The analysis of vitamin c

Experiment33‐1

Analysis for Vitamin C



A. Introduction

Vitamin C, known chemically as ascorbic acid, is an important component of a
healthy
diet. In the mid-eighteenth century the British navy found that the
addition of citrus fruit
to the sailors' diet prevented the malady called scurvy.
Humans are one of the few mem
bers of the animal kingdom unable to synthesize
vitamin C, resulting in the need for regu
lar ingestion in order to remain healthy. The
National Academy of Sciences has established
the threshold of 60 mg/day for adults as
the Recommended Dietary Allowance (RDA).
Linus Pauling, a chemist whose many
contributions to chemical bonding theory should be
well-known to you,
recommended a level of 500 mg/day to help ward off the com
mon cold. He had
also suggested that large doses of vitamin C are helpful in preventing
cancer.
The
vitamin C content of foods can easily be determined by oxidizing ascorbic acid,
C
6

H
8
O
6
to dehydro-L-ascorbic acid, C
6
H
6
O
6
:




O
O
+2H
+
+2e
-
O
O
O
O
OHOH
C
H
C
C

H
C
H
H
OH
OH
OHH
OHH










Vitamin C Dehydro-L-ascorbic acid



This reaction is very slow for ascorbic acid in the dry state, but occurs readily when in
contact
with moisture. A reagent that is particularly good for the oxidation is an
aqueous solution of
iodine,
I
2
. Since

I
2

is not very soluble in water, we
dissolve it in a solution of potassium
iodide, KI, in which the I2 exists mainly as I
3
-
,
a complex ion. The reaction with ascorbic acid
involves
I
2
, which is reduced to I
-
ion:


)(2)(2
2
aqIaqIe
−−
⎯→⎯+


In the overall reaction, one mole of ascorbic acid requires one mole of
I
2
for
complete

oxidation.
When the red-colored
I
2
solution is added to the ascorbic
solution, the characteristic iodine
color disappears because of the above reaction.
Experiment33‐2

Although we could use the first permanent appearance of the yellow color of dilute
iodine to mark the end point of the titration, better
results are obtained when starch is
added as an indicator. Starch reacts with
I
2
to form an
intensely colored blue
complex. In the titration I
2
reacts preferentially with ascorbic acid, and
so its
concentration remains very low until the ascorbic acid is all oxidized. At that
point, the
I
2
concentration begins to go up and the reaction with the indicator
occurs:

)()(
22

complexIstarchstarchaqI −⎯→⎯+

yellow
blue

Because an I
2
solution cannot be prepared accurately by direct weighing, it is necessary
to
standardize the I
2
against a reference substance of known purity. We will use pure
ascorbic
acid for this reference, or primary standard. After standardization you can use
the iodine
solution for the direct determination of vitamin C in any kind of sample.

B.

Procedure

B-1. Standardization of the Iodine Solution
Obtain from the storeroom a buret and-an unknown vitamin C sample. Weigh out
accurately
on the analytical balance three ascorbic acid samples of approximately 0.10
g into clean
250-InL Erlenmeyer flasks. Dissolve each sample in approximately 100 mL
of water.
Clean your buret thoroughly. Draw about 100 mL of the stock I
2

solution from
the carboy
in the laboratory into a 400-mL beaker and add approximately 150 mL of
water. Stir thor
oughly and cover with a piece of aluminum foil. Rinse the buret with
a few milliliters of the I
2
solution three times. Drain and then fill the buret with the I
2

solution.
After taking an initial reading of the buret (you may find looking toward a light source
will
make it easier to see the bottom of the meniscus), add 1 mL of starch indicator to
the first
ascorbic acid sample and titrate with the iodine solution. Note the change of the
I
2
color as
you swirl the flask gently and continuously during the titration. Continue the
addition of the
iodine, using progressively smaller volume increments, until the sample
solution just turns a
distinct blue. After reading the buret, titrate the other tvio sample
solutions-being sure to
add the starch indicator and to read your buret before and after
each titration.

B-2. Analysis of an Unknown Containing Vitamin C
Given your experience with the standardization reaction, you should be able to

devise an
analogous procedure to determine the vitamin C content of your unknown
sample. You will
need to select a sample size, and you may need to carry out an
initial treatment of the sample.
In particular, if your instructor assigns you a fruit
juice sample, it will be desirable to first
f
ilter the sample through cheese cloth, followed
by rinsing of the filter with water.

It may be helpful in choosing the sample sizes to calculate an iodine solution
Experiment33‐3

parameter
called the titer-the number of mg of ascorbic acid which reacts with 1 ml- of
iodine solu
tion. This number is easily found from the I
2
concentration and the mass
relationship in the
reaction. It is desirable to have the volume of I
2
for each titration be at
least 15 mL. Using a
small initial sample will give you an indication of how much to
scale up for your final
titrations.
Report your results in per cent vitamin C, if a solid sample was used. For liquid
samples,

report mg of vitamin C per 100 mL. In each case, calculate the size sample
required to give
the RDA of vitamin C.



Experiment33‐4

C. Data and Calculations

C-1. Standardization of Iodine
Solution


Sample I II
III

Mass of Ascorbic Acid
Sample (g)


Moles of Ascorbic Acid

Initial buret reading (mL)

Final buret reading (mL)

Volume of I
2
added (mL)

Moles of I
2
consumed

Molarity of I
2
(M)
Titer of I
2
(MM Ascorbic
Acid = 176 g/mole)



Experiment33‐5

C-2. Unknown Sample

Mass or volume of unknown

Initial buret reading (mL)
Final buret reading (mL)
Moles of iodine added

Moles of vitamin C in
sample


Mass of vitamin C in sample
(mg)

Percent vitamin C in solid
sample


Concentration of vitamin C
in liquid sample


Amount of sample which
will furnish RDA





Experiment33‐6

D.

Analysis for Vitamin C Pre-Lab


1. Write a balanced equation for the reaction between I
2
and ascorbic acid. Identify the
oxidizing agent and the reducing agent.

2. A solution of I2 was standardized with ascorbic acid. Using a 0.1000-g sample of
pure
ascorbic acid, 25.32 mL of I
2
were required to reach the starch end point.

a. What is the molarity of the iodine solution?






b. What is the titer of the iodine solution (
mg asc/mL)
?




3. A sample of fresh grapefruit juice was filtered and titrated with the above I
2
solution.
A 1
00 mL sample of the juice took 9.85 mL of the iodine solution to reach the starch
end

point.

a. What is the concentration of vitamin C in the juice in mg vitamin C/100 mL of juice
(
mg/ 100 mL)
?







b. What quantity of juice will provide the RDA amount of vitamin C (mL)?

Experiment33‐7


Acknowledgements



Masterton, W. L.; Slowinski, E. J.; Wayne, C. W.
Chem. Principles in the
Laboratory.
1997,
257-261.