Barry A. Coles

Photoelectrochemical ESR: Part I. Experimental

Abstract A flow system for simultaneous photoelectrochemical electron spin resonance studies is described. The relevant theory is presented and the sensitivity of the technique evaluated. Experiments involving the dye fluorescein are shown to be in good agreement with the theory. It is shown that it is possible to generate radicals by irradiation of electrochemically generated species and to induce photochemical reactions by irradiation of electrochemically generated radicals.

Microwave-enhanced anodic stripping detection of lead in a river sediment sample. A mercury free procedure employing a boron-doped diamond electrode

Microwave activation of electrochemical processes has recently been introduced as a novel technique for the enhancement and control of processes at the electrode-solution interface and is employed here to improve the analytical detection of Pb2+. Instead of the conventional mercury-based accumulation and stripping procedure, mercury-free boron-doped diamond electrodes are employed. The deposition and anodic stripping detection by square-wave voltammetry of Pb2+ in a 0.1 M HNO3 solution is shown to be strongly enhanced by microwave activation at boron-doped diamond electrode. The temperature at the electrode-solution interface is calibrated with reversible redox couple Fe3+/Fe2+ (4 mM Fe3+, 4 mM Fe2+) in 0.1 M HNO3 and a standard addition procedure is developed for the sensitive detection of Pb2+ concentrations from 1 µM to 5 µM. The limit of detection by square-wave voltammetry after 20 s deposition time was found to be 0.1 µM and 1.0 µM with microwave activation and without microwave activation, respectively. Then, the Pb content in a water sediment sample detected by anodic stripping voltammetry at boron-doped diamond electrodes is shown to be in good agreement with two other independent analytical procedures based on ICP mass spectroscopy and on sono-cathodic stripping voltammetry.

Microwave activation of electrochemical processes: convection, thermal gradients and hot spot formation at the electrode|solution interface

Microwave activation of electrochemical processes is possible by self-focussing of intense microwave radiation at the electrode|solution (electrolyte) interface of an electrode immersed in a solution and placed in a microwave cavity. Considerable changes in voltammetric current responses are observed experimentally for the one-electron reduction of Ru(NH3)63+ in aqueous 0.1 M KCl and for the stepwise two-electron reduction of the methylviologen dication (MV2+) in aqueous 0.1 M NaCl. The formation and interconversion of two distinct forms of solid deposits, MVam0 and MVcryst0, on a mercury electrode surface is investigated, both in the presence of microwave activation and with conventional heating. It is shown that microwave activation achieves (i) high temperatures in the vicinity of the electrode, (ii) thermal desorption of deposits from the electrode surface and (iii) limiting currents an order of magnitude higher compared to those induced by conventional isothermal heating to the same electrode temperature.A simple physical model based on Joule heating of the aqueous solution phase is employed in a finite element simulation (FIDAPTM) procedure to explain the differences observed experimentally between conventional heating and microwave activation. Based on the comparison of simulation and experimental data, a considerable thermal gradient and ‘hot spot ’ region in the diffusion layer of the electrode, together with convective mass transport are proposed.

High temperature electrochemical studies using a channel flow cell heated by radio frequency radiation

Abstract Systems involving one and two electron redox reactions were studied under hydrothermal conditions using a novel hydrodynamic method based on a conventional channel flow cell where the working electrode was heated by radio frequency radiation. The diffusion activation parameters obtained with the radio frequency channel cell were compared with independent data from microelectrode high temperature experiments and analysed in terms of the Stokes–Einstein equation.

Accurate and precise zinc isotope ratio measurements in urban aerosols.

We developed an analytical method and constrained procedural boundary conditions that enable accurate and precise Zn isotope ratio measurements in urban aerosols. We also demonstrate the potential of this new isotope system for air pollutant source tracing. The procedural blank is around 5 ng and significantly lower than published methods due to a tailored ion chromatographic separation. Accurate mass bias correction using external correction with Cu is limited to Zn sample content of approximately 50 ng due to the combined effect of blank contribution of Cu and Zn from the ion exchange procedure and the need to maintain a Cu/Zn ratio of approximately 1. Mass bias is corrected for by applying the common analyte internal standardization method approach. Comparison with other mass bias correction methods demonstrates the accuracy of the method. The average precision of delta(66)Zn determinations in aerosols is around 0.05 per thousand per atomic mass unit. The method was tested on aerosols collected in Sao Paulo City, Brazil. The measurements reveal significant variations in delta(66)Zn(Imperial) ranging between -0.96 and -0.37 per thousand in coarse and between -1.04 and 0.02 per thousand in fine particular matter. This variability suggests that Zn isotopic compositions distinguish atmospheric sources. The isotopic light signature suggests traffic as the main source. We present further delta(66)Zn(Imperial) data for the standard reference material NIST SRM 2783 (delta(66)Zn(Imperial) = 0.26 +/- 0.10 per thousand).

Microwave Activated Voltammetry: The Thermally Enhanced Anodic Stripping Detection of Cadmium

In situ microwave activation of electrochemical processes in a novel electrochemical cell, in which intense microwave radiation is focused locally into the region at the electrode surface–solution interface, is shown to allow high-temperature voltammetry experiments at 100 µm Pt disk electrodes. Factors such as the cell geometry and the deposition of a thin film of mercury are shown to influence the microwave effect.The detection of trace metals or impurities by anodic stripping voltammetry is a routinely applied procedure with applications especially in rapid online monitoring, in remote place analysis, or for extremely dilute samples. For cadmium detection by anodic stripping voltammetry microwave radiation is demonstrated to strongly affect the accumulation process but not the stripping process. Calibration of the effects induced by microwave radiation on the experimentally observed voltammetric data, based on the equilibrium potentials for the Fe(CN)64–/3– and the Ru(NH3)63+/2+ redox systems, demonstrates that the data obtained are consistent with a thermally enhanced process. The temperature achievable at the electrode–solution interface before boiling and cavitation occurs, is shown to be strongly dependent on the type of electrode material and surface morphology. At a mercury film electrode deposited on platinum temperatures in excess of 150 °C can be applied in voltammetric experiments in a constant heating mode.

The theory of EC reactions at tubular electrodes

Abstract The theory for EC reactions at channel and tubular electrodes is developed. It is shown that first-order rate constants in the range 10−2 s−1

Microwave Activation of Electrochemical Processes: Square‐Wave Voltammetric Stripping Detection of Cadmiumin the Presence of the Surfactant Triton X

A novel electrochemical cell based on a flow through system in a microwave cavity is shown to allow voltammetric experiments under microwave conditions. Focusing of the microwave radiation at the electrode-solution (electrolyte) interface creates conditions of extreme localized heating with an inverted thermal gradient within the diffusion layer of the electrode and convective flow. The deposition and anodic stripping detection by square-wave voltammetry of Cd2+ in a 0.1 M acetate buffer solution is shown to be strongly enhanced by microwave activation. The temperature at the electrode-solution interface is calibrated with the reversible redox couple Ru(NH3)63+/2+ in 0.1 M acetate buffer. The effect of microwave activation on the square-wave voltammetric responses for the reduction of Ru(NH3)63+ and for the detection of Cd2+ in 0.1 M acetate buffer solution are studied. In the presence of a nonionic surfactant, Triton X-100, which blocks the electrochemical Cd2+ response under conventional conditions, microwave activation is shown to have a considerable effect in enhancing the sensitivity for Cd2+ detection.

Thermal activation of electrochemical processes in a Rf-heated channel flow cell: experiment and finite element simulation

Abstract A novel approach to thermoelectrochemistry is presented which involves the direct heating of the working electrode in a channel flow cell system by eddy currents caused by 8 MHz radio frequency (Rf) radiation. For the model redox systems Fe(CN)63−/4− and Ru(NH3)63+/2+ it is shown that it is possible to perform electrochemical experiments with simultaneous thermal activation at temperatures close to the boiling point of the electrolyte solution. Quantitative analysis of data obtained from thermoelectrochemical studies in the Rf-heated channel flow system is possible with the help of a computer model. Numerical simulation results obtained with a finite element program (FIDAP™) for the complex heat and mass flow during voltammetric experiments at the heated electrode are shown to be in quantitative agreement with experimental data. Both the increase in the rate of diffusion as well as the change in the flow pattern in the heated low viscosity region of the channel are shown to contribute significantly to the enhanced mass transport. After confirming the quantitative agreement of the numerical model with the data obtained for the oxidation of Fe(CN)64− and the reduction of Fe(CN)63− in 0.1 M KCl, the activation energy for the Ru(NH3)63+/2+ redox system diffusion in 0.1 M KCl is determined.

Photoelectrochemical electron spin resonance. Part 4.—The ‘photo-ECE’ reaction and the reduction of 1-halogenoanthraquinones

The reduction of 1-bromoanthraquinone in acetonitrile solvent in the presence of tetrabutylammonium perchlorate, at irradiated (565 nm) gold electrodes is shown to produce the radical anion of anthraquinone. A combination of photoelectrochemical hydrodynamic voltammetry, in situ electrochemical ESR and preparative experiments are used to deduce that the mechanism is a ‘photo-ECE’ reaction, in which the first electron transfer produces the radical anion of 1-bromoanthraquinone. On irradiation, this anion dissociates. Bromide ions and anthraquinone are formed and the latter undergoes a further one-electron reduction at the electrode resulting in the detection of significant photocurrents. The H atom abstracted in the formation of the anthraquinone is shown to arise from the tetrabutylammonium cations and not from the solvent as previously suggested in analogous processes. The related species, the 1-chloroanthraquinone radical anion, is shown to be stable towards irradiation on the electrochemical timescale, whereas 1-iodoanthraquinone is found to undergo reduction at gold in the absence of light in an analogous mechanistic manner to 1-bromoanthraquinone in the light.