The solution is a solvent free buffer solution containing stabilized pNPP using sodium azide as preservative. In kinetic assays p-nitrophenol appears yellow and is measured at 405 /650 nm. If stopped reactions are preferred, add equal amounts of 1.0 M sodium hydroxide solution and read the absorbance at 405 /650 nm. The stability life time is 3 years from date of manufacturing offering improved background characteristics.