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G-6
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 be completely neutralized by a base in a titration reaction. This process can be
explained by Le Châtelier’s Principle. As the concentration of H3O
+
decreases in solution, because these ions
are being neutralized by the addition of OH
-
, more of the weak acid will dissociate to compensate for this shift
in equilibrium. The dissociation of the weak acid will continue until all of the HA has dissociated and all the
H3O
+
ions are neutralized.
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)
K
in
=
[H3O
+
][In
-
]
[HIn]
The color of an indicator is sensitive to the pH because HIn (acid form) and In
-
(base form) have different
colors. The color of the solution containing the indicator will depend on the relative proportions of the In
-
and
the HIn forms, which will determine the [H3O
+
]:
[H3O
+
] = K
in
x
[HIn]
[In
-
]
