Practical investigations
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The question is to plan an investigation about the percentage purity of iron in iron cast by redox titration and I think I have to use potassium manganate in the presence of acid but I'm not sure what the formula is for cast iron can you help?
Jenkin writes:
Cast iron is impure iron so it doesn’t have a formula. Typically it contains from 92 to 95% of iron together with carbon, manganese, sulphur, phosphorus and silicon.
To find the % of iron in a sample of cast iron, the technique is to take a weighed sample, dissolve it in an excess of dilute sulphuric acid and titrate the solution of Fe2+ with a standard solution of potassium manganate(VII). The endpoint is reached when one drop of the KMnO4 solution produces a permanent pink colour. The equations are
Fe (s) + 2H+(aq) -> Fe2+(aq) + H2(g) (dissolving the sample), and
MnO4-(aq) + 5Fe2+(aq) + 8H+(aq) -> 5Fe3+(aq) + Mn2+(aq) + 4H2O(l) (during the titration).
There is an important practical detail: whilst reacting with the sulphuric acid the solution must be kept out of contact with atmospheric oxygen which would oxidise the Fe2+ to Fe3+. One way of doing this is to have the iron and acid in a flask fitted with a Bunsen valve, which will allow gas to escape but not to enter. The solution will be protected from oxidation by the hydrogen above it.
A suitable quantity of the iron is about 0.15 g if you are using 0.02 mol dm-3 KMnO4. You should use about 100 cm3 of approximately 1.0 mol dm-3 sulphuric acid.
We are not qualified to give health and safety advice on React. It is important to discuss the experiment with your teacher and to carry out a full risk assessment before you start work.
Risk assessment
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updated: 11 May 2006
