Question

Why do you use phenolphthalein as an indicator in titration of KHP and NaOH?

Answer 1

Because it works.

Explanation:

There are several other indicators we could conceivably use for this titration. Phenolphthalein is cheap, and widely available, and works for the purpose. I always used to like phenolphthalein inasmuch as its acidic form is COLOURLESS, and its endpoint is very recognizable from either direction.

Answer 2

Phenolphthalein is usually used since it is easy to detect visually a slight pink color from a colorless liquid solution.