21 Titration: Reactions of
Acids and Bases
The Self-ionization of Water
pure water at 25oC, both H3O+ and
OH- ions are found at concentrations
of 1.0 x 10-7 M
 Kw is called the ion-product constant
Kw = [H3O+] [OH-]
 In pure water at 25oC, Kw = 1.0 x 10-14
 In
The Self-ionization of Water
 Kw
is very useful because it applies
not only to pure water, but to every
water solution at 25oC, even acidic or
basic solutions.
 Thus, if a solution has an H3O+
concentration of 1.0 x 10-2 M, then the
OH- concentration must be 1.0 x 10-12
M. i.e. [OH-] = Kw / [H3O+]
pH
 In
1909, Danish biochemist Soren
Sorensen proposed a simple way to
express the concentrations of H3O+
ions based on logarithms. This scale
is known as the pH scale.
 The pH of a solution is -1 times the
Log of the H3O+ concentration in
moles per Liter.
 pH = -log[H3O+]
pH =
+
-log[H3O ]
 Because
the pH scale is a logarithmic
scale, each one-unit change in pH
represents a 10-fold change in the
concentration of H3O+ ions.
 The pH of a solution can be measured
by using acid-base indicators such as
litmus paper, or by using an electronic
device called a pH meter.
pH Scale
pH
[H3O+] Concentration
0
100 M
1
10-1 M
2
10-2 M
3
10-3 M
Banana
4
10-4 M
Coffee
5
10-5 M
Saliva
6
10-6 M
Pure water
7
10-7 M
Blood
8
10-8 M
9
10-9 M
Borax
10
10-10 M
Lime water
11
10-11 M
12
10-12 M
Bleach
13
10-13 M
1.0 M NaOH
14
10-14 M
.1 M HCl
Lemon juice
Buffers
A
buffer is a mixture that is able to
release or absorb H+ ions, keeping a
solution’s pH constant.
 Most common buffers are mixtures of
weak acids with their conjugate bases.
i.e. a buffer of acetic acid and the acetate
anion keeps the pH near 4.74
 The amount of acid or base that a buffer
can neutralize is called the buffer
capacity. [See examples on page 634]
Acid-Base Titration
 The
concentration of a weak acid or a
weak base can be easily calculated from
the results of a procedure called an
acid-base titration.
 An acid-base titration is a carefully
controlled neutralization reaction.
Acid-Base Titration
 To
conduct a titration, a standard
solution is slowly added to the unknown
solution until neutralization is complete –
called the equivalence point.
 The point at which the indicator changes
color is called the end point of the
titration.
Performing a Titration
 To
run a titration, the standard solution
is slowly added to the unknown solution.
 As the two solutions mix, the base in
one solution neutralizes the acid in the
other solution, a reaction that runs
nearly to completion.
Performing a Titration
 The
reaction between acetic acid and
sodium hydroxide is:
HC2H3O2 + NaOH
H2O + NaC2H3O2
The point at which the indicator changes
color is called the end point of the titration.
 Total # mol of H+ = Total # mol of OHThis equation is key to calculating the
concentration of an acid or base using data
from a titration.
Performing a Titration
 The
reaction between oxalic acid and
sodium hydroxide from Lab #57 is:
H2C2O4 + 2 NaOH
2H2O + Na2C2O4
Note that it takes 2 mol of base to neutralize
each 1 mol of acid.
This equation is key to calculating the
concentration of an acid or base using data
from a titration.
Titration
Strong Acid / Strong Base
HCl + NaOH
H2O + NaCl
Weak Acid/ Strong Base
HC2H3O2 + NaOH
H2O + NaC2H3O2
Weak Base / Strong Acid
NH3 + HCl
NH4Cl
[Fig 19-14, 19-15, & 19-16 on pgs. 640-642]