 The mixture is purified by passing it over copper at to remove the and the , and then KOH and are used to remove the acids and moisture by sorption.Radon is condensed by liquid nitrogen and purified from residue gases by sublimation.Radon commercialization is regulated, but it is available in small quantities for the calibration of 222Rn measurement systems, at a price, in 2008, of almost per milliliter of radium solution (which only contains about 15 picograms of actual radon at any given moment).