Potentiodynamic sensors

Modern dynamic electrochemical techniques offer additional enhancement of the information acquisition process, including selectivity and detection limit. Instead of holding the potential of the working electrode at a constant value, the potential is varied in some specific way. In that approach, we have a choice of several nonsteady-state electrochemical techniques. They are all derived from the basic current-voltage concentration relationship (Section 5.1). A complete discussion of these electroanalytical techniques can be found in electrochemistry textbooks (Bard and Faulkner, 2001). 

The most popular electroanalytical technique used at solid electrodes is Cyclic Voltammetry (CV). In this technique, the applied potential is linearly cycled between two potentials, one below the standard potential of the species of interest and one above it (Fig. 7.12). In one half of the cycle the oxidized form of the species is reduced in the other half, it is reoxidized to its original form. The resulting current-voltage relationship (cyclic voltammogram) has a characteristic shape that depends on the kinetics of the electrochemical process, on the coupled chemical reactions, and on diffusion. The one shown in Fig. 7.12 corresponds to the reversible reduction of a soluble redox couple taking place at an electrode modified with a thick porous layer (Hurrell and Abruna, 1988). The peak current ip is directly proportional to the concentration of the electroactive species C (mM), to the volume V (pL) of the accumulation layer, and to the sweep rate v (mVs 1). 

The constant /3 contains a partitioning coefficient of the analyte between the solution and the modifying layer, as well as the constants related to the bulk electrolysis in a small volume (i.e., thin) cell (Bard and Faulkner, 2001). If the electroactive species are confined to the electrode, if the couple is perfectly reversible, and if the extraction is fast on the time scale of the experiment, the peaks in the cyclic voltammogram occur at the same potential and the areas (charge) below the cathodic and anodic branches are equal, as is the case in Fig. 7.12. Obviously, any deviations from these conditions are reflected in the shape of the CV curve. Nevertheless, even then the relationship between the peak current iv and the bulk concentration of the electroactive species can be reproducible. In the determination of Fe2+ using the above procedure, the linear calibration between 5 x 10 8 and 5 x 10-6 M concentration has been obtained. 

If more electroactive species are present, the corresponding number of peaks is obtained. Moreover, when the standard potentials for the oxidation (and reduction) of those species are sufficiently separated (AT 0 100 mV), the scanning of potential provides additional selectivity. Nevertheless, the same caveat regarding the relationship between selectivity and applied potential, as discussed above, applies. The overall selectivity of this sensor is due to the combination of two factors selective complexation of Fe2+ with 2,2 -bipyridile and the oxidation of the complex at the characteristic potential (+1.0 V vs. SCE). The value and the popularity of cyclic voltammetry and other electrodynamic techniques lie in the fact that they contain information not only about the concentration, but also about the chemical reactions accompanying the charge transfer. 

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There are three types of electrochemical sensors potentiometric, amperometric, and potentiodynamic sensors. 

Potentiodynamic sensors are based on the measurement of the current response of the working electrode under no mass transport limiting conditions. Potentiodynamic methods typically involve accumulation (or preconcentration) steps, such as in stripping voltammetry for analyzing trace metals in solution. 

Potentiodynamic gas sensors have a schematic structure that is practically equal to that of amperometric gas sensors. They are -> electrochemical cells that measure a -> current signal directly related to the concentration of the analyte, but are not necessary operated in a region where -> mass transport is limiting. They are typically employed to detect less reactive species such as benzene and halogenated hydrocarbons that require a previous accumulation step at a suitable -> adsorption potential to be then reduced or oxidized according to a given potential scan [iii]. The adsorption time can be automat-... 

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