David Millard
Naomi Bryner (partner)
Experiment 5: Iodimetric Titation of Vitamin C11
Introduction:
Vitamin C, also known as ascorbic acid, in a supplemental source will be tested via titration.
The amount of ascorbic acid will be compared to the manufacture’s label to determine if the
amount is consistent.
Reactions:
IO3− + 8I + 6H + ↔ 3I3− + 3H2 O
2−
−
I3− + 2S2 O2−
3 ↔ 3I + S4 O6
C6 H8 O6 + I3− + H2 O ↔ C6 H8 O7 + 3I − + 2H +
Procedure:
Standardizing Thiosulfate Solution








As a group, prepare a starch indicator solution.
o Make paste, 5.0g of soluble starch in 50mL of water.
o Pour paste into 500mL of boiling water, boil until clear.
Prepare approximately 0.07M Na2S2O3 by dissolving about 8.70g of Na2S2O3∙H2O in
500mL of freshly boiled water containing 0.05g of Na2CO3.
Store solution in a tightly amber bottle.
Prepare approximately 0.01M KIO3 by weighing about 1.0g of solid reagent and
dissolving it in a 500mL volumetric flask.
Pipet 50.00mL of KIO3 solution into an erlenmeyer flask.
Add 2.0g of solid KI and 10mL of 1M H2SO4.
Immediately titrate with thiosulfate until the solution has lost almost all of its color (pale
yellow) and then add 5mL of starch indicator and complete the titration to a clear
endpoint.
Repeat until you have three good trials.
Analysis of Vitamin C


Weigh out an appropriate amount of ascorbic acid for an approximate 30mL titration, and
dissolve it in 50mL of 0.3M H2SO4 with stirring.
Add 2.0g of solid KI and add 50mL of standard KIO3.

Titrate with standard thiosulfate as above, adding 2.0mL of starch indicator just before
the end point.
Repeat procedure using two weighed vitamin C tablets.

Data:
Standardizing Thiosulfate
KI (g)
2.0060
2.0047
2.0192
2.0010
Sample 1
Sample 2
Sample 3
Average
Thiosulfate Molarity
Thiosulfate (mL)
35.02
35.01
35.09
35.04
0.0562
Titrating Ascorbic Acid (used 150mL KIO3)
KI (g)
Thiosulfate (mL)
Ascorbic Acid
(g)
Weight percent
of ascorbic acid
Sample 1
2.0056
30.61
0.5312
18.08
Sample 2
2.0308
30.40
0.5212
18.62
Sample 3
2.0018
31.28
0.5144
18.02
Average
2.0123
30.76
0.5223
18.24
KI (g)
Thiosulfate (mL)
Tablet mass (g)
2.0018
2.0018
2.0018
13.89
14.47
14.18
0.5057
0.5039
0.5048
Titrating Vitamin C (used 100mL KIO3)
Sample 1
Sample 2
Average
Weight percent
of tablet
35.36
34.92
35.14
Calculations:
Finding molarity of KIO3:
g KIO3 ∗
mol KIO3 mol KIO3
=
= Molarity KIO3
g KIO3
L
Example:
1.0029g ∗
1mol KIO3
0.004686mol KIO3
=
= 0.0094M KIO3
214.00277g KIO3
0.50L
Finding molarity of Thiosulfate:
mol KIO3
mol I3−
mol S2 O2−
mol S2 O2−
3
3
g KIO3 ∗
∗
∗
=
= Molarity S2 O2−
3
−
g KIO3
mol KIO3
mol I3
L
Example:
1.0029g KIO3 ∗
1mol KIO3
3mol I3−
2mol S2 O2−
0.0281mol S2 O2−
3
3
∗
∗
=
−
214.00277g KIO3 1mol KIO3
1mol I3
0.50L
= 0.0562M S2 O2−
3
Finding grams of Ascorbic Acid:
mol I3−
mol AA
g AA
M Na2 S2 O3 ∗ L Na2 S2 O3 ∗
∗
∗
= g Ascorbic Acid
mol I3− mol AA
mol S2 O2−
3
Example:
3mol I3−
1mol AA 176.12g AA
0.0562M Na2 S2 O3 ∗ 0.03504L Na2 S2 O3 ∗
2− ∗ 1mol I − ∗ 1mol AA
2mol S2 O3
3
= 0.5204g Ascorbic Acid
Finding weight percent of ascorbic Acid
Excess I3-:
mol S2 O2−
mol I3−
3
M Na2 S2 O3 ∗ L Na2 S2 O3 = mol Na2 S2 O3 ∗
∗
= I3− excuess moles
2−
mol Na2 S2 O3 mol S2 O3
Example:
0.0562M Na2 S2 O3 ∗ 0.03061L Na2 S2 O3 ∗
1mol S2 O2−
1mol I3−
3
∗
1mol Na2 S2 O3 2mol S2 O2−
3
= 0.000860I3− excess moles
Moles of I3-:
mol I3−
M KIO3 ∗ LKIO3 = mol KIO3 ∗
= mol I3−
mol KIO3
Example:
3mol I3−
0.0094M KIO3 ∗ 0.050LKIO3 = 0.00047mol KIO3 ∗
= 0.00141mol I3−
1mol KIO3
Moles of I3-:
mol I3− − excess mol I3− = mol I3−
Example:
0.00141mol I3− − 0.000860mol I3− = 0.00055mol I3−
Finding Mass of Ascorbic Acid:
mol I3− ∗
mol AA g AA
= g AA
mol I3− mol AA
Example:
0.00055mol I3− ∗
1mol AA 176.12g AA
∗
= 0.0969g AA
1mol I3−
1mol AA
Finding weight percent of Ascorbic Acid:
g AA in tablet
∗ 100 = wt%
g tablet
Example:
0.0969g
∗ 100 = 18.08%
0.5223g
Finding weight percent from tablet:
g Ascorbic Acid
∗ 100 = wt% Ascorbic Acid
g tablet
Example:
0.500g AA
∗ 100 = 80.33%
0.6224g
Finding standard deviation:
2
√∑(xi − xavg ) = Standard Deviation
n−1
Conclusion:
The overall objective of this lab was to determine the amount of ascorbic acid in a tablet and
compare it to the manufacturers claim. We found the amount in the vitamin C tablet to be on
average 35.14% (±0.3137) compared to the claim of the pills containing 80.33%. This is a
significant difference. However, there were many errors in this lab. First, the amount of H2SO4
was not consistent with the molarity. Also, the formula used to determine the molarity of
thiosulfate was later determined to be incorrect, but the incorrect number was used to carry out
the remainder of the lab. This error accounted for a larger amount of ascorbic acid to be used in
the second titration. The undissolved solids from the tables could have been any kind of
impurity from the manufacture such as dirt or some kind of additive to help with the taste or the
digestion of the pill.