1

Quantitative Analysis of Cobalt (II) Ions

Collect

◼ One test tube and six 50 mL Erlenmeyer

flasks (wash clean, oven dry, and cool to r.t.)

◼ Test tube (take 8 mL Co2+ standard soln)

◼ 10 mL Volumetric flask

◼ 2 mL Graduated pipet and pipet filler

◼ Cork stoppers (6)

◼ Cuvette (rinse with water, do not brush it;

return after class)

◼ Dropper

◼ Lens tissue (2)

◼ Take ca. 80 mL of D.I. water with 100 mL

beaker

◼ Unknown cobalt(II) ion solution: give by TA

2021/01/24 revised

2

Objective & Skills

Objective

▪ Determine trace amounts of cobalt(II) ions by quantitative

analysis of [Co(SCN)42-] with spectrophotometer

max: 620 nm

Skills◼ Pipet, volumetric flask, spectrophotometer, and plotting

Light pink

Co2+(aq)

Blue

+ 4SCN－(aq) [Co(SCN)4]2-(aq)

3

absorbed

Transmitted

Absorption Spectrum and Color

Absorption spectrum of Co(SCN)42-(aq)

Complementary color

Beer’s Law

▪ Transmittance: T = P/P0

▪ Absorbance:

A = - logT = e b cA: 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)

4x

x

x

x

x

y = mx + b

A1

C1

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Step 1: Prepare Blank Solution (without Co2+)

(1) 0.80 mL 6 M HCl

(2) 2.0 mL 50% KSCN

(3) 4.8 mL acetone

Add to volumetric flask from dispenser

◼ Firmly stopper the

flask

◼ Invert repeatedly

◼ Assure mixing well

and solids disappear

◼ Transfer soln into

clean Erlenmeyer

flask

◼ Stopper to prevent

evaporation

Add D.I. water

to 10 mL mark

Dropper

Mark

Pre-set

volume of

disperser

White ppt.

appears

6

Step 2: Prepare Cobalt(II) Sample Solution with Standard Solution

II. Unknown analysis:

Accurately measure x mL unknown

Co2+ soln with 2 mL pipet

* 0.5 ≦ x ≦ 2.0

I. Calibration curve:

Accurately measure indicated volume of 0.10

mg/mL Co2+ standard soln with 2 mL pipet

1) 0.50; 2) 1.00; 3) 1.50; 4) 2.00 mL

To 10 mL volumetric flask

A clean Erlenmeyer flask with stopper on

Add D.I. water till 10 mL mark

Mix thoroughly

Transfer to

Add 0.80 mL 6 M HCl

2.0 mL 50% KSCN

4.8 mL acetone

To 10 mL volumetric flask

Add 0.80 mL 6 M HCl

2.0 mL 50% KSCN

4.8 mL acetone

A clean Erlenmeyer flask with stopper on

Add water till 10 mL mark

Mix thoroughly

Transfer to

Standard sample solutions

• Color intensity increases with concentration

• Transparent without white precipitate

• Checked by TA 7

Formula of Standard Cobalt(II) Sample Solution

Sample

No.

0.10 mg/mL

Co2+ (mL)6 M HCl

50%

KSCNAcetone

0 0

0.8 mL 2.0 mL 4.8 mL

1 0.50

2 1.00

3 1.50

4 2.00

Unknown 0.5 ≦ x ≦ 2.0

0 1 2 3 4 Transparent blue

sample solution

(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

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(4) Wavelength

setting

(2) Cuvette

holder

(5) Blank

(3) A/T/C Mode

(1) Power

switch

Step 3: Calibration of Spectrophotometer

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Calibration: Blank Adjust

Notice:

• Hold the top of cuvette

• Rinse with water

• Do not brush it

• Do not use a test tube

1) Align cuvette in same

direction to control the

path of the light

2) Close lid

3) Press “Blank” button to

blank adjust

Blank soln

1) Rinse cuvette twice

with blank soln

2) Add soln until 1/2 height

3) Wipe with lens tissue

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Step 4: Absorbance of Standard and Unknown Cobalt(II) Sample Solution

◼ 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

0 1 2 3 4

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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 Co2+

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.

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Plot the Calibration Curve

▪ 5-points plot (blank soln included)

▪ X-Y scattering plot (label x,y axes)

▪ Show linear regression (y = mx + b) and R2

▪ The absorbance of the unknown solution should be in

the range of the calibration curve

y = 32.5x + 0.0052

R² = 0.9997

00.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.005 0.01 0.015 0.02 0.025

Ab

so

rba

nc

e

[Co2+] mg/ mL

Cobalt Solution Calibration Curve

Ax

C

Full version report

13

In your report, it should include

1) Make and print out chart 1

2) Make and print out diagram 1

3) Calculate original Co2+ 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)

Chart 1

2[Co ]

2.0 mL0.10 mg/mL

10.0 mL

0.020 mg/mL

+

=

=

Diagram 1

Linear regression

R2

EXCEL X-Y scattering plot and add regression line

Notice of Results

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Ex. Deliver 5.00 mL solution:

▪ Wash a 10 mL pipet thoroughly.

▪ Rinse twice with small portion of sample solution.

▪ Press valve A of pipet filler and squeeze bulb to

expel the air inside and create a vacuum.

▪ Insert the top of pipet into pipet filler, press valve S

to draw liquid to pass the graduation mark.

▪ Remove pipet filler and quickly cover the top of

pipet with index finger and adjust liquid level to the

mark of 5.00 mL.

▪ Hold pipet 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 the flask.

▪ Stay for 15 s to drain the solution.

▪ Do not blow out remaining liquid, pipet is calibrated

for this to remain.

▪ Wash thoroughly after use. 14

0 1

2

3

4

5

6

7

8 9

10 ml Ex

20

C

o

25 ml

EX 20

C

o o

0 1

2

3

4

6

7

8 9

0 1

2

3

4

5

6

7

8 9

安全吸球

排氣閥

吸液閥

A

SE

排液閥

安全吸球

5

6

7

8

9

10

Totally Delivery

Aspirate

valve

Bulb

Suction

valve

Empty

valve

Pipette

filler

T12 - Graduated Pipet

Transfer

pipetPipet

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T13 - Volumetric Flask

▪ Wash a 10 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 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.

標線刻度

100 ml

±0.15 In

20oC

Mark

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Dispenser

1. Check if the pre-set volume is

correct, do not change the

volume setting afterwards.

2. Position the volumetric 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）

I. Prelab report

✓ Objective

✓ Principle

✓ Chemicals

✓ Procedures

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II. Exp. Records

✓ Observation

✓ Operation

✓ Reaction condition

✓ Data and results

III. Final report

✓ Data analysis

✓ Error analysis

✓ Questions and

discussion

✓ Feedback and

suggestion

Full Version Report (within 4 pages)

▪ 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

Category Guidelines

I. Prelab

report

1. Indicate the main principles and chemical equations in summary.

2. List the chemicals’ physical, chemical properties, and toxicity.

3. Use the flow chart to explain the procedures concisely.

II. Lab

record

4. Record the data with correct significant figures and units.

5. Record the observations, operation, and reaction condition in

details.

III. Results

&

discussion

6. Process the data correctly, including the equations and calculation.

7. Indicate the final results with correct significant figures and units.

8. Analyze the results and make the appropriate discussion.

9. Plot the results with appropriate XY axis and indicate each curves.

10. Provide the concrete outcome and constructive suggestion.

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Scoring Rubrics for Lab Report