21.4. Acid-Base Neutralization http://www.ck12.org
Practice Problem- What is the concentration of a Ba(OH) 2 solution, if 17.25 mL is required to neutralize 19.10 mL of 0.520 M
HBr?
Titration Curves
As base is added to acid at the beginning of a titration, the pH rises very slowly. Nearer to the equivalence point,
the pH begins to rapidly increase. If the titration is a strong acid with a strong base, the pH at the equivalence point
is equal to 7. A bit past the equivalence point, the rate of change of the pH again slows down. Atitration curve
is a graphical representation of the pH of a solution during a titration. The figure below (Figure21.11) shows
two different examples of a strong acid-strong base titration curve. On the left is a titration in which the base is
added to the acid, so the pH progresses from low to high. On the right is a titration in which the acid is added to the
base. In this case, the pH starts out high and decreases during the titration. In both cases, the equivalence point is
reached when the moles of acid and base are equal, and the pH is 7. This also corresponds to the color change of the
indicator.
FIGURE 21.11
A titration curve shows the pH changes
that occur during the titration of an acid
with a base. On the left, a strong base
is being added to a strong acid. On the
right, a strong acid is being added to a
strong base. In both cases, the pH has a
value of 7 at the equivalence point.Titration curves can also be generated for weak acid-strong base or strong base-weak acid titrations. The general
shapes are similar, but the graph has additional inflection points, and the pH at the equivalence point is not exactly
- In a weak acid-strong base titration, the pH is greater than 7 at the equivalence point, and it is less than 7 at the
equivalence point for a strong acid-weak base titration.
Indicators
As noted earlier, an acid-base indicator is a substance that displays different colors when in the presence of an acid
or a base. How does that work? An indicator is a weak acid that ionizes within a known pH range, usually about
2 pH units. We can represent the protonated form of the indicator molecule as HInd and the deprotonated form as
Ind−. The following equilibrium exists for the indicator.
? HInd(aq)
(colorless)⇀↽H+(aq)+Ind−
(pink)(aq)