Determination of Vitamin C by an Iodometric Titration
Purpose: The goal of this lab is to determine the concentration of Vitamin C in
juices and real lemon. A redox titration, involving an iodometric method, will be
used to do the analysis. The samples will be classified by their Vitamin C content.
Introduction: Although most mammals can synthesize Vitamin C, or ascorbic acid
(C6H8O6), from sugars, man must ingest considerable quantities of this substance.
The National Academy of Sciences recommends the consumption of 60mg of
ascorbic acid per day. Vitamin C deficiency, which typically causes abnormalities in
bone and teeth, was first characterized in sailors in the 18th century. These
abnormalities were eliminated by compelling sailors to eat limes, a source of
Vitamin C. Many vegetables also contain large quantities of Vitamin C, but ascorbic
acid is commonly destroyed by many cooking processes, and hence citrus fruits are
regarded as the most reliable source of Vitamin C.
Vitamin C can be determined in food by use of an oxidation-reduction
reaction. The redox reaction is preferable to an acid-base titration because a
number of other species in juice can act as acids, but relatively few interfere with
the oxidation of ascorbic acid by iodine. The solubility of iodine is increased by
complexation with iodine to form triiodide:
I2(aq) + I-1  I3-1
Triiodide then oxidizes Vitamin C to dehydroascorbic acid:
C6H8O6 + I3-1 + H2O  C6H6O6 + 3I-1 + 2H+1
The endpoint is indicated by the reaction of iodine with starch suspension, which
produces a blue-black product. As long as Vitamin C is present, the triiodide is
quickly converted to iodide ion, and no blue-black iodine-starch product is
observed. However, when all the Vitamin C has been oxidized, the excess triiodide
(in equilibrium with iodine) reacts with starch to form the expected blue-black
color.
Experimental Procedure:
1. Preparation of iodine solution
Dissolve 5.00g of potassium iodide (KI) and 0.268g of potassium iodate (KIO3) in
200mL of distilled water in a 500mL volumetric flask. Add 30mL of 3M H2SO4 and
dilute to a final volume of 500mL with distilled water. Mix thoroughly.
2. Preparation of Vitamin C standard solution
Dissolve 0.250g of Vitamin C in 100mL of water. Dilute to volume in a 250mL
volumetric flask
3. Standardization of the iodine solution with the Vitamin C standard solution:
Add 25mL of Vitamin C solution into a 125mL Erlenmeyer flask. Add 10 drops of a
1% starch solution to the flask. Rinse your buret twice with 5-10mL of the iodine
solution, and then fill it. Record your initial buret volume in your data table. Titrate
the solution until the endpoint is reached (the first sign of blue color that remains
after at least 20 seconds of swirling). Record the final volume. Repeat the titration
at least three times. Results should agree to 0.1mL.
4. Titration of juice samples:
Add 25mL of your beverage sample into a 125mL Erlenmeyer flask. Repeat the
iodometric titration until you have three good measurements (again to 0.1mL).
5. Titration of real lemon:
Add 10mL of real lemon into a 125mL Erlenmeyer flask. Repeat the iodometric
titration until you have three good measurements (again to 0.1mL).
Data:
Prepare separate data and data analysis tables for the standardization, real juice,
and lemon sections as part of your pre-lab. Your data table for the standardization
should include the following:
 Mass of vitamin C used
 Initial and final volume readings
Your data analysis table for the standardization should include the following:
 Volume of titrant used
 Concentration (g/L) of standardized iodine solution based on each
titration
 Average concentration for iodine solution in g/L
Data tables for real juice and lemon sections should include the following:
 Initial and final volume readings
Data analysis tables for real juice and lemon sections should include the following:
 Volume of titrant used
 Grams of Vitamin C
 Moles of Vitamin C
 Molarity of Vitamin C
 Concentration of Vitamin C in g/L
 Average concentration for the real juice and lemon
Discussion:
1. As you complete the experiment, place the average concentration (g/L) of
your lemon samples on the Smart Board. Determine a class average and
compare that data with the manufacture’s data.
2. In addition, place the average concentration (g/L) of your beverage on the
Smart Board
3. Rank each beverage (real juice) from lowest to highest Vitamin C
concentration. Is this what you would expect?
4. Would you expect juices that have been exposed to the air for several days to
have higher, lower, or about the same levels of ascorbic acid as unexposed
fruit juices? Explain.