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.