# pH indicators

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**Understand how the colour change of pH indicators is related to acid-base chemistry.**

Alizarin yellow is a pH indicator which is yellow when protonated and blue when deprotonated.- Write a K
_{a}expression and equilibrium equation showing the dissociation of alizarin yellow, using the symbols HIn and In^{-}. - In terms of equilibrium concentration, what happens to the pH indicator at the titration endpoint?
- Explain, in terms of equilibrium, why the solution turns yellow when [H
_{3}O^{+}] is high.

- Write a K