tantan
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Zn from US pennies
Modern US pennies are copper-plated zinc.
Is it possible to remove copper from them
with no (or very little) effect on zinc inside?
Thanks in advance.
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thunderfvck
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I'm sure HCl will eat through the copper plating and react also with the Zn. BUt that's when you ahve to move in and remove the pennies from
their acid bath.
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DDTea
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It is very easily done. Instead of HCl, I used FeCl3 (it's an etchant sold at Radio Shack for the purpose of etching copper!). I'm sure
HCl would work, and be a cheaper alternative, but I had good results with FeCl3. And yes, you do have to remove the pennies eventually and was them
with water, and then you have your zinc.
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Saerynide
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I electrolysed off the copper. I was left with a silver coloured penny 
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tantan
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Thanks for the answers. It seems that precise timing is inevitable.
I'll first try the FeCl3 method whenever I have a chance.
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unionised
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Zinc reacts rather vigourously with FeCl3.
Copper barely reacts with HCl at all, and only in the presence of air. It will generate coper chloride which reacts with zinc to precipitate copper
and form zinc chloride.
The problem with any of these methods is that zinc is generally a lot more reactive than copper. You could use a file or wire brush to remove the
copper. You might also try melting the zinc. The solubillity of copper in zinc is quite high so you wouldn't get a pure product. What do you want
the zinc for? The presence of a little copper might not matter.
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Mendeleev
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The HCl will not do anything to the copper, it will react with the zinc however. It's a great method to make hollow pennies, scrape a bit of the
copper off, and then dip in HCl. I simply torched the penny until the copper plating melted of and left me with a zinc circle.
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tantan
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Thanks, I'm well aware of all what unionised wrote.
That's why I asked to this forum in hope of some trick.
If it were zinc-plated copper, it would be very
easy to remove Zn and leave Cu intact.
I want my zinc to still look similar to a penny, and
the only reason for this is that I'm an element collector!
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tantan
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Thanks, Mendeleev.
It seems interesting to make some hollow pennies!
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Hang-Man
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HCL will eat through copper if you add H2O2 to it  . I scratch the sides of the
pennies with a file and dump several in HCL and leave them there for a day to two. Then take the liquid and add to it either to Mg(I have a shitload)
or Al. The Al will not react without a bit of a kick from you, so heat it untill it starts boiling and then the Al will react vigorously leaving Zn
powder.
Oh, and nitric acid will take off the Cu and leave the Zn will it not?
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Mr. Wizard
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I have removed the copper from zinc US pennies by soaking them in a solution of ' Lime-Sulfur ', a Dormant Disease Control spray used on
fruit trees, it is a yellow liquid that smells strongly of sulfur. The active ingredient is Calcium Polysulfides. I don't know the optimum
mixture, but 20 cc or so of this mixed with 80 cc of water, will make a yellow solution which will quickly form a layer of copper sulfide on a penny.
Experiment for the best concentration. Making the calcium polysulfide should be an easy project too. The black sulfide layer is very loose on the
coin, and gentle shaking will remove it, exposing more copper to the sulfide solution. The underlying zinc is not attacked in an overnight soak, but
places where the copper is covered by another coin are also intact, so the coins will have to be stirred around, or occasionally removed and washed,
using a toothbrush will remove most of the loose copper sulfide. When you are done you have a bright shiny zinc coin. You will also have some black
copper sulfide.
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Theoretic
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Nice method that, polysulfide dissolution, very neat. And you can also regenerate the polysulfide by melting the sulfide with sulfur, right? For this,
the sulfide better be sodium sulfide, which is easily meltable, even the 9 hydrate. Fusing Na2S 9 hydrate was used to make sodium polysulfides - a
crude mixture, but will work for the method.
Hold on, isn't CaS supposed to precipitate?
What do you get when you reduce CuS with carbon, Cu and CS2?  Holy CS2...
And if you reduce it with H2, you get Cu and H2S, which can be oxidized to sulfur, can be used to reform the polysulfide.
The oxidation of H2S by air can be greatly speeded up by using a little bit of HNO3 as a catalyst:
2HNO3 + H2S => 2NO2 + 2H2O + S.
4NO2 + 2H2O + O2 => 4HNO3.
Now that I've been blabbering on, I can also give an alternative method.
Dissolve sulfur in petrol, drop in the pennies and stir (thus free sulfur can be used, but with a not-very-pleasant-solvent). Free sulfur can be
bought from a gardening shop, I assume is cheaper than polysulfides.
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Mr. Wizard
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I haven't tried the direct sulfur in petrol ( gasoline?), but it would have advantages if it worked, but it isn't very soluble in most
common hydrocarbons. Benzene, CS2, or alcohol seem more suited. As a side note, copper wire, or wool was sometimes used in small oil wells in Texas,
to remove the sulfur, or H2S from "drip gas". Drip gas is a cheap form of unrefined gasoline removed from gas wells, or the gas from oil
wells. Left in the gasoline, it would eat the copper or other metals out of the bearings in the engine.
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Theoretic
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If sulfur is soluble in alcohol ( ), then it of course would be soluble in
petrol, since it dissolves better in nonpolar solvents, and alcohol is much more polar than petrol (yes, gasoline is what I mean), no?
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Cyrus
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And I think turpentine will dissolve sulfur too, plus you get the nice foresty pine smell, which might offset the smell of sulfur.
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Polverone
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followup on Cu-removal and the structure of pennies
I found that soaking and shaking modernt US pennies in sodium polysulfide solution does indeed remove most of the visible outer layer of coppy by
converting it to copper sulfide. The last clinging bits of copper sulfide were removed by shaking the pennies with wet table salt.
My shiny apparently copper-free pennies were then used for various purposes. I discovered, upon dissolving one in a mixture of acetic acid, sodium
chloride, and ammonium nitrate, that the liquid turned a distinctly coppery shade of blue-green!
More careful experimentation with drops of hydrochloric acid placed on a penny one-by-one showed that it retained an outer shell of copper-bearing
alloy and an inner core of apparently pure zinc. When dropped in aqueous HCl or other acids, the alloy portion dissolves and purer copper redeposits
upon the zinc below, so that there are dark, flaky deposits of copper draped over the dissolving bulk metal.
Various other zinc- or copper-attacking mixtures can be applied to the metal, eventually yielding wafers of zinc with no noticeable copper content
remaining, but I have not yet found a simple and reliable way to strip off the outer alloy while retaining most of the pure zinc core.
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The_Davster
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I made a hollow penny from a Canadian penny. A small scratch mark was made down to the zinc layer. After the penny sat in 31.45% HCl for about 3
days the solution it was in had turned green. I proceed to think "Oh shit was there some nickel in that penny?" I then opened up the penny
(wearing gloves) and the inside had a green coating.
So I am worried, I do not want hollow pennies if they are carcinogenic. I checked my coin collecting book and Canadian pennies are composed of pure
copper on pure zinc, no mention of nickel. Can anyone else think of a source of that green color other than nickel contamination?
I guess now if I want hollow pennies I will have to go with American pennies, even though they do cost more than Canadian ones...
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Polverone
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HCl will dissolve copper in the presence of oxidizing agents... including dissolved oxygen from the air.
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tantan
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Mr. Wizard, thank you very much for your brilliant method.
Also, thanks to Polverone for reporting the result of your
experiment. I plan to perform mine in an ultrasonic bath
to facilitate the reaction by continuously removing any CuS
formed. I'll report the result whenever I have a chance to try.
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thiadiazole
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My method isn’t as elegant as the others. But Cu melts at 1083 °C and Zinc melts at 419 °C. if a couple of pennies are placed in a large spoon
and heated with a propane torch the zinc will melt and can be poured away from the still solid copper. its a little wasteful but fast and the zinc is
pure.
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JohnWW
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Unfortunately, New Zealand and Australia did away with bronze 1c and 2c pieces altogether several years ago, due to inflation. At the same time, the
$1 and $2 bills were replaced with gold-colored brassy coins, but I am not sure of their composition. The smallest coin now used here is the 5c
cupro-nickel piece. I still have a few of them, and also a fair number of the large pre-decimal (pre-1967) pennies (British and Australian coins were
used up to 1933 in New Zealand, and some survived in circulation until 1967). The bronze coins were almost pure Cu, with a very small amount of Sn.
Except possibly for the current $1 and $2 coins, no coins here had Zn in them.
John W.
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Mr. Wizard
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Melting the zinc carries a small amount of dissolved copper with the liquid zinc. It will only be a little bit, but enough to contaminate many
reactions. The impure zinc actually has a lower melting point than the pure zinc. Distillation of the zinc would be a better method of purifying it.
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Theoretic
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"I checked my coin collecting book and Canadian pennies are composed of pure copper on pure zinc, no mention of nickel. Can anyone else think of
a source of that green color other than nickel contamination?"
Copper chloride is green due to the CuCl4-- complex ion, unlike other copper salts (although a dilute solution is blue also).
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