Buffer Solutions
(2015/11/18 revised)
Collect:
 250 mL volumetric flask (1)
 10 mL graduated pipet (2), pipet filler
 50 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
1
Objective & Skills
Objective
 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
2
Introduction I: Buffer Solution
 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
Base
added

H A (a q ) + O H - (a q )
H 2 O (l) + A - (a q )
Its conjugated base to neutralize the added acid
Acid
added
A - (aq) + H + (aq)
H A (aq)
 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
3
Introduction II: pH of Buffer
HA(aq)

H+(aq)
+
A-(aq)
[H  ][A  ]
Ka 
[HA]
Henderson-Hasselbalch equation
K a [HA]
[H ] 
[A  ]

[HA]
pH  log[H ]  logK a  log(  )
[A ]


[A ]
pH  pK a  log(
)
[HA]
4
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 concentration
of two components are higher
 Calculation:
Buffer Capacity(meq/L  pH)
1
1000(mL/L)
(drop/pH)  Vdrop(mL/drop)  N(eq/L) 

slope
30(mL)
5
Introduction IV: pH Meter
 Three 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+
varies
 Thermoprobe: measure the temperature of solution
 Voltmeter: measure the potential difference between
the two electrodes
6
Introduction V: pH Meter
Em = K − 2.3RT(pH)/nF

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 constant
Em = mT(pH) + K
Em
Second standard
solution
+mV
0
First standard
solution
pH
4
7
14
- mV
The Relationship Between Measured
Cell Potential and pH value
7
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
5 drops of acid
Record all
pH values
50 mL beaker
5 drops of base
8
Procedure 1. Calibrate pH-meter
NT$ 4500 !!
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

pH Electrode
Thermoprobe
Cap of electrode
Control knobs
Rearward of pH meter
9
Procedure 1. Calibrate pH-meter
(1) Collect standard buffer solution
(2) Start calibrating pH meter
Slope
button
Calib
button
pH 4.0
pH 7.0
Immerse thermoprobe and
electrode into pH 7.00 buffer
solution
 Adjust Calib button until pH 7.00

Rinse thermoprobe and electrode
with DI water
 Immerse in pH 4.00 buffer solution
 Adjust Slope button until pH 4.00

10
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
11
Procedure 2. Prepare Solution
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
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
12
Preparation of Test Solutions
(1) Concentration vs.
buffer capacity
(2) [OAc-]/[HOAc]
vs. buffer capacity
(Not done in this exp)
Test
solution
0.050 M
HOAc (mL)
0.050 M
Distilled
NaOAc (mL) 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
The pH change of water is drastic; test it last
13
Procedure 3. Calculate the Average
Volume of 1 Drop of HCl/NaOH

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
1.0 M HCl
1.0 M NaOH
Vavg,HCl =_____ mL/drop
Vavg,NaOH =____ mL/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
14
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
Turn the stir on
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


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
Repeat above steps with solution (b)~(e)
15
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
16
Example of Data Analysis
a
b
c
0.05 M
HOAc
0.05 M
NaOAc
0.05 M
HOAc/NaOAc
-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
No. of
drop
Add
acid
Avg. pH
of acid
and base
Add
base
d
e
0.017 M
Distilled
HOAc/NaOAc
water
17
Plots of pH vs. Drops of Acid/Base

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
pH change
11.00
0.05M HOAc
pH value
9.00
0.05M NaOAc
7.00
0.05M
HOAc/NaOAc
5.00
0.017M
HOAc/NaOAc
3.00
DI water
1.00
-8
-6
-4
-2
0
2
HCl / NaOH drops
4
6
8
18
Calculation of Buffer Capacity


Use the central 5 points of solution (c) and (d)
Show linear regression and R2 value
Buffer Capacity
4.90
linear (d)
y = 0.085x + 4.638
4.80
R2 = 0.9968
(a) 0.05M (1:1)
(b) 0.017M(1:1)
linear (d)
Linear (c)
pH value
4.70
linear (c)
y = 0.03x + 4.642
R2 = 0.9912
4.60
4.50
4.40
-3
-2
-1
0
1
HCl/NaOH drops
2
3
B uffer C apacity (m eq/L pH )
=
1
1000(m L/L)
(drop/pH )  V drop (m L/drop)  N (eq/L) 
slope
30(m L)
0.050 M (1:1) buffer capacity = 22 meq/L‧pH
0.017 M (1:1) buffer capacity = 7.8 meq/L‧pH
19
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
20