Voltammetry at partially covered electrodes: Part II. Linear potential sweep and cyclic voltammetry

Effect of inhomogeneity of the electrode surface on the linear potential sweep and cyclic voltammograms is investigated theoretically and experimentally using model electrodes partially covered with photoresist layer. Good agreement between the theoretical and experimental results is obtained.     

The electrooxidation of methanol and related compounds at ruthenium dioxide-coated electrodes

The underlying metal was observed to corrode when a ruthenium dioxide-coated titanium electrode was anodized in an aqueous methanol solution. With a similarly coated platinum electrode peaks were observed on the voltammogram below 1.0 V which were attributed to methanol oxidation on the underlying metal. This effect was more pronounced when the electrode was subjected during cycling to potentials close to 0 V. Rapid oxidation of methanol on RuO₂ was observed at potentials above 1.0 V, the rate at a given potential increasing in an approximately linear manner with increasing alcohol concentration. The rate of reaction also increased with increasing temperature and increasing surface roughness. Tafel slope values were rather high (>100 mV decade^?1) and a mechanism involving anodically generated species such as OH_ads was proposed to account for these results. The variation of activity with pH was similar to that reported earlier for oxygen evolution at these anodes and this was again explained in terms of partial deactivation of the surface due to a combination of proton loss and phosphate ion adsorption at intermediate pH values. The release of carbon dioxide from aqueous solutions of higher alcohols at 25°C confirmed the high oxidizing power of RuO₂ anodes.     

On-line FFT faradaic admittance measurements application to A.C. cyclic voltammetry

A measurement approach is described and data are presented which demonstrate the ability to effect a.c. cyclic voltammetric measurements with the on-line digital FFT approach to faradaic admittance data acquisition. The equipment utilized enables complete faradaic admittance spectra to be obtained at an effective spectrum acquisition rate of 10 s^?1, so that the d.c. potential range encompassed by a typical cyclic wave can be encompassed with adequate resolution in the E_dc dimension in ≥6 s, approximately. The instrument features dynamic, computerized measurement and compensation of the non-faradaic ohmic resistance and double-layer capacitance contributions to the acquired total cell admittance. Measurements with quasi-reversible systems yield the expected faradaic admittance and phase angle responses over a quite generous bandwidth. Applications to mercury and platinum electrodes are illustrated.     

Hydrodynamic voltammetry at channel electrodes: Part VII. Current transients at double channel electrodes

Expressions for the transient current at the downstream electrode in response to galvanostatic electrolysis at the upstream electrode in the channel flow cell were derived by applying double Laplace transformation when the electrode reaction at the upstream electrode is kinetically controlled. The ratio of the transient current to the steady state current or the transient collection efficiency was calculated as a function of electrode geometry and θ

, where U_m is the mean flow velocity in the channel cell, D the diffusion coefficient of the electroactive species, b the half height of the channel, x₁ the length of the upstream electrode and t the time elapsed since the beginning of the galvanostatic electrolysis at the upstream electrode. Curves for the transient collection efficiency can be applied to evaluating the amount of adsorption at the upstream electrode when metal at the electrode is anodically dissolved in solution. Digital simulation was carried out. Transient curves, obtained analytically, were in good agreement with those evaluated from the digital simulation. In order to allow one to draw transient curves readily, we derived a simple approximate equation.     

Polymer films on electrodes: Part III. Digital simulation model for cyclic voltammetry of electroactive polymer film and electrochemistry of poly(vinylferrocene) on platinum

The electrochemistry of electrodeposited poly(vinylferrocene) (PVF) and poly(vinylferrocene acrylonitrile) (PVFAN) films on platinum electrodes was studied in acetonitrile solutions using perchlorate or p-toluenesulfonate as the counter ion by cyclic voltammetry. A model is proposed for the cyclic voltammetric behavior of polymeric films on electrode surfaces. The model incorporates non-equivalent redox sites with interconversion between such sites, electron-transfer kinetics at substrate/film interface and diffusion within the film. Parameters are obtained which yield a good fit to the experimental results.     

Hydrodynamic voltammetry at channel electrodes: Part III. Theory of kinetic currents

A theory of kinetic currents at a channel electrode is developed without any assumption on the magnitude of the rate constant of the preceding chemical reaction. An approximate equation with reasonable accuracy which relates the limiting current to the rate constant, flow velocity and the electrode geometries, is presented. The validity of the equation derived from the concept of the thin reaction layer is also discussed.     

Convolution and finite difference approach: Application to cyclic voltammetry and spectroelectrochemistry

A method combining finite difference analysis of concentration profiles and convolution of faradaic current is proposed for the study of electrochemical systems involving fast electron transfers and secondary homogeneous chemical reactions by large amplitude techniques such as cyclic voltammetry. It amounts to carrying out the fitting of experimental data with the formal kinetics of the assumed mechanism automatically inside the computer. The conditions of applicability are those where the system is under mixed diffusion-chemical reaction kinetic control. Accuracy is first evaluated in simulated experiments testing separately and then simultaneously the two main steps of the treatment: finite difference resolution and numerical convolution. It is shown that the method not only provides a determination of the rate constant of the rate determining step but may also serve to mechanism diagnosis. The practical applicability of the method is illustrated on the example of the electrodimerization of acetophenone in acetonitrile. Extension of the method to the treatment of data obtained by combined electrical and spectrochemical experiments is discussed.     

The cathodic reduction of HCl dissolved in LiCl+KCl+ZnCl2 melts

The cathodic reduction of HCl dissolved in molten LiCl+KCl eutectic with the presence of up to 19 mol %ZnCl₂ was investigated chronopotentiometrically and voltammetrically. On glassy carbon electrodes the reduction is a reversible, one-electron charge transfer forming soluble product and without kinetic complications. While the ZnCl₂ causes an increase in solubility of the HCl, it also causes a decrease in the diffusion coefficient of the reduced species. Results obtained when using a platinum working electrode contrasted with those for the same solvent when using a glassy carbon electrode. The difference is probably due to adsorption occurring on the working electrode, the degree of adsorption approximating to monolayer coverage. The adsorption can be explained by either an AR, SR model or an SAR model and while the accuracy is insufficient to enable differentiation betwen the two, the latter mechanism is favoured.     

Hydrodynamic voltammetry at channel electrodes: Part II. Theory of first-order kinetic collection efficiencies

The theory of first-order kinetic collection efficiencies at the double channel electrode is developed for the following two schemes: (I) A±n₁e→B (at the generator electrode), B→^kP (in solution), B±n₂e→Y (at the detector electrode), (II) A±n₁e→B, B→^kA, B±n₂e→Y. The exact expressions for the kinetic collection efficiencies are obtained as ascending and asymptotic series with respect to the kinetic parameter. Further, approximate formulae in exponential forms are given, which hold within an error of about 2% for conventional electrode geometry. Finally, the validity of the approximate procedure, which has been used previously to obtain the kinetic collection efficiencies for fast homogeneous reactions, is discussed in comparison with the present theory.     

Hydrodynamic voltammetry at channel electrodes: Part IV. Theory of chronopotentiometry for reversible electrode reactions

The theory of chronopotentiometric measurements at channel electrodes is developed for reversible electrode reactions, by rigorously solving the corresponding time-dependent boundary value problem, where the non-uniform accessibility of the channel electrode surface is taken into consideration. The theoretical equations of the transition time and the potential-time curve are derived as functions of (I_d/I), where I denotes the applied current intensity and I_d the limiting diffusion current obtained at steady-state. Finally, the time variation of the current density distribution at the electrode surface is given.     

X-ray absorption spectroscopy of transition-metal trichalcogenides

The sulfur K and metal L_III absorption spectra of transition-metal trichalcogenides (TMTC's) were measured. The matrix element effect plays an important role in these spectra. It was considered that the structures up to 5 eV above the absorption edge reflect the chalcogen antibonding band, the metal nonbonding d_z² band, and the metal d bands, and that the higher energy structures are derived from the metal s and p bands. The chalcogen antibonding band arises from chalcogen pairing and the metal d, s, and p bands are the mixture bands with chalcogen p orbitals. Evidence that shows that the lowest conduction band of the group IV TMTC's is the chalcogen antibonding band is presented. The overlap of the metal d and metal s bands is promoted by increasing the atomic number of chalcogen atoms.     

Ellipsometry of DNA adsorbed at mercury electrodes: A preliminary study

The adsorption of DNA at a mercury electrode can be detected ellipsometrically. Native DNA is probably more weakly absorbed at positive charges than at negative charges. Denatured DNA is more strongly adsorbed in the reduced form than in the protonated form and is desorbed at large negative charges. Adsorption of both forms is characterized by small negative values of δΔ/δ. It is suggested that these results should be interpreted in terms of an anisotropic film.     

Nucleophilic displacement reactions on fluorine. I. General phenomenon and lone-pair nucleophiles

S_N2 displacement reactions involving halogen atoms are progressively more facile in going from F to I. Diverse and hitherto largely mechanistically uncorrelated examples of S_N2(F) reactions are presented and its is suggested that they form part of a general phenomenon in fluorine chemistry.     

Limiting diffusion currents in hydrodynamic voltammetry: Part IV. Stationary disk and ring electrodes in a rotational flow produced by a rotating disk

A new electrolysis cell for the use in hydrodynamic voltammetry is devised, in which a stationary disk or ring electrode is immersed coaxially in a rotational flow produced by a uniformly rotating disk (rotor) and works as an indicator electrode. Theoretical equations of the limiting diffusion currents at such a disk or ring electrode are derived. The theoretical predictions for the dependence of the limiting diffusion current upon various experimental variables, i.e., the rotation speed of the rotor, the bulk concentration of depolarizer, the geometrical parameter of the electrode and the viscosity of the solution, are verified experimentally for the reduction of ferricyanide ions.     

Second excited singlet state emission spectroscopy of thiocarbonyls. I. Thiophosgene

Well resolved emission spectra have been recorded after dye laser excitation of low-lying vibronic levels in the $\text{B}$(¹A₁) state of Cl₂CS. The effects of substrate pressure, added SF₆ and excitation wavelength on the spectra are reported. A Franck-Condon analysis suggests that the excited state CS bond length is 0.5 Å longer than in the ground state.     

A study of aluminium monofluoride and aluminium trifluoride by high-temperature photoelectron spectroscopy

The Hel photoelectron spectrum of gaseous AIF(X¹Σ⁺) has been recorded and the first three cationic states have been assigned with the aid of PNO/CEPA calculations. The first band shows vibrational structure and analysis of the component separations and relative intensitives leads to values of ω_c = 1040 ± 40 cm^?1 and r_c = 1.59 ± 0.01 Å in the AIF⁺ (X²Σ⁺) state; the corresponding theoretical values are 960 cm^?1 and 1.60 Å respectively. The first adiabatic ionization potential, 9.73 ± 0.01 eV, allows a determination of the quantum defect, δ, in a number of previously observed Rydberg states of AIF. The Hel photoelectron spectrum of gaseous AIF₃ has also been obtained. It is assigned on the basis of ab initio molecular orbital calculations and comparison with the corresponding BF₃ spectrum.     

Optical emission spectroscopy with a microwave-induced plasma in a sealed microtube

A microwave system is used to induce a plasma inside a sealed microtube containing a sample or standard, and the emission from the analyte is measured. The detection limits are about 4 ng for S, Cl and Br, and 100 ng for Cd and Sn, in approximately 30-mul samples. Both gaseous and liquid samples can be analysed.