Vitamin C Content in Fruit Juices and Vegetables

Aim

To compare the vitamin C (ascorbic acid) content of different fruit juices or vegetable extracts, such as orange, lemon, guava and tomato, by finding the volume of each needed to decolourise a fixed volume of DCPIP solution.

Variables

• Manipulated variable: the type of fruit juice or vegetable extract tested (e.g. orange, lemon, guava, tomato).
• Responding variable: the volume of juice or extract needed to just decolourise a fixed volume of DCPIP solution; a smaller volume needed indicates a higher vitamin C concentration.
• Controlled variables: the volume and concentration of DCPIP solution used each time, the rate at which the juice is added (drop by drop), and the temperature.

Materials and apparatus

• DCPIP solution
• Several fruit juices or vegetable extracts (e.g. orange, lemon, guava, tomato)
• Syringe, burette or graduated dropper
• Test tubes
• White tile
• Stopwatch

Procedure

1. Measure a fixed volume of DCPIP solution (e.g. 2 cm3) into a test tube placed on a white tile so the colour change can be seen clearly.
2. Fill a syringe, burette or graduated dropper with the first fruit juice or vegetable extract.
3. Add the juice drop by drop into the DCPIP solution, swirling the test tube constantly after each drop.
4. Stop adding the juice and record the volume used as soon as the blue colour of the DCPIP just disappears.
5. Repeat the procedure with a fresh, fixed volume of DCPIP solution for each of the other juices or extracts, keeping the DCPIP volume and dropping technique the same each time.

Expected results

The volume of juice or extract needed to decolourise the fixed volume of DCPIP differs between samples, depending on how much vitamin C each one contains. Typical results are shown below.

Conclusion

The fruit juice or vegetable extract that decolourises the DCPIP solution using the smallest volume contains the most vitamin C per unit volume, since vitamin C is the substance in the juice that reduces (decolourises) the blue dye. Comparing the volumes needed across several samples, tested using the same volume and concentration of DCPIP each time, gives a fair ranking of their relative vitamin C content, meeting the aim of the experiment.

Safety

• DCPIP solution can stain skin and clothing, so wear an apron and wipe up spills at once.
• Use the syringe, burette or dropper carefully and keep the working area dry to avoid slips.
• Do not drink the juices used in the laboratory, as the glassware and reagents may have contaminated them, and wash your hands after the experiment.

Common mistakes

Paper 3-style questions

Question 1 (hypothesis). A student compares the vitamin C content of fresh orange juice and orange juice that has been boiled for ten minutes. State a hypothesis for this comparison.

Model answer. Boiled orange juice needs a larger volume to decolourise the same volume of DCPIP than fresh juice, because heat destroys some vitamin C, so the boiled juice has a lower vitamin C content.

Question 2 (variables and fair test). State the manipulated and responding variables, and explain two things the student must keep the same for a fair comparison.

Model answer. The manipulated variable is whether the juice is fresh or boiled; the responding variable is the volume of juice needed to decolourise the DCPIP. To be fair, the student must keep the volume and concentration of DCPIP the same each time and add the juice drop by drop at the same rate, so that any difference is caused only by the vitamin C content.

Question 3 (tabulation and inference). Fresh juice needed 1.2 cm³ and boiled juice needed 2.4 cm³ to decolourise the DCPIP. Tabulate these results and state the inference.

Model answer. A table with the columns 'Type of juice' and 'Volume needed to decolourise DCPIP (cm³)' records fresh 1.2 and boiled 2.4. The inference is that fresh juice contains more vitamin C, because it needed only half the volume to decolourise the same amount of DCPIP; the vitamin C acts as a reducing agent that decolourises the blue dye.

Source:SRC-DSKP-EN

Frequently asked questions