Potentiometric sensors Nernst equation

In other words, at the nonpolarized interface, the interfacial potential Eeq is uniquely tied by the Nernst equation (5.8) to the activity ai of the charged species crossing the interface. This is the key relationship in potentiometric sensors (Chapter 6). 

It is important to note that the electrode potential is related to activity and not to concentration. This is because the partitioning equilibria are governed by the chemical (or electrochemical) potentials, which must be expressed in activities. The multiplier in front of the logarithmic term is known as the Nernst slope . At 25°C it has a value of 59.16mV/z/. Why did we switch from n to z when deriving the Nernst equation in thermodynamic terms Symbol n is typically used for the number of electrons, that is, for redox reactions, whereas symbol z describes the number of charges per ion. Symbol z is more appropriate when we talk about transfer of any charged species, especially ions across the interface, such as in ion-selective potentiometric sensors. For example, consider the redox reaction Fe3+ + e = Fe2+ at the Pt electrode. Here, the n = 1. However, if the ferric ion is transferred to the ion-selective membrane, z = 3 for the ferrous ion, z = 2. 

The ion-selective membrane is the key component of all potentiometric ion sensors. It establishes the preference with which the sensor responds to the ion of interest in the presence of various other ionic components of the sample. By definition, the ion-selective membrane forms a nonpolarized interface with the solution. If the interface is permeable to only one ion, the potential difference at that interface is expressed by the Nernst equation (6.6). If more than one ion can permeate, the interface can be anything between the liquid junction and the mixed potential. The key distinguishing feature is the absolute magnitude of the total exchange current density. 

Symmetrical placement of the ion-selective membrane is typical for the conventional ISE. It helped us to define the operating principles of these sensors and most important, to highlight the importance of the interfaces. Although such electrodes are fundamentally sound and proven to be useful in practice, the future belongs to the miniaturized ion sensors. The reason for this is basic there is neither surface area nor size restriction implied in the Nernst or in the Nikolskij-Eisenman equations. Moreover, multivariate analysis (Chapter 10) enhances the information content in chemical sensing. It is predicated by the miniaturization of individual sensors. The miniaturization has led to the development of potentiometric sensors with solid internal contact. They include Coated Wire Electrodes (CWE), hybrid ion sensors, and ion-sensitive field-effect transistors. The internal contact can be a conductor, semiconductor, or even an insulator. The price to be paid for the convenience of these sensors is in the more restrictive design parameters. These must be followed in order to obtain sensors with performance comparable to the conventional symmetrical ion-selective electrodes. 

Because each enzyme sensor has its own unique response, it is necessary to construct the calibration curve for each sensor separately. In other words, there is no general theoretical response relationship, in the same sense as the Nernst equation is. As always, the best way to reduce interferences is to use two sensors and measure them differentially. Thus, it is possible to prepare two identical enzyme sensors and either omit or deactivate the enzyme in one of them. This sensor then acts as a reference. If the calibration curve is constructed by plotting the difference of the two outputs as the function of concentration of the substrate, the effects of variations in the composition of the sample as well as temperature and light variations can be substantially reduced. Examples of potentiometric enzyme electrodes are listed in Table 6.5. 

Potentiometric sensor is based on a redox reaction that occurs at the electrode-electrolyte interface in an electrochemical cell. If a redox reaction Ox + Ze Red takes place at an electrode surface, it is called a halfcell reaction. In the above reaction. Ox is the oxidant. Red is the reduced product, e in the electron, and Z is the number of electrons transferred in the reaction. At thermodynamic quasiequilibrium conditions, the Nernst equation is applicable and can be expressed as ... 

If two such electrodes are separated by a thin layer of only zirconia, the application of a potential will lead to the pumping of oxygen from the cathode to the anode. This device can be used as an amperometric sensor for oxygen if a diffusion barrier restricts the flux of oxygen to the cathode. Note that similar devices are also often used as potentiometric sensors according to the Nernst equation (i.e., the lambda-probe in cars with catalytic converters). In this case one side of the cell has to act as a reference, e.g., by using ambient air. 

Potentiometric detection in microfiuidic systems is based on a detection electrode bearing sensing membranes, which allow the buildup of a measurable potential, which follows the Nernst equation. Such electrodes are known as ion-selective electrodes. For useful detection in electroseparation methods, the selectivity should extend to a range of ions so that universal potential sensors can be formed. 

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