Sciencemadness Discussion Board - Copper from pre-1981 US pennies

Copper from pre-1981 US pennies

ScienceBum - 5-6-2016 at 14:01

Howdy. I'm doing a video demo on numismatics and I'm covering U.S. pennies in particular, most of them after 1982 are just copper-plated zinc coins. But the ones 1981 and before are actually a roughly 95% copper/zinc alloy, and I was wondering if there was some chemical way to extract the copper for them for the demo or if it /had/ to be melted down and separated that way?

So I'm looking for something like a substance that can dissolve zinc and copper both, and later on adding something that can precipitate out the copper and leave zinc in solution. Are there any chemicals like that that you guys know of? And if not would the copper/zinc alloy pennies still dissolve in nitric acid for example?

Any information on this would be appreciated, thanks so much <3

Orenousername - 5-6-2016 at 15:01

You can probably just score the pennies so that the zinc is exposed and soak them in HCl

Of course some CuCl2 will form as well but it will be negligible without an appropriate oxidizing agent.

[Edited on 6-5-2016 by Orenousername]

j_sum1 - 5-6-2016 at 16:31

Yes but why?

If you want copper then electrical wire is a really good source and high purity.

pepe - 5-6-2016 at 16:40

Quote: Originally posted by j_sum1

Yes but why?

If you want copper then electrical wire is a really good source and high purity.

Pretty much this.

Also I would be wary of destroying US currency in an attempt to show some science. Sure its just a penny but it is a federal offence. Dont mean to be "that guy" but I would hate to see a fellow chemistry enthusiast get nabbed for something silly.

Orenousername - 5-6-2016 at 16:45

To be fair he did say it was for a demonstration...

Also, in high schools around the US the "golden penny" demonstration is fairly common, heating a Cu/Zn penny until they alloy to form bronze. Granted the teachers are federal workers, exempt from law

S.C. Wack - 5-6-2016 at 19:00

Quote: Originally posted by pepe

Also I would be wary of destroying US currency in an attempt to show some science. Sure its just a penny but it is a federal offence. Dont mean to be "that guy" but I would hate to see a fellow chemistry enthusiast get nabbed for something silly.

The prohibition contained in § 82.1 against the treatment of 5-cent coins and one-cent coins shall not apply to the treatment of these coins for educational, amusement, novelty, jewelry, and similar purposes as long as the volumes treated and the nature of the treatment makes it clear that such treatment is not intended as a means by which to profit solely from the value of the metal content of the coins.

pepe - 5-6-2016 at 19:08

Quote: Originally posted by S.C. Wack

Quote: Originally posted by pepe

Good to know. I just went with the assumption that 5 and 1 cent coins would be lumped in with all currency in terms of the legal status of deliberate destruction. Im glad to see that it includes "amusement" as an accepted purpose. I guess it makes sense to regulate the treatment by quantity.

careysub - 6-6-2016 at 06:33

It is not illegal to destroy US currency, as long as your are not melting down pennies and nickels for their metal content. Period.

It is illegal to alter currency and then pass it on as currency.

This prohibition is to protect against a form of counterfeiting, altering existing bills. It is technically illegal to draw a mustache on George Washington and then spend the dollar bill, but no one ever gets prosecuted for this. Treasury only cares if you are trying to alter value or in some sense create currency.

ScienceBum - 6-6-2016 at 09:33

Thanks guys, yeah I know about the US currency laws (and this is for a youtube demo vid I plan on posting later). I also know that I can get copper easier from wires or by just straight up buying it, but really I'm interested in the chemistry for this. I think it'd be nifty if there was an easy way to extract the copper for future labs I could do, but if it's really not feasible then no worries..

Also, in response to Orenousername where you said I could just score the pennies to expose the zinc, that's true but not all of the zinc would be exposed since it's alloyed and not plated right? So for example, if nitric acid dissolves copper and zinc (it should since it's an acid and they're both metal right?) what would be a possible way to precipitate out the copper in pure form? Anything that does that?

Thanks for your replies <3

Orenousername - 6-6-2016 at 10:33

Quote: Originally posted by ScienceBum

Thanks guys, yeah I know about the US currency laws (and this is for a youtube demo vid I plan on posting later). I also know that I can get copper easier from wires or by just straight up buying it, but really I'm interested in the chemistry for this. I think it'd be nifty if there was an easy way to extract the copper for future labs I could do, but if it's really not feasible then no worries..

Also, in response to Orenousername where you said I could just score the pennies to expose the zinc, that's true but not all of the zinc would be exposed since it's alloyed and not plated right? So for example, if nitric acid dissolves copper and zinc (it should since it's an acid and they're both metal right?) what would be a possible way to precipitate out the copper in pure form? Anything that does that?

Thanks for your replies <3

Given enough time HCl would probably dissolve all the zinc. Nitric acid would dissolve both because it's an oxidizing acid. HCl + H2O2 would work as well. You could probably precipitate copper hydroxide out with sodium hydroxide. It is commonly prepared from NaOH and copper sulfate so will probably work with other salts, but Zn(OH)2 is quite insoluble in water as well.

[Edited on 6-6-2016 by Orenousername]

Texium - 6-6-2016 at 12:22

Ok, here's what you do:

Dissolve the whole thing in nitric acid. This leaves you with a blue solution containing a mixture of zinc nitrate and copper(II) nitrate. Alternatively, use a mixture of hydrochloric acid and hydrogen peroxide in place of nitric acid. Next, add a solution of sodium hydroxide until the solution is no longer acidic, and then continue to do so until all of the copper precipitates as the hydroxide and the solution becomes colorless. Zinc is amphoteric, so it stays in solution as colorless zincate anions. Filter out the copper(II) hydroxide (which by this point is probably starting to turn brown as it becomes copper(II) oxide), and redissolve it in an acid of your choice. At this point, you should be able to show the presence of zinc in the colorless solution by adding a drop of dilute ferrocyanide solution. A white precipitate should form, possibly tinted slightly blue or brown from excess copper remaining in the solution.

Orenousername: Kindly stop posting assumptions that may mislead people. Scoring an alloyed penny has absolutely no effect on how HCl would effect it, and HCl would not dissolve all of the zinc "given enough time" as most of the zinc is trapped well within the core of the penny, surrounded by copper at the atomic level. The only way to get good separation is by dissolving it entirely. I'm calling you out on this now because you've done it in a few other threads too and it's starting to become an annoying pattern.

Orenousername - 6-6-2016 at 12:47

Quote: Originally posted by zts16

Orenousername: Kindly stop posting assumptions that may mislead people. Scoring an alloyed penny has absolutely no effect on how HCl would effect it, and HCl would not dissolve all of the zinc "given enough time" as most of the zinc is trapped well within the core of the penny, surrounded by copper at the atomic level. The only way to get good separation is by dissolving it entirely. I'm calling you out on this now because you've done it in a few other threads too and it's starting to become an annoying pattern.

I'm sorry, I am unfamiliar with much of this stuff. The 0.8% copper in the zinc alloy enough to produce this effect? Is there a place where I can read about the generalities of this kind of chemistry? Or will I have to hunt through hundreds of research papers..

I am not meaning to mislead people, although I should probably make it clear that I'm inexperienced and don't know what i'm talking about.

unionised - 6-6-2016 at 12:51

Quote: Originally posted by Orenousername

... ZnCl2 is quite insoluble in water as well.

Nope

unionised - 6-6-2016 at 12:53

You could just hacksaw a penny in half and show the "silvery" colour of the bit in the middle. Then drop it into dilute acid and leave it to attack the zinc. Eventually you might even get some sort of "shell" of copper.

Orenousername - 6-6-2016 at 12:55

Quote: Originally posted by unionised

Quote: Originally posted by Orenousername

... ZnCl2 is quite insoluble in water as well.

Nope

Sorry, meant zinc hydroxide.

DraconicAcid - 6-6-2016 at 15:15

Quote: Originally posted by pepe

Im glad to see that it includes "amusement" as an accepted purpose. I guess it makes sense to regulate the treatment by quantity.

There are machines made to stretch coins and imprint them with a stamp to serve as a souvenier of some place. Those were probably the "amusement" they had in mind.

S.C. Wack - 6-6-2016 at 16:58

Quote: Originally posted by ScienceBum

So I'm looking for something like a substance that can dissolve zinc and copper both, and later on adding something that can precipitate out the copper and leave zinc in solution.

I believe the correct answer is: copper, copper sulfate in sulfuric acid, and electrons...What is copper, copper sulfate in sulfuric acid, and electrons?...It would be shocking if there were no videos of penny anodes in beakers out there.

The Composition of the Cent

Following is a brief chronology of the metal composition of the cent coin (penny):

The composition was pure copper from 1793 to 1837.
From 1837 to 1857, the cent was made of bronze (95 percent copper, and five percent tin and zinc).
From 1857, the cent was 88 percent copper and 12 percent nickel, giving the coin a whitish appearance.
The cent was again bronze (95 percent copper, and five percent tin and zinc) from 1864 to 1962.
(Note: In 1943, the coin's composition was changed to zinc-coated steel. This change was only for the year 1943 and was due to the critical use of copper for the war effort. However, a limited number of copper pennies were minted that year. You can read more about the rare, collectible 1943 copper penny in "What's So Special about the 1943 Copper Penny.")
In 1962, the cent's tin content, which was quite small, was removed. That made the metal composition of the cent 95 percent copper and 5 percent zinc.
The alloy remained 95 percent copper and 5 percent zinc until 1982, when the composition was changed to 97.5 percent zinc and 2.5 percent copper (copper-plated zinc). Cents of both compositions appeared in that year.

Orenousername - 6-6-2016 at 21:32

Oh god, I just reread the original post and sorry guys. I thought he was talking about the post 1981 pennies...

Ignore me pls.

ScienceBum - 7-6-2016 at 11:05

So I'm currently testing out the H2O2/HCl mixture theory since I don't have nitric, and it's become a rather bright green color (the peroxide is 3% unfortunately). You guys think the green is from the copper reacting or is that more likely something like chlorine?

DraconicAcid - 7-6-2016 at 11:05

The green is chloro complexes of copper. It's working.

AJKOER - 7-6-2016 at 14:39

Whatever you can do to increase surface area will also speed things up. This includes turning the penny (for educational purposes) into powder, cutting it into pieces or pounding it to make it flatter (this could add a bit of drama/action to your video).

In place of H2O2, one can try employing a fish tank air pump. That is, use dioxygen/air directly in the presence of H+ if there is some Cu2O presence (pretreat with NaOCl could increase the oxide coating, then rinse) along with NaCl. This direct use of oxygen/air generally works in the case of the metal salts containing Fe, Cu and Co (not sure about Ni and not Zn dominated by Zn(ll) salts) when in their lower valent state in acid. Source: See my comments and provided references and links available at: http://www.sciencemadness.org/talk/viewthread.php?tid=66247#...

Organic acids found in vinegar and lemons, for example, can also be useful when combined with an O2 source and especially some NaCl (or sea salt, read "Ion Partitioning at the liquid/vapor interface of a multi-component alkali halide solution: A model for aqueous sea salt aerosol" comment at http://www.science.gov/topicpages/s/sea+salt+components.html and also "Tropospheric Chemistry of Internally Mixed Sea Salt and Organic Particles: Surprising Reactivity of NaCl with Weak Organic Acids" ) to raise the 'activity coefficient' (see http://www.sciencemadness.org/talk/viewthread.php?tid=65082#...) and promotes the complexing of cuprous salts.

Applying heat, of course, to the reaction mixture also helps as does using a microwave (source: http://pubs.acs.org/doi/abs/10.1021/ar400309b ).

[Edited on 8-6-2016 by AJKOER]