Zone Diagram Research Articles

Kinetics of electrochemical reactions mediated by redox polymer films: New formulation and strategies for analysis and optimization

A new formulation of the kinetics of electrochemical reactions mediated by redox films is proposed for the case of an irreversible cross-exchange reaction in the context of rotating disc electrode voltammetry. It involves the normalization of the substrate concentration versus its value at the film boundary interface. The overall kinetics are then shown to depend on only two dimensionless parameters. Procedures for analysing the effects of the rotation speed and the other experimental parameters are proposed. The construction of a kinetic zone diagram summarizing the variation of the kinetic control with the various parameters is useful for defining the optimal performances of the film.

Impedance zone diagram of an oxidation mechanism in an adsorbed phase wtth chemical recombination

After calculating the faradaic impedance of a mechanism comprising a stage of anodic oxidation of an electroactive species, followed by superficial recombination of an adsorbate, the sizes of the various elementary impedances of the species were defined. An impedance zone diagram was plotted. It enables the numer of faradaic impedance loops to be determined in relation to the value of the kinetic parameters of the model.

Impedance zone diagram of an oxidation mechanism in an adsorbed phase with chemical recombination

After calculating the faradaic impedance of a mechanism comprising a stage of anodic oxidation of an electroactive species, followed by superficial recombination of an adsorbate, the sizes of the various elementary impedances of the species were defined. An impedance zone diagram was plotted. It enables the numer of faradaic impedance loops to be determined in relation to the value of the kinetic parameters of the model.

Étude du comportement électrochimique du couple Fe(CN) 63−−Fe(CN) 64− sur électrode de platine et de carbone vitreux

The different models for the transfer processes of redox systems are compared by plotting a potentiostatic zone diagram. This diagram clearly shows the different steady state behaviours of the redox systems. It is used to compare the rapidity of the Fe(CN) 6 3−−Fe(CN) 6 4− system on a platinum electrode and on a vitreous carbon electrode. The reduction process of the ion Fe(CN) 6 3− is found to be comparable with a nernstian system on a platinum electrode under the given experimental conditions; however, this is no longer the case for a vitreous carbon electrode.

Product distribution in preparative scale electrolysis: Part V. EC reaction schemes followed by competition between dimerization and first-order deactivation or further electron transfer

The problem of product distribution is analyzed for two reaction schemes involving the dimerization of the product formed upon electron transfer followed by a first-order chemical reaction. The first of these involves a competition between dimerization and first-order deactivation. The product distribution then depends upon a single competition parameter containing the rates of the three chemical reactions and the operational parameters, substrate concentration and diffusion layer thickness for the potentiostatic electrolysis regimes and current density for galvanostatic electrolysis. In the second reaction scheme dimerization competes with further electron transfer. The latter reaction may occur at the electrode and/ or in the solution leading to a three-cornered Dim-ECE-Disp competition. A detailed analysis of the limiting Dim-ECE and Dim-Disp behavior is carried out as a function of a single competition parameter having a different expression for each case. This allows the product distribution to be predicted as a function of the intrinsic (rates) and operational (concentration, diffusion layer thickness, current density) parameters for the two limiting situations. In the general case a zone diagram is given which represents the effect of the parameters on the outcome of the three-cornered competition. The reductive cleavage of aromatic and aliphatic organic molecules is discussed in this context, illustrating the application of the results to experimental systems.

Étude d'un mécanisme d'oxydation anodique en phase adsorbée à l'aide de diagrammes de zone I: Comportement potentiostatique

It is shown that it is possible with the help of a kinetic zone diagram to state quantitatively the condition to be fulfilled by the rate constants and the potential of the electrode in order that the kinetic control be either the diffusion process, the charge transfer or the chemical desorption process for a multistep oxidation reaction A - o → A - A - + s → A,s + e A,s → A + s

ECE and disproportionation: Part VI. General resolution. Application to potential step chronoamperometry

The competition between homogeneous and heterogeneous electron transfers in ECE-DISP mechanisms: A+eB BC C+eD B+C→A+D is analyzed in the context of potential step chronoamperometry. Starting from the expressions of the apparent number of electrons for the limiting situation, ECE, DISP1, DISP2, the transition between two of these “pure” mechanisms is systematically investigated. The general system is then analyzed for large values of the kinetic parameters, i.e., in the particular case of a stationary state arising from mutual compensation of diffusion and chemical reaction (“pure kinetic” conditions). This provides an estimation of the range of parameters where numerical analysis of the general case is actually necessary together with tests of accuracy of the numerical computation procedures. In this framework the analysis of the most general case involving no particular assumption about the magnitude of the thermodynamic and kinetic parameters leads to a three-dimensional kinetic zone diagram which allows to predict the effects of the intrinsic (equilibrium and rate constants) and operational (time, concentration) parameters on the displacement of the system from one limiting situation to the other. On these bases, the practical effectiveness of single-step techniques in discriminating between the limiting mechanisms is discussed. It is shown that the discrimination between DISP2 and either ECE and DISP1 can be easily carried out in terms of either concentration or time dependence of the current response. The discrimination between ECE and DISP1 is much more difficult and requires a high experimental accuracy to be effective.

ECE and disproportionation Part V. Stationary state general solution application to linear sweep voltammetry

Consideration of ECE type schemes: A+1eB BC C+1eD involving a chemical reaction interposed between two electron-transfer steps has been very successful in explaining the occurrence of the multi-electron processes so often encountered in organic electrochemistry. As soon as the second electron transfer is regarded as easier than the first one the interference of the disproportionation reaction: B+CA+D in the overall kinetics must however be considered. Besides the ECE case two other limiting mechanisms DISP1 and DISP2 are thus also obtainable corresponding respectively to reaction [2] or reaction [4] being the r.d.s. The polarization problem is stated and solved under “pure kinetic” conditions, i.e., when a stationary state deriving from mutual balance of diffusion and chemical reaction is established. The results are applied to peak location and peak shape analysis in linear sweep voltammetry. It is shown that the effect of the experimental parameters: rate and equilibrium constants, sweep rate and initial concentration can be rationalized with the use of only two dimensionless parameters in the general case. A finite difference approach is described in the genral case whereas in most of the particular cases (pure mechanisms, or transition between two of the limiting cases) integral equations are obtainable. A kinetic zone diagram is then constructed in the space of the two dimensionless parameters. It allows to predict how a variation in each of the experimental parameter shifts the system from one limiting mechanism to the other.

Ece and disproportionation: Part V. Stationary state general solution application to linear sweep voltammetry

Consideration of ECE type schemes: A+1eB BC C+1eD involving a chemical reaction interposed between two electron-transfer steps has been very successful in explaining the occurrence of the multi-electron processes so often encountered in organic electrochemistry. As soon as the second electron transfer is regarded as easier than the first one the interference of the disproportionation reaction: B+CA+D in the overall kinetics must however be considered. Besides the ECE case two other limiting mechanisms DISP1 and DISP2 are thus also obtainable corresponding respectively to reaction [2] or reaction [4] being the r.d.s. The polarization problem is stated and solved under “pure kinetic” conditions, i.e., when a stationary state deriving from mutual balance of diffusion and chemical reaction is established. The results are applied to peak location and peak shape analysis in linear sweep voltammetry. It is shown that the effect of the experimental parameters: rate and equilibrium constants, sweep rate and initial concentration can be rationalized with the use of only two dimensionless parameters in the general case. A finite difference approach is described in the genral case whereas in most of the particular cases (pure mechanisms, or transition between two of the limiting cases) integral equations are obtainable. A kinetic zone diagram is then constructed in the space of the two dimensionless parameters. It allows to predict how a variation in each of the experimental parameter shifts the system from one limiting mechanism to the other.

Deglaciation Chronology of Lofoten – Vesterålen – Ofoten, North Norway

A deglaciation chronology of the last ice sheet in Lofoten - Vesterålen - Ofoten based on regional shore displacement is presented. An isobase system and a shore zone diagram have been constructed. The heights of raised shore features refer to present mean tide level, and have been adjusted according to the variation in height of recent forms. Owing to this variation in particular, and deformation of features due to shore displacement, a shore zone diagram has been substituted for the traditional shore line diagram. Seven stages in the ice recession have been traced. The outermost probably occurred at a time close to the maximum of the last ice sheet. During its retreat, the ice became increasingly directed by topography. The situation of deposits in the landscape was determined by this. The regional shore displacement seems to indicate that the highest mountains in the area were nunataks during the last glacial maximum.