SnailsAttack
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Mass-production of copper acetate from metallic copper and vinegar
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.
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SnailsAttack
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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]
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njl
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| 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.
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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.
Reflux condenser?? I barely know her!
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DraconicAcid
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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.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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njl
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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).
Reflux condenser?? I barely know her!
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Texium
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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.
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SnailsAttack
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Yeah that's what I said
Texium is right about the MRS being responsible for copper's inertness
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DraconicAcid
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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.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Amos
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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]
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Fulmen
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The acid isn't the oxidizer, it's the atmospheric oxygen. So the net reaction should be :
2Cu + O2 + 4HAc = 2CuAc₂ + 2H₂O
We're not banging rocks together here. We know how to put a man back together.
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Rainwater
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| 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.
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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]
"You can't do that" - challenge accepted
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SnailsAttack
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| 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.
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good to know. thanks
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Texium
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Thread Moved
26-11-2022 at 10:36 |
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