Wednesday, April 15, 2009

AS Practical Guide II

Guide to Potassium manganate(VII) titration

· For KMnO4 titration, no indicator is added because KMnO4 acts as its own indicator and it would change to pale pink colour at the end point.
· KMnO4 titration is used to determine the concentration of Fe2+ (Iron(II) ions) , H2O2 (hydrogen peroxide) and C2O42- (ethanedioate).
· KMnO4 titrations must be carried out in acidified condition because in neutral or alkaline solution, brown solid MnO2 is formed and as a result it would be difficult to detect the end point.
· To acidify, sulphuric acid is used and not other acids such as hydrochloric acid and nitric acid because the chloride ions or nitrate ions would also be oxidised.
· Most KMnO4 titrations should be carried out at room temperature except titration involving C2O42- which is normally carried out at 70 oC. For ethanediote, any temperature lower than 70 oC could cause the precipitation of MnO2.
· Titration using KMnO4 solution should be carried out slowly. A brown precipitate of MnO2 is formed when KMnO4 is dropped from the burette too quickly.
Guide to Iodine-thiosulphate titration
· Starch is used as an indicator in this titration
· The starch solution should not be added at the beginning of the titration. The reason is at this stage, there is a high concentration of iodine and iodine would be adsorbed onto the starch molecule.
· The starch solution should be added towards the end of the titration when the reaction mixture turns pale yellow. (Warning: not too pale as you may overshot the endpoint)
· The starch solution will produce a dark blue colour with iodine.
· At the end point ,the solution turns colourless.
· Be careful, the solution may become blue again due to the atmospheric oxidation of iodide (from potassium iodide). You shouldn’t continue titrating again.

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