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| Orange Juice |
Vitamin C,
a water-soluble ion of ascorbic acid, is essential for normal growth, repair
and development in humans. Unlike most other vertebrates, humans and some other
primates are incapable of synthesizing their own vitamin C, with the result
that it is a necessary requirement in our diet.
In this
activity the relative vitamin C content of a range of juices and drinks will be
examined using a technique known as titration. Titration involves adding one
reactant to another until an end point, often indicated by a colour change, is
reached.
Vitamin C and its Reaction with the TriIodide Ion
In this
experiment the end point will occur when a starch solution containing the
beverage sample changes from clear to a bluish-black in the presence of iodine
solution. The more vitamin C a sample contains, the greater the number of drops
of iodine that will be required. This is due to the following reaction between
the ascorbate ion and the triIodide (I3-) ion present in iodine solution:
C6H8O6 +
I3- + H2O --> C6H6O6 + 3I- + 2H+
As long as
the vitamin C (C6H8O6 ) is being oxidized to dehydro ascorbic acid (C6H6O6),
the iodine is not available to react with the starch solution in the sample
tested. When all of the vitamin C has reacted, however, any further added
iodine reacts with the starch to form a blue-black complex, thus indicating the
end-point of the reaction.
Vitamin C Comparison – Materials and Teaching Method
Step 1-
Making up the Starch Solution
This step
should be carried out by the teacher or laboratory technician prior to the
lesson. The volume of starch solution prepared should be enough for around 6-7
groups of students.
Materials
and Equipment Required:
•
1.5 litres near boiling water
•
6 teaspoons starch
•
Large 2 litre beaker or plastic bottle
Mix the
starch with the hot water and pour into a large beaker or 2 litre plastic
bottle. Allow the solution to cool.
Step 2 –
Testing Selected Beverages With Iodine and Starch Solution
The
following materials and equipment are required per group of four students:
•
Around 50ml starch solution
•
Dropper bottle or similar of Lugol’s iodine solution
•
Small beaker or jar
•
Teaspoon
•
Samples of beverages/juices such as different brands of orange
juice, lemon juice, soft drinks, apple juice, milk, blackcurrant juice and
cranberry juice
Students
should be instructed to copy down the following directions, which could be
followed by a teacher-led explanation:
•
Add 1 tsp starch solution to 20ml of the selected beverage in a
small beaker.
•
Add drops of iodine to the beaker until the liquid remains
blue-black in colour.
•
The liquid that takes the most drops to remain blue-black contains
the most vitamin C.
A suitable
results table should also be drawn up to include the following columns:’ liquid
tested’, ‘number of drops of iodine needed to keep the solution a blue-black
colour’ and ‘ranking of liquids in order of least vitamin C to most vitamin C’.
Typical
results using these quantities range from about 4 drops of iodine in the
liquids containing little to no vitamin C (such as soft drinks and milk) to
around 20 drops in commercially prepared orange juices.
Vitamin C Comparison – Follow-Up Questions and Activities
The
following questions could be written on the board after students write up the
experiment and their observations:
1.
What is the scientific name for Vitamin C?
2.
Explain why starch solution is added to the liquids that are tested.
3.
Which of the liquids tested contained the most vitamin C? The least?
4.
Write the equation for the reaction that occurs between the iodine
solution and the vitamin C in the liquids tested.
For more
advanced students, an actual determination of vitamin C concentration can be
conducted using a dissolved vitamin C tablet as a standard solution. Assuming
the tablet contains 250 mg vitamin C (read the label to check this), the number
of drops of iodine needed to reach the end-point can be compared to the results
for other liquids.
If, for
instance, 5 drops of iodine are needed to reach the end-point with the vitamin
C solution and 10 drops are needed when a 20 ml orange juice sample is tested,
the orange juice must contain twice as much vitamin C as the standard, that is,
500 mg per 20 ml. This can then be divided by 20 to arrive at a vitamin C
concentration of 25 mg/ml.
References
•
University of
Canterbury College of Science, 'Determination of Vitamin C Concentration by
Titration', http://www.outreach.canterbury.ac.nz/chemistry/documents/vitaminc_iodine.pdf
•
Scharf, W. and
Malerich, C.,Baruch College, New York, 'Determination of Vitamin C of Citrus
Juices',http://www.baruch.cuny.edu/wsas/academics/natural_science/chm_1000/vitiamin_C.pdf
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