Buffer Class Notes – M Jeffrey
A buffer solution maintains a nearly constant pH of a system even when a small amount
of a strong acid or base is added to the system.
Buffer solutions are used to calibrate pH meters as well as to control the rate and extent
of a reaction that is pH sensitive.
Human blood pH must be between 7.30 and 7.40 in order to carry blood to our cells. If
our blood was not buffered, the acid absorbed from a glass of orange juice could be lethal.
The buffering system in our blood involves HCO3- ions and H2CO3.
Another example is a buffered aspirin.
Common Ion Effect
When a reaction has reached equilibrium and an outside source adds one or more of the
ions that is already in solution, the reverse reaction will occur at a faster rate to
reestablish equilibrium. (Le Chatelier’s Principle)
“The common ion” is the one already present in equilibrium before a substance is added
that increases the concentration of that ion.
Na+ (aq) + Cl- (aq)
NaCl
The addition of 12M HCl adds both H+ and Cl-, both at a concentration of 12M. Why?
The reaction will shift left causing NaCl to come out of solution.
A buffer solution combines a weak acid or a weak base with one of its salts. This
produces a common ion effect where both the acid (base) and its conjugate are present in
comparable quantities.
Examples:
A weak acid and its salt:
HF and NaF
A weak base and its salt: NH3 and NH4Cl
A typical buffer solution is one made from acetic acid (HC2H3O2) and sodium acetate.
HC2H3O2
H+
+ C2H3O2-
NaC2H3O2  Na+ + C2H3O2 A buffer solution with a pH in the acidic range (1-7) can be made from a solution of a
weak acid and its salt.
A buffer solution with a pH in the basic range (7-14) can be made from a solution of a
weak base and its salt.
If a small amount of a strong acid added to a buffered solution, most of the added H+ will
combine with an equivalent amount of weak base of the buffer, to form the conjugate
acid of that weak base.
If a small amount of a strong base added to a buffered solution, most of the added OHwill combine with an equivalent amount of weak acid of the buffer, to form the conjugate
base of that weak acid.
The key to a buffer solution is that the weak acid (or base) only dissociate to a small
degree. The salt on the other hand is completely dissociated.
The resulting buffer solution has a large proportion of weak acid (HC2H3O2), a large
portion of the common ion (C2H3O2-) and a small proportion of the H+ ion.
HC2H3O2
a lot
H+ + C2H3O2a little
a lot
The buffering capacity (or the effectiveness of the buffer solution) depends on the
amount of acid and conjugate base from which the buffer is made. The larger the amount,
the larger the buffering capacity.
A typical notation for a buffering system would be:
CH3COOHNa/ CH3COOH
Text Page 719 16.2
or CH3COO-/ CH3COOH