Phenol red solution is widely used as a pH indicator in biological and microbiological applications. It changes color depending on the acidity or alkalinity of the solution, making it ideal for monitoring pH changes in cell cultures and microbiological media. It is commonly added to cell culture media to visually indicate the pH status—appearing yellow in acidic conditions, red at neutral pH, and pink to purple in alkaline environments. This allows researchers to quickly assess the health of cell cultures or detect microbial activity. Phenol red is also used in water quality testing and some clinical diagnostic procedures to detect pH shifts in samples. Its sensitivity in the physiologically relevant pH range (around 6.8 to 8.4) makes it especially valuable in laboratory settings involving living cells or tissues.