PURPOSE: To use a titled apparatus under high temp. and pressure by providing an indicator electrode for measuring a hydrogen concn. having constitution wherein an internal electrode and an internal electrolyte are contained in a vessel which is formed of an H+-sensitive glass or an oxygen ion-conductive solid electrolyte, and using a specified metallic composite for a reference electrode.

CONSTITUTION: In the hydrogen ion concn. measuring apparatus, an indicator electrode 10 is a hollow body of a hydrogen ion-sensitive membrane 10A, and is made of a hydrogen ion-sensitive glass membrane or stabilized zirconia. An internal electrode 11 of silver coated with silver chloride or the like and an electrolyte 12 such as a KCl soln. etc. are packed in the hollow sensitive membrane body 10A. Whereas, a reference electrode 20 is constituted of silver or silver alloy or a metallic composite 20 coated with silver (alloy) on the surface, and is covered with an insulating film 21. Since the electrodes 10 and 20 are constituted to resist high temp. and pressure, the direct pH measurement of an aq. soln. such as the primary cooling water or the boiler water in a nuclear power plant at high temp. and pressure can be made possible.