Phenol red solution is widely used as a pH indicator in biological and medical applications. It is commonly added to cell culture media to monitor pH changes, where it shifts from yellow in acidic conditions to red in neutral and pink to purple in alkaline environments. This color change allows researchers to quickly assess the health and metabolic activity of cells in culture. It is also used in diagnostic tests, such as the detection of bacterial enzyme activity or in assays measuring carbon dioxide production. Due to its sensitivity and reversible color transition, phenol red is a reliable tool for real-time pH monitoring in liquid media without the need for electronic instruments.