Phosphatase substarte (4-Nitrophenyl phosphate disodium hexahydrate, p-nitrophenyl phosphate disodium hexahydrate) is widely used as a small molecule phosphotyrosine-like substrate in activity assays for protein tyrosine phosphatases. It is a colorless substrate that upon hydrolysis is converted to a yellow 4-nitrophenolate ion that can be monitored by absorbance at 405 nm.