Zbigniew Galus

On the present understanding of the nature of inhibition of electrode reactions by adsorbed neutral organic molecules

Summary The discussion of the present state of understanding of the inhibition of electrode reactions by adsorbed organic compounds has been presented. It has been shown that the different models of the mechanism of inhibition may be reduced to the concept of competitive adsorption of activated complex, solvent and inhibitor molecules with different adsorption isotherms taken to describe adsorption of activated complex. On the basis of a statistical analysis of the literature data it has been shown that the inhibition may be well described by three different model equations originated from Flory-Huggins, Frumkin or Blomgren-Bockris adsorption isotherms for activated complex. Because of marked experimental errors both for the determination of the rate constant of the electrode process and the surface coverages of the electrode by the inhibitor, as well as a result of the impossibility or the proper correction of the rate constants for double layer effect it has been proved that the choice of the model of inhibition is not an ambiguous procedure if the measurements are carried out in the range of surface coverages 0

A reconsideration of the kinetic data for the Fe(CN)63−/Fe(CN)64− system

Abstract The electrode kinetics of the Fe(CN) 6 3− /Fe(CN) 6 4− system was studied using a rotating platinum disc electrode. A significant increase of the standard rate constant was observed for a low depolarizer concentration. This phenomenon was explained in terms of the partial blocking of the electrode surface at higher reactant concentration by poorly soluble species which arise from Fe(CN) 6 3− or Fe(CN) 6 4− under the influence of the electrode field. The results of this paper suggest that the rate constants of the electrode reaction of the Fe(CN) 6 3− /Fe(CN) 6 4− system published so far are too low.

On the mechanism of electrode reactions occurring in the presence of foreign neutral adsorbable organic compounds

Abstract With the use of the thermodynamics of the surface phase the distinction between inhibitive properties of the equilibrium and non-equilibrium surfactant adsorption on energetically uniform and non-uniform surface is shown. It was pointed out that these properties additionally depend on the position of the reactant inside or outside the adsorbed layer. The respective equations are derived. On the basis of thermodynamic arguments the critical reassessment of the present understanding of the inhibition phenomena is performed. The possibility of the appearance of a limiting rate at higher coverages of the electrode by surfactant is discussed.

Electrochemical redox mechanism of uranium in acidic perchlorate solutions

Summary The reduction and oxidation processes of uranium(VI), (V), (IV) and (III), in aqueous perchlorate solutions have been studied using the dropping mercury electrode, the hanging mercury drop electrode and the thin-layer technique. The experiments were performed in acidic media (0 −1 . The data obtained show that under certain conditions, the reduction process of uranium(V) is influenced by the chemical reaction of uranium(V) with uranium(III), resulting in the formation of uranium(IV). In the presence of this reaction the character of the polarographic and chronovoltammetric curves is changed. The disappearance of the uranium(III) oxidation peak on the chronovoltammetric curves obtained with the hanging mercury drop electrode can also be explained by this reaction. The kinetic parameters of the reduction of uranium(V) to (IV) were determined as k f,h 0 =(4.0±0.4)×10 −8 cm s −1 , and α=0.46±0.01. A detailed mechanism of the reduction of uranium(V) is proposed.

Ion pair formation effects in the reversible electroreduction of nitrobenzene in N,N-dimethylformamide

Summary From polarographic measurements p K ass values of ion pairs formed by the nitrobenzene radical anion N − and cations M + have been estimated. Application of the function (R+δ) −1 distinctly improved the empirical relation between E 1/2 ipf and ionic potentials verifying the assumption of the considerable influence of solvation effects on ion pair formation. This is also supported by a thermocycle treatment of solvation and association of ions and ion pairs.

A new development in polynuclear inorganic films: Silver(I)/“crosslinked” nickel(II)-hexacyanoferrate(III, II) microstructures

Abstract Ag(I)-stabilized Ni(II)—hexacyanoferrate(III, II) can be electrochemically produced on common electrode substrates; during reductive potential cycles (between 0.85 and 0 V vs sce ) in the colloidal mixture of the respective components, the mixed polynuclear films are grown. The results are compared with the Ag-free Ni(II)—hexacyanoferrate system. It is shown that the overall rigidity and stability of the novel material is significantly increased; further, addition of Ag(I) results in much more restricted flow of a cation of supporting electrolyte (favoring potassium over other alkali cations).

Manifestation of steric factors in electrode kinetics: Investigations of the deposition and dissolution kinetics of the Cd2+/Cd(Hg) system in presence of adsorbed aliphatic alcohols and acids

Abstract The role of the steric factors in the electrode reactions occurring on the interface covered by aliphatic alcohols and acids has been investigated, taking as an example the Cd 2+ /Cd(Hg) system. It has been shown that this process takes place inside the inner layer and the activated complex is created by the replacement of r ≠ water molecules from the surface. From studies carried out at surfactant concentrations corresponding to the plateau of the adsorption isotherm the steric factors r ≠ / r i have been determined ( r i is the number of solvent molecules replaced from the surface by one molecule of adsorbed surfactant). By comparison with investigations performed at lower coverages the magnitude of r ≠ and r i has been estimated. The influence of different factors on the experimental value of these parameters has been discussed.

Kinetics and mechanism of the Cr3+/Cr2+ electrodereaction in concentrated perchlorates and chlorides

Summary The electrode reaction Cr(III)+e→Cr(II) was studied in concentratedsolutions of NaClO4, Ca(ClO4)2, NaCl and CaCl2. In perchlorate solutions the formal potentials, standard rate constants and electrode reaction orders were determined. From these data the hydration numbers were estimated and the mechanism of the electrode system studied was discussed. The rate constant of the Cr(III)/Cr(II) system is practically independent of the perchlorate concentration. The electrode reaction proceeds much faster in concentrated calcium chloride. In addition the Cr(II)/Cr system at a mercury electrode in concentratedsolutions of sodium perchlorate and calcium chloride was studied.

Mixed-valence electron hopping, redox conduction and migration effects in solid-state electrochemistry of transition metal hexacyanoferrates

A model is proposed and supported with experimental data to describe solid-state electrochemical responses of transition metal hexacyanoferrate powders: K2NiIIFeII(CN)6/KNiIIFeIII(CN)6, KlnIIIFeII(CN)6/InIIIFeIII(CN)6 and Berlin Green (K1-x{FeIII[FeIII(CN)6]}x·{FeIII[FeII(CN)6]}1-x, where x < 1). The materials are prepared at various degrees of mixed valency, i.e. ratios of hexacyanoferrate(III, II) sites, contain different amounts of structural K+-counterions, and differ in the extents of hydration. The powders are sandwiched between two glassy carbon slide electrodes and investigated in the absence of an external supporting electrolyte. The model is based on the assertion that the hexacyanoferrate(III, II) sites are fixed, and electron self-exchange (hopping) between them is fast. The nature of the current—potential responses is strongly dependent on the systems degree of hydration, the related potassium mobility and the possibility of coupling electron transfer with the K+ counterion transport. When the materials are largely hydrated (e.g. contain 7–10 structural H2O), and comprise potassium at high concentrations (as in nickel hexacyanoferrates), the applied potential differences produce diffusional concentration gradients and lead to redox transitions. A typical redox conductivity pattern is in the form of well-defined voltammetric peaks, symmetrical around 0 V, and proportional to the degree of mixed valency. Transport of K+ seems to be rate limiting. Chronocoulometry yields effective diffusion coefficients for charge propagation in the nickel(II) hexacyanoferrate(III, II) and indium(III) hexacyanoferrate(III) systems in the range 10−10 to 10−9 and 10−11 to 10−10 cm2 s−1, respectively. Drying of the materials freezes the counterions and, as the systems are mixed-valence semiconductors, makes a sitewise electron hopping, an ohmic mechanism, predominant. The ohmic effects may also appear in the hydrated systems, owing to migration, when the population or mobility of structural K+ ions is decreased.

Electrochemical identity of copper hexacyanoferrate in the solid-state: evidence for the presence and redox activity of both iron and copper ionic sites

Abstract We describe the electrochemical behavior of solid copper hexacyanoferrate (powder) investigated in the absence of contact with a liquid external supporting electrolyte. In addition to a typical hexacyanoferrate(III,II) redox transition, the system is characterized by a second electrode process which appears at more negative potentials. Diagnostic experiments, which include spectrochemical and solid-state voltammetric characterization, support our view that the latter redox reaction most likely originates from reduction of the systems lattice copper(II) ions. The overall dynamics of charge transport in the material, when discussed in terms of effective diffusion, is moderately high (on the level of 10 −8 cm 2 s −1 ). Comparison has been also made to the electrochemical behavior of solid Prussian blue and to the voltammetric characteristics of conventional electrodes modified with thin films of copper hexacyanoferrate.