o-Cresolphthalein Complexone is primarily used as a colorimetric reagent in analytical chemistry for the determination of calcium and magnesium ions in various samples. It forms a distinct colored complex with these ions, allowing for their quantification through spectrophotometric methods. This reagent is particularly valuable in clinical diagnostics for measuring calcium levels in blood and urine, aiding in the assessment of bone health and metabolic disorders. Additionally, it is employed in water quality testing to monitor hardness, which is influenced by the presence of calcium and magnesium. Its sensitivity and specificity make it a reliable tool in both laboratory and industrial settings for precise ion analysis.