1. Tomatoes are rich in vitamin C. () Suggest the problem(s) that might arise if
ID: 1086403 • Letter: 1
Question
1. Tomatoes are rich in vitamin C. () Suggest the problem(s) that might arise if a student used the procedure in this experiment to determine the vitamin C content in a tomato. (2) Would you expect the vitamin C concentration of tomato juice to be greater or less than the vitamin C concentration of a fresh tomato? Briefly explain. 2. A student failed to carry out all the steps in this procedure. Briefly comment on the effect each of the following oversights would have, if any, on her calculated number of milligrams of ascorbic acid in a grapefruit. () The student neglected to add metaphosphoric acid to the grapefruit sample, prior to titration.Explanation / Answer
1) From the later part of the questions ,it is clear that redox titration method implementing the use of DCP was employed for the determination of vitamin C in tomatoes and tomato juice
DCP or 2,6-dichloroindophenol is an intense colored titrant known for its specific ability to oxidize only vitamin C
This titrant shows color change in different mediums, its dark blue in neutral and basic medium and red in acidic medium,so can be used as self indicator for titration.
Vitamin C is oxidized as per half -rxn:
Vit C(red) -->Vit C(oxidized) +2e- +2H+
reduction half rxn:
DCP(ox) +2e- +2H+ -->DCP(red)
overall redox: DCP(ox)+Vit C(red) --->Vit C(oxidized) +DCP(red)
Vit C(colorless)+DCP--->products are colorless,so the solution stays colorless untill all the ascorbic acid or Vit C is oxidized,At the end point ,when all the Vit C is consumed ,the solution turns pink
The DCP titrant needs to be standardized using std Ascorbic acid solution to get its exact concentration.
problems faced during titration:
1) The juice extracted from a tomato must be filtered as,it may show higher volume due to the pulp content and give wrong concentration value of Vit C when calculated using equation.
(Concentration of DCP)* volume of DCP= (Concentration of Vit C)* volume of Vit C
2) the color change at end point is not sharp but beige color,so one trial titration must be done to observe the color change at end point.The trial version of the color must be preseved for comprision in other titrations.Remember the color must hold for 30 seconds or so,and if it fades away,means you have not yet reached the end point
3)color at the end point must also be compared with that forthe standard Vit C
2) The vit C of tomato juice must be greater than that of a fresh tomato as the former is a concentrated and filtered version of the contents of a tomato. As vit C is water soluble so the juice having more of liquid part has higher concentration of the vit
Tomato may contain higher content of substances other than Vit C like lycopene,sodium,protein,sugar due to the thickness of the added pulp part.
3)
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