Indicators as Weak Acids

[Jacqueline Antoun 2D]

Why do indicators have to be weak acids?? Can they also be weak bases??

[Shannon Han 2B]

I think it's because the release of the H+ from the weak acid results in a structural change. The course reader says it absorbs light differently. Therefore, you need the release of an H+ for the indicator to be effective and that's why they are weak acids.

[zhajee]

Indicators can be either weak acids or weak bases. Taking a weak acid as an example, the acid and its conjugate base have different colors. At a low pH, the concentration of H3O+ is high and so equilibrium lies to the left. When at a high pH, the concentration of H3O+ is low and equilibrium lies to the right, so the color will change accordingly. Indicators can't be strong acids/bases because they dissociate completely and reversible equilibrium cannot be introduced so the color won't change.

[David Y]

The release of H+