Sodium 1H-indol-3-yl phosphate (3-indoxyl phosphate sodium salt) is a specialty chemical primarily utilized as a chromogenic substrate for alkaline phosphatase (ALP) and other phosphatases in biochemical assays. Enzymatic dephosphorylation releases 3-indoxyl, which rapidly oxidizes and dimerizes to form an insoluble indigo blue precipitate, enabling high-sensitivity detection in histochemical and cytochemical staining, immunohistochemistry, ELISA, Western blotting, and other enzyme-linked techniques. It is also used in enzymatic studies of phosphate hydrolysis and as a synthetic intermediate for certain indole derivatives in pharmaceutical research and medicinal chemistry.