Measurement of pH and potential in supercritical water

Two of the most important parameters in defining the chemical properties of supercritical water used in fossil power plants and the corrosion properties of metals immersed in these environments are the electrochemical potential and pH, both measured at the temperature of operation. The broad overall goal of this program was to develop reference electrodes and pH sensors that would operate at temperatures and pressures corresponding to supercritical water. The program was intended to establish the proof of concept, and demonstrate the viability of fabricating and using sensors for operation under these extreme conditions. During this study SRI fabricated an yttria-stabilized zirconia (YSZ) pH sensor with a Hg/HgO internal reference and proved the thermodynamic viability of using this as a pH sensor at temperatures above the critical point of water (i.e., [ge]374[degree]C). A novel all-solid-state Ag/AgCl, (Cl[sup [minus]], glass) reference electrode was also fabricated in which the solid electrolyte is a chloride-ion-doped glass having a constant chloride ion activity. The electrode was tested as a valid reference electrode at 374[degree]C. It is rugged and leakproof and is expected to be stable over long periods of time because of the extremely slow diffusion of Cl[sup [minus]] ions through the solid glass matrix.