Download - 實驗 15 Buffer Solutions - 國立臺灣大學
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
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25 ml
EX 20
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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