Chemical amounts (A2)
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I'm doing a practical of trying to find out the percentage by mass of iron(II) and (III) in the same solution of 200cm3. I know the procedure but need to know some suitable concentrations and volumes of solutions and zinc to use through the experiment.
Ulex writes:
Your basic method is likely to be titration with potassium manganate(VII). See p.458/9 in the Nuffield Students’ Book for an experiment which introduces the method. You will probably need to plan to do the following things.
1. Make or obtain a solution of potassium manganate(VII) of exactly known concentration. This isn’t as easy as it sounds because the solid consists of almost black crystals and the solution has a very dark purple colour so it is very difficult to know when all the solid has dissolved. Accurately-known solutions are commercially available in a variety of concentrations.
2. Titrate known volumes (pipette) of your solution with your potassium manganate(VII) solution to find the iron(II) content.
3. You will have to reduce a known volume of the acidified mixture to iron(II) using, say, powdered zinc, make up the solution to a known volume and then titrate portions of this with potassium manganate(VII) solution to find the total iron content of the mixture.
It is very difficult to recommend suitable quantities to use based on the information you supply. I think the selection of suitable quantities is part of the problem! If I were in your position I would tackle the problem empirically. Do a trial rough titration using, say, 10.0 cm3 of your solution with 0.1M potassium manganate(VII) – not forgetting to add at least 10 cm3 of 1M sulphuric acid before you start to titrate. If the titre turns out to be very small, repeat with a less concentrated manganate solution.
Continue with this process until you get a sensible titration result. Then work accurately in the conventional way. As for the quantity of zinc to use, this is much less of a problem because all that matters is that you have an excess. You can check visually that there is zinc left over after the reduction.
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updated: 21 August 2003
