The titration of a weak base (NH3) with a strong acid (HCl) is shown below. Note
that the
curve has
the same
shape as the weak acid-strong base curve, but it is inverted. Thus,
the regions of the curve have the same features, but the
pH decreases throughout
the process:
Curve for a weak base-strong acid
titration.
Titrating 40.00
mL of 0.1000
M NH3 with a solution of
0.1000 M HCl leads
to a curve whose shape is the same as that of the weak acid-strong
base curve,
but inverted. The midpoint of the buffer region occurs when [NH3] = [NH4+].
Methyl red is a suitable indicator
here.
1. The initial
solution is that of a weak base, so the pH starts out
above 7.00.
2. The
pH decreases gradually in the buffer region, where significant amounts of base
(NH3) and conjugate acid (NH4+) are
present. At the midpoint of the buffer region, the pH equals the pKa of the ammonium ion.
3. After
the
buffer region, the curve drops vertically to the equivalence point, at
which all the NH3 has reacted and the solution
contains only NH4+ and Cl-. Note that the pH at the equivalence
point is
below 7.00
because Cl- does not react with water
and NH4+ is
acidic:
NH4+(aq) + H2O(l)
⇌ NH3(aq)
+ H3O+(aq)
4. Beyond
the
equivalence point, the pH decreases slowly as excess H3O+
is
added.
For this titration also, we must be more
careful in choosing
the indicator
than for a strong acid-strong
base titration.
Phenolphthalein
changes colour too
soon and too slowly to indicate the equivalence point;
but methyl red lies on the steep portion of the curve and straddles the equivalence point, so
it is a perfect choice.
