quantitative analysis of vitamin c

1

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

2

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

4

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 ==−

5

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

6

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

7

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

8

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

9

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

10

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

11

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)

0 1

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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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