MEASURING VITAMIN C CONTENT
Introduction
Vitamin C (aka ascorbic acid) is an essential nutrient and is found in varying
concentrations in many fruit and vegetables.
There are two commonly used procedures for measuring the vitamin C
content: titration with DCPIP and titration with iodine.
DCPIP (aka dichlorophenolindophenol) is a blue coloured compound that is readily reduced to the
colourless DCPIPH in the presence of a suitable reducing agent such as vitamin C:
DCPIP
blue
DCPIPH
colourless
Vitamin C reacts with DCPIP in a 1:1 ratio, so by recording the volume of an unknown vitamin C solution
required to just decolourise a given volume of a known solution of DCPIP, we can calculate the amount of
vitamin C it must have contained.
An alternative method involves reacting the vitamin C with an accurately known solution of iodine and
titrating it with sodium thiosulphate to determine how much unreacted iodine remained. This is known as
iodometry. First, an accurately known solution of iodine is created by reacting iodate ions with iodide ions
in the presence of acid:
IO3-(aq) + 5I-(aq) + 6H+(aq)  3I2(aq) + 3H2O(l)
Next, the iodine is reacted with vitamin C:
Ascorbic Acid + I2(aq)  Dehydroascorbic Acid + 2H+(aq) + 2I-(aq)
Finally the remaining iodide is titrated with sodium thiosulphate to determine its concentration:
2S2O32-(aq) + I2(aq)  S4O62-(aq) + 2I-(aq)
Apparatus and Materials
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Standard laboratory glassware
Sieve
Burette
1% starch solution
Concentrated sulphuric acid
Graduated pipettes and fillers
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1g dm-3 solution of DCPIP
Two oranges
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0.10 M sodium thiosulphate solution
0.0010 M potassium iodate solution
0.0050 M potassium iodide solution
DCPIP Procedure
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Cut an orange in half and squeeze the juice through a sieve and into beaker.
Fill a burette with the orange juice.
Using a graduated pipette, transfer 10.0 cm3 of DCPIP solution into a 100 cm3 conical flask.
Place the flask on a white tile and add the orange juice drop-wise until the blue colour disappears.
Record the volume of orange juice added and repeat until two consistent readings are obtained.
Iodometry Procedure
Generate your well-known iodine solution in a boiling tube by reacting 2.0 cm3 potassium iodate solution with
3.0 cm3 potassium iodide solution, measuring the solutions with a graduated pipette.
7. Add three drops of concentrated sulphuric acid to cause a colour change from colourless to yellow-brown.
8. Add a few drops of starch solution to form a deep-blue/black solution.
9. Using a graduated pipette, add 1.0 cm3 of orange juice and swirl to mix.
10. Add sodium thiosulphate solution from a burette drop-wise until the blue-black colour just disappears.
11. Record the volume of sodium thiosulphate used and repeat until two consistent sets of results are obtained.
6.
Analysis
a. Calculate the molar concentration of the DCPIP solution and determine the number of moles present in 10.0 cm 3
of it.
b. Use your answer to a to determine the molar concentration of Vitamin C in the orange juice and thus work out
the mass of vitamin C present in 1.0 dm3 of orange juice.
c. Determine the number of moles of iodine produced in the first step of the iodometry procedure.
d. Use the results of the sodium thiosulphate titration to determine the number of moles of iodine that must have
remained after reacting with the orange juice.
e. Use your answers to c and d to calculate how much iodine was used up by the vitamin C in the orange juice and
thus the number of moles of vitamin C present in 1.0 cm3 of juice and its molar concentration.
f. Use your answer to e to determine the mass of vitamin C present in 1.0 dm3 of orange juice.
g. How much orange juice would you need to drink to fulfil your recommended daily allowance (RDA) of vitamin C?
RDA values can be found online.
h. Which method do you think is the best and why?
i. Think of a research question you could ask relating to vitamin C content and briefly outline how you might
investigate it.