Potentiometric sensors cell voltage

Potentiometric sensors are based on the measurement of the voltage of a cell under equilibrium-like conditions, the measured voltage being a known function of the concentration of the analyte. Potentiometric measurements involve, in general, Nernstian responses under zero-current conditions that is, the measurement of the electromotive force of the electrochemical cell. 

Numerous applications have been developed in the field of chemical analysis using potentiometric measurements as indicators, including the production of potentiometric sensors and titration devices. In this chapter, we will focus on the defining principles of these potentiometric methods at zero current when these systems are in thermodynamic equilibrium, which is not necessarily true for all potentiometric measurements. In particular, the following description is confined to electrochemical cells with no ionic junction. In practice, these results will also be applied to many experimental cases in which ionic junction voltages can be neglected . 

Typical construction and its output characteristics of ZrO -based sensors are shown in Fig. 2.7. The cell in oxygen sensors is usually shaped like a test tube where the inner and outer surfaces are each coated with ultrathin layers of porous platinum which act as the cathode and anode electrodes. The output of this potentiometric sensor is due to the combined effect of chemical and electrical processes. At high temperatures >650 °C, zirconium dioxide exhibits two mechanisms (Park et al. 2009) (1) ZrO partly dissociates to produce oxygen ions, which can be transported through the material when a voltage is applied and (2) ZrO behaves like a solid eleetrolyte for oxygen. 

Obviously the arbitrary inclusion of ion conductors in the circuit may cause considerable effects. In such cases the difference between the chemical potentials of the ions may not be ther-modynamicaJly defined but may exhibit appreciable values. The time-dependence of irreversible contributions is often not very great, so that pseudostationary cell voltages are measured. The glass electrode and the Daniell element Cu ICUSO4I ZnS04 Zn are examples from aqueous electrochemistry. Such considerations are very important for the performance and selectivity of potentiometric sensors [541,542]. In the case that there are several electrode processes, the phenomenon of mixed potentials must be taken into account (see footnote 59). 

Potentiometric mode There is no essentially different principle involved from that on which the fuel cell is based. The distinction is that in the case of the fuel cell the required output is power whereas with the sensor it is either a small voltage or small current that monitors some chemical characteristic of the ambient. 

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 ... 

The majority of solid electrolyte sensors are based on proton conductors (Miura et al. 1989, Alberti and Casciola 2(X)1). Metal oxides that can potentially meet the requirements for application in solid electrolyte sensors are listed in Table 2.7. These proton condnctors typically do not have high porosity but rather can reach 96-99% of the theoretical density (Jacobs et al. 1993). Similar to oxygen sensors, solid-state electrochemical cells for hydrogen sensing are typically constructed by combining a membrane of solid electrolyte (proton conductor) with a pair of electrodes (electronic conductors) Most of the sensors that use solid electrolytes are operated potentiometrically. The voltage produced is from the concentration dependence of the chenucal potential, which at eqnihbrium is represented by the Nemst equation (Eq. 2.3). 

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