Practical investigations
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I am doing a Chemistry prac investigating the amount of Fe(II) present in iron tablets. I am investigating this using a titration with potassium permanganate, after adding distilled water and 2M suphuric acid to the ground iron tablet in the titration flask. According to the amount of iron that is given on the bottle, the titre should be about 3 ml. However, I cannot reach an end-point because the permanent pink colour that indicates the end point of the reaction is never reached. No matter how much potassium permanganate I add, the colour fades out (sometimes this takes overnight). The colour still fades out even when titres of 100 and 200 ml are added. I was wondering what may have occurred to cause this unforeseen consequence. Thanks heaps.
Ulex replies
I think I can see what the problem is, so long as I have understood your procedure correctly. As I understand it you are crushing up a tablet, putting it in a titration flask and adding distilled water and sulphuric acid and then titrating the mixture. Have I got it right?
If so, the problem is this. The iron(II) sulphate is not the only substance in the tablet. When you powder the tablet and add water, the iron(II) sulphate is not going to dissolve instantaneously. So when you add the potassium manganate(VII) solution, it reacts with the Fe2+(aq) which has dissolved and you get an apparent end point. After a few moments, more Fe2+ dissolves, so the pink colour disappears. This cycle goes on, gradually taking longer and longer.
It would be a better technique to crush up the tablet with water and sulphuric acid in a pestle and mortar, being careful not to spill any. Then transfer the mixture to a standard volumetric flask, wash the pestle and mortar with several small portions of water, and add the washings to the flask. Make up to the mark with more sulphuric acid and water. Then leave the stoppered mixture overnight, so that as much iron(II) sulphate as possible dissolves, and the remnants of the tablet sink to the bottom of the flask. Then titrate pipetted portions of the mixture with potassium manganate(VII). You should have much less trouble with the end point disappearing.
All this depends on my having understood you correctly in the first place!
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Risk assessment
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updated: 14 December 2006
