Transition elements
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How do I prepare solutions of iron(III) and sodium thiocyanate of known concentrations, to use for colorimetry to determine the percentage of iron in iron tablets? I know that I need to convert the iron(II) to iron(III) but I don't know how.
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Ulex replies........
You have to do this empirically because it all depends on the expected answer. You know what mass of iron is supposed to be in each tablet (packet label) so when you extract the iron by grinding up the tablet with water, you will know roughly what the concentration of the solution is after you have made it up to a known volume. Make up your standard comparison solutions so that they cover a range of concentrations extending both above and below what you expect the final answer to be.
Sodium or potassium thiocyanate may be used in concentration 0.1 mol dm-3.
To make sure that the iron is in the +3 state, before you make up the volume, add a few drops of concentrated nitric acid (CORROSIVE) and boil the solution (FUME CUPBOARD). Oxides of nitrogen are evolved. Allow to cool and neutralise the solution with sodium hydrogencarbonate, testing with indicator paper. Then make up the solution with pure water in a volumetric flask.
You MUST check all this with your teacher before carrying out this procedure.
Risk assessment
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updated: 01 February 2008
