Chemistry Reference
In-Depth Information
HA
3
H
A
The equilibrium constant for this ionisation is given by
[H
]
[A
]
K
a
——-------
[HA]
Taking logarithms of both sides and separating the hydrogen ion term gives
[A
]
log
K
a
log[H
]
log —--
[HA]
Multiplication throughout by
1 gives
[A
]
log
K
a
log[H
]
log —--
[HA]
or
[A
]
p
K
a
pH
log —--
[HA]
which rearranges to give
[A
]
pH
p
K
a
log —--
[HA]
Since the acid in question is weak, the number of A
ions derived from
dissociation of the acid itself is very small compared with the number
derived from the fully ionised salt. This means that [A
] is approximately
equal to total concentration [SALT]; and similarly [HA], since the acid is
weak and predominantly unionised, is approximately equal to the total acid
concentration [ACID]. The equation can now be rewritten as
[SALT]
pH
p
K
a
log
—----
[ACID]
(1.7)
The Henderson-Hasselbalch equation can also be derived from considera-
tion of the ionisation of a weak base, B, which ionises in aqueous solution
as follows:
B
H
2
O
3
BH
OH
In this case the [SALT] term can be replaced by the concentration of the
conjugate acid of the weak base, [BH
], which, in effect, yields the same
equation.
