Determination of the concentration of vitamin C by using the DCPIP - DOC

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Determination of the concentration of vitamin C by using the DCPIP - DOC Powered By Docstoc
					                Determination of the concentration of vitamin C
               in fruits/vegetables by using the DCPIP test (2010)
Principle : Vitamin C is a strong reducing agent. It can decolorize the blue dye
DCPIP.
             The amount of vitamin C
             in a food sample
             can be estimated by
             the amount of it
             that is used to decolourize
             a fixed amount of DCPIP.

Apparatus :
Test tube, clean and dry
Test tube rack
5 ml syringe or a good quality squeezing pipette
1 ml pipette
pipette filler


Materials :
________% vitamin C solution, (standard for calibration) ; prepared from
commercial vitamin C tablet (0.02 %)
________ % DCPIP solution, freshly prepared
    Possible samples (depending on supply)
● grape juice, paper pack

●   lemon juice, diluted from fresh squash (Citrus fruit)
●    orange juice, diluted from fresh squash (Citrus fruit)
●    orange juice, diluted from bottled drink Mr. Juicy (Citrus fruit)
●   kiwi fruit juice, diluted from fresh squash (non-citrus fruit, but high in vitamin C)
●   green pepper extract / juice
●   Choi sum (vegetable squash extract)


Procedure COPY these sentences onto your SBA workbook. In the future, you will
need to write experimental procedure by yourself.
1. Use a 1 ml pipette and a pipette filler to prepare a test tube with 1 ml of blue DCPIP
   solution. Prepare a few in a batch (一批)     .
2. Fill a 5 ml syringe with the standard vitamin C solution. (Or use a dropper)
3. Slowly (gently) add the vitamin C solution into the DCPIP test tube drop by
   drop. [ PRECAUTIONS : Hold the dropper vertically. Make sure that the drop
   size is constant.]
4. After adding one drop, DO NOT shake the contents.
5. Stop dropping when the blue colour disappears.
    [The end point is taken when the blue solution suddenly becomes colourless.]
6. Record the number of drops of vitamin C solution required to decolourise the
   DCPIP solution.
7. Repeat the experiment twice (= two times).
8. Enter the results into the table below. [Each student must enter his/her data sheet.]
9. Repeat the experiment for other fruit juices.
10. Rinse the dropper / syringe with water and then a new fruit juice to reduce
      contamination and ensure accuracy.
Results :

                           The number of drops of fruit juice
  Fruit juice
                                                                       Calculated
   sample /                     required to decolourise
   standard                                                           Concentration
                                    1 ml of DCPIP solution
  vitamin C
                                                                      of vitamin C
    solution        Trial one       Trial two   Trial three AVERAGE

 1.     standard
 vitamin C

 2.


 3.


 4.


 5.



Calculation : [accurate to 2 decimal places.]


Since we know the number of drops of standard vitamin C solution
needed to decolourise a fixed volume of DCPIP,
by using simple proportion,
we can calculate
the amount of vitamin C
in an unknown sample
if we get comparable data.

 Concentration of                         Average number of drops of standard
 vitamin C in a fruit                     vitamin C solution x ______ %
                                =
 juice sample
                                          Number of drops of fruit juice sample


Complete the questions in the SBA workbook and worksheet p.33 (of this
chapter 5).


[Related key words : reducing property ; oxidation ; heat treatment ;
decolourisation ; other colour pigments ; non-citrus fruit ; sample ; inverse
relation ; calibration]

				
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posted:6/21/2012
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