…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


{ 5 comments… read them below or add one }

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.

Watson August 21, 2017 at 11:01 pm

We used to etch PCBs with FeCl using a heat lamp over the tray. It was highly dependent on the temperature. I think some of the problems, like taking so long, would be helped a lot by keeping the solution warm.

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