E. Kirowa-Eisner

Ensembles of microelectrodes: Digital simulation by the two-dimensional expanding grid method: Cyclic voltammetry, iR effects and applications

Microelectrodes differ from infinitely large planar electrodes by the radial component of diffusion. The diffusion toward ensembles of microelectrodes resembles semi-infinite linear diffusion in two limiting cases: at short times, when the thickness of the diffusion layer is small compared to the radius of the individual electrodes, and at long times when the diffusion layers of adjacent electrodes fully overlap. Cyclic voltammograms are calculated by digital simulation with the use of a two-dimensional expanding space grid, which is efficient with respect to computer time. Ensembles of microelectrodes have an advantage over infinitely large electrodes for the measurement of heterogeneous rate constants only in the intermediate time domain, when the diffusion layer is large compared to the radius of the electrode, but there is no significant overlap. The sensitivity in analytical applications is increased when significant overlap of adjacent diffusion layers is reached. For ensembles of microelectrodes with R1<0.1 μm, the sensitivity is up to three orders of magnitude greater than in normal pulse polarography. The influence of the ohmic potential drop at the microelectrodes is estimated and found to be smaller than for infinitely large electrodes. The ohmic potential drop is nonuniformly distributed over the surface of a microelectrode (lower at the edge than at the center). At short times (t

Thermodynamics and kinetics of upd of lead on polycrystalline silver and gold

The formation of underpotential deposition (upd) layers of lead on polycrystalline gold and silver was reinvestigated. Two peaks are observed on gold and only one on silver. On gold, the formation and removal of the upd layers are reversible, up to sweep rates of 0.4 V s−1, while on silver, the reversibility is limited to 0.04 V s−1. The shapes of the peaks on both metals can be explained in the framework of the Frumkin isotherm, with different interaction parameters. The conditions for mass transport involvement have been derived. Measurements can be conducted up to sweep rates of 1.0 V s−1 in 12 mM solutions without significant involvement of mass transport limitation. On the other hand, when deposition occurs under purely mass transport limitation, the surface responds as an ensemble of ultra microelectrodes. The shape of the anodic stripping peak is independent of the surface coverage reached during deposition, and the anodic peak potential always occurs when half of the atoms initially adsorbed have been dissolved during the positive sweep.

Electroreduction of 1‐Methyl‐2‐, 3‐, and 4‐Carbomethoxypyridinium Ions

The character of both polarographic waves of the three isomers 1‐methyl‐2‐, 3‐, and 4‐carbomethoxypyridinium ions (2+, 3+ , and 4+, respectively) has been studied in acetonitrile and in methanol. The first wave is due to a one‐electron reduction under DC and normal pulse polarographic (NPP) conditions. Whereas a stable free radical 4ċ is reversibly formed in the reduction of 4+ and a dimer is irreversibly formed in the reduction of 3+, the 2+ reversibly forms radical 2ċ at low concentration and short pulse width and a dimer under DC conditions. The equilibrium constant and the rate constants for the dimerization reaction dimer have been determined by the reverse‐pulse polarographic method (RPP). The radicals 2ċ and 4ċ have been identified by RPP, and the formal potentials of the couples 2+/2ċ and 4+/4ċ are reported. The second reduction wave of the three compounds is diffusion controlled in protic media (methanol) and yields dihydropyridine as a product. In acetonitrile, the pyridinium salts (n+) exhibit quite complicated behavior, which is dependent on the nature of the pyridinium ion (position of the carbomethoxy group on the ring) and its concentration, the nature of the cations present in the solution, and the time scale in which the electrochemical method is applied. The 3+ isomer does not undergo a second reduction step in this solvent. The anions 2− and 4− react with the cations to form (dihydropyridines), , , or the corresponding dimers n‐n (reaction with the starting material n+). The latter reaction causes a decrease in the amplitude of the second wave and is the most favored reaction in the presence of the indifferent bulky tetrabutylammonium cation, under which conditions the anions (2− and 4−) are most stable and are easily detected. The formal potential of the couples 2ċ/2− and 4ċ/4− are reported.

The silver-silver perchlorate reference electrode in propylene carbonate

Summary A Ag/Ag+ reference electrode in propylene carbonate (PC) was developed. Simple preparation, fast response, good stability and high reproducibility characterize it. Nernst slopes for this electrode were determined at different concentrations of supporting electrolyte. Positive deviations from the expected slope of 2.3 RT/F were found. The activity coefficient γAg+ increases with increasing concentration of supporting electrolyte, at constant silver ion concentration, in qualitative agreement with the positive deviations from the theoretical Nernst slopes. The exchange current density of Ag/Ag+ (20 mM) is 2 × 10−5 A cm−2. The effect of the addition of small amounts of water to the solvent was found to be of minor importance.

Mechanism of the electroreduction of glycol aldehyde on the DME

Summary The reduction of glycol aldehyde (GA) in alkaline solution on the dropping mercury electrode was studied. The limiting current is controlled by mass transport and by a preceding chemical reaction (the base-catalysed dehydration of hydrated glycol aldehyde). The plot of i 1 vs . pH goes through a maximum at pH 12.7 due to catalysis of the dehydration process on the one hand and ionization of GA on the other hand. Emphasis was placed on determining the kinetic parameters at low current densities (at the foot of the wave) where the reaction is completely activation controlled. The mechanism involves quasi-equilibrium in a first charge transfer and proton addition steps followed by a second charge-transfer which is rate-determining. The Tafel slope is explained in terms of the recently derived combined isotherm which takes into account the potential dependence of adsorption of intermediates due to charge-transfer and due to competition with the solvent.

Temperature dependence of the transfer coefficient for the hydrogen evolution reaction on the DME

The correlation between the transfer coefficient α for the hydrogen evolution reaction with temperature T was studied at the dropping mercury electrode (DME). The transfer coefficient, corrected for the diffuse double layer, was found to be independent of temperature for LiCl and KCl as the supporting electrolytes. In the case of CsCl, a small residual dependence of α on T is observed. This is most probably an artefact, resulting from lower reliability of the ф2 correction owing to specific adsorption of Cs+ ions. The transfer coefficients have a slight dependence on the concentration of the electroactive species in the range of 0.3–100 mM HCl. In all cases the transfer coefficient is strictly independent of potential. In 3 mM HCl and at 25°C, the transfer coefficients and half-wave potentials for LiCl, NaCl, KCl and CsCl are 0.53, 0.52, 0.52 and 0.50, and − 1.574 V/SCE, − 1.589 V/SCE, − 1.599 V/SCE and − 1.618 V/SCE respectively. Arrhenius plots, corrected for diffuse-double-layer effects, at different values of the corrected potential E − ф2 converge at T = 0, as expected if α is independent of temperature. The electrochemical enthalpy of activation varies from 30 to 60 kJ mol−1 in the range of potentials where the polarographic wave is measured. High reliability of the measurements is achieved due to the excellent reproducibility and periodic renewal of the surface of the DME. At low HCl concentration (0.3–10 mM) the activation current is calculated from the mixed activation- and diffusion-controlled current over the entire polarographic wave. At higher concentrations, only data at the foot of the wave were used in order to eliminate the effect of the polarographic maxima.

The effect of size and location of adsorbed intermediates in the double layer on the tafel slope the reduction of hydroxylamine on the dropping mercury electrode

Abstract The reduction of hydroxylamine on the dropping mercury electrode was studied. Kinetic parameters were determined from measurements at the foot of the polarographic wave. Tafel plots were extended to higher current densities with the use of the Koutecký correction. This molecule was chosen as an example of a relatively small species for which the effect of competition with water is small and the importance of the location of the adsorbed intermediate in the double layer can be demonstrated. The experimentally observed value of b = 96 ± 3 mV (combined with reaction orders of unity both with respect to hydroxylamine and hydrogen ions) was found to be consistent with the notion of an adsorbed intermediate whose reduction is brought about by a potential difference φ M - φ x , where φ x has an intermediate value between φ M and φ s . These results were compared with earlier results found in a study of the reduction of nitroalkanes under similar conditions. The advantages of making measurements on the DME at the foot of the polarographic wave are emphasized.

The temperature dependence of the Tafel slope—I. Instrumentation, calibration and a study of the reduction of hydroxylamine on the dme

Abstract The dropping mercury electrode ( dme ) is used to obtain very accurate current-potential measurements for the reduction of hydroxylamine as a function of temperature. The experimental set-up is fully computerized to obtain a large number of experimental points for proper statistical analysis of the results (100–150 points on each Tafel line). The Nernst slope for the reversible reduction of Cd 2+ and Tl + is used as an internal standard to determine the overall reliability of the experimental setup. Agreement to within 0.4–0.6% with theory is observed over the whole range of temperatures measured, from 1°C to 94°C. The Tafel lines for the reduction of hydroxylamine are found to be linear over 4 decades of current, showing that the transfer coefficient and the symmetry factor are strictly independent of potential (over 0.35–0.45 V). Tafel slopes are measured with a standard deviation of 0.4–1.0 mV and are found to follow the expected temperature dependence, with the transfer coefficient (and symmetry factor) independent of temperature. This is the first reported case of a complex electrode reaction for which the transfer coefficient was measured with high accuracy (0.619 ± 0.004) and found to be independent of temperature.

Reverse pulse voltammetry in ec2 reactions: Electrochemistry of 1-alkyl-4-t-butylpyridinium ions

Abstract The applications of the reverse pulse method are extended in order to distinguish between different paths of electrodimerization. The method for determination of rate constants of the chemical reaction coupled with charge transfer (ec2) is further developed. Measurements of K′d vs. t p provide valuable information for the determination of rate constants. The reverse pulse method is applied in the study of the electrodimerization of 1-alkyl-4-t-butylpyridinium ions (1-alkyl=−CH3; −CH2CH3; −CH(CH 3)2) in acetonitrile. It is found that the electrodimerization mechanism is consistent with the sequence: Py+ + e = Py ⇋ 1 2 Py2. The formal potential Eo′ is measured directly for 1-CH(CH3)2, and indirectly for the other pyridium ions. All values are identical within experimental error (Eo′ = −1.835 V vs. Ag/Ag+). The values of kd and km are measured. With an increase in the size of the l-alkyl group, kd decreases and km increases. This is consistent with the expected “stabilization” of the radicals due to the increase of the l-alkyl group size which hinders the dimerization site (2- and 6-positions).

The effects of adsorption conformation of intermediates on the kinetic parameters of an electrode process mechanism of the electroreduction of chloropyridines on the DME

Abstract The mechanism of reduction of 2-and 4-chloropyridine and 2,3-dichloropyridine in ethanol-water solvent was investigated at the DME. The electrode processes were studied by measurements of Tafel slopes, reaction orders and pH effects. Although the reduction process, involving the positions 2 and 4 to the nitrogen should proceed by the same mechanism, different Tafel slopes were observed for the investigated compounds. These differences and the absolute values of the Tafel slopes are explained by the combined adsorption isotherm on the basis of the adsorption conformation of the intermediates. The proposed mechanism involves: (a) first protonation and electronation in a quasi-equilibrium step resulting in an adsorbed chloropyridine radical (intermediate I) parallel to the electrode surface; (b) cleavage of the C-Cl bond in the rate-determining step, resulting in an adsorbed cation radical (inter-mediate II) which by analogy to π-mers and dimers can be parallel or perpendicular to the electrode, depending on the electrode charge. In the perpendicular orientation of intermediate II, the three investigated compounds have different adsorption conformations, yielding different Tafel slopes as a result of differences in the adsorption parameters m and ∂φx/∂φM. In the parallel orientation of intermediate II the monochloropyridines have the same adsorption parameters and hence the same Tafel slope. The Tafel slopes computed via the combined adsorption isotherm are in agreement with the experimental values.