I think I used a dilute solution in EtOH with a few drops conc. HCl. I dissolved it first in conc. HCl then add EtOH. Its not very critical with concentration. I should say that it is a good thing to be carefull and keep it moist. I also had a very old 250g jar of mCPBA that was 100% pure, no water or chlorobenzoic acid added. This is dangerous and it was before people realize that it should be kept moist and dilueted.