A potentiometric sensor includes a housing, in which a reference half cell space and a separated, measuring half cell space are formed. The reference half cell space contains a reference electrolyte and at least one part of a first potential sensing electrode for sensing a reference potential, and the measuring half cell space is liquid-tightly sealed by a measuring membrane, and contains an inner electrolyte and at least one part of a second potential sensing electrode for sensing a measuring half cell potential. A passageway extends through a wall of the housing, and opens into the reference half cell space, and is sealed relative to a medium surrounding the housing. The potentiometric sensor has a way for producing through the passageway an electrolytic contact between the reference electrolyte and the medium surrounding the housing.