Nernstian systems

FIGURE 1.3. Cyclic voltammetry of a Nernstian system involving the reduction of free-moving molecules. Concentration profiles of A ( ) and B (—) alongside the potential scan. 

Potential Step and Double Potential Step Chronoamperometry of Nernstian Systems... 

Cyclic Voltammetry of Two-Electron Nernstian Systems. Disproportionation... 

The sum of the surface concentrations of the two forms of the catalyst Tp and Tq is equal to the total catalyst surface concentration, T°. In the case of a Nernstian system,... 

The formal potential of the couple, E°, is often calculated from Equation 23.11, which is valid for a Nernstian system. 

We have already seen that a system that is always at equilibrium is termed a reversible system thus it is logical that an electrochemical system in which the charge-transfer interface is always at equilibrium be called a reversible (or, alternatively, a nernstian) system. These terms simply refer to cases in which the interfacial redox kinetics are so fast that activation effects cannot be seen. Many such systems exist in electrochemistry, and we will consider this case frequently under different sets of experimental circumstances. We will also see that any given system may appear reversible, quasire-versible, or totally irreversible, depending on the demands we make on the charge-transfer kinetics. 

Cyclic ac voltammograms for completely nernstian systems are easy to predict on the basis of results from the previous section. The mean surface concentrations, Co(0, Om Cr(0, Om adhere to (10.5.3) and (10.5.4) unconditionally hence at any potential they are the same for both the forward and reverse scans. The cyclic ac voltammogram should therefore show superimposed forward and reverse traces of ac current amplitude dc-We expect a peak-shaped voltammogram that adheres in every way to the conclusions reached in Section 10.5.1 about the general ac voltammetric response to a reversible system at a planar electrode. 

These concepts are used in several ways. We may speak of chemical reversibility when the same reaction (e.g., cell reaction) can take place in both directions. Thermodynamic reversibility means that an infinitesimal reversal of a driving force causes the process to reverse its direction. The reaction proceeds through a series of equilibrium states however, such a path would require an infinite length of time. Electrochemical reversibility is a practical concept. In short, it means that the Nernst equation ([Eq. (I.3.I5)] can be applied also when lEI > Eg,. Therefore, such a process is called a reversible or Nernstian reaction (reversible or Nernstian system, behavior). This is the case when the activation energy is small consequently ks and jo are high. 

In this chapter, only the response of a reversible system will be described. Linear sweep ac voltammetry of a Nernstian system is considered. 

These two concepts (control by mass transport and nernstian system) as terms are not strictly synonymous. Quasi-fast redox couples (see section 4.3.2.6) close to equilibrium conditions are nernstian, but they are not strictly controlled by mass transport. 

Similarly, the term reversible Is not synonymous with nernstian. Even for an E mechanism, it is possible to find cases where nernstian systems are reversible only in a very narrow potential range close to equilibrium. 

The solution of the problem for a Nernstian system requires a modified formulation of the concentration ratio at the electrode surface in the reverse period... 

The cyclic voltammetric currents were normally not the main source of attention but rather the semi-integral Ij of the current was calculated from this the semi-differential or square root of time deconvolution dlj/dt. This dlj/dE were used for the clearest displays of results. The main difference in application of the latter pair depends on whether a potential ramp linear with time is applied at die working electrode surface i.e. hardware compensated at tte potentiostat if necessary for resistance between working counter electrodes. In this case using the linearly varying potential as an axis dlj/dt dl/dE are similar in shape either provides a suitable display. If uncompensated resistance remains then in Nernstian systems I is a function of the appropriate E (E ) suitable compensation via -i.R can be sqiplied post csqiture... 

Nernstian system with surface-confined species =0 ocv 1 =90.6ln... 

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