go. Say Thanks to your Sister-in-Law for providing some background on the Chemistry behind how this works!

Stephen Hobley March 3, 2011 at 6:03 pm

Yeah, but Muriatic Acid is not that safe to handle…

Mr.Wizard March 3, 2011 at 6:03 pm

Thanks, that’s awesome! Keep em coming.

Tom March 3, 2011 at 6:16 pm

Acetic acid in aqueous solution (vinegar) is an equilibrium reaction producing H+ ions and acetate. Adding table salt (NaCl) forms sodium acetate forcing the equilibrium to produce more H+ ions and acetate, strengthening the acid. At least that’s what I remember from my high school chemistry many many years ago.

Phlogiston March 3, 2011 at 6:55 pm

Good job, I hope it lowers the treshold for others to etch pretty PCBs.

Tell your sister the chloride serves to form CuCl4(2-) complex ions. It also serves this function in ferric chloride etchant:

2Fe(3+) + Cu + 4Cl(-) –> 2Fe(2+) + CuCl4(2-)

This helps to keep the copper in solution (since CuCl2 is not particularly soluble), and the CuCl4(2-) can itself oxidise more copper (forming Cu(+) ions)

CuCl4(2-) + Cu –> 2CuCl2(-)

The mixture of salt (NaCl) with vinegar is not very much safer than dilute muriatic acid BTW, nut ,uriatic acid is not very dangerous really. Your stomach is filled with it. If you get bits on your skin or chlothes, just wash them off right away with a plenty of water and you’ll be fine. You should be concerned about the heavy metals in used etching solution (Copper, and perhaps lead if you put in boards with older on them) more so than the acid or ferric chloride.

Phlogiston March 3, 2011 at 7:04 pm

One more thing: this mixture is also known to be capable of dissolving gold… (unlike ferric chloride), so you may want to watch any jewelry you are wearing to make sure it stays nice and shiny.

mixing three safe ingredients doesn’t necesarily yield a safe mixture !!
(and vice versa)

Vetter March 3, 2011 at 7:06 pm

Okay, this all makes sense and as far as your sister-in-law goes, nice to know you’ve got backstops for your MacGyveristic metalings.

I have a question for future reference: Your solution of Vinegar, Hydrogen Peroxide and Salt uses the off the shelf 3% concentration of H2O2. How would you alter the formula when using ‘food’ 10% grade H2O2?

Stephen Hobley March 3, 2011 at 7:18 pm

Yes I was curious as to whether the resulting chemical compound and/or mixture was as dangerous as HCl.

Stephane March 3, 2011 at 7:36 pm

I have not etched pcb for a long time but i will certainly test this method.
There is something i am not sure.
In the video, you say 100ml vinegar and 100ml peroxide. But the video shows you putting peroxide for a much shorter time than vinegar.
I could be wrong as english is not my native language, but just to be sure, can you confirm the proportions?

Stephen Hobley March 3, 2011 at 7:38 pm

Yes it’s a 50/50 mix – the vinegar took longer to pour since it was a bigger bottle and more prone to spilling.

JohnMc March 3, 2011 at 8:04 pm

Not to down play the science, but lemons or limes and plain table salt has been the bartenders friend for polishing brass and copper for centuries. The fruits of course a natural source of acetic acid. Which begs the question — Stephen you up for trying a large grapefruit on your next board?? :)

But appreciate the tip. We will give it a go.

Stephen Hobley March 3, 2011 at 8:07 pm

Sure – I tried vinegar and salt – but that seemed to do nothing (that was sample #2).

I was told that monosodium glutamate can be used in place of vinegar too…

Anon March 3, 2011 at 9:05 pm

Virtually -any- acid can be used in place of acetic acid (vinegar). MSG is the salt of glutamic acid whose pKa is 4.4 – so an order of magnitude less than acetic acid.

Mike Stone March 3, 2011 at 9:14 pm

I etch with Cupric Chloride (which starts as hydrochloric acid and hydrogen peroxide), and the reaction is basically the same.

Etchants work by oxidizing the copper. The cuprous ion (Cu+1) isn’t soluble in pure water, but will dissolve in a chloride solution. The chloride ions (Cl-1) cluster around the cuprous ions, and since the chloride ions *are* soluble in water, they sort of give the copper a boost.

It takes a lot of chloride ions to hold the cuprous ions in solution, but the excess of chlorine will further oxidize the cuprous ions to cupric ions (Cu+1 -> Cu+2). The neat bit is that cupric atoms will oxidize metallic copper (Cu0 + Cu+2 -> 2Cu+1), so the solution just keeps making itself stronger.

A fully-oxidized cupric solution will be bright green, and when you put copper into it, you’ll see what looks like a black fog rolling off the metal. That fog is made of cuprous ions, which are really a reddish-brown, but appear black because the cupric solution filters out all the red light. The etchant solution will get darker and darker as more copper dissolves, eventually getting to a point where it looks like Guinness/old motor oil. By then, most of the copper is in the cuprous state, and the solution won’t etch very well.

The cool thing is that you can ‘regenerate’ the solution by adding a few more drops of hydrogen peroxide. The peroxide will oxidize all the cuprous ions to cupric, and the solution will go from opaque brown to clear green again. The solution will also regenerate by absorbing oxygen from the air, but that takes several hours.

Stephen Hobley March 3, 2011 at 9:22 pm

I just tried lemon juice – and this was barely working.

Stephen Hobley March 3, 2011 at 9:23 pm

Rebuttal from the Science Desk:

“The oxygen atoms in hydrogen peroxide are electron deficient when compared to oxygen atoms in species like water, which is why we see hydrogen peroxide accept electrons to form water. This is a bit of a tricky substance because it can act as an oxidizing or a reducing agent depending on its environment. This may help explain. In an oxidation-reduction reaction we see electrons transferred from one species to another. We use something called an “oxidation state” to assign the electrons in a molecule to each atom…it does not reflect formal charge, but is more of a bookkeeping tool for keeping track of electrons. In a bond between two species like O and H, the electrons are assigned to the most electronegative atom (that atom which has a higher affinity for electrons). In all peroxides the oxidation state for oxygen is -1 because the oxygen exists as an O2(2-) ion, but the oxygen in water has an oxidation state of -2, a gain of one electron. We do see hydrogen peroxide sometimes act as a reducing agent (donating electrons) in the presence of a strong oxidizing agent. H2O2 (arrow) 2H+ + O2 + 2e-. “

DeadlyDad March 3, 2011 at 10:12 pm

FYI, if you want ‘live’ hydrogen peroxide, pick up some sodium percarbonate (i.e. the main component of Oxiclean) and mix it in hot water.

Skeptic March 3, 2011 at 10:15 pm

Too good to be true. You used too many crackpot “tells” to pull me in.

Paul March 4, 2011 at 1:12 am

So Allyson is your wifes brothers wife?

James March 4, 2011 at 3:24 am

Skeptic.. what.. the hell? I was semi skeptical too, at least to the ease of use. I’m literally doing this experiment right now, however, with a smaller scale of solution and two pennies (1 pure unmarked control penny and 1 pcb-etching painted penny).. it’s working pretty well, and quickly.
Your post is about as smug and arrogant as it gets… Perhaps you would find a better home on youtube comments instead. Why don’t you try the experiment itself instead of cloaking yourself in smarmy self-satisfaction… “HAHA INTERNET MAN, YOU WON’T FOOL ME LIKE THESE OTHER SHEEPLE.”

God I hate people.

Peter March 4, 2011 at 4:58 am

when looking for ways to produce a patina on steel, the net yields the same ingredients you list, vinegar, hydrogen peroxide and salt. I lacked the peroxide and took a different route to try and produce the correct sort of rust on my bike http://www.refurbishforfun.blogspot.com I still wonder would it have been different with the peroxide. I can’t find anyone to answer the question (my sisters are nurses – and also married).

Kilrathi March 4, 2011 at 9:29 am

“I was told that monosodium glutamate can be used in place of vinegar too…”

Just so you know, I am highly allergic to this stuff, anaphalxis(?sp) anaphalactic shock(?sp), hospital, and actually died on the table no heart beat no nothing.
If this stuff in mild forms on garlic bread or salads(since it is used as a preservative) can cause me to stop breathing…
Wouldnt atomizing it in this solution then via the gases then inhaling it be a crud load worse? If it does this then this would be something to watch out for… if one will tend to use this method.

Though I think MSG was banned in the USA or not banned but jsut removed out of foods and I dont think it gets made anymore does it here in the US? Other countries maybe though.

Phip March 4, 2011 at 12:00 pm

“The main safety concern is the long term storage. Hydrogen peroxide decomposes (thermodynamically favorable) to water and oxygen gas, so if stored over time, you have a potentially dangerous product.”

Is this a typo? I don’t generally think of water and oxygen gas being major safety concerns.

Ryan March 4, 2011 at 2:33 pm

Lee Valley has some glue spreaders that may help remove the “gunk” without harm to the PCB
http://www.leevalley.com/en/wood/page.aspx?p=48479&cat=1,110,42967

Frode March 4, 2011 at 6:31 pm

I have a hypothesis of what’s happening:
Cu + 2NaCl + H2O2 + 2HCH2COOH -> Cu + 2Na+ 2Cl- + 2HO* + 2HCH2COO- + 2H+
Cu + 2H+ + 2Cl- + 2HO* -> CuCl2 + 2H2O (this is a well known etching reaction)
2Na+ + 2HCH2COO- -> 2HCH2COONa
Therefore:
Cu + 2NaCl + H2O2 + 2HCH2COOH -> CuCl2 + 2H2O + 2HCH2COONa
Where:
HCH2COONa + H2O HCH2COOH + NaOH

If this hypothesis is correct, the vinegar is partally a catalyst. To what extent depends on the equalibrium reaction. Also note that NaOH (Caustic soda) is a possible product in this reaction, and that it is a strong base by definition. It’s not directly dangerous to the envroniment as of I know, but CuCl2 – on the other hand – is not the best thing you can spread into the nature.

I suggest that you should use a mix of hydrogenperoxide and hydrochloric acid. Then you only get CuCl2 and water, and you don’t need to care about to what degree Caustic Soda is being formed.

Frode March 4, 2011 at 6:32 pm

Correction due to filter issues:

“HCH2COONa + H2O HCH2COOH + NaOH”
should be:
“HCH2COONa + H2O [insert equalibrium sign here] HCH2COOH + NaOH”

Aleks Clark March 4, 2011 at 7:29 pm

@Kilrathi

yea nobody is really allergic to MSG. Has been studied by the FDA et al. Stay away from peanuts. Oh and wifi. I heard people who were sensitive to MSG were also sensitive to magical radio waves. Probably best to stay off the internet all together, you never know! You also might want to avoid these foods: http://en.wikipedia.org/wiki/Glutamic_acid_(flavor)#Concentration_in_foods

You can find it at the grocery store sold as flavour enhancer (and it really works, it gives food the “savory” taste, aka umami)

Andrew March 4, 2011 at 7:38 pm

@Phip oxygen is plenty dangerous.

jenningsthecat March 4, 2011 at 8:27 pm

Etching the baord upside-down may give you better results. Try using some loops of insulated wire to hold the board at its corners; this will allow you to easily agitate the board, as well as holding the copper side downwards so the precipitates formed by etching fall OFF OF the copper instead of ONTO it.

Some brushing may still be required to keep the copper free from deposits that decrease etching speed, but the need for brushing off will probably be lessened; this means there will be less chance of scratching the resist.

Kris Lee March 4, 2011 at 8:48 pm

Does this solution produces any (toxic?) gases?

Hashish March 5, 2011 at 12:05 pm

What are you using to mask the copper?

Phlogiston March 5, 2011 at 5:50 pm

@Kris Lee
It may produce small amounts of chlorine gas, which is toxic indeed. However, the rate of evolution is extremely slow and should not normally pose any problem. If you decide to store the solution, don’t keep it in a closed container (to prevent overpressurising it) and keep it in a well ventilated place. (But since it is so easy to make, perhaps it is best to just make however much you need and discard it when done).

@Kilrathi
A -very- unlikely story, because glutamate is a common amino acid and your body contains a lot of it. (monosodium glutamate is just the sodium salt of glutamate). It hasn’t been banned anywhere.

ondrej March 6, 2011 at 7:16 am

I was using this many years ago :) Once I ran out if HCl (1:1:4 – HCl:H2O2:H2O) so I tried to replace hydrochloric acid by H2SO4 (didn’t work at all) then citric acid (didn’t work either). Finally I tried vinegar, which worked, but it was verrrry slow so I added NaCl which good catalysis (which I learned during electroplating experiments at school) and it worked!

anon March 6, 2011 at 8:46 am

Steve, what do you use to mask the copper? It seems to me you’re using those old dry transfer sheets with tracks, pads and symbols one could find in electronics stores about 15-20 years ago. Do you have a source for them? I cannot find them locally anymore or on Ebay; maybe they’re known by a different name.

Stephen Hobley March 6, 2011 at 11:00 am

I tried those – but they were expensive – I’ve been using a mixture of techniques lately. It merits a proper article – I need to publish something properly.

Stephen Hobley March 6, 2011 at 11:01 am

I think it’s triggering the production of NaCl so it’s not officially catalysis – but you’re right, it speeds things up a lot.

Filip March 6, 2011 at 2:26 pm

Since it is an electrochemical reaction I think that the NaCl provides “bulk ions” that helps the oxidizer to contact the metal surface.
The bulk ions helps the molecule of interest to pass through the inner Helmholtz plane and participate in a reaction at a solid surface. This is why you add sodium carbonate or other salts to a solution of dyes when you color clothes in your washing machine.

I don’t remember exactly how this works, cause I only heard about it briefly in class, in a lecture about ion selective electrodes (analytical chemistry). So this might be quite wrong, if I remember I will dig up the book and wright back to you!

MrFusion March 7, 2011 at 4:15 am

Try soaking a small sponge with your solution and then rubbing that on the board.

Should speed up the process (ie minute or two), and use a lot less solution (ie tablespoon or two).

Check out: http://www.instructables.com/id/Sponge-Ferric-Chloride-Method-Etch-Circuit-Bo/

Cheers!

Pelle March 8, 2011 at 10:06 am

If this was painfully slow, could it be that the white vinegar was too diluted? I got quite god “boiling” when adding salt (from fast food sachets) but it felt I needed to add a lot of it for anything to happen.

Also, as I’m diluting both the peroxide and the vinegar, would I get a more aggressive etchant with stronger mixes? Say 6% peroxide 10% vinegar? Or is that not to recommend?

Erich March 8, 2011 at 10:56 am

A former electroplating shop laboratory analyst tells me that the mixture used is of the type called a “chloride etch” in the trade.

HCl plus copper reacts quite slowly. HCl will remove surface oxide straight away but the bright metal will only react very slowly thereafter.

Adding hydrogen peroxide oxidises the surface of the copper, which is then very quickly attacked by the HCl.

If you don’t have HCl handy, acetic acid and NaCl will still produce (rather dilute but nevertheless effective) HCl sufficient to do the job.

!

MazE March 8, 2011 at 8:31 pm

We usually suggest our clients to use vinegar and salt to make copper cookware shine …

Anthony March 9, 2011 at 4:18 pm

This links in pretty nicely with another web-based experiment I saw recently: Cleaning pennies with Taco Sauce. His experiment identifies vinegar and salt as the principal ingredients.

http://www.cruftbox.com/cruft/docs/cleaningcopper.html

At the end of his discussion, he writes a professor who had previously written a paper on the subject and gets the following reply:

“In water, under any conditions, salt breaks down into sodium ions and chloride ions. (Even in the solid, it is in that form, so it would be better to say that the water just allows the ions to separate.)

It’s a little more complicated than that. The copper in the tarnish would be in the 2+ form. Chloride ions do not bond strongly to this form. However, they do bond fairly strongly to the 1+ form. A reaction that would be unfavorable in the absence of chloride ions, Cu + Cu2+ *–> 2Cu1+, becomes favorable when the chloride is present to tie up the Cu1+ form (as [CuCl2]-).

The first process can be viewed as transfer of an electron from Cu (metallic copper) to Cu2+. This is followed by reaction of the Cu1+ with the chloride. The resulting copper-chloride complex is soluble in water. It’s the electron-transfer aspect of the chemistry (the conversion of metallic copper and the Cu2+, to Cu1+) that makes the role of the salt a little surprising.”

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