Gas Membrane Sensors

The idea of separating the gas sample by a gas-permeable membrane from the actual internal sensing element is common to several types of electrochemical and some optical sensors. The potentiometric Severinghaus electrode and the amperometric oxygen Clark electrode have already been discussed. Actually, most types of sensors can be used in this configuration and the conductometric sensor is not an exception (Bruckenstein and Symanski, 1986). 

The theory of operation of the conductometric gas membrane sensor has been experimentally verified in detail for CO2 and SO2, and sensors for H2S and NH3 based on the same principle have also been made. The basic transport and equilibration processes are the same as in the Severinghaus electrode (Section 6.2.2). Upon entering the aqueous solution inside the cell, the gas dissociates to its constituent ions. Because each dissociated species contributes to the overall conductivity, the specific conductance A of the cell is 

The experimental conductance Gceii depends on the cell geometry as characterized by the cell constant k. Therefore, the output of the sensor is related to the partial pressure of the gas as in (8.19). 

The magnitude of the dissociation constant A plays an important role in the response characteristics of the sensor. For a weakly dissociated gas (e.g., CO2, K = 4.4 x 10-7), the sensor can reach its equilibrium value in less than 100 s and no accumulation of CO2 takes place in the interior layer. On the other hand, SO2, which is a much stronger acid (K = 1.3 x 10-2), accumulates inside the sensor and its rep-sonse time is in minutes. The detection limit and sensitivity of the conductometric gas sensors also depend on the value of the dissociation constant, on the solubility of the gas in the internal filling solution, and, to some extent, on the equivalent ionic conductances of the ions involved. Although an aqueous filling solution has been used in all conductometric gas sensors described to date, it is possible, in principle, to use any liquid for that purpose. The choice of the dielectric constant and solubility would then provide additional experimental parameters that could be optimized in order to obtain higher selectivity and/or a lower detection limit. 

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