http://www.ck12.org Chapter 21. Acids and Bases
According to Le Châtelier’s principle, the addition of H+ions (as in a low pH solution) drives the equilibrium to
the left and the protonated HInd predominates. The addition of OH−(as in a high pH solution) decreases the H+
concentration and drives the equilibrium to the right, so the deprotonated form Ind−predominates. To be useful as
an indicator, the two forms must be different colors. In the case of phenolphthalein, the protonated form is colorless,
while the deprotonated form is pink.
The figure below (Figure21.12) shows a variety of acid-base indicators that can be used in titration experiments.
FIGURE 21.12
Indicators are substances that change
color at a certain pH. A wide variety of
indicators is available.Depending on the pH at the equivalence point, the appropriate indicator must be chosen. For example, bromophenol
blue has a yellow color below a pH of about 3 and a blue-violet color above a pH of about 4. Bromophenol blue
would not be a good choice as the indicator for a strong acid-strong base titration, because the pH is 7 at the
equivalence point. However, it could be used for a strong acid-weak base titration, where the pH at the equivalence
point is lower.
Most indicators, as shown above (Figure21.12), have two colored forms. A universal indicator makes use of
a mixture of indicators so that an entire rainbow of colors is displayed when going from low pH to high pH (
Figure21.13). A universal indicator is used to make pH paper, which can be used to quickly test solutions for their
approximate pH.
Some naturally occurring substances act as acid-base indicators. Red cabbage juice, for example, can be used as a
natural indicator (Figure21.14).
Lesson Summary
- Acids and bases neutralize each other, forming a salt and water. A strong acid-strong base neutralization
results in a neutral solution with a pH of 7. - A titration is an experiment in which a controlled acid-base neutralization reaction is used to determine the
unknown concentration of an acid or a base. The equivalence point is reached when the number of hydrogen
ions is equal to the number of hydroxide ions. - A titration curve shows the pH of a solution during the course of a titration.
- Acid-base indicators are used in a titration in order to detect the end point of the titration.