Practical investigations
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I'm currently doing my advanced higher chemistry project, and have run into a few difficulties. I'm trying to determine the iron content of an iron tablet by a colorimetric method. I've tried converting the Fe2+ to Fe3+ using potassium permanganate the adding a drop of KSCN, but the mauve of the excess KMnO4 gets in the way. I'm now trying H2O2 to oxidise the Fe2+ as it’s colourless but I'm really confused about concentrations and methods. Please help!
Igloo replies
I can see why you are having problems with potassium permanganate. Acidified hydrogen peroxide is an ideal alternative oxidising agent in the circumstances. If you are unsure as to whether all the Fe2+ ions have been oxidised, why not carry out a test on a drop of your mixture? Potassium ferricyanide (potassium hexacyanoferrate(III), K3Fe(CN)6, gives an intense blue colouration in the presence of Fe2+ ions (but not with Fe3+ ions). So, if no blue colour is formed, you can assume that your oxidation process is complete. If this isn’t the case you will need to increase volumes or concentrations of the hydrogen peroxide and/or acid appropriately.
Incidentally the oxidation process you are carrying out with hydrogen peroxide tends to be relatively slow, so you will need to heat up your mixture in a hot water bath and wait for several minutes before testing.
In short, the volumes, concentrations and conditions necessary must come from personal practical experience, so your methods may well follow from procedures of trial and error. This is what investigations are all about!
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 25 January 2006
