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1.3.3Unit 1: Lifestyle, Transport, Genes and Health

Core Practical 2: Investigate the Vitamin C Content of Food and Drink — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 21 of 29 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Principle of the Method

Vitamin C
  • Vitamin C The chemical name is ascorbic acid, a good reducing agent that donates electrons easily and is readily oxidised.
DCPIP
  • DCPIP This is a blue dye that is reduced by ascorbic acid and turns colourless when it reacts.
Titration
  • Titration Vitamin C solution is added from a burette or pipette to a fixed volume of DCPIP until the blue colour just disappears.
End point
  • End point The smaller the volume of solution needed to decolourise the DCPIP, the higher the concentration of vitamin C.

Part 2 of 5Method

1
Make up a series of vitamin C solutions of known concentration from a stock solution, using a serial dilution.
2
Measure a fixed volume, for example 1 cm³, of DCPIP solution into a test tube using a pipette.
3
Add vitamin C solution drop by drop, or from a graduated pipette, shaking after each addition until the blue colour just disappears.
4
Record the volume used for each known concentration and plot a calibration curve of volume against concentration.
5
Repeat with the fruit juice, using the same procedure, and use the calibration curve to read off the vitamin C concentration of the juice.

Part 3 of 5Validity and Improvements

Validity and Improvements
Control variables
  • Control variables Use the same volume and concentration of DCPIP and the same temperature, and add the solution at the same rate each time.
Repeats
  • Repeats Repeat the titration at least three times and use a mean of concordant results to improve reliability.
Coloured juices
  • Coloured juices Dark coloured juices can hide the end point, so dilute them or use a colorimeter to detect the loss of blue.
Vitamin C loss
  • Vitamin C loss Vitamin C breaks down with heat, light and air, so use fresh samples and avoid heating or leaving solutions exposed.

Part 4 of 5Worked Example

Question

  • 1.0 cm³ of DCPIP is decolourised by 0.5 cm³ of a vitamin C standard of concentration 1.0 mg per cm³.
  • The same volume of DCPIP needs 2.0 cm³ of orange juice.
  • Calculate the vitamin C concentration of the juice.

  1. Mass of vitamin C needed to decolourise 1 cm³ DCPIP = 0.5 × 1.0 = 0.5 mg.
  2. The same mass is present in 2.0 cm³ of juice.
  3. Concentration = 0.5 ÷ 2.0 = 0.25 mg per cm³.

Answer The juice contains 0.25 mg of vitamin C per cm³.

Part 5 of 5Exam Checklist & Tips

1.3.3 – Key Points for the Exam

  • DCPIP A blue dye that is decolourised when reduced by vitamin C.
  • Ascorbic acid The chemical name for vitamin C.
  • End point The point at which the colour just disappears.
  • Calibration curve A graph of known concentrations used to find unknown ones.

Common Mistakes

  • Adding the vitamin C solution too quickly so that the end point is overshot.
  • Changing the volume of DCPIP between tubes, which makes the results incomparable.
  • Believing that a larger volume of juice means more vitamin C, when it means less.
  • Heating the juice, which destroys the vitamin C.

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