Potentiometric Nonequilibrium

The nonequilibrium potentials measured in solid-electrolyte cells are established by electrochemical reactions, just as for equilibrium-based sensors. For example, in an environment containing CO and O2, the CO could be oxidized chemically according to the following reaction  

However, on an oxygen ion conduction electrolyte surface, the oxidation of CO can occur electrochemically by 

the sum of these two electrochemical reactions is the chemical reaction in 

Equation (13.8). Before reaching equilibrium, these two reactions will reach a 

One of the advantages of mixed potential sensors is that it is possible for both electrodes to be exposed to the same gas. The elimination of a need to separate the two electrodes simplifies the sensor design, which in turn reduces fabrication costs. Although this simpler planar design is often used, the electrodes are sometimes separated to provide a more stable reference potential. As with equilibrium potentiometric sensors, the minimum operating temperature is often limited by electrolyte conductivity. However, the maximum operation temperatures for nonequilibrium sensors are typically lower than those of equilibrium sensors, because the electrode reactions tend towards equilibrium as the temperature increases. This operating temperature window depends on the electrode materials, as will be discussed later in the chapter. 

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The liquid junction is a practical necessity in most electrochemical measurements and in almost all potentiometric measurements. It is a site of the dreaded liquid junction (or diffusion) potential Ej. Because this potential is the result of the diffusion, which is a nonequilibrium process, any potentiometric measurement, which uses a liquid junction is a nonequilibrium measurement, by definition. The origin of Ej is explained in Fig. 6.5. 

Sensors for which the output is an open-circuit voltage are referred to as potentiometric sensors, and can be used for a wide variety of species [13-18]. The measured voltage can be established by a thermodynamic equilibrium or by a nonequilibrium steady state between electrochemical reactions at the electrode. 

Although a thermodynamic analysis of the functioning of the respiratory chain should be conducted using methods of nonequilibrium thermodynamics, in practice it is the so-called midpoints potentials determined by the method of potentiometric titration with the use of mediators13 that are used for characterizing the redox properties of the components of the respiratory chain. The method devised for this purpose is founded on the supposition that the mediator is oxidized or reduced only by the chain components of interest to us. The correctness of this supposition is usually controlled by independent methods, as for instance, the spectroscopic or ESR method. 

Nonequilibrium Potentiometric Sensors (or Mixed Potential Sensors)... 

Non-equilibrium processes at the sample/membrane interface and across the bulk membrane bias the selectivity and detection limits of the electrodes. Elimination of these nonequilibrium effects by operating the electrodes under complete equilibrium conditions will be of both practical and fundamental significance. While non-equilibrium responses are useful for potentiometric polyion-selective electrodes, it is not obvious whether potentiometry based on mixed ion-transfer potentials is a better transduction mechanism than amperome-try/voltammetry based on selective polyion transfer (65, 66). Ion-transfer electrochemistry at polarized liquid/liquid interfaces is introduced in Chapter 17 of this handbook. 

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