Alternative coulometric signal readout based on a solid-contact ion-selective electrode for detection of nitrate

Traditional potentiometric NO₃^--selective electrodes suffer from a fundamental limitation of the Nernst slope (59.1 mV/dec at 25 °C) due to the relationship between the potential and the logarithmic of ionic activity. Herein, a coulometric signal readout is proposed instead of the potentiometric response for detection of NO₃^- based on an ordered mesoporous carbon (OMC)-based solid-contact ion-selective electrode (ISE). The mechanism for obtaining the coulometric signal is based on the electrical double layer capacitance of OMC compensating the potential change at the ion-selective membrane/solution interface during the measurements under the control of a constant applied potential. Under the optimized conditions, the coulometric signal for the OMC-based solid-contact NO₃^--ISE shows two linear responses in the activity range of 1.0 × 10^-6-8.0 × 10^-6 M and 8.0 × 10^-6-8.0 × 10^-4 M, and the detection limit is 4.0 × 10^-7 M (3σ/s). The proposed coulometric response also shows excellent reproducibility and stability in the presence of O₂ and CO₂ and light on/off. Additionally, the coulometric response shows acceptable and reliable results for detection of NO₃^- in mineral water as compared to the traditional potentiometric response and the ion chromatography. This work provides a promising alternative signal readout for detection of ions by using solid-contact ion-selective electrodes.