Transition elements
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I am researching the iron content of different green vegetables. I am puzzled as to why you need to boil the vegetable in sulphuric acid before you titrate with KMnO4. I have searched text books and the internet and found nothing. Please help.
Igloo replies ...
Dilute sulphuric acid is needed in any case for the subsequent titration with KMnO4, since it is one of the reactants.
Using it at an earlier stage in the procedure is important for two reasons:
(1) The acid helps to break down complex organic structures within the vegetable, thus ensuring that all the Fe2+ ions (including those “locked” up in the large molecules) are released into solution.
(2) A more important reason is the need to avoid any oxidation of Fe2+ ions to Fe3+ ions by oxygen from the air before the KMnO4 titration is carried out.
Use of the values in Table 6.1 of the Nuffield Data Book should help you to see that iron (II) is a much stronger reducing agent under alkaline conditions (see entry 20) than it is under acid conditions (see entry 67). The extent of oxidation from iron(II) to iron(III) tends increases dramatically as the pH rises; hence the need to ensure that the mixture remains acidic throughout the vegetable boiling process.
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updated: 28 April 2004
