quantitative analysis of vitamin c
TRANSCRIPT
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Quantitative Analysis of Vitamin C
Collect:• Magnetic stirring bar (given out and collected by assistant TA)
• One pipet filler and a 25 mL transfer pipet
• Two 125 mL Erlenmeyer flasks
Prepare:• One clean and oven dried 100 mL beaker (for taking 30 mL
KIO3 solution)
Prepare before experiment:• Each group prepare your own vitamin C tablets and fruit juice
or fruit
• Weigh vitamin C tablet
➔ crush the tablet
➔ dissolve in 50 mL of distilled water
2021/03/02 revised
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Objective & Techniques
Objective
▪ Analyze the vitamin C content in
– Vitamin C tablets
– Fruit juice or fruit
Techniques
▪ Hot plate/magnetic stirrer
▪ Dispenser
▪ Transfer pipet
▪ Buret and titration
▪ Volumetric flask
▪ Vitamin C (L-ascorbic acid, C6H8O6) is a reducing agent
▪ Use KIO3 as titrant to titrate a sample solution containing
NaI in acidic medium, to produce I2:
IO3- + 5I- + 6H+ → 3I2 + 3H2O (1)
▪ I2 can undergo a rapid redox reaction with ascorbic acid:
▪ At the equivalence point, any I2 produced thereafter will
react with starch indicator to produce a blue-black
complex to indicate the end point3
Introduction-Redox Titration
O
HO OH
O
HO HH
HOH H + I2
O
O O
O
HO HH
HOH H + 2 I- + 2 H+
抗壞血酸
(2)
L-Ascorbic acid
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Introduction-Stoichiometry
▪ From the stoichiometric calculation:
IO3- + 5I- + 6H+ → 3I2 + 3H2O (1)
O
HO OH
O
HO HH
HOH H + I2
O
O O
O
HO HH
HOH H + 2 I- + 2 H+
抗壞血酸
(2)
Ascorbic acid
3(mol)IO(mol)I(mol)OHC 32686 == −
3
(mol)OHC
3
(mol)I
1
(mol)IO 68623 ==−
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Procedure I: Vit. C Tablet with Low Dose (below 250 mg)
Weigh one of
Vit. C tabletCrush with tools 125 mL
Erlenmeyer
flask
1. Dissolve 1 tablet with 50 mL H2O
- Vit. C < 250 mg/tablet
2. Prepare 2 sample
50 mL D.I. water
Stir and dissolve
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Add 2 mL of 1 M NaI(aq)
2 mL of 1 M HCl(aq)
1 mL 2% Starch
Blue-black complex
(end point)
4. Take another Vit. C sample
➔Repeat the procedure and perform a duplicate titration
➔Calculate the ascorbic acid content (mg/tablet) and the
average value
3. Titrate with KIO3
Titrate with
0.025 M KIO3
Read Vi, Vf
to 0.01 mL
Procedure I: Vit. C Tablet with Low Dose
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Crush with tools
50 mL H2O
Procedure II: Vit. C Tablet with High Dose (above 250 mg)
Stir and
dissolve
• Stir with stirrer
• DO NOT TURN
the heat on
100 mL Volumetric flask
(2) Rinse beaker several times
(3) Add water to the mark
(1) Transfer to volumetric flask Mix soln thoroughly
Transfer to a beaker
1. Dissolve and dilute sample to 100 mL
Take and record wt.
of one tablet
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Use 25 mL transfer pipet
Take 25.0 mL
Vit. C solution
Titrate with
0.025 M KIO3
Calculate the ascorbic
acid content of the
tablet (mg/tablet)
2. Measure 25 mL sample soln and titrate
Read and
record Vi, Vf
Carry out
duplicate titration
Procedure II: Vit. C Tablet with High Dose (above 250 mg)
Take another
25 mL portion
+
2 mL of 1 M NaI(aq)
2 mL of 1 M HCl(aq)
1 mL 2% Starch
Read Vi
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Use 25 mL transfer pipet
Take 50.0 mL fruit juice
Titrate with
0.025 M KIO3
Blue-black complex
(end point)
Calculate the ascorbic
acid content (mg/100 mL)
1. Take 50 mL sample solution and titrate
Read and
record Vi, Vf
Carry out a
duplicate titration
Procedure III: Vit. C Content of Fruit Juice
+
2 mL of 1 M NaI(aq)
2 mL of 1 M HCl(aq)
1 mL 2% Starch
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Calculation
低劑量 ΔV mL 0.025 M 3 176
Stoichiometry:
(mg/ tablet)
[IO3-]
Low-dose:
高劑量100 mL
ΔV mL 0.025 M 3 17625 mL
(mg/ tablet)High-dose:
1. Vitamin C Tablet: mg/ tablet
2. Fruit Juice: mg/100 mL
100 mL
ΔV mL 0.025 M 3 17650 mL
= ? (mg)
3(mol)IO(mol)I(mol)OHC 32686 == −
Brief Report
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Notice
▪ The buret and transfer pipet must be cleaned,
inverted clamped, and left to dry after the
experiment.
▪ Magnetic stirring bars should be returned to
the assistant TA
▪ Recycle the juice bottle and do not throw it in the bathroom’s garbage bin.
▪ Use a clean and oven dried 100 mL beaker to take KIO3(aq) to
avoid diluting the concentration.
▪ Take 30 mL of KIO3(aq) for titration, do not take too much to reduce
the waste.
▪ Add 1 mL of starch indicator to signal the end point for each titration.
▪ Wash the volumetric flask thoroughly after use.
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▪ Wash a 25 mL-transfer pipet thoroughly.
▪ Rinse twice with small portion of sample solution.
▪ Press valve A of pipette filler and squeeze bulb to
expel the air inside and create a vacuum.
▪ Insert the top of pipette into pipette filler, press
valve S to draw liquid to pass the graduation mark.
▪ Remove pipette filler and quickly close the top of
pipette tube with index finger and adjust liquid
level to the mark.
▪ Hold pipette vertically and transfer liquid into
container. (One hand hold pipet and the other
hand hold container to operate.)
▪ Remove index finger and free fall the drop on tip
by touching it to the wall of container.
▪ Stay for 15 s to drain the solution.
▪ Do not blow out remaining liquid. Pipet is
calibrated for this to remain.
▪ Wash immediately after use.
T12 Video on Youtube (Click)
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10 ml Ex
20
C
o
安全吸球
排氣閥
吸液閥
排液閥
移液吸管
25 ml
EX 20
C
o
45o
25 ml
EX 20
C
o
移液吸管
25 ml
EX 20
C
o
A
SE
Transfer
pipet Pipet
Aspirate
valve
Bulb
Suction
valve
Empty
valve
Pipette
filler
T12- Transfer Pipet
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T13 - Volumetric Flask
▪ Wash a 100 mL volumetric flask thoroughly
and rinse with D.I. water.
▪ Transfer the solution to flask with the help of
a funnel.
▪ Fill the flask about half full and swirl the
contents to achieve solution.
▪ Bring the liquid level almost to the mark.
▪ Use a dropper to add solvent to the mark.
▪ Stopper the flask and invert repeatedly to
assure uniform mixing.
▪ Pour the solution in beaker for later use.
▪ Wash immediately after use.
標線刻度
100 ml
±0.15 In
20oC
Mark
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T14 - Titration
▪ Rinse a 25 mL-buret with D.I. water thoroughly.
▪ Rinse twice with ca. 5 mL titrant.
▪ Transfer the titrant to buret via funnel.
▪ Drain slowly until the tip is free of air bubbles and
completely filled with liquid.
▪ Read and record initial (Vi) and final volume (Vf) of
buret to 0.01 mL.
▪ Place buret tip well inside the receiving flask
during titration.
▪ Swirl the flask with right hand and control the
stopcock with left hand (process done by one
person).
▪ Rinse the buret with tap water, inverted clamped,
and left to dry after the experiment.
T14 Video on
Youtube (Click)
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Dispenser
1. Check if the pre-set volume is
correct, do not change the
volume setting afterwards.
2. Position the flask under the tip
of dispenser.
3. Lightly pull the piston pump up
and down several times to get
rid of the bubbles (done by TA).
4. Lightly pull piston pump up to
the top, then slowly push down
to obtain the measured solution.
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