Investigation of potential inversion in the reduction of 9,10-dinitroanthracene and 3,6-dinitrodurene

In the case of two-electron reduction of a compound, potential inversion refers to the situation where introduction of the second electron occurs with greater ease than the first. That is E⁰₁?E⁰₂<0 where E⁰₁ and E⁰₂ are the standard potentials for the two steps of reduction. The extent of potential inversion in the reduction of 9,10-dinitroanthracene, 1, and 3,6-dinitrodurene, 2, has been assessed by steady-state microelectrode voltammetry, cyclic voltammetry under conditions of near-reversible behavior, cyclic voltammetry under conditions of quasireversible behavior and electrochemical impedance spectroscopy (EIS). The studies were conducted in acetonitrile at 298 K. Cyclic voltammetry under quasireversible conditions and EIS were most sensitive to small changes in E⁰₁?E⁰₂. The value of E⁰₁?E⁰₂ for 1 was found to be ?107 mV and that for 2 was ?280 mV.     

Influence of the coordination sphere on the mechanism of cobalt nucleation onto glassy carbon

Electrolytic phase formation of cobalt onto a glassy carbon electrode (GCE) was investigated using linear sweep voltammetry and the potential step technique in aqueous 10^?2 M CoCl₂+1 M NH₄Cl at pH 4.6 and 9.5. Thermodynamic, voltammetric and spectrophotometric analysis of the solutions showed that the predominant chemical species of Co(II) in solution were the Co(H₂O)₆²⁺ ion at pH 4.6 and the Co(NH₃)₅²⁺ complex at pH 9.5. Voltammetric analysis showed that the experimental equilibrium potential of the Co(NH₃)₅²⁺/Co(0) system was more negative than the Co(H₂O)₆²⁺/Co(0) couple. However the electrocrystallization overpotential for cobalt deposition on GCE from the aquo complex was higher than for the amine complex. Analysis of the current–time transients obtained at each pH, indicated that distinct mechanisms of nucleation are involved during the early stages of cobalt deposition. In the case of Co(H₂O)₆²⁺ reduction, the transients were described theoretically in terms of 3D nucleation with diffusion controlled growth. For cobalt deposition from the Co(NH₃)₅²⁺ species, the transients were explained by a combination of three different kinds of parallel nucleation processes, 2D progressive nucleation, 2D instantaneous nucleation and 3D progressive nucleation, each of which was limited by lattice incorporation of cobalt ad-atoms.     

Alkaline-earth cation transfer assisted by monensin across the water ∣ 1,2-dichloroethane interface in the presence of alkali cations

The electrochemical transfer of alkaline-earth cations (Me²⁺) assisted by monensin (HX) across the water ∣ 1,2-dichloroethane (DCE) in the presence of alkali cations (M⁺) is reported. A shift in the peak potential for Ba²⁺ transfer and an increase in ΔE_p from 0.030 to 0.060 V are the principal effects produced by the presence of M⁺. These findings depend on pH and the relative concentrations of both cations. The following chemical exchange reaction M_(w)⁺+HX_(c)+H₂O?MX_(o)+H₃O⁺ coupled to the electrochemical transfer of Ba²⁺ accounts for ΔE_p=0.060 V of the peak observed as a consequence of the net charge transfer of +1 at the interface. These results are in agreement with the hyper-Nernstian slope of 60 mV found for Ca²⁺ and Ba²⁺ ion selective electrodes based on antibiotics with carboxylic groups at intermediate pH values. A theoretical equation for the dependence of Δ_o^wφ_1/2 for Me²⁺ with pH and M⁺ activity was developed.     

Study of the Cu|Cu(NO3)2·H2O|Cu electrochemical thermocouple at low temperatures

The thermoelectric properties of the non-isothermal Cu|Cu(NO₃)₂|Cu cell have been studied. It has been established that the value of the thermoelectromotive force (E) changes linearly with temperature up to the freezing point of the solution. At lower temperatures than this, at concentrations where the frozen phase consists of pure ice, E increases due to the increase in the concentration of the solution, whereas at compositions where the frozen phase is Cu(NO₃)₂·XH₂O, the decrease in Cu(NO₃)₂ concentration results in a decrease of E. At temperatures lower than the freezing temperature, the potential difference measured between the poles of the cell, in addition to the thermo-emf—due to the concentration change during freezing—includes also the electric potential difference caused by the concentration difference between the half-cells. The slope of the E–ΔT straight lines (E′) increases with the concentration of cupric nitrate solutions.     

A new approach to the estimation of electrocrystallization parameters

To overcome the drawbacks in estimating electrocrystallization parameters using traditional methods, we propose a genetic algorithm using a novel crossover operator based on the non-convex linear combination of multiple parents to estimate the electrocrystallization parameters A (the nucleation rate constant), N₀ (the nucleation density) and D (the diffusion coefficient of Zn²⁺ ions) simultaneously in the general current-time expression of Scharifker and Mostany for nucleation and growth by fitting the whole current transients for zinc electrodeposition onto glassy carbon electrode immersed in the acetate solutions. By running the algorithm, we obtained for different step potentials, D values close to 2.10×10^?6cm² s^?1, which are comparable to reported values. The values of A obtained for all step potentials are identical, 1.41×10⁹ s^?1, which indicates that zinc deposition onto glassy carbon electrode follows three-dimensional instantaneous nucleation and growth. In addition, from the values of N₀ obtained, one can observe that an increase in step potential leads to a higher N₀. These results show that our algorithm works stably and effectively in solving the problem of estimating the electrocrystallization parameters, and more importantly, it can be extended easily to a general algorithm to estimate multiple parameters in an arbitrary chemical model.     

Influence of the OPDA additions on impedance response and on anion exchange property of the modified PANI

The addition of the ortho-phenylenediamine (OPDA) to aniline solution resulted in a slower polymer synthesis rate and in the modified PANI structural change. The interaction of both monomers produced the change in PANI morphology, which was confirmed by atomic force microscope (AFM). The stainless steel (SS) corrosion protective properties of PANI and modified PANI coatings were examined in chloride (Cl⁻) media by open circuit potential (E_o.c. vs. t) monitoring. The presence of Cl⁻ in supporting electrolyte resulted in two different E_o.c. vs. t potential regions. The different E_o.c. “drop” in two specific potential regions was attributed to the facilitated and hindered Cl⁻ flux through PANI towards the SS surface. The Cl⁻ flux was found to be governed by the potentially dependant “pump” and “barrier” effects, which were explained to be governed by the PANI anion exchange property. It has been found that the polymer structure has a significant influence on the electroactive behavior of the modified PANI. The influence of the polymer structure on the “barrier” and “pump” effects was additionally investigated by adding OPDA. The denser polymer structure obtained by addition of OPDA resulted in “weakened pump” and “enhanced barrier” effects, which produced hindered Cl⁻ flux. The anion exchange process detected by electrochemical impedance spectroscopy (EIS) in the low frequency region and the PANI anion exchange property were found to be under the influence of the OPDA additions. An increase in OPDA addition resulted in a lower polymer ability to replace the anions, which consequently hindered the process detected in the low frequency region. The method proposed in this paper offers the possibility to predict the modified PANI corrosive protective properties of SS in Cl⁻ media using EIS measurement, which is less time consuming than the long-lasting E_o.c. vs. t measurements.     

Analysis of the hydrogen electrode reaction mechanism in thin-layer cells. 1. Theory

This work develops the equations that relate the kinetic parameters of the hydrogen electrode reaction (HER) with the current density (j) vs. potential (E) dependences of a thin-layer cell (TLC). Two operation modes of the TLC are analyzed. The first one proposes to examine the j(E) dependence of the hydrogen evolution reaction (her) on one electrode while the paired electrode oxidized the dissolved H₂ back to H⁺ under diffusion control. The second mode proposes to analyze the j(E) dependence of the hydrogen oxidation reaction (hor) on one electrode while the other generates dissolved H₂ from H⁺ under diffusion control. In both cases, as very high mass-transport rates are reached for distances in the micrometer range, the j(E) curves are sensitive to the complete set of kinetic parameters even for very large reaction rates. Possible ways to incorporate these equations in the theoretical formalisms of well-established TLC-based techniques such as scanning electrochemical microscopy are discussed.     

Adsorption of human serum albumin on electrochemical titanium dioxide electrodes: Protein–oxide surface interaction effects studied by electrochemical techniques

The adsorption of Human Serum Albumin (HSA) on a semiconductor TiO₂ electrochemical oxide was investigated using Cyclic Voltammetry (CV), Capacity–Potential curves (C–E) and time-resolved techniques as a function of electrode potential and protein concentration. The presence of HSA adsorbed on the electrode surface modifies the voltamperometric behavior of the hydrogen evolution reaction (her) and also produces a major modification in the diffusional layer thickness of the H⁺ ions. The adsorbed amounts of HSA were analyzed throughout different adsorption isotherms. The experimental data were modeled with a modified Langmuir type isotherm, considering a weak chemisorption on a surface with heterogeneity in site-energy distribution with some degree of attractive lateral interactions between the adsorbed protein molecules. The effect of the adsorption potential (E_ads) was investigated polarizing the electrode at −0.70, −0.50 and −0.08 V vs. SCE. The capacity response obtained from the different impedance experiments was determined by the space charge region in the semiconductor. It was possible to correlate processes produced by the protein adsorption on the surface (occurring in the electrolyte side of the interface) with changes in the semiconductor properties of the TiO₂ (in the oxide side of the interface). The adsorption of HSA produces an increase in donor concentration (N_D) of the semiconductor and a shift of the E_fb to more negative values. These effects are more pronounced with an increase in the protein concentration. The relative change in N_D is lower and the change in E_fb is higher when the adsorption occurs at less negative applied potentials. Adsorption kinetics and thermodynamic parameters were calculated for a wide protein concentration range.     

Comparative analysis of alkali and alkaline-earth cation transfer assisted by monensin across the water ∣ 1,2-dichloroethane interface

In the first part of this paper the electrochemical transfer of alkali cations (M⁺) assisted by monensin (HX) across the water ∣ 1,2-dichloroethane (DCE) interface at pH<5, combined with a chemical exchange reaction at 5<pH<9, is proposed as the only mechanism responsible for the transfer of these cations. At pH>9 the current is voltammetrically negligible. An equation for the dependence of Δ_o^wφ_1/2 on pH and Na⁺ concentration is developed. In the second part of the paper the transfer of alkaline earth cations assisted by the same ionophore is studied. The electrochemical reactions (Me_(w)²⁺+HX_(o)?MeHX²⁺_(o)) and (Me_(w)²⁺+X^?_(o)?MeX⁺_(o)) are responsible for the peaks observed at pH<5.0 and at pH>9.0, respectively. As expected, in both cases ΔE_p=0.030 V while at intermediate pH the electrochemical exchange reaction (Me²⁺_(w)+HX_(o)?MeX⁺_(o)+H⁺_(w)) is proposed. The net charge transfer of +1 at the interface accounts for ΔE_p=0.060 V for the peak observed ΔE_p in agreement with the hyper-Nernstian slope of 60 mV found for Ca²⁺ and Ba²⁺ ISE based on antibiotics with carboxylic groups at intermediate pH values. The higher selectivity for Ba²⁺ and the tendency in selectivity at alkaline pH found in the ISEs are also observed and thus explained according to the mechanism proposed.     

Anion recognition and electrochemical characteristics of the self-assembled monolayer of nickel(II) azamacrocyclic complex

Anionic recognition of the self-assembled monolayer of dinickel(II) (2,2-bis(1,3,5,8,12-pentaazacyclotetradec-3-yl)-diethyl disulfide) perchlorate (1) was studied electrochemically. The dinickel(II) complex 1 adsorbs on gold electrodes from methanol solutions and yields stable, self-assembled electroactive monolayers (SEMs); the SEM of 1 shows a reversible redox wave at 0.82 V in aqueous 0.1 M NaNO₃ corresponding to the Ni^3+/2+ redox reaction. The surface coverage, Γ, of the self-assembly of 1 determined by cyclic voltammetry is constant (Γ=(1.4±0.08)×10^?10 mol cm^?2) with change in the deposition time (2–36 h) and the concentration of 1 in methanol solution (0.2–5 mM) and is equivalent to a monolayer coverage of the nickel macrocyclic complex. The capacitance of the monolayer of 1 was determined from the double-layer capacitance measurements by chronoamperometry; the monolayer of 1 is assigned to be well-solvated by observing that the dielectric constant of the self-assembly domain (?_film=74) is nearly equal to that of water (?_water=78). Electrochemical investigations reveal that the monolayer of 1 can sense electrochemically various non-electroactive anions, NO₃^?, CF₃COO^?, SO₄^2?, H₂PO₄^?, HPO₄^2?, ClO₄^?, PF₆^? and SCN^?, from the variation of the formal potential, E°′, in aqueous solutions of different anions. The E°′ of the monolayer of 1 is 0.82 V in aqueous 0.1 M NaNO₃ and shifts to a less positive potential, 0.55 V, in aqueous 0.1 M Na₂SO₄; the shift in the E°′ was reversible on exchanging the monolayer of 1 between 0.1 M NaNO₃ and 0.1 M Na₂SO₄. The shift in the E°′ of the monolayer has been explained by an axial coordination of electrolyte anions with the trivalent nickel ion. The redox reaction of the SEM of 1 is not observed in aqueous solutions of 0.1 M NaClO₄ and 0.1 M NaSCN; but the redox activity was retained on changing the monolayer electrode to an aqueous solution of 0.1 M Na₂SO₄ or 0.1 M NaNO₃. The monolayer of 1 could detect electrochemically the biologically important phosphate anion, adenosine triphosphate (ATP), at submillimolar concentrations; on addition of 1 mM ATP, the formal potential of the monolayer shifts towards the less positive potential region by about 250 mV. The CVs of the SEM of 1 were recorded in aqueous solutions containing different concentrations of NaH₂PO₄ or Na₂SO₄, keeping the ionic strength of the electrolyte solution constant with added NaNO₃. The E°′ of the monolayer shifts to the less positive potential region with an increase in the concentration of H₂PO₄^? or SO₄^2? anion in solution phase, and the analysis of cyclic voltammetric results reveals that the nickel(III) complex forms a 1:1 complex with SO₄^2? anion but a 1:2 complex with H₂PO₄^? anion.     

Temperature-dependence of oxygen reduction activity at a platinum electrode in an acidic electrolyte solution investigated with a channel flow double electrode

The temperature dependence of the oxygen reduction reaction (orr) activity at a platinum electrode in 0.1 M HClO₄ electrolyte solution was investigated with a channel flow double electrode at 20–90 °C. We demonstrate that an apparent rate constant (k_app) for the orr can be evaluated from the hydrodynamic voltammograms by correcting the oxygen concentration in the electrolyte solution. The apparent activation energy for the k_app was found to be 38 kJ/mol at ?0.525 V vs. E⁰ (0.760 V vs. RHE at 30 °C). Production of H₂O₂ was not detected at potentials more positive than 0.3 V in the temperature region examined. One capacitive semicircle was observed in electrochemical impedance spectroscopy for the orr, suggesting the fast complete reduction of intermediates to H₂O following the rate determining step of the first electron-transfer.     

Effect of pH and the extent of micellization on the redox behavior of non-ionic surfactants containing an anthraquinone group

Cyclic voltammetry has been employed for the study of aqueous electrochemistry of the surfactants, α-(anthraquinonyloxyhexyl)-ω-hydroxy-oligo(ethylene oxide) (ACPEG) and α-anthraquinonyl-ω-hydroxy-oilgo(ethylene oxide) (APEG), which have wide differences in surface activity. Potential–pH diagrams have been constructed and the various features of the diagrams have been analyzed in the light of the change in solution equilibria and the difference in the extent of micellization. The redox potentials of the surfactants have been found to exhibit strong pH dependence. The electrode reaction involves two-electron reduction of anthraquinone (AQ) to its dianion (AQ^2?), which is highly sensitive to the pH of the solution. At controlled pH, potential–pH plots allow the establishment of the values of the ionization constants for dihydroanthraquinone (AQH₂) and its monoanion (AQH^?) as pK_a¹=7.83 and pK_a²=11.38, respectively. Under unbuffered conditions, the effective pH close to the electrode surface controls the potential of the electrode process. The changeover from the H⁺-available to the H⁺-depleted electrode process gives rise to a sudden jump in potential. In highly alkaline solutions, AQ forms an adduct with hydroxyl ion, which causes a linear decrease in the potentials with increase in pH. The different extent of micellization results in a difference in the peak current and the half wave potentials (E_1/2) for ACPEG and APEG but causes no significant change in the shapes of the E_1/2–pH diagrams. This has been explained in terms of the disruption reaction of the micelles, preceding the electrochemical reaction.     

Liquid junction potential between electrolyte solutions in different solvents: further study on the component related to ion solvation

The liquid junction potential (LJP) between electrolyte solutions in different solvents was studied by paying attention to the component related to ion solvation (component (b)). The actual variation in component (b) was obtained as the variation in corrected emfs (E_corrected) of a cell and compared with the theoretical values (E_j(b)_calc). New data for the E_corrected ? E_j(b)_calc relation were obtained using amphiprotic ethylene glycol (EG) and formamide (FA) as solvent on one side of the junctions. The E_corrected ? E_j(b)_calc relations at miscible FA/ and EG/organic solvent junctions were nearly linear with average slopes of 0.32 and 0.33, respectively, in contrast to 0.46 for H₂O/organic solvents, 0.26 for MeOH/aprotic solvents, and ～0 between two aprotic solvents. On the other hand, for partially miscible FA/ and EG/nitrobenze (NB), the slopes were 0.70 and 0.84, respectively, approaching unity with the decrease in miscibility. Some factors that may influence component (b) were pointed out and were discussed in detail by considering the phenomena at the junctions. Additional data that support the method to estimate the actual values of component (b) by (slope) × E_j(b)_calc were obtained. By appropriate corrections for component (b), the component due to solvent–solvent interactions was shown to be electrolyte-independent also at the junctions containing EG, FA and N-methylformamide on one side.     

Four gbpC Gene Homologues in Streptococcus sobrinus

We previously identified the glucan-binding protein C gene, gbpC, solely involved in dextran-dependent aggregation (ddag) of Streptococcus mutans. Recently, we identified two gbpC gene homologues, gbpC and dbl, in Streptococcus sobrinus, and suggested that the dbl gene was very likely responsible for ddag of this species. However, homology searches with the gbpC or dbl genes against the incomplete TIGR S. sobrinus 6715 database suggested that this strain may have other gbpC homologues. PCR-based chromosomal walking from the gbpC and dbl genes revealed two additional homologous genes designated as gbpC2 and dblB. Proteins encoded by these genes exhibited alpha-1,6 glucan-binding activities. Therefore, gbpC2 and dblB are also logical candidates responsible for the ddag phenotype.     

Electrodeposition of Al–Ni intermetallic compounds from aluminum chloride-N-(n-butyl)pyridinium chloride room temperature molten salt

Electrodeposition of aluminum–nickel intermetallic compounds (particularly Ni₃Al) has been carried out onto platinum and mild steel cathodes from a 2:1 (mole ratio) aluminum(III) chloride-N-(n-butyl)pyridinium chloride (BPC) molten bath saturated with nickel(II) chloride at room temperature. A single phase of Al–Ni alloy is difficult to obtain by controlled-potential and controlled-current methods; however, it can be obtained by pulse current plating. The electrodeposition of nickel from an AlCl₃–BPC–NiCl₂ (6.14:3.07:0.09 mole ratio) molten bath occurs via an instantaneous nucleation mechanism in the very initial stage of the crystal growth. The deposition reaction mechanisms of nickel in this molten bath are revealed by electrochemical analysis. The experimental Tafel slope of 42 mV dec^?1 and the calculated transfer coefficient $\text{α}\text{→}{}_{\mathrm{<mtext>c</mtext>}}$of 1.5 suggest that the rate determining step is a charge transfer reaction of an adsorbed bare monovalent cation to the metallic state. The effect of the cycle regime on the electrodeposition of Al–Ni alloys has been investigated. The current efficiency for the deposition of alloys is about 99%.     

Preparation of a novel clay/metal complex hybrid film and its catalytic oxidation to chiral 1,1′-binaphthol

An ITO electrode modified with a hybrid film of chiral metal complex (Λ-[Os(phen)₃]²⁺) and a clay (montmorillonite) has been prepared for the purpose of chiral sensing. As a first step, a floating monolayer of amphiphilic Os(II) complex, [Os(phen)₂(dC18bpy)](ClO₄)₂ (phen=1,10-phenanthroline, dC18bpy=4,4^′-dioctadecyl-2,2^′-bipyridyl), was formed on an aqueous dispersion of sodium montmorillonite. The monolayer acted as an organic part for the hybridization of clay particles in an aqueous phase. The hybrid film of clay and amphiphilic metal complex was transferred onto an indium tin oxide (ITO) substrate by the vertical dipping method. The next step was to immerse the electrode in chloroform, during which the amphiphilic Os(II) complex was removed from the clay surface. Thereafter the electrode was immersed in an aqueous solution of 0.5 mM Λ-[Os(phen)₃](ClO₄)₂ and rinsed with water. Cyclic voltammetric measurements were performed at each step of the above procedures. When the observed curves were simulated on the basis of a double-layered modified electrode, the electron transfer rate constant (k₁) for Λ-[Os(phen)₃]²⁺/Λ-[Os(phen)₃]³⁺ was determined to be 0.25 s^?1. This Os^II/Os^III redox couple was found to mediate the electrochemical oxidation of chiral 1,1^′-2-binaphthol in a stereoselective way: i.e., the S-isomer was oxidized at a 1.4 times higher rate than the R-isomer.     

Kinetics of the electrodeposition of PbSn alloys: Part I. At glassy carbon electrodes

The electrodeposition of lead, tin and lead–tin alloys on glassy carbon has been studied by electrochemical techniques. Potentiostatic I–t transients were recorded to obtain the nucleation mechanism, while cyclic voltammetry was used to characterize the system. The alloy composition was determined by differential pulse anodic stripping voltammetry. Since the redox potentials for lead and tin are similar in the non-complexing electrolyte used, HBF₄, a peak-fitting program was used. In the fitting procedure the half-peak width obtained in the single metal systems were retained for the alloy, while the height and the position of the peaks were allowed to change. Structural information on the electrodeposited layers was obtained by X-ray diffraction, and scanning electron microscopy was used to determine the surface morphology. The experimental results clearly show that the deposition of lead, tin and lead–tin alloys on a glassy carbon substrate is a diffusion-controlled process with a three-dimensional (3D) growth mechanism. The nucleation is instantaneous, and the number of nucleation sites increases with increasing overpotentials, dE/d log(N₀)?60 mV decade^?1. The deposition of tin results in well-defined crystals with tetragonal shape and large areas of free glassy carbon surface. The crystallites have different sizes, which indicates fast surface diffusion of small units. In the presence of lead the microstructure of the electrodeposited tin changes drastically, resembling the microstructure of pure lead. Even small amounts of lead inhibit the deposition of tin on tin and prevent the formation of dendrites. From the stripping analysis it can be concluded that although tin and lead seem to be deposited side by side in an eutectic type alloy, some influence on the stripping potential is observed.     

Numerical simulations of linear scan anodic stripping voltammetry at a modified square array of hemispherical microelectrodes located in a thin-layer cell

The finite element method has been used to simulate LSASV measurement at a loosely packed square array of recessed microdiscs and of hemispherical mercury microelectrodes located in a thin-layer cell. The mercury volume (radius=7.5 μm), the protective gel (thickness=300 μm) and the solution medium (thickness=200 μm) have been meshed. The concentrations of the reduced and oxidised species were linked both by the Nernst equation and the equality of the fluxes at the mercury|gel interface. Both overlapping of adjacent diffusion layers and depletion occurring in this finite box decrease the amount of species being reduced during the deposition step. However, the peak width (W^1/2=39 mV) and peak potential (E_p?E^′0=?34.6±1 mV) of the resulting stripping current are independent of the deposition time. During the stripping step, the metal concentration at the surface of the electrode is much higher than the initial concentration. The time to reach a uniform concentration in the gel again after the voltammetric reoxidation step has been determined, for both single microelectrode and a square array, as a function of the deposition time, thus giving the maximum frequency of successive measurements that should be used. Agreement between simulated and experimental results confirms that diffusion is the predominant transport process occurring in such thin-layer cell. In addition, comparison with analytical expressions has been done whenever possible.     

Electrochemical oxidation in different media of bis(p-phenylene crown ether) as a symmetrical molecule bearing two identical redox centers

The oxidation of the long chain polyether p-disubstituted benzene derivative (BO5O5) and the related macrocyclic paracyclophane (BBO5O5) have been investigated by cyclic voltammetry at different concentrations (10⁻³–3×10⁻² M) and potential scan rates (0.05–250 V s⁻¹). At about 10⁻³ M in CH₃CN or CH₂Cl₂ containing a perchlorate salt as the supporting electrolyte, BO5O5 was oxidized reversibly into moderately stable radical cations. The formal potential E°′ corresponding to this first oxidation step was found to be shifted towards more positive values when the cation was changed from Bu₄N⁺ to Na⁺, and this variation may be ascribed to the coordination of the alkali cation by the polyether chains. At higher concentrations of the reactant and in a thoroughly dried medium, no deposit of a polymeric material onto the electrode surface was electrogenerated. At low scan rates and under the same conditions, the oxidation of BBO5O5 gave rise to two closely spaced, irreversible processes. The radical cations involved in each step were thought to be highly reactive towards nucleophilic species present in solution. At high scan rates, a single reversible one-electron transfer was observed, the formal potential of which varied with the nature of the electrolyte cation in the same way as for BO5O5. The heterogeneous charge-transfer rate constants estimated for BO5O5 and BBO5O5 were compared with the values predicted by the Marcus theory. As for BO5O5, attempts to electropolymerize BBO5O5 were unsuccessful.     

Spatially limited diffusion coupled with ohmic potential drop and/or slow interfacial exchange: a new method to determine the diffusion time constant and external resistance from potential step (PITT) experiments

We have analyzed chronoamperometric curves, I(t), after small-amplitude potential steps ΔE (PITT technique) for the model of linear diffusion of a species inside an electroactive film, taking into account ohmic effects in the external media (solution and electrode) as well as a finite rate of the interfacial exchange. For its short-time interval, t?τ_d (τ_d is the diffusion time constant, corresponding to unlimited diffusion from the interface), three approximate analytical expressions have been proposed. One of these represents an interpolation formula between the value of the current at the start of the diffusion process, I(0)=ΔE/R_ext (after the end of the EDL charging), and the Cottrell equation: $\text{I?I(0)/(1+Λ(}\text{π}\text{t/τ}{}_{\mathrm{<mtext>d</mtext>}}\text{)}{}^{\mathrm{1/2}}\text{)}$, Eq. (9) where Λ=R_d/R_ext is the ratio of diffusion and external (solution, electrode, etc.) resistances. Its comparison with the exact analytical solution derived recently by [Montella, J. Electroanal. Chem. 518 (2002) 61] shows the ability of this simple approximation to reproduce qualitatively the current-time dependence within the short-time interval for a wide range of Λ including the case where the external resistance is dominant. Another similar formula, Eq. (10), but with fractional exponents results in even better agreement with the exact result. Both analytical expressions enable one to evaluate the parameters, τ_d and Λ, of a real system by linear fitting of its experimental data in the corresponding coordinates, e.g., in coordinates, [I(t)t^1/2]^?1 vs. t^?1/2 for the above analytical expression. The treatment of experimental data in these coordinates also allows one to determine the upper limit of the “short-time range” in which the analytical approximation is applicable so that only the points for this time interval are used for the fitting procedure. We have also derived an analytical expression (11) for the same short-time interval, which reproduces the exact solution with a much higher precision. Since the use of this formula does not allow one to extract parameters of the process from experimental data by the simple linear fitting, we have proposed an original procedure of the data treatment to determine τ_d and Λ without complicated calculation or optimization schemes.