Abstract: Electrochemical sensors and methods for their use are provided for the detection of the pH and/or oxygen concentration in an aqueous medium. The subject sensors comprise a closed-end cylinder shaped fluoride based solid-state electrolyte having an inner and outer surface. In contact with the inner surface of the electrolyte is a solid reference electrode. In a preferred embodiment, the electrolyte is described by the formula La.sub.0.95 X.sub.0.05 F.sub.2.95, wherein X is an alkaline earth metal. In addition to finding use in the detection of the pH or oxygen concentration of a medium where one these factors is known, at least two of the subject sensors may be used in conjunction, where the reference electrode differs between the two sensors, to determine the pH and oxygen concentration of a medium where both of these values are unknown.

Abstract: An electrochemical sensor for determination of the activity of a certain metal in a metal or alloy being tested comprises a solid electrolyte consisting of a solid solution of strontium fluoride and lanthanum fluoride to which is added a dispersed phase of a fluoride of such certain metal, a reference electrode of known concentration of such certain metal placed in contact with one face of the electrolyte, an opposite face of the solid electrolyte being in contact with the metal or alloy being tested, and a device for measuring the chemical potential difference between the reference electrode and the metal or alloy being tested.