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Core Practical 2

Investigate the vitamin C content of food and drink.
OBJECTIVES
● To be able to calculate the vitamin C concentration of fruit juices using the titration method
● To solve problems set in practical contexts
● To process and analyse data using appropriate mathematical skills
SAFETY
● Wear eye protection.
● Avoid skin contact with the DCPIP and test tube solutions.
● Do not taste the fruit juice.
MATHS SKILLS
● Recognise and use expressions in decimal and standard form.
● Find arithmetic means.
● Understand the terms mean, median and mode.
● Solve algebraic equations.
● Plot two variables from experimental or other data.
EQUIPMENT
● eye protection
● 1% DCPIP solution
● 1% vitamin C solution
● a selection of fruit juices
● test tubes
● test tube rack
● small beakers
● small syringes 1 cm3 and 5 cm3

DIAGRAM

figure A Dropping fruit juice solution into DCPIP

PROCEDURE
It is possible to determine the concentration of vitamin C in a solution by using dichlorophenolindophenol (DCPIP). Vitamin C is an antioxidant, so it reduces the DCPIP causing a colour change. By using a solution of vitamin C with a known concentration, it is possible to calculate the concentration of vitamin C in other solutions, for example, in fruit juices (see figure A).
1. Use the 5 cm3 syringe to draw up 5 cm3 of 1% DCPIP. Shake the syringe to expel any air bubbles.
2. Add 1 cm3 of DCPIP to a test tube.
3. Use a clean 5 cm3 syringe to draw up 5 cm3 of the 1% vitamin C solution.
4. Add the vitamin C solution to the test tube containing the DCPIP, one drop at a time. After each drop, shake the test tube slightly to ensure the solutions have mixed.
5. Continue to add vitamin C solution until the blue colour of the DCPIP disappears.
6. Record the volume of vitamin C solution added. You can find this volume by subtracting the value on the syringe from the original 5 cm3 in the syringe.
7. Repeat steps 2–6 twice more and calculate a mean value for the volume of 1% vitamin C solution needed to decolorise 1 cm3 of DCPIP.
8. Add 1 cm3 of DCPIP to a clean test tube.
9. Use a clean syringe to draw up 5 cm3 of a fruit juice. Add the fruit juice to the DCPIP one drop at a time. Record the volume of juice needed to cause the blue colour to disappear.
10. Repeat steps 8 and 9 twice more, then calculate the volume of fruit juice needed to decolorise 1 cm3 of DCPIP.
11. Repeat steps 8–10 with the other fruit juices.

ANALYSIS OF RESULTS
1. Write down the average volume of vitamin C solution needed to decolorise DCPIP (based on your results).
2. 1 cm3 of the vitamin C solution contains 10 mg of vitamin C. You can use this information and your calculation of the average volume of 1% vitamin C solution required to decolorise the DCPIP to calculate the concentration of vitamin C in the fruit juices. Use the formula:

For example, if the
volume of 1% vitamin C solution needed to decolorise the DCPIP was 1.4 cm3 and the volume of fruit
juice used was 2.6 cm3, the calculation would be:

Use the formula to calculate the concentration of vitamin C in each of the fruit juices used.
LEARNING TIPS
● Ensure that the drops of vitamin C solution or fruit juice land directly in the DCPIP and do not stick to the side of the test tube. Otherwise, your results will not be accurate.
● Acidic fruit juices will not completely decolorise the DCPIP; instead, the solution will turn pink. This should be taken into consideration.