Collect:

◼250 mL volumetric flask (1)

◼10 mL graduated pipet (2), pipet filler

◼50 mL beaker (4)

◼Dropper (2) (not broken)

◼Magnetic stir bar (give by GTA)

Prepare:

◼ Wash and oven dry two test tubes

◼10 and 50 mL graduated cylinders

◼ 250 mL beaker1

Buffer Solutions

2021/01/25 revised

Objective

◼ Prepare buffer solutions of HOAc/NaOAc

◼ Investigate the factors affecting buffer capacity

➢ [A-]/[HA] ratio

➢ Concentration of A- and HA

Skills

◼ Use of pH-meter

◼ Preparation of solution

◼ Dilution of conc. acid

◼ Determine the buffer capacity2

Objective & Skills

3

▪ Buffer: solution that maintains a fairly constant pH value upon addition of a small portion of acid or base.

▪ A buffer contains

◆ A weak acid HA to neutralize the added base

◆ Its conjugated base A- to neutralize the added acid

▪ To prepare a buffer solution that could be:◆ Higher concentrations of strong acids or strong bases

Ex.: 1 M HCl, 1 M NaOH

◆ Mixture of a weak acid and its conjugate base

Ex.: HF/NaF

◆ Mixture of a weak base and its conjugate acid

Ex.: NH3/NH4Cl

Buffer Solution

Add base: HA(aq) + OH-(aq) → H2O(l) + A-(aq)

Add acid: A-(aq) + H+(aq) → HA(aq)

4

◼Henderson-Hasselbalch equation

[HA]

]][A[HKa

−+

=

][A

[HA]K][H a

−

+ =

)][A

[HA]log(logK]log[HpH a −

+ −−=−=

)[HA]

][Alog(pKpH a

−

+=

pH of Buffer

HA(aq) ⇌ H+(aq ) + A-(aq)

5

Buffer Capacity

▪ Definition: The number of moles of OH- or H+ that causes 1 L of the buffer to undergo one unit change in pH

▪ A buffer has higher buffer capacity when the concentrationof two components are higher

▪ A buffer has the greatest capacity when the concentration ratio of the two components is 1 ([A

-]/[HA] = 1)

▪ A buffer with pH = pKa ± 1, shows better buffer capacity

▪ Calculation:

30(mL)

1000(mL/L)N(eq/L)(mL/drop)V(drop/pH)

slope

1

pH)eq/LCapacity(m Buffer

drop =

6

pH Meter

pH meter consists of three parts:

◼ pH electrode

➢ Reference electrode (usually made of silver with

silver chloride), the potential is a fixed value

➢ Indicator electrode (usually made of glass), the

potential changes with the concentration of H+

◼ Thermoprobe: used to measure the temperature of soln

◼ Voltmeter: used to measure the potential difference (Em)

between the two electrodes

Fig. T16-1 pH meter in lab (SUNTEX SP-701)

7

pH Meter – Em vs. pH

Em = K − 2.3RT(pH)/nF

Em = mT(pH) + K

◼ Em: measured cell potential

◼ K: constant, determined by

the type of electrode used

◼ R: gas constant

◼ T: absolute temperature of

the solution

◼ pH: pH value of solution

◼ n: number of moles of

electrons transferred in the

reaction

◼ F: Faraday constantThe relationship between measured

cell potential and pH value

7.00 14.00

First standard

solution

Second standard

solution

0pH

+mV

- mV

Em

4.00

8

Outline of Procedures

1. Calibrate pH-meter 2. Prepare solution 3. Calculate the volume of

one drop of HCl/NaOH

4. Prepare test solutions

and record pH value

50 mL beaker

5 drops of acid

5 drops of base

Record all

pH values

9

Procedure 1. Prepare pH-Meter ◼ Push the “POWER” button,

warm up for 10 minutes

◼ Remove the electrode cap by

rotating it

◼ Use washing bottle to rinse the

electrodes

◼ Blot dry with a tissue

◼ Press “HOLD” when cleaning

the electrodes and the screen

will freeze

NT$ 4000 !!

Cap of electrode Control knobs Rearward of pH meter

pH Electrode

Thermoprobe

10

◼ Rinse thermoprobe and electrodes

◼ Immerse in pH 4.00 buffer solution

◼ Adjust Slope knob until meter shows

‘4.00’

◼ Immerse thermoprobe and electrodes

into pH 7.00 buffer solution

◼ Adjust Calib knob until meter shows

‘7.00’

pH 7.00

CalibSlope

pH 4.00

(1) Collect pH 7.00 and 4.00 standard buffer solution

(2) Switch to “Temp” function to check if the temp is close to r.t.

(3) Switch to “pH” function

Procedure 1. Calibrate pH-Meter

11

Notice in Using pH Meter

◼ Use 50 mL beaker for testing

◼ Place the electrode on the holder

◼ When testing, both thermoprobe and

the electrode should be placed in soln

◼ The salt bridge of electrode should be

fully immersed in the test solution

◼ Position the electrode properly so that the

stirring bar will not strike the electrode

◼ Turn the magnetic stir on

◼ Rinse the electrode with D.I. water and

blot dry with tissues when changing the

test soln

◼ Immerse the electrode in clean D.I.

water when not in use

◼ Immerse the electrode in 3 M KCl soln

when not in use for long period of time

Salt

bridge

12

Odd no. groups:

Prepare 0.05 M HOAc

◼ Use pipet to get 0.74 mL

of conc. HOAc (17 M)

◼ Transfer to volumetric

flask that contains some

D.I. water

◼ Dilute to 250 mL

◼ pH of soln is ~3

Even no. groups:

Prepare 0.05 M NaOAc

◼ Obtain 1.70 g of

NaOAc·3H2O

◼ Dissolve in ca. 100 mL

D.I. water

◼ Transfer to volumetric

flask

◼ Rinse the beaker

several times

◼ Dilute to 250 mL

◼ pH of soln is ~ 8

Procedure 2. Prepare Solution

◆ Two groups share one set of solution

◆ Label the solution to avoid mistaken

◆ Measure out each soln with 10 mL graduated pipet as Table 1

13

Test

solution

0.050 M

HOAc (mL)

0.050 M

NaOAc (mL)

Distilled

water (mL)

(a) 30 0 0

(b) 0 30 0

(c) 15 15 0

(d) 5 5 20

(e) 0 0 30

(f) 5 25 0

(g) 10 20 0

(h) 20 10 0

(i) 25 5 0

Preparation of Test Solutions

Note: the pH change of water is drastic; test it last

(1) Concentration vs.

buffer capacity

(2) [OAc-]/[HOAc]

vs. buffer capacity

(Not done in this exp)

◼ Measure the volume of 50 drops of 1.0 M HCl or NaOH

solution with a 10 mL graduated cylinder

◼ Calculate the average volume of each drop

◆ Droppers must be in good condition and not broken in

order to prevent the volume of the drops varied too much

Procedure 3. Determine the Average Volume of 1 Drop of HCl/NaOH

Vavg, HCl = _____ mL/drop

Vavg, NaOH = ____ mL/drop

1.0 M HCl 1.0 M NaOH

14

15

Procedure 4. Measure the pH of Test Solution while Adding Acid/Base

Obtain 1.0 M HCl with dropper

Add one drop into soln. (c)

Mix thoroughly

and record pH values

Repeat adding acid for 5 times

Repeat above steps with soln. (a)(b)(d)(e)

Prepare another beaker of soln. (c)

Obtain 1.0 M NaOH with dropper

Add drops progressively into soln.

and record change in pH values◼ Set up the apparatus

◼ Immerse the thermoprobe and

electrode in soln

◼ Position the electrode in the soln so

that the stirring bar will not strike the

electrode

◼ Stir and mix the soln continuously

Turn the stir on

16

After Experiment

▪ Rinse and check pH electrode before use

▪ Immerse the electrode into clean D.I. water

(or Place electrode in plastic-cap that containing 3 M KCl)

▪ Turn the pH meter off

▪ Hand in magnetic stirring bar to TA

▪ Waste liquids (salts) can be discarded in sink after

neutralization

17

Example of Data Analysis

No. of

drop

a b c d e

0.050 M

HOAc

0.050 M

NaOAc

0.050 M

HOAc/NaOAc

0.017 M

HOAc/NaOAc

Distilled

water

5 2.39 5.60 4.47 4.15 11.49

4 2.47 5.72 4.51 4.26 11.40

3 2.59 5.85 4.55 4.36 11.27

2 2.71 6.07 4.58 4.46 11.09

1 2.82 6.37 4.61 4.56 10.79

0 3.00 7.71 4.65 4.64 5.49

1 3.15 10.78 4.67 4.73 3.01

2 3.33 10.93 4.70 4.80 2.71

3 3.48 11.22 4.73 4.89 2.55

4 3.59 11.39 4.76 4.99 2.43

5 3.65 11.51 4.80 5.09 2.35

Avg. pH

of acid

and base

Add

acid

Add

base

Chart 1

18

Plots of pH vs. Drops of Acid/Base

1.00

3.00

5.00

7.00

9.00

11.00

-8 -6 -4 -2 0 2 4 6 8

pH

va

lue

HCl / Na OH drops

pH cha nge

0.05M HOAc

0.05M NaOAc

0.05M

HOAc/NaOAc

0.017M

HOAc/NaOAc

DI water

◼ Use x-y scattering plot in Excel with pH values as y-axis, and the drops as x-axis

◼ Label clearly the x, y-axis and the lines

◼ Put plots of a, b, c, d, and e solns all in one graph for comparison

◼ The data sheet and graph must be handed in along with the final lab report

* Use the average pH value as zero point for calculation and graphing

* If the initial pH values vary widely for each two trials, then adding base and adding

acid can be plotted separately

Diagram 1

0.050 M (1:1) buffer capacity = 22 meq/L‧pH (not theoretical value)

0.017 M (1:1) buffer capacity = 7.8 meq/L‧pH (not theoretical value)19

Example of Calculation of Buffer Capacity

linear (c)

y = 0.03x + 4.642

R2 = 0.9912

linear (d)

y = 0.085x + 4.638

R2 = 0.9968

4.40

4.50

4.60

4.70

4.80

4.90

-3 -2 -1 0 1 2 3

pH

va

lue

HCl/Na OH drops

Buffe r Ca pa city(a) 0.05M (1:1)

(b) 0.017M(1:1)

linear (d)

Linear (c)

◼ Use the central 5 points of soln (c) and (d)

◼ Show linear regression and R2 value

No. c d

Drops0.050 M

HOAc/NaOAc

0.017 M

HOAc/NaOAc

-2 4.58 4.46

-1 4.61 4.56

0 4.65 4.64

1 4.67 4.73

2 4.70 4.80

Diagram 2Chart 2

N = 1.0

drop

Buffer Capacity (meq/L pH)

1 1000(mL/L)= (drop/pH) V (mL/drop) N(eq/L)

slope 30(mL)

(Vavg. of HCl + Vavg. of NaOH) / 2

I. Prelab report

✓ Objective

✓ Principle

✓ Chemicals

✓ Procedure

20

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)

▪ Hand in the report in the next week of the class.

▪ Hand in lab report on time, including prelab report,

records, and final report

▪ Late submission: -5 points within a week, get 0 points 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.

21

Scoring Rubrics for Lab Report

22

T13 - Volumetric Flask

▪ Wash the volumetric flask thoroughly and

rinse with distilled water.

▪ Transfer the solution to flask with the help

of a funnel.

▪ Fill the flask with D.I. water about half full

and swirl the contents to homogeneous.

▪ Bring the liquid level almost to the mark.

▪ Use a dropper to add water to the mark.

▪ Stopper the flask and invert repeatedly to

assure uniform mixing.

▪ Pour the solution in beaker for later use.

▪ Wash thoroughly after use.

標線刻度

100 ml

±0.15 In

20oC

Mark

T13 Video on Youtube (Click)

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 drain the soln by

touching the tip to the wall of the flask

▪ Stay for 10-15 s to drain the solution

▪ Do not blow out remaining liquid, pipet is calibrated

for this to remain

▪ Wash thoroughly after use 23

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

Partially Delivery

Aspirate

valve

Bulb

Suction

valve

Expel

valve

Pipette

filler

T12 - Graduated Pipet

Transfer

pipetPipet