I want to prepare PNP solution to analyze acid and alkaline phophatase activity in soil, and for p-nitropheny phosphate solution i could not find the Disodium p-nitropheny tetrahydrate
Yes, you can use the hexahydrate salts also, but for calculation purpose the molecular weight of hexahydrate salts (371.15) you have to consider. We are regularly using it in our laboratory and getting excellent results.