Determination of the concentration of vitamin Cin fruits/vegetables by using the DCPIP test (2008)

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 decolorize

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 :

* 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

Procedure

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.

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 :

Juice sample /

standard

vitamin C

solution

The number of drops of fruit juice

required to decolorise

1 ml of DCPIP solution

Calculated

Concentration

of vitamin CTrial one Trial two Trial three AVERAGE

1.

2.

3.

4.

5.

Calculation : [accurate to 2 decimal places.]

Since we know the number of drops of standard vitamin C solutionneeded to decolourise a fixed volume of DCPIP,by using simple proportion,we can calculatethe amount of vitamin Cin an unknown sampleif we get comparable data.

Complete the questions in the Activity Book TWO,

p.53

[Key words : reducing property ; oxidation ; heat

treatment ; decolourisation ; other colour pigments ;

Concentration of vitamin C in a fruit juice sample

Average number of drops of standard vitamin C solution x ______ %

Number of drops of fruit juice sample

=

non-citrus fruit ; sample ; inverse relation ;

calibration]