A new potentiometric gas sensor-the air-gap electrode

The concept and construction of a newly developed gas sensor, the air-gap electrode, is described. The hydrophobic gas-permeable membrane generally employed in gas electrodes is replaced by an air gap which separates the electrolyte layer from the sample solution. By avoiding the membrane construction and utilizing a very thin layer of electrolyte at the surface of the indicator electrode, a very high speed of response is obtained. Furthermore, the life time of the sensor is substantially increased, as the electrode does not come into direct physical contact with the sample solutions; thus there is no interference from surfactants, particulate matter or organic solvents. The electrolyte layer can be easily renewed or even changed according to the requirements of a particular analysis, so that the same electrode can be used for measurements of a variety of gases. Two different techniques of applying the air-gap electrode are described. The airgap electrode was used to determine the carbon dioxide and ammonia contents of a series of samples, using sample volumes from 50–150 μl to several ml. The electrode was also used to determine the ammonium ion content in serum and plasma samples, yielding highly satisfactory results. The speed of response was of the order of 1–3 min. Some potential applications of the sensor are mentioned.