How about adding an excess of Cu metal to dilute (38%) nitric acid, filtering under suction to remove unreacted Cu and other insoluble junk, then
evporate until crystals form? I've tried it; it works! Just beware of the noxious fumes of NO2- literature states that only concentrated nitric acid
produces brown NO2 fumes; but from my experience, even dilute (38%) nitric acid produces NO2 when it reacts with Cu. |