Alkali metal sulfates are basically applicable to SO
x sensors as solid electrolytes. The electro-motive force (emf) of the solid electrolyte cells is expressed by the Nernst equation. The lowering of the emf is noticed at a large SO
x pressures difference between the anode and the cathode. This is caused by the physical permeation of gases in the electrolyte through cracks and pores which arise from volume change during the III-I phase transition and from low sinterability, The use of NASICON (Na
3Zr
2Si
2PO
12) which is easily sintered to high densities provides an SO
x sensor with no permeability. The generation of emf is essentially due to the formation of Na
2SO
4 both on the anode and on the cathode. The solid reference electrodes for SO
x sensors are reviewed briefly. A small insert-type CO
2 sensor has been constructed using Na
2CO
3 with a tip of NASICON or β-alumina, and is proved to function satisfactorily. The activity of Na
2O in NASICON or β-alumina is taken as a reference state in this sensor.
View full abstract