Determination of Vitamin C Content by Redox Titration
Background
Vitamin C, or ascorbic acid, is a water-soluble antioxidant that plays a vital role in protecting the body from
infection and disease. Ascorbic acid is not synthesized by the human body and therefore must be acquired
from dietary sources such as fruits and vegetables.
The chemical structure and antioxidant (reducing) action of ascorbic acid are illustrated in the redox halfreaction below
Oxidation
+ 2H+ + 2eBased on the above equation, it is possible to determine the amount of vitamin C in solution by reacting it with
a suitable oxidizing agent. In this case we use iodine (complexed as triiodide), which is in turn generated by a
redox reaction involving iodate (IO3-) and iodide (I-) ions. The iodate reacts with iodide to form iodine according
to the following reaction:
IO3- + 5I- + 6H+  3I2 + 3H2O
Iodine is relatively insoluble, but this can be improved by complexing the iodine with excess iodide in-situ to
form triiodide:
I2 + I- ↔ I3Triiodide oxidizes vitamin C to form dehydroascorbic acid and re-forms iodide in the process.
+
I3- +
H2O
→
+ 3I- + 2H+
As long as vitamin C is present in the solution, the triiodide is converted to the iodide ion very quickly.
However, when the all the vitamin C is oxidized, iodine and triiodide will be remain in solution. They will then
react with starch to form a blue-black complex. The blue-black color is the endpoint of the titration.
Question
What percentage of a tablet of chewable Vitamin C is pure ascorbic acid?
Materials
20 mL pipette and pipette bulb
retort stand
mortar and pestle
1 250 mL Erlenmeyer flask
Vitamin C tablet
burette and burette clamp
0.5% starch solution
beakers
funnel
0.002 M KIO3 solution
white paper
100 mL volumetric flask
1.0 M HCl solution
0.6 M KI solution
wash bottle
Procedure
1. Measure the mass of the entire Vitamin C tablet.
2. Crush the Vitamin C tablet using the mortar and pestle,
3. Accurately weigh approximately 200 mg of the powder and transfer it to a 100 mL volumetric flask. Add
approximately 5 mL of 1.0 M HCl to the flask when half-full to help the powder dissolve.
4. Make up to the 100 mL mark with distilled water. Two groups may share one flask depending on
availability of equipment.
5. Pipette a 20 mL aliquot of the Vitamin C solution into a 250 mL Erlenmeyer flask and add approximately
10 mL of distilled water. Remember to use proper technique.
6. Then add approximately 5mL of 0.6M KI, and 1 mL of starch solution to the Erlenmeyer.
7. Titrate the sample with the 0.002 M KIO3- solution until the first distinct and persistent traces of a darkblue can be seen.
8. Repeat the titration 2 more times.
9. Dispose of all chemicals, rinse all equipment with water, wash down the bench with wet paper towel
and return all equipment.
Analysis
1. Write the balanced half reactions between iodate and iodide, and between ascorbic acid and the
liberated iodine.
2. Identify the atoms that are being oxidized and reduced in the reaction and label the oxidation numbers
for these atoms.
3. Calculate the average volume of iodate solution used.
4. Calculate the number of moles of iodate reacting and the number of moles of iodine liberated.
5. Calculate the number of moles of ascorbic acid oxidized in the Erlenmeyer flask. Throughout all of
these calculations, take care to mind the correct mole to mole ratios.
6. Calculate the concentration of the ascorbic acid in the volumetric flask in mol/L.
7. Calculate the number of milligrams of Vitamin C in the original tablet.
8. What percentage of the tablet is in fact Vitamin C?
9. Is the number reasonable? What are some explanations for any percentage error?