Practical: Vitamin C Content (Edexcel A (SNAB) A Level Biology)

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Cara Head

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Cara Head

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1.2.11 Practical: Vitamin C Content

  • Vitamin C is found in green vegetables, fruits, and potatoes
  • It is essential for a healthy diet
  • The chemical name for vitamin C is ascorbic acid
    • Ascorbic acid is a good reducing agent and therefore it is easily oxidised
  • Methods for the detection of vitamin C involve titrating it against a solution of an oxidising agent called DCPIP
    • DCPIP is a blue dye that turns colourless in the presence of vitamin C
    • Note that titration is a method of chemical analysis that involves determining the quantity of a substance present by gradually adding another substance

Apparatus

  • Vitamin C solutions
  • 1% DCPIP solution
  • Distilled water
  • Range of fruit juices
  • Measuring cylinder
  • Pipette
  • Stop watch
  • Test tubes

Method

  1. Make up a series. e.g. six, of known vitamin C concentrations
    • This can be done by serial dilution
  2. Use a measuring cylinder to measure out 1 cm3 of DCPIP solution into a test tube
  3. Add one of the vitamin C solutions, drop by drop, to the DCPIP solution using a graduated pipette or burette
    • Shake the tube for a set period of time after adding each drop
    • It is important to keep the shaking time the same for each concentration; this is a control variable
  4. When the solution turns colourless record the volume of vitamin C solution added
  5. Repeat steps 2-4 for the same concentration twice more and calculate a mean
  6. Repeat steps 2-5 for each of the known concentrations
  7. Results can be plotted as a line of best fit showing the volume of vitamin C needed to decolourise DCPIP against the concentration of vitamin C
    • This is a calibration curve and can be used to find the concentration of vitamin C in unknown samples such as fruit juices
  8. Repeat steps 2-6 using fruit juices of unknown concentration; these can be compared to the calibration curve to estimate concentration of vitamin C in each juice sample

Risk assessment

  • DCPIP is an irritant 
    • Avoid contact with the skin 
    • Wear eye protection

The DCPIP test for Vitamin C

Drops of vitamin C solution of known concentration can be added to DCPIP to determine the volume required for the DCPIP to be decolourised

Results

  • The volume of vitamin C solution required to decolourise DCPIP should decrease as the concentration of the vitamin C solution increases
  • The results of the experiment can be plotted on a graph of volume of vitamin C needed to decolourise DCPIP against the concentration of vitamin C
    • The line of best fit for such a graph is known as a calibration curve; unknown substances can be compared to it to gain an estimate of their vitamin C concentration
  • This calibration curve produced from this experiment can be used to estimate the concentration of vitamin C in fruit juices 

vitamin-c-dcpip-results

A graph of volume of vitamin C needed to decolourise DCPIP against vitamin C concentration can be used as a calibration curve to estimate the vitamin C concentration of unknown substances

Calculating the mass of vitamin C

  • It is possible to use the results of the practical described above to calculate the mass of vitamin C present at various points in the investigation
    • This is because we know that 1 cm³ of 1 % vitamin C solution contains 10 mg of vitamin C
  • The mass of vitamin C needed to decolourise DCPIP can be calculated as follows:

mass of vitamin C to decolourise 1cm³ of DCPIP = 10 mg × volume of vitamin C used

  • Calculating the mass of vitamin C needed to decolourise DCPIP then allows us to calculate the mass of vitamin C present in a fruit juice sample:

mass of vitamin C in fruit juice sample = mass of vitamin C to decolourise 1cm³ of DCPIP × volume of fruit juice used

Worked example

In an investigation into vitamin C concentration a mean of 0.6 cm3 of a 1 % vitamin C solution was needed to decolourise a 1 cm3 DCPIP sample. A mean of 1.4 cm3 of a fruit juice was needed to decolourise the same volume of DCPIP.

Calculate the mass of vitamin C needed to decolourise 1 cm3 DCPIP, and the mass of DCPIP present in the fruit juice sample.

Step 1: calculate the mass of vitamin C needed to decolorise 1 cm3 DCPIP

mass to decolourise DCPIP = 10 mg × volume of vitamin C used 

= 10 x 0.6

= 6 mg

Step 2: calculate the mass of vitamin C in the fruit juice sample

mass in fruit juice sample = mass of vitamin C to decolourise 1cm³ of DCPIP × volume of fruit juice used

= 6 x 1.4

= 8.4 mg

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Cara Head

Author: Cara Head

Expertise: Biology

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding