Dissolve in water and keep solution at 4 °C for a few weeks. Run and deduce a blank for each of your assays. If the blank turns yellow (A410 >0,05), make fresh solution. Most likely, the solution will keep better at - 20 °C, but keeping it at 4 °C avoids th hassle of thawing it each time you need it. Also, don't forget that the pK of p-nitrophenol is 7.15, so that the extinction coefficient is highly sensitive to small pH variations around pH 7-8. For better accuracy and sensitivity, it is better to alkalinize by stopping the reaction with 1/10 vol 1 M sodium carbonate before reading. However, if you want to run kinetics with continuous assays, use chloro-nitrophenyl glucoside, since chloronitrophenol has a lower pK and is fully deprotonized at neutral pH

Thank you for your answer very informative