triplepoint
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ID of precipitate formed in CuAc from pennies
First, the legal issues:
Disclaimer: I do not claim any expertise in federal statutes and regulations about destroying pennies. Don’t rely on me for legal advice. Do your
own research or use a different source of copper.
That said, I have done some limited research on whether it is legal to use pennies for non-monetary purposes like this. I have satisfied myself for
my purposes that it appears to be legal. Prohibitions on defacing currency, etc, appear to be primarily aimed at stopping people from: (1) passing
off smaller denominations as larger ones (to use a crude example, adding a “0” to a $1 bill and passing it off as a $10 bill, and (2) shaving or
“clipping” coins – removing some of the metal from a coin and passing it for the value of the original coin. This would be a real issue in the
case of a coin made of precious metal, like gold or silver. It is illegal to remove some of the metal from a coin so you can keep it, then spend the
shaved or clipped coin. This debases the currency because these coins used to be made of a known weight of precious metal. Shaving or clipping the
coin meant that people would not be able to trust the value of the coin and it would impair the conduct of financial transactions
Even if I am wrong, I suspect that federal law enforcement personnel have more important priorities to investigate and prosecute, like whether the
toilets in our bathrooms use too much water per flush or whether we are using contraband light bulbs to light our houses.
The observed reaction:
Copper acetate formed by reaction of copper pennies with hydrogen peroxide (H2O2) and acetic acid (AcOH) forms blue solution
with a finely divided white precipitate. Gentle heating of the mixture accelerates the process, but it will occur even at room temperature. I used
household strength ingredients (3% H2O2 and 5% AcOH), but I believe that the specific strength is not significant.
My understanding of the reaction is as follows: Cu(s) + H2O2(aq) + 2H+(aq) = Cu2+(aq) + H2O(l).
Some of the white precipitate apparently remains in solution, because after removing the excess copper and filtering the blue (copper acetate)
solution, some additional white precipitate appears.
When the copper acetate crystallizes, it excludes the white precipitate.
Attempts to identify the white precipitate:
Test 1 I first gently heated H2O2 in order to see whether the precipitate was the result of a contaminant in
the H2O2. Result: no precipitate. So, the H2O2 alone was not the source of the precipitate.
Test 2 I next performed the same process with the AcOH. Result: no precipitate. The AcOH alone is now ruled out as the source of
the precipitate.
Test 3 I then tested H2O2 together with the AcOH in order to determine whether the precipitate was caused by a
side reaction. Result: no precipitate.
Note that H2O2 and AcOH form peracetic acid (CH3CO3H). The formula is H2O2 +
CH3CO2H ==> CH3CO3H + H2O. That does not mean that there is no
CH3CO3H formed. Presumably there is, but it remains in colorless solution and does not account for the precipitate.
Test 4 Since I have now excluded the H2O2 and the AcOH, either alone or in combination with each other, as
sources of the white precipitate, I next turned to the question of whether it was caused by a contaminant in the pennies. Well, I knew that the
pennies I was using were (mostly) copper. Prior to 1982, US pennies were made of copper. They were switched partway through the 1982 production year
to a thin cladding of copper over zinc. A little research disclosed that the pre-1982 copper was actually an alloy that was mostly copper.
The precise composition has varied somewhat from time and most recently (prior to 1982) was 95% copper and 5% zinc. I therefore performed a synthesis
of zinc acetate to compare the result with the mystery precipitate. Zinc metal placed in a solution of H2O2 and AcOH forms
{drumroll}: a finely divided white precipitate that looks very much like my mystery precipitate.
Conslusions:
I am satisfied for my purposes that this white precipitate is (mostly) zinc acetate.
It is entirely possible that this precipitate is actually zinc acetate mixed with acetates of other metals in alloy with the copper (since the
composition of the penny varied from time to time and I did not segregate years when plunking pennies into the reaction vessel). The most likely
additional acetate is tin acetate, as tin was definitely used in the copper alloy at times. This variation in composition would also be consistent
with my observation that the quantity of the white precipitate varied from batch to batch, while the gross quantities of the reactants remained the
same between batches.
I did not perform any qualitative analyses of the substance because I am confident enough to satisfy my curiosity that this precipitate is primarily
zinc acetate.
I hope I haven’t bored you to death with this post. It is a very simple, basic example of the detective work involved in chemistry.
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bbartlog
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I would always sort my pennies when doing reactions of this type... of course, the zinc present even in pre-1982 pennies means that I still can't make
pure copper compounds directly by this method.
| Quote: | | Prohibitions on defacing currency, etc, appear to be primarily aimed at |
Some of those laws were aimed at the folks that melted down pre-1964 quarters (and I imagine other old mostly-silver coinage) en masse, back when they
were still in circulation. Used to be a bit of an underground business sometime in the 1970s.
The less you bet, the more you lose when you win.
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liquidlightning
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The precipitate also forms with copper wires, and even a couple small pieces of a 99.999% copper ingot.
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triplepoint
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To clarify, when I said I didn't sort, I meant by year other than pre- and post- 1982.
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triplepoint
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Hmmm. You leave me only 2 choices: pretend I didn't see your post or continue investigating. Haven't decided which yet.
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watson.fawkes
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Or 3) You could
verify the claim, see if it matches your own experiments. You could also try it with a bit of zinc ingot.
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liquidlightning
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Just performed the experiment. Made it via electrolysis with copper/graphite electrodes. The precipitate still formed. I think that this might be
cuprous acetate. It forms as per the following.
Cu + Cu(CH3COO)2 ---> 2 CuCH3COO
And found this on the wiki of Cupric Acetate
| Quote: |
Unlike the copper(II) derivative, copper(I) acetate is colourless and diamagnetic. |
However, it only formed for me with the electrolysis method after I boiled it all down and accidentally burned a little. The amount that burned
decomposed into copper and acetic acid. This copper probably reacted with the cupric acetate. Wiki mentions this is a way cuprous acetate can be made.
| Quote: |
Heating a mixture of anhydrous copper(II) acetate and copper metal affords cuprous acetate |
I'll perform some tests with copper powder. Will report back when I do.
[Edited on 4-7-2012 by liquidlightning]
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liquidlightning
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Yes that was it. Also I think Copper (II) Acetate may decompose to Cuprous Acetate if not dissolved in slightly acidic conditions.
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triplepoint
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I don't believe cuprous acetate could account for the substance I have been seeing, at least not via the routes described. My solutions are all
acidic when this occurs. It appears without heating and upon gentle, low level heating. However, that could account for its appearance from
apparently pure copper. Perhaps it is still formed when conditions of ph and temperature are less than perfect, just in smaller quantities.
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unionised
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I suspect that our bronze-age ancestors might have offered some insight into this.
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