o-Cresolphthalein complexone is widely used as a colorimetric reagent in analytical chemistry, particularly for the determination of calcium and magnesium ions in various samples. Its ability to form a stable, colored complex with these metal ions makes it a valuable tool in clinical diagnostics, where it is employed to measure calcium levels in blood and urine, aiding in the diagnosis and monitoring of bone and metabolic disorders. Additionally, it is utilized in water quality testing to assess hardness, which is crucial for industrial processes and environmental monitoring. The reagent’s sensitivity and specificity ensure accurate and reliable results in these applications.