Chemistry Reference
In-Depth Information
concentration in a similar way to pH
log [H
]. The term pOH is used
much less frequently in the literature than pH but it follows that if pOH for
0.01
M
NaOH
2 and pH
pOH
14, the pH of the solution in the
example above
12.
The buffer capacity (
b
) for this buffer can now be calculated as
No. of moles of NaOH added
b
——————-------------
Change in pH observed
0.01
b
——-----
(4.79
4.7)
0.01
b
——
0.09
b
0.11
Since buffer solutions work best at a pH equal to the p
K
a
of the acid or base
of which they are composed, consideration of the p
K
a
will determine choice
of buffer for a given situation. The p
K
a
of acetic acid is 4.7, and therefore
an acetic acid-acetate buffer would be useful for buffering a solution to a
pH of approximately 5. Similarly, an alkaline buffer can be obtained by
using ammonia solution, which will buffer to a pH of approximately 10
(p
K
a
of ammonia
9.25).
If a buffer is required to control the pH of a neutral solution, use is
made of the second ionisation of phosphoric acid. Phosphoric acid is a
triprotic acid, which requires three equivalents of NaOH as follows:
H
3
PO
4
NaOH
3
Na
H
2
PO
4
H
2
O
p
K
a
2.12
Na
H
2
PO
4
3
(Na
)
2
HPO
2
H
2
Op
K
a
NaOH
7.21
(Na
)
2
HPO
2
NaOH
3
(Na
)
3
PO
3
H
2
Op
K
a
12.67
A mixture of sodium dihydrogenphosphate, Na
H
2
PO
and disodium
hydrogenphosphate, (Na
)
2
HPO
2
, will function as a buffer and control
the pH to a value of approximately 7.0. In this example, the species with
the greater number of available hydrogen atoms functions as the acid,
i.e. Na
H
2
PO
4
, while the (Na
)
2
HPO
2
functions as the salt.
The choice of buffer to use in a given situation therefore depends on
the p
K
a
of the acid or base involved. As a general rule, buffer solutions work
well within plus or minus one pH unit of the p
K
a
. Beyond these values, the
buffer capacity is too small to allow effective buffer action.
