Transition elements
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I am investigating the percentage of copper present in pre-1992, UK one penny coins. I am using concentrated nitric acid to dissolve the coin. However, I am having difficulty in one part of my experiment where I am to precipitate copper ions using potassium iodide. I am neutralising my solution using calcium carbonate before filtering off the excess. I understand that I am then to precipitate copper iodide by the addition of potassium iodide and then titrate with sodium thiosulphate. However, I am unsure of the quantities of these solutions required as well as the general experimental procedure.
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Ulex says
I think the best way to proceed is as follows. Add calcium carbonate powder a little at a time until there is no further effervescence. How much you need depends on how much excess nitric acid you used. Then add a spatula measure of potassium iodide crystals. Iodine is produced which, as you say, ‘dissolves’ with the aid of the excess iodide ions. The equation is:
Cu2+(aq) + 2I-(aq) -> CuI(s) + 1/2 I2(aq)
Make up the resulting mixture to a known volume using a standard volumetric flask. Let any excess calcium carbonate settle. Titrate pipette volumes of this solution with sodium thiosulphate of known concentration (try 0.1M for example and change this concentration if the titres are ridiculous!) using starch solution as indicator in the conventional way.
You may be tempted to filter off copper(I) iodide and weigh it but I strongly advise not going at it this way. Gravimetric procedures are notoriously tricky to apply in practice.
Risk assessment
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updated: 12 February 2008
