Phenolphthalein Disulfate Potassium Salt Hydrate is a chromogenic substrate primarily used in biochemical and enzymatic assays for the detection and quantification of sulfatase enzymes, such as arylsulfatase. In these assays, the enzyme hydrolyzes the sulfate groups, releasing free phenolphthalein, which undergoes a color change from colorless to pink in alkaline conditions (pH 8.2–10.0). This visible or spectrophotometric change (absorbance at 540 nm) enables precise measurement of enzyme activity. It is widely applied in research, diagnostic kits, and pharmaceutical development for studying sulfatase-related processes. Its solubility, stability, and sensitivity make it an essential reagent in specialized laboratory protocols.