Titrations of Weak Acids and
Weak Bases
Chapter 17
Titration of a Weak Acid with a
Strong Base
• The initial pH (initial
acid: use Ka of the
acid and the initial
concentration of the
acid to calculate this
pH (ICE Table)
• Major substance
present is the acid not
dissociated
Titration of a Weak Acid with a
Strong Base
• Between the initial pH and
the equivalence point
(buffer mixture): prior to
equivalence point, the
acid is neutralized, and its
conjugate base is being
formed
– Thus mixture contains a
mixture of the acid and
conjugate base
– To calculate pH:
• Henderson-Hasselbach
equation
Titration of a Weak Acid with a
Strong Base
• Half way between start and equivalence point is
the half-equivalence point
– At this point, the concentration of acid =
concentration of the conjugate base
• pH = pKa for the acid
• Major substances present:
– H3O+, anion of the acid, cation of the base, and undissociated
acid
Titration of a Weak Acid with a
Strong Base
• At each point below the equivalence point, the
pH of the solution during titration is determined
from the amounts of the acid and its conjugate
base present at that particular time.
– If [A-]>[HA], pH > pKa
– If [A-]<[HA], pH < pKa
Titration of a Weak Acid with a
Strong Base
• Unlike in the previous
case, the conjugate base
of the acid affects the pH
when it is formed.
• The pH at the equivalence
point will be >7 because
the conjugate base will be
a major substance
present since the acid and
base neutralize each
other
• Phenolphthalein is
commonly used as an
indicator in these
titrations.
Titration of a Weak Acid with a
Strong Base
• After the equivalence
point (excess base):
excess base is
present and only thing
that determines the
pH
Titration of a Weak Acid with a
Strong Base
With weaker acids,
the initial pH is higher
and pH changes near
the equivalence point
are more subtle.
3 differences between strong acidstrong base & weak acid-strong
base titrations
• The solution of the weak acid has a higher
initial pH than a solution of a strong acid of
the same concentration
• The pH change at the rapid-rise portion of
the curve near the equivalence point is
smaller for the weak acid than it is for the
strong acid
• The pH at the equivalence point is above
7.00 for the weak acid-strong base titration
Titration of a Weak Base with a
Strong Acid
• The pH at the
equivalence point in
these titrations is < 7.
– The conjugate acid is
still present enough to
decrease the pH
• Methyl red is the
indicator of choice.
Titrations of Polyprotic Acids
• In these cases there
is an equivalence
point for each
dissociation of a
proton.
• The number of
titration curves in the
graph tells how many
labile protons there
are that can be
removed from the
acid
Choosing Indicators
• Choose an indicator solution whose pKa is
closest to the pH at the equivalence point
• Because the pH changes so sharply with little
change in the concentration of the acid or base
added near equivalence point as long as the
indicator changes in that range it will work
• Choose an indicator that changes color on the
steep part of the curve