Potentiometric sensors zero current

In potentiometric sensors, an electrical potential between the working electrode and a reference electrode is measured at zero current conditions in a solution containing ions that exchange with the surface. The first potentiometric MIP sensor was prepared in 1992 by Vinokurov (1992). The substrate-selective polyaniline electrode was electrosynthesized with polypyrrole, polyaniline, and aniline-p-aminophenol copolymers. The development of an MIP-based potentiometric sensor was reported in 1995 by Hutchins and Bachas (1995). This potentiometric sensor has high selectivity for nitrite with a low detection limit of (2 + l)x 10 M (Fig. 15.10). 

Potentiometric measurements are done under the condition of zero current. Therefore, the domain of this group of sensors lies at the zero-current axis (see Fig. 5.1). From the viewpoint of charge transfer, there are two types of electrochemical interfaces ideally polarized (purely capacitive) and nonpolarized. As the name implies, the ideally polarized interface is only hypothetical. Although possible in principle, there are no chemical sensors based on a polarized interface at present and we consider only the nonpolarized interface at which at least one charged species partitions between the two phases. The Thought Experiments constructed in Chapter 5, around Fig. 5.1, involved a redox couple, for the sake of simplicity. Thus, an electron was the charged species that communicated between the two phases. In this section and in the area of potentiometric sensors, we consider any charged species electrons, ions, or both. 

The mixed potential accounts for a large portion of reported artifacts in the unorthodox potentiometric sensors, particularly biosensors, and can be rightfully called evil potential . The physical origin of such artifacts can be illustrated using a simple example. Let us assume that a multiple electron transfer takes place simultaneously at the interface of a lump of Zn immersed in dilute HC1. Because this metal is not externally connected the net current is zero. The redox reactions taking place are as follows. 

Measurement of the cell potential of a potentiometric sensor should be carried out under zero-current or quasiequilibrium conditions. A high input impedance electrometer is commonly used. The advantage of potentiometric sensor is that the sensor output and the cell potential do not depend on the electrode surface area. This can make the manufacturing of practical sensors simpler. A major disadvantage of... 

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. 

Potentiometric sensors operate at thermodynamic equihbrium conditions. Thus, in practical potentiometric sensing, the potential measurement needs to be made under zero-current conditions. Consequently, a high-input impedance electrometer is often used for measurements. Also, the response time for a potentiometric sensor to reach equilibrium conditions in order to obtain a meaningful reading can be quite long. These considerations are essential in the design and selection of potentiometric sensors for biomedical apphcations. 

Electrochemical sensors can use amperometry, potentiometry, or conductometry as transduction principle [32], Potentiometric sensors make use of the development of an electrical potential at an electrode surface in contact with ions that exchange with the surface. The potential is measured under zero-current conditions against a reference electrode and is proportional to the logarithm of the analyte activity in the sample. Potentiometric sensors are limited to the measurement of charged species or of gases that dissociate to yield charged species in an electrolyte. Ion-selective electrodes are one example of this sensor type. 

By extension, the term electrochemistry stretches to include systems which have no controlled exchange of electrical energy with the exterior. The overall electric current is zero the electrochemical system Is at open circuit. The term combines two very different situations. The first applies to any system in thermodynamic equilibrium, that is in which no transformation of matter occurs. This is the case with many potentiometric sensors. The second situation covers systems that are likely to react spontaneously, namely with a transformation of matter and the internal exchange of electric energy, such as in corrosion. The concepts of electrochemistry are the suitable tools to describe such systems. 

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 . 

Potentiometry is a method in which the electrochemical cell potential is measured at equilibrium at which the current is zero. The properties of the interface region differ from the bulk properties. A potential is established at the phase boundaries, e.g., between the solution and the electrode surface. The potential of electrochemical cells is the sum of all interface potentials including electrode/electrolyte interface and liq-uid/Uquid interface (i.e., the two electrolyte solutions of different compositions that are in contact with each other). Ideally the measured potential should depend only on the potential between the interfaces of interest for analytical purpose. This is typically accomplished by keeping all other interfaces constant through a suitable electrode construction. Potentiometric sensors (e.g., ion selective electrodes) usually consist of a manbrane that contains ion exchangers, lipophilic salts, and plasticizers, and the transmembrane potential gives the activity of the analyte ion in solution. 

Next we have to define the boundary and the initial conditions. For so called zero flux sensors there is no transport of any of the participating species across the sensor/enzyme layer boundary. Such condition would apply to, e.g., optical, thermal or potentiometric enzyme sensors. In that case the first space derivatives of all variables at point x are zero. On the other hand amperometric sensors would fall into the category of non-zero-flux sensors by this definition and the flux of at least one of the species (product or substrate) would be given by the current through the electrode. 

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