Question: how to prepare dpph solution?
Answer: Hence to make a 0.1 mM solution , you will need to dissolve 39.4 mg of DPPH in methanol/ethanol. A simple way to check is for 1 M in 1000 mL you need 394 g, for 1 mM you need 394 mg, so for 0.1 mM you would require 39.4 mg. However, I would suggest making a higher stock 10 mM and then dilute it before use.