0
10
20
30
40
50
Buffers
and
Acid/Base
Titration
Reaction of Weak Bases with Water
The base reacts with water, producing its
conjugate acid and hydroxide ion:
CH3NH2 + H2O  CH3NH3+ + OH-
Kb = 4.38 x 10-4


[CH 3 NH 3 ][OH ]
Kb  4.38 x 10 
[CH 3 NH 2 ]
4
Kb for Some Common Weak Bases
Many students struggle with identifying weak
bases and their conjugate acids.What patterns
do you see that may help you?
Formula
Conjugate
Acid
Kb
NH3
NH4+
1.8 x 10-5
Methylamine
CH3NH2
CH3NH3+
4.38 x 10-4
Ethylamine
C2H5NH2
C2H5NH3+
5.6 x 10-4
Diethylamine
(C2H5)2NH
(C2H5)2NH2+
1.3 x 10-3
Triethylamine
(C2H5)3N
(C2H5)3NH+
4.0 x 10-4
Hydroxylamine
HONH2
HONH3+
1.1 x 10-8
Base
Ammonia
Hydrazine
H2NNH2
H2NNH3+
3.0 x 10-6
Aniline
C6H5NH2
C6H5NH3+
3.8 x 10-10
Pyridine
C5H5N
C5H5NH+
1.7 x 10-9
Reaction of Weak Bases with Water
The generic reaction for a base reacting
with water, producing its conjugate acid and
hydroxide ion:
B + H2O  BH+ + OH

[ BH ][OH ]
Kb 
[ B]
(Yes, all weak bases do this – DO NOT
endeavor to make this complicated!)
Buffered Solutions
A solution that resists a change in
pH when either hydroxide ions or
protons are added.
Buffered solutions contain either:
 A weak acid and its salt
 A weak base and its salt
Acid/Salt Buffering Pairs
The salt will contain the anion of the acid,
and the cation of a strong base (NaOH, KOH)
Weak Acid
Formula
of the acid
Hydrofluoric
HF
Formic
HCOOH
Benzoic
C6H5COOH
Acetic
Carbonic
Propanoic
Hydrocyanic
CH3COOH
H2CO3
HC3H5O2
HCN
Example of a salt of the
weak acid
KF – Potassium fluoride
KHCOO – Potassium formate
NaC6H5COO – Sodium benzoate
NaH3COO – Sodium acetate
NaHCO3 - Sodium bicarbonate
NaC3H5O2 - Sodium propanoate
KCN - potassium cyanide
Base/Salt Buffering Pairs
The salt will contain the cation of the base,
and the anion of a strong acid (HCl, HNO3)
Formula of
the base
Example of a salt of the weak
acid
NH3
NH4Cl - ammonium chloride
Methylamine
CH3NH2
CH3NH2Cl – methylammonium chloride
Ethylamine
C2H5NH2
C2H5NH3NO3 - ethylammonium nitrate
Aniline
C6H5NH2
C6H5NH3Cl – aniline hydrochloride
Base
Ammonia
Pyridine
C5H5N
C5H5NHCl – pyridine hydrochloride
Titration of an Unbuffered Solution
13
12
11
10
A solution that is
0.10 M CH3COOH
is titrated with
0.10 M NaOH
9
pH
8
7
6
5
4
3
2
1
0.00
5.00
10.00
15.00
20.00
25.00
milliliters NaOH (0.10 M)
30.00
35.00
40.00
45.00
Titration of a Buffered Solution
13
12
11
A solution that is
0.10 M CH3COOH and
0.10 M NaCH3COO is
titrated with
0.10 M NaOH
10
9
pH
8
7
6
5
4
3
2
1
0.00
5.00
10.00
15.00
20.00
25.00
milliliters NaOH (0.10 M)
30.00
35.00
40.00
45.00
Comparing Results
Gra ph
pH
Buffered
Unbuffered
mL 0.10 M NaOH
Comparing Results
Buffered
13
13
12
12
11
11
10
10
9
9
8
8
7
pH
pH
Unbuffered
6
6
5
5
4
4
3
3
2
1
0.00
7
2
5.00
10.00
15.00
20.00
25.00
milliliters NaOH (0.10 M)
30.00
35.00
40.00
45.00
1
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
milliliters NaOH (0.10 M)
In what ways are the graphs different?
In what ways are the graphs similar?
40.00
45.00
Henderson-Hasselbalch Equation
 [ A ] 
 [base] 
  pK a  log 

pH  pK a  log 
 [acid ] 
 [ HA] 
 [ BH  ] 
 [acid ] 
  pKb  log 

pOH  pKb  log 
 [base] 
 [ B] 
This is an exceptionally powerful tool, and it’s use
will be emphasized in our problem solving.
Weak Acid/Strong Base Titration
13
12
11
10
9
Endpoint is above
pH 7
pH
8
7
A solution that is
0.10 M CH3COOH
is titrated with
0.10 M NaOH
6
5
4
3
2
1
0.00
5.00
10.00
15.00
20.00
25.00
milliliters NaOH (0.10 M)
30.00
35.00
40.00
45.00
Strong Acid/Strong Base Titration
13
12
11
10
9
pH
8
7
Endpoint is at
pH 7
A solution that is
0.10 M HCl is
titrated with
0.10 M NaOH
6
5
4
3
2
1
0.00
5.00
10.00
15.00
20.00
25.00
milliliters NaOH (0.10 M)
30.00
35.00
40.00
45.00
Strong Acid/Strong Base Titration
13
12
A solution that is
0.10 M NaOH is
titrated with
0.10 M HCl
11
10
9
pH
8
7
Endpoint is at
pH 7
It is important to
recognize that
titration curves are
not always
increasing from left
to right.
6
5
4
3
2
1
0.00
5.00
10.00
15.00
20.00
25.00
milliliters HCl (0.10 M)
30.00
35.00
40.00
45.00
Strong Acid/Weak Base Titration
13
12
11
10
9
pH
8
7
6
5
Endpoint is below
pH 7
4
A solution that is
0.10 M HCl is
titrated with
0.10 M NH3
3
2
1
0.00
5.00
10.00
15.00
20.00
25.00
milliliters NH3 (0.10 M)
30.00
35.00
40.00
45.00
Selection of Indicators
Indicator
Indicator Transitions
Low pH color
Transition pH range High pH color
Gentian violet (Methyl violet 10B)
yellow
0.0–2.0
blue-violet
Leucomalachite green (first transition)
Leucomalachite green (second transition)
yellow
green
0.0–2.0
11.6–14
green
colorless
Thymol blue (first transition)
red
1.2–2.8
yellow
Thymol blue (second transition)
yellow
8.0–9.6
blue
Methyl yellow
Bromophenol blue
red
yellow
2.9–4.0
3.0–4.6
yellow
purple
Congo red
blue-violet
3.0–5.0
red
Methyl orange
Bromocresol green
Methyl red
Methyl red
red
yellow
red
red
3.1–4.4
3.8–5.4
4.4–6.2
4.5–5.2
orange
blue
yellow
green
Azolitmin
Bromocresol purple
Bromothymol blue
Phenol red
Neutral red
Naphtholphthalein
Cresol Red
red
yellow
yellow
yellow
red
colorless to reddish
yellow
4.5–8.3
5.2–6.8
6.0–7.6
6.8–8.4
6.8–8.0
7.3–8.7
7.2–8.8
blue
purple
blue
red
yellow
greenish to blue
reddish-purple
Phenolphthalein
Thymolphthalein
Alizarine Yellow R
Litmus
colorless
colorless
yellow
red
8.3–10.0
9.3–10.5
10.2–12.0
4.5-8.3
fuchsia
blue
red
blue
Source: Wikipedia