You can use a .45 micron, but when you want to say the solution is "Sterile" lab setting, .2 micron is used, if you search on Millpore or Whattman MFG sites, they list chemicals and temps and sterile factors of such filters to be used for research purposes. 0.2 micron should be used whenever possible, if .45 is all you got, it works too, but .2 is best.