Practical investigations

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Hi, I am planning an investigation into the iron content in organic and inorganic forms.
1) Will vitamin C release Fe²⁺ ions from the oxalate.
2) In what form/state is Fe found in vegetables- how can I find this out?
Will it affect whether I use sulphuric acid in which to dissolve it? Will it affect whether I can use KMnO₄ to indicate its presence during a titration?

Igloo replies ...
I am not sure what you mean in your first question. The oxalates present in vegetables, such as rhubarb and spinach, form complexes with Fe²⁺ ions, and this affects the ability of iron to be absorbed after the digestion process. Vitamin C tends to decompose these complexes, releasing aqueous iron ions into solution.
 
Iron is mostly present in its second oxidation state (Fe²⁺) in vegetables. If you wanted to confirm this, add a dilute solution of ammonia or sodium hydroxide to an aqueous vegetable extract, and the formation of a green precipitate confirms the presence of Fe²⁺ ions. There are also likely to be some Fe³⁺ ions present too (though not many relatively) and the best way to identify these is to add a dilute solution of acidified potassium thiocyanate. A red colour confirms the presence of Fe³⁺ ions. This is a very sensitive test indeed.
 
If you want to determine the total iron content of your vegetable extract, it is wise to add some small pieces of granulated zinc and a little sulphuric acid, and allow the reduction of Fe³⁺ to Fe²⁺ ions to take place before filtering off the excess zinc. The solution can then be titrated with acidified manganate(VII) ions as usual.
 
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Risk assessment
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updated: 06 December 2006