CHEMICAL SENSORS FOR ACID-BASE-ACTIVE GASES - APPLICATIONS TO CO2 AND NH3

It is shown how acid-base interactions of complex gases such as CO2, NH3 or H2O with solids can be used to measure the partial pressures of these gases. Two examples developed in our laboratory are presented: i) The interactions of acid-base-active gases such as NH3 (Lewis base) or BF3 (Lewis acid) with the ionic conductors AgCl or CaF2 cause ionic space charge regions giving rise to a change of the ionic surface conductivity. The response time can be significantly improved by applying a microelectrode impedance method. ii) The use of a potentiometric arrangement the cell reaction of which is a pure acid-base reaction, leads to a fast, precise and drift-free CO2-sensor. Since the reference electrode is not affected by CO2 itself, the entire cell can be exposed to the CO2 and O-2 containing atmosphere without the necessity of sealing the reference electrode. The chemical potential of oxygen even though important for defining the component activities at each electrode cancels in the overall reaction of our cells. The performance of two cells is presented. While the measuring electrode consists of Na2CO3, the reference electrodes discussed are made of Na2Ti6O13/TiO2 or Na2Ti6O13/Na2Ti3O7 two-phase mixtures, both being stable with respect to a reaction with CO2. The sensors are very fast, response times are of the order of seconds.