Non-faradaic currents

The electric current connected with chemical conversion is termed the faradaic current in contrast to the non-faradaic current required to charge the electrical double layer. The equation for the electrode reaction is formulated similarly to that for the half-cell reaction (Section 3.1.4) for the cathodic reaction, the electrons are placed on the left-hand side of the equation and, for the anodic reaction, on the right-hand side. 

This type of current, which originates from chemical processes which obey Faraday s law, is called a faradaic current, to distinguish it from non-faradaic currents which, as we shall see in Section 5, arise from processes of a strictly physical nature. 

In the course of an electrochemical experiment the experimental conditions are carefully controlled to minimize the onset of non-faradaic currents as much as possible. 

Based on the way in which an electrode process has been illustrated, until now it would seem reasonable to assume that the only source of electron flow between the electrode and the species in solution might be attributed to faradaic processes of the type Ox + e = Red. It has already been mentioned in Section 2.3, however, that non-faradaic currents exist. Let us discuss their origin. 

Section 4.2.4, the faradaic currents fall off more slowly with time in that they decrease as a function of the square root of time (i oc l/V ). Consequently, the time dependence is a diagnostic test to discriminate between faradaic and capacitive (i.e. non-faradaic) currents. 

To appreciate that a majority of non-faradaic currents are caused by the effects of adsorption, capacitance and the electrode double-layer, or by competing side reactions such as solvent splitting. 

In the previous section, we introduced the way that coulometry can be employed as an analytical tool, looking speciflcally at some simple forms of the technique. We saw that the charge passed was a simple function of the amount of material that had been electromodified, and then looked at ways in which the coulometric experiment was prone to errors, such as non-faradaic currents borne of electrolytic side reactions or from charging of the double-layer. 

A fast rate of mass transport is useful to the electroanalyst because the faradaic component of the charge is made greater, while the non-faradaic current is not affected. In addition, note that /non-faradaic will be small anyway since it is in proportion to electrode area. 

We saw earlier that the limit of the electrode sensitivity was the ratio of faradiac to non-faradaic currents. The net result of using a microelectrode is to increase this ratio, and thus allow the analyst to analyse solutions of lower concentration - perhaps as low as 10 mol dm . ... 

To appreciate that to minimize non-faradaic currents, the polarography solution should be purged of oxygen and contain a surfactant depolarizer in low concentration. 

In the previous chapter, it was emphasized that the non-faradaic component of current should be minimized. Is non-faradaic current a problem in polarography ... 

Figure 6.17 A Randles-SevCik plot of 7p against Data refer to the oxidation of aqueous ferrous ion at a stationary platinum wire electrode. The non-linearity at the higher scan rates represents the demand for flux at the working electrode being too great since i is too fast, while the non-zero intercept is caused by non-faradaic currents contributing...

The simplest reason for a non-zero intercept is non-faradaic current, the causes of which are discussed in Section 6.8.2. 

Because the oxidation of an analyte is effected with a mediator, the potential of the working electrode need not be as anodic as the potential needed for the first type of sensor, implying that anions such as ascorbate (see above) will remain unnoticed as electro-inactive contaminants. By remaining unoxidized, the magnitude of the non-faradaic current is kept to a minimum. 

All of the electroanalytical techniques described in this present chapter have made use of the general relationship, faradaic current a analyte concentration , according to Faraday s laws. It is therefore important that such non-faradaic currents be minimized. First, the resistance of the solution can be minimized by adding an inert electrolyte to the solution in swamping concentration. (Adding a swamping electrolyte also decreases the extent of mass transport by migration.)... 

A calibration curve would be a safer method of determining Canaiyte. since compensation for non-faradaic currents can then be made. 

As always, a superior method would have been to construct a calibration curve, and read off the values of Canaiyte since we are more likely to discern non-faradaic currents in this way. It is also advisable to start any series of analyses by varying the flow rate over as wide a range as possible in order to observe those flow regimes which are reproducible and can therefore be employed, and those which should be avoided. 

Although the hybridization of single-stranded DNA to its complement results in detectable changes in electrochemical properties, particularly in support of non-Faradaic current, the DNA bases may also demonstrate redox behavior that gives rise to Faradaic currents. The electrochemical behavior of DNA has been studied over the past few decades. Differential pulse voltammograms show clearly defined peaks for the reduction of cytosine and adenosine. Electrochemical characterization of guanine by cyclic voltammetry has shown... 

A second type of current arises due to the presence of the electrochemical double layer (Sect. 1.2). Additionally, a current may flow due to the adsorption or desorption (Sect. 1.2) or species O and R as well as electroinactive species. In these instances, no chemical reaction occurs and consequently electrons are not transferred across the electrode-solution interface. However, a current may flow elsewhere and this current is called a non-faradaic current. 

Both faradaic and non-faradaic currents may flow when an electrode reaction occurs. Thus, the total current which flows is often the sum of the faradaic and non-faradaic contributions to the current. Most often, it is the faradaic current that is of interest. Many electrochemical techniques have been developed which minimize or eliminate this non-faradaic contribution to the current, but discussion of these is beyond the scope of the present chapter. 

In Sect. 1.2, we chose to ignore the effects of the double layer as well as complexities due to adsorption and desorption processes. Similarly, we will also choose to ignore the presence of non-faradaic currents, though in practice they may be important. Hence, throughout this chapter only faradaic currents will be considered. 

The two interfacial processes, charge transfer and double-layer charging, proceed in parallel and so the total current density is the sum of the faradaic and the charging (sometimes called non-faradaic) current densities... 

We are therefore in a position to compare Faradaic and non-Faradaic currents and at 10-5 M they are of comparable magnitude (using Cj 20/xFcm-2). However, ic decays with t1/3 whereas iF increases with t1/6. This is, of course, an important reason for the sampling of current towards the end of drop life, which is used, for example, in the pulse polarography techniques. Pulse techniques are also useful when the integral capacitance, Ci, varies with potential in these cases graphical elimination of ic can be difficult. 

The non-faradaic current can be immediately obtained by differentiating the expression of the non-faradaic charge given by Eqs. (6.150) or (6.155). In this case, the well-known expression (see Sect. 1.9),... 

Another more sophisticated approach is to make a Fourier Transform analysis of the response in the way proposed by Bond et al. [84, 85]. In this case, the perturbation is a continuous function of time (a ramped square wave waveform) which combines a dc potential ramp with a square wave of potential that can be described as a combination of sinusoidal functions. Under these conditions, the faradaic contribution to the response generates even harmonics only (i.e., the non-faradaic current goes exclusively through odd harmonics). Thus, the analysis of the even harmonics will provide excellent faradaic-to-non-faradaic current ratios. 

Another point is that the reduction and oxidation potential limits (electrochemical window) are defined as the potentials at which the current density reaches a predefined value that is arbitrarily chosen [40, 48], Ue et al. also mention that the same problem arises in the choice of the sweep rate [40]. For example Egashira and coworkers obtained a log I- U line shifted to a higher position at a faster potential scan in comparison to a slower scan because of non-Faradaic currents such as the larger charging currents of the double-layer, and the decomposition of impurities [41]. The last factor affecting the electrochemical window is the electrode itself, its composition and its morphological surface structure, which defines the electrocatalytic properties [40]. 

This type of electrode is a source or sink of electrons, permitting electron transfer without itself entering into the reaction, as is the case for the first or second type of electrodes. For this reason they are called redox or inert electrodes. In reality the concept of an inert electrode is idealistic, given that the surface of an electrode has to exert an influence on the electrode reaction (perhaps small) and can form bonds with species in solution (formation of oxides, adsorption, etc.). Such processes give rise to non-faradaic currents (faradaic currents are due to interfacial electron transfer). This topic will be developed further in subsequent chapters. 

To conclude this section, it should be emphasized that the steady state vol-tammograms described above are quite different from the first scan of the cyclic voltammetry of these systems. During the first polarization of these electrodes to low potentials, pronounced reduction processes of solution components are observed. As a result of these processes, a stable precipitate forms on the electrodes as insoluble films, and hence the above steady state voltammetric behavior reflects electrochemistry which is surface film controlled. The outer, solution side of these films is probably porous, leading to the high interfacial capacity which is reflected by the relatively high non-Faradaic currents which characterize these voltammograms. The next section describes in detail the initial voltammetric behavior of these systems and the surface film formation on the electrodes. 

To many analysts the major limitation of electrochemical detection for liquid chromatography (LCEC) is its limited applicability to gradient elution techniques. Amperometric electrochemical detectors exhibit both the best and the worst characteristics of solute property and bulk property detectors. While the Faradaic current arises only from the solute, the non-Faradaic current arises from... 

The two major classes of voltammetric technique 4 Evaluation of reaction mechanisms 6 General concepts of voltammetry 6 Electrodes roles and experimental considerations 8 The overall electrochemical cell experimental considerations 12 Presentation of voltammetric data 14 Faradaic and non-Faradaic currents 15 Electrode processes 17 Electron transfer 22 Homogeneous chemical kinetics 22 Electrochemical and chemical reversibility 25 Cyclic voltammetry 27 A basic description 27 Simple electron-transfer processes 29 Mechanistic examples 35... 

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