Chemistry 232 Determination of Vitamin C by an Iodometric Titration

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Chemistry 232
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 60 mg of ascorbic acid per day. Vitamin C deficiency, which typically
causes abnormalities in bones and teeth, was first characterized in sailors in the eighteenth 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 iodide to form triiodide:





I
-
2(aq) + I- º I3





(1)

Triiodide then oxidizes vitamin C to dehydroascorbic acid:



C
-
6H8O6
+ I3 + H2O → C6H6O6 + 3I- + 2H+


(2)


vitamin C


dehydroascorbic acid







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 5.00 g potassium iodide (KI) and 0.268 g potassium iodate (KIO3) in 200 mL of distilled
water in a 400 mL beaker. Add 30 mL of 3 M sulfuric acid. Then pour the solution into a 500 mL
graduated cylinder, and dilute to a final volume of 500 mL with distilled water. Mix thoroughly and transfer
to a 600 mL beaker. Do not put this solution in a volumetric flask!!!!

2. Preparation of vitamin C standard solution.

Dissolve 0.250 g vitamin C in 100 mL water. Dilute to volume in a 250 mL volumetric flask.


Determination of Vitamin C by an Iodometric Titration

3. Standardization of the iodine solution with the vitamin C standard solution.

Add 25.00 mL of vitamin C solution into a 125 mL Erlenmeyer flask. Add 10 drops of 1 % starch
solution. Rinse your buret twice with 5 -10 mL of iodine solution, and then fill it. Record your initial buret
volume. Titrate the solution until the endpoint is reached (the first sign of blue color that remains after at
least 20 s of swirling). Record the final volume. Repeat this titration at least three times. Results should
agree to 0.1 mL.

4. Titration of juice samples.

Add 25.0 mL of your beverage sample into a 125 mL Erlenmeyer flask. Repeat the iodimetric
titration until you have three good measurements (again to 0.1 mL). Use the Q-test to check for bad data.

5. Titration of Real Lemon.

Add 10.0 mL of Real Lemon into a 125 mL Erlenmeyer flask. Repeat the iodimetric titration until
you have three good measurements (again to 0.1 mL). Use the Q-test to check for bad data.

Data Analysis:
1. What is the reaction to produce iodine from iodate and iodide? Draw the structures of the organic
compounds given in Equation (2).

2. (a) Prepare tables of all your titration data. Include the mass of vitamin C used, and the
volume of
titrant used for each titration. Calculate the molarity of the standardized
iodine solution based on
each titration. Average your concentration results to obtain the most accurate value and determine the
standard deviation.
(b) Calculate the amount of vitamin C in your beverage and Real Lemon including the
volume of
titrant used, the moles of vitamin C present, the molarity of vitamin C, and the concentration of vitamin C
in g/L

3. Collect data from all the other groups in the class. Rank each beverage from lowest to highest vitamin
C concentration.

4. Average all of the data for the Real Lemon for the whole class and compare that data with the
manufacturer’s data.

Additional Problems:
1. (a) A standard iodine solution was standardized against a 0.4123 g primary standard As4O6 by

dissolving the As4O6 in a small amount of NaOH solution, adjusting the pH to 8, and titrating,
requiring 40.28 mL iodine solution. What is the concentration of the iodine
solution?



As4O6 (s) + 6 H2O → 4 H3AsO3



H
-
3AsO3 + I3 + H2O → H3AsO4 + 3I- + 2H+

(b) The purity of a hydrazine (N2H4) sample is determined by titration with triiodide. A
sample of
the oily liquid weighing 1.4286 g is dissolved in water and diluted to 1 L in a
volumetric flask. A 50.00
2

Determination of Vitamin C by an Iodometric Titration

mL aliquot is taken with a pipette and titrated with the standard iodine solution in (a), requiring 42.41 mL.
What is the percent purity by weight
of the hydrazine?




N
-
2H4 + 2 I3 → N2 + 6 I- + 4 H+

2. A 0.200 g sample containing copper is analyzed iodometrically. Copper(II) is first reduced to
copper(I) by iodide according to the following reaction:
2Cu2+ + 4I- → 2CuI(s) + I2
and the liberated I2 is titrated against thiosulfate. What is the percent copper in the sample if
20.0 mL of 0.100 M Na2S2O3 is required for titration of the liberated I2 according to the
following reaction?





I
2-
2-
2 + 2S2O3 → 2I- + S4O6

3. Triiodide ions are generated in solution by the following reaction:
IO -
-
3 + 6 H+ + 8 I- → 3 I3 + 3 H2O

If a 25.00 mL sample of 0.0100 M KIO3 is reacted with an excess of KI and it requires
32.04 mL of Na
-
2S2O3 solution to titrate the I3 ions formed, what is the molarity of the
Na2S2O3? Use the equation below.
I -
2-
2-
3 + 2 S2O3 º 3 I- + S4O6

Report: This is a short report. Be sure to include the following:
Tabulate results for standardization and juice analysis (including statistics).
Class results for various juices.
Answers to questions
Signed pledge


PLEDGE:
My signature at the end of this report is my pledge that this work is my own. I have neither
given nor received help from other students.





Signature ________________________________________





Date _____________


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