Potentiometric gas sensors

New polymer membrane-based ISEs for nitrate and carbonate exhibit detection limits and selectivities that may be applicable for ocean measurements. In addition, a number of these ISEs can be used as internal transducers for the design of useful potentiometric gas sensors. For example, dissolved C02 can be detected potentiometrically by using either a glass membrane electrode or a polymer-based carbonate ISE, in conjunction with an appropriate reference electrode, behind an outer gas permeable membrane. Novel differential pC02 sensors based on two polymer membrane-type pH sensors have also been developed recently. 

One could immobilize the urease layer on top of a Severinghaus electrode for CO2 or NH3 (Section 6.3.2) and use the device as an enzymatic-potentiometric gas sensor. The primary disadvantage of such an arrangement would be its slow response time. A more direct way is through the detection of the ionic species resulting from the hydrolysis of ammonia and carbon dioxide. 

Fig. 6.40 Potentiometric gas sensor based on yttrium-stabilized zirconia (YSZ)...

Gas sensors — (a) Gas sensors with liquid electrolytes — Figure 2. can regarded as an Oxygen concentration Cell. They Scheme of a potentiometric gas sensor work depending on electrode material in a broad range... 

M. Kleitz and M. Siebert, Electrode reactions in potentiometric gas sensors, in T. Seiyama (Ed.), Chemical Sensor Technology, Vol. 2. Elsevier, Amsterdam, 1989, pp. 151-71. 

Amperometric sensing of gases is based on solid ion-conducting materials, as described for potentiometric gas sensors. Solid-state amperometric gas sensors measure the limiting current (ij) flowing across the electrochemical cell upon application of a fixed voltage so that the rate of electrode reaction is controlled by the gas transport across the cell. The diffusion barrier consists of small-hole porous ceramics. The limiting current satisfies the relationship ... 

Pasierb, P., and Rekas, M. 2009. Solid-state potentiometric gas sensors-current status and future trends. Journal of Solid State Electrochemistry 13, 3-25. 

Guillet, N., Lalauze, R. and Pijolat, C. (2004) Oxygen and carbon monoxide role on the electrical response of a non-Nernstian potentiometric gas sensor proposition of a model. Sens. Actuators B. 98 (2-3), 130-9. 

Di Bartolomeo, E., Grilli, M.L. and Traversa, E. (2004) Sensing mechanism of potentiometric gas sensors based on stabilized zirconia with oxide electrodes. J. Electrochem. Soc., 151 (5), H133-9. 

Zhuiykov, S., Mathematical modeling of YSZ-based potentiometric gas sensors with oxide sensing electrodes Part II Complete and numerical models for analysis of sensor characteristics. Sens. Actuators B, Chem. 120 (2007) 645-656. 

FIGURE 2.5 Equivalent scheme of the boundary impedance of the SE-YSZ (a) at the presence and (b) at the absence of the current in the circuit. (Reprinted from Zhuiykov, S., Mathematical modelling of YSZ-based potentiometric gas sensors with oxide sensing electrodes part II Complete and numerical models for analysis of sensor characteristics. Sensors and Actuators B, Chem. 120 (2007) 645-656, with permission from Elsevier Science.)... 

Based on the fact that the potentiometric gas sensors establish thermodynamic equilibria at the interfaces with gases, let s consider this process for the YSZ-based sensor measuring the NO2 concentration in a humidified atmosphere. The discharge reactions at the TPB (at x > 8) can be shown as follows ... 

Potentiometric gas sensors for the reaction products, NH3 and CO2, have also been employed. Since these measurements are based on gas diffusion through a hydrophobic membrane, no direct disturbances by sample constituents occur. As early as 1969, Guilbault et al. coupled immobilized urease with a carbon dioxide sensor. Anfalt et al. (1973) applied an ammonia gas sensitive electrode to urea assay. A major drawback of these sensors is their long response time which is due to the slow diffusion of the gases. Since it takes several additional minutes to reach a new baseline after each measurement, only a few samples can be processed per hour. Guilbault et al. (1985) therefore tried an NH3 electrode, the interned buffer of which was exchanged after each measurement (double injection electrode). This approach led to a substantial decrease of the washing time. 

A further disadvantage of potentiometric gas sensors is the difference between the pH optima of the electrode and urease. Thus, NH3 electrodes are operated at a pH around 8 while the optimal pH of the urease reaction is at pH 7. 

M. Josowitz, J. Janata, K. Ashley and S. Pons, Electrochemical and ultraviolet-visible spectroelectrochemical investigation of selectivity of potentiometric gas sensors based on polypyrrole, Anal. Chem., 1987, 59, 253 T. Sata. Possibility for potentiometric humidity sensor of composite membranes prepared from anion-exchange membrane and conducting polymer, Sensor Actuators, B, 1995, 23, 63 K. Ogura, H. Shiigi, T. Oho and T. Tonosaki, A C02 sensor with polymer composites at ordinary temperature, J. Electrochem. Soc., 2000,147, 4351. 

J. F. Coetzee and C. Gunaratna, Potentiometric Gas Sensor for the Determination of Free Chlorine in Static or Flow Injection Analysis Systems. Anal. Chem., 58 (1986) 650. 

The use of potentiometric gas sensors without calibration by realizing equilibrium cell potential differences demands that above all the following is guaranteed ... 

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