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CHM 337: Quantitative Analytical Chemistry
Lab #5: Iodimetric Titration of Vitamin C
Introduction:
This lab used a redox titration to identify whether a supplemental source of ascorbic acid
(Vitamin C) met the manufacturer’s specifications. Redox titrations involve oxidation/reduction
reactions between the analyte and the titrant. This experiment involved two parts, a
standardization and the reaction of a mild reducing agent (ascorbic acid) with triiodide via an
indirect titration. The reaction proceeded via the following reactions:
−
−
+
IO−
3 + 8I + 6H ↔ 3I3 + 3H2 O
2−
−
I3− + 2S2 O2−
3 ↔ 3I + S4 O6
I3− + H2 O + C6 H8 O6 ↔ C6 H8 O7 + 3I − + 2H +
Methods:
A starch indicator was made by another group for use by the entire lab section. A 0.7M
Na2S2O3 solution was made by adding 8.8389g Na2S2O3 to 500mL freshly boiled distilled water
which contained 0.0500g of Na2CO3 in a 500mL volumetric flask. This solution was transferred
to an amber bottle. A 0.01M KIO3 solution was made by dissolving 1.2644g KIO3 in 500mL of
distilled water in a 500mL volumetric flask. A 1.0M H2SO4 solution was made by adding 17mL
of 6M H2SO4 to 100mL of distilled water in a 100mL volumetric flask.
The thiosulfate (Na2S2O3) solutions was standardized by pipetting 50mL of KIO3 solution
into three Erlenmeyer flasks and adding roughly 2g of solid KI and 10mL of 1.0M H2SO4 to each
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flask. The samples were titrated using thiosulfate solution until dark yellow in color. At this
point, 2mL of the starch indicator was added and the titration completed. This procedure was
repeated for three trials.
Three samples, each of roughly 0.062g of ascorbic acid, were each dissolved in 50mL of
1.0M H2SO4. Roughly 2g solid KI and 50mL KIO3 were added to each sample. Each sample was
titrated using the procedure outlined above. This was repeated with all three samples. This
procedure was repeated using three samples of roughly 0.081g of a crushed, commercial Vitamin
C tablet instead of pure ascorbic acid.
Results and Calculations:
Standardization of
thiosulfate
Solid KI (g)
Thiosulfate (mL)
Trial #1
2.040
24.4
Ascorbic acid trials
Trial #1
Solid KI (g)
2.0550
Ascorbic acid (g)
0.0696
Thiosulfate (mL)
25.9
Mean Thiosulfate (mL)
Standard Deviation Thiosulfate (mL)
Vitamin C trials
Trial #1
Solid KI (g)
1.9905
Vitamin C tablet (g)
0.0969
Thiosulfate (mL)
25.8
Wt% ascorbic acid
87.7%
Mean Thiosulfate (mL)
Standard Deviation Thiosulfate (mL)
Mean Wt% ascorbic acid
Standard Deviation Wt% ascorbic acid
Trial #2
2.001
24.3
Trial #3
1.993
24.1
Trial #2
2.0373
0.0663
25.0
25.6
±0.493
Trial #3
1.9883
0.0791
25.8
Trial #2
2.0071
0.0960
24.8
88.5%
25.4
±0.513
89.9%
±3.09%
Trial #3
2.0108
0.0910
25.5
93.4%
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Molarity of thiosulfate (Na2S2O3):
8.8389g Na2 S2 O3 1000mL 1mol Na2 S2 O3
∗
∗
= 0.10037M Na2 S2 O3
500mL H2 O
1L
176.12g Na2 S2 O3
Molarity of KIO3:
1.2644g KIO3 1000mL
1mol KIO3
∗
∗
= 0.011817M KIO3
500mL H2 O
1L
214.001g KIO3
Theoretical Mass Needed of Ascorbic Acid for 30mL titration:
30mL ∗
1L
0.07mol Na2 S2 O3
1mol S2 O2−
1mol I3−
1mol KIO−
3
3
∗
∗
∗
∗
−
1000mL
1L
1mol Na2 S2 O3 2mol S2 O2−
3mol
I
3
3
∗
1mol ascorbic acid 176.12g ascorbic acid
∗
= 0.062g ascorbic acid
1mol KIO−
1mol ascrobic acid
3
Actual Mass Needed of Ascorbic Acid for 30mL titration:
1L
0.10037mol Na2 S2 O3
1mol S2 O2−
1mol I3−
1mol KIO−
3
3
30mL ∗
∗
∗
∗
∗
−
2−
1000mL
1L
1mol Na2 S2 O3 2mol S2 O3
3mol I3
∗
1mol ascorbic acid 176.12g ascorbic acid
∗
= 0.088g ascorbic acid
1mol KIO3−
1mol ascrobic acid
Theoretical Mass of Vitamin C tablet needed for 30mL titration:
0.5g ascorbic acid 0.062g ascorbic acid
=
→ x = 0.081g tablet
0.6531g tablet
x
Actual Mass of Vitamin C tablet needed for 30mL titration:
0.5g ascorbic acid 0.088g ascorbic acid
=
→ x = 0.11g tablet
0.6531g tablet
x
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Excess moles I3-:
1 mol S2 O3 2−
1 mol I3 −
Molarity Na2 S2 O3 ∗ Volume Na2 S2 O3 ∗
∗
1 mol Na2 S2 O3 2 mol S2 O3 2−
0.10037mol Na2 S2 O3
1 mol S2 O3 2−
1 mol I3 −
=
∗ 0.0258L Na2 S2 O3 ∗
∗
1L
1 mol Na2 S2 O3 2 mol S2 O3 2−
= 0.00129mol I3−
Initial moles I3-:
1mol I3−
Molarity KIO3 ∗ Volume KIO3 ∗
1mol KIO3
0.011817mol KIO3
3mol I3−
=
∗ 0.050L KIO3 ∗
= 0.0017726mol I3−
1L
1mol KIO3
Moles I3-:
initial moles I3− − excess moles I3− = 0.0017726mol − 0.00129mol = 0.00048mol I3−
Mass of Ascorbic Acid:
moles I3− ∗
1mol ascorbic acid 176.12g ascorbic acid
∗
1mol I3−
1mol ascorbic acid
= 0.00048mol I3− ∗
1mol ascorbic acid 176.12g ascorbic acid
∗
1mol I3−
1mol ascorbic acid
= 0.085g ascorbic acid
Weight percent of ascorbic acid in tablet:
wt% =
mass ascorbic acid
0.085g ascorbic acid
× 100% =
∗ 100% = 87.7% ascorbic acid
mass tablet
0.0969g tablet
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Discussion:
The undissolved solid from the tablets was most likely the binding matrix used to bind
the components into a solid tablet. The mean amount of Vitamin C in the tablet (89.9%)
exceeded the manufacturer’s claim.
Conclusion:
In this experiment, the starch indicator could be added to the solution sooner to prevent
over-titration, which occurred in the trials of several groups. This experiment seems like a fairly
accurate method of determining the amount of Vitamin C contained in a commercially available
tablet, as the values of ascorbic acid returned in each sample were comparable. In this reaction,
the ascorbic acid acts as a reducing agent and is oxidized. The iodine acts as an oxidizing agent
and is reduced.