Abstract: The present invention relates to an electrode for measuring the nitrate concentration in aqueous solutions. The electrode consists of a container with an internal reference system and a membrane which is a combination of an organic nitrate sensor phase of a quaternary ammonium salt as electroactive material, an organophilic, hydrophobic solvent for the quaternary ammonium salt with substantial viscosity and polyvinyl chloride or polyurethane as non-porous material. The quaternary ammonium salt is chosen from the group consisting of: tetraoctyl ammonium nitrate, tetranonyl ammonium nitrate, tetradecyl ammonium nitrate, tetraundecyl ammonium nitrate, tetradodecyl ammonium nitrate, tetratridecyl ammonium nitrate and tetratetradecyl ammonium nitrate; and the solvent consists of dialkylphthalate or a dialkyladipate of a solubility parameter of 9 .+-. 2 (cal/cm.sup.3).sup.1/2.