Quantitative Analysis of Cobalt (II) Ions (2020/05/07 revised) Collect ◼ 50 mL Erlenmeyer flask (6) (wash clean ➔ dry ➔ cool) ◼ Test tube (1) (wash clean and dry to take 8 mL Co 2+ standard soln) ◼ 10 mL Volumetric flask (1) ◼ 2 mL Graduated pipet (1) ◼ Pipet filler (1) ◼ Cork stoppers (6) ◼ Cuvette (1) (rinse with water, do not brush it; return after class ) ◼ Dropper (1) ◼ Lens tissue (2) ◼ Take ca. 80 mL of D.I. water with 100 mL beaker ◼ Unknown cobalt(II) ion solution: 6 mL (given out by TA) 1
Objective & Skills Objective ▪ To determine trace amounts of cobalt(II) ions by quantitative 2- ] with spectrophotometer analysis of [Co(SCN) 4 + 4SCN － (aq) Co 2+ (aq) [Co(SCN) 4 ] 2- (aq) Light pink Blue max : 620 nm Skills ◼ Pipet, volumetric flask, spectrophotometer 2 ◼ Plotting
Absorption Spectrum and Color 2- (aq) Absorption spectrum of Co(SCN) 4 absorbed Transmitted Complementary color 3
Beer’s Law ▪ Transmittance: T = P/P 0 ▪ Absorbance: A = - logT = e b c A: Measured absorbance e : Extinction coefficient b: Path length c: Concentration of the absorbing substance ▪ Concentration is directly proportional to absorbance for dilute solution ▪ Calibration curve: Abs (y axis) vs. c (x axis) y = mx + b x x A 1 x x x 4 C 1
Step 1: Prepare Blank Solution (without Co 2+ ) Pre-set volume of (1) 0.80 mL 6 M HCl (2) 2.0 mL 50% KSCN disperser (3) 4.8 mL acetone Add to volumetric flask from dispenser Add D.I. water to 10 mL mark Dropper ◼ Firmly stopper the ◼ Transfer soln into Mark flask clean Erlenmeyer ◼ Invert repeatedly flask ◼ Assure mixing well ◼ Stopper to prevent White ppt. and solids disappear evaporation appears 5
Step 2: Prepare Cobalt(II) Sample Solution with Standard Solution Calibration curve: Unknown analysis: Accurately measure indicated volume of 0.10 Accurately measure x mL unknown mg/mL Co 2+ standard soln with 2 mL pipet Co 2+ soln with 2 mL pipet * 0.5 ≦ x ≦ 2.0 1) 0.50; 2) 1.00; 3) 1.50; 4) 2.00 mL To 10 mL volumetric flask To 10 mL volumetric flask Add 0.80 mL 6 M HCl Add 0.80 mL 6 M HCl 2.0 mL 50% KSCN 2.0 mL 50% KSCN 4.8 mL acetone 4.8 mL acetone Add water till 10 mL mark Add D.I. water till 10 mL mark Mix thoroughly Mix thoroughly Transfer to Transfer to A clean Erlenmeyer flask with stopper on A clean Erlenmeyer flask with stopper on 6
Formula of Standard Cobalt(II) Sample Solution Sample 0.10 mg/mL 50% 6 M HCl Acetone Co 2+ (mL) No. KSCN 0 0 1 0.50 2 1.00 0.8 mL 2.0 mL 4.8 mL 3 1.50 4 2.00 0.5 ≦ x ≦ 2.0 Unknown Transparent blue 0 1 2 3 4 sample solution Standard sample solutions • Color intensity increases with concentration • Transparent without white precipitate • Checked by TA 7
Step 3: Calibration of Spectrophotometer (1) Turn on power and warm up for 20 min. (2) Empty cuvette holder (3) Press “Mode” button and set to A (Absorbance) (4) Set the analytical wavelength (i.e. 620 nm) (5) Press “BLANK” button to zero set (4) Wavelength setting (2) Cuvette holder (1) Power switch (3) A/T/C Mode (5) Blank 8
Calibration: Blank Adjust 1) Rinse cuvette twice with blank soln 2) Add soln until 1/2 height 3) Wipe with lens tissue Blank soln 1) Align cuvette in same direction to control the Notice: path of the light • Hold the top of cuvette 2) Close lid • Rinse with water Press “Blank” button to 3) • Do not brush it blank adjust • Do not use a test tube 9
Step 4: Absorbance of Standard and Unknown Cobalt(II) Sample Solution 0 1 2 3 4 ◼ Start testing from lower concentration ◼ Rinse cuvette with test solution twice ◼ Fill cuvette with soln at 1/2 height ◼ Wipe cuvette with lens tissue ◼ Place cuvette in cuvette holder ◼ Align cuvette in same direction ◼ Close lid and read the Absorbance * Use same cuvette to make whole measurements 10
Notice of Experiment ◆ Familiar with the operation of pipet, volumetric flask, and spectrophotometer first. ◆ Use 2 mL graduated pipet to measure 0.10 mg/mL Co 2+ standard solution accurately. Do not use graduated cylinder instead. ◆ Write down all the observation such as color, precipitate, endo- or exothermic… etc. ◆ After class, waste solution contains heavy metals and organic solvent that must be recycled. ◆ Wash the Erlenmeyer flasks used and put into oven to dry. ◆ Wash and clean up all the apparatus including pipet. ◆ Hand the cuvette to TA; do not leave in the spectrophotometer. ◆ Turn off the spectrophotometer and put dust cover on. 11
Plot the Calibration Curve Full version report ▪ 5-points plot (blank soln included) ▪ X-Y scattering plot (label x,y axes) ▪ Show linear regression (y = mx + b) and R 2 ▪ The absorbance of the unknown solution should be in the range of the calibration curve Cobalt Solution Calibration Curve y = 32.5x + 0.0052 Absorbance R² = 0.9997 0.7 0.6 0.5 A x 0.4 0.3 0.2 0.1 0 0 0.005 0.01 0.015 0.02 0.025 C [Co 2+ ] mg/ mL 12
Notice of Results In your report, it should include 1) Make and print out chart 1 2) Make and print out diagram 1 3) Calculate original Co 2+ concentration of unknown with suitable significant figures and units Note: Concentration of unknown solution should take into account its dilution factor ( x mL diluted to 10.0 mL) Diagram 1 Chart 1 Linear regression R 2 + 2 [Co ] EXCEL X-Y scattering plot 2.0 mL and add regression line = 0.10 mg/mL 10.0 mL 13 = 0.020 mg/mL 13
T12 - Graduated Pipet Ex. Deliver 5.00 mL solution: Totally Delivery ▪ Wash a 10 mL pipet thoroughly. ▪ Rinse twice with small portion of sample solution. Pipette 安全吸球 ▪ filler Press valve A of pipet filler and squeeze bulb to Aspirate expel the air inside and create a vacuum. A 排氣閥 valve ▪ Insert the top of pipet into pipet filler, press valve S 安全吸球 Bulb 10 ml S Ex to draw liquid to pass the graduation mark. E Suction 吸液閥 valve ▪ Remove pipet filler and quickly cover the top of 25 排液閥 Empty pipet with index finger and adjust liquid level to the ml valve mark of 5.00 mL. 5 ▪ Hold pipet vertically and transfer liquid into 6 7 container. (One hand hold pipet and the other 8 hand hold container to operate.) 9 ▪ Remove index finger and free fall the drop on tip 10 by touching it to the wall of the flask. Transfer ▪ Pipet Stay for 15 s to drain the solution. pipet ▪ Do not blow out remaining liquid, pipet is calibrated for this to remain. 14 ▪ Wash thoroughly after use.
T13 - Volumetric Flask ▪ Wash a 10 mL volumetric flask thoroughly and rinse with D.I. water. 標線刻度 Mark ▪ Transfer the solution to flask with the help of a funnel. ▪ Fill the flask about half-full and swirl the In ml contents to achieve solution. 100 ± 0.15 20 o C ▪ Bring the liquid level almost to the mark. ▪ Use a dropper to add solvent drop by drop to the mark. ▪ Stopper the flask and invert repeatedly to assure homogeneous. ▪ Pour the solution in flask for later use. ▪ Wash immediately after use. 15
Dispenser 1. Check if the pre-set volume is correct, do not change the （ 1 ） volume setting afterwards. 2. Position the volumetric flask （ 3 ） （ 2 ） 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. 16
Full Version Report (within 4 pages) III. Final report I. Prelab report II. Exp. Records ✓ Data analysis ✓ Observation ✓ Objective ✓ Error analysis ✓ Operation ✓ Principle ✓ Questions and ✓ Reaction condition ✓ Chemicals discussion ✓ Data and results ✓ Procedures ✓ Feedback and suggestion ▪ Submit your lab report at the class next week. ▪ Hand in lab report on time, including prelab report, records, and final report. ▪ Late submission: -5 points within a week; get 0 point over a week. ▪ Miss contents: -5 points per category 17
Scoring Rubrics for Lab Report Category Guidelines 1. Indicate the main principles and chemical equations in summary. I. Prelab 2. List the chemicals’ physical, chemical properties, and toxicity. report 3. Use the flow chart to explain the procedures concisely. 4. Record the data with correct significant figures and units. II. Lab 5. Record the observations, operation, and reaction condition in record details. 6. Process the data correctly, including the equations and calculation. 7. Indicate the final results with correct significant figures and units. III. Results 8. Analyze the results and make the appropriate discussion. & discussion 9. Plot the results with appropriate XY axis and indicate each curves. 10. Provide the concrete outcome and constructive suggestion. 18
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