A Robust Sensor for Inline pH Measurements
Real-time accurate pH monitoring is crucial in pharmaceuticals, breweries, agriculture, and water treatment industries. Traditional glass-bulb pH sensors use an ion-selective glass membrane and a liquid-based reference electrode (RE). The pH sensor operates by allowing the hydrogen ions from the test solution to exchange with sodium ions bound to silicate groups on the glass membrane. In the process, it generates a small voltage. This voltage reflects the pH of the solutions. These sensors typically follow Nernstian behavior, providing fast response and high repeatability over a pH range of 1–14. However, they are fragile and prone to wear in harsh conditions, which reduces sensitivity over time.
This study presents the fabrication and characterization of an innovative all-solid-electrode pH sensor that promises stable and accurate performance even in demanding and challenging conditions.
The proposed sensor replaces fragile glass bulbs with robust metal oxides like antimony (Sb) as the sensing electrode. Metal oxides are stable, resist ion interference, are cost-effective, and perform reliably in harsh conditions with fast response and long lifespan, making them ideal for sensing applications.
The pH sensor utilizes a 99.8% pure antimony (Sb) rod as the sensing electrode. The solid reference electrode (SRE) is made by coating an Ag/AgCl (silver/silver chloride) electrode with Potassium chloride (KCl)-saturated polyvinyl chloride (PVC) and pure PVC. This design ensures a stable, analyte-independent potential, which is essential for accurate pH measurement. PVC is chosen for its durability, chemical resistance, and proven performance in ion-selective applications.
Both the antimony and solid reference electrodes are encased in a 3D-printed housing made of a robust, chemically inert polymer. The sensing electrode extends from the main casing, while the reference electrode is housed in a separate, detachable compartment, allowing easy replacement. This design ensures the protection of the electrodes and enables their immersion in the test solution.
We evaluated the sensor's performance by measuring its Open Circuit Potentials (OCPs) in standard buffer solutions ranging from pH 3.94 to 9.04. The sensor showed a stable and linear response, with a sub-Nernstian sensitivity of −43.18 mV/pH. It maintained consistent OCP readings over time, exhibited minimal hysteresis (~5 mV) during titration cycles, and stabilized within 60 seconds after pH changes. Repeated testing confirmed its repeatability, and long-term evaluation showed less than ±0.15 pH variation over four days, demonstrating strong stability.
Beyond laboratory validation, the sensor’s efficacy has been demonstrated in real-world applications. It was successfully employed to monitor the pH of idli batter during its fermentation, a process where precise pH control is crucial for consistent product quality. Furthermore, its application in soil pH analysis proved highly accurate, with readings showing an error of less than ±0.15 pH compared to conventional laboratory pH meters.
The all-solid-electrode pH sensor offers a durable, maintenance-friendly solution ideal for continuous inline monitoring in industries like food processing, water treatment plants, pharmaceuticals, and agriculture. This innovative solution reduces maintenance requirements and minimizes contamination of the test medium, paving the way for more efficient and reliable pH monitoring across diverse applications.