Electroactive Form adsorption

The prewave is caused by the adsorption on the surface of the electrode of some of the electroactive material. If the adsorbed form is easier to reduce then it will be reduced at a lower potential than the rest of the electroactive form in the solution. The adsorbed form will thus form a separate prewave at more positive potentials (for a reduction or cathodic process) than the main wave. Since there are only a limited number of sites available for adsorption, they will become fully occupied above a certain concentration. Thus the height of the adsorption prewave will become constant after a certain concentration and any further increase in concentration will only increase the height of the main wave. 

Adsorption postwaves are similar to the prewaves but are caused by adsorption of the electrolysis product, or when the adsorption of the original electroactive form makes it more difficult to reduce. Usually adsorption of a species releases energy which is then available to the electrolysis step, making it easier to reduce the species. 

The first group of events involves convective or molecular diffusion to the electrode, possibly with some chemical transformation of the dominantly present species into an electroactive form, and the adsorption step. 

Many organic electrode processes require the adsorption of the electroactive species at the electrode surface before the electron transfer can occur. This adsorption may take the form of physical or reversible chemical adsorption, as has been commonly observed at a mercury/water interface, or it may take the form of irreversible, dissociative chemical adsorption where bond fracture occurs during the adsorption process and often leads to the complete destruction of the molecule. This latter t q)e of adsorption is particularly prevalent at metals in the platinum group and accounts for their activity as heterogeneous catalysts and as... 

If the electrolyte components can react chemically, it often occurs that, in the absence of current flow, they are in chemical equilibrium, while their formation or consumption during the electrode process results in a chemical reaction leading to renewal of equilibrium. Electroactive substances mostly enter the charge transfer reaction when they approach the electrode to a distance roughly equal to that of the outer Helmholtz plane (Section 5.3.1). It is, however, sometimes necessary that they first be adsorbed. Similarly, adsorption of the products of the electrode reaction affects the electrode reaction and often retards it. Sometimes, the electroinactive components of the solution are also adsorbed, leading to a change in the structure of the electrical double layer which makes the approach of the electroactive substances to the electrode easier or more difficult. Electroactive substances can also be formed through surface reactions of the adsorbed substances. Crystallization processes can also play a role in processes connected with the formation of the solid phase, e.g. in the cathodic deposition of metals. 

Chemical reaction steps Even if the overall electrochemical reaction involves a molecular species (O2). it must first be converted to some electroactive intermediate form via one or more processes. Although these processes are ultimately driven by depletion or surplus of intermediates relative to equilibrium, the rate at which these processes occur is independent of the current except in the limit of steady state. We therefore label these processes as chemical processes in the sense that they are driven by chemical potential driving forces. In the case of Pt, these steps include dissociative adsorption of O2 onto the gas-exposed Pt surface and surface diffusion of the resulting adsorbates to the Pt/YSZ interface (where formal reduction occurs via electrochemical-kinetic processes occurring at a rate proportional to the current). 

Differential pulse voltammetry and electrochemical impedance have demonstrated that G, A, guanosine, and their oxidation products are electrostatically adsorbed on GC and GC(ox) surfaces [47,49]. The strength of adsorption of the DNA bases on the GC surface were found to be similar [49]. Strongly adsorbed G dimers were formed on GC between G and the adsorbed G oxidation products, which slowly cover and block the surface. The appHcation of ultrasound led to removal of the adsorbed species. The effect of this was mainly to enhance transport of electroactive species and to clean the electrode in situ, avoiding electrode fouling. 

If only adsorbed forms of ox and red are electroactive, then the voltammetric peaks are sharp and symmetrical (the current rises from zero to a maximum value and then falls again to zero, and there is little or no peak separation).39 41 The symmetrical, sharp peak results from the fixed amount of the reactant that is adsorbed on the electrode. The values of ip, Ep, and the peak width depend on the type of adsorption isotherm involved and relative strength of the adsorption of oxidized and reduced species on the electrode surface. If the adsorption is described by a Langmuir isotherm, Epc = p a and the peak current is described by... 

Voltammetry provides a powerful insight into the effect of the applied potential on the surface coverage, the free energy of adsorption, and the associated kinetics for electroactive films that form on electrode surfaces by irreversible adsorption. 

The adsorption of Cu2+ ions on the Ti02 forms electroactive surface states within the band gap of the oxide, whose energy position was determined by the electrolyte electroreflection method [285, 286]. These copper-induced surface states were established to be located ca. 1.1 eV below the conduction band edge. Information concerning the subbands of the surface states in the Ti02 electrodes modified with Ag, Pd, Pt and Au one can find in [286, 310, 311], as well as in Chapter 6 of this book. 

Surface redox reactions — or surface -> electrode reactions, are reactions in which both components of the -> redox couple are immobilized on the electrode surface in a form of a -> monolayer. Immobilization can be achieved by means of irreversible -> adsorption, covalent bonding, self-assembling (- self-assembled mono-layers), adhesion, by Langmuir-Blodgett technique (- Langmuir-Blodgett films), etc. [i]. In some cases, the electrode surface is the electroactive reactant as well as the product of the electrode reaction is immobilized on the electrode surface, e.g., oxidation of a gold, platinum, or aluminum electrode to form metal oxide. This type of electrode processes can be also considered as surface electrode reactions. Voltammetric response of a surface redox reaction differs markedly from that of a dissolved... 

Usually, for a potential-decay experiment, the system is at steady state just before the circuit is opened. Therefore the value of K(0) to be used to define the initial conditions for solution of the differential equations is the potential at which the system was held prior to the transient. The initial value of 6 is the corresponding steady-state value, obtained by inserting K(0) into Eq. (54), setting Eq. (54), equal to zero, and solving for 6. It is this 6 that is required for evaluation of the adsorption behavior of the electroactive intermediate. The required differential kinetic equations can be solved numerically for various mechanisms and forms of transients t) t) or V t) derived. 

Let us consider the case where adsorbed O, but not dissolved O, is electroactive (31-33). This could be the case when the sweep rate, u, is so large that O does not have time to diffuse appreciably to the electrode surface [i.e., Do(dCo(0, t)/dx =o drQ(t)/dt]. Alternatively, the wave for adsorbed O could be shifted to potentials well before the reduction wave for dissolved O. The conditions for such behavior will be given below. There are also cases where adsorption is so strong that the adsorbed layer of O can form even when the solution concentration is so small that the contribution to the current from dissolved O is negligible. We also assume that within the range of potentials of the wave, the F s are independent of E. Under these conditions, (14.3.1) becomes... 

In applications of voltammetry to biological samples, it is often the sample rather than the sensitive voltammetric analyzer that is the limiting factor. Getting the sample into a form that can take full advantage of the instrument capability may be the hardest part of the analysis. For this reason, the sample is usually treated prior to analysis. Such treatment releases the trace metals bound to sample components, and minimizes fouling of electrode (by adsorption of certain sample components) or background currents (from other electroactive constituents). The precision and bias of the data obtained by voltammetric analysis of biological samples will be more dependent on how well the sample is decomposed than with many other analytical techniques (e.g., atomic absorption spectroscopy which relies on atomization of the metal from the solution). 

The adsorbed species causing any given adsorption effect can either be the electroactive species of interest or some other species in the solution. One species can itself give rise to several different adsorption phenomena, as one chemical species is capable of existing in several different adsorbed forms. 

The adsorption of ions and molecules on the surface of mercury electrodes is a thoroughly investigated phenomenon [51 ]. Surface-active substances are either electroactive [52] or electroinactive [53]. The former can be analyzed by adsorptive stripping voltammetry [54]. This is the common name for several electroanalytical methods based on the adsorptive accumulation of the reactant and the reduction, or oxidation, of the adsorbate by some voltammetric technique, regardless of the mechanisms of the adsorption and the electrode reaction [55, 56]. Frequently, the product of the electrode reaction remains adsorbed to the electrode surface. Hence, the term stripping should not be taken literally in all cases. Besides, some adsorbates may be formed by electrosorption reactions, so that their reduction includes covalently bound mercury atoms. The boundary between adsorption followed by reduction, on the one hand, and electrosorption, on the other, is not strictly defined. Moreover, it is not uncommon that, upon cathodic polarization, the current response is caused by a catalytic evolution of hydrogen, and not by the reduction of the adsorbate itself [57]. However, what is common to all methods is a hnear relationship between the surface concentration of the adsorbate and the concentration of analyte at the electrode surface ... 

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