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, typically appearing yellow below pH 6.8, red at neutral to slightly alkaline pH (around 7.4 to 8.2), and pink to magenta above pH 8.2. This property makes it ideal for monitoring pH changes in cell culture media, where maintaining proper pH is critical for cell growth and viability. It is commonly added to cell culture media to visually indicate the health of the culture—yellow suggests excessive acidity due to contamination or overgrowth, while a deep pink color may indicate the medium has become too alkaline. Phenol red is also used in diagnostic procedures, such as the phenol red thread test, which measures tear secretion in ophthalmology to assess dry eye conditions. Its non-toxic nature at working concentrations and clear visual response make it a valuable tool in both research and clinical settings.