The electroneutrality approximation in electrochemistry

The electroneutrality approximation assumes that charge separation is impossible in electrolytic solutions. It has a long and successful history dating back to 1889 and may be justified because of the small absolute values for the permittivities of typical solvents. Dimensional analysis shows that the approximation becomes invalid only at nanosecond and nanometre scales. Recent work, however, has taken advantage of the capabilities of modern numerical simulation in order to relax this approximation, with concomitant advantages such as avoiding paradoxes and permitting a clear and consistent 'physical picture' to describe charge dynamics in solution. These new theoretical techniques have been applied to liquid junction potentials and weakly supported voltammetry, with strong experimental corroboration for the latter. So long as dynamic processes are being studied, for which analytical solutions are unavailable in any case, numerical simulation is shown to render electroneutrality unnecessary as an a priori assumption. © 2011 Springer-Verlag

Electrochemical formation and investigation of a self-assembled [60]fullerene monolayer.

The formation and characterisation of a C(60) monolayer at the electrode|electrolyte interface has been studied by cyclic voltammetry, potential step chronoamperometry and ac voltammetry. The presence of the monolayer is evidenced by the presence of a very sharp peak P in the voltammogram, attributed to the faradaic phase formation of an ordered monolayer, and of a reduction post peak Q associated with the reduction of adsorbed species. The chronoamperograms exhibit a well-defined maximum, characteristic of a nucleation and growth mechanism. By comparison with existing models of phase transitions, a progressive polynucleation and growth mechanism is demonstrated. The monolayer is proposed to consist of a 2D fulleride salt. It is suggested that the formation of the monolayer can take place for a broad range of solution compositions, but requires an atomically smooth substrate such as mercury

Measurement of the diffusion coefficients of [Ru(NH3)(6)](3+) and [Ru(NH3)(6)](2+) in aqueous solution using microelectrode double potential step chronoamperometry

Microelectrode double potential step chronoamperometry is reported which allows the measurement of the diffusion coefficients of [Ru(NH3) 6]3+ and [Ru(NH3)6]2+ in aqueous solutions containing various concentrations of the supporting electrolyte, KCl, KNO3 or K2SO4. In the case of K2SO4, ion-pairing of hexaammineruthenium cations is inferred where for KCl and KNO3 no such effects were noted. The triply charged ion was found to have a significantly lower diffusion coefficient than the doubly charged ion with a ratio of ∼0.71 in the KCl and KNO 3 media. © 2010 Elsevier B.V. All rights reserved

Cyclic voltammetry in the absence of excess supporting electrolyte offers extra kinetic and mechanistic insights: comproportionation of anthraquinone and the anthraquinone dianion in acetonitrile.

Less is more: The use of voltammetry in the total or partial absence of supporting electrolyte provides kinetic and mechanistic information beyond that available from familiar fully supported voltammetry. For example, unsupported voltammetry demonstrates that the comproportionation of anthraquinone and anthraquinone dianion proceeds at a diffusionally controlled rate in acetonitrile, a result which could not be determined from "diffusion- only" voltammetry. Copyright © 2010 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim

Quantitative Voltammetry in Weakly Supported Media: Effects of the Applied Overpotential and Supporting Electrolyte Concentration on the One Electron Oxidation of Ferrocene in Acetonitrile

Chronoamperometry is reported on the one electron oxidation of 3 mM ferrocene in acetonitrile using a 300 (± 5 μm radius gold hemisphere electrode. Varying concentrations of supporting electrolyte (tetra- nbutylammonium perchlorate) are used ranging from the fully supported (100 mM) to the almost completely unsupported (0.1 mM). The response is simulated using the Nernst-Plank-Poisson system of equations and excellent agreement between theory and experiment noted, so vindicating a recent theory (J. Phys. Chem. C, 2008, 112, 13716) for voltammetry in weakly supported media. © 2009 American Chemical Society

Quantitative Voltammetry in Weakly Supported Media. Two Electron Transfer, Chronoamperometry of Electrodeposition and Stripping for Cadmium at Microhemispherical Mercury Electrodes

Double potential step chronoamperometry for the two-electron transfer amalgam-forming deposition and stripping of cadmium at a hemispherical mercury electrode in aqueous solution containing different concentrations of supporting electrolyte is compared with simulated results using the Nernst-Planck-Poisson system of equations. The latter were generated using an approximation of zero electric field at the interface [J. Phys. Chem. C 2008, 112, 13716-13728]; the approximation gives a good agreement between theory and experiment, so validating its use for the description of macroelectrode and microelectrode (but not nanoelectrode) problems involving low or negligible levels of supporting electrolyte. © 2009 American Chemical Society

How Much Supporting Electrolyte Is Required to Make a Cyclic Voltammetry Experiment Quantitatively "Diffusional"? A Theoretical and Experimental Investigation

Theory is presented for cyclic voltammetry at a hemispherical electrode under conditions where the electric field is nonzero and migration is significant to mass transport. The nonlinear set of differential equations formed by combining the Nernst-Planck equation and the Poisson equation are solved numerically, subject to a zero-field approximation at the electrode surface. The effects on the observed voltammetry of the electrode size, scan rate, diffusion coefficient of electroactive and supporting species, and quantity of supporting electrolyte are noted. Comparison is drawn with experimental voltammetry for the aqueous system [Ru(NH3) 6]3+/2- at a Pt macroelectrode with varying levels of supporting electrolyte KCl. The approximations concerned are shown to be applicable where the ratio of supporting (background) electrolyte to bulk concentration of electroactive species (support ratio) exceeds 30, and general advice is given concerning the quantity of supporting electrolyte required for quantitatively diffusion-only behavior in macroelectrode cyclic voltammetry. In particular, support ratios are generally required to be greater than 100 and certainly substantially greater than 26, as has been suggested for the steady-state case. © 2009 American Chemical Society

Voltammetry in the absence of excess supporting electrolyte - ECE-DISP1 reactions: The electrochemical reduction of 2-nitrobromobenzene in acetonitrile solvent

The reduction of 2-nitrobromobenzene in acetonitrile has been studied in the presence and near absence of supporting electrolyte at a mercury hemispherical microelectrode. In the presence of a high concentration of tetra-n-butylammonium perchlorate, voltammetric studies over a wide range of voltage scan rates spanning the transition from convergent to linear diffusion confirms the reduction mechanism as DISP1, rather than ECE, as previously thought by the majority of earlier studies. A rate constant of approximately 23 s-1 at 298 K was inferred for the loss of bromide ion from the 2-nitrobromobenzene radical anion. In the presence of weak support, with concentration ratios of electrolyte to reactant of 5 and 0.5, the Nernst-Planck-Poisson system of equations were used to model the coupled diffusion-migration-reaction. It was found the both the mechanism of reaction and the derived homogeneous kinetics were unchanged in the near absence of supporting electrolyte. © 2011 Elsevier B.V. All rights reserved

Potential step chronoamperometry at hemispherical mercury electrodes: The formation of thallium amalgams and the measurement of the diffusion coefficient of thallium in mercury

Results for the measurement of the diffusion coefficients of Tl0 in a Tl/Hg amalgam (DHg), and Tl+ in 1.0 M KF aqueous solution (Daq) are presented. The values of DHg = (1.2 ± 0.1) × 10-5 cm2 s-1 and Daq = (2.4 ± 0.2) × 10-5 cm2 s-1 at 22 ± 1 °C are obtained by means of comparison of experimental deposition and stripping chronoamperometric transients with theoretical simulations for a 12.5 μm radius hemispherical mercury drop in which Tl is first deposited and then stripped. © 2008 Elsevier B.V. All rights reserved

An experimental comparison of the Marcus-Hush and Butler-Volmer descriptions of electrode kinetics applied to cyclic voltammetry. The one electron reductions of europium (III) and 2-methyl-2-nitropropane studied at a mercury microhemisphere electrode

We present a comparative experimental evaluation of the Butler-Volmer and Marcus-Hush models using cyclic voltammetry at a microelectrode. Numerical simulations are used to fit experimental voltammetry of the one electron reductions of europium (III) and 2-methyl-2-nitropropane, in water and acetonitrile, respectively, at a mercury microhemisphere electrode. For Eu (III) very accurate fits to experiment were obtained over a wide range of scan rates using Butler-Volmer kinetics, whereas the Marcus-Hush model was less accurate. The reduction of 2-methyl-2-nitropropane was well simulated by both models, however Marcus-Hush required a reorganisation energy lower than expected. © 2011 Elsevier B.V. All rights reserved