Exercise 5
Illinois Central College
CHEMISTRY 130
Laboratory Section:__________
Page 1
Name:_________________________
Chemical Stoichiometry: The Determination of Vitamin C
Objectives
This experiment will illustrate the stoichiometric relationships between reactants in a
chemical process. This stoichiometric relationship represents the mole:mole ratios of reactants
and products to one another in a balanced chemical reaction.
This experiment also uses the concept of Molarity, which represents the concentration of a
solution in moles of solute per liter of solution and allows the chemist to relate the volume of a
given solution to the number of moles of dissolved solute present. That is,
moles solute = (volume of solution used, L) x (Molarity of the solution)
We will also address the concept of the percent purity of an unknown. This is calculated by
dividing the amount of pure substance found in a sample by the actual sample mass and
multiplying this quotient by 100.
Background
Vitamins are a group of organic substances required in the diet of man and animals. Vitamin C,
or ascorbic acid, was first isolated as a pure substance by Albert Azent-Gyorgi and Charles King
in 1928. It can be obtained from citrus fruits, tomatoes, potatoes, and fresh vegetables,
particularly from red and green peppers. It is now made synthetically. All animals except man,
other primates, and guinea pigs, are capable of synthesizing their own ascorbic acid; man must
obtain it from food. The long-established use of citrus fruits (particularly limes) to prevent the
occurrence of shipboard scurvy found ready explanation when it was established that the
condition is connected with a deficiency of Vitamin C. This anti-scorbutic use is the origin of the
common name, ascorbic acid. Well-balanced diets provide adequate amounts of the vitamin as
measured by the Recommended Daily Allowance (RDA) of about 75 mg per day. The use of
large, regular, supplementary doses of Vitamin C (megavitamin therapy) in prophylaxis and
therapy of the common cold and other medical conditions has been strongly advocated,
particularly by Linus Pauling. Such use is the subject of lively controversy with the final verdict
not yet in sight.
Ascorbic acid is a cheap, comparatively simple, water-soluble, organic compound having the
molecular formula, HC6H706 . It is a weak mono-protic acid and may be titrated with a known
concentration of sodium hydroxide solution, NaOH(aq), to a phenolphthalein end-point in order to
determine the amount of Vitamin C present. It oxidizes readily in air, particularly at higher
temperatures, and hence the the vitamin is easily destroyed in cooking and in long storage. The
oxidation of ascorbic acid by iodate salts is rapid and quantitative, that is, if we monitor the
amount of iodate used to oxidize a sample containing vitamin C, we can determine the exact
amount in the sample.
Exercise 5
Page 2
We shall use a procedure in which a solution sample of ascorbic acid is first titrated with NaOH
(sodium hydroxide) and then, after reacidification, with potassium iodate. The two relevant
equations are:
HC6H7O6(aq) + NaOH(aq)
3 HC6H7O6(aq) + KIO3(aq)
NaC6H7O6(aq) + H2O
3 C6H6O6(aq) + KI(aq) + 3 H2O
Note that the mole:mole relationship of ascorbic acid to sodium hydroxide is 1:1, meaning
one mole of ascorbic acid reacts with one mole of base. On the other hand the mole:mole
relationship of ascorbic acid to potassium iodate is 3:1, so that three moles of ascorbic acid
are oxidized by one mole of potassium iodate.
Phenolphthalein serves as an indicator in the titration with sodium hydroxide. When we have
added sufficient NaOH to neutralize the ascorbic acid in our sample, the phenolphthalein will
change from clear to pink indicating the endpoint of the reaction. The reaction with potassium
iodate titration utilizes starch as the indicator. When all of the ascorbic acid has reacted with the
KIO3, the excess KIO3 oxidizes the KI produced in the reaction (which is colorless) to I2 which
forms a deep blue color with the starch indicating that the reaction is complete.
Procedure
A. Prepare a solution of 10 mL of 1 M HCl and 1 mL of starch solution in a 50 mL beaker and
set it aside. Accurately weigh out a sample of ascorbic acid (about 0.250 g) and place it in a
250 mL Erlenmeyer flask. Add about 50 mL of distilled water and swirl the flask until the
mixture is homogeneous. Add 4 drops of phenolphthalein indicator
to the Erlenmeyer flask and titrate rapidly, while swirling the flask,
with standard (0.100 M) NaOH to the pink endpoint. (As you
approach the endpoint, the pink color will begin to persist. At that
point, begin to add the titrant drop by drop such that the last drop
added causes a persistent pink color that will not swirl away.)
Quickly note your titer (the amount of solution used from your
Swirl the liquid
during the titration
burette) then reacidify the sample by adding the HCl/starch
and wash the walls
mixture you previously prepared. (The former will convert the
with distilled water
from the wash bottle.
ascorbate ion back to unionized ascorbic acid. The latter will act as
the endpoint indicator in the second titration.) Speed is of the
essence, since solutions of ascorbic acid oxidize rapidly in the
presence of sodium hydroxide. Titrate with the standard (0.0300
M) KI03 solution until the first appearance of a blue endpoint. (If
the starch is omitted, the endpoint will be signaled, less accurately, by the appearance of a
yellow color.) Repeat entire procedure with a second sample.
B. Obtain a Vitamin C tablet from the dispensing table. Weigh the tablet, then crush it using a
mortar and pestle and rinse all of the resulting powder (using distilled water) into a 250 mL
Erlenmeyer flask. Carry out the procedure outlined in Part A using the crushed tablet
instead of the ascorbic acid sample. Repeat the entire procedure with a second tablet of the
same unknown.
Exercise 5
Illinois Central College
CHEMISTRY 130
Laboratory Section:__________
Name:_________________________
REPORT SHEET
Chemical Stoichiometry: The determination of Vitamin C
A.Determination of Purity of Ascorbic Acid
Titration with NaOH (0.100 M)
Trial 1
Trial 2
Trial 1
Trial 2
Mass of ascorbic acid
Initial buret reading
Final buret reading
Volume of NaOH used
Moles of NaOH used
Moles of ascorbic acid neutralized
Grams of ascorbic acid
Percent purity of ascorbic acid
Average purity of ascorbic acid
Titration with KIO3 (0.0300 M)
Initial buret reading
Final buret reading
Volume of KIO3 used
Moles of KIO3 used
Moles of ascorbic acid reacted
Grams of ascorbic acid
Percent purity of ascorbic acid
Average purity of ascorbic acid
Page 3
Exercise 5
Page 4
B. Determination of Ascorbic Acid in Vitamin C Tablets
Titration with NaOH (0.100 M)
Trial 1
Trial 2
Trial 1
Trial 2
Mass of tablet
Initial buret reading
Final buret reading
Volume of NaOH used
Moles of NaOH used
Moles of ascorbic acid neutralized
Grams of ascorbic acid
Milligrams of ascorbic acid per tablet
Percent ascorbic acid in tablet
Average percent ascorbic acid
Titration with KIO3 (0.0300 M)
Initial buret reading
Final buret reading
Volume of KIO3 used
Moles of KIO3 used
Moles of ascorbic acid neutralized
Grams of ascorbic acid
Milligrams of ascorbic acid per tablet
Percent ascorbic acid in tablet
Average percent ascorbic acid
On a separate sheet, show set-ups and calculations for Trial 1 for both the powder and
tablet.
Exercise 5
Illinois Central College
CHEMISTRY 130
Laboratory Section:__________
PRELAB: Exp. 5
Page 5
Name:_________________________
Chemical Stoichiometry: The Determination of Vitamin C.
SHOW YOUR WORK
1. A 0.258 g sample of a powder containing some ascorbic acid required 14.50 mL of 0.100 M
NaOH for complete neutralization. Calculate the purity of the ascorbic acid.
moles of NaOH used in the titration __________________
moles of ascorbic acid reacted
__________________
grams of ascorbic acid in sample
__________________
Percent purity of the powder
__________________
2. A tablet of Vitamin C weighed 0.306 g and required 15.70 mL of 0.0300 M KIO3 solution
for oxidation to the blue starch-iodine endpoint. Calculate the percent ascorbic acid in the
tablet.
moles of KIO3 used in the titration _________________
moles of ascorbic acid reacted
_________________
grams of ascorbic acid in tablet
_________________
Percent ascorbic acid in tablet
_________________
Exercise 5
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