Phenol red (phenolsulfonphthalein or PSP), a water-soluble dye that is used to measure pH in laboratories.
The pH scale ranges from 0 to 14. A pH of 7 means neutral. pH of less than 7 is acidic. pH of more than 7 is basic.
Phenol red is red at neutral pH.
Phenol red is purple at basic or alkaline pH.
Phenol red is orange at acidic pH.