1
BUFFERS AND BUFFER CAPACITY
By Dr. Jan Pláteník
Principle of buffering
A buffer solution is a solution that resists changes in pH either when diluted or when limited
amounts of acid or base are added to it. Such a solution can be prepared by combining a
weak acid and its salt with a strong base (conjugated base) or, analogously, a weak base
and its salt with a strong acid (conjugated acid).
For example:
Acetate buffer: CH
3
COOH
(the weak acid)
+ CH
3
COONa
(the salt, conjugated base)
Phosphate buffer: NaH
2
PO
4
(the weak acid)
+ Na
2
HPO
4
(the salt, conjugated base)
Tris buffer:
(Tris: Tris [2-amino-2-(hydroxymethyl)-propan-1,3-diol)], an organic base)
The Henderson-Hasselbalch equation describes the behavior of such a buffer and for the
mixture of a weak acid and its salt with a strong base (conjugated base) it has the form:
pK
a
negative logarithm of the dissociation constant for the weak acid,
c
s
substance concentration of the salt (conjugated base),
c
ac
substance concentration of the weak acid (conjugated acid).
Graphically the Henderson-Hasselbalch equation plotted as the acid : conjugated base ratio
vs. pH of buffer actually constitutes the titration curve of the weak acid (see figure on the
next page).
Note also that for the acid : base ratio 1:1 the pH of buffer just equals the pK
a
(this is valid
for uni-univalent systems, e.g., CH
3
COOH/CH
3
COONa, NaH
2
PO
4
/Na
2
HPO
4
).
ac
s
a
c
c
logpKpH +=
