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Gas Oxidation and Reduction In a typical amperometric CO gas sensor, the molecules of CO are directly oxidized at the anode surface to produce CO2. The current generated on the sensing electrode is related to the rate of CO reaction, which can be limited by reaction rate or diffusion (Stetter and Li 2008). 13) In cases when the analyte gas is difficult to reduce or oxidize, a catalyst may be included in the surface as an integral composite material or as a modifying coating. For example, in an amperometric CH4 sensor with a carbon nanotube electrode loaded with palladium, the Pd catalyst facilitates CH4 to gain electrons from the nanotube.

Schematic diagram of the polymer/acid electrolyte AGS for hydrogen. (Reprinted with permission from Korotcenkov 2009. ) electrodes (ISEs) that are based on redox or ion-exchange reactions (Holzinger et al. 1997; Yamazoe and Miura 1998; Bobacka et al. 2008) and are typically used in detection of analytes in liquids, such as the gas sensors described here. The simplest configuration for the potentiometric gas sensor consists of two electrodes in contact with an electrolyte, similar in arrangement to the two-electrode version of an amperometric sensor except that the measurement is taken at zero current (or as near to zero as possible) and all reactions must be in equilibrium to observe the themodynamic potential.

Therefore, Eq. 3c) or 4  CHEMICAL SENSORS. 4) The activity effect of electrons is neglected because the electron concentration in the solid electrode is a constant. pA is the partial pressure of A and p0 is 1 atm. 4b) and Eq. 5a) is for a one-electron transfer reaction and will vary depending on the number of electrons involved; the sign of the potential will depend on whether the analyte is oxidized or reduced on the electrode surface. Potentiometry is most frequently explained based on this equation, a Nernst-type equation which relates the partial pressure of the analyte and the electrode potential.

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