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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 KIO3

solution)

Prepare before experiment：• Weigh 1 vitamin C tablet crush the tablet dissolve in 50 mL of distilled water (2015/09/26 revised)

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Objective & TechniquesObjective 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

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Introduction 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 point

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 From the stoichiometric calculation:

3(mol)OHC

3(mol)I

1(mol)IO 68623

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

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Procedure I: Vit. C Tablet with Low Dose (below 250 mg)

Weigh one Vit. C tablet

Crush 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 distilled 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 sampleRepeat the procedure and perform a duplicate titrationCalculate the ascorbic acid content (mg/tablet) and the

average value

3. Titrate with KIO3

Titrate with 0.025 M KIO3

Record Vi, Vfto 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 into 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 mLVit. C solution

Titrate with 0.025 M KIO3

Calculate the ascorbic acid content of the tablet (mg/tablet)

2. Obtain 25 mL sample soln and titrate

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 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

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:3

(mol)OHC3

(mol)I1(mol)IO 68623

(mg/ tablet)[IO3-]

Low-dose:

高劑量100 mL

∆V 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

∆V mL 0.025 M 3 17650 mL

= ? (mg)

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Notice

Use a clean and oven dried 100 mL beaker to take KIO3(aq) to avoid changing the concentration.

Take 50 mL of KIO3(aq) for the titration, do not take too much to reduce the waste.

Wash the volumetric flask thoroughly after use. 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.

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Manipulation of Titration

Wash and Clean a 25 mL-buret thoroughly. Rinse twice with 5 mL 0.025 M KIO3(aq). Transfer the solution to buret with funnel. Drain slowly until the tip is free of air bubbles

and completely filled with liquid. Take 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).

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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.

（2）

（1）

（3）

（4）

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Transfer Pipet

Wash a 25 mL-transfer pipet thoroughly. Rinse twice with small portion of sample

solution. Draw liquid past the graduation mark

with pipet filler. Adjust liquid level to the mark. Transfer liquid into Erlenmeyer flask. Drain the drop on tip by touching it to the

wall of the 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.

量吸管

10mlEx

移液吸管

25ml

安全吸球排氣閥

吸液閥

A

SE

排液閥

45o

Transfer pipet

Pipet

Pipet filler

Exhaust valve

Suction valve

Ejection valve

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Volumetric Flask Wash a 100 mL volumetric flask thoroughly

and rinse with distilled 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

In20oC

Mark