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Unformatted text preview: er all of the ascorbic acid in the sample of interest has been
consumed. The titration flask initially contains some added KI and starch solution. The
first small excess of NBS oxidizes some iodide ions to molecular iodine, which then
reacts with more I- to produce the triiodide ion, I3-. The polymeric amylose (starch)
molecules wrap themselves around the triiodide ions and form a blue colored aggregate
to signal the endpoint.
Ascorbic acid solutions are susceptible to air oxidation, which can be inhibited in various
ways, including acidification. The air oxidation of ascorbic acid is catalyzed by metal
ions increasing the rate of degradation of ascorbic acid significantly. Oxalic acid forms
complexes with most metal ions and prevents their functioning as an oxidation catalyst.
Oxalic acid is commonly employed to increase the longevity of ascorbic acid. The last
segment of this experiment will test these factors involved in the degradation of ascorbic
Please read the appropriate chapter in your textbook, as pre-laboratory preparation for this
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This note was uploaded on 04/09/2013 for the course CHE CHE 2C taught by Professor Nasiri during the Spring '07 term at UC Davis.
- Spring '07