“…For the measurements of pH in hightemperature aqueous systems such as nuclear power plants, four types of pH sensors have been proposed, including platinumbased hydrogen electrodes, 1 palladium hydride electrodes, 2,3 metal/metal oxide electrodes, 4,5 and ceramic membrane electrodes (e.g., the yttria-stabilized zirconia (YSZ) membrane electrode). [6][7][8][9] In case of hydrogen electrodes, however, it is not easy to practically use them because molecular hydrogen should exist in a test system. 10 For palladium hydride electrodes, they cannot be used in a system containing oxidants.…”