Electrode coverage

Prefers to the electrode coverage with n-butanol. ox = oxalate ion. 

Gihoy D, Conway BE. 1968. Surface oxidation and reduction of platinum electrodes— coverage kinetic and hysteresis studies. Can J Chem 46 875-876. 

The adsorption coefficient or / is a function of the standard Gibbs adsorption energy alone when the molecules do not interact and the adsorption occurs without limitation, i.e. at very low electrode coverage. This dependence is expressed by the relationship... 

The above relationships were derived for low electrode coverages by the adsorbed substance, where a linear adsorption isotherm could be used. Higher electrode coverages are connected with a marked change in the surface charge. The two-parallel capacitor model proposed by Frumkin and described by the equation... 

It is very simple to determine the value of = T/Tm for a strongly adsorbed substance in electrolysis with a dropping mercury electrode. If a much smaller amount of substance is sufficient for complete electrode coverage than available in the test solution, then the surface concentration of the surface-active substance T is determined by its diffusion to the electrode. 

Fig. 3-35 Growth of electrode coverage with adsorbates on smooth and platinized Pt electrodes at 25°C in 5 M H2SO4 with 0.1 MCH3OH. 

A shadow-mask technique has been applied for the local metal deposition to exclude metal residues on other designs processed on the same wafer (Fig. 4.2b). Such metal residues may be caused by imperfections in the patterned resist due to topographical features on the processed CMOS wafers or dust particles. The metal film is only deposited in those areas on the wafer, where it is needed for electrode coverage on the microhotplates. This also renders the lift-off process easier since no closed metal film is formed on the wafer, so that the acetone has a large surface to attack the photoresist. Another advantage of the local metal lift-off process is its full compatibility with the fabrication sequence of chemical sensors based on other transducer principles [20]. 

The thin layers formed in ABS (pH 5.3) always presented a better coverage of the HOPG surface with DNA molecules than layers formed in pH 7.0 PBS [65]. Comparing the thickness and the electrode coverage of the layers obtained with both ss and ds DNA at different pHs on applying a potential of -I- 0.300 V it was concluded that the layer obtained at pH 5.3 presented a self-assembled lattice that was more relaxed and extended on the surface. The results that were obtained by AFM corroborate previous observations that the best binding efficiency of dsDNA on hydrophobic surfaces occurs at approximately pH 5.5 [65]. 

Andrade and Molina [46] have performed electrochemical impedance studies of mercury electrodes with hematite particles adhered at different electrode potentials. Adhesion of such particles was strong and the decrease in the impedance was accompanied by an increase in the number of attached particles. Experimental results were analyzed in terms of an equivalent circuit including the constant phase element (CPE), the magnitude of which appeared to be directly related to the electrode coverage. A pore model for the metal/hematite particles interface has been proposed. 

Differential Electrochemical Mass Spectrometry (OEMS) was also used for methanol stripping experiments, which can give some information on the electrode coverage by species coming from the adsorption and oxidation of methanol. First, it can be seen from the CVs and the MSCVs recorded on a coreduced PtogRuo2/C catalyst as an example (Fig. 19) that the coverage of the electrode is much lower from methanol adsorption (curves 2) than that from CO adsorption (curves 1). 

Because the formation of an inner-sphere precursor state involves specific chemical interactions between the reactant and the electrode surface, it is difficult to calculate the precursor-complex equilibrium constant. However, such states can be sufficiently stable so that the electrode coverage by the precursor complex approaches unity (i.e., a monolayer of adsorbed reactant is formed). In these circumstances, the observed rate becomes independent of the bulk reactant concentration, and k, can be obtained directly from combined with the estimated close-packed surface concentration. An analogous situation exists for stable precursor complexes formed in homogeneous solution ( 12.3.3.1). 

There are also scientific difficulties in approaching the problem of the entities formed in passivation. This is particularly so in the region of low electrode coverage, where there is no possibility of clearly distinguishing between the adsorbed or (monomolecular or less) oxide layer without an experimental method that is sufficiently sensitive and applicable situ. Experimental approach to the problem was for a long time connected with the measurement of electrochemical parameters, which usually measured an average current density and thus gave little information about the local current distribution, the nature and thickness of the passive film, and the film distribution over the surface. An... 

Figure 29. Rate of reduction of diphenyl ammonium ions versus electrode coverage with organic substance (according to Ref. 181).

Both Nomura (78, 79) and Bruckenstein (80, 81) have used bulk wave devices oscillating at a liquid interface for electrochemical studies. In one study it was reported that 0.02 monolayer electrode coverage could be detected for a metal of molecular weight 100 (81). These studies indicate that selective detection can occur if the analyte is electroactive such that oxidation or reduction causes a surface mass change and if no other sample matrix component is electroactive at the same electrical potential. 

On the other hand, EMIRS is an unique tool to identify "in situ", as a consequence of fingerprinting through their vibrational spectra, the adsorbed intermediates produced, even at very low electrode coverages (a few percent), by the chemisorption of small organic molecules, either the reactive species, or the poisoning species. In most cases the latter were identified as adsorbed CO, either linearly- or bridge- and multi-bonded to the electrode surface. 

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