7.21: Indicators

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"Boil Them Cabbage Down" is an old bluegrass song. Many people enjoy the music, but chemistry students also enjoy the product of boiled cabbage. Extracting the anthocyanin dye from cabbage leaves with boiling water gives a solution that is red when acidic, purple when neutral, and green to yellow when basic.

Indicators

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 \(\ce{HIn}\) and the deprotonated form as \(\ce{In^-}\). The following equilibrium exists for the indicator:

\[\ce{HIn} \left( aq \right) \rightleftharpoons \ce{H^+} \left( aq \right) + \ce{In^-} \left( aq \right)\nonumber \]

According to Le Chatelier's principle, the addition of \(\ce{H^+}\) ions (as in a low pH solution) drives the equilibrium to the left and the protonated \(\ce{HIn}\) predominates. The addition of \(\ce{OH^-}\) (as in a high pH solution) decreases the \(\ce{H^+}\) concentration and drives the equilibrium to the right and the deprotonated \(\ce{In^-}\) 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 shows a variety of acid-base indicators that can be used in titration experiments.

Figure \(\PageIndex{1}\): pH indicators. (CC BY-NC; CK-12)

Depending on the pH at the equivalence point, the appropriate indicator must be chosen. For example, bromphenol blue has a yellow color below a pH of about 3 and a blue-violet color above a pH of about 4. Bromphenol 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. Instead, it could be used for a strong acid-weak base titration, where the pH at the equivalence point is lower.

Most indicators have two colored forms. Universal indicator displays the entire rainbow of colors from low pH to high pH (see figure below). Universal indicator is used to make pH paper, which can be used to quickly test solutions for their approximate pH.

Figure \(\PageIndex{2}\): Universal indicator tape. (CC BY-NC; CK-12)

Summary

An acid-base indicator is a substance that displays different colors when in the presence of an acid or a base.
Examples of acid-base indicators are given.