measuring the vitamin c content of foods and fruit juices _ nuffield foundation.pdf
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7/23/2019 Measuring the Vitamin C content of foods and fruit juices _ Nuffield Foundation.pdf
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Nuffield Foundation » Teachers » Practical Biology » > Health and disease » What's in our food? » Measuring the Vitamin C content of foods and fruit juices
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Standard techniques
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Practical Work for Learning
For the class – set up by technician/ teacher:
Vitamin C solution, 1% (Note 1)
DCPIP solution, 1% (Note 2)
For each group of students:
Burette
Pipette, graduated
Pipette filler
Fruit juice samples
Measuring the Vitamin C content of foods and fruit
juices
Class practical
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.
Lesson organisation
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
Health and Safety and Technical notes
The chemicals used in this investigation are LOW HAZARD (Notes 1 and 2).
Read our standard health & safety guidance
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 cm 3 of water.
Ethical issues
There are no ethical issues with this procedure. Consider what to do if your results give very different measures
than those quoted by manufacturers.
Procedure
SAFETY: Take care with fragile glassware such as burettes.
Preparation
a Make up a 1% solution of vitamin C with 1 g of vitamin C in 100 cm 3; this is 10 mg cm –3.
b Make up a 1% solution of DCPIP.
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7/23/2019 Measuring the Vitamin C content of foods and fruit juices _ Nuffield Foundation.pdf
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Page last updated on 24 November 2011
Investigation
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 cm 3 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.
Teaching notes
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
Downloads
Download the student sheet Measuring the Vitamin C content of foods and fruit juices (56 KB) with questions
and answers.
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asuring the Vitamin C content of foods and fruit juices | Nuffield F... http://www.nuffieldfoundation.org/practical-biology/measuring-vitami...
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