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

by Stephen Hobley on March 2, 2011

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


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


{ 88 comments… read them below or add one }

JohnBailey March 18, 2011 at 4:56 pm

Tried it last night. My first ever home etched PCB. Worked great.
I tried a toilet cleaner “recipe” a few weeks ago, and wasted a bottle of Peroxide trying to get it working. Not enough acid I think. only 5% or so HCL in the cleaner. All it did was turn the copper pink.

It did take this method about an hour to etch all the unwanted copper off, but I was using a pretty bare board pattern, so quite a bit of copper. And I think I may have got the salt part wrong. More experimentation in my future I think.

Thanks for the article.

Leif - KC8RWR April 13, 2011 at 10:05 am

I tried this twice but all I managed to do was turn the copper pink. The second time I let it sit over 24 hours. I’m going to revisit it though and try one more time, measuring the ingredients very carefully. I’m also going to try changing salts. The salt I have is just labeled salt and doesn’t specify that it is sodium chloride as opposed to calcium, potassium or some other salt. Aside from that I am stumped.

Stephen Hobley April 13, 2011 at 11:03 am

What are the strengths of your peroxide and vinegar?

If you look at the three tests I did one of them went the same way yours did – the metal just went pink. I forget which one this was, but it could have been just the peroxide and vinegar.

Buzai Andras May 2, 2011 at 6:24 pm

Wouldn’t this produce Chlorine(http://en.wikipedia.org/wiki/Chlorine), which is a very poisonous gas (has been used by millitary as a bio weapon)?
I am no chemistry expert, I am just asking. Before I try this I just want to make sure that during etching no harmfull substances are released :).

Stern May 13, 2011 at 5:10 pm

Buzai: You do not have to worry about the generation of chlorine gas. Only in very oxidizing conditions would chlorine gas be generated. So unless you somehow accidently spill some highly concentrated hydrochloric acid and manganese dioxide in there, your solution is safe !

tmk May 19, 2011 at 2:07 am

How long does the process take?

I followed the instructions pretty carefully and after about 45 minutes all i have is a orangey-pink board sitting in a nice blue bath of water. It’s quite a nice color, really.

I’m in the US, and i used white vinegar (says diluted with water to 5%), and hydrogen peroxide (3% solution), and I’ve been adding salt like a madman.

Every time I add salt it bubbles fiercely, which is fun. After a while adding salt doesn’t produce any more bubbling, but if you add more hydrogen peroxide, it bubbles.

It doesn’t seem like yours was bubbling much, so i don’t know what i did differently or incorrectly. Any thoughts?

tmk May 19, 2011 at 2:32 am

only thing i can think of is that the salt i used was ‘iodized’ (morton’s iodized salt)… could that be affecting the reaction?

I’m now out of vinegar so i can’t try the sea-salt i have on hand.. Maybe another time


dsfjkdsaj June 13, 2011 at 3:13 am

Stephen wrote:

“I just tried adding a sprinkle of salt to the (already) potent mix of Hydrogen Peroxide and Muriatic Acid”

Muriatic acid is another name for hydrochloric acid, and, according to the Wikipedia article on it:

“Upon mixing hydrochloric acid with common oxidizing chemicals, such as sodium hypochlorite (bleach, NaClO) or potassium permanganate (KMnO4), the toxic gas chlorine is produced.”[1]

Hydrogen peroxide, as your sister-in-law mentions elsewhere in this post, is an oxidizing agent.

So please be careful! Hydrochloric acid is dangerous enough by itself. You really don’t want to be breathing in chlorine gas on top of it.


Ian June 20, 2011 at 5:06 am

I’ve tried this twice now and it has worked both times. It does take about 45 minutes to etch the board and I have small areas that are spotty with remnants of copper but it’s easy enough to clean these up. It might be the board I used which wasn’t very good quality.

I’ve always been too chicken to use the stronger solutions like muriatic acid so this is great thanks!

Is it possible to reuse the mix or is it better to use a new batch each time?

Ian July 29, 2011 at 12:45 pm

I’ve just had another go today with the hydrogen peroxide/vinegar mix and unfortunately it didn’t work properly. I used photo resist board this time to do a nice neat layout using a UV box and a laser printed pattern, then developed it with caustic soda. I rinsed everything carefully, then added it to the solution. It did start to bubble quite nicely and turn yellow as before. However, after a while the reaction stopped and I ended up with a discoloured copper board. The board I am using this time is better quality and I think the copper is thicker. I am going to try ferric chloride and I think the board should be OK to try again.

ardencaple August 8, 2011 at 9:15 am

In fact, this is just the starter reaction of a very interesting etching chemistry.
(I just found this today)

A full explanation is here:


(I just found this today)

Kaz August 17, 2011 at 10:51 am

This etching method is known already. Google around.

Muqarrab August 21, 2011 at 3:23 am

Hey Buddy, I m so Happy, I tried it last night…. It took Almost 20 hours, but final product was worth it, i m a beginner and tried anhydrous ferric chloride solution but it didn’t etched at all..Nobody told me to use HCL Along with that.. well that’s an other matter. Thanx for the product..

David September 5, 2011 at 4:48 am

Stephen, could you please post an update, because i think we’re mixing things up here…. Sometimes, you say you use Muriatic Acid (HCl), and on other times, you talk about vinegar.

The Hcl, H2O2 and NaCl “recipe” works well, and that’s common knowledge (as of today). But what you did is replacing the Hcl with vinegar. Am I right ?

Stephen Hobley September 5, 2011 at 7:41 am

I replaced the muriatic acid with vinegar, and found that this could etch copper too.

Byron September 21, 2011 at 2:13 am

My roommate showed me this a while back, but I just got around to trying it out. It isn’t as fast a ferric chloride, but works really well and isn’t nearly as nasty. It took me about 25min to etch a 4.5in x 2in double-sided board with about 40% of the copper masked off. My board was 1/2oz copper so the 1oz (Radio Shack) boards should take a bit longer. I constantly agitated the board with a toothbrush and added a good sprinkling of salt about every minute. One note I wasn’t sure about at first: the Vinegar and H2O2 are how you buy them, no need to dilute them any more.

Works great, I won’t be going back to Ferric Chloride. Thanks a bunch for the info.

cat December 7, 2011 at 5:03 am

imho the bubbling when poring salt in a acidic solution comes from agents (calciumcarbonate) added to kitchen salt to keep it corny (ie. not forming lumps of salt due to air humidity).
so pouring LOTS of salt in the acid may nearly neutralize it.

Joseph December 10, 2011 at 1:38 pm

Etched my first couple of PCB boards the other day using this method and it worked great. Didn’t bother measuring the vinegar or hydrogen-peroxide, just mixed approximately the same amount in a paper bowl (not much more than it took to cover the surface of the copper), added some salt, and used a watercolor paint brush to periodically remove the gunk from the copper’s surface.

I used un-iodized salt so, as one commenter suggested, that may be the issue for those who say they only manage to turn their boards pink. Also, the reaction goes better if you don’t skimp on the salt.

Tom December 16, 2011 at 9:58 pm

If I may add a bit of chemistry… :)

The reason why the addition of chloride ions helps this reaction forward is because rather than forming “CuCl2″ you form a number of rather more complicated kinds of compounds.

You see copper is what chemists call a transition metal, transition metals are characterized by their ability to react reversibly with molecules like water, ammonia, chloride ions (Cl-) and tons of others. When you dissolve CuCl2 in water a situation arises in which a number of different complex ions exist, one of them is the tetrachlorocuprate ion in which four chloride ions and one Cu2+ ion have reacted to form a very soluble ion. This species is more prominent in solution when more chloride ions are present which is clearly the case when you add the table salt (NaCl). The high solubility of the ion means that any copper that is oxidized by the hydrogen peroxide is quickly whisked away never to return. This of course speeds up the reaction.

Another added advantage of a higher chloride concentration is the fact that chloride adheres to the surface of the raw copper and makes it more susceptible to oxidation by “preparing” the copper atoms near the surface to become “CuCl2″ and other similar kinds of complex ions.

Just saying…

Stephen Hobley December 17, 2011 at 2:55 am

And said well… ;-)

Andrey February 27, 2012 at 12:24 pm

Buzai Andras:
if you mix ferric chloride with ammonium persulphate, the amount of Clorine gas produced is huge !!!!! it’s poison even for plants in that room!

both substances are used to etch PCB, so don’t you ever mix them!

Stephen Hobley February 27, 2012 at 12:39 pm

That sounds a bit like “don’t think of an elephant”

D.plomat March 5, 2012 at 7:06 am

I’ll try your solution as it seems to produce the same result but with much less nasty chemicals than mine.

I use a mixture of (added in this precise order just before etching):
1/3 water
1/3 130 vol. (35% IIRC) Hydrogen peroxyde
1/3 25% Muriatic acid

Has the same advantage of transparency where you can see the progress (well, except the bubbling), etchs in less than a minute, produces copper chloride (autocatalytic) and warms a bit so it improves efficiency with each etched PCB until it slowly self decomposes (either HCl or H2O2 seems depleted some hours after mix even if not used).

On the down side i must wear protective goggles (never had a spill, but never know…) and avoid breathing the fumes (Chlorine, doesn’t produce a lot so in exterior or well ventilated room it’s ok but better avoid taking a deep breath right above).

Maybe at those concentrations it’s the HCl that adds in Chlorine ions in excess as salt does in your solution. Will try adding some salt to see if it etchs faster (but then it would be too fast for precise control)

Anyway thanks to your method i’ll keep it simple+cheap+clean+efficient AND get rid of the ‘nasty’ aspects of the process and outdoor etching sessions during cold winter.

a fellow electronics hobbiest March 8, 2012 at 5:14 pm

this is absolutly brilliant!!! ill be sure to try it some time! i have a question though and would love it if you could relpy promptly. i read on another blog that, while this solution works just as well as ferric chloride, it must be disposed of as such. i dont know how. ive never etched before and thus, never used ferric chloride before. so i am in uncharted waters here. i read on yet another blog that you can dispose of ferric chloride down a drain, only after adding sodium carbonate to raise the ph levels to a friendly 7.0 or 8.0. will this also work with your solution? again, if you could reply fairly quickly, i would greatly appreciate it.
do keep up the good work!

many thanks


Garrick March 26, 2012 at 6:19 am

I just tried this for a small board, and it didn’t do anything except fizz fine bubbles for a half hour. I eventually found I should rub the brown layer off until I got the pink copper. I had to do that every few minutes. The amount of force I had to rub with started to damage the resist traces. But it eventually worked.

Shannon April 12, 2012 at 8:27 pm

Hi! I have a question about this recipe. I have recently read that ferric chloride can be used to etch steel as well as copper. Would there be anything dangerous about using this mixture to attempt to etch steel? I don’t know enough about Chemistry yet to determine this for myself.

Wayne Sallee April 24, 2012 at 4:31 pm

@ Shannon:
Using ferric chloride to etch steal would be like using coper chloride to etch coper. No it’s not any more dangerous, but rater the ferric chloride would be a bit exhausted for etching steal. “Ferric” means iron”. Ferric chloride does not fume the air as much as hcl because it’s bound to the iron, but apparently when using it to etch coper, the chlorine likes the coper better than the iron so it let’s go of the iron, making the water muddy looking, and bods to the coper producing a soluble coper chloride. So if you want to etch iron, you might as well use a different acid than ferric chloride. You could use hcl acid. Then when it reached saturation you could use that to etch copper.

Wayne Sallee

Clinton June 24, 2012 at 5:17 pm

The solution works fine just slow. This is a safe alternative. Thanks again

KJ September 7, 2012 at 12:29 pm

Thanks, will give this a trial and effort shot. Never etched a pcb before, meets all three areas that must be achieved (and still maintain safety)…. cheap,cheaper and my favorite CHEAPEST. Thanks again.

Andy September 22, 2012 at 11:29 pm

I used this method (in the US, as well). I tried it on a tiny little board and it worked pretty well. I then tried it on a 6″x9″ board with quite a bit of copper to remove and, after adding salt until it would work anymore, I refreshed the bath with new vinegar and peroxide. It wound up taking about 30 minutes. I use the photo resist method to make my boards now (used to use toner transfer) and to keep the edges clean and sharp, IMHO, faster etching is better. I’m switching back to the environmentally unfriendly ferric chloride. Makes me sad, but it’s faster.

Cass September 30, 2012 at 5:24 pm

Do I just draw the lament pattern on the copper square with a marker pen ??

Cass September 30, 2012 at 5:25 pm

How do I get that pattern on the copper

Ultrafro January 9, 2013 at 2:22 pm


How are you storing the contents? Is it possible to keep the solution in an air-tight container (such as tupperware)? I read elsewhere that peroxide/acid solutions will build pressure over time and be dangerous if kept in a sealed container. Any science behind that, or were you able to use air-tight containers?


Kharvyu` January 28, 2013 at 3:40 pm

Allyson is a genius!

David Greg Taylor January 16, 2014 at 1:44 pm

Hi Stephen,

In researching nontoxic intaglio printmaking, I came across this blog post, and using the information, came up with a way to do etchings without the toxic acids, like nitric, or the very unpleasant staining of ferric chloride. I especially like buying my etching solution at the grocery store. Thanks for the inspiration!

Here is the finished product that came out of this experiment in intaglio:

David Greg Taylor January 16, 2014 at 8:37 pm

Since posting earlier in the day, I’ve come across a lot of research that warns of the danger of an environment of peracetic acid, which is the combination of vinegar and hydrogen peroxide. Not much better than nitric acid. It does appear that ferric chloride may be quite a mess to deal with, but it doesn’t kill you. At least I got one smoking’ cool etching out of it. :)

Al February 21, 2014 at 6:22 am

I know that vinegar loses strength when it gets old. Which could explain some people’s lack of success. I had to throw out a gallon jug of vinegar that had lost all smell and taste. A whole tablespoon tasted like plain water. It had been opened and sitting in my pantry for several years. Use a fresh bottle of vinegar and peroxide. I’m going to try this method. Thanks.

Ki Fredeen May 10, 2016 at 11:40 pm

Recently, I have talked to my AP Chem teacher and done some research and I have found that the table salt is unlikely to be involved in equilibrium, since equilibrium (when the rate of ionization of copper is equal to the rate of formation of solid copper) is only affected by the concentrations of the reactants and products of the reaction.
The reaction that is dissolving the copper is a redox reaction, involving the exchange of electrons between the solid copper and the H2O2 in acidic solution to create dissolved copper ions (paired with the acetate ions left over) and water from H+ ions and the H2O2. The formula is Cu + 2H+ ion + H2O2 + > Cu+2 ion + 2H2O. Notice that neither chlorine or sodium are involved in this reaction, so the addition of sodium or chloride ions will not effect equilibrium.
Notice that the reaction requires the flow of electrons between the copper and the peroxide, and in distilled water (It can be safely assumed that companies use distilled to ensure the purity of their products) no electrons can be transmitted without the presence of dissolved ions. The vinegar provides some ions, but vinegar is a weak acid, and less than one percent of the molecules actually release their hydrogen and acetate ions. So in reality, the reaction is taking place, but is hindered by the slow flow of electrons caused by a lack of ions.
The addition of the salt makes the solution more conductive, allowing for a faster transfer of electrons between the copper and the peroxide, speeding up the pace of the reaction to a more visible pace. This is why iron/steel rusts faster in salt water, because the salt dissolved in the water enables the required electrons to flow.

I hope this has helped people better understand the role of salt in this reaction.

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