Chapter VI - Optically transparent thin layer spectroelectrochemistry (Part 4)

Non-ideal behavior of the Nernst plots
This article continues with the non-ideal behavior of Nernst plots for non-overlapping spectra and the electrode process for product deposition.

3. Non-overlapping spectra

The above discussed cases are all cases where the spectra of electroactive substances overlap, and the following case will be discussed when only one substance has light absorption [7-1].

a) For backward reaction (EC mechanism)

Since the total concentration of the substance is constant, then
The Nernst equation (7-6) of the electrochemical reaction equation (7-1) can be written as equation (7-7)
The Nernst plot is also a straight line. According to its slope, the electron transfer number n can be obtained, but its intercept is EOA/B' + (RT/nF) ln(1 + K), which can be used to obtain the equation (7- 2) of equilibrium constant K.

b) For forward reaction (CE mechanism)
If the absorbance change of substance B is monitored, the expression of its Nernst equation is
The Nernst plot is also a straight line with a slope of RT/nF. The slope of the Nernst plot differs from the equation potential in equation (7-9) when the equilibrium constant of the preceding reaction is small.If the equilibrium constant is large, that is, K >> 1, the effect of the preceding reaction is almost negligible.
For these systems mentioned above, which affect only the equilibrium concentrations of reactants and products, but not other electrochemical reactions, it is still possible to obtain n from the slope of the Nernst plot, but its intercept does not correspond to its equation potential.

c) Electrode process with two consecutive electron transfers (EE mechanism)
For the electrode process containing two consecutive electron transfers, already discussed in the previous paper.
When equilibrium is reached at potential E, there is If only substance O produces light absorption, that is, δ = 1, we can obtain
where
Since P varies with potential, the Nernst plot is not a straight line

d) For ECE mechanism The same can be obtained
Also because P varies with potential, the Nernst plots are not linear.
The Nernst plots are generally nonlinear for such systems where there are other electrochemical processes that affect the equilibrium concentration of the monitored substances.

4. Electrode process of product deposition

Consider the thin layer cell containing only the oxidizing substance O at the beginning, the concentration is CO*, and the initial absorbance of the solution is

Before the deposition of the product, when E gradually becomes negative, CO gradually decreases and CR gradually increases, which conforms to the Nernst equation, the material balance of the relationship equation is
When the potential reaches ES, CR is equal to the solubility product SR of substance R.
When the potential steps to a more negative potential than ES, the concentration of O adjacent to the electrode surface will continue to decrease, and some of the reduced material R will be deposited on the electrode surface, because the concentration of R on the adjacent electrode surface cannot exceed SR, and at equilibrium, its concentration can be expressed by the Nernst equation as (7-21).
Therefore, when E exceeds ES, CO decreases, but CR remains constant for SR, so when E > ES, the absorbance is
where θ(E) = exp[n(E - EO*)F/RT].
At the potential of E = ES, CR = SRwe can get
At a potential of E < ES, then we have
If it is assumed that the reduced substance is deposited on the electrode and does not produce light absorption, which can be achieved by using a mesh electrode, we have when E → -∞
Combine the above equations. This gives us
where
or for E > ES
With a larger value of E, the θ(E)(1 - xf) >> x(1 - f) equation (7-32) approaches a straight line
In the E - ln{r(E)} plot, the slope of this asymptotic lines gives the value of n. The intercept on the E axis for ln{r(E)} = 0 is given by
When E < ES
As E becomes very negative, so that θ(E) << l/(f - x), the above equation (7-36) also approaches a straight line
The slope of the straight line is the same as in equation (7-34), and the intercept at ln{r(E)} = 0 is
Therefore, if x = εR/εO is known, the values of EO' and f can be obtained from the above two intercepts i. When εR = 0, the formula (7-32) can be simplified to the following formula
The intercept and slope directly give the values of n and EO' , and the horizontal distance between the line and the asymptote of equation (7-37) is -lnf.

It was found that the laser dye 1-methyl-4(5-phenyl-2-oxazolyl)pyridinium p-toluenesulfonate was strongly deposited on the Au electrode after reduction.

Fig. 7-1 shows the absorption spectra of 2.36 mmol/L of the dye in 0.4 mol/L KCl aqueous solution at different applied potentials, with an absorption maximum at 368 nm for the oxidized state, which also shows that εR = 0. That is, there is no absorption in the reduced state, but only in the oxidized state.
Fig. 7-2 shows E as a function of ln[r(E)]. The experimental points measured at a wavelength of 368 nm are indicated by +. The solid lines are the theoretical curves of Eqs. (7-33) and (7- 36). The potential pair ln[r(E)] was plotted using the absorbance value at 368 nm, and the measured potential E0 = -1.014 V, SR = 0.35 mmol/L, C* = 2.36 mmol/L [7-2]. The dashed line is the asymptote of Eqs. (7-34) and (7-37).

Reference

[7-1] S. Dong and Y. Xie, J. Electroanal. Chem. 335, 197(1992)
[7-2] W. T. Yap, E. A. Blubaugh, R. A. Durst and R. T. Burke, J. Electroanal. Chem. and Interfacial Electrochemistry, 160, 73(1984)