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G-5
the equivalence point has been added. The inverse log of the negative of the pH value at this point will equal
the K
a
value (i.e. K
a
= 1 x 10
-pH
)
Note that this procedure cannot be used for a strong acid. The procedure for weak acids depends on the
existence of a point along the titration curve where [A
-
]
= [HA]. When a strong acid is added to water, it is
essentially all dissociated. There is no point along the titration curve of a strong acid where [A
-
]
= [HA]. The
concentration of undissociated HA is extremely small and cannot be accurately determined, so we will assume
that for strong acids, like HCl
(aq)
, [HA] is zero. For this reason, K
a
values are not defined for strong acids (the
denominator of the relation is essentially zero), therefore, the K
a
value, by definition, would approach infinity.
NOTE: It is important to understand that the terms “strong” and “weak” do not refer to the
concentrations of the species in solution, but only to the extent of dissociation in water.
A solution of a weak acid will indirectly react, (be neutralized), with a strong base in a titration reaction. This
process can be explained by Le Châtelier’s Principle. The strong base reacts with hydronium ions, H3O
+
(aq)
.
As the concentration of H3O
+
(aq)
decreases in solution, more of the weak acid will dissociate to compensate for
this stress on the equilibrium. Owing to the large equilibrium constant for the reaction of OH
-
(aq)
with H3O
+
(aq)
,
at the equivalence point, the dissociation, (neutralization), of the weak acid is essentially complete.
III. Acid-Base Indicators
Often you might know the identity of the acid in solution, but not its concentration. To determine the
concentration, you will need to titrate the sample of acid to its equivalence point using a standardized base
solution. If you know how the pH curve is changing in the vicinity of the equivalence point (for example see
Figures 1 and 2), these titrations can be carried out without a pH meter. Instead, an acid-base indicator can
be used.
Acid-base indicators are weak organic acids that have conjugate bases of a different color. They change color
in specific pH ranges, in fact, this color change occurs when the pH is close to the pK
a
value of the indicator,
pK
in
. The sharp change in color of the indicator signals the end point of the titration. More details are given
in your text book.
There are a number of different indicators and it is important to select the proper one for the particular titration.
For the end point to be useful, it must occur at a volume of titrant very close to that of the equivalence point of
the titration and the color change of the indicator must be dramatic enough to be detected. Since indicators are
weak acids, they are only partially dissociated in aqueous solution.
HIn
(aq)
+ H2O
(l)
H3O
+
(aq)
+ In
-
(aq)
(9)
