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.

Quote: Originally posted by bbartlog

I would always sort my pennies when doing reactions of this type...

Quote: Originally posted by liquidlightning

The precipitate also forms with copper wires, and even a couple small pieces of a 99.999% copper ingot.

Hmmm. You leave me only 2 choices: pretend I didn't see your post or continue investigating. Haven't decided which yet.

Quote: Originally posted by triplepoint

You leave me only 2 choices: pretend I didn't see your post or continue investigating.