Electrode reactions of dissolved species

Electrode Reactions of Dissolved Species on Stationary Planar Electrodes... 

In cyclic voltammetry, a potential range is swept in forward and reverse direction once or several times. Figure 5.17 schematically shows the cyclic voltammogram of an electrode reaction involving dissolved species such as for example Fe " and Fe " ions. In the anodic sweep Fe is oxidized to Fe, in the reverse direction theFe " is reduced yielding a cathodic current. In simple cases, the shape of the voltammogram and the difference between the potential maxima allow one to identity the reaction mechanism [8], but in corrosion, these criteria are usually of limited usefiilness and shall therefore not be further discussed here. 

The response of a reversible reaction (2.146) depends on two dimensionless adsorption parameters, Pr and po. When pR = po the adsorbed species accomplish instantaneously a redox equilibrium after application of each potential pulse, thus no current remains to be sampled at the end of the potential pulses. The only current measured is due to the flux of the dissolved forms of both reactant and product of the reaction. For these reasons, the response of a reversible reaction of an adsorbed redox couple is identical to the response of the simple reaction of a dissolved redox couple (2.157), provided Pr = po- As a consequence, the real net peak current depends linearly on /J, and the peak potential is independent of the frequency. If the adsorption strength of the product decreases, i.e., the ratio increases, the net peak current starts to increase (Fig. 2.73). Under these conditions, the establishment of equilibrium between the adsorbed redox forms is prevented by the mass transfer of the product from the electrode surface. Thus, the redox reaction of adsorbed species contributes to the overall response, causing an increase of the current. In the hmiting case, when ]8o —0, the reaction (2.146) simplifies to reaction (2.144). 

Greiver and Zaitseva [67GRE/ZAI] observed that elemental selenium dissolves in a concentrated, highly alkaline aqueous solution of mono- and polysulphides under oxygen-free conditions. The colour of the solution changes from colourless/yellow over to intense red-brown. Based on some relatively crude stoichiometric data, the authors claimed that the dominating species in such solutions is SSe. The standard potential of the couple Se, S/SSej" was reported to be - 0.383 V at 298.15 K in the same reference. However, the electrode reaction of the cell employed for this determination was never established and neither was its reversibility proved. This datum (2Se(s) S(s) 2e SScj" logic - 12.95) is therefore not accepted by the present review. 

The problem is a complex one for not only is there convection of the liquid by electroosmosis but if the dissolved contaminants are themselves charged, then an additional electromigration velocity will be imposed on them. Moreover, to the extent that concentration gradients are set up, there will also be transport of dissolved species by diffusion. In addition, there are chemical reactions in the bulk fluid and at the electrodes, together with adsorption or desorption at the soil surface. 

Note that many steps are involved in an EC reaction, such as the electron transfer reaction, transport of molecules from the bulk solution to the electrode surface and chemical reactions coupled to the electron transfer reaction. As with any multi step reaction, the rate of the overall reaction is generally determined by the rate of the slowest step (the rate-limiting step), and it is important to identify this step. In the analytical electrochemistry of dissolved species, the limiting step is typically the transport of molecules to the electrode surface through the solution. However, there are many instances where this is not the case and where the rate of the heterogeneous electron transfer reaction is important, for example in corrosion electrochemistry. 

The potential of a half-cell reaction between dissolved species is called the oxidation-reduction potential or redox potential. It describes the oxidative strength of a solution. For example, the electrode reaction (2.88) sets the redox potential of an aqueous solution containing dissolved oxygen. 

Equation 10.2, which involves consumption of the metal and release of electrons, is termed an anodic reaction. Equation 10.3, which represents consumption of electrons and dissolved species in the environment, is termed a cathodic reaction. Whenever spontaneous corrosion reactions occur, all the electrons released in the anodic reaction are consumed in the cathodic reaction no excess or deficiency is found. Moreover, the metal normally takes up a more or less uniform electrode potential, often called the corrosion or mixed potential (Ecotr)-... 

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