Chemical amounts (A2)
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'Estimation of iron(II) and iron(III) in a mixture containing both' is one of my pieces I have to investigate. The thing is, I haven't got a clue on how to do it! Please would you be able to give me a helping hand and tell me how it all works and how I would go about doing it? All I get told is that I am provided with 200 cm3 of a solution containing 1.1 to 1.3 g of iron ions as a mixture of the two. It has present at least 30% by mass of the two ions. I know have to give detailed method on how to work out the amount present, but I am unable to do so, so please could you help?
You know roughly how much iron is present so you need to find out two things:
1 Exactly how much iron is present altogether
2 How much of it is iron(II).
I would expect that your basic method is to titrate samples of your mixture with potassium manganate(VII) using the method used for analysing ‘ferrous sulphate’ tablets given in Expt.19.1c in the Nuffield Chemistry Students’ Book. Try to get hold of a copy if you haven’t got one. This would enable you to find out how much iron(II) is present.
To find the total iron present you might take a sample of your mixture and reduce all the iron in it to Fe2+ using granulated zinc and sulphuric acid. Then titrate samples of the resulting solution and find the ‘total iron’ content.
The actual details of how to carry out the analysis will require some careful thought – particularly in deciding what concentration of potassium manganate(VII) to use.
Your first move is to do some research on the reaction between potassium manganate(VII) and iron(II) ions. Good hunting!
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Risk assessment
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updated: 15 December 2006
