Why would you want to adjust the pH?
When you gas do it for a little bit then freeze, if you didnt over gas the solution after filtering out all of the crystals the NP wont be acidic, or wont be very acidic anyway....
even so the small amount of acidity will combine with the alkalinity of the freebase to produce salt, just needs to be gassed a little more to see them fall out of solution....
any water will affect the x-tal process, so just dont add anything containing water...
-AC