Core practical 2: vitamin C content of food and drink

The principle

DCPIP is a blue dye. Vitamin C (ascorbic acid) is a reducing agent, so it turns DCPIP colourless. The more vitamin C a solution contains, the less of it is needed to decolourise a fixed volume of DCPIP.

Method

1. Put a fixed volume of DCPIP (for example 1 cm3) into a test tube.

2. Add a vitamin C standard of known concentration (for example 1 mg cm-3) drop by drop from a burette or syringe, swirling, until the blue colour just disappears. Record the volume.

3. Repeat with each juice, using fresh DCPIP each time.

4. Repeat each test at least three times and calculate a mean.

Control: volume and concentration of DCPIP, the end point (judge against a white tile), swirling, temperature.

Calculation

The same mass of vitamin C is needed to decolourise 1 cm3 of DCPIP every time.

• Mass of vitamin C needed = volume of standard used x concentration of standard.
• Concentration in juice = mass needed / volume of juice used.

Shortcut: juice concentration = standard concentration x (volume of standard / volume of juice).

Limitations: the end point is hard to judge in coloured juices, and vitamin C oxidises in air, so use fresh samples.

Worked example

2.0 cm3 of 1.0 mg cm-3 vitamin C decolourises 1 cm3 of DCPIP. Orange juice takes 5.0 cm3. Find the vitamin C concentration in the juice.

1. Mass of vitamin C needed = 2.0 x 1.0 = 2.0 mg
2. Concentration in juice = 2.0 / 5.0

Answer: 0.4 mg cm-3

Check yourself

1. 1.5 cm3 of a 1.0 mg cm-3 vitamin C standard decolourises 1 cm3 of DCPIP. Apple juice takes 6.0 cm3. What is the vitamin C concentration of the apple juice in mg cm-3?
   Show answer

   0.25 mg cm-3. Mass needed = 1.5 mg; 1.5 / 6.0 = 0.25 mg cm-3.

2. A standard of 0.5 mg cm-3 needs 4.0 cm3 to decolourise 1 cm3 DCPIP. A fruit drink needs 10.0 cm3. Find its vitamin C concentration in mg cm-3.
   Show answer

   0.2 mg cm-3. Mass needed = 0.5 x 4.0 = 2.0 mg; 2.0 / 10.0 = 0.2 mg cm-3.

3. What colour change shows the end point when vitamin C is added to DCPIP?
   • Colourless to blue
   • Blue to colourless
   • Blue to brick red
   • Orange to blue-black
   Show answer

   Blue to colourless. Vitamin C reduces blue DCPIP to colourless.

4. Juice A needs 3 cm3 and juice B needs 8 cm3 to decolourise the same volume of DCPIP. Which is correct?
   • Juice B contains more vitamin C per cm3
   • They contain the same amount
   • Juice A contains more vitamin C per cm3
   • Neither contains vitamin C
   Show answer

   Juice A contains more vitamin C per cm3. Less juice needed means a higher concentration.

5. Which is a control variable in this practical?
   • The volume and concentration of DCPIP
   • The type of juice
   • The volume of juice needed
   • The vitamin C concentration of the juice
   Show answer

   The volume and concentration of DCPIP. DCPIP must be the same each time so results are comparable.

6. Why can results for some juices be unreliable?
   • DCPIP is colourless
   • Vitamin C never reacts with DCPIP
   • The juice colour makes the end point hard to see
   • The standard has no vitamin C
   Show answer

   The juice colour makes the end point hard to see. Coloured juices mask the colour change.

7. Why should juices be tested soon after opening?
   • Vitamin C is oxidised by air over time, lowering its concentration
   • Vitamin C increases over time
   • DCPIP reacts with sugar
   • Juice gets thicker
   Show answer

   Vitamin C is oxidised by air over time, lowering its concentration. Exposure to oxygen breaks down vitamin C.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.