A dual-ion-selective electrode system for real-time monitoring of dissolved ammonia

Abstract

A robust, all-solid-state potentiometric sensor was developed for the selective detection of dissolved ammonia (NH₃) in aqueous and gas-equilibrated environments. The sensor design is based on a coupled configuration of a nonactin-based ammonium-selective electrode (NH₄⁺-ISE) and a hydrogen ion-selective electrode (H⁺-ISE), enabling direct measurement of NH₃ activity through the equilibrium: NH₄⁺ ⇌ NH₃ + H⁺. The resulting electrochemical cell exhibited a near-Nernstian response over a wide dynamic range, with a detection limit below 10 ppm and a response time under 6 seconds. In contrast to conventional membrane-based gas sensors, the dual-electrode system showed minimal signal drift and eliminated the need for gas-permeable membranes or internal filling solutions. Sensor performance was evaluated under various pH and ionic strength conditions, confirming matrix-independent behavior and suitability for direct application in complex environmental samples such as seawater and wastewater. The sensor also demonstrated excellent reversibility and real-time monitoring capability during dynamic NH₃ fluctuation experiments in a freshwater aquaculture system, successfully tracking diurnal changes linked to photosynthetic and respiratory activity. A comparison with a commercial Severinghaus-type ammonia gas probe revealed significantly enhanced stability, faster response, and improved reproducibility for the proposed device. This dual-ion-selective electrode system offers a practical and high-performance platform for on-site NH₃ detection in environmental, aquacultural, and biological monitoring applications.