Nemstian reactions electrode processes

In the periodically modulated version of this experiment (32), the laser heating is carried out sinusoidally at a frequency of 5 to 20 Hz, and the resulting sinusoidally varying current, is detected with a lock-in amplifier, as in hydrodynamic modulation. The variation of A/q with E is called thermal modulation voltammetry (TMV). Near, A/p shows a peak whose magnitude for a nemstian reaction is a function of the ratio of the en-tropic energy of the electrode reaction divided by the activation energy of the mass transport process. While the method is capable of extracting thermodynamic information about a reaction, both the theory and the experimental set-up is sufficiently complex that it has not yet found widespread use. 

The extent or degree of completion of a bulk electrolytic process can be predicted for nemstian reactions from the applied electrode potential and a suitable form of the Nernst equation. 

Because any potentiometric electrode system ultimately must have a redox couple (or an ion-exchange process in the case of membrane electrodes) for a meaningful response, the most common form of potentiometric electrode systems involves oxidation-reduction processes. Hence, to monitor the activity of ferric ion [iron(III)], an excess of ferrous iron [iron(II)] is added such that the concentration of this species remains constant to give a direct Nemstian response for the activity of iron(III). For such redox couples the most common electrode system has been the platinum electrode. This tradition has come about primarily because of the historic belief that the platinum electrode is totally inert and involves only the pure metal as a surface. However, during the past decade it has become evident that platinum electrodes are not as inert as long believed and that their potentiometric response is frequently dependent on the history of the surface and the extent of its activation. The evidence is convincing that platinum electrodes, and in all probability all metal electrodes, are covered with an oxide film that changes its characteristics with time. Nonetheless, the platinum electrode continues to enjoy wide popularity as an inert indicator of redox reactions and of the activities of the ions involved in such reactions. 

With respect to the heterogeneous electron-transfer process, reversible (nemstian) systems are always at equilibrium. The kinetics are so facile that the interface is governed solely by thermodynamic aspects. Not surprisingly, then, the shapes and positions of reversible waves, which reflect the energy dependence of the electrode reaction, can be exploited to provide thermodynamic properties, such as standard potentials, free energies of reaction, and various equilibrium constants, just as potentiometric measurements can be. On the other hand, reversible systems can offer no kinetic information, because the kinetics are, in effect, transparent. 

Even though this section has been developed with the assumption that the electrode reaction is a one-step, one-electron process, many of the conclusions apply generally for chemically reversible multi-electron mechanisms. The nemstian limit is still described by (10.3.8) and Figure 10.3.2, but with a given by... 

The equivalent circuit corresponding to an uncomplicated electrochemical reaction (i.e., a one-step CT process) is shown in Figure 15.1. An important advantage of ac voltammetry is that it allows relatively easy evaluation of the solution resistance ( J and double layer capacitance (C4). These elements can be separated from the and components, which together make faradaic impedance. Without simplifying assumptions, the analysis of faradaic impedance even for a simple ET reaction is rather complicated (9). The commonly used assumptions are that the dc and ac components of the total current can be uncoupled, and the dc response is Nemstian because of the long dc time scale. The latter assumption is reasonable because ac voltammetry is typically used to measure fast electrode kinetics. The ac response of the same electrochemical process may be quasi-reversible on the much shorter ac time scale. Quasi-reversible ac voltammograms are bell-shaped. 

Subscripts are used to provide additional information. The subscript for reversible (meaning that both forward and reverse processes are fast enough to maintain equilibrium or Nemstian conditions at the surface), and i represents irreversible (only the forward reaction is significant) i and r are limiting cases of q, or quasi-reversible (meaning that both the forward and reverse processes take place but are not fast enough to be considered at equilibrium). Thus, in an E Cj mechanism the electrode reaction is fast and reversible and the chemical reaction is irreversible. 

Equation 1.109 has the same form as the Nemst equation. It indicates that the surface concentrations of species involved in the Faradaic process are related to the electrode potential by an equation of the Nemst form when the exchange current density is very large. Such electrode reactions are often called reversible or Nemstian, because the principal species obey thermodynamic relationships at the electrode surface. 

The non-nemstian Ni(III)/Ni(lI) peaks [11] (Fig. 2) are very similar to those reported for Ni electrodes, in which the process has been attributed to the reaction [12]... 

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