Novel CeO₂-based screen-printed potentiometric electrodes for pH monitoring

Nuclear waste repositories are being installed in deep excavated rock formations in some places in Europe to isolate and store radioactive waste. In France, the Callovo-Oxfordian formation (COx) is a possible candidate for nuclear waste storage. This work investigates the applicability of CeO(2)-based oxides (CeO(2), Ce(0.8)Sm(0.2)O(2) and Ce(0.8)Zr(0.2)O(2)) for monitoring the pH of the COx pore water (T=25°C). The study is limited to the pH range between 5.5 and 13.2, which includes the pH values that have been encountered or are anticipated in the COx formation during its evolution as radioactive waste repository due mainly to alkalinisation, an increase in salinity, and a decrease in redox potential. Screen-printing was done to assemble electrodes and rapidly generate data sets. The electrochemical behavior of CeO(2)-based screen-printed electrodes (CeO(2)-based SPEs) was determined by cyclic voltammetry and electrochemical impedance spectroscopy. The use of the electrodes for pH sensing was then evaluated by potentiometric measurements. The feasibility of measuring pH with CeO(2)-based SPEs was first tested in NH(4)Cl/NH(3) buffer solutions, leading to electrode calibration over the widest range of pH, from around neutral to basic pH. Experiments were then conducted in NaHCO(3)/Na(2)CO(3) buffer samples similar to conditions prevailing in the COx formation. Ce(0.8)Zr(0.2)O(2) SPEs exhibit a near-Nernstian behavior (sensitivity -(51±2)mV/pH) in the pH range of 5.5-13.2 at 25°C. Electrode response was slightly affected by the direction of the pH change. Electrode reliability was clearly demonstrated for pH monitoring. Probes based on the same components, but more durably designed, could be considered for pH measurements in radioactive waste repositories.