Michael Sharp

Determination of the charge-transfer kinetics of ferrocene at platinum and vitreous carbon electrodes by potential steps chronocoulometry

Potential step chronocoulometric methods were used to measure kinetic parameters for the oxidation of ferrocene at platinum vitreous carbon electrodes in acetonitrile. The formal rate constants observed, kof ∼ 0.02 cm s−1, were independent of electrode material and of the supporting electrolyte chosen (LiClO4, TEAP or TEABF4). After correcting for the effects of double layer structure, the experimental activation energy for electron transfer was found to be ΔG≠ (exp) = 0.03 ±0.01 eV molecule−1 which agreed within 20% with a theoretical value ΔG≠ (theor) = 0.26 eV molecule−1 calculated for activation by solvent repolarization alone from the Born equation.

Aspects of charge propagation through Nafion/Os(bipy)32+/3+ films on glassy-carbon electrodes

Abstract According to a kinetic model, the maximum currents, iL, max that can be obtained from the mediated oxidation of ferrocyanide at glassy-carbon electrodes covered with Nation® 1 /Os(bipy)33+ films are determined jointly by film conductivity and by a saturation in the rate of the surface reaction. Values of iL, max, predicted on the basis of chronocoulometrically measured charge-propagation currents, iE, agreed with those actually recorded by rotating-disc techniques. These observations confirmed the validity of the model and further indicated that iE for these systems was the same for both transient and steady-state conditions, in contradiction with earlier published results. Significant electron hopping between neighbouring redox sites within these coatings was revealed by the dependence of the apparent charge-transport diffusion coefficient, DE(app), on mediator concentration, cT. Attempts were made to explain these results in terms of parallel conduction pathways, variable electron-hopping distances, and migrational effects.

Impedance characteristics of a modified electrode

Abstract Impedance measurements have been made on a graphic electrode covered by poly(4-vinylpyridine) films containing IrCl 2− 6 / IrCl 3− 6 species. The results obtained can be accounted for qualitatively by a simple redox couple model. However, the quantitative agreement is not good and indicates that refinements to the simple model will be necessary.

A comparison of the charge transfer kinetics between platinum and ferrocene in solution and in the surface attached state

Abstract Linear and cyclic potential sweep techniques were used to measure the kinetics of electron transfer between a platinum electrode and ferrocene, both in solution and chemically attached to its surface. The temperature dependencies of the rate constants conformed with Arrhenius behaviour in both cases although the observed activation energies were not equal. The kinetics of the reaction involving dissolved ferrocene were satisfactorily accounted for in terms of Marcus theory. For the surface reaction, interpretation in terms of absolute reaction rate theory was abandoned in favour of an approach based on elastic electron tunnelling through a surface layer where quantitative agreement between calculated and observed preexponential factors in the rate equation was obtained.

Simple preparation and characteristics of stabilized, protonated poly-4-vinylpyridine films containing electroactive anions on glassy carbon electrode surfaces

Abstract Stable, cross-linked, polyelectrolyte coatings were prepared directly on glassy carbon electrode surfaces by reacting poly-4-vinylpyridine with alkyl dihalides in the solid phase. The electrochemical properties of hexachloroiridate(III/IV) and hexacyanoferrate(II/III) couples incorporated in these films are summarised. The analysis of limiting current data from rotating disc electrode experiments provided values for the rate constant for oxidation of Fe 2+ substrate by polymer-bound IrCl 2− 6 under conditions where reaction was confined to the coating/solution interface. Differences between these values and those pertaining to homogeneous solutions and to the interior of the polymer phase are briefly discussed in terms of the diffusion-controlled encounters of the reacting species prior to charge transfer. In these kinetic studies, high substrate concentrations produced limiting currents which did not conform with the predictions of relevant theory. The causes of these discrepancies could not be specified.

Ion-selective electrodes based on 7,7,8,8-tetracyanoquinodimethane-radical salts

Abstract Ion-selective solid-state electrodes were prepared from ion-radical salts of TCNQ. The copper-selective and the silver-selective electrodes showed only a few interferences and behaved according to Nernsts law over 5 and 6 decades of activities, respectively. The tetraethylammonium compound resulted in an electrode selective to this ion compared to ammonium and other tetraalkylammonium ions. The same was true for the tetraphenylarsonium electrode. The electrode response was reasonably fast and the stability acceptable for use as sensors in various applications.

Studies of solid-state ion-selective electrodes prepared from semiconducting organic radical-ion salts

Abstract The primary redox reactions for solid-state ion-selective electrodes prepared from electronically semiconducting salts of 7,7,8,8-tetracyanoquinodimethane (tcnq) can be identified by considering the redox properties of their constituent ions or molecules. Three different processes involving the couples, M n+ /M 0 , 2tcnq o /(tcnq - ) 2 and (tcnq - ) 2 /2tcnq 2- are possible depending on salt composition. Ionic product values determined by potentiometric and atomic absorption methods are in excellent agreement for several such salts; K s ( K 2 tcnq 2 )=5.8±1.2·10 -11 (pot.), 1.7±1·10 -11 (a.a.s.); K s (Cdtcnq 2 ) = 3.0±0.5·10 -9 (pot.), 2.9±0.3·10 -9 (a.a.s.); K s (Pbtcnq 2 ) = 1.3±0.3·10 -10 (pot.), 0.96±0.2·10 -10 (a.a.s.); and indicate that the lower activity limit for electrode response is controlled by the solubility of the sensor material itself. Comparisons of predicted and observed standard electrode potentials provide quantitative support for an ion-exchange mechanism of interference. The behaviour of electrodes prepared from Cu 2 tcnq 2 (copper(I)) and Cutcnq 2 (copper(II)) is explained on the basis of an interference mechanism and considerations of solid-state equilibria.

AC-impedance studies of carbon electrodes coated with poly-4-vinylpyridine films containing the Fe(CN)63−/4− redox couple

Abstract Impedance measurements are reported for the redox couple Fe(CN) 6 4− /Fe(CN) 6 3− trapped in a poly(4-vinylpyridine) film on a carbon electrode. The impedance plane display shows a charge-transfer resistance coupled with an interfacial double-layer capacity. At lower frequencies there is no evidence of a Warburg impedance. Instead there is a transition to a low-frequency capacity. Analysis of the results has enabled the heterogeneous rate constant for electron transfer from the electrode surface to the redox couple to be evaluated.

The determination of electron transfer rates for several quinonoid compounds at platinum and gold electrodes

Abstract Rapid scan cyclic voltammetric methods were used to determine the standard heterogeneous rate constants of single electron transfer reactions involving several quinonoid molecules at platinum and gold electrodes. Measurements were made in acetonitrile, dimethylformamide, dimethylsulphoxide and propylene carbonate solutions. Corresponding activation energies were evaluated on the basis of theoretical treatments of Marcus and of Levich, Dogonadze and Chizmadzhev and were compared with theoretically calculated values. Discrepancies between theory and experiment are discussed qualitatively in terms of reactant and solvent structures.

Some solid-state perchlorate selective electrodes

Abstract Solid perchlorate salts derived from the radical-cations of several p -diamines have been examined with respect to their use in solid-state anion-sensitive electrodes. Nernstian behaviour was approached over activity ranges 10 -1 –10 -3.5 M perchlorate for three of the materials studied and selectivity for perchlorate ions over some common anions observed. Electrode response was fast and the stabilities of potentials acceptable. For the electrode containing o -tolidine perchlorate, an explanation of the observed behaviour has been attempted on the basis of acid-base and redox equilibria.