Measuring the Vitamin C content of foods and fruit juices
Measure the vitamin C content of a sample of fruit juice by measuring the volume of the sample required to decolourise a solution of DCPIP. Calibrate the results by comparison with a known concentration of vitamin C.
Demonstrate the effect of vitamin C on DCPIP. Get students to test 2 or 3 of the juices you provide to practise the technique. Ask students to develop a hypothesis to test, and to investigate it systematically. Your classroom organisation may depend on the equipment you have available. A burette of DCPIP may be the focal point for each working group.
Apparatus and Chemicals
For the class – set up by technician/ teacher:
Vitamin C solution, 1% (Note 1)
DCPIP solution, 1% (Note 2)
For each group of students:
Fruit juice samples
Health and Safety and Technical notes
The chemicals used in this investigation are LOW HAZARD (Notes 1 and 2).
1 Vitamin C solution: See CLEAPSS Hazcard. CLEAPSS Recipe card recommends a concentration of 0.1%; this protocol suggests 1%. This solution is LOW HAZARD.
2 DCPIP (2,6-dichlorophenol-indophenol) is LOW HAZARD; see CLEAPSS Hazcard. CLEAPSS Recipe card recommends a concentration of 0.1%; this protocol suggests 1% so dissolve 1.0 g of dye in 100 cm3 of water.
There are no ethical issues with this procedure. Consider what to do if your results give very different measures than those quoted by manufacturers.
SAFETY: Take care with fragile glassware such as burettes.
a Make up a 1% solution of vitamin C with 1 g of vitamin C in 100 cm3; this is 10 mg cm–3.
b Make up a 1% solution of DCPIP.
c Pipette 2 cm3 of vitamin C solution into a test tube.
d Using a graduated pipette or a burette, add 1% DCPIP drop by drop to the vitamin C solution. Shake the tube gently after adding each drop. Add DCPIP solution until the blue colour of the final drop does not disappear.
e Record the exact amount of DCPIP solution that was added.
f Repeat the procedure and calculate an average result.
g Repeat with the fruit juices to be tested. If more than 5 cm3 of DCPIP are completely decolourised, dilute the fruit juice and repeat the test. If the fruit juice has a strong colour that will interfere with determining the end point, dilute the juice before testing.
h Calculate the amount of vitamin C in the standard solution in mg cm–3. Calculate how much vitamin C there is in each of the fruit juices in mg cm–3.
In acidic conditions, DCPIP does not decolourise completely, but remains pink. With strongly acidic juices such as lemon juice this could confuse determination of the endpoint.
If you are testing lots of different solutions, it is easier to put the DCPIP in a burette and titrate it into measured samples of fruit juice, rather than cleaning a burette several times in one lesson. If you have plenty of graduated pipettes, you could measure each juice into a measured sample of DCPIP and observe the point at which the DCPIP loses its colour.
Hypotheses to test could include…
- fresh juices have more vitamin C than long-life
- juice ‘not from concentrate’ is best in terms of vitamin content
- fruit squashes have less vitamin C than fruit juices
- if heat destroys vitamin C, then heat-treated long-life juices will have lower concentrations
- if heat destroys vitamin C, then boiled fruit juice will have lower concentrations than unboiled
- manufacturers generally provide reliable information about their products
- vitamin C degrades in vitamin tablets, and old tablets will have less than fresh ones
Health and safety checked, September 2008
Download the student sheet Measuring the Vitamin C content of foods and fruit juices (56 KB) with questions and answers.