Food Test for Vitamin C (DCPIP Test)

Aim

To test a food sample for the presence of vitamin C using DCPIP solution, and to compare the vitamin C content of different food samples.

What it tests for

The DCPIP test detects vitamin C (ascorbic acid) in food. DCPIP (2,6-dichlorophenolindophenol) is a blue dye that is reduced and loses its colour when it reacts with vitamin C, which is a reducing agent.

The number of drops of food extract needed to decolourise a fixed amount of DCPIP gives a rough, semi-quantitative comparison of vitamin C content between samples.

Variables

• Manipulated variable: the type of food extract tested, for example orange juice, lemon juice, and guava juice.
• Responding variable: the number of drops of food extract needed to decolourise a fixed volume of DCPIP solution.
• Controlled variables: the volume and concentration of DCPIP solution, the size of each drop, and the temperature of the extract.

Materials and apparatus

• Food extracts or fruit juices to be tested
• 0.1% DCPIP solution (blue)
• A standard vitamin C (ascorbic acid) solution for comparison
• Test tubes and a test-tube rack
• A graduated dropper or syringe, and a measuring cylinder
• A white tile or white paper as a background to judge the colour
• A filter funnel and filter paper for preparing extracts

Reagent and method

1. Prepare a food extract, such as fruit juice, by squeezing or crushing the food and filtering if necessary.
2. Place about 2 cm3 of blue DCPIP solution into a clean test tube.
3. Using a dropper, add the food extract to the DCPIP solution one drop at a time.
4. Shake or swirl the test tube gently after each drop.
5. Continue adding drops and count them until the blue colour of the DCPIP just disappears.

Positive result

A positive result is shown when the blue DCPIP solution becomes colourless (decolourised) after a number of drops of food extract are added. Fewer drops needed to decolourise the DCPIP indicates a higher concentration of vitamin C in the food extract, while more drops needed indicates a lower concentration.

Expected results

The drop count for each extract can be tabulated and used to rank the samples by vitamin C content.

Inference and conclusion

DCPIP is decolourised because vitamin C is a reducing agent that reduces the blue dye to a colourless form. The fewer drops of extract needed to decolourise a fixed volume of DCPIP, the higher the concentration of vitamin C in that extract.

Comparing the drop counts therefore ranks the samples by vitamin C content, as long as the same volume and concentration of DCPIP is used each time. Because vitamin C is destroyed by heat, a boiled sample needs more drops than a fresh one, which shows why fresh fruit and gentle cooking preserve more vitamin C.

Paper 3-style questions

Question 1 (hypothesis). A student compares fresh orange juice with orange juice that has been boiled. State a suitable hypothesis.

Model answer. Fresh orange juice contains more vitamin C than boiled orange juice, so it needs fewer drops to decolourise the same volume of DCPIP.

Question 2 (variables). State the manipulated, responding and one controlled variable for this comparison. Model answer. Manipulated variable: whether the juice is fresh or boiled.

Responding variable: the number of drops needed to decolourise the DCPIP. Controlled variable: the volume of DCPIP solution (also its concentration, the drop size, and the temperature at testing).

Question 3 (tabulation and graph). The student tests four fruits. How should the results be recorded and displayed?

Model answer. Record the drops needed for each fruit in a table, then draw a bar chart of drops against fruit; a shorter bar means higher vitamin C content.

Question 4 (inference). Fresh juice needed 8 drops and boiled juice needed 20 drops to decolourise the DCPIP. What can you infer?

Model answer. The fresh juice contains more vitamin C than the boiled juice, because it needed fewer drops; boiling has destroyed some of the vitamin C. This supports the hypothesis.

Safety

• Wear safety goggles, because DCPIP solution and acidic fruit juices can irritate the eyes.
• Add the extract with a clean dropper and avoid touching your mouth with it.
• Handle glassware carefully to avoid breakage, and clean up spills promptly.
• Wash your hands after handling the reagents and dispose of used solutions as directed.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions