Quantitative Analysis of Vitamin C (2020/05/07 revised) 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 KIO 3 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 1
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 2
Introduction-Redox Titration ▪ Vitamin C (L-ascorbic acid, C 6 H 8 O 6 ) is a reducing agent ▪ Use KIO 3 as titrant to titrate a sample solution containing NaI in acidic medium, to produce I 2 : - + 5I - + 6H + → 3I 2 + 3H 2 O IO 3 (1) ▪ I 2 can undergo a rapid redox reaction with ascorbic acid: HO H HO H H H O O O O HO HO + 2 I - + 2 H + + I 2 (2) H H H H O O HO OH 抗壞血酸 L-Ascorbic acid ▪ At the equivalence point, any I 2 produced thereafter will react with starch indicator to produce a blue-black complex to indicate the end point 3
Introduction-Stoichiometry ▪ From the stoichiometric calculation: - + 5I - + 6H + → 3I 2 + 3H 2 O IO 3 (1) HO H HO H H H O O O O HO HO + 2 I - + 2 H + (2) + I 2 H H H H O O HO OH 抗壞血酸 Ascorbic acid − IO (mol) I (mol) C H O (mol) = = 3 2 6 8 6 1 3 3 − = = C H O (mol) I (mol) IO (mol) 3 4 6 8 6 2 3
Procedure I: Vit. C Tablet with Low Dose (below 250 mg) 1. Dissolve 1 tablet with 50 mL H 2 O - Vit. C < 250 mg/tablet 2. Prepare 2 sample 50 mL D.I. water Stir and dissolve Weigh one of Crush with tools 125 mL Vit. C tablet Erlenmeyer flask 5
Procedure I: Vit. C Tablet with Low Dose 3. Titrate with KIO 3 Titrate with 0.025 M KIO 3 Read V i , V f to 0.01 mL Add 2 mL of 1 M NaI(aq) Blue-black complex 2 mL of 1 M HCl(aq) (end point) 1 mL 2% Starch 4. Take another Vit. C sample ➔ Repeat the procedure and perform a duplicate titration ➔ Calculate the ascorbic acid content (mg/tablet) and the 6 average value
Procedure II: Vit. C Tablet with High Dose (above 250 mg) 1. Dissolve and dilute sample to 100 mL 50 mL H 2 O Stir and dissolve • Stir with stirrer Take and record wt. Crush with tools • DO NOT TURN of one tablet the heat on (1) Transfer to volumetric flask Mix soln thoroughly (2) Rinse beaker several times Transfer to a beaker (3) Add water to the mark 100 mL Volumetric flask 7
Procedure II: Vit. C Tablet with High Dose (above 250 mg) 2. Measure 25 mL sample soln and titrate 2 mL of 1 M NaI(aq) Use 25 mL transfer pipet 2 mL of 1 M HCl(aq) ＋ 1 mL 2% Starch Take 25.0 mL Vit. C solution Read V i Titrate with Take another Calculate the ascorbic 0.025 M KIO 3 25 mL portion acid content of the tablet (mg/tablet) Carry out Read and record V i , V f duplicate titration 8
Procedure III: Vit. C Content of Fruit Juice 1. Take 50 mL sample solution and titrate 2 mL of 1 M NaI(aq) 2 mL of 1 M HCl(aq) ＋ Use 25 mL transfer pipet 1 mL 2% Starch Take 50.0 mL fruit juice Titrate with Blue-black Carry out a Calculate the ascorbic 0.025 M KIO 3 complex duplicate titration acid content (mg/100 mL) (end point) Read and record V i , V f 9
Calculation Condensed Report − = = Stoichiometry: C H O (mol) I (mol) IO (mol) 3 6 8 6 2 3 1. Vitamin C Tablet: mg/ tablet Δ V mL 低劑量 Low-dose: 0.025 M 3 176 (mg/ tablet) - ] [IO 3 100 mL Δ V mL 高劑量 0.025 M 3 176 High-dose: (mg/ tablet) 25 mL 2. Fruit Juice: mg/100 mL 100 mL Δ V mL 0.025 M 3 176 = ? (mg) 50 mL 10
Notice ▪ Use a clean and oven dried 100 mL beaker to take KIO 3 (aq) to avoid diluting the concentration. ▪ Take 30 mL of KIO 3 (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. ▪ 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. 11
T12- Transfer Pipet ▪ Wash a 25 mL-transfer pipet thoroughly. T12 Video on Youtube (Click) ▪ Rinse twice with small portion of sample solution. Pipette ▪ Press valve A of pipette filler and squeeze bulb to filler Aspirate expel the air inside and create a vacuum. A 排氣閥 valve ▪ Insert the top of pipette into pipette filler, press Bulb 安全吸球 10 ml S valve S to draw liquid to pass the graduation mark. Ex Suction E 吸液閥 valve ▪ Remove pipette filler and quickly close the top of 25 pipette tube with index finger and adjust liquid Empty 排液閥 ml 25 valve level to the mark. ml ▪ Hold pipette vertically and transfer liquid into 25 ml container. (One hand hold pipet and the other hand hold container to operate.) ▪ Remove index finger and free fall the drop on tip 45 o by touching it to the wall of container. ▪ Stay for 15 s to drain the solution. Transfer 移液吸管 pipet ▪ Do not blow out remaining liquid. Pipet is Pipet calibrated for this to remain. 移液吸管 ▪ Wash immediately after use. 12
T13 - Volumetric Flask ▪ Wash a 100 mL volumetric flask thoroughly 標線刻度 Mark 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 In ml 100 ± 0.15 20 o C 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. 13
T14 - Titration T14 Video on ▪ Rinse a 25 mL-buret with D.I. water thoroughly. Youtube (Click) ▪ 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 (V i ) and final volume (V f ) 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. 14
Dispenser 1. Check if the pre-set volume is （ 1 ） correct, do not change the volume setting afterwards. （ 3 ） （ 2 ） 2. Position the flask under the tip of dispenser. （ 4 ） 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. 15
Recommend
More recommend
