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I'm a highschool student and as part of an investigatory project, I have decided to analyse which metals are present in our everyday coins.

I chose this because in my previous academic year, I had learnt how to analyze the cations present in inorganic salts and I employed the same systematic analysis method that I had learnt to test the presence of metal cations in some coins.

Obviously, I cannot expect the coin analysis to go exactly like the salt analysis. I would have to ignore certain tests like the 'flame test'.

Could you please tell me what precaustions I should take for this experiment? I wish to write up a clear and systematic process by which any student / coin collector can easily do in a laboratory with only common chemicals and salts.

All advice, resources and tips will be most appreciated.

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  • $\begingroup$ Just for completeness I'll add that any sort scrapping or acid test would damage the coin which would hurt its numismatic vale. XRF is nondestructive. $\endgroup$ – MaxW Nov 27 '17 at 6:40
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1) The coins must be dissolved (at least partially). There is a good amount of metals that won't dissolve in common acids (hydrochloric acids), some will require nitric acid, some will require aqua regia and some will require even worse, though that's unlikely. So, most likely you will have to deal with concentrated nitric acid, that produce nitric oxide (fox's tail) when react with metals. Given that you must have fume hood. In case something undissolved remains, you can try aqua regia, and if even that fail, you have something rare, like nioubium, with no common method of wet analisys.

2) Most metals used in coins have toxic salts. So, be cautios, and avoid skin contact. Latex gloves and protective goggles are not vital, but certainly recommended unless you believe you can be accurate enough. Usual safety requirements for work with conc. acids applies as well.

3) In case of non-destructive analisys, X-ray fluorescent spectrometry may be employed. It will give accurate analysis in almost any realistic case, but requires quite costly equipment. It is not extremely costly per analysis, though.

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    $\begingroup$ As for the dissolving, if you cannot use strong acids, it might be worth trying anodic oxidation. As for the detection, go ahead with your cation analysis, just the sample preparation is more involved. $\endgroup$ – ssavec Oct 22 '14 at 11:03
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    $\begingroup$ I am of the opinion, that protective goggles are vital, always, any experiment. You never know what might go wrong and losing eyesight is just terrible. $\endgroup$ – Martin - マーチン♦ Oct 22 '14 at 11:51
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    $\begingroup$ @Nick the only common coin metal identifiable with flame test is copper, and honestly, flame test is not the most reliable way to check for copper. I would scrap the flame test. $\endgroup$ – permeakra Oct 22 '14 at 14:59
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    $\begingroup$ @Nick: If you mean normal flame test, just scrape the coin and put the dust into flame, you are about to use the worst color and intensity detector around, i.e. your eye. Maybe using camera and then analyze the spectrum could be slightly better (see Atomic absorption spectroscopy). But still, you are about to analyze the mixture of metals, not to identify unknown pure metal. This is much more difficult task and some metal may mask others, some do not show at all, etc... $\endgroup$ – ssavec Oct 22 '14 at 15:30
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    $\begingroup$ @Nick: file or sandpaper? Both in diamond version, if you want to have your sample cleaner. $\endgroup$ – ssavec Oct 22 '14 at 16:03
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I know this does not answer what you specifically asked for, but it may be a good way to get quick analyses of coins.

Try to get a quick scan with an X-Ray Fluorescence gun (XRF gun), if you want to use a modern, non-destructive analytical technique. It is common for firms and certain types of stores to have one such gun, for example jeweller stores keep such guns to check the gold or silver purity, or toy stores may keep a gun to check for dangerous elements in imported toys.

Maybe you could talk yourself into getting a free scan for your school project. They may have pre-installed metal calibration programs, and a scan takes 30 seconds. if your coin is plated with some metal, and the rest of the coin is some other metal-alloy, you can see this with an XRF gun if you scratch your coin on one side before analysis.

XRF is a nice technique, and you should be able to have a good idea of how it works (in principle) with theory from high school chemistry or physics. Modern chemical analysis is not "wet chemistry", or flame tests; it is instrumental analysis (although wet techniques are still used).

To conclude, see if you can find a local store or university or someone who has an XRF gun. That would give you the proportions (%) of the major components and a standard deviation of the analysis (error or uncertainty).

Good luck!

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  • $\begingroup$ Aww, I appreciate this answer very much. thanks. This is a good tip (and hence in the scope of this question). Maybe an XRF gun will be able to find the chromium that I couldn't. $\endgroup$ – Nick Nov 13 '14 at 14:34
  • $\begingroup$ I don't know where you are from (and hence not what kind of coins you're analyzing), but the US Government does not mention any chromium in their coins (link at bottom). I have no idea how they're reporting the results, and maybe the coins have trace amounts of chromium. If so, XRF may not detect it; at least not an XRF gun (there are stationary XRF instruments used for laboratory analyses - the guns are meant for quick in-field analyses). Then again, maybe you would find something. I am sure the coins are not as pure as reported here. tinyurl.com/67mo73b $\endgroup$ – Yoda Nov 13 '14 at 14:43

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