Sensors for Acid-Base-Active Gases

Abstract

AbstractThe theoretical background for potentiometric and conductometric (bulk and surface) sensors is briefly discussed with regard to the usual application, viz. the detection of redox-active gases, and the importance of understanding chemical diffusion is stressed in this context. It is then shown how the analogous concept applied to acid-base interactions leads to powerful possibilities to measure partial pressures of acid-base active gases such as CO2, NH3 and H2O. Two examples of novel sensor principles are discussed: i) The response of the surface ionic conductivity of AgCl or CaF2 upon changes of partial pressures of (Lewis) basic or acid gases can be used to sense NH3 and BF3. ii) In the major part of the paper it is shown how the deliberate use of overall acid-base cell reactions in potentiometric arrangements leads to a fast and precise CO2-sensor. For this purpose, open reference electrodes based on Na2Ti6O13 or Na2SnO3 have been constructed. Owing to the criteria developed in the text the whole cell can be exposed to a CO2 and O2 containing atmosphere without the necessity of sealing. Detailed results are given.