Fabrication of thick film sensitive RuO2-TiO2 and Ag/AgCl/KCl reference electrodes and their application for pH measurements

• Pastes for RuO2-TiO2 sensitive and thick film reference electrodes were developed.
• Thick film potentiometric and conductimetric pH sensors were fabricated.
• Microstructure, composition, potential and impedance of sensors were investigated.
• Dependence of sensor surface composition on pH was proved by XPS studies.
• Influence of interfering ions and sensor application in water were discussed.

In this work thick film potentiometric and conductimetric sensors based on mixed metal oxides RuO2 and TiO2 were fabricated and investigated. For potentiometric sensors, pH sensitive RuO2-TiO2 electrode and a reference Ag/AgCl/KCl electrode were developed and deposited on an alumina substrate by screen printing method. For conductimetric pH measurements, the sensitive RuO2-TiO2 paste was screen printed on the surface of an interdigitated conducting electrode. The results of the potentiometric, impedance spectroscopic, SEM, EDS, XRD and XPS studies of the fabricated sensors were presented and discussed. The thick film pH sensor with both the reference and sensitive electrodes screen printed on the same alumina substrate shows a sensitivity of 56.11 mV/pH, close to the theoretical value, a very fast response shorter than 15 s, long lifetime and reusability. Its applicability for measurements of pH of water was confirmed. The electrochemical impedance spectroscopy method was used for characterization of the properties of the sensitive RuO2-TiO2 film in a frequency range of 10 Hz–2 MHz. The sensor conductance was found to show a linear dependence on the pH value in the range of 4–11. Comparison of X-ray photoelectron spectra of the sensors before and after treatment in various solutions revealed some differences in surface composition dependent on pH value. The cost of the fabricated thick film pH sensor was significantly reduced by using for the sensitive electrode besides RuO2 a much cheaper second oxide TiO2, and due to a simple method of the reference electrode preparation.