Edexcel A (SNAB) A Level Biology:复习笔记1.2.11 Practical: Vitamin C Content

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
  4. Shake the tube for a set period of time using a stop watch
    • It is important to keep the shaking time the same for each concentration; this is a control variable
  5. When the solution turns colourless record the volume, in number of drops, of vitamin C solution added
  6. Repeat steps 2-5 for the same concentration twice more and calculate an average
  7. Repeat steps 2-6 for each of the known concentrations
  8. 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

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

Risk assessment

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

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

 

 

 

转载自savemyexams

更多Alevel课程
翰林国际教育资讯二维码