 |
 |
3 of 12 |  |
 |
 |
I-3
buffering capacity of a buffer occurs, when the concentration of the weak acid equals the concentration
of the conjugate base. On a titration curve this corresponds to the point where pH = pK
a
(i.e. the half
equivalence volume).
Buffer capacity may be calculated using the following formulas:
Acid capacity of a Buffer (mol L
-1
) =
moles of acid needed to decrease the pH by 1 unit
volume of buffer used (L)
Base capacity of a buffer (mol L
-1
) =
moles of base needed to increase the pH by 1 unit
volume of buffer used (L)
Generally, buffers will maintain a relatively constant pH as long as both HA and A
-
are present in the
solution in significant concentrations and if a substantial volume of the buffer is used. The capacity of
the buffer, therefore, is determined by the amounts (moles) of HA and A
-
present in the solution. If one
of these species has been virtually completely neutralized through the addition of OH
-
or H3O
+
, then the
buffer is considered to be exhausted and a dramatic pH change will be observed if additional OH
-
or
H3O
+
is added to the solution.
The action of a buffer solution is visible in the titration curve of a weak acid and a strong base. In
Figure 2 in Experiment H, the pH changes only by about 2 units between the addition of 5 and 20 mL
of 0.50 M OH
-
. In this region, the solution in the titration flask is a buffer solution.
1/2 Eq. vol.
Eq. vol. = 25.0 mL
Eq. Pt.
Titration of HA using 0.50 M NaOH
Volume of NaOH added (mL)
30
20
10
0
pH
14
12
10
8
6
4
2
0
BUFFER REGION
small
change
in pH
