of Vitamin C (2020/05/07 revised) Collect: Magnetic stirring bar - - PowerPoint PPT Presentation

1

Quantitative Analysis

• f 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
• r fruit
• Weigh vitamin C tablet

➔ crush the tablet ➔ dissolve in 50 mL of distilled water

(2020/05/07 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 point

3

Introduction-Redox Titration

O HO OH O HO H H HO H H

+ I2

O O O O HO H H HO H 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 H H HO H H

+ I2

O O O O HO H H HO H H

+ 2 I- + 2 H+

抗壞血酸 (2)

Ascorbic acid

3 (mol) IO (mol) I (mol) O H C

3 2 6 8 6

 = =

−

3 (mol) O H C 3 (mol) I 1 (mol) IO

6 8 6 2 3

= =

−

5

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

Weigh one of

• 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 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.

• f 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

• f 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 3 176 25 mL

(mg/ tablet) High-dose:

• 1. Vitamin C Tablet: mg/ tablet
• 2. Fruit Juice: mg/100 mL

    100 mL ΔV mL 0.025 M 3 176 50 mL

= ? (mg)

3 (mol) IO (mol) I (mol) O H C

3 2 6 8 6

 = =

−

Condensed 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.

12

▪ 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)

10 ml Ex

安全吸球 排氣閥 吸液閥 排液閥 移液吸管

25

ml

45o

25

ml

移液吸管

25

ml

A S E

Transfer pipet Pipet

Aspirate valve Bulb Suction valve Empty valve Pipette filler

T12- Transfer Pipet

13

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

14

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)

15

Dispenser

• 1. Check if the pre-set volume is

correct, do not change the volume setting afterwards.

• 2. Position the flask under the tip
• f 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）