Experiment 5: Iodimetric Titration of Vitamin C11
Purpose: To determine how much, if any ascorbic acid (vitamin C) is actually found in supplemental
sources of vitamin C.
Procedure:
Part 1: Standardization of Thiosulfate Solution
1. Group part: prepare starch indicator solution:
a. 5g soluble starch paste in 50mL water. Pour into 500mL boiling water and boil until
clear.
2. Prepare about 0.07M Na2S2O3 by dissolving 8.70g Na2S2O3*5H2O in 500mL boiling water
containing 0.05g Na2CO3
a. Prepare approximately 0.01M KIO3 by dissolving 1.0g solid reagent in a 500mL flask
3. Standardize: pipet 50mL KIO3 into an Erlenmeyer flask
a. Add 2.0g KI(s) and 10mL of 0.5M H2SO4
b. Titrate with thiolsulfate until color is gone
c. Repeat titration until three good trials
Part 2: Analysis of Vitamin C
1. Weigh amount ascorbic acid for a 30mL titration
a. Dissolve in 50mL 0.3M HCl
2. Add 2g KI (s) and 50mL standard KIO3
a. Titrate with standard thiosulfate and add 2mL starch indicator before endpoint
3. Repeat using two vitamin C tablets
a. Determine the weight percent of vitamin C, standard deviation, and accuracy
b. Three good trials
Reactions:
𝐼𝑂3− + 8𝐼 + 6𝐻 + ↔ 3𝐼 − + 3𝐻2 𝑂
𝐼3− + 2𝑆2 𝑂3− ↔ 3𝐼 − + 𝑆4 𝑂62−
𝐶6 𝐻6 𝑂6 + 𝐼3− + 𝐻2 𝑂 ↔ 𝐶6 𝐻8 𝑂7 + 3𝐼 − + 2𝐻 +
Data:
Part 1
Mass Na2CO3=0.0522g
Mass Na2S2O7*5H2O=8.7278g
Mass KIO3(s) =1.0015g
Trial
Mass KI
Thiosulfate added
Molarity Thiosulfate
1
2
3
Average
2.0088g
2.0041g
2.0014g
2.0048g
38.0mL
37.3mL
37.4mL
37.7mL
0.0472M
0.0756M
0.0754M
0.0751M
Part 2:
Ascorbic acid treated with H2SO4
Trial
1
Mass KI
1.9999g
Mass Ascorbic Acid
2.005g
Thiosulfate added
9.3mL
Wt % A.A.
90.5%
Amount Vitamin C
0.3004g
0.3122g
0.3063g
Thiosulfate added
3.8mL
2.5mL
3.15mL
Wt% vitamin C
74.30%
74.25%
74.28%
KI titrated with Thiosulfate
Trial
2
3
Average
Amount KI
2.0011g
2.0002g
2.00065g
Calculations:
Mass ascorbic acid needed:
0.0759𝑚𝑜𝑙 𝑆2 𝑂3 ∗
0.03𝐿
1𝑚𝑜𝑙 𝐶6 𝐻6 𝑂6 176.1259𝑔
∗
∗
= 0.2005𝑔 𝐶6 𝐻6 𝑂6
2𝑚𝑜𝑙𝑆2 𝑂3
1𝑚𝑜𝑙 𝐼3−
1𝑚𝑜𝑙
Ascorbic Acid
1𝑚𝑜𝑙
1
∗
= 0.0094𝑀 𝐾𝐼𝑂3
214𝑔 𝐾𝐼𝑂3 0.5𝐿
3𝑚𝑜𝑙 𝐼 − 1𝑚𝑜𝑙 𝑆2 𝑂3
0.0094𝑀 𝐾𝐼𝑂3 ∗ 0.005𝐿 ∗
∗
= 0.00282𝑚𝑜𝑙𝑆2 𝑂3
1𝑚𝑜𝑙 𝐼𝑂3 1𝑚𝑜𝑙 𝐼3−
0.00282𝑚𝑜𝑙 𝑆2 𝑂3
= 0.0742𝑀𝑆2 𝑂3
0.038𝐿
1.0015𝑔 𝐾𝐼𝑂3 ∗
Mol 𝐈𝟑−
1𝑚𝑜𝑙 𝑆2 𝑂3
1𝑚𝑜𝑙 𝐼3
∗
= 3.49𝑥10−4 𝑚𝑜𝑙 𝐼3− 𝑒𝑥𝑐𝑒𝑠𝑠
1𝑚𝑜𝑙 𝑁𝑎2 𝑆2 𝑂3 2𝑚𝑜𝑙 𝑆2 𝑂3
3𝑚𝑜𝑙 𝐼3−
(0.0094𝑀)(0.05𝐿) = 4.7𝑥10−4 𝑚𝑜𝑙 𝐾𝐼𝑂3 ∗
= 0.00141𝑚𝑜𝑙 𝐼3−
1𝑚𝑜𝑙 𝐾𝐼𝑂3
(0.0093𝐿 𝑆2 𝑂3 ) ∗ (0.0751𝑀 𝑆2 𝑂3 ) ∗
0.00141𝑚𝑜𝑙 − 3.49𝑥10−4 𝑚𝑜𝑙 𝑒𝑥𝑐𝑒𝑠𝑠 = 0.001031 𝑚𝑜𝑙 𝐼3−
Grams Ascorbic Acid:
0.001031 𝑚𝑜𝑙 𝐼3− ∗
1 𝑚𝑜𝑙 𝐴. 𝐴. 176.1256𝑔
∗
= 0.1816𝑔 𝐴. 𝐴.
1 𝑚𝑜𝑙 𝐼3−
1 𝑚𝑜𝑙 𝐴. 𝐴.
Weight percent of Ascorbic Acid:
𝑔 𝑐𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑
0.1816𝑔
=
𝑥100 = 90.5%
𝑔 𝑠𝑡𝑎𝑟𝑡𝑒𝑑 𝑤𝑖𝑡ℎ 0.2005𝑔
Weight percent of Vitamin C:
0.0038𝐿𝑆2 𝑂3 ∗ 0.0751𝑀𝑆2 𝑂3 ∗
1𝑚𝑜𝑙 𝑆2 𝑂3
1𝑚𝑜𝑙 𝐼3
∗
= 0.000143𝑚𝑜𝑙 𝑒𝑥𝑐𝑒𝑠𝑠 𝐼3−
1𝑚𝑜𝑙 𝑁𝑎2 𝑆2 𝑂3 2𝑚𝑜𝑙 𝑆2 𝑂3
(0.0094𝑀)(0.05𝐿) = 4.7𝑥10−4 𝑚𝑜𝑙 𝐾𝐼𝑂3 ∗
3𝑚𝑜𝑙 𝐼3−
= 0.00141𝑚𝑜𝑙 𝐼3−
1𝑚𝑜𝑙 𝐾𝐼𝑂3
0.00141𝑚𝑜𝑙 𝐼3− − 0.000143𝑚𝑜𝑙 𝑒𝑥𝑐𝑒𝑠𝑠 𝐼3− = 0.001267𝑚𝑜𝑙 𝐼3−
Grams Vitamin C calculated:
0.001267 𝑚𝑜𝑙 𝐼3− ∗
1 𝑚𝑜𝑙 𝐴. 𝐴. 176.1256𝑔
∗
= 0.2232𝑔 𝑉𝑖𝑡𝑎𝑚𝑖𝑛 𝐶
1 𝑚𝑜𝑙 𝐼3−
1 𝑚𝑜𝑙 𝐴. 𝐴.
Percent yield Vitamin C
0.2232𝑔
∗ 100% = 74.30%
0.3004𝑔
Weight percent claimed by manufacturer:
0.500𝑔 𝑉𝑖𝑡𝑎𝑚𝑖𝑛 𝐶
∗ 100% = 81.62%
0.6126𝑔 𝑡𝑎𝑏𝑙𝑒𝑡
Standard Deviation:
2
2
2
√∑(𝑥𝑖 − 𝑥𝑎𝑣𝑔 ) = √(74.30 − 74.28) + (74.25 − 74.28) = 0.036
𝑛−1
1
Questions:
1. Equations:
𝐼𝑂3− + 8𝐼 + 6𝐻 + ↔ 3𝐼 − + 3𝐻2 𝑂
𝐼3− + 2𝑆2 𝑂3− ↔ 3𝐼 − + 𝑆4 𝑂62−
𝐶6 𝐻6 𝑂6 + 𝐼3− + 𝐻2 𝑂 ↔ 𝐶6 𝐻8 𝑂7 + 3𝐼 − + 2𝐻 +
2. The undissolved solid from the tablets were fillers and additives which may include calcium
carbonate
3. The amount of vitamin C in the tablet was close to the manufacturers claim. The average
percent Vitamin C was 74.28% and the claimed percent was 81.62%.
Conclusion:
The goal of this lab was to determine how much Vitamin C was in a Vitamin C tablet and compare it to
the manufactures claim. This was done by titrating different samples with Thiosulfate. Overall, the
percent Vitamin C recovered was close to the claimed amount. The average percent Vitamin C was
74.28% and the claimed percent was 81.62%. The standard deviation was relatively low at only 0.036.
Error from this lab may have come from not dissolving the tablet all the way before titrating it with the
Thiosulfate.