o-Cresolphthalein is primarily used as a pH indicator in various chemical and biological applications. It is commonly employed in titrations to determine the endpoint of acid-base reactions, particularly in the pH range of 8.2 to 9.8, where it changes color from colorless to pink. This property makes it useful in laboratory settings for analyzing the acidity or alkalinity of solutions. Additionally, it is utilized in clinical diagnostics, particularly in the measurement of calcium levels in blood serum. In this context, it forms a colored complex with calcium ions, allowing for quantitative analysis through spectrophotometric methods. Its stability and distinct color change make it a reliable tool in both research and medical laboratories.