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SPECIAL TOPIC I
pH,
p
K
a
,
and Buffers
This is a continuation of the discussion on acids and bases found in Chapter 2 of the text. First, we will see how the
pH of solutions of acids and bases can be calculated. We will look at three different kinds of solutions.
1.
A solution made by dissolving a strong acid or a strong base in water.
2.
A solution made by dissolving a weak acid or a weak base in water.
3.
A solution made by dissolving a weak acid and its conjugate base in water.
Such a solution is known as a
buffer solution
.
Before we start, we need to review a few terms.
An acid is a compound that loses a proton, and a base is a compound that gains a proton.
The degree to which an acid (HA) dissociates is described by its acid dissociation constant
1
K
a
2
.
HA
H A
=
[H A
[HA]
a
+
+ −
−
+
K
] [ ]
The strength of an acid is indicated by its acid dissociation constant
1
K
a
2
or by its
p
K
a
.
p
K
a
=
-
log
K
a
The stronger the acid, the
larger
its acid dissociation constant and the
smaller
its
p
K
a
.
For example, an acid with an acid dissociation constant of
1
*
10
-
2
1
p
K
a
=
2
2
is a stronger acid than one
with an acid dissociation constant of
1
*
10
-
4
1
p
K
a
=
4
2
.
While
p
K
a
is used to describe the strength of an acid, pH is used to describe the acidity of a solution. In other
words, pH describes the concentration of hydrogen ions in a solution.
pH
=
-
log
3
H
+
4
The smaller the pH, the more acidic the solution:
acidic solutions have pH values
6
7;
a neutral solution has a
pH
=
7;
basic solutions have pH values
7
7.
A solution with a
pH
=
2
is more acidic than a solution with a
pH
=
4.
A solution with a
pH
=
12
is more basic than a solution with a
pH
=
8.
Determining the pH of a Solution
To determine the pH of a solution, the concentration of hydrogen ion
3
H
+
4
in the solution must be determined.