The thermodynamic viability of yttria-stabilized zirconia pH sensors for high temperature aqueous solutions

Abstract

The thermodynamic viability of the yttria-stabilized zirconia sensor (YSZS) [H2O, H+/ZrO2(Y2O3)/HgO/Hg] for the measurement of pH in high temperature aqueous solutions is evaluated by measuring potentials for this electrode and a conventional hydrogen electrode (HE) against a common reference electrode in a variety of solutions [0.01m H3PO4, 1m Na2SO4, 0.01m B(OH)3+0.01m KOH, and 0.01m KOH] at temperatures from 298.15K (25°C) to 573.115K (300°C). In order to compare theoretical and experimental potentials for the cell $$Pt/H_{2,} H^ + , H_2 O/ZrO_2 (Y_2 O_3 )/HgO/Hg (i.e.,HE/YSZS)$$ it was necessary to apply corrections for solution non-ideality (reflected in the activity of water) and salting in/salting out of hydrogen in solutions of finite ionic strength. Plots of the potential of the YSZS and the HE against a common external pressure balanced reference electrode (EPBRE) demonstrate the Nerstian response of the zirconia electrode, and an analysis of the cell HE/YSZS shows that the yttria-stabilized zirconia sensor is thermodynamically viable. Accordingly, the YSZS can be used as a primary standard for the measurement of pH. The data indicate that the precision of the YSZS for measuring pH in high temperature aqueous solutions is better than ±0.05 pH unit at 300°C (573.15K), provided that the activity of water and the isothermal liquid junction potential associated with the reference electrode are accurately known.