Potentiometric DO detection in water by ceramic sensor based on sub-micron RuO2 sensing electrode

Abstract

An alumina sensor using sub-micron RuO2 sensing electrode (SE) was fabricated and examined for potentiometric dissolved oxygen (DO) detection in water at a temperature range of 9–35 °C. The electromotive force (emf) response at these temperatures was linear to the logarithm of DO concentration in the range from 0.6 to 8.0 ppm (log[O2], −4.71 to −3.59). RuO2-SE displays a Nernstian slope of −41 mV per decade at pH 8.0. It was also found that the response/recovery time to DO changes were sluggish as the water temperature cools down. Response time T90 to DO changes increased from 8 min at a temperature of 23 °C to about 30 min at a temperature of 9 °C. The proton conductivity of hydrous RuO2 appears to be due to the dissociative adsorption of water and the formation of acidic OH groups in Ru (III,IV) cluster ions. In strong alkaline solutions, the sensor’s emf exhibited a mixed potential of fast and slow electrochemical reactions involving DO, RuO42− and OH− ions. The results also revealed that as pH of the solution increases to pH 10.0–13.0, the response/recovery rate becomes faster, stabilizing more or less quickly depending upon the solution alkalinity. Scanning electron microscopy, energy dispersive X-ray-analysis and impedance spectroscopy techniques were used to examine respectively the morphology, crystalline structure and electrochemical behaviour of sub-micron RuO2 oxides.