Skills applied: CP1 sugars and starch, CP2 vitamin C

CP1: semi-quantitative tests

Method recap (see Core practical 1, Unit 1): heat equal volumes of sample and Benedict's reagent in a boiling water bath for the same time; iodine for starch.

To make it semi-quantitative, make colour standards from glucose solutions of known concentration (serial dilution) and match the unknown to the closest standard.

Better: filter off the precipitate and read the absorbance (or transmission) of the remaining solution in a colorimeter, plot a calibration curve of absorbance against known concentration, then read the unknown's concentration off the curve.

Controlled: volume of reagent and sample, heating time and temperature, same colorimeter filter, zero with a blank.

The biuret protein estimation (recommended additional practical) works the same way: colour standards of known protein concentration.

CP2: vitamin C with DCPIP

Method recap (see Core practical 2, Unit 1): vitamin C decolourises blue DCPIP. Add vitamin C solution from a burette or pipette, drop by drop, to a fixed volume of DCPIP, swirling, until the blue colour just disappears.

1. Standardise: find the volume of a known vitamin C solution (for example 1.0 mg cm⁻³) needed to decolourise 1 cm³ DCPIP.

2. Repeat with the juice.

3. Mass of vitamin C that decolourises 1 cm³ DCPIP = standard concentration x standard volume. Juice concentration = that mass ÷ juice volume.

Issues: judging the end point (use a white tile and a colour standard), DCPIP turns pink in acid juices, vitamin C oxidises over time and with heat.

Typical exam angles

• Explain why the same volume of DCPIP and the same shaking are used.
• Explain why fresh juice is tested immediately.
• Calculate concentration and percentage decrease over storage.
• Suggest how to make colour matching less subjective (colorimeter, several observers, white background).

Worked example

0.50 cm³ of 1.0 mg cm⁻³ vitamin C decolourises 1 cm³ of DCPIP. 2.0 cm³ of orange juice decolourises 1 cm³ of DCPIP. Calculate the vitamin C concentration of the juice.

1. Mass decolourising 1 cm³ DCPIP = 1.0 x 0.50 = 0.50 mg
2. Juice concentration = 0.50 mg ÷ 2.0 cm³ = 0.25

Answer: 0.25 mg cm⁻³

Check yourself

1. 0.40 cm³ of 1.0 mg cm⁻³ vitamin C decolourises 1 cm³ DCPIP; 1.6 cm³ of apple juice also decolourises 1 cm³ DCPIP. Calculate the vitamin C concentration of the juice in mg cm⁻³.
   Show answer

   0.25 mg cm⁻³. Mass = 1.0 x 0.40 = 0.40 mg; 0.40 / 1.6 = 0.25 mg cm⁻³.

2. 1 cm³ DCPIP is decolourised by 0.50 mg vitamin C. A drink needs 5.0 cm³ to decolourise 1 cm³ DCPIP. What volume of drink, in cm³, would contain 20 mg of vitamin C?
   Show answer

   200 cm³. Concentration = 0.50 / 5.0 = 0.10 mg cm⁻³; 20 / 0.10 = 200 cm³.

3. A calibration line for glucose is absorbance = 0.25 x concentration (in mmol dm⁻³). A sample has absorbance 0.62. Calculate its glucose concentration.
   Show answer

   2.48 mmol dm⁻³. 0.62 / 0.25 = 2.48 mmol dm⁻³.

4. How are colour standards for CP1 made?
   • By mixing paints to match the unknown
   • By heating Benedict's reagent for different times
   • By testing glucose solutions of known concentrations with Benedict's reagent
   • By adding iodine to starch
   Show answer

   By testing glucose solutions of known concentrations with Benedict's reagent. Known concentrations give reference colours.

5. Why is the precipitate removed before reading Benedict's results in a colorimeter?
   • The remaining blue copper(II) ions in solution give a reading related to sugar concentration
   • The precipitate would dissolve the cuvette
   • Colorimeters only work with red light
   • To make the solution more concentrated
   Show answer

   The remaining blue copper(II) ions in solution give a reading related to sugar concentration. Less blue left means more reducing sugar.

6. What is the end point in the DCPIP titration?
   • When the solution turns brick red
   • When a precipitate forms
   • When the blue colour of DCPIP just disappears
   • When the solution turns blue-black
   Show answer

   When the blue colour of DCPIP just disappears. Vitamin C reduces DCPIP to colourless.

7. Why should juice be tested soon after it is prepared?
   • Vitamin C is oxidised over time, lowering the measured content
   • DCPIP only works on fresh juice
   • Juice becomes more acidic and turns DCPIP blue
   • Sugar in the juice increases
   Show answer

   Vitamin C is oxidised over time, lowering the measured content. Vitamin C breaks down with time, heat and air.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018): Unit 3, Unit 6 and Appendix 6 mathematical skills
• Pearson Examiners' Report WBI13/01, June 2024
• Pearson mark scheme WBI13/01, June 2024 (via Physics & Maths Tutor)

Last updated 8 November 2026. Spotted a mistake? Tell us.