Finding the End Point with Indicators

An indicator is an acid or a base whose An acid-base indicator is itself an acid or base whose various protonated species have 

Below pH 1.7, the predominant species is red between pH 1.7 and pH 8.9, the predominant species is yellow and above pH 8.9, the predominant species is blue (Color Plate 4). For simplicity, we designate the three species R, Y-, and B2.  

Acid-base indicator color changes are featured in Demonstration 11-1. Box 11-2 shows how optical absorption by indicators allows us to measure pH. 

One of the most common indicators is phenolphthalein, usually used for its colorless-to-pink transition at pH 8.0-9.6. 

In strong acid, the colorless form of phenolphthalein turns orange-red. In strong base, the red species loses its color.8 

---