Orlando 1
Quantitative Analytical Chemistry
Joseph Orlando
Lab #5: Iodimetric Titration of Vitamin 𝐂 𝟏𝟏
Purpose:
In this experiment we first standardized thiosulfate to use to determine the amount of ascorbic
acid (vitamin C) present in a supplemental source. We compared the experimental data to the
manufacturer’s specifications to see if they were met.
IO3- + 8 I + 6 H+ -----> 3 I- + 3 H2O
I3- + 2 S2O32- -----> 3 I- + S4O62C6H8O6 + I3- -----> C6H8O7 + 3 I- + 2 H+
Procedure:
Standardization of Thiosulfate Solution
1. Prepare starch indicator; make a paste of 5g soluble starch in 50mL H2O then pour the paste
into 500mL boiling H2O, boil until clear.
2. Prepare 0.07M Na2S2O3 by dissolving ~8.7g Na2S2O3 in 500mL freshly boiled H2O containing
0.05g of Na2CO3. Store in tightly capped amber bottle. Prepare 0.01M KIO3 by weighing ~1g
solid reagent and dissolving in 500mL flask.
3. Standardize thiosulfate solution. Pipet 50mL KIO3 solution into an Erlenmeyer flask. Add 2g
solid KI + 10mL 0.5M H2SO4. Titrate with thiosulfate until solution becomes pale yellow. Add
2mL starch solution and complete titration. Repeat this process three times.
Analysis of Vitamin C
1. Weigh vitamin C for a ~30mL titration and dissolve in 50mL 0.3M H2SO4.
2. Add 2g solid KI and 50mL standard KIO3. Titrate with standard thiosulfate as above, add 2mL
starch before endpoint. Repeat using two weighed vitamin C tablets.
3. Determine wt% of vitamin C in tablet and report standard deviation and accuracy of
titrations.
Orlando 2
Data:
Standardization of Thiosulfate Solution
KI(g)
Thiosulfate added(mL)
Trial 1
2.0581
0
Molarity of Thiosulfate
Solution
0
Trial 2
2.0405
39.52
0.072M
Trial 3
2.0020
38.45
0.074M
Average
2.0335
38.98
0.073M
Standard Deviation
0.0014
Ascorbic Acid Titration
Trial 1
KI(g)
Ascorbic Acid(g)
2.0003
0.1998
Thiosulfate
added(mL)
9.29
Weight % of
Ascorbic Acid
27.03%
Weight % of
Vitamin C
72.53%
Vitamin C Titration
Vitamin C(g)
KI(g)
Trial 1
0.2645
1.9998
Thiosulfate
added(mL)
8.79
Trial 2
0.3142
2.0038
15.62
75.89%
Average
0.2894
2.0018
12.21
74.21%
Standard Deviation
2.376
Equations:
𝑴𝒐𝒍𝒂𝒓𝒊𝒕𝒚 𝒐𝒇 𝑲𝑰𝑶𝟑 = 1.0183𝑔 𝐾𝐼𝑂3 𝑥
1 𝑚𝑜𝑙
1
𝑥
= 0.0095𝑀
214 𝑔𝑟𝑎𝑚𝑠 0.5 𝐿
Orlando 3
𝑴𝒐𝒍𝒂𝒓𝒊𝒕𝒚 𝒐𝒇 𝑵𝒂𝟐 𝑺𝟐 𝑶𝟑 = 0.0095𝑀 𝐼𝑂3− 𝑥 0.05 𝐿 𝑥
3 𝑚𝑜𝑙 𝐼 − 2 𝑚𝑜𝑙 𝑆2 𝑂3
1
−𝑥
− 𝑥
1 𝑚𝑜𝑙 𝐼𝑂3
1 𝑚𝑜𝑙 𝐼3
0.03952𝐿
= 0.072𝑀
𝑨𝒎𝒐𝒖𝒏𝒕 𝒐𝒇 𝑨𝒔𝒄𝒐𝒓𝒃𝒊𝒄 𝑨𝒄𝒊𝒅 𝑵𝒆𝒆𝒅𝒆𝒅 = 0.0095𝑀 𝐼𝑂3− 𝑥 0.05𝐿 𝑥
0.073𝑀 𝑁𝑎2 𝑆2 𝑂3 𝑥
3 𝑚𝑜𝑙 𝐼3−
= 0.001425 𝑚𝑜𝑙 𝐼3−
1 𝑚𝑜𝑙 𝐼3−
0.03𝐿
= 0.001095 mol 𝐼3− 𝑒𝑥𝑐𝑒𝑠𝑠
2
0.00141 − 0.001095 = 0.000306 𝑚𝑜𝑙 𝐼3−
𝑾𝒆𝒊𝒈𝒉𝒕 𝑷𝒆𝒓𝒄𝒆𝒏𝒕 𝒐𝒇 𝑨𝒔𝒄𝒐𝒓𝒃𝒊𝒄 𝑨𝒄𝒊𝒅
= 0.00929𝐿 𝑥 0.073𝑀 
1 mol S2 O3
1 mol I3

 3.39  10  4 excess I3
1 mol Na 2 S 2 O3 2 mol S2 O 2
Grams of Ascorbic Acid (Calculated)
mol of I3− ×
1 mol A. A. molar mass A. A
×
= grams of A. A
1 mol I3−
1 mol
Percent Yield of Ascorbic Acid
Grams calculated
× 100 = % A. A.
Grams started with
Conclusion:
The purpose of this experiment was to determine whether the amount of Vitamin C in a store
bought tablet matched or exceeded the amount of Vitamin C that was claimed to be in the
tablet by the manufacturer. The calculated weight percent of vitamin C in the tablets was
74.21%±2.376. Some product was most likely lost during transfer which would lower the weight
percent. The calculated weight percent of vitamin C seems to be very close to the
manufacturers claim.
Post Lab Questions:
1. What was the undissolved solid from the tablets?
The undissolved solid from the tablets was calcium carbonate.
2. Did the amount of vitamin C in the tablet meet or exceed the manufacturer’s claim?
The amount of vitamin C experimentally calculated was an average of 74.21% and
somewhat met the manufacturers claims although it did not exceed their claims. Human
error caused by transferring of the product may have caused this discrepancy in data.