Precautions and tips for Qualitative Inorganic Analysis of Coins

Question

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.

Answer 1

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.

Answer 2

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!

Answer 3

I am thinking of three methods, you could put the coin in a scanning electron microscope if it has a X-ray spectrometer attached and then get a spectrum of the X-rays which come off the coin.

If you had access to a TRIGA nuclear reactor then you could try neutron activation combined with gamma spectroscopy. If you use a short irradation time in the reactor and count quickly after activation then you can measure things like copper. It is generally non destructive but it is possible to induce long lived radioactivity in the sample. I suspect these days that very few people will have access to a TRIGA reactor.

I think that a good option would be to digest the coin in aqua regia and then dilute and filter it into a volumetric flask which has been soaked in acid to make sure that the metal contamination level is low. I would then suggest ICPOES as a method of making a measurement of the metal concentrations in the digested coin.

In some cases such as copper it might be possible to take the digested coin and use reagents such as dimethyl phen to make strongly coloured complexes, you can then measure the concentration of the complex with a UV/vis machine and then calculate the concentration of the metal in the flask. From the mass of the coin you can then estimate the metal content in the original coin.