Jon C. Ball

Anodic stripping voltammetry of copper at insonated glassy carbon-based electrodes: application to the determination of copper in beer

The suitability of ultrasound-assisted anodic stripping voltammetry (sono-ASV) for the detection of total copper content in beer using both mercury thin film and glassy carbon electrodes has been investigated. An immersion horn probe is introduced into a small thermostatted conventional three electrode cell (20 cm3) opposite the working electrode: an ex situ mercury plated Nafion®-coated mercury film electrode or a bare glassy carbon electrode. Minimal sample pre-treatment is required which consists of acidification of the beer with dilute nitric acid and out-gassing with argon. After the deposition of copper (as the metal or its amalgam) on the electrode in the presence of ultrasound, a square wave scan is employed to get the analytical signal. In the absence of ultrasound, electrode passivation by organic species and lower rates of mass transport prevent the observation of any measurable signals. In situ cavitational cleaning of the electrode by insonation maintains the electrode activity. Total copper content levels in the range of 100 to 300 µg Cu L–1 were determined by sono-ASV using both electrode substrates and showed excellent agreement with values provided by an independent method. This highlights the validity of the sono-ASV method as a useful electroanalytical technique in hostile media.

Sonoelectrochemical and sonochemical effects of cavitation: correlation with interfacial cavitation induced by 20 kHz ultrasound

Sonoelectrochemical measurements at macro-electrodes under extreme conditions with a very short distance between ultrasonic horn tip and electrode and different ultrasound intensity levels are shown to result in violent cavitation detected in form of current peaks superimposed on the average limiting current. Analysis of the current data obtained for the oxidation of ferrocene in dimethylformamide (0.1 M NBu4PF6) at a 4 mm diameter Pt disc electrode and for the reduction of Ru(NH3)6(3+) in aqueous 0.1 M KCl at a 6 mm diameter Pt disc electrode consistently indicate a change of the physicochemical nature of sonoelectrochemical processes under extreme conditions. The sonoelectrochemical measurement of the rate constant for the carbon bromide bond cleavage of a 3-bromobenzophenone radical anion electrogenerated at a glassy carbon electrode in dimethylformamide solution in the presence of power ultrasound is shown to yield evidence for a breakdown of the conventional mass transport model of a planar diffusion layer under extreme conditions. The change can be correlated to the number of current data points deviating more than 10% from the mean of the current due to violent cavitation processes superimposed onto the average limiting current. Further, a study of the sonochemical destruction of aqueous dilute cyanide solution (in 0.1 M NaOH) demonstrates a correlation between the electrochemically detected cavitation violence and the sonochemical activity. Factors that govern the violence of interfacial cavitation appear to be directly proportional to the factors that make cavitation in the bulk solution chemically efficient.

Voltammetry of Electroactive Oil Droplets. Part II: Comparison of Experimental and Simulation Data for Coupled Ion and Electron Insertion Processes and Evidence for Microscale Convection

Modelling electrochemical processes at the three phase junction between electrode–aqueous electrolyte–oil droplet presents a considerable challenge due to the complexity of simultaneous electron transfer between electrode and droplet, ion uptake or expulsion between droplet and aqueous phase, the interaction of redox centers at high concentration, and transport processes accompanying the electrochemical process. For the case of oxidation of para-tetrahexylphenylenediamine (THPD) microdroplet deposits on basal plane pyrolytic graphite electrodes or random arrrays of microelectrodes (RAM) three models may be envisaged which proceed via A) exchange of ions between droplet and aqueous electrolyte with the electrochemical process commencing at the electrode–oil interface, B) rapid electron transport over the oil–aqueous electrolyte interface and the electrochemical process commencing from the oil–aqueous electrolyte interface inwards, and C) slow electron transport across the oil–aqueous electrolyte interface and the electrochemical process commencing solely from the triple interface. Numerical simulation procedures for these three models, which allow for interaction of redox centers via a regular solution theory approach, are compared with experimental data. A positive interaction parameter, Z=1.4, consistent with a dominant ionic liquid–ionic liquid and neutral oil–neutral oil type interaction is determined from experimental data recorded at sufficiently slow scan rates. The overall mechanism, which governs the voltammetric characteristics at higher scan rates, is shown to be apparently consistent with the triple interface model C). However, the rate of diffusional transport determined by comparison of experimental with simulation data is orders of magnitudes too high. Additional convection processes, possibly of the Marangoni type, appear to be responsible for the fast rate observed for the redox process. The significance of the models presented in the context of microdroplet deposits for other related electrochemical systems is discussed.

The Use of Sonotrodes for Electroanalysis:Sono-ASV Detection of Lead in Aqueous Solution

Sonotrodes were made by implanting a working electrode into the tip of an ultrasonic horn. These were investigated for the use in ultrasound assisted anodic stripping voltammetry (ASV) of Pb2+ and Cu2+ in aqueous solutions to which Hg2+ was added to permit the formation of lead/copper amalgam in a plating step. Insonation not only allows this preconcentration step to take place under conditions of unusually high mass transport but also causes enrichment of the trace metals in the form of intermetallic compounds leading to sharp stripping responses. The effect of increasing insonation time on the oxidation peak sizes, shapes and positions was studied. Comparison was made with the voltammetry of Pb2+ and Cu2+ observed at platinum electrodes in the absence of Hg2+ where broad signals for both were seen. The nature of the deposition of mercury on platinum electrodes was studied by both voltammetry and atomic force microscopy (AFM). The growth of mercury droplets with time under ‘silent’ plating conditions was seen. In anodic stripping experiments using platinum sonotrodes the ratio of Hg2+ to Pb2+ in the codeposition was examined along with the total amount of charge deposited for a range of Pb2+ concentrations. Sharp anodic stripping responses were obtained down to a Pb2+ concentration level of 2 μgL−1 making sono-ASV a potentially valuable technique for the detection of Pb2+ in solution. In contrast glassy carbon sonotrodes were found to be unsatisfactory since the adhesion of mercury under insonation was poor.

Voltammetry of Electroactive Oil Droplets. Part I: Numerical Modelling for Three Mechanistic Models Using the Dual Reciprocity Finite Element Method

Diffusion problems encountered for the case of electrochemical conversion of microdroplets of an electroactive oil deposited on an electrode are studied by the dual reciprocity finite element simulation method (DRM). Three plausible mechanistic models are compared. For a microdroplet of electroactive material A) the electrochemical reaction commences from the oil–electrode surface interphase; B) there is rapid charge conduction over the droplet surface so that ion insertion occurs from the droplet–aqueous electrolyte interface; C) the electrochemical reaction occurs only at the three phase boundary oil–aqueous electrolyte–electrode. Cyclic voltammograms and potential step transient responses are simulated. The dimensionless peak current and peak separation as a function of dimensionless scan rate are presented and a comparison of the three mechanistic models shows distinctly different behaviors. The normalized transient currents as a function of normalized time are given and compared for the three mechanistic models. The results provide a clear basis for the mechanistic characterization of real systems in terms of the three models considered.

Anodic stripping voltammetry at flow-through electrodes: theory and experiment

Abstract Numerical simulations based on the Time Dependent Backwards Implicit method are used to implement the theory of anodic stripping voltammetry peaks at mercury thin film channel electrodes. First the peakshape is found to be a sensitive function of both electrolyte flow rate and voltage sweep rate. Criteria are established to characterise the conditions under which symmetric or asymmetric peakshapes are expected. Second the mercury film is shown to be stripped in a spatially non-uniform fashion with the flow inducing the upstream part of the film to be depleted before, and at less positive potentials than that downstream. Implications for practical ASV are noted. Experiments on the ASV detection of Pb 2+ at a well-characterised channel electrode are reported and the waveshapes measured as a function of solution flow rate are found to be in excellent agreement with theory.

Photoelectrochemistry of bromonitrobenzenes: mechanism and photoelectrochemically-induced halex reactions

The theory for homogeneous ECrevE, ECrevCE and ECECE mechanisms at macroscopically large channel electrodes is derived. The photoelectrochemical reductions of para -bromonitrobenzene and 2,4-dibromonitrobenzene in acetonitrile solutions and at macroscopic platinum channel electrodes are studied using irradiation at 330 and 470 nm, corresponding to absorption bands in the corresponding radical anions. The former compound is shown to follow a homogeneous ECrevCE pathway in acetonitrile solutions containing tetrabutylammonium-based supporting electrolytes; by changing the supporting electrolyte to a salt of the tetramethylammonium cation, the mechanism is changed qualitatively and follows an ECEE pathway. The photoelectrochemical reduction of 2,4-dibromonitrobenzene in acetonitrile solution containing supporting electrolytes derived from the tetrabutylammonium cation is shown to follow an overall ECECE mechanism, with both chemical steps being chemically-reversible, and with the loss of the ortho -bromo in the dark. In the presence of chloride supporting electrolytes, it is shown that light-induced rupture of a C � /Br bond occurs reversibly with the competing formation of a C � /Cl bond. Unoptimised bulk photoelectrosynthesis indicates that some halogen exchange occurs, demonstrating the viability of a novel approach to halex reactions. # 2002 Elsevier Science B.V. All rights reserved.