Phenol red is widely used as a pH indicator in various biological and chemical applications, with a color transition from yellow at pH 6.8 to red at pH 8.4, and pink to dark red above pH 8.4. In cell culture, it helps monitor the pH of the medium, turning yellow under acidic conditions and pink to red in alkaline environments. It is also employed in microbiology to detect pH changes in bacterial growth media and in water quality testing to measure acidity or alkalinity. Additionally, phenol red is used in medical diagnostics, such as in the phenol red thread test to assess tear production and diagnose dry eye syndrome. Its color-changing properties make it valuable in titration experiments and as a visual indicator in laboratory settings.