BCIP, toluidine (5-Bromo-4-chloro-3-indoxyl phosphate, p-toluidine salt) is a widely used chromogenic phosphatase substrate, which forms a dark blue (λmax 615 nm) precipitate on enzymatic hydrolysis. It is often used with the oxidant NBT (nitro blue tetrazolium chloride), which facilitates the precipitation.  Store at 4°C and protect from light C15H15BrClN2O4P MW: 433.6 [6578-06-9]