104T buffer [相容模式] -...
Transcript of 104T buffer [相容模式] -...
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Collect:250 mL volumetric flask (1) 10 mL graduated pipet (2), pipet filler50 mL beaker (4)Dropper (2) (not broken)Magnetic stir bar (given out and collected by GTA)
Prepare: 10 and 50 mL graduated cylinders 250 mL beaker
Buffer Solutions(2015/11/18 revised)
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Objective & SkillsObjective Prepare buffer solutions of HOAc/NaOAc Determine the factors that affect buffer capacity [A-]/[HA] ratio A- and HA concentration
Skills Operation of pH-meter Preparation of solution Dilution of conc. acid Test the buffer capacity
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Buffer: solution that maintains a fairly constant pH value upon addition of a small amount of acid or base.
A buffer contains A weak acid to neutralize the added base
Its conjugated base to neutralize the added acid
To prepare a buffer solution, it could be: Higher concentrations of strong acids or strong bases
i.e. 1 M HCl, 1 M NaOH A mixture of a weak acid and its conjugate base
i.e. HF/NaF A mixture of a weak base and its conjugate acid
i.e. NH3/NH4Cl
Introduction I: Buffer Solution
A-(aq) + H+
(aq) HA(aq)Acid added
H A (aq)+ O H -(aq) H 2O (l) + A -
(aq)Base added
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Henderson-Hasselbalch equation
[HA]]][A[HKa
HA(aq) H+(aq) + A-
(aq)
][A[HA]K][H a
)][A
[HA]log(logK]log[HpH a
)[HA]
][Alog(pKpH a
Introduction II: pH of Buffer
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Introduction III: 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 the greatest capacity when the concentration ratio of the two components is 1 ([A-]/[HA] = 1)
A buffer with pKa - 1 ≤ pH ≤ pKa + 1, shows better buffer capacity
A buffer has higher buffer capacity when the concentrationof two components are higher
Calculation:
(mL)(mL/L)(eq/L)(mL/drop)(drop/pH)
pH)(meq/L
301000NV
slope1
Capacity Buffer
drop
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Introduction IV: pH MeterThree components of pH meter:pH electrode
Reference electrode (usually made of silver and silver chloride), the potential is a fixed value
Indicator electrode (usually made of glass), the potential changes when the concentration of H+
variesThermoprobe: measure the temperature of solutionVoltmeter: measure the potential difference between
the two electrodes
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Introduction V: pH Meter
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 through the electrodes during a reaction
F: Faraday constantThe Relationship Between Measured Cell Potential and pH value
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First standard solution
Second standard solution
0pH
+mV
- mV
Em
4
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Outline of Procedures
1. Calibrate pH-meter 2. Prepare solution 3. Calculate the volume of one drop of HCl/NaOH
4. Prepare test solutionsand record pH value
50 mL beaker
5 drops of acid
5 drops of base
Record allpH values
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Procedure 1. Calibrate pH-meter Push the “POWER”
button, warm up for 10 minutes
Remove the electrode cap by rotating it
Use washing bottle to clean the electrodes
Blot dry with a tissue
NT$ 4500 !!
Cap of electrode Control knobs Rearward of pH meter
pH Electrode
Thermoprobe
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Rinse thermoprobe and electrode with DI water
Immerse in pH 4.00 buffer solution Adjust Slope button until pH 4.00
Immerse thermoprobe and electrode into pH 7.00 buffer solution
Adjust Calib button until pH 7.00
pH 7.0
Calibbutton
Slope button
pH 4.0
(1) Collect standard buffer solution(2) Start calibrating pH meter
Procedure 1. Calibrate pH-meter
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Notice in Using pH Meter Use 50 mL beakers 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 When changing the test solution, the
electrode should be rinsed with distilled water and blot dry with tissues
When the electrode is not in use, it needs to be immersed in clean distilled water
When the electrode is not in use for long period of time, it should be immersed in 3 M KCl solution
Salt bridge
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Odd 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 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
2 groups share one set of solution Label after solution is made in case of a mix up According to table 1, measure out each soln with 10 mL graduated pipet
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Testsolution
0.050 MHOAc (mL)
0.050 MNaOAc (mL)
DistilledWater (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
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)
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Measure the volume of 50 drops of 1 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 to prevent the volume of the drops that come out of the dropper to be vary too much
Procedure 3. Calculate 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
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Procedure 4. Measure pH of Test Solution while Adding Acid/Base
Obtain 1.0 N of HCl with dropper
Add one drop into solution (a)
Mix thoroughly and record pH values
Repeat adding acid for 5 times
Repeat above steps with solution (b)~(e)
Prepare another beaker of solution (a)
Obtain 1.0 N of NaOH with dropper
Add drops progressively into solution and record change in pH
values
Set up apparatus Immerse the thermoprobe and
electrode in solution Position the electrode in the soln so
that the stirring bar will not strike the electrode
Stir and mix the solution continuously
Turn the stir on
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Experiment Completed
Clean and check pH electrodes Immerse the electrode into clean DI 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
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Example of Data Analysis
No. of drop
a b c d e0.05 M HOAc
0.05 M NaOAc
0.05 M HOAc/NaOAc
0.017 M HOAc/NaOAc
Distilledwater
-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 10.790 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
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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 v
alue
HCl / NaOH drops
pH change
0.05M HOAc
0.05M NaOAc
0.05M
HOAc/NaOAc
0.017M
HOAc/NaOAc
DI water
Graph with pH value as y-axis, and the drops as x-axis
Choose x-y scattering plot in Excel
Put plots of a, b, c, d, and e solutions all in one graph for comparison
The graph must be handed in along with the final lab report
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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/NaOH drops
Buffer Capacity(a) 0.05M (1:1)
(b) 0.017M(1:1)
linear (d)
Linear (c)
0.050 M (1:1) buffer capacity = 22 meq/L‧pH
0.017 M (1:1) buffer capacity = 7.8 meq/L‧pH
Use the central 5 points of solution (c) and (d) Show linear regression and R2 value
drop
Buffer Capacity (m eq/L pH)1 1000(m L/L)= (drop/pH) V (m L/drop) N(eq/L)
slope 30(m L)
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Additional Information
The pH values of the separately prepared solutions may be different at first, but if the pH does not vary too much, use the average value for calculation and graphing
If the values vary widely, then adding base and adding acid can be plotted separately
Label clearly the x and y-axis and the lines Use Microsoft Excel for graphing and finding the least
square regression lines