Practical: Vitamin C Content (Edexcel AS Biology (A) SNAB): Revision Note

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

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 

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