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
- Make up a series. e.g. six, of known vitamin C concentrations
- This can be done by serial dilution
- Use a measuring cylinder to measure out 1 cm3 of DCPIP solution into a test tube
- 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
- When the solution turns colourless record the volume of vitamin C solution added
- Repeat steps 2-4 for the same concentration twice more and calculate a mean
- Repeat steps 2-5 for each of the known concentrations
- 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
- 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
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