The Control of Oxygen Activities in Argon-Oxygen Mixtures by

High-temperature electrochemical cells based on stabilized zirconia as a solid electrolyte with porous platinum electrodes may be used either to monitor oxygen activities by open-circuit measurements or else to pump oxygen into or out of stationary or streaming as mixtures. In this work, a feed-back control unit (potentiostat) in combination with a zirconia-base solid electro-lyte tube was used to simultaneously control and measure the oxygen partial pressure in a flowing inert carrier gas (argon). The oxygen activity of "tank " argon was controlled to desired levels between 2 • 10-2 and 10-16 atm oxygen. A four-electrode double cell arrangement is shown to be superior to a three-electrode arrangement. The ZrO2-base electrolytes exhibit a Po2-independ-ent predominant oxygen ion conduction over a range of oxygen pressures from above 1 atm to about 10-25 atm (depending on temperature). The oxygen ion con-ducting electrolytes may be used in combination with two separated metall ic (generally platinum) elec-trodes, which are at equil ibrium with different oxygen partial pressures to form a high-temperature galvanic cell as follows solid j P02', Pt oxygen ion conducting Pt, P02" electrolyte [I] where P02 ' and P02 " represent the oxygen partial pres-sures at the cathode and anode, respectively, with Po2 ' < Po2". At open circuit, the voltage of this eel1 indicates the ratio in oxygen chemical potentials at the electrodes according to the equatio