Vitamin C, or ascorbic acid, takes part in a reversible reaction with iodine in solution. This can be used to compare vitamin C levels in various beverages.
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
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:
What is the scientific name for Vitamin C?
Explain why starch solution is added to the liquids that are tested.
Which of the liquids tested contained the most vitamin C? The least?
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.
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