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I am investigating the vitamin C content in cooked and uncooked vegetables. Raw broccoli used up less iodine than cooked broccoli. I repeated this with other vegetable - Brussel Sprouts & turned up with the same results. I am not sure where I am going wrong.
This is my method. I used 100 g broccoli with 100 ml distilled water to liquidise. I made it up to a 100 ml in a volumetric flask, adding any distilled water as required. 20 ml of the extract and 150 ml of water with 1 ml starch solution in a beaker. Since this was a large volume, I used 20 ml of this solution to titrate.
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Igloo writes ...
You don’t mention how you adapted the technique to study the cooked vegetable, but I assume that you discarded the cooking water and simply liquidised the cooked vegetable before using the same method as described above. Presumably you still used 100 g of the raw vegetable before cooking, rather than 100 g of the cooked variety. If so, then yes, I am surprised that your samples required more iodine after the cooking.
The problem could well be with the iodine. Fellow students have written in frequently to ask why their titration results with iodine are not as expected. Iodine is not a particularly discriminating oxidising agent, and is known to oxidise other material in the vegetable apart from vitamin C. This still doesn’t explain why your results are greater with the cooked vegetable, but maybe there are processes occurring during the cooking which produce more oxidisable material. This is simply an idea of mine, but it would explain why you are having problems.
My strong recommendation is that you use DCPIP instead. This is a far more discriminating oxidising agent, and vitamin C is pretty much the only material which it will oxidise during the titration. What is more, experiments using uncooked and cooked samples of vegetables with DCPIP definitely work! If you want more details as to the techniques involved have a look at our web site and the food science students’ booklet at:
http://www.chemistry-react.org/go/default/Topic/SpecialStudies/Topic_92.html
Experiment 2 on pages 75 and 76 take you through the routine as it applies to cabbage. This can easily be adapted for use with broccoli or any other vegetable.
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updated: 21 February 2008
