Current-concentration relationships, potential voltammetry

Information is obtained with this type of electrochemical sensor from either the combined cur-rent/potential-concentration relationship (voltammetry) or from the current-concentration relationship alone (amperometry). 

Stirred-solution voltammetry utilizes current-voltage relationships that are obtained at a stationary electrode immersed in a stirred solution. In order to understand this aspect of electrochemistry, it is extremely useful to consider a typical current-voltage curve (voltammogram) in terms of the concept of concentration-distance profiles presented in the preceding section. The discussion will consider the potential, rather than the current, as the controlled variable. 

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). 

Voltammetric MEMS biosensors (voltammetry) are based on measuring the current-voltage relationship in an electrochemical cell consisting of electrodes in a solution. After a potential is applied to the sensor, current, which is proportional to the concentration of the electroactive species of interest, is measured. Amperometry is a special case when potential is maintained so as to be constant with time. 

Fig. 2.13 Current versus overpotential curves showing the effect of experimental parameters in the presence of forced convection, according to the relationship = /cL lnFc. (a) Electrode size (and shape). Ideally, in the presence of a uniform current-density distribution, Deviations may be due to edge effects, non-uniformity of flow (e.g. entrance length effects) or contributions from natural convection, (b) Concentration of electroactive species in the reactor. ii should be proportional to c. It is sometimes convenient to test this by incremental increases in c . The background curve is represented by = 0. (c) Relative velocity of the electrolyte or electrode, cc where x is a constant which depends upon the geometry and flow conditions, x may vary slightly over different ranges of Reynolds number. The limiting-current plateau may shorten and tilt as velocity increases, due to the increasing importance of electron transfer to the overall reaction kinetics. The maximum on the 1 curve may arise due to unsteady-state mass transport and is akin to a peak in linear sweep voltammetry, i.e. it may arise due to an excessive rate of potential change.

The theory of linear-sweep voltammetry (LSV) applied to heterogeneous redox reactions accompanied by the nondissociative adsorption of the reactant or the product is developed. The basic criterial relationships of LSV in this case are invariant with respect to the type of adsorption isotherm and the number of adsorption sites occupied by one species. The degree of irreversibility of the discharge-ionization step can be evaluated from the effect of the potential scan rate on the peak potential. The nature of the adsorbate can be deduced from the effect of the reactant concentrations on the peak current. 

Fig. 16.6 and -0.52 V in Fig. 16.4), the onset of current occurs at a more negative potential, and the current minimum occurs at a much more positive potential in Fig. 16.4 compared to Fig. 16.6, i.e., the peak width is much larger (0.125 V in Fig. 16.6 vs. 0.045-0.065 V in Fig. 16.4). Second, although the concentration in Fig. 16.6 is a factor of 100 lower than the lowest concentration in Fig. 16.4, the background-corrected peak current (-0.3 pA) is only a factor of-5 lower (-1.4 pA). There is only a factor of 2.5 difference in the deposition times (2 vs. 15 min). Both of these differences are related to the higher sweep rate. The increased peak width may be ascribed to kinetic factors [17], and the increased relative current can be ascribed to the simple linear relationship expected for peak current (sweep voltammetry) vs. concentration for a surface-bound redox couple [18].

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