1Updated 2/16/2019
HOW DOES A BUFFER MAINTAIN PH?
A buffer is a special solution that stops massive changes in pH levels. Every buffer that is made has a certain buffer capacity, and
buffer range. The buffer capacity is the amount of acid or base that can be added before the pH begins to change significantly. It can be
also defined as the quantity of strong acid or base that must be added to change the pH of one liter of solution by one pH unit. The
buffer range is the pH range where a buffer effectively neutralizes added acids and bases, while maintaining a relatively constant pH.
INTRODUCTION
The equation for pH also shows why pH does not change by much in buffers.
Where,
is the concentration of the conjugate base
is the concentration of the acid
When the ratio between the conjugate base/ acid is equal to 1, the pH = pK . If the ratio between the two is 0.10, the pH drops by 1 unit
from pK since log (0.10) = -1. If a ratio increases to a value of 10, then the pH increases by 1 unit since \log (10) = 1. The buffer
capacity has a range of about 2. This means when a buffer is created, the pH can be changed by -1 by acid or +1 by base before the pH
begins to change substantially. After the addition of base to raise the pH by 1 or more, most of the conjugate acid will have been
depleted to try to maintain a certain pH, so the pH will be free to increase faster without the restraint of the conjugate acid. The same
goes for the addition of acid, once the conjugate base has been used up, the pH will drop faster since most of the conjugate base has
been used up.
EXAMPLE 1
What is the effect on the pH of adding 0.006 mol HCL to 0.3L of a buffer solution that is 0.250M HC H O and 0.560 M
NaC H O ? pK = 4.74
Calculate the starting amount of C H O
Calculate the starting amount of HC H O
=
K
a
[ ][ ]
H
+
A
−
[
HA
]
(1)
pH
=
p
+ log
K
a
[ ]
A
−
[
HA
]
(2)
A
−
HA
a
a
232
2 3 2 a
pH
= 4.74+ log = 4.74+ 0.35= 5.09
0.560
0.250
(1)
+
⇌
H
+
O
C
2
H
3
O
−
2
H
3
O
+
C
2
H
3
O
2
H
2
(2)
232-
0.300
L
× 0.560
M
= 0.168
mol
C
2
H
3
O
−
2
(3)
232
