An in-situ pH measuring system for high temperature aqueous solutions was developed with YSZ membrane tubes and a new pressure balanced regulator system. Air was filled inside the tube, the pressure of which being balanced with that of high temperature aqueous side by the regulator system. The Pt coated inside surface functioned as an oxygen electrode. Owing to the regulator system and some improvement in the shape of the tube and its cooling position, the damage of the full stabilized membrane tubes was almost prevented. In the temperature range from 150 to 275°C, the potential of the YSZ tube electrode was confirmed to show the one to one correlation to the potential of the Pt oxygen electrode in air-saturated aqueous solutions. The pH measurement by the YSZ tube was also confirmed not to be affected by other redox couples contained in solutions.
