Oxygen Ion--Conducting Ceramics: A New Application in High-Temperature--High-Pressure pH Sensors.

Membrane electrodes fabricated from yttria-stabilized zirconia, a representative oxygen ion-conducting ceramic, show a linear voltage response to pH over the range 3 to 8 at 285 degrees C and a pressure of 1200 pounds per square inch (82 atmospheres). Test units have been operated continuously at 285 degrees C without failure for periods as long as 9 days. Unlike sensors which are based on electron transfer couples, such membrane electrodes are insensitive to changes in the oxidation-reduction environment and, in turn, exert no influence upon the environment. Such ceramic membranes can therefore be used for the direct measurement of the pH of geothermal brines, of water in nuclear reactors, and in high-temperature thermodynamic studies on aqueous systems.