Controlled atmospheres are employed in electrochemistry whenever oxygen, water, or other constituents of the air may interfere with the reaction under study. The problem is basically twofold: to supply and contain an atmosphere of suitable composition, and to exclude air. For our present purposes it is sufficient to confine the discussion to inert atmospheres: that is, gases devoid of oxygen and water (and sometimes nitrogen). The use of high-purity argon, helium, or nitrogen is most common.