Electrodes adsorption

The pzc of a pc-Cd renewed by cutting was determined in dilute fluoride and sulfate solutions by capacitance measurements.645,646 The C, E curves exhibited distinct minima whose depth increased with increasing dilution of the solution (Table 15). This value is ca. 30 mV more negative than that for polished electrodes and reflects the more disturbed surface stmcture of a renewed electrode. Adsorption of aliphatic alcohols and acids has also been studied on these electrodes.645,646... 

At Pt electrodes, adsorption of oxygen species is supposed to be controlled by the Temkin isotherm in the low overpotential region [Damjanovic and Bockris, 1966], whereas in the higher overpotential region, the absence of an oxide layer leads to... 

BB-SFG OF ELECTROCATALYSIS AND ELECTRODE ADSORPTION ON SINGLE METAL ELECTRODES... 

Leung L-WH, Wieckowski A, Weaver MJ. 1988. In situ infrared spectroscopy of well-defined single-crystal electrodes Adsorption and electrooxidation of carbon monoxide on plati-nuk(lll). J Phys Chem 92 6985-6990. 

Solutions in a conductance cell are often stirred to hasten salt dissolution, to promote solution mixing, or to prevent temperature gradients. Some workers observe an upward drift in measured resistances of unstirred solutions 12-17) while others report a downward drift unless the unstirred solution is mixed by shaking of the cell immediately before the measurement9-18-26>. The magnitude of this change is often 0.1 % or more. The effect has not been observed in other cases 8>10). The source of this problem has been variously attributed to temperature variations, electrode adsorption effects and solvent impurities, although the problem has not been analyzed in detail. In all but one of the above cases 12> the resistance of the stirred solution was taken as the true value. 

As Figure 21 shows, even closer are the two oxidations in triferrocenylphosphate, [O = P 0(C5H4)Fe(C5H5) 3]. The small spurious peak present at about + 0.2 V in the reverse scan of the cyclic voltammogram is likely due to electrode adsorption phenomena, given the ascertained stability of the corresponding trication.34... 

As far as the electrode adsorption phenomena caused by P° y erro cenes are concerned, Figure 26 shows the cyclic voltammetric response o the illustrated fluorenyl-ferrocene polymer (Fw 7000). 

As seen the complex displays either a single ferrocenyl-centred oxidation (which however looks like it is partially chemically reversible) or a single cobaltocenium-centred reduction (which is affected by electrode adsorption), thus testifying that no interaction exists among the different metallocene units. 

As illustrated in Figure 43, [Ni(dmit)2] displays a chemically reversible -/2- reduction, whereas the -/0 oxidation step is affected by electrode adsorption phenomena.79... 

As illustrated in Figure 45, this dianion undergoes two successive one-electron oxidations, the first of which is clearly chemically reversible, whereas the second is affected by electrode adsorption phenomena.81... 

A radiotracer method was elaborated for the study of the adsorption phenomena on nickel electrodes formed by electrodeposition of nickel ( nicke-lized nickel electrode). Adsorption studies were carried out with labeled HCl, Fi2S04, H3PO4, thiourea, phenylacetic, and acetic acids in 0.1 mol dm HCIO4 supporting electrolyte. The potential dependence of the adsorption of these species was determined in the potential range —200 to +200 mV vs. relative hydrogen electrode (RHE). It was found... 

Lipkowski and coworkers [332] have applied several electrochemical techniques to study adsorption of A -dodecyl-A, N-dimethyl-3-ammonio-l-propanesulfonate, a model zwitterionic surfactant, on Au(lll) electrode. Adsorption of this compound proceeded via a few states. The first two states were observed at potentials close to the zero charge potential. At low bulk concentrations, a nearly flat film of the adsorbed molecules was formed, which was converted into a hemimicellar state at higher concentrations. The second state at negative potentials corresponded to... 

Electrode mechanism and analytical determination of cocaine and its metaboKtes, such as benzoylecgonine, ecgonine, and ecgonine methyl ester have been studied [192] at HMDE. These compounds were adsorbed at the electrode. Adsorption of benzoylecgonine was stronger than that of cocaine. Consequently, adsorptive stripping voltammetric method for the determination of cocaine was based on reduction of benzoylecgonine. 

The triply branched compound 56+ shows a reversible three-electron reduction process and, at more negative potential, a process strongly affected by electrode adsorption. A comparison with the behavior of dumbbell 34+ enables to assign these processes to the simultaneous first and second reduction of its three bipyridinium units. 

The concentration of the electroactive species of interest is typically in the range 0.1 to 5 mM. The upper concentration limit is chosen to minimize IR, the contribution of the migration current due to the electroactive species and electrode adsorption effects associated with the presence of high concentrations of electroactive species. At concentrations below 0.1 mM, background current terms may start to become significant relative to the Faradaic current. 

A mercury-sulfate electrode served as a reference electrode. All electrode potentials are referred to the potential of the reversible hydrogen electrode in the same electrolyte and at the same temperature as the test electrode. Adsorption measurements were performed in the 0.5 M H2SO4 solution prepared using special purity B-5 sulfuric acid and water doubly-distilled. To remove oxygen dissolved in the electrolyte, pure helium or argon was bubbled through acid solution. 

One-electron processes can be conveniently studied in electrochemical experiments where the oxidation of most simple main group organometallics and hydrides occurs at rather low potentials but generally irreversibly (Eq. (2)) [10,12,27,28]. High reactivity of such cleavage products can even affect the electrochemical process through electrode adsorption. 

Aryl-ether-based dendrons were used as stoppers in a series of rotaxanes containing one to three bipyridinium units surrounded by polyether macrocycles (compounds 74-76) [150], Electrochemical investigation in acetonitrile shows that a first reversible cathodic wave is due to the bipyridinium units, which are all reduced simultaneously and independently at the same potential. Electrode adsorption problems prevented accurate measurements for a second reduction process, also due to the bipyridinium units. On oxidation, two not fully reversible processes are observed, involving both the polyether macrocycle surrounding the bipyridinium units and the dioxybenzene units of the dendritic stoppers. 

Modified electrodes for this analytical purpose have mostly been formed by electrode adsorption of the mediator systems on the electrode surface or by electropolymerization [24,116]. Recently, for example, NAD(P)H oxidations have been performed on platinum or gold electrodes modified with a monolayer of pyrroloquinoline quinone (PQQ) [117] or on poly(methylene blue)-modified electrodes with different dehydrogenases entrapped in a Nafion film for the amperometric detection of glucose, lactate, malate, or ethanol [118]. In another approach, carbon paste electrodes doped with methylene green or meldola blue together with diaphorase were used for the NADH oxidation [119]. A poly(3-methylthio-phene) conducting polymer electrode was efficient for the oxidation of NADH [120]. By electropolymerization of poly(aniline) in the presence of poly(vinylsulfonate) counterions. 

The electrocatalytic activity of various nickel macrocycles in aqueous solution was studied. Cyclic voltammograms indicate that 7 / S -NiHTIM2+, NiMTC2+ and NiDMC + are better catalysts than Ni(cyclam)2+ in terms of more positive potentials and/or their larger catalytic currents [26], Bulk electrolyses with 0.5 mM Ni complexes confirm that these complexes are excellent catalysts for the selective and efficient CO2 reduction to CO. The macrocycles with equatorial substituents showed increased catalytic activity over those with axial substituents. These structural factors may be important in determining their electrode adsorption and CO2 binding properties. 

The two metals act as the plates of a condenser, one of the plates being used for adsorption of the gas and the other being a reference electrode. Adsorption of gas causes a change in contact potential equal to the change in work function. Disappearance of the electrical field between the two plates may be detected either by the steady condenser method or the vibrating condenser method ... 

---