Sciencemadness Discussion Board

Mass-production of copper acetate from metallic copper and vinegar

SnailsAttack - 10-2-2022 at 07:21


This post is a study of various methods I've tested to produce copper(II) acetate.

= Synthesis of copper acetate

Copper acetate is unlike most other metal acetates in that it cannot be synthesized by the direct action of vinegar on plain metallic copper, like so:

Cu + 2CH₃COOH -> Cu(CH₃COO)₂ + H₂

This reactions does not proceed. Or at least not at ambient conditions over the course of a week.

The reason for this might be that copper is lower on the metal reactivity series than hydrogen, and therefore it can't swap with vinegar's hydrogen ion without a catalyst (of which several are known).


= Salt-catalyzed vinegar bath method

The fastest and most efficient way to produce copper acetate is by soaking metallic copper in a vinegar bath that also contains a small amount of salt. I believe the chlorine ions act as a catalyst. I have also tried sodium sulphate (without any vinegar) but it had no effect on the copper whatsoever over the course of a week.

The bath will turn blue within a few minutes(?) and should be depleted of free vinegar within 12 hours, after which the solution appears to generate large amounts of basic copper carbonate or a similar compound through rapid degradation of the remaining copper metal. (further testing required)

The copper acetate can be separated from the salt by repeated recrystallizations, mechanical separation (picking out all the individual crystals with a pair of tweezers or whatever) and by exploiting the fact that salt is much more soluble than copper acetate.

The salt-containing crystals will tend to manifest as turquoise, fungi-looking structures. I haven't yet pinned down precisely what they are, perhaps a double salt, but I call them "strudels". You'll know them when you seen them.


= Peracetic acid method

High-purity copper acetate can be synthesized directly from metallic copper and hydrogen peroxide via the net reaction:

2Cu + 4CH₃COOH + 4H₂O₂ -> 2Cu(CH₃COO)₂ + 6H₂O + O₂

This reaction takes only about 20 minutes to finish at room temperature, but proceeds instantaneously at near-boiling (be cautious of the resulting offgas). The reaction mechanism probably works via formation of peracetic acid:

CH₃COOH + H₂O₂ <-> CH₃CO₃H + H₂O

2Cu + 4CH₃CO₃H -> 2Cu(CH₃COO)₂ + 2H₂O + O₂

Originally I believed that the reaction proceeded via the following formula:

Cu + 2CH₃COOH + H₂O₂ -> Cu(CH₃COO)₂ + 2H₂O

But this didn't explain the bubbles that were evolved as the reaction works. On an internet thread, one user proposed that "the copper(II) ion is a catalyst for hydrogen peroxide, and the copper metal serves as a nucleation site for gas to form, thus making it appear that the metal itself is the catalyst."

I believed this for a while, because it's a very clever explanation. But it's not true, because it wouldn't make sense for hydrogen peroxide to catalyze the formation of copper acetate without inherently being consumed as a direct constituent in the reaction.

This could be verified by testing for an accelerated decomposition of hydrogen peroxide in the presence of copper(II), or by testing the offgas for the presence of hydrogen which should not be produced in the peracetic acid route.



= Electrolytic method

This method is the most work. If you have the equipment to perform electrolysis, copper acetate can be produced by using copper electrodes in a vinegar bath:

CH₃COOH -> H⁺ + CH₃COO⁻

Cathode:
2H⁺ -> H₂

Anode:
2CH₃COO⁻ + Cu -> Cu(CH₃COO)₂

The reaction proceeds faster with the addition of a small amount of sodium sulphate, probably due to the formation of copper(II) hydroxide which reacts with the vinegar to produce copper acetate. The reaction is modified like so to incorporate the electrolysis of water:

CH₃COOH -> H⁺ + CH₃COO⁻
H₂O -> H⁺ + OH⁻

Cathode:
2H⁺ -> H₂

Anode:
2CH₃COO⁻ + Cu -> Cu(CH₃COO)₂
2OH⁻ + Cu -> Cu(OH)₂
4OH⁻ -> 2H₂O + O₂

The electrodes decay in unique ways. The anode tapers to a needle point (careful of that) while the cathode flakes apart as it grows fractal-like branches of what I believe to be maybe copper(I) oxide or otherwise metallic copper.

I'm not experienced with electrochemistry and these formulas may be innaccurate.

Most of the copper acetate can be separated from the sodium sulphate electrolyte by repeated recrystallizations and mechanical separation, and the remaining sodium sulphate can be purified by heating until all the leftover copper acetate is decomposed to insoluble copper oxides.

Sodium sulphate is much more soluble than copper acetate, and that can be exploited to separate them as well.

SnailsAttack - 10-2-2022 at 07:24




Copper(II) acetate monohydrate crystals.




A picture of the setup described in the salt-catalyzed vinegar bath method.

[Edited on 2/10/2022 by SnailsAttack]

njl - 10-2-2022 at 08:03
Quote:

I believed this for a while, because it's a very clever explanation. But it's not true, because it wouldn't make sense for hydrogen peroxide to catalyze the formation of copper acetate without inherently being consumed as a direct constituent in the reaction.


It's not catalytic, it's consumed in the reaction. The way I see it is this: the peroxide is there to oxidize the copper metal into copper ions (as CuO). This will happen without acetic acid. Once the copper is oxidized it's simply a matter of exchanging the oxygen with acetate or whatever other anion you want. This is easily done with acids because the product is the desired salt and water. Peroxide is not catalytic here. That being said, there is a catalytic reaction between copper salts and peroxide that accelerates decomposition into H2O and O2.

What's missing from my explanation is exactly why acetic acid won't oxidize copper on it's own (at an appreciable rate). For some reason I get the feeling it isn't just acid strength, but I'm not sure.

DraconicAcid - 10-2-2022 at 08:31

Acetic acid (or any other simple acid) isn't a strong enough oxidizing agent to oxidize copper. End of story.

The reason you get any reaction at all is because you're doing the reaction in air.

2 Cu + 4 CH3CO2H + O2 --> 2 Cu(CH3CO2)2 + 2 H2O

Hydrogen peroxide can also act as an oxidizing agent, but some of it will decompose rather than reacting with the copper, because metal salts catalyze its decomposition.

Halide ions will catalyze the reaction between copper and oxygen by allowing copper(II) ions to react with copper metal to form soluble copper(I) complexes, which can then diffuse to the surface of the solution where all the oxygen is.

njl - 10-2-2022 at 10:45

Quote: Originally posted by DraconicAcid  
Acetic acid (or any other simple acid) isn't a strong enough oxidizing agent to oxidize copper. End of story.


This is just another way of saying what I said, which is that the reaction won't happen. I understand that acetic acid alone can't oxidize copper, I don't understand why. The high school chemistry class answer is because it's not a strong enough oxidizing agent. I don't understand why the anion matters if H+ is being reduced to H2, and the metal is being oxidized (yet it clearly does).

Texium - 10-2-2022 at 11:09

The anion doesn't matter. HCl, H2SO4 etc can't oxidize copper either. Think about it from the opposite perspective. Copper metal, unlike iron, zinc, aluminum, etc is not a strong enough reducing agent to reduce H+ to H2. Look up a standard reduction potentials table, and you can see that copper sits right below hydrogen.

SnailsAttack - 10-2-2022 at 14:51

Quote: Originally posted by njl  
It's not catalytic, it's consumed in the reaction.


Yeah that's what I said

Texium is right about the MRS being responsible for copper's inertness

DraconicAcid - 11-2-2022 at 09:01

Quote: Originally posted by njl  
I don't understand why the anion matters if H+ is being reduced to H2, and the metal is being oxidized (yet it clearly does).


The only time the anion matters is if the anion is itself an oxidizing agent (such as nitrate), or if it's a halide. Halides will form soluble complex ions with copper(I) and copper(II), catalyzing the reaction with atmospheric oxygen.

Amos - 11-2-2022 at 11:04

If all of the acetic acid is consumed in the aforementioned reactions, you'll start to get what I believe are actually basic acetates forming as a precipitate. They're not very easy to re-dissolve or do anything useful with, so it's really pretty important to have excess acetic acid there.

Vinegar and copper just isn't a good idea. The title of the post is "mass production" but I only see maybe a gram or two of crystals surrounded by messy basic precipitates. You'll never make household vinegar work on a large scale without way too much work. Glacial acetic acid is extremely affordable and is cleaner and faster to work with in every sense. And in most places copper sulfate is also readily available, so that copper metal can be skipped altogether and basic copper carbonate can be used as the precursor to copper acetate. All these workarounds and secondary reagents being used to beg the copper to react only need to be brought into the conversation if someone is just too lazy to order some GAA or make basic copper carbonate, and do it a better way.

[Edited on 2-11-2022 by Amos]

Fulmen - 11-2-2022 at 16:38

Quote: Originally posted by njl  
I understand that acetic acid alone can't oxidize copper, I don't understand why


The acid isn't the oxidizer, it's the atmospheric oxygen. So the net reaction should be :

2Cu + O2 + 4HAc = 2CuAc₂ + 2H₂O

Rainwater - 12-2-2022 at 02:54

Quote: Originally posted by SnailsAttack  

the cathode flakes apart as it grows fractal-like branches of what I believe to be maybe copper(I) oxide or otherwise metallic copper.


From my experiments, you are correct. This was minimized by using a thin piece of wire as the cathode and bagging it to collect the fall off.

A direct reaction seems problematic. Wiki has a description of a production method

http://www.sciencemadness.org/smwiki/index.php/Copper(II)_ac...
http://www.sciencemadness.org/talk/viewthread.php?tid=11593

[Edited on 12-2-2022 by Rainwater]

SnailsAttack - 15-2-2022 at 18:55

Quote: Originally posted by Amos  
If all of the acetic acid is consumed in the aforementioned reactions, you'll start to get what I believe are actually basic acetates forming as a precipitate.

I've heard about "basic" copper acetate before but I think it could also be copper(II) hydroxide produced by hydrolysis of the copper acetate. There's not much information about this on the internet.
Quote: Originally posted by Amos  
The title of the post is "mass production" but I only see maybe a gram or two of crystals surrounded by messy basic precipitates. You'll never make household vinegar work on a large scale without way too much work.

Oh I've got way more than what's in the pictures, probably 10 grams by now. Hypothetically I should be able to produce 40 grams of copper acetate per every 0.47 L of vinegar I buy. Separation is easy since the copper acetate crystals dissolve realllyyy slowly.
Quote: Originally posted by Rainwater  

From my experiments, you are correct. This was minimized by using a thin piece of wire as the cathode and bagging it to collect the fall off.

good to know. thanks