Oxides, electrode/solution interface

Controlled-potential (potentiostatic) techniques deal with the study of charge-transfer processes at the electrode-solution interface, and are based on dynamic (no zero current) situations. Here, the electrode potential is being used to derive an electron-transfer reaction and the resultant current is measured. The role of the potential is analogous to that of the wavelength in optical measurements. Such a controllable parameter can be viewed as electron pressure, which forces the chemical species to gain or lose an electron (reduction or oxidation, respectively). 

Accordingly, the resulting current reflects the rate at which electrons move across the electrode-solution interface. Potentiostatic techniques can thus measure any chemical species that is electroactive, in other words, that can be made to reduce or oxidize. Knowledge of the reactivity of functional group in a given compound can be used to predict its electroactivity. Nonelectroactive compounds may also be detected in connection with indirect or derivatization procedures. 

Table 8.76 shows the main characteristics of voltammetry. Trace-element analysis by electrochemical methods is attractive due to the low limits of detection that can be achieved at relatively low cost. The advantage of using standard addition as a means of calibration and quantification is that matrix effects in the sample are taken into consideration. Analytical responses in voltammetry sometimes lack the predictability of techniques such as optical spectrometry, mostly because interactions at electrode/solution interfaces can be extremely complex. The role of the electrolyte and additional solutions in voltammetry are crucial. Many determinations are pH dependent, and the electrolyte can increase both the conductivity and selectivity of the solution. Voltammetry offers some advantages over atomic absorption. It allows the determination of an element under different oxidation states (e.g. Fe2+/Fe3+). 

A comparison of the products of AP hydrolysis of HQDP (HQ), PP, and 1-NP using cyclic voltammetry revealed that HQ produced well-defined peaks, and that the oxidation of HQ is reversible. More importantly, no apparent passivation of the electrode surface was observed even at high millimolar concentrations after 50 scans. Following a series of investigations, this non-fouling nature of HQ was attributed to the non-accumulation of its oxidation products on the electrode surface and the good diffusional properties of HQ at the electrode-solution interface. Another positive feature of HQDP as a substrate for AP is a tenfold greater oxidation current response of HQ compared to those obtained in the presence of PP or 1-NP. Overall, HQDP provides a suitable and attractive alternative substrate system for AP in the development of amperometric immunosensors. 

During reduction, electrons travel/rom the power pack, through the electrode, transfer across the electrode-solution interface and enter into the electroactive species in solution. Conversely, during oxidation, electrons move in the opposite direction, and are conducted away from the electroactive material in solution and across the electrode-solution interface as soon as the electron-transfer reaction occurs. (Incidentally, these different directions of electron movement explains why an oxidative current has the opposite sign to a reductive current, cf. Section 1.2.)... 

We can state this argument in reverse - alteration of the potential at the electrode solution interface will itself cause the ratio of a(O) to a(R) to alter to that dictated by the Nernst equation, and the conversion of material from its reduced to its oxidized forms (or back) requires the production (or consumption) of charge. In fact, we can write a variant of the Nemst equation (equation (3.8)), as follows ... 

The plots shown in Figure 6.3 show the variations of the concentrations of both reduced and oxidized forms ofTl (of reactant Tt in Figure 6.3(a) and product Tl in Figure 6.3(b)). Each concentration is depicted as a function of the distance from the electrode solution interface where oxidation is effected these curved traces are often termed concentration profiles. Each of the figures incorporate a series of concentration profiles, drawn as a function of time, to show how the Nemst layer increases in thickness during electrolysis, because the extent of electromodification has increased with time. 

The rotating disc electrode is constructed from a solid material, usually glassy carbon, platinum or gold. It is rotated at constant speed to maintain the hydrodynamic characteristics of the electrode-solution interface. The counter electrode and reference electrode are both stationary. A slow linear potential sweep is applied and the current response registered. Both oxidation and reduction processes can be examined. The curve of current response versus electrode potential is equivalent to a polarographic wave. The plateau current is proportional to substrate concentration and also depends on the rotation speed, which governs the substrate mass transport coefficient. The current-voltage response for a reversible process follows Equation 1.17. For an irreversible process this follows Equation 1.18 where the mass transfer coefficient is proportional to the square root of the disc rotation speed. 

The slowest step, or rate-determining step, can be either (a) electron transfer at the electrode-solution interface or (b) formation of atoms at the electrode surface. The activation polarization component of the overpotential, r)a, is related to the actual rate of oxidation or reduction, i, and the exchange current density ... 

From what has been described so far, there can be a flow of cathodic current, or of anodic current at an electrode/solution interface, according to the value (and particularly the sign) of the overpotential, i.e., of the displacement from equilibrium of the electric potential of the electrode. The equilibrium referred to is that of some specific interfacial electron transfer reaction (e.g., the cathodic reduction of 02 (02 + 4H+ + e —> 2H20)) or the anodic oxidation of ethylene, C2H4 + 4H20 — 2C02 + 12H+ + 12e. 

The formation or dissolution of a new phase during an electrode reaction such as metal deposition, anodic oxide formation, precipitation of an insoluble salt, etc. involves surface processes other than charge transfer. For example, the incorporation of a deposited metal atom (adatom [146]) into a stable surface lattice site introduces extra hindrance to the flow of electric charge at the electrode—solution interface and therefore the kinetics of these electrocrystallization processes are important in the overall electrode kinetics. For a detailed discussion of this subject, refs. 147—150 are recommended. 

Since the electrochemical reduction or oxidation of a molecule occurs at the electrode-solution interface, molecules dissolved in solution in an electrochemical cell must be transported to the electrode for this process to occur. Consequently, the transport of molecules from the bulk liquid phase of the cell to the electrode surface is a key aspect of electrochemical techniques. This movement of material in an electrochemical cell is called mass transport. Three modes of mass transport are important in electrochemical techniques hydrodynamics, migration, and diffusion. 

The important concept in these dynamic electrochemical methods is diffusion-controlled oxidation or reduction. Consider a planar electrode that is immersed in a quiescent solution containing O as the only electroactive species. This situation is illustrated in Figure 3.1 A, where the vertical axis represents concentration and the horizontal axis represents distance from the electrodesolution interface. This interface or boundary between electrode and solution is indicated by the vertical line. The dashed line is the initial concentration of O, which is homogeneous in the solution the initial concentration of R is zero. The excitation function that is impressed across the electrode-solution interface consists of a potential step from an initial value E , at which there is no current due to a redox process, to a second potential Es, as shown in Figure 3.2. The value of this second potential is such that essentially all of O at the electrode surface is instantly reduced to R as in the generalized system of Reaction 3.1 ... 

To impose the diffusion-controlled conversion of O to R as described earlier, the potential E impressed across the electrode-solution interface must be a value such that the ratio Cr/Cq is large. Table 3.1 shows the potentials that must be applied to the electrode to achieve various ratios of C /Cq for the case in which Eq R = 0. For practical purposes, C /C = 1000 is equivalent to reducing the concentration of O to zero at the electrode surface. According to Table 3.1, an applied potential of -177 mV (vs. E° ) for n = 1 (or -88.5 mV for n = 2) will achieve this ratio. Similar arguments apply to the selection of the final potential. On the reverse step, a small C /Cq is desired to cause diffusion-controlled oxidation of R. Impressed potentials of +177 mV beyond the E° for n = 1 (and +88.5 mV for n = 2) correspond to Cr/Cq = 10"3. These calculations are valid only for reversible systems. Larger potential excursions from E° are necessary for irreversible systems. Also, the effects of iR drop in both the electrode and solution must be considered and compensated for as described in Chapter 6. 

It is clear that the passage of charge across the electrode—solution interface may cause oxidation or reduction. Since the amount of chemical change is governed by Faraday s laws, such processes are termed faradaic. A link exists between the electrode potential, E, and the concentrations (properly activities) of compounds of the electrode process. In general... 

The observation of molecular luminescence at electrode solution interfaces results from high-energy annihilation reactions between electrochemically generated radical ions that result in the formation of an electronically excited species [6-16], The radical ions can be generated at two separate electrodes in close proximity to one another or at the same electrode by alternating between reductive and oxidative potentials. This is particularly useful when the radical ions are unstable since they can be produced in situ immediately prior to, or during, the reaction. The general mechanism of an ECL reaction is as follows. 

Electron Transfer in Electrochemistry. In electrochemical cells electron transfer occurs within the electrode-solution interface, with electron removal (oxidation) at the anode, and with electron introduction (reduction) at the cathode. The current through the solution is carried by the ions of the electrolyte, and the voltage limits are those for electron removal from and electron insertion into the solvent-electrolyte [e.g., H20/(H30+)(C10j ) (Na )(-OH) ... 

Electrode Materials and Their Electrochemical Behavior. There is abundant evidence that the rate of electron transfer across an electrode-solution interface is dependent on the physical and chemical properties of the electrode material. The term electrocatalysis has been coined for this effect, and studies of the oxidation of hydrocarbons61 and the reduction of water and hydronium ion62 have provided ample evidence for its existence. 

Types of electrode/solution interface studied include oxide films on metals, monolayer deposits obtained by underpotential deposition, adsorption, and spectroelectrochemistry in thin-layer cells. 

The causes of overvoltage are very complex. Basically, the phenomenon is caused by difficulties in transferring electrons from the species in the solution to the atoms on the electrode across the electrode-solution interface. Because of this situation, %° values must be used cautiously in predicting the actual order of oxidation or reduction of species in an electrolytic cell. 

D.N. Furlong, D.E. Yates and T.W. Healy, Fundamental Properties of the Oxide-aqueous Solution Interface, In Stud. Phys. Theor. Chem. 11 (No electrodes conduct, met. oxides. Part B (1981) 367. (Review, structure, double layer.)... 

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