Conductometric Sensor for Atmospheric Carbon Dioxide Determination

A novel conductometric sensor is described for atmospheric gaseous species that dissolve in water to form conducting solutions. The sensor consists of a hydrophobic gas porous membrane which separates the gas sample from a thin layer of water. The gaseous species, e.g., carbon dioxide, diffuses through the membrane and rapidly equilibrates in the water layer. Conductivity electrodes positioned within the thin layer measure its conductance. The theory relating the square of the equilibrium conductance to the partial pressure of carbon dioxide, P_co2, is given. The configuration of the sensor as a CO₂ detector requires that chemical filters be employed to remove interferences from SO₂, NO₂, and NH₃. Linearized calibration curves are obtained over a CO₂ range of 0–3%, with response time on the order of 20 s or less. The portable conductometric sensor is a hand-held unit measuring 3 in. × 2.5 in. × 5 in.