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

• 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