Measurement of pH in subcritical and supercritical aqueous systems

The use of yttria-stabilized zirconia (YSZ) electrodes of the type Hg/HgO/ZrO2(Y2O3)/H+, for measuring pH in aqueous solutions at high subcritical (150 374°C) is reviewed. The construction and operation of the YSZ and reference electrodes employed in these studies are described and their application in measuring the pH of a variety of technologically-important aqueous system is discussed. We show that the YSZ electrode is thermodynamically viable at temperatures into the supercritical region, and that it is a primary pH sensor in that calibration is not necessary, provided that the activity of water is known. However, highly accurate pH measurements at high subcritical and supercritical temperatures will require the development of more accurate reference electrodes.