R. de Levie

On the impedance of electrodes with rough interfaces

Abstract Published treatments of the impedance of “blocking” electrodes with self-similar rough surfaces can be generalized to include the effects of slow faradaic processes. A procedure is outlined to transform the measured half-cell impedance to the shape of the equivalent impedance on a microscopically smooth planar electrode. The analysis supports a fractional phase shift (rather than a constant one) due to interfacial roughness, and can be used to obtain a measure of such roughness from the experimental data.

On the negative faradaic admittance in the region of the polarographic minimum of In(III) in aqueous NaSCN solution

Summary A general relation between the faradaic admittance and the d.c. polarographic current has been derived. Its applicability is demonstrated in the case of the negative faradaic admittance of In(III) in 1 M NaSCN, by calculating the d.c. polarogram from the measured faradaic admittance. The inverse process, the calculation of the faradaic admittance from the d.c. polarogram, is equally feasible in principle, but is less precise. Rate constants for the reduction of In(III) in 1 M NaSCN have been obtained over a 0.8 V wide range of potentials. The procedure used makes it possible to obtain rate constants of fast processes ( k of the order of 0.1 cm sec −1 ) from low-frequency measurements where instrumental errors can be kept to a minimum.

Condensed thymine films at the mercury/water interface: Part I. General features

Abstract The general features of the condensed films formed by thymine at the mercury/water interface are described. The kinetics of their formation are those of nucleation and growth, i.e., of phase formation. The slow nucleation is responsible for the observed capacitance hysteresis. The interfacial capacitance in the presence of a condensed thymine film is remarkably insensitive to thymine or electrolyte concentration, to the nature of the electrolyte or to temperature, but the region of potentials over which the condensed film is stable depends strongly on these variables. At low temperatures, additional condensed films are observed.

Instrument for the automatic measurement of the electrode admittance

Summary An instrument is described which will automatically plot the in-phase and quadrature components of the electrode admittance in the fashion of a polarogram. Correction for uncompensated series resistance is achieved by positive feedback, and synchronous rectification is used for signal detection. The limitations of the instrument are discussed, and illustrations of its applicability are given.

Transport of ions of one kind through thin membranes

SummaryThe equation for steady-state movement of ions of one kind through planar membranes has been solved. Numerical results are given, as well as profiles of potential, field and concentration. For small deviations from the equilibrium potential, an essentially constant intrinsic membrane conductance is obtained, which can be calculated from equilibrium properties. For larger deviations from equilibrium, the intrinsic membrane conductance is still essentially constant for symmetrical interfacial concentrations of the permeable ion, but varies significantly with potential for asymmetric interfacial concentrations, especially if these concentrations are small. In the latter case, one can often use the constant field approximation, for which explicit expressions are presented.

Condensed thymine films at the mercury/water interface: Part III. Thermodynamic analysis

Abstract Direct measurements of the interfacial tension, the charge density and the differential capacitance of mercury in contact with thymine in aqueous 1.0 M NaCl solution show that one can obtain two different sets of thermodynamic parameters, one corresponding to the interface before the onset of nucleation, the other pertaining to that same interface after nucleation and growth have covered it with a condensed monolayer.

Two-dimensional phase transitions at electrochemical interfaces

A two-state Ising model has been applied to the two-dimensional condensation of tymine at the mercury-water interface. The model predicts a quadratic dependence of the transition potential on temperature and on the logarithm of the adsorbate concentration. Both predictions have been confirmed experimentally.

Double-layer dynamics in the adsorption of tetrabutyl ammonium ions at the mercury—water interface: I: Survey

Abstract The literature shows that the adsorption behavior of tetrabutyl ammonium ions exhibits several rather unusual aspects. We report here that, below room temperature, the capacitance exhibits a pit region at low tetrabutyl ammonium concentrations. However, this capacitance pit may disappear at higher adsorbate concentrations. Capacitance—potential curves also provide evidence for the existence of distinct interfacial “states” outside the pit region. The appearance of needle-like capacitance peaks is associated with the existence of one such interfacial state, which clearly involves anion co-adsorption. Finally, the region of tetrabutyl ammonium adsorption is strongly influenced by competitive specific adsorption of both anions and cations, and provides direct evidence for the specific adsorption of common alkali cations at sufficiently negative potentials.

Condensed thymine films at the mercury/water interface: Part II. Effects on electrode kinetics

Abstract Condensed films provide ideal systems to distinguish between electron transfer with or without adsorption, i.e., between so-called inner- and outer-sphere electrode reactions. The one-electron reductions of europium(III) and vanadium(III) at an electrode covered with a condensed thymine film are unaffected by the film pressure, and therefore do not involve direct contact with the electrode, whereas the rate of the one-electron reduction of Co(NH 3 ) 3+ 6 varies with film pressure, as do the two-electron atom transfer reductions of cadmium(II) and lead(II). Apparently, the reduction of Co(NH 3 ) 3+ 6 involves direct contact with the mercury.

Condensed thymine films at the mercury/water interface: Part V. Phase transitions

Abstract The topography of the various phase transitions in a supersaturated thymine solution is described. Most transitions leading from a non-condensed to a condensed film involve nucleation and growth, via a metastable intermediate surface state, as do some transitions between condensed films. The latter have unusually high Avrami slopes and, sometimes, involve multiple intermediate states. Other transitions exhibit exponential transients. Of the three pit regions observed, that at the most positive potentials is present only in super-saturated solutions, is clearly associated with three-dimensional phase formation, and does not lead to a well-defined steady-state interfacial capacitance. Most transitions leading to a non-condensed final state exhibit exponential transients.