Iodometric Titration of Vitamin C11
Purpose:
The purpose of this lab is to determine the amount of ascorbic acid in a sample and compare it to
the reported value.
Procedure:
Standardisation of Thiosulfate
Prepare a starch indicator through making a paste of 5 g soluble starch in 50 mL water. Pour paste
into 500 mL of boiling water and boil until clear. Prepare approximately 0.07 M Na2S2O3 (8.70 g
Na2S2O3 • 5H2O in 500 mL freshly boiled water containing 0.05 g Na2CO3); store solution in an amber
bottle. Prepare 0.01 M KIO3 (1 g solid dissolved in 500 mL flask). For standardisation, pipet 50 mL of
KIO3 solution into an Erlenmeyer flask and add 2 g solid KI and 10 mL 0.5 M H2SO4; immediately
titrate with Thiosulfate until solution loses almost all colour, then add 2 mL of starch indicator and
complete titration. Obtain three good trials.
Analysis of Vitamin C
Weigh out appropriate amount of C11 for approximately 30 mL titration and dissolve in 50 mL 0.3 M
H2SO4. Add 2 g solid KI and 50 mL of standard KIO3. Titrate with standard Thiosulfate as above,
adding 2 mL starch indicator just before the end point. Repeat this procedure using two weighed
vitamin C tablets.
Reactions:
IO3- + 8I + 6H+ ----> 3I3- + 3H2O
I3- + 2S2O32- ----> 3I- + S4O62C6H8O6 + I3- + H2O ----> C6H8O7 + 3I- + 2H+
Data:
Mass Na2CO3 = 0.0507 g
Mass Na2S2O3 • 5H2O = 8.7021 g
Mass KIO3 = 0.9992 g
Standardisation of Thiosulfate
Trial
KI (g)
Thiosulfate added (mL)
Molarity of Thiosulfate
Solution
1
2.0013
36.62
0.0765
2
1.9911
37.39
0.0749
3
1.9898
37.70
0.0742
Average
1.9941
37.24
0.0752
Analysis of Vitamin C
Ascorbic Acid titrated with
Trial
Ascorbic Acid (g)
KI (g)
Thiosulfate added
(mL)
1
0.1985
1.9908
8.98
Weight % of
Ascorbic Acid
and KI titrated with Standard Thiosulfate
Trial
Vitamin C (g)
KI (g)
Thiosulfate added
(mL)
1
0.2082
2.0007
11.92
2
0.2088
2.0008
11.52
3
0.2087
2.0003
12.51
Average
0.2086
2.0006
11.65
Weight % of
Vitamin C
Calculations:
Molarity of
= mol
0.9992 g ×
= 0.004669 mol
Molarity of
Amount of Ascorbic Acid (A.A.) Needed
Weight % of A.A (also used for Vitamin C)
L of Thiosulfate added × M of Thiosulfate solution ×
0.00898 L × 0.0765 M ×
mol of
= mol
= 0.000343 mol excess
= excess
0.0765 M×
Weight % Claimed by Manufacturer
Standard Deviation of Vitamin C:
=
Conclusions:
The lab was designed to determine the average weight percent of Vitamin C in a table and compare
that to the reported value given by the manufacturer. Our average weight percent for Vitamin C was
80.93 while the reported value for the weight percent of Vitamin C was 74.96. As one can see, our
calculated value significantly surpassed the reported value from the manufacturer; this is most likely
attributed to the solid in the solution not dissolving completely. Another source of error could be the
amount of Thiosulfate that was added for the Vitamin C trial, it was less than anticipated which
could mean an error on our part somewhere leading up to that titration.