Potentiometric sensor for measuring wellbore effluent

A potentiometric sensor for wellbore applications having a permanent aqueous contact between measuring and reference electrode, whereby contact is ensured by discharging the internal solution from the reference electrode directly onto a measuring (ion-sensitive membrane such as a glassbulb pH electrode, and protecting the reference junction of the reference electrode with water wet porous material, such as sintered glass or a membrane. The sensor is advantageously used with oil-water mixtures under high pressure and/or temperature.