Determination of the concentration of vitamin C in 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 |The number of drops of fruit juice |Calculated | |vitamin C solution |required to decolorise |Concentration of | | |1 ml of DCPIP solution |vitamin C | | |Trial one |Trial two |Trial three |AVERAGE | | |1. | | | | | | |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. Complete the questions in the Activity Book TWO, p. 53 [Key words : reducing property ; oxidation ; heat treatment ; decolourisation ; other colour pigments ; non-citrus fruit ; sample ; inverse relation ; calibration] 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 =.