ELECTRICAL SENSING OF COMPLEX GASEOUS SPECIES BY MAKING USE OF ACID-BASE PROPERTIES

Whereas the interaction of elemental gaseous species such as O2, H-2, Cl2, etc. with a second phase is in the first place a redox interaction, many more complex gas molecules such as NH3, CO2, H2O interact over a wide range of conditions primarily in an acid-base kind of way. It is shown that concentrating on this viewpoint leads to a new sensor principle in the (surface) resistive sensing mode (exemplified for NH3) as well as to a considerably simplified set-up in the galvanic sensing mode (exemplified for CO2). In the first case ionic space charge effects indicate the acid-base interaction of an ionic or mixed conductor with the gas. Results are given for NH3 (and (CN)2) detected by AgCl. The latter case is exemplified for CO2, where the anode and cathode of electrochemical cells such as M2CO3(s), CO2/solid electrolyte/M2XO3(s), XO2(s) can be simultaneously exposed to the same CO2 and O2 containing atmospheres, and thus sealing problems are avoided as long as the difference of the Gibbs energies of formation for CO2 and XO2 is smaller than the difference of the values for the respective M2O-compounds (M2CO3, M2XO3). Results for X=Zr and M=Na are discussed. In this way the problem of the reference electrode for galvanic CO2 sensors may be solved elegantly.