Robert de Levie

Fractals and rough electrodes

Abstract The recent literature on the effect of electrode roughness on electrochemical measurements is reviewed. Some implicit assumptions and approximations in the model of Nyikos and Pajkossy are examined, as well as its extension to include charge transfer processes. Explicit expressions are then presented for the effects of diffusion and of coupled homogeneous chemical reactions on the electrode impedance measured at the equilibrium potential. Finally, it is shown that (and how) the apparent transfer coefficient depends on electrode roughness.

On the coupling of interfacial and diffusional impedances, and on the equivalent circuit of an electrochemical cell

Abstract It is shown that the Warburg (diffusion) impedance is coupled to the charge transfer resistance, even to the extent that the sign of the Warburg impedance is determined by that of the charge transfer resistance. Experimental data obtained with the polarographic reduction of In (III) in aqueous NaSCN solution illustrate the existence of a negative Warburg impedance. The coupling between the diffusional and interfacial impedances (that is, between homogeneous mass transport and heterogeneous mass consumption or production) appears to be quite a general one, present with both redox and desad phenomena. Several graphical representations of the frequency-dependence of the electrode admittance or impedance are discussed, and the advantages of the complex capacitance plane are illustrated.

Thiocyanate electrocatalysis of the reduction of In(III)

Summary The slow step in the thiocyanate-catalyzed electro-reduction of In(III) on Hg has been identified as the surface reaction of In 3+ with two adsorbed thiocyanate ions. This appears to be the only reaction pathway available in thiocyanate solutions when the pH is sufficiently low to prevent hydrolysis. The reaction rate is proportional to the square of the surface activity sa of adsorbed SCN −

How to Use Excel in Analytical Chemistry and in General Scientific Data Analysis

Part I. Introduction to Using the Spreadsheet: 1. How to use Excel Part II. Statistics and Related Methods: 2. Introduction to statistics 3. More on least-squares Part III. Chemical Equilibria: 4. Acids, bases, and salts 5. Other ionic equilibria Part IV. Instrumental Methods: 6. Spectrometry, chromatography, and voltammetry Part V. Mathematical Methods: 7. Fourier transformation 8. Standard mathematical tools 9. Numerical simulation of chemical kinetics Part VI. Spreadsheet Programming: 10. Macros.

Dipicrylamine transport across an ultrathin phosphatidylethanolamine membrane

Abstract Admittance measurements at zero applied direct voltage are reported for solutions containing 10 −4 to 10 −7 M dipicrylamine, in contact with a black phosphatidylethanolamine membrane. The experimental data can be interpreted quantitatively in terms of a model involving slow phase transfer and fast desorption—adsorption kinetics. The adsorption of dipicrylamine makes it possible to distinguish experimentally between ion transport across the membrane itself and that across the water/membrane interfaces.

Membrane admittance measurements under computer control

Abstract The application of the elegant Fourier transform method of Smith et al. 3,4 to measurements on ultrathin, “black” lipid membrane is described. A digital computer generates a waveform containing many frequencies, applies it to the measurement cell containing an artificial, ultrathin membrane, collects the resulting electrical response, extracts the cell admittance by a substitution method, and facilitates subsequent numerical data analysis. The performance of the instrument is illustrated.

On the measurement of small photo-currents on a dropping mercury electrode

Summary An instrument is described with which small photo-currents on a dropping mercury electrode can be measured automatically. A mercury arc lamp in conjunction with interference filters is used as a source of essentially monochromatic light. Chopping of the light beam and synchronous rectification of the resulting alternating current makes it possible to measure photo-currents, even if these are several orders of magnitude smaller than the simultaneously flowing double-layer charging current. Results obtained with the instrument are given, and are discussed in the light of recent theories of electron photo-emission.

Hydrogen bonding and two-dimensional condensation in uracils

Abstract The molecular organization in compact uracil films at the mercury/water interface is suggested to be a planar array of hydrogen—bonded molecules, similar to that found in the solid state.

On the theory of the faradaic admittance with reactant adsorption

Summary Explicit expressions are derived for the faradaic admittance of metal/metal ion and redox electrodes with reactant adsorption, without making a priori assumptions regarding the dependence of the rate constants on potential. The results exhibit two types of coupling: that between redox and desad processes, and that between interfacial and transport impedances.

The effect of halide ions on the condensation of adenine at the mercury-water interface

Abstract The interface between mercury and concentrated aqueous solutions of adenine exhibits a well-studied capacitance pit, A. In the presence of I − , Br − or Cl − , a second pit region, B, can be observed at more positive potentials. In the presence of Br − anions, the formation of this second capacitance pit is relatively slow, and can be interrupted by switching the potential from region B to region A, in which case the as yet unused fraction of the mercury surface will be covered by an adenine film of type A, coexisting with the earlier-formed film B. Thus it is possible to prepare surfaces fully covered by a controlled mixture of two different condensed phases. This coexistence is metastable because, on a much longer timescale, film B will dissolve at potentials in region A. We also report on some capacitance oscillations at the negative limit of the adenine pit, A, at low temperatures and/or high adenine concentrations.