Phenol Red sodium salt is widely used as a pH indicator in biological and cell culture applications. It changes color in response to pH, typically appearing yellow below pH 6.8, red at neutral pH around 7.4, and pink to purple above pH 8.2. This makes it ideal for monitoring the pH of culture media, especially in cell and tissue culture, where maintaining proper pH is critical for cell health. It is commonly added to cell culture media to visually indicate metabolic activity—such as acid production from glucose metabolism—allowing researchers to assess cell growth and medium condition without additional testing. Due to its sensitivity and reversible color change, it is also used in various diagnostic procedures and biochemical assays where real-time pH monitoring is needed. Despite its utility, Phenol Red has weak estrogenic activity, so it is sometimes omitted from media used in hormone-sensitive experiments or long-term endocrine studies.