Nemstian reactions voltammetry

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. 

As in the case of differential double potential pulse techniques like DDPV, slow electrochemical reactions lead to a decrease in the peak height and a broadening of the response of differential multipulse and square wave voltammetries as compared with the response obtained for a Nemstian process. Moreover, the peak potential depends on the rate constant and is typically shifted toward more negative potentials (when a reduction is considered) as the rate constant or the pulse length decreases. SWV is the most interesting technique for the analysis of non-reversible electrochemical reactions since it presents unique features which allow us to characterize the process (see below). Hereinafter, unless expressly stated, a Butler-Volmer potential dependence is assumed for the rate constants (see Sect. 1.7.1). 

Cyclic Square Wave Voltammetry (CSWV) is very useful in determining the reversibility degree and the charge transfer coefficient of a non-Nemstian electrochemical reaction. In order to prove this, the CSWV curves of a quasi-reversible process with Kplane = 0.03 and different values of a have been plotted in Fig. 7.17. In this figure, we have included the net current for the first and second scans (Fig. 7.17b, d, and f) and also the forward, reverse, and net current of a single scan (first or second, Fig. 7.17a, c, e) to help understand the observed response. 

Kilmartin and Wright [97K1L/WRI] recently studied the development of thin CuSeCN(s) layers on metallic copper in 0.1 M SeCN solutions by cyclic voltammetry. The peaks observed were assumed to relate to the formation and removal of CuSeCN(s) according to the reaction Cu(cr) + SeCN CuSeCN(s) + e . The shapes of the volt-ammograms do, however, indicate that the electrode reaction exhibits non-Nemstian behaviour. The authors also noted that during their experiments, a possible formation of elemental selenium occurred on the electrode surface. Therefore, the recorded electrode potential characteristics cannot be regarded as well established and these data cannot be used for calculating the standard electrode potential of the above redox couple and the solubility product of CuSeCN(cr). 

A solution of volume 50 cm contains 2.0 X 10 M Fe and 1.0 X 10 M in 1 M HCl. This solution is examined by voltammetry at a rotating platinum disk electrode of area 0.30 cm. At the rotation rate employed, both Fe and have mass-transfer coefficients, m, of 10 cm/s. (a) Calculate the limiting current for the reduction of Fe under these conditions, (b) A current-potential scan is taken from +1.3 to —0.40 V vs. NHE. Make a labeled, quantitatively correct, sketch of the i-E curve that would be obtained. Assume that no changes in the bulk concentrations of Fe and Sn " occur during this scan and that all electrode reactions are nemstian. 

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. 

Linear scan voltammetry (LSV) and cyclic voltammetry (CV) (see Chapter 11) are among the most common electrochemical techniques employed in the laboratory. Despite their utility, however, they are not particularly well suited to careful measurements of diffusion coefficients when using electrodes of conventional size. We will briefly discuss techniques for measuring D with LSV and CV, but the reader should be cautioned that these measurements under conditions of planar diffusion (i.e., at conventional electrodes) are probably useful to only one significant digit, and then only for nemstian systems with no coupled homogeneous reactions and with no adsorption. For more reliable results with LSV and CV, UMEs should be used. 

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